This manuscript has been reproduœd from the microfilm master. UMI films the

text directly from the original or copy submitted. Thus, some thesis and

dissertation copies are in typewriter face, while others may be from any type of

cornputer printer.

The quality of this reproduction is dependent upon the quality of the copy

submitted. Broken or indistinct print, colored or poor quality illustrations and

photographs, print bleedthrough, substandard margins, and improper alignment

can adversely affect reproduction.

In the unlikely event that the author did not send UMI a complete manuscript and

there are missing pages, these will be noted. Also, if unauthorized copyright

material had to be rernoved, a note will indicate the deletion.

Oversize materials (e-g., maps, drawings, charts) are reproduced by sectioning

the original, beginning at the upper left-hand corner and wntinuing from left to

right in equal sections with small overlaps. Each original is also photographed in

one exposure and is induded in reduced form at the back of the book.

Photographs included in the original manuscript have been reproduced

xerographically in this copy. Higher quality 6" x 9" black and white photographic

prints are available for any photographs or illustrations appearing in this copy for

an additional charge. Contact UMI directly to order.

Bell & Howell Information and Leaming 300 Norai Zeeb Road, Ann Arbor, MI 48106-1346 USA

8OO-S2l-O6OO

Compositional and Carbon Isotopic Characteristics of Abiogenically Derived Hydrocarbons at the Kidd Creek Mine, Timmins, Ontario

Timothy David Westgate

Thesis submitted in conformity with the requirements of the degree of Master of Science Graduate Department of Geology

University of Toronto

O Copyright by Timothy David Westgate 1998

National Library I * m of Canada Bibliothèque nationale du Canada

Acquisitions and Acquisitions et Bibliographie Services senrices bibliographiques

395 WelIington Street 395. rue Wellington Ottawa ON K1A ON4 Ottawa ON K1A ON4 Canada Canada

The author has granted a non- exclusive licence aUowing the National Library of Canada to reproduce, loan, distribute or sel1 copies of ths thesis in microfom, paper or electronic formats.

The author retains ownership of the copyright in this thesis. Neither the thesis nor substantial extracts fiom it may be printed or othenvise reproduced without the author's permission.

L'auteur a accordé une licence non exclusive permettant à la Bibliothèque nationale du Canada de reproduire, prêter, distribuer ou vendre des copies de cette thèse sous la forme de microfiche/£ih, de reproduction sur papier ou sur format électronique.

L'auteur conserve la propriété du droit d'auteur qui protège cette thèse. Ni la thèse ni des extraits substantiels de celle-ci ne doivent être imprimés ou autrement reproduits sans son autorisation.

Compositional and Carbon Isotopic Characteristics of Abiogenically Derived Hydrocarbons at the Kidd Creek Mine, Timmins, Ontario

Timothy David Westgate Degree of Master of Science, 1998, Graduate Department of Geology, University of Toronto

Abstract

Volumetrically sipifkant quantities of hydrocarbon-rich gas are stored in the

crystalline rocks of the Canadian Shield at Kidd Creek Mine, Timrnins, Ontario.

Measurements of gas discharge rates, compositions and isotopic signatures indicate that;

1) large quantities of gas discharge onto the 6800 fi. Ievel of the mine; 2) lower explosive

limits of gas mixtures are on average >17% lower than that calculated based on a pure

CH4 phase in air; 3 ) gases are not significantly altered by secondary processes such as

mixing, migration and oxidation subsequent to borehole completion; and 4) there is low

hydraulic communication among the fractures hosting the gases.

C& hydrocarbons typically become more depleted in I3c with increasing

molecular weight. The relative concentrations of Ci-C4 hydrocarbons follow a Schulz-

Flory distribution. These distinct isotopic and compositional signatures indicate that the

gases are not produced by conventional thermogenic processes. Rather, they suggest that

the gases are fomed by processes of inorganic synthesis via water-rock interaction.

Acknowled, amen&

1 would first like to thank my thesis supervisor, Dr. Barb Shenvood Lollar for the

oppominity to conduct this research project and for her continued guidance,

encouragement and insight throughout the various stages of this project.

1 would also like to extend my gratitude to my supervisory cornmittee, Dr. Don

Chipley and Dr. Ed Spooner for their criticai reviews and comments on this manuscript.

Thanks also go to Neil Amer and Greg (Hed) Slater for their invaluable assistance in the

lab and in underground field work. 1 would also like to acknowledge the staff in the

Geology Ofice at Kidd Creek Mine for their assistance in the mine and for providing the

much needed geological information. I owe my sanity to the rest of the grad students in

the Department: Mike (T. McStaggart), Brenda, Heather, Chris, Jason, Man, Kim, and

Nawojka, (and the good people at the GSU) al1 of whom never hesitated to twist my arm.

I am especially thankful to Andrew Coniy for clrafting figure 2.2.

I am grateful for financial support from Falconbridge Mining Ltd. and NSERC

which made this research possible. 1 am also grateful to the University of Toronto for the

scholarship that provided me with rneans to live over the course of my studies.

Last but not least, I would like to thank my parents, John and Cora Westgate, and

the rest of my family, Vicki, Andrew, Jane, Alec, David, Heather, Joshy, and Maddy for

their continued interest and unwavering support throughout my studies. 1 would also like

to thank al1 the members of my extended family, Joaq Roy, Ted, Fran, Andrea, Shannon,

Sarah, and Jarnie and rny friends, Dave (B.T.C.), Charlene, Aaron (Dr.) and Deb for

providing me with a life outside of the realm of isotope geochemistry! Above all, I would

like to dedicate this work to Ken-y, for her encouragement, understanding, fnendship and

love over the years.

Abstract

Acknowledgments

Table of Contents

Chapter 1. Introduction

1.1 Introduction 1.2 Charactenstics of Hydrocarbon Gases of Biogenic and Abiogenic Ongin 1.3 Occurrences and Processes of Abiogenic Hydrocarbon synthesis

1 -3.1 Mid-Ocean-Ridge Hydrothermal Environrnents 1.3.2 Ophiolite Sequencas 1.3.3 Expenmental Evidence for the Abiogenic Synthesis of Hydrocarbon Gases 1.3 -4 Shield Gases

1.4 Research Objectives 1.5 Thesis Structure 1 -6 References 1.7 List of Figures

Figure 1. i Figure 1.2

Chapter 2. Spatial distribution and temporal evolution of hydrocarbon gases discharging from boreholes at the Kidd Creek Mine, Timmins, Ontario, Canada.

2.1 Abs tract 2.2 Introduction

2.2.1 Geological Setting and Sampling Location 2-3 1Methodology

2.3.1 Sample Collection 2.3.2 Compositional Analysis 2.3.3 Isotopic Analysis

2.4 Results 2.4.1 Compositional Results 2 -4.2 Carbon Isotope Results

2.4.3 Gas Discharge Rates and Reservoir Estimates 2.5 Discussioii of Results

2.5.1 Bacterial Gas Component 2.5.2 host Rock - Gas Relationships

2.6 Sumrnary artd Conclusions 2.7 References 2.8 List of Figures

Figure 2.1 Figure 2.2 Fiawe 2.3 Figure 2-4 Figure 2.5 Figure 2-6 Figure 2-7

2.9 List of Tables Table 2.1 TabIe 2.2 Table 2.3 Table 2.4

Chapter 3. Carbon isotopic and compositional evidence for inorganic synthesis of hydrocarbon gases in crystaliïne rock environments

3.1 Abstract 3 -2 Carbon Isotope Distributions in C 1 -C2 Hydrocarbons 3.3 Hydrocarbon gases at Kidd Creek Mine 3 -4 References 3.5 List of Figures

Figure 3.1 Figure 3.2

3 -6 List of Tables Table 3.1

3 -7 List of Appendices Appendix 3.1 Appendix 3.2

Chapter 4. Summary of Conclusions 4.1 Conclusions 4.2 References

Chapter 1 : Introduction and Literature Reviav.

Chapter 1: Introduction and Literature Review

1.1 INTRODUCTION

The origin of methane (CH4) and associated light hydrocarbons (Cd%, C3H8,

C a i o ) in subsurface deposits has traditionally been attributed to two processes, bacterial

production and thermal alteration of sedimentary organic matter. These two processes are

commonly referred to as bacteriogenic and thermogenic respectively (Schoell, 1988).

Bacterial gas is primariIy cornposed of CK, which is produced by methanogenic

microorganisms via two principal pathways: CO? reduction and acetate fermentation

(Schoell, 1980; Wolternate et al., 1984; Jenden and Kaplan, 1986; Martens et ai., 1986;

Burke et al., 1988). Thexmogenic gases are cornposed of variable mixtures of C h and

higher (C2+) hydrocarbons that are produced through the themochemical cracking of high

molecular weight sedimentary organic matter. Hydrocarbons produced both via

bacteriogenic and thermogenic processes are considered to be biogenic since the ultimate

carbon source is organic (Welhan, 1988).

Altematively, both CH4 and light (C2+) hydrocarbons can be produced via inorganic

or abiogenic processes. By definition, processes of abiogenic CH4 production are

considered to be those that do not involve a high molecular weight organic precursor

(Welhan, 1988). Abiogenic methanogenic processes include: re-equilibration of

magrnatic COz during cooling p e l h a n and Craig, 1979; Gerlach, 1980; Holloway, 1984;

Kreulen, 1987; Kelley, 1996); the Fischer-Tropsch synthesis (Lancet and Anders, 1970);

hydrolysis of mafic and ultrarnafic rocks (Apps, 1985; Bemdt et al., 1996); and

metamorphkm of carbonate or graphite-bearing rocks (Giardini and Salotti, 1969;

Holloway, 1984). These processes have been invoked to account for CH4 occurrences in

Chap ter 1 : In iroducrion and Literature Reviav.

several environments including: unsedimented mid-ocean ridges such as the Mid-Atlantic

Ridge, the East Pacific Rise and the Southwest Indian Ridge (Welhan and Craig, 1979;

Weihan and Craig, 1983; Vanko and Stakes, 199 1; Bougault et al., 1993; Charlou and

Donval, 1993; Kelley, 1996); obducted ophiolite sequences such as the Semai1 Nappe in

Oman (Fria et al., 1992) and the Zambales Ophiolite in the Philippines (Abrajano et al.,

1988; 1990); fluid inclusions associated with aikaiic granitic intrusives in south

Greenland (Petersilie and Sorensen, 1970; Komerup-Madsen et al., 19SS), and Quebec,

Canada (Salvi and Williams-Jones, 1997); and the crystalline rocks of the Canadian

Precarnbnan Shield (Sherwood Lollar et al., 1993a). As wili be discussed in the

following section, the genetic classification of biogenic and abiogenic gases has been

facilitated by the application of both compositional and stable isotopic analyses.

1.2 GENETIC CHA~CTERIZATION OF HYDROCARBON GASES OF BIOGENIC AND

ABIOGENIC ORIGIN

In a series of papers, Schoell (1 980; 1983; 1988) and Whiticar and Faber (1986)

developed a frarnework for the genetic classification of thermogenic and bacteriogenic

gases using compositional and isotopic signatures. This characterization scheme is

largely based on three parameters: 5"cC~4, and the relative abundances of higher

C2+ hydrocarbons. All isotopic values in this thesis are given in conventional delta (6)

notation in per mil (%O). 6% values are expressed with respect to PDB standard and 6 ' ~

values are expressed with respect to SMOW standard. It is possible to distinguish

between bactenogenic and thermogenic sources because bacterially derived gases are

Chapter 1 : Introducrion and Literamre Review, 3

predorninantly composed of CH4, whereas gases of thermogenic ongin also contain a

sipificant component of C2+ hydrocarbons. As result the CI/ (CI fC3) ratio of a

bacteriogenic gas is typically > 1000, whereas themogenically denved gases typically

have values ~ 5 0 (Bernard et al., 1977). Isotopically, it is possible to distinguish between

these two sources as well. Empirically denved data sets indicate that CH4 produced via

thermogenic or bacteriogenic pathways have characteristic ranges of S " C ~ H ~ and 8 ' ~ ~ ~ ~

isotope signatures. ïhemogenic gases typicaily have ?ii3cCH4 signatures between -25 and

-6OXo and s ~ H ~ ~ ~ values between -100 and -300%0. Bacteriogenic C& typicaliy has

values more negative than -60960 and s ~ H ~ ~ ~ values more negative than -150%0

( F i 1 ) In comparison, the compositional and isotopic characteristics of abiogenic

gases are not as well defined. Abiogenic Cl& derived fiom mantle carbon sources

through either direct mantle degassing or hi& temperature re-equilibration of mantle-

denved CO?, is generally thought to have high 6% values (-6.9 to -1 8.0%0; Abrajano et

al., 1988; 1990; Welhan, 1988; Fritz et al., 1992). Volatiles denved fiom the mantle can

typically be identified through the analysis of helium isotopes. Helium isotope ratios are

a sensitive means of identifjmg m a d e volatiles because mantle denved He is enriched in

the 'primitive' 3 ~ e isotope which was trapped in the earth's interior during planetary

accretion (Lupton, 1983). Typical crustal and mantle end mernber 3 ~ e / ' ~ e isotopic

values are 2.5~10" and 1.2x10-~, respectively (O'Nions and Oxburgh, 1983). He isotope

ratios are generally expressed as R R A ratios where R is the ' ~ e / ' ~ e ratio of the sarnple

and RA is the 3 ~ e / ' ~ e ratio of the present atmosphere. The R/RA ratios of stable

continental cmst are commonly in the range of ~ 0 . 0 1 - 0.1 whereas any gas with > 1%

C hap ter 1 : Introduction and Literature Rmevrew. 4

rnantle derived component will have a R/% ratio 20.1. Consequently, mantle derived

13 C - e ~ c h e d C& is typically associated with He which is enriched in 3 ~ e . However,

while " c - e ~ c h e d CH4 may be characteristic of mande-derived abiogenic CH4, isotopic

enrichment is not necessarily characteristic of kinetically controlled processes of

abiogenic CH4 formation from shallow crustal carbon sources (Le dissolved inorganic

carbon, graphite). Lyon and Hulston (1984) calculated a kinetic isotope fractionation

factor for the Fischer-Tropsch synthesis (A'~c(c&-co~)= -35.9%0) based on the

synthesis of C& h m CO2 + H2 at 400°C. However, there is too much variability in the

613c of the various carbon sources, and not enough known about fiactionation factors

involved with other abiogenic reaction mechanisms to establish a characterisric range of

isotopic signatures empirically. Nonetheless, abiogenic C h can ofien be recognized

based on the fact that it has isotope signatures that do not fa11 within the ranges

established for conventional biogenic sources.

In addition to the 613c signatures of CH4, the carbon isotope signatures of higher

rnolecular weight hydrocarbons (Cz to C4) can yield insight into the processes responsible

for their synthesis. CI to C4 hydrocarbons of thermogenic ongin have a characteristic

carbon isotope distribution whereby the hydrocarbons become more enriched in " C with

increasing molecular weight. This orderly isotopic distribution is accounted for by kinetic

fractionation effects that a i s e due to differences in the physical chemistry of ''c and 13c

which cause "c-"C bonds to be weaker, and thus break easier, than 1 3 ~ - 1 2 ~ bonds. This

charactenstic distribution has been modeled rnathematically (Smith et al., 197 1 ; Waples

and Toniheim, 1978; James, 1983; Chung et al., 1988; Galimov, 1988; Clayton, 1991;

Chap ter 1 : Introduction and Literature Reviav. 5

Rooney et al., 1995) and verified experirnentally by pyrolysis experiments in which light

hydrocarbons are produced via thermal decomposition of higher molecular weight

organic matter (Fig. 1 -2). Moreover, such distributions are ubiquitous among

thermogenically derived gases throughout the world and are, thus, considered to be

diagnostic of gases produced via this process (DesMarais et al., 1988).

Alternatively, CI+ hydrocarbons produced by the polymerization of lower molecular

weight homologues have the opposite carbon isotope distribution to that found in

thermogenic gases. This trend, such that the 613c values becorne more negative with

increasing molecular weight, has been produced experimentally in hydrocarbons

synthesized by spark discharge in a methane atmosphere (Fig 1.2; DesMarais et al., 198 1;

Chang et al., 1983). Again, this distribution results from kinetic isotope effects which

cause the light isotope ("c) to react faster than the heavy isotope ("c). By definition,

processes of abiogenic synthesis do not derive light hydrocarbon gases from the

degradation of higher molecular weight organic source matter. Instead, it is generally

thought that abiogenic processes synthesize higher molecular weight species frorn

precursors such as Cgaphire, COt, and CH4. It is possible, therefore that an isotopic

depletion trend similar to that observed in hydrocarbons produced by spark discharge

processes may be charactenstic of abiogenically derived hydrocarbons produced via other

abiogenic mechanisms. While relatively few nanirally occumng hydrocarbons having

such isotopic depletion trends have been identified, those that have appear to be produced

via abiogenic processes. Reported occurrences include: hydrocarbons obtained fiom the

Murchison meteorite (Yuen et al., 1 984) and hydrocarbons trapped in fluid inclusions

associated with the Khibiny massif, on the Kola Peninsula, Russia (Khtarov et al., 1979)

Cha pter 1 : Introdrrcrion and Literatzire Reviav.

and the Ilimaussaq cornplex, South Greenland (Konnerup-Madsen et ai., 1988).

The preceding discussion outlined how cornpositional and isotopic characteristics of

gases can be used to detemine their origins, as well as provide insight into the processes

responsible for their synthesis. The followinp is a more detailed discussion of known

occurrences of abiogenic hydrocarbon gases, their isotopic and compositional

charactenstics, and the processes responsible for their synthesis.

1.3 OCCURRENCES AND PROCESSES OF ABIOGENIC HYDROCARBON SYNTHESIS

1.3.1 Mid-Ocean-Ridge Hydrotherrnal Environrnents

C& and other light hydrocarbons are ubiquitous components of hydrothermal fluids

discharging from mid-ocean ndge (MOR) volcanic enviro~ments. In sorne MOR

environments, such as the Guaymas Basin, in the Gulf of California there are sipnificant

inputs of sedirnentary organic matter. The 6% values of the C& from these sedimented

MOR environments (-43.0 to -5 1 .O%*) and the low Ci/(Ci+C3) ratios are mutually

consistent with gases denved from the thermal alteration of sedimentaiy organic matter

(Weihan and Lupton, 1987; Lilley et al., 1993). In cornparison, CH4 is also an important

constituent of hydrothermal fluids venting from MOR environments that do not receive

any significant inputs of sedimentary organic matter such as the East Pacific Rise (EPR).

The 6 " ~ values of CH4 (-18.0 to -15.0 %O) gas venting fiom these sediment-starved MOR

environments are significantly more emiched than conventional biogenic gases (Weihan

and Lupton, 1987; Welhan, 1988). Gases in these environments have been attributed to

abiogenic ongins involved with the re-equilibration of '3~-enriched carbon derived fiorn

Chapter 1 : ~nrroducrion and Literarure Review. 7

a magmatic or mantle source. Isotope ratios of He in basalts and hydrothermal fluids are

also consistent with a mantle source, having ' ~ e f % e isotope ratios that are 8 t h e s higher

than atmospheric He (Lupton, 1983; Welhan, 1988). Moreover, carbon isotopes in C h

and CO2 record equilibrium temperatures in the 500-600°C rang, indicative of a high

temperature history, and consistent with tzmperatures in hydrothemai MOR

environrnents (Welhan, 1988). Analysis of magmatic volatiles occluded in basaltic

glasses indicate that CH4 and Hz are present in proportions similar to that found in EPR

hydrothermal fluids, suggesting that these gases are not the result of direct magmatic

outgassing but rather, are derived from the leaching of magmatic volatiles occluded in

fresh basaltic rocks (Welhan and Craig, 1983; Welhan, 1987). Based on this evidence,

the unusually '3~-enriched CH4 in the hydrothemal fluids discharging from the EPR

were interpreted in terms of high temperature re-equilibration with '3~-enriched

magmatic CO2 pnor to extraction from the rocks. More recently, detailed examination of

fluids occluded in the oceanic cmst have provided funher insight into the abiogenic

processes responsible for the synthesis of hydrocarbon gases in these environments.

The composition, and carbon and hydrogen isotope signatures of COr, CH4, Hz and

H20 in fluid inclusions in rock from a -50 l m section of drill core from the Southwest

Indian Ridge (SWIR) have been analyzed by Kelley (1996) and Kelley and Fruesh-Green

(1995). This core was recovered from a layer of the oceanic cmst that consists primanly

of mid-ocean ndge basalts overlying gabbroic rocks that extend d o m to the upper

mantle. This work provided the first direct analysis of CO2-Ca-H20-NaCI bearing

fluids in this section of the oceanic cmst. Based on analyses of fluid inclusions in both

the magmatic and alteration minera1 assembiages, two distinct phases of CE& generation

Chapter 1 : Introduction and Lilerature Review. 8

were identified with the first phase occumng dunng cooling and re-equilibration of

magmatic COZ, and the second phase occumng during sea water alteration of the oceanic

cmst.

The first phase of CH4 production is characterized by inclusions bearing

C02+CH4+H20 fluids which typically contain between 30 and 50 mol % CO2 and up to

33 mol % Cl&. Isotopic compositions of these fluids are consistent with a magrnatic

ongin (8%co2 = - 1.9 to -5.5%0; 6 ' ~ ~ z ~ = -33 to -65960). The range of 613cCH4 (- 18.8 [O -

27-49/00) values is consistent with C a formed by re-equilibration of the CO2 phase since

the reactions that produce the CH4 tend to favor the production of light "CI& This re-

equilibration was attributed to reducing conditions that are created either during the

precipitation of graphite or by the inward diffusion of Hz. Based on equilibrium

calculations, Holloway (1984) found that the presence of graphite placed important

constraints on the carbon-oxygen-hydrogen (C-O-H) system. Significantly, these

calculations show that in the presence of graphite, CH4 exists as a stable species under

geologically common oxidation conditions and temperatures. The inward difision of H2

would also tend to lower f 0 2 values towards more reducing conditions, which could also

cause the re-equilibration of CO2 to C h through the reaction:

The source of the Hz gas under these conditions could be either from direct magnatic

degassing or through serpentinization reactions associated with the alteration of the

ultramafic and mafic mineral assemblages (Neai and Stanger, 1983; Apps, 1985;

Abrajano et al., 1988; 1990). Serpentinization reactions are also thought to be important

Chapter 1 : Introduction and Lirerature Revr'avav 9

in the generation of abiogenic CH4 in the 'second phase' inclusions found in the oceanic

crust (Vanko and Stakes, 199 1 ; Kelley, 1996).

The second phase of CH4 production defmed by Kelley (1996) is characterized by

CH4+H20*H2*graphite bearing fluid inclusions with C& concentrations up to 40 mol %.

While the origins of these fluids are not well defined, they are compositionally and

isotopicaily consistent with fluids produced during seawater alteration of mafic rock and

serpentinization-type reactions associated with the alteration of ultramafic rocks in the

EPR and Southern Juan de Fuca Ridge (Kelley and Frueh-Green, 1995; Kelley, 1996).

The formation of CH4 and Ht during serpentinization reactions results from the reducing

conditions that are created during the hydrolysis of ferromagnesian silicates. Hydrolysis

of these minerals leads to the oxidation of ferrous (Fe'? iron to femc ( ~ e ' 3 iron (Moody,

1976; Apps, 1985; Neal and Stanger, 1983). This oxidation is coupled to a reduction

reaction in which Hz and CH4 can be formed through such reactions as (Abrajano et al.,

1988; 1990):

olivine + H20 + CO or COt = magnetite + serpentinite + CH4 + brucite + Hr (2).

Sirnilar geochemical mechanisms have also been invoked to account for the occurrence of

reduced gases in obducted ophiolite scquences.

1 J.2 Ophiolite Gases

Several studies have reported the occurrence of reduced gases issuing from fractures

in obducted ophiolite sequences such as the Zambales Ophiolite, Philippines and the

Semai1 nappe, Oman (Neal and Stanger, 1983; Abrajano et al., 1988; 1990; Fritz e t al.,

Chapter 1 : Introdz!ction and Lirerature Review. 10

1992). Gases from the Zambales ophiolite are composed predominantly of CH4 (ca. 55

vol %) and free Hz (m. 40 vol %) (Abrajano et al., 1988; 1990). The gases emanating

from the Semai1 nappe have significantly lower Cl& concentrations (CU. <5 %) and free

Hz concentrations that are substantially higher (up to 97 vol %) meal and Stanger, 1983;

Fritz et al., 1992). The 6 " ~ values of the C h fiom the Zambales Ophiolite s r ' Semail

nappe are both highly enriched in " C -6-9 to -14.7%0), arguing against a

conventional biogenic origin. Instead, these gases have been attributed to

serpentinization reactions similar to those descnbed above. While the carbon source for

such reactions is not clear, examination of the noble gas inventory of the Zambales gas

seep does show a significant mantle-derived component with ' ~ e & e ratios of -4RA

(Abrajano et al., 1990). This evidence, in conjunction with the 13~-enriched CH4 suggest

that the gases could be derived, at least in part, from volatiles (e-g., CO2, CO, CH4)

trapped in the ultramafic protolith.

1.3.3 Experimental Evidence for the Abiogenic Synthesis of Hydrocarbon Gases

In addition to these field studies, the geochemical mechanisms involved in the

production of reduced gases dunng the alteration of mafic and ultrarnafic rocks have been

elucidated through a number of experimental and theoretical studies. Apps (1983) tested

the thermodynamic feasibility of abiogenic C& synthesis during the hydrdysis of

ultramafic and mafic rocks. Reac tions were simulated using the reaction progress

computer code FASTPATH. At 25OC and 1 atm. pressure he found that in the presence

of CO2 denved from the decomposition of carbonates the abiogenic production of CH4

Chapter 1 : Introduction and Liîerature Review.

via the reaction:

was thermodynamîcally feasible. Janeclq and Seyfkied (1986) have verified this in an

experiment in which pendotites were reacted with seawater at 300°C and 500 bar for a

penod of two years. At the end of the study significant amounts of CH4 were measured

(66pmoUkg). This study indicated that C& was produced dunng serpentinization

reactions; however, since oniy one measurement was made at the end of the experiment it

was difficult to make any interpretations regarding the reaction mechanisms involved.

Unfortunately, neither these experiments nor subsequent studies investigated the isotopic

fractionation factors involved in these reactions.

More recently, Bemdt et al. (1996) conducted experimental serpentinization

reactions in order to defuie better the reaction mechanisms involved in the production of

these reduced gases. In this experiment, olivine Poss) was reacted with a CO2 beanng

NaCl fluid at 300°C and 500 bar in a reaction ce11 that was designed to permit the

periodic withdrawal of fluid sarnples throughout the course of the expenment. Their

results indicated that fiee Hz gas, CH4 and other light (Cz+) hydrocarbons were produced

during the serpentinization of the olivine. Berndt et al. (1996) invoked the Fischer-

Tropsch synthesis (see reaction 1 above) since this non-equilibnum processes could also

account for the formation of CzHs and CjHs hydrocarbons (He~ci -Ol ive and Olive,

1976; Satterfield and HUE, 1982). The distribution of hydrocarbons produced during this

expenment is characteristic of the Fischer-Tropsch synthesis, in which the ratios of

hydrocarbons with successive C numbers are nearly constant:

Chapter 1 : In~roduction and Literature Review-

This compositional distribution is referred to as the Schulz-Flory distribution in which the

log of gas concentration is linearly related to the carbon number (Schulz, 1935; Flory,

1936). While this distribution is not necessarily diagnostic of gases produced

abiogenically because it is theoretically possible for such distributions to be produced

during the random breakage of C-C bonds during themochemical degradation of

hydrocarbons, it is nonetheless charactenstic of hydrocarbons produced by

polyrnerization processes such as the Fischer-Tropsch synthesis (He~ci -Ol ive and Olive,

1976; Satterfield and Huff, 1982).

1-3.4 Canadian ShieId Gases

Volumetncally significant and regionally extensive accumulations of CI& and C2+

hydrocarbons occur across the Canadian Precarnbrian Shield. Gases in these

environments are stored in pressurized pockets and fracture systems in association with

Ca-Cl brines and saline groundwaters. The gases are composed predominantly of CH4, in

association with CzH6 and N2, as well as minor amounts of higher molecular weight

hydrocarbons (C3H8 and CJIIO) and He. At several sites on the Shield, gases have also

been found to contain significant concentrations of free H2 (up to 26 %) (Sherwood Lollar

et al., 1993a,b; Montgomery, 1994). The 6l3ccH4 and B'HCH~ values of gas ~a.&~leç

collected fiom sites on both the Canadian and Fennoscandian Shields do not fit into the

established ranges for either bactenogenic or thermogenic sources. 6I3cC~4 values for

these gases are, in general, too hi& and Ci/(C2 + C3) ratios ~ O O low to be accounted for

Chapter 1 : Introdztction and Literantre Rmèw. 13

entirely by bacteriogenic processes, and 62~CH4 values are typically more depleted in the

heavy isotope than conventional thermogenic gases (Shewood Lollar et al., 1993a;

Montgomery, 1994). Several gas samples were found to have very high 6I3c signatures

(up to -22.4%0) which approach that of rnantle-derived CH4 from the EPR (see section

1.3.1). However, ' ~ e l ' ~ e isotope ratios of gases from the Canadian and Fennoscandian

Shields (RA >0.03) are spical of cmstal values, indicating no significant mantle

component. Based on this unique set of compositional and isotopic signahues Sherwood

Lollar et al. (1993a) suggested that these gases were produced via processes of abiogenic

synthesis associated with water-rock interaction within the crystalline rocks. At many

sites the processes and reaction mechanisrns were unclear; at three sites in particular,

however, Shenvood Lollar et al. (1 993a) found that ' ~ - d e ~ l e t e d isotopic signatures for

CH4 and H2 were similar to isotope signatures found for CH4 and Hz produced via

serpentinization of ultramafic rocks at the present time in ophiolite sequences in the

Philippines and Oman. The widespread occurrence of altered ultramafic rocks at these 3

Shield sites further supports serpentinization as the source of CJ& and Hz in these areas.

As was discussed in section 1.2 above, examination of the carbon isorope distribution

among the CI to C4 hydrocarbons can also provide insight into the processes responsible

for their synthesis. While earlier work has provided evidence for an abiogenic ongin

based on 613cCH4 and 6 ' ~ ~ ~ ~ signahues and Ci/(C2+C3) ratios, the isotope signatures of

the higher hydrocarbon (C1 - C4) phases were not extensively characterized pnor to this

thesis.

In addition to abiogenically denved gases Sherwood Lollar et al. (1993b) showed

Chapter 1 : h~dzrct ion and Literature Reviav. 14

that a component of bacteriogenic gas may be present at several sites on the Canadian and

Fe~oscandian Shields. Through investigation of in situ biofilms Doig (1994) and Doig

et al- ( 1 995) dernonstrated that viable communities of methanogenic microorganisms

exist in exploration boreholes in the Canadian Shield. Using gas production rates

measured in a laboratory culture of these biofilms Doig et al. (1995) estimated that

between 3 and 17% of the total C& discharging from the boreholes could be produced by

ongoing processes of bacterial methanogenesis. This study was unable to resolve,

however, whether this bacteriogenic gas was produced by microorganisms indigenous to

the crystalline rock environrnent or by surface microbes introduced by mining operations

and drillinp.

Montgomery (1994) attempted to correlate gas occurrences with the structural and

geological features of the host rock at two mines on the Canadian Shield. Examination of

gashost rock relationships is significant for the Canadian Shield mining comrnunity since

it may allow them to identiQ possible gas bearing bost rocks. Moreover, such

relationships may provide further insight into the geochemical mechanisms responsible

for gas genesis. This earlier study utilized records made by mine personnel of where gas

was encountered during exploration drilling. Unfortunately, the anecdotal rather than

systematic approach to recording these gas occurrences was inherently incornplete and, as

a result, no quantitative relationships between gas occurrences and host rock

characteristics could be discerned.

Chapter 1 : Introduction and Literaîure Reviav.

1.4 Research Objectives

The present study was camied out fiom 1995 to 1997 on the 6800 ft. Level (L.6800)

of the Kidd Creek Mine, Timmins, Ontario. During this hme period the mine undertook

a major prograrn of exploration drillhg and expansion and was willing to facilitate a

systematic gas sampling program. Over 2 years boreholes were sampled as soon as

possible after completion and then resampled at replar intervals. The smtegy was to

examine the spatial and temporal distribution of gas occurrences, discharge rates,

compositions and carbon isotopic signatures for al1 available boreholes on L.6800 of the

Kidd Creek Mine and to monitor changes in approximately twelve of them through time.

The use of Gas Chromatograph-Combustion-Isotope Ratio Mass Spectrometry (GC-C-

IRMS) made it possible to establish the first systematic data set of 613c values for ethane,

propane, and butane in this environment, in addition to methane.

The research objectives of this study are as follows:

Chapter 2 is entitled: Spatial disiribution and temporal evolution of hydrocarbon

gases al the Kidd Creek Mine, Timmins, Ontario, Canada. This chapter outlines the

results of a detailed examination of spatial and temporal distributions of gas occurrences,

discharge rates, compositions and carbon isotopic signatures. The objectives are to: i)

examine the compositions of these gases and the implications they have with respect to

their lower explosive limit; ii) assess the volumes of gas stored in the rock and the degee

of hydraulic communication between gas pockets or reservoirs; iii) evalxate relationships

among gas occurrences, compositions, carbon isotopic signatures and the geological

Chapter 1 : Introdzrction and Literature Review. 16

andor structural features of the host rock; iv) detemine if secondary processes such as

mixing with a bacteriogenic gas component significantly alter the composition or isotope

signatures of the gases subsequent to borehole completion.

Chapter 3 is entitled: Carbon isotopic and compositions[ evidence for ïnorganic

synthesis of hydrocarbon goses Ni cystalline rock environments. This chapter shows the

results of the carbon isotope analysis of the Ci to C4 hydrocarbons. As was discussed in

section 1.3, examination of the distribution of carbon isotopes among CI to C4

hydrocarbons can provide insight into the processes responsible for their synthesis. This

chapter also presents the results of the compositional analyses of the C 1 -C4 hydrocarbons.

Similar to isotopic distributions, the relative concentrations of these Ci -C4 hydrocarbons

are also regulated by the processes responsible for their synthesis. This isotopic and

compositional àata provides new evidence in support of an abiogenic origin for Shield

gases at the Kidd Creek Mine.

1.5 Thesis Structure

This thesis bas been written as two papers (Chapter 2 and 3). An introductory

chapter, (Chapter 1) summarizing the compositional and isotopic charactenstics of

abiogenic gases, outlining previous work on Shield gases and other occurrences of

abiogenic gases; and a concluding chapter, (Chapter 4) summarizing the conclusions of

the two papers, completes the required thesis format. References, figures, and tables are

included at the end of each chapter. Chapter 2 was written in the format required for

submission to "The Canadian Journal of Earth Sciences". Chapter 3 was written in the

Chapter 2 : Inmduction and Literatur-e Reviav. 17

format required for subrnission as a ''Letter to Nature" (authorship for pubiications shown

below). Given the space limitations required by Nature, the coinpiete set of

compositional and isotopic plots of the data for chapter 3 is included in Appendices 3.1

and 3.2. For both chapters 2 and 3, references and headings have been refomatted to

maintain consistency throughout the thesis.

Chapter 1 : Inrroduction and Lirerature Review-

Chapter 2:

Authors: T.D. Westgate and B. Shenuood Lollar

Title: Spatial distribution and temporal evohtion of hydrocarbon gases ut the Kidd

Creek rMine, Timm ins, On tario. Canada

Current publication status: In preparation for subrnission to "Canadian Journal of Earth

Sciences"

Chapter 3:

Authors: T.D. Westgate and B. Sherwood Lollar

Title: Carbon isotopic and cornposilional evidence for inorganic synthesis of

hdvdrocarbo gases in cr ystalline rock enviuonments.

Current publication status: In preparation for subrnission to "Nature".

Chapter I : Introduchon and Lireraîure Reviav.

Abrajano T. A., Sturchio N., Kennedy B. M., Lyon G. L., Muehienbachs K., and Bohlke J. K. (1 990) Geochernistry of reduced gas related to serpentinization of the Zarnbales ophiolite, Philippines. Applied Geochernistry 5,625-630.

Abrajano T. A., Sturchio N. C., Bohike J. K., Lyon G. L., Poreda R. J., and Stevens C. LM. (1988) Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: deep or shallow ongin? Chernical Geology 71,211-222.

Apps J. A. (1985) Methane formation during hydrolysis by mafic rock. University of California.

Bernard B., Brooks J. M., and Sackett W. M. (1977) A geochemical mode1 for characterization of hydrocarbon gas sources in marine sediments. Ofshore Techno fogy Conference, Nineth, 43 5-43 8.

Bemdt M. E., Allen D. E., and Seyfned W. E. J. (1996) Reduction of CO2 during serpentinization of olivine at 300C and 500 bar. Geology 24(4), 35 1-354.

Bougault H., Charlou J.-L., Fouquet Y., H.D. N., Vaslet N., Appriou P., Baptiste P. J., Rona P. A., Dmitriev L., and Silantiev S. (1993) Fast and slow spreading ridges: structure and hydrothermal activity, ultrarnafic topographic highs, and CH4 Output. Journal of Geophysical Research 98(B9), 9643-965 1 .

Burke R. A. J., Martens C. S., and Sackett W. M. (1988) Seasonal variations of D/H and 13C/12C ratios of biogenic methane in surface sediments of Cape Lookout Bight, USA. Science.

Chang S., Des Marais D. J., Mack R., Miller S. L., and Strathearn G. E. (1983) Prebiotic organic syntheses and the origin of life. In Earth 's Earliest Biosphere, Its Ongin and Evohtion (ed. J . W . Schopf), pp. 53-92. Princeton University Press.

Charlou J. L. and Donval J. P. (1993) Hydrothermal methane venthg between 12N and 26N along the Mid-Atlantic ridge. Journal of Geophysical Research 98,9625-9642.

Chung H. M., Gormly J. R., and Squires R. M. (1988) Ongin of gaseous hydrocarbons in subsurface environments: Theoretical considerations of carbon isotope distribution. Chernical Geolqy 71,97-103.

Clayton C. (1 99 1) Carbon isotope fractionation during natural gas generation from kerogen. Marine and Petroleurn Geology 8,23 2-240.

Chapter 1 : Introduction and Lirerature Review. 20

DesMarais D. J., Donchin J. H., Nehring N. L., and Truesdell A. H. (1981) Molecular carbon isotopic evidence for the ongin of geothermal hydrocarbons. Nature 292, 826- 828,

DesMarais D. J., Stallard M. L., Nehring N. L., and Truesdell A. H. (1988) Carbon isotope geochemistry of hydmcarbons in the Cerro Prieto geothermal field, Baja California Norte, Mexico. Chenzical Geology 71, 159- 167.

Doig F. (1 994) Bactenal Methanogenesis in Canadian Shield Groundwaters. MSc., University of Toronto.

Doig F., Sherwood Lollar B., and Fems F. G. (1995) Microbial comrnunities in deep Canadian S hield groundwaters - An in situ bio film experirnent. Geornicrobio logy Journal 13,9 l-lOZ*

Flory P. J. (1936) Molecular size distribution in linear condensation polymen. Journal of the American Chemical Sociey 58, 1877-1 885.

Fritz P., Clark 1. D., Fontes J.-C., Whiticar M. J., and Faber E. (1992) Deuterium and ['c evidence for low temperature production of hydrogen and methane in a highly alkaline groundwater environment in Oman. Water-rock Interaction, 793-796.

GaIimov E. M. (1988) Sources and mechanisms of formation of gaseous hydrocarbons in sedimentary rocks. Chernical Geology 71,77-95.

Gerlach T. M. (1980) Chemical characteristics of the volcanic gases fiom Nyiragongo lava lake and the jeneration of CH4-nch fluid inclusions in alkaline rocks. Joumal of Votcanic and Geotherntal Research 8, 1 77- 1 89.

Giardini A. A. and Salotti C. A. (1969) Kinetics and relations in the calcite-hydrogen reaction and relations in the dolomite-hydrogen and siderite-hydrogen systems. American MineraZogist 54, 1 1 5 1 - 1 1 72.

Henrici-Olive G. and Olive S. (1976) The Fischer - Tropsch synthesis: molecular weight distribution of primary products and reaction mechanism. Angew. Chem. Int. Ed. Engl. 15(3), 136-141.

Holloway J. R. (1984) Graphite-CH4-H20-COz equilibria at low grade metarnorphic conditions. Geology 12,455-458.

James A. T. (1983) Correlation of natural gas by use of carbon isotopic distribution between hydrocarbon components. American Association of Petroleurn Geologists Bulletin 67, 1 176- 1 19 1.

Janecky D. R. and Seyfhed W. E. J. (1986) Hydrothemal serpentinkation of periodotite within the oceanic cmst: Expenmental investigations of mineralogy and major elernent chemistry. Geochimica et Cosmochimica Acta 50, 1357- 13 78.

C hapter 1 : Introduciion and Literarure Review. 2 1

Jenden P. D. and Kaplan 1. R. (1986) Cornparison of microbial gases from the Middle Amerka Trench and Scripps Submarine Canyon: implications for the onmgin of natural gas. Applied GeochemrSiry 1 ,63 1.

KeIley D. S. (1996) Methane-nch fluids in the oceanic crust. Journal of Geophysical Research 101(B2), 2943-2962.

Kelley D. S. and Frueh-Green G. (1995) Methane concentrations and isotopic compositions in layer 3 of the oceanic crust. EOS, AGU fall meeting, 675.

Khitarov N. L, Kravtsov A. L, Voitov G. L, Fridmm A. L, Ortenbeg N. A., and Pavlov A. S. (1979) Free emanation gases of the Khibiny massif ( in Russian). Sovetskaia geologiia 2,62-73.

Ko~erup-Madsen J., Kreulen R., and Rose-Hansen J. (1988) Stable isotope characteristics of hydrocarbon gases in the alkaline Ilimaussaq cornplex, south Greenland. Bull. Mineral. 106,642-653.

Kreulen R. (1987) Themodynamic cdculations of the C-O-H system applied to fluid inclusions: are fluid inclusions unbiased samples of ancient fluids? Chernical Geology 61,59-64.

Lancet H. S. and Anders E. (1970) Carbon isotope fractionation in the Fischer-Tropsch synthesis of methane. Science 170, 980-982.

Lilley M. D., Butterfield D. A., Olson E. J., Lupton J. E., Macko S. A., and McDuff R. E. (1993) Anomalous CH4 and NH4+ concentrations at an unsedimented mid-ocean- ridge hydrothemal system. Naiure 344,4547.

Lupton J. E. (1983) Terrestrial inert gases:isotope tracer studies and clues to primordial components in the mantle. Annual Reviews in Earth Planetary Science 11 ,3 7 1-4 14.

Lyon G. L. and Hulston J. R. (1984) Carbon and hydrogen isotopic compositions of New Zealand geothermal gases. Geochimica et Cosmochimica Acta 48, 1 16 1 - 1 17 1.

Martens C. S., Blair N. E., Green C., and DesMarais D. J. (1986) Seasonal variations in the stable carbon isotopic signature of biogenic methane in a coastal sediment. Science 233, 1300-1 303.

Montgomery J. (1994) An isotopic study of CH4 and associated N2 and H1 gases in Canadian Shield mining environments. MSc, University of Toronto.

Moody J. B. (1976) Serpentinkation: a review. Lithos 9, 125-1 38.

Neal C. and Stanger G. (1983) Hydrogen generation from m a d e source rocks in Oman. Earth and Planetary Science Letiers 60,3 15-32 1.

Chapter 1 : Introduction and Literature Review. 22

O'Nions R. K. and Oxburgh E. R. (1983) Heat and helium in the earth. Nature 306(5942), 429-43 1.

Petenilie 1. A. and Sorensen H. (1970) Hydrocarbon gases and bituminous substances in rocks from the Ilrnaussaq alkaline intrusion, south Greenland. Lithos 3, 59-76.

Rooney M. A., Claypool G. E., and Chung H. M. (1995) Modeling thermogenic =as generation using carbon isotope ratios of natural gas hydrocarbons. Chemical Geology 126, 2 19-232.

Salvi S. and Williams-Jones A. E. (1997) Fischer-Tropsch synthesis of hydrocarbons during sub-solidus alteration of the Strange Lake peralkaline granite, QuebecILabrador, Canada- Geochimica et Cosmochimica Acta.

Satterfield C. N. and Huff G. A. J. (1982) Carbon number distribution of Fischer-Tropsch products formed on an iron catalyst in a s1un-y reactor. Journal of CataZysis 73, 187- 197.

Schoell M. (1980) The hydrogen and carbon isotopic composition of methane frorn natural gases of various ongins. Geochimica Cosmochimica Acta 44,649-66 1.

Schoell M. (1983) Isotope techniques for tracing migration of gases in sedimentary basins. Jorrrnal of Geological Society London 140,4 15-422.

Schoell M. (1 988) Multiple origins of methane in the earth. Chemical Geology 71, 1-1 0.

Schulz C. V. (1935) Ueber die beziehungen zwischen reakionsgschwinddigkeit and zusammenstzung des reaktionprodukts bei makropolymerisations-vorgangen. Zeirschrifr frlr Physlralische Chemie B30,3 79-3 98.

Sherwood Lollar B., Frape S. K., Weise S. M., Fritz P., Macko S. A., and Welhan J. A. (1 993a) Abiogenic methanogenesis in crystalline rocks. Geochimica et Cosmochimica Acta 57,5087-5097.

Sherwood Lollar B., Frape S. K., Fritz P., Macko S. A., Welhan J. A., Blomqvist R., and Lahermo P. W. (1993b) Evidence for bacterially generated hydrocarbon gas in Canadian Shield and Fennoscandian Shield rocks. Geochimica et Cosmochimica Acta 57,5073-5085.

Smith J. E., Erdman J. G., and Morris D. A. (1971) Migration, accumulation, and retention of petroleum in the earth. 8th WorZd Petroleum Congress Proceedings, 13- 26.

Vanko D. A. and Stakes D. S. (1991) Fluids in oceanic layer 3: evidence from veined rocks, hole 735B, Southwest Lndian Ridge. Proceedings of rhe Oceanic Drilling Program, Scientifc Results 1 18, 1 8 1 -2 1 5.

Chapter 1 : introduction and Lireratrire Reviav. 23

Waples D. W. and Toniheim L. (1978) Mathematical models for petroleum-forming processes:carbon isotope fractionation. Geochimica et Cosmochimica Acta 42, 467- 472.

Welhan J. A. ( 1987) Methane and hydrogen in mid-ocean-ridge basalt glasses: analysis be vacuum cnishing. Canadian Journal of Earth Sciences 25, 38-48.

Weihan J. A. (1 988) Origins of methane in hydrothermal systems. Chernical Geology 71, 133-198.

Welhan I. A. and Craig H. (1979) ~Methane and hydrogen in east pacific rise hydrothermal fluids. Geological Research Letrers 6(11), 829-83 1.

Welhan J. A. and Craig H. (1 983) Methane, hydrogen and helium in hydrothennal fluids at 21 degrees N on the east Pacific Rise. In hydrothermal processes at seafloor spreading centres (ed. P. A. Rona, K. Bostrom, L. Laubier, and K. L. J. Smith), pp. 3 9 1-409. Plenum.

Welhan J. A. and Lupton J. E. (1987) Light Hydrocarbon Gases in Guaymas Basin Hydrothermal Fluids: Thermogenic Versus Abiogenic Origin. The Arnerican Association ofPetroleum Geologists Bulletin 71(2), 2 15-223.

Whiticar M. J. and Faber E. (1986) Methane oxidation in sedirnent and water colurnn environrnents - Isotope evidence. Organic Geochemistry 10, 759-763.

Woltemate I., Whiticar M. J., and Schoell M. (1984) Carbon and hydrogen isotopic composition of bacterial methane in a shallow freshwater lake. Limnology and Oceanography 29(5), 985-992.

Yuen G., Blair N., DesMarais D. J., and Chang S. (1984) Carbon isotope composition of low molecular weight hydrocarbons and monocarboxlic acids from Murchison meteorite. Nature 307, 252-254.

Chapter 1 : introduction and Literature Reviav.

Figure 1.1: Ranges of 8"cCH4 and S ~ H isotopic signatures for bactenogenic and

thexmogenic gases after Schoell(1988).

Figure 1.2: Plots of 8I3c values of individual C i - C hydrocarbons against their carbon

number. Shown are the results of two separate experiments where hydrocarbons were

synthesized from the thermal decomposition of hexane and a spark in a methane

atmosphere from Des Marais et al. (1981).

C hapter I : Introduction and Lirerature Review.

Figure 1.1 Ranges of s"c,, and. S'H, isotopic signatures for bacteriogenic (B) and thermogenic (T) gases after Schoell, (1 988). BR and B, refer to bacterial CO, reduction and fermentation pathways respectively.

Chapter 1 : Introduction and Literahire Reviav.

Carbon no.

Figure 1.2 Plots of 6I3c values of individual C i - C hydrocarbons against their carbon number. Shown are the results of two separate experiments where hydrocarbons were synthesized from the thermal decomposition of hexane (O) and a spark in a methane atmosphere (H). (Frorn Des Marais et al.; 198 1)-

Chapter 2: Spatial Distrt-bution and Temporal Evolution of Hydrocarbon Gases. ..

Chapter 2: Spatial Distribution and Temporal Evolution of Hydrocarbon Gases

Discharging from Boreholes at the Kidd Creek Mine, Timmins, Ontario, Canada

Volumetrkally significant quantities of hydrocarbon-rich gas stored in fractures and

stmctural cavities in the crystalline rocks of the Canadian Precambrian Shield are released

through exploration boreholes into the Kidd Creek Mine, Timmins, Ontario. Gas flow

rates from individual boreholes on the 6800 ft. Level (L.6800) of the mine range from

c1Umin to 30.9Umin. The estimated total volume of gas discharging into L.6800 from

al1 boreholes is 2.5 x 10'~/month. Gases are composed primarily of CH4 (70.5 - 84.6%),

Cz&, (5.6 - 13.3%), N1 (3.9 - I3.8%), Cz+ hydrocarbons (6.26 - 17.0%), and free H1 (0.13

- 12.7%). The relatively high concentrations of free H2 and Cz+ hydrocarbons found in

these gases result in calculated lower explosive limits (LEL) of gas mixtures (3.91 to

4.92%) which are on average >17% lower than that calculated previously based on a pure

CH4 phase in air (5.3%). The results show that variations in the relative concentrations of

H2 and C2+ hydrocarbons throughout the mine and across the Canadian Shield must be

assessed in order to calculate explosive hazard.

The 613~CH4 signatures and compositions of gases discharging from individual

boreholes on L.6800 did not change significantly over the two year snidy period. This

indicates that: 1) secondary processes such as mixing and oxidation do not modi& gases

subsequent to borehole completion; and 2) gases discharging from older boreholes will

have compositions and 613cCH4 signatures that reflect that of the 'pristine' Shield gas

which flowed from the borehole immediately afier drilling. In contrast, gas samples from

different boreholes have variable compositions and isotopic signatures. Geochemical

Chapter 2: Spatial Distriburion and Temporal Evolution of Hydocarbon Gases ... 28

controls regulating this spatial variability are not clear and do not appear to be related to

differences in the host rock type in which the gas was encountered. However, such a

pattern of compositional and isotopic variability does support a mode1 of low hydraulic

communication between the fractures which host the gases.

Chapter 2: Spatial Distribution and Temporal Evolution of Hydrrocarbon Gares .-- 29

2.2 ~NTRODUCTION

Occurrences of hydrocarbon-rich gases in the crystalline rocks of the Canadian

Precambrian Shield are both volumetrically significant and pervasive (Sherwood Lollar et

al., 1993a;b). Gases in these environments are stored in pressurized pockets and fracture

systems in association with saline groundwaters and Ca-CI rich brines. The high total

dissolved solidç and 6180 and 8 ' ~ signatures of these groundwates reflect the

predominating influence of long term water-rock interaction on the geochemistry of these

systems (Frape and Fria, 1987; Guha and Kanwar, 1987; McNutt et al., 1990; Bottomley

et al., 1994). Earlier studies indicate that the compositions, 6I3c and 6% isotopic

signatures of Shield gases do not substantially overlap with the expected range for

conventionai biogenic sources (Le. thermogenic or bactenogenic) as defined by Schoell

(1988). Sherwood Lollar et al. (1993b) suggested that a large component of the gases

encountered on the Canadian Shield is derived from inorganic or abiogenic processes.

Mechanisms of abiogenic synthesis include; re-equilibration of magmatic CO2 (Welhan,

1988; Kelley, 1996); the Fischer-Tropsch synthesis (Lancet and Anders, 1970); hydrolysis

of mafic and uitramafic rocks (Apps, 1985; Berndt et al., 1996); and heating or

metamorphism of carbonate rocks (Giardini and Salotti, 1969; Holloway, 1984). Such

processes have been invoked to account for C& occurrences in other environments

including; unsedimeoted mid-ocean hydrothemal ridges such as the Mid Atlantic Ridge,

the East Pacific Rise and the Southwest Indian Ridge (Welhan and Craig, 1979; Welhan

and Craig, 1983; Vanko and Stakes, 199 1; Bougault et al., 1993; Charlou and Donval,

1993; Kelley, 1996); and obducted ophiolite sequences such as the Semai1 Nappe in

Chapter 2: Spatial Distribution and Temporal Evolution of Hydrrocarbon Gases-.. 30

Oman (Fritz et al., 1992) and the Zambales Ophiolite in the Philippines (Abrajano et al.,

1988; Abrajano et al., 1990)-

In addition to gas produced via abiogenic procesçes, possible bactenal contributions

to the Shield gases have been investigated. Biofilm cultures taken in exploration

boreholes at both Copper Cliff South in Sudbury, Ontario and the Kidd Creek Mine in

Tirnmins, Ontario showed viable communities of methanogenic microorganisms (Doig,

1994; Doig et ai., 1995). Based on rates of CH4 production in bactenal cultures from the

mine groundwaten, Doig et al. (1 995) estimated that bacterial CH4 could account for 3 to

17% of the total CH4 discharging from the boreholes at Copper Cliff South and the Kidd

Creek Mine. It is not easily determinable whether microbial populations in the rnining

boreholes and groundwaters have been introduced by recent rnining activities, or were

indigenous to these deep crystalline rock environments. The ongin of microbial

populations in the deep subsurface and their diversity and function are the subject of a

number of international research programs (Chapelle and Lovley, 1990; Fredrickson et

al., 199 1 ; Pedersen, 1993; Pedersen, 1997).

Montgomery (1994) made the first attempt to examine the relationship between gas

occurrences and the structural and geological characteristics of the host rock at Copper

Cliff South Mine using mine records of where gas was encountered along each borehole.

This approach was seriously hampered by the historically unsystematic and therefore

incomplete approach to recording gas occurrences in the drilling logs and was unable to

distinguish any definitive associations between gas occurrences and the host rock type.

Carrïed out from 1995 to 1997, the present study at the Kidd Creek Mine provided a more

systematic approach to the investigation of gashost rock relationships. Through close

Chapter 2: Spatial Dis~riburion and Temporal Evolution of Hydrocarbon Grises... 3 1

collaboration with the Geology Office at the Kidd Creek Mine detailed gas sampling

throughout one level of the mine (L.6800) was carried out in conjunction with the dnlling

program. Gas samples were taken shortly after completion of new drillholes and holes

were penodically resarnpled throughout the 2 year time frame of the study. The strategy

was to examine the spatial and temporal variation in gas occurrences, discharge rates,

compositions, and isotopic signatures for al1 available boreholes on the 6800 fi. level.

The objectives of this research were to use this database 1) to examine the compositions

of the gases and the implication they have with respect to their lower explosive limit

&EL) and therefore safety impact; 2) to assess the volumes of gas stored in the rock and

the degree of hydraulic communication between gas pockets or reservoirs; 3) to evaluate

relationships among gas occurrences, compositions, carbon isotopic signatures and the

geological andor structural features of the host rock; and 4) to determine if secondary

processes such as mixing with a bacteriogenic gas cornponent significantly alter

composition or isotope signatures of the gases subsequent to borehole completion.

2.2.1 Geological Setting and Sampling Location

Figure 2.1 shows the location of The Kidd Creek Mine. Kidd Creek, one of the

largest volcanogenic massive suifide deposits in the world, is situated in the Southem

Volcanic Zone of the Abitibi greenstone belt, 24 km north of Tirnrnins, Ontario. The

mine is located in an overturned volcano-sedimentary sequence which has been termed

the Kidd Volcanic Cornplex (KVC). The KVC is characterized by steeply dipping,

northeriy trending, Archean (-2700 Ma; (Nunes and Pyke, 1981)) felsic, mafic and

ultramafic units interlayered with minor metasedimentary units al1 of which have reached

Chapter 2: Spatial Distribution and Temporal Evolution of Hydrocarbon Gases-.. 32

the greenschist grade of regional metamorphism (Walker et al., 1975; Maas et al., 1986;

Bleeker and Parrish, 1996). Al1 gas samples were collected from the 6800 fi. level

(2072m below land surface) which, at the time of sampling, was the deepest level of the

mine. Exploration boreholes on this Ievel are typically drilled perpendicular to the

bedding of the steeply dipping units. Consequently, almost every borehole intersects the

same sequence of units in the following order: andesite/diorite intrusive, felsic volcanics

consisting of massive rhyolites and rhyolitic volcaniclastics, and altered ultramafics.

Figure 2.2a is a schematic diagram showing the location of drilling stations and the

generalized layout of the geoiogy on Kidd Creek L.6800. Figures 2.2b-h are section views

of dnlling stations indicating borehole orientation and lithological logs.

2.3 METHODOLOGY

2.3.1 Sample Collection

Gases in the Canadian Shield occur in association with brine and saline

groundwaters trapped in pressurized (>5000 kPa) fracture systems within the host rock.

In the course of ddling, these fracture systems are intersected, triggering depressunzation

of the fluids and gas release. Gas samples were collected from the collars of 3 1 freely

discharging boreholes on L.6800, The Kidd Creek Mine. 7 of these boreholes were

resampled up to 5 times over the course of the study in order to evaluate any

compositional or isotopic changes that occur in the gases over time. The fust gas samples

were typically obtained within 2 months of the borehole being completed and are

assumed to reflect most closely the compositional and isotopic signatures of the Shield

gas endmembers. If methanogenic bacterial populations are introduced into the

Chapter 2: Spatial Distribution and Temporal Evolution of Hydrocarbon Gases ... 33

subsurface during dnlling and mining operations, then variations in the compositions and

isotopic signatures of the gases issuing from these boreholes over time should reflect

mixing of these later components with original Shield gas endrnernbers.

Several rnethods were utilized for collecting gas from the collars of the boreholes.

Boreholes discharging gas and bnne simultaneously were sampled using a "jas stripper"

device to separate the gas phase from the water, similar to that described by Fritz et al.

(1987). Gas sarnples were then collected in previously evacuated glass flow-through

vessels. For holes discharging gas but not bnne an evacuated glass flow-through vessel

was comected directly to the c o l l a of the borehole using plastic tubing and either an

inflatable packer device which could be inserted into the collar of the borehole or

threaded steel fittings attached directly to the borehole collar. For both the above

methods, once the vessel was comected to the borehole, gas was allowed to flush through

the entire sampling assembly to reduce atmosphenc contamination. In the laboratory,

aliquots of the gas samples were transferred via vacuum line to lOmL evacuated glass

sarnple vials which were capped with butyl blue stoppers and crimp seals. The stoppers

were pre-treated by boiling in 0.1 M NaOH for 1 hour. Samples for compositional and

isotopic analyses were withdrawn directly through the butyl blue stopper using a gas tight

syringe following established laboratory practices.

2.3.2 Cornpositional Analysis

Compositional analyses of gas samples were performed at the Stable Isotope

Laboratory in the University of Toronto using a Varian 3300 Gas Chromatograph

equipped with a Themal Conductivity Detector (TCD) and a Flame Ionization Detector

Chapter 2: Spatial Distribution and Temporal Evolurion of HYdrocarbon Gases.. . 34

(FID). Inorganic components, Hz, He, Oz, and Nz, were separated isothermally at 40°C

with He carrier gas using a Motecular ~ i e v e @ 5A 45/60 mesh 16 fi. x 1/8 inch diameter

column with a TCD detector. Hydtocarbon analyses ( C a , Cz&, C3H8, i-CJHia, n-

CJIla) were perfomed with a h carrier gas using a Porapak Q" 80/100 mesh, 6ft. x 118

inch column with a Fm detector using the following temperature program: initiai

temperature 35OC hold 1.8 minutes, increase to 90°C at lj0C/minute hold O minutes,

increase to 14S°C at S°C/minute hoid 3 minutes. Analyses were run in triplicate and mean

values are reported. Reproducibilities of al1 analyses are within k5 Yo.

2.3.3 Isotopic Analysis

Stable carbon isotope analyses of hydrocarbons were conducted by Gas

Chromatograph-Combustion-Isotope Ratio Mass Spectrometry (GC-C-IRMS) at the

. University of Toronto using a gas source Finnigan MAT 252 mass spectrometer

interfaced with a Varian 3400 capillary GC. Compound separation, purification,

combustion to COz, and analysis were conducted via this miniaturized on-line system.

The 3400 is fitted with a 25m Poraplot Q" column (i.d. 0.32 mm). The Ci and Cz

hydrocarbons were separated isothermally at 30°C. The following temperature program

was used to optimize the separation of C3, i-C4 and n-Ca for the iast i 1 sarnples taken in

May 1997: initial temperature 50°C hold 1 minute, increase to 90°C at 10°C/minute hold

O minutes, increase to 145°C at S°C/minute hold O minutes, increase to 200°C at

20°C/minute hold 2 minutes. Accuracy and reproducibility on al1 analyses are better than

0.5%0 expressed with respect to PDB standard.

Chapter 2: Spatial Distribution and Tempoml Evolution of Hydrocarbon Gases-.. 35

2.4 RESULTS

2.4.1 Compositional Results

Gas compositions are presented in Table 2.1. Results are corrected for air

contamination by assuming that al1 O2 (typically < 2 vol. %) is a result of contamination

during sampling and by subtracting a corresponding amount of N2 (Sherwood Lollar et

al., 1993b). Resultant compositions are nomalized to 100%. The assumption that al1 0 2

is derived from amiospheric contamination is valid given the highly reducing conditions

that prevail in these crystalline rock environrnents. The primary components of the gas

are CH4 NI and ethane (C&) which typically comprise >90 vol. % of the gas. The

balance of the gas is composed of higher hydrocarbons (C3Hs, i -Ca io , n-CJHlo), He and

variable concentrations of free Hz-

The concentration of Hz in the gases from the 6800 fi. Ievel is typically higher than

values previously reported for shallower levels of the mine (Fig. 2.3). In 5 sarnples taken

between LA600 and L.6100 of The Kidd Creek Mine, Montgomery (1994) reported K2

concentrations no higher than 1.7 vol. % (typically <0.01 to 1.7 vol.%). In contrast, on

the 6800 fi. level of The Kidd Creek Mine free HZ concentrations range from 0.17 to 12.7

vol.%. More than half of the boreholes have H2 concentrations >1 vol.% and 16% have

Hz concentrations >5 VOL%. While such elevated H2 concentrations have been found at a

number of sites in the Fennoscandian Shield, only one mine investigated on the Canadian

Shield has shown comparable levels (Shenvood Lollar et al., 1993a; Sherwood Lollar et

al., 1993b): at Copper Cliff South in Sudbury, Ontario, Hz concentrations between 8.6

and 38 vol.% have been reported for 8 boreholes (Sherwood Lollar et al., 1993b;

Montgomery, 1994). High concentrations of free H2 gas have been encountered in other

Chapter 2: Spatial Distribution and Temporal Evolution of Hydrocarbon Gases ... 36

crystalline rock environrnents such as obducted ophiolite sequences and mid-ocean ridge

hydrothermal systems where they are attributed to reactions related to the serpentinization

of mafic and ultramafic rocks (Abrajano et al., 1988; Abrajano et al.: 1990; Fritz et al.,

1992; Kelley, 1996).

Gases from Kidd Creek have CH4 and Cz+ concentrations that are among the

highest that have been reported for Shield gases (Fig. 2.4). The high concentration of

these hydrocarbon species, along with elevated Hz concentrations have important

implications regarding mine safety. The explosive hazard of a flamrnable gas mixture in

air is assessed by calculating the lower explosive limit &EL) for the gas mixture. LEL

values for flammable gases have been determined expenmentally and u e typically

expressed as a minimum percentage-by-volume that will propagate a flame if ignited.

The LEL values for pure gas components (CH4, CzH6, C3H8, C&o, Hz) in air are given in

Table 2.2 (Lewis and Elbe, 1987). For gas mixtures such as those found in The Kidd

Creek luine, LEL is calculated via the foilowing equation:

The non-flamrnable components are disregarded and concentration of each flarnrnable

component (Le., C a , C2H6, C3Hs, C4Hio, Hz) is normalized with respect to the total

concentration of flammable species in the gas.

Pnor to this study, at mines on the Canadian Shield LEL values of 5.3% were

assumed based on the assumption that the gases were composed o d y of Ch. LEL values

calculated for specific boreholes on L. 6800 are shown in Table 2.1. LEL values range

Chapter 2: Spatial Distribution and Temporal Evolurion of Kydrocarbon Gases. .. 37

between 3.9 1 and 4.92%, an average >17% reduction in the LEL as compared to a pure

C a phase in air. Tbe presence of significant concentrations of higher hydrocarbons and

H2 with lower LEL values significantly lowen the LEL value that should be applied to

assess the explosive nature of the gas mixtures discharging in the Canadian Shield mines.

In particular, for some mines such as Kidd Creek, different LEL values may be

appropriate for different leveis of the mine based on the results described earlier

indicating higher relative concentrations of H2 in the deeper levels of the mine. For the

Canadian Shield mining community as a whole, assessment of the relative concentrations

of flarnmable species such as C a , C2+, and Hz as well as charactenzation of non-

flarnmable species such as Nr in the gas is clearly an important criteria for accurate

assessment of explosive hazard.

2.1.2 Carbon Isotope Results

Carbon isotopic compositions are shown in Table 2.3. The range of 613&4 values

(-40.7 to -32.2960; n=3 1) is slightly more enriched in I3c than the range of values reported

earlier (-4 1.8 to -37.6 %O; n=5; (Sherwood Lollar et al., 1993b; Montgomery, 1994)) for

gases fiom shallower levels of the Kidd Creek Mine. Interestingly, as shown in Figures

2.2b-h and 2.5 in nearly ali cases C& obtained from the deepest boreholes have 6I3cCH4

values that are 2 to 5%0 more enrïched in " C than C& kom more shallow boreholes at

the same drilling station. E s pattern of isotopic variation with depth may account for

the slightly more enriched range of 6 1 3 ~ values found in the gases from the 6800 A. level

Chapter 2: Spaiiai Distribution and Temporal Evolzition of H@ocarbon Gases--- 38

as compared to the gas sarnples from L. 4600 to 5300 in the earlier studies (Shewood

Loltar et al., 1993b; Montgomery, 1994).

While the effects of migration on 613cCH4 signatures are not well established

(James, 1983; Schoell, 1983b; Ricchuito and Schoell, 1988), expenmental results and

mathematical simulation indicate that migration may cause shifis in G ~ ~ C C H ~ values of 1

to 4.7 9/00 towards more depleted values (Fuex, 1980). However, both migration and other

secondary effects such as mixing with bacterial gas also result in an increase in C1I(C2 f

C3) ratios of the affected gas (Schoell, 1983b; Ricchiuto and Schoell, 1988). Since no

such compositional trends with depth occur concomitant with the iso topic trend with

depth illustrated in Fig. 2.5, these processes cm be mled out.

While vaiiability ir both 8I3cC~4 values and compositions was found among

different boreholes, the values and compositions of the gases discharging from

each individual borehole did not change significantly over the duration of the sttidy. Only

two boreholes (6070, 6299) had 6 l 3 c = ~ 4 shifts in excess of the error associated with

analyses (0.5%0). This indicates that gas samples obtained from older boreholes will

generally have compositions and isotopic signatures which reflect that of the 'pristine'

Shield gas which flowed from the hole immediately after drilling was cornpleted. Most

importantly, it indicates that boreholes tap a large number of hydraulically isolated

fracture systems with distinct differences in gas geochemistry. The results imply no

significant mixing between gas pockets either before or after the fracture systems are

opened by drilling.

Chapter 2: Spatial Distribution and Temporal Evolution of Hydrocarbon Gases ... 39

2.4.3 Gas Discharge Rates and Reservoir Estimates

As of the last sampling date (May 1997) there were a total of 174 boreholes on L.

6800 for which access was available to 44. Of these, 34 holes (77%) were producing gas.

No discerning charactenstics could be identified in gas producing versus non-gas

producing boreholes (Le., no consistent differences were observed in borehole length,

orientation, rock type intersected or fracture density, as illustrated for typical boreholes in

Figure. 2.2b-h). Given that gas discharge rates typically decrease over time (Table 2.4), it

seems likely that the 10 boreholes which were not producing gas may have produced gas

at some point but have 'dried up' pnor to sampling.

The rates of gas discharge from the boreholes are variable, ranging from < 1L/min.

to as high as 30.9Umin (Table 2.4). The highest discharge rates were measured shonly

after borehole completion, and over 10 months the range of measured discharge rates

decreased. While some boreholes had consistently high discharge rates and showed no

indication of abatement, over the 10 month period, gas discharge rates for most boreholes

decreased over time (Fig. 2.6; Table 2.4). No apparent relationship exists between gas

discharge rates and the number of frachxes indicated on the borehole core logs.

However, the fact that certain boreholes discharge gas for extended periods of time while

other boreholes only discharge gas for a short penod suggests that the system is

characterizcd by both fracture networks that contain volumetrically large quantities of gas

as well as smaller isolated gas pockets. Furthemore, the heterogeneity of flow rates

suggests that the difTerent fracture systems are not hydraulically connected, a conclusion

supported by the distinct compositions and isotope signatures for gases discharging fiom

different boreholes discussed earlier.

Chapter 2: Spatial Distriburion and Temporal Evolution of Hydrocarbon Gases ... 40

Using average gas flow rates for each borehole, the total volume of gas that

discharged from each of the holes during the 10 month study penod can be determined

(Table 2.4). Based on these values, the estimated total volume of gas that discharged into

the mine workings from these boreholes over the 10 month penod is 3.6 x lo7 L. Based

on the observation that 77% of boreholes investigated were producing gas, and on an

average discharge rate of 4.4 Ymin, an estimated discharge of gas to the mine workings

of 2.5 x 1 o7 Umonth can be extrapolated for the 174 boreholes on L. 6800. This estimate

is obviously subject to a substantial degree of error given the large number of

assumptions; however, it provides an order of magnitude estimate of the gas flux into the

mine workings on this level. While this disc harge rate is volumetricall y insignificant

when cornpared to those encountered in sedimentary rock environments, within the

confines of the mininj environment the volumes of explosive gas are large eenough to

pose significant safety problems.

25 DISCUSSION OF RESULTS

2.5.1 Bacterial Gas Component

Results from one borehole (#6070) (Tables 2.1; 2.3) that showed identifiable shifts

in compositions and isotopic signatures over t h e facilitate assessrnent of the degree of

mixing of Shield gas with a bacterial gas component introduced post-borehole

completion. Addition of a bacterial gas tends to cause a decrease in the 613c sipature

towards more depleted values with a concomitant increase in the Ci/(C2 + C3) ratio

because bacterial gas is largely composed of CHI (CI/(C2 + C3) -10 000) which

characterïstically has 6 ' ' ~ values that are highly depleted in I3c relative to other sources

C hapter 2: Spatial DisHbution and Tempord Evolution of Hydrocarbon Gases.. . 41

(6I3c < -60.0 9/00; (Schoell, 1983a; Schoell et al., 1988)). The e s e s discharging fiom

borehole kr6070, one of the oldest boreholes for which time series data were available,

had a rninor but significant compositional and isotopic shift. The 613cCH4 values of the

gas fiom this hole became 2.2 960 more negative while the C1/(C2 + C3) ratio increased by

2.2 over a 19 month period. As illustrated in Figure 2.7, the direction of these shifts is

consistent with the addition of a small component of bacterial gas. Since this shift occurs

subsequent to the boreholes being dnlled it appears that the production of bacterial gas is

promoted by microbial contamination associated with the drillhg of the borehole.

Estimates of the 6% isotopic signatures of the bacterial gas endmember and the

volume of gas required to cause the observed isotopic shifi in the Shield gas endmember

in borehole 6070 can be made by examining theoretical mixing lines between a shield gas

endmember and a bacterial gas endmember. Fig. 2.7 shows that based on an initial 6070

Cl/(Clf C3) ratio and carbon isotopic signature of 5.63 and -37.3960 respectively (Tables

2.1; 2.3), the 6I3c and CI/(C2+C3) ratios of subsequent samples fiom borehole $6070 do

not fa11 on a mixing line (line 1) with a typical bacterial gas ezhember isotopic

composition (Le. 613cCH4 < -60%0). In fact, the variation in #6070 is consistent with

mixing of an initial (6070) endmember with a bacterial endmember with a 613cCH4

between -40 and -50%0 and a C1/(C2+C3) ratio = 10000 (line 2). This range of 6 " ~

values is more depleted than conventional bacterial CK. Hence, while the observed

temporal variation in 6070 is definitely a function of mixing, the origin and controls on

the isotopic signatures of the more Cl+-nch endrnember are not clear. Potential

candidates include: 1) another Shield endmember, 2) a bacterial component related to

Chap ter 2: Spatial Distribution and Temporal Evolution of Hvdrocarbon Gares ... 42

drilling contamination, or 3) a bacterial component derived from in situ methanogenic

microbial populations. Distinguishing between in situ deep subsurface rnicrobial

populations in crystalline rock and microbes introduced due to drilling and exploration is

not a simple process (Pedenen, 1993; Doig et al., 1995; Pedersen, 1996; Pedersen, 1997).

There are a vanety of sources of bactenal contamination in these environrnents including

the fresh surface waters used as a drilling fluids and the greases used as drilling

Iubricants. The fact that variation in the compositional and isotopic signature of gases

discharging From borehole 6070 has occmed subsequent to borehole completion supports

the hypothesis that the bactenal gas component in these boreholes is due to bactenal

populations introduced dunng mining operations and drilling.

Interestingly, laboratory cultivation of microbial populations obtained from Kidd

Creek indicate that bacterial generated gases in these environments may have relatively

enrïched 6I3ccH4 signatures. Doig et ai. (1995) was able to obtain a 6I3cCH4 value from a

laboratory culture of biofilms collected at Kidd Creek and found a value of -43.0i0.5%0.

Using the same mixing mode1 given in Fig. 2.7 estimates of the volume of bacterial gas

being produced can be made. Assurning a bacterial endrnernber with a s ~ ~ c ~ ~ ~ equal to

that found by Doig (1994) (Le. -43.Ok0.5%0) and a Ci/(& + C3) = 10 000, a bacterial gas

contribution of approximateiy 30 % can be calculated. Estimates of bactenal CH4 flux

rates have also been made by Doig (1994). Based on the assumptions: 1) that there is the

sarne concentration of methanogens per cm' on the surface of the borehole as were found

on the biofilms; 2) that the biofilm covers the entire length of the borehole; and 3) that the

CH4 production rate is both unifom along the length of the borehole and equal to that

Chapter 2: Spatial Distriburion and Temporal Evolution of Hydrocnrbon Gases ... 43

found in the laboratory cultures, it was estimated that between 3 to 17% of the total CH4

discharging from a borehole could be bacterially generated. While this range is lower

than that of the present study, both of these estimates are only approximate and as such

are subject to a large degree of error. Nonetheiess, both snidies suggest that only a small

cornponent of gas discharging from certain boreholes is denved from bactenal

methanogenic processes.

It should also be aoted that sirnilar shifts in the 6I3c values and the C[/(Cz + C3)

ratios could result from physical processes associated with the migration of the gases

through the fractured rock. However, the effects of migration on gas compositions and

isotope signatures are still â matter of controversy (Fuex, 1977; Schoell, l983b; James,

1953; Ricchiuto and Schoell, 1988) and, as a result, the author favors the rnixing scenario

given the direct evidence for methanogenic bacteria in these environments.

2.5.2 Host Rock - Gas Relationships

The majonty of the boreholes show no significant variation in compositions and

isotopic signatures over time, and distinct isotopic signatures from borehole to borehole.

What controls this spatial variability in the isotope signatures and compositions of these

gases? This snidy atternpted to assess the degree to which particular host rock types

control gas compositions and isotopic signatures by comparing the gas charactenstics

with the borehole core logs. Since these boreholes are uncased, however, it is impossible

to constrain where along the iength of the borehole the gas producing regions occur.

Furthermore, the mine keeps no systematic record of the location of gas occurrences

during borehole drilling. As a result, it is not possible to determine precisely which rock

Chap ter 2: Spatial Distribution and Temporal Evolution of Hydrocarbo Gases--.

units the gas is discharging from, making it impossible to sample gas fkom a specific rock

type-

Given this absence of direct evidence of gas locations in the boreholes this snidy

took an alternative approach. The relationships arnong gas composition, isotopic

signatures and the host rock were examined by comparing the gas charactenstics to the

length of the borehole passing through a particular rock type. Most of the boreholes that

were sampled were dnlled in the same direction, approximately perpendicular to the

bedding. As a result, the boreholes al1 inteeect a sirnilx sequence of rock types (Le.,

andesite/dioritc, massive rhyolite, rhyolitic volcaniclastics and altered ultrarnafic, as

shown in Figure 22b-h). However, the thickness of a particular rock type intersected in

each borehole varies depending on the dip of the borehole and the location on the mine

level fkom which the hole was started. As a result, it is possible to compare the isotopic

signatures and compositions of the gas samples with the amount of each rock type

intersected by the borehole. Also, because only certain boreholes penetrated through the

entire stratigraphie sequence to the altered ultramafic rocks, it was also possible to

compare the gases fiorn holes which penetrated this unit with gases from boreholes that

did not. No definitive relationships were revealed between the thickness of a given rock

type intersected by a borehole and the gas composition or isotope signature. Likewise,

boreholes that penetrate the ultramafic rocks have no systematic differences as compared

to gases fiom boreholes that only intenect the andesite/dionte, massive rhyolite, and

rhyolitic volcaniclastics. It is possible that such correlations between gas and rock are

impossible to evaluate since the fractures which host the gas may intersect several

different rock types. Consequently, gases may migrate along these fractures and across

Chapter 2: Spatial Distribution and Temporal Evolution of Hydrocarbon Gases ...

lithologc boundaries, thus making it impossible to correlate the charactenstics of the gas

to a single host rock type.

While The Kidd Creek Mine keeps no systematic records of where along the length

of the borehole the gas producing regions were, the drillers daily reports occasionally

mention the depth along a borehole where gas was encountered since gas sensors would

cause the drill rig to autornatically shut down. While only 6 such instances are recorded

in the drillers reports, on each of these occasions the gas was encountered while dnlling

in the rhyolite. Montgomery (1 994) also indicated based on discussions with drillers that

the gas occurrences were rnost common in the rhyolite, in particular the rhyolitic

volc;?nicIastics. CVhile the srna11 number of such records and the anecdotal nature of t t i is

information are not extremely reliable, inhiitively such relationships seem reasonable

since the rhyolite tends to be more highly fractured than surrounding mirs and as such

would provide a structural trap where gases could accunulate. It is not, however,

possible to detemine whether the rhyolite represents a source of gas in this environment

or if it merely provides a structural trap where gases generated elsewhere tend to

accumulate.

The ideal means of examining the controls on gas compositions and isotopic

signatures as a function of host rock type, would be to sarnple the gases directly from the

different rock units. This would require either a system of packers that could seal off

various sections of the borehole where a single rock type occurs, or cased boreholes that

have open intervals within a single rock type. While such systems do exist, the cost of

installing them in a mine would be prohibitive since these systems were designed for

large diarneter water wells, not narrow diarneter exploration boreholes. However, while

Chapter 2: Spatial Disrribution and Temporal Evolution of Hydrocarbon Gmes ... 46

no relationships could be found between gas and host rock type, the spatial distribution of

the isotope signatures and compositions of the gases does provide insight into the

characteristics of the fracture systems that host the gases.

Previous hydrogeological studies at other sites on the Canadian Shield found that at

shallower depths (G00m below surface) the fracture systems are highly interconnected

and therefore have relatively hi& hydraulic conductivities (Frape and Fritz, 1987). With

increasing depth the hydraulic conductivity decreased, however, and while brines were

encountered in fractures at depths greater than 2000 meters below surface, no evidence

could be found to indicate that these Fractures were hydraulically active until disturbed by

mining operations. Later work by McNutt et al. (1990) confimed these findings using

the 8 7 ~ r / 8 6 ~ r isotope ratios of the brines. This work found that brine sarnples from

different boreholes at the same mine had distinct Sr isotope signatures, attesting to the

lack of hydraulic communication between them. Similady, the results of the present

study show considerable variability in the compositions and carbon isotope signatures of

gases from different boreholes. In contrast, the gas samples obtained from the same

borehole during different times are isotopically and compositionally consistent. This

pattern of isotopic signatures and compositions further supports a mode1 of Iow hydraulic

communication among- fractures, consistent with the previous work on brines in these

environments.

2.6 SUMMARY AND CONCLUSIONS

Measurement of gas discharge rates at Kidd Creek indicates that voIumetncally

significant quantities of gas are stored in the crystalline rocks at the Kidd Creek mine. In

C hap ter 2: Spatial Distribution and Temporal Evohtion of Hydrocarbon Gares- .. 47

general, gas discharge rates decreased over the course of the study penod; however,

several boreholes maintained consistently high discharge rates and showed no si@ of

abatement. The heterogeneity of gas discharge rates and the spatial and temporal

distribution of compositions and carbon isotope signatures al1 indicate that the fiacnires

which host these gases are not hydraulically connected. Consequently, it cannot be

assumed that the gases stored in deeper fracture systems will be drained by boreholes

drilled at hîgher elevations. This indicates that new pockets of gas will likely be

encountered as the mine develops deeper regions. As a result, these gases present a long

term hazard to mining operations in this environment. High overall concentrations of

flammable species as well as increased concentrations of C2+ hydrocarbons and free H2

relative to other sites on the Canadian Shield create a significant safety hazard at Kidd

Creek. Furthemore, concentrations of free Hz are higher on the 6800 ft. level than

shallower levels. This resuits in lower LEL values at depth than are encountered at higher

elevations, indicating that different LEL values may be appropriate in different areas of

the mine.

This study also indicates that gases discharging from most boreholes on the 6800 fi.

level have compositions and 6 I3cc~4 signatures that do not undergo significant change

over a period of time of up to 19 months subsequent to the borehole being drilled. A

relatively rninor, but consistent shift in the 613c values and Ci/(C2 + C3) ratios for one

borehole appears to be a function of mixing; however, the origin of the other end member

is unclear. Importantly, however, this study shows that the compositions and isotopic

signatures of the gases do not change appreciably from that of the 'pristine' Shield gas

that initially discharged from the borehole immediateiy after drilling was compteted.

Chapter 2: Sparial Dishibution and Temporal Evohtion of Hydrocarbon Gnses ... 48

The results of this study show no definitive relationships between gas compositions

and isotopic signatures and the geological and structural feanires of the host rock. [t is,

however, possible that the approach used to sample the gases in this study did not provide

the resoiution required to discem such gashost rock relationships. As a result, it may be

necessary to develop a more sophisticated sampling technique in the funire which would

make it possible to sample gases directly from the individual host rock units.

Chapter 2: Spatial Distribution and Temporal Evolution of Hydroocarbon Gases--- 49

Abrajano T. A., Sturchio N., Kennedy B. M., Lyon G. L., Muehlenbachs K., and Bohlke 5. K. (1990) Geochemistry of reduced gas related to serpentinization of the Zambales ophiolite, Philippines. Applied G e o c h e m i s ~ 5, 625-630.

Abrajano T. A., Sturchio N. C., Bohlke J. K., Lyon G. L., Poreda R. J., and Stevens C. M. ( 1 988) Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: deep or shallow origin? Chernical Geology 71,2 1 1-222.

Apps J . A. (1 985) Methane fornation during hydrolysis by mafic rock. University of California,

Bernard B., Brooks J. M., and Sackett W. M. (1977) A geochemical mode1 for characterization of hydrocarbon gas sources in marine sediments. Ofshore Technology Conference, Nineth, 435-438.

Bemdt M. E., Allen D. E., and Seyfned W. E. J. (1996) Reduction of CO2 during serpentinization of olivine at 300C and 500 bar. Geology 24(4), 35 1-354.

Bleeker W. and Parrish R. R. (1 996) Stratigraphy and U-Pb zircon geochronology of Kidd Creek: implications for the formation of giant volcanogenic massive sulphide deposits and the tectonic history of the Abitibi greenstone belt. Canadian Journal of Earth Science 3, 12 13- 123 1.

Bottomley D. J., Gregoire D. C., and Raven K. G. (1994) Saline groundwaters and brines in the Canadian Shield: Geochemical and isotopic evidence for a residual evaporite component. Geochimica et Cosrnochimica Acm 58(S), 1483 - 1498.

Bougault H., Charlou J.-L., Fouquet Y., H.D. N., Vaslet N., Appriou P., Baptiste P. J., Rona P. A., Drnitriev L., and Silantiev S. (1993) Fast and slow spreading ridges: structure and hydrothennal activity, ultrarnafic topographic highs, and CH4 Output. Journal of Geophysical Research 98(B9), 9643 -965 1.

Chapelle F. H. and Lovley D. R. (1990) Rates of rnicrobial metabolism in deep coastal plain aquifers. Applied and Environmental Microbiology 56, 1865- 1874.

Charlou J. L. and Donvai J. P. (1993) Hydrothemal methane venting between 12N and 26N along the Mid-Atlantic ridge. Journal of Geophysical Research 98, 9625-9642.

Doig F. (1994) Bacterial Methanogenesis in Canadian Shield Groundwaters. MSc., University of Toronto.

Doig F., Sherwood Lollar B., and Fems F. G. (1995) Microbial communities in deep Canadian Shield groundwaters - An in situ biofilm experiment. Geomicrobio/ogy Journal 13,9 1-1 02.

Chapter 2: Spatial Distribution and Temporal Evolution of Hydrocarbon Guses-.. 50

Frape S. K. and Fritz P. (1 987) Geochemical trends fiom groundwaters fiom the Canadian Shield. In Saline Waters and Gases in C q m a l h e Rock, Vol. 33 (ed. P. Fritz and S. K. Frape), pp. 19-38. Geological Association of Canada Speciai Papers.

Fredrickson J. K., BaIkwilI D. L., Zachara J. M., Li S. W., Brockman F. J., and Simmons M. (199 1) Physiological diversity and distribution of heterotrophic bacteria in deep Creiaceous sediments of the Atlantic Coastal Plain. Applied and Environmental Microbiology 57,4024 1 1.

Fritz P., Clark 1. D., Fontes J-C., Whiticar M. J., and Faber E. (1992) Deutenum and 13c evidence for low temperature production of hydrogen and methane in a highly alkaline groundwater environment in Oman. Water-rock Interaction, 793-796.

Fritz P., Frape S. K., and Miles M. (1987) Methane in the crystalline rocks of the Canadian S hield. Isotope Geochemi s~ of Groundwater and Fracture Material in Plutonic Rock, 2 1 1-224.

Fuex A. N. (1 977) The use of stable carbon isotopes in hydrocarbon exploration. Journal of Geochemical Exploration 7, 155- 188.

Fuex A. N. (1980) Experirnental evidence against an appreciable isotopic fkactionation of methane dunng migration. In Advances in Organic Geochemistry 1979 - Physics and Chernistry of the Earth, Vol. 12 (ed. A. G. Douglas and J. R. Maxwell), pp. 725-732. John Wiley and Sons.

Giardini A. A. and Salotti C. A. (1969) Kinetics and relations in the calcite-hydrogen reaction and relations in the dolomite-hydrogen and sidente-hydrogen systems. American Mineralogist 54, 1 1 5 1 - 1 1 72.

Guha J. and Kanwar R. (1 987) Vug brines - fluid inclusions: a key to the understanding of secondary gold enrichment processes and the evolution of deep brines in the Canadian Shield. In Saline Woter and Gasm in Crystalline Rocks, Vol. 33 (ed. P. Fritz and S. K. Frape), pp. 95-102. Geological Association of Canada Special Volume.

Holloway J. R. (1 984) Graphite-Ch-H2O-CO2 equilibria at low grade metarnorphic conditions. Geology 12,455-458.

James A. T. (1983) Correlation of natual gas by use of carbon isotopic distribution between hydrocarbon components. Amencan Association of Petroleum Geologists Bulletin 67, 1 176- 1 19 1.

Kelley D. S. (1996) Methane-rich fluids in the oceanic cmst. Jormzal of Geophysical Research 101(B2), 2943-2962.

Lancet H. S. and Anders E. (1970) Carbon isotope fractionation in the Fischer-Tropsch synthesis of rnethane. Science 170,980-982.

Chapter 2: Spatial Dism'bution and Temporal Evolution of ffydrocarbon Gases- .. 5 1

Lewis B. and Elbe G. v. (1 987) Combustion,flames and erplosi0n.s of gases. Academic Press.

Maas R., McCulloch M. T., Campbell 1. H., and Coad P. R. (1986) Sm-Nd and Rb-Sr dating of an Archean massive sulfide deposit: Kidd Creek, Ontario. Geology 14('), 585-588.

McNun R. H., Frape S. K., Fritz P., Jones M. G., and MacDonald 1. M. (1990) The 87SrB6Sr values of Canadian Shield brines and fracture minerals with applications to groundwater mixing, fracture history, and geochronology. Geochimica et Cosmochirnica Acta 54,205-2 15.

Montgomery J. (1994) An isotopic study of C a and associated N2 and Hz gases in Canadian Shield rnining environments. MSc, University of Toronto.

Neal C. and Stanger G. (1983) Hydrogen generation from mantle source rocks in Oman. Earth and Planetary Science Letters 60,3 15-32 1.

Nunes P. D. and Pyke D. (198 1) Time-stratigraphie correlation of the Kidd Creek orebody with volcanic rocks south of Timrnins, Ontario, as inferred from zircon U-Pb ages. Economic Geology 76,944-95 1.

Pedenen K. (1993) The deep subterranean biosphere. Enrrh-Science Reviews 34,243- 260.

Pedersen K. (1 996) Investigations of subterranean bactena in deep c,ystalline bedrock and their importance for the disposal of nuclear waste. Canadian Journal of Microbiology 42(4), 3 82-39 1.

Pedersen K. (1 997) Microbial life in deep granitic rock. FEMS Microbiology Reviews 20, 3 99-4 14.

Ricchiuto T. and Schoell M. (1988) Origin of natural gases in the -4pulian Basin in south Italy: A case history of muting of gases of deep and shallou origin. Organic Geochemistry 13(1-3), 3 1 1-3 18.

Schoell M. (1983a) Genetic characterization of natural gases. Amencan Association of Petroleum Geologists Bulletin 67(l2), 222 5-223 8.

Schoell M. (1983b) Isotope techniques for tracing migration of gases in sedimentary basins. Journal of Geological Society London 140,4 1 5-422.

Schoell M. (1988) Multiple origins of methane in the earth. Chemical Geology 71, 1-10.

Schoell M., Tietze K., and Schoberth S. M. (1988) Origin of methane in lake Kivu (east central Africa). Chernical Geology 71,257-265.

Chapter 2: Spatial Disbibution and Temporal Evohion of Hydrocarbon Gases ... 52

Sherwood Lollar B., Frape S. K., Fritz P., Macko S. A., Welhan J. A., Blomqvist R., and Lahermo P. W. (1993a) Evidence for bacterially generated hydrocarbon gas in Canadian Shield and Fernoscandian Shield rocks. Geochr'mica et Cosmochimica Acta 57,5073-5085,

Sherwood Lollar B., Frape S. K., Weise S. M., Fritz P., Macko S. A., and Welhan I. A. (1 993b) Abiogenic methanogenesis in crystalline rocks. Geochimica et Cosmochimica Acta 57,5087-5097.

Vanko D. A. and Stakes D. S. (199 1) Fluids in oceanic layer 3: evidence from veined rocks, hole 7333, Southwest Indian Ridge. Proceedings of the Qceanic Drilling Program, Scienrific Results 1 18, 1 8 1-2 1 5.

Walker R. R., Matulich A., Amos A. C., Watkins J. J., and Mannard G. W. (1975) The geology of the Kidd Creek mine. Economic Geology 70,80-89.

Welhan J. A. (1988) Ongins of methane in hydrothemal systems. Chernical Geology 71, f 83-1 98.

Welhan J. A. and Craig H. (1 979) Methane and hydrogen in east pacific nse hydrothennal fluids. Geo[ogical Research Letters 6(11), 829-83 1.

Welhan J. A. and Craig H- (1983) Methane, hydrogen and helium in hydrothemal fluids at 2 1 degrees N on the east Pacific Rise. In hydrothermal processes at seafloor spreading centres (ed. P. A. Rona, K. Bostrom, L. Laubier, and K. L. J. Smith), pp. 3 9 1 -409. Plenum.

Chapter 2: Spnn'al Distribution and Temporal Evolution of Hydrocnrbon Guses-.. 53

2.8 LIST OF FIGURES

Figure 2.1: Geographical location of Kidd Creek Mine, 27 km north of Timmins,

Ontario.

Figure 2.2: Schematic geology plan of Kidd Creek L.6800 with section views of ddling

stations (4S, lS, 2N, 6N, 9N, Deep A) showing borehole lithologies and 613cc~4 of gas

discharging from them. Also s h o w are boreholes which were not producing gas.

Figure 2.3: Comparison of major component compositional data ( H I , C a , N2) h m

L.6800 Kidd Creek Mine with previously obtained values from L.4600 to L.6 100.

Figure 2.4: Comparison of major component compositions of gases fiom Kidd

Creek(this study) with gases from other sites on the Canadian Shield.

Figure 2.5: Plot of 613cCH4 VS. borehole termination depth for boreholes from 5 drilling

stations on Kidd Creek L.6800-

Figure 2.6: Plot of gas discharge rates for 4 boreholes measured on each of 3 sarnpling

dates.

Figure 2.7: Plot of CI/(C2+C3) vs. 613cCH4 showing gas samples from Kidd Creek. Also

shown are mixing lines between a Shield endmember and a conventional bactenal

endmember and a mixing line between a Shield endmember and an endmember with a

613cc~4 equai to that found in laboratory cultures of Kidd Creek methanogens (Doig,

1 994)(Modified after; (Bernard et al., 1 977)).

Chapier 2: Spatial Distribution and Temporal Evohtion of Hj,&ocarbon Gases ... 54

Figure 2.1 Geographical location of Kidd Creek mine, 27 km north of Timmins, Ontario.

C hap ter 2: Spatial Distriburion and Temporal Evolurion of Hydrocnrbon Guses-.. 55

Figure Z.2a Schematic geology plan of Kidd Creek L.6800 showing location of dnlling stations 4S, IS, ZN, 6N, 9N, Deep A). Section lines correspond to section view maps in figs. 2.2b-h.

Chapter 2: Spatial Dishibution and Temporal EvoZtrrion of Hydrocarbon Gases .-- 56

Figure 2.2b Section view of boreholes from dnlling station 4s on L.6800. For hotes where samples were obtained the 6"~, values of the gas discharging from the hole is shown. For holes not sampled it is indicated whether they were producing gas or not.

Chapter 2: Spatial Disrriburion and Temporal EvoZution of Hydrocurbon Gases ... 57

Figure 2 . 2 ~ Section view of boreholes fiom drilling station 1s on L.6800 (See Fig 2.2b for caption and legend).

Chapter 2: Spatial Dishibution and Temporal Evolz~rion of Hydrocarbon Guses-.. 58

2N - Section C-C'

Fig 2.2d Section view of boreholes fiom dnlling station 6N on L.6800 (See Fig 2.2b for caption and legend).

Chaprer 2: Spatial Distribution and Temporal Evolution of ffvdrocarbon Gases ...

6N - Section D-D' Collar Azimuth 46"

Fig 2.2e Section view of boreholes ftom drillhg station 6N on L.6800 (See Fig 2.2b for caption and legend).

Chapter 2: Spatial Dishibution and Temporal Evohtion of Hydrocarbon Gases ... 60

, 9N - Section E-E" Collar Azimuth 35"

Figure 2.2f Section view of boreholes from dnlling station 9N on L.6800 (See Fig 2.2b for caption and legend).

Chapter 2: Spatial Disrribution and Temporal Evoiution ojHydrocarbon Gares... 61

9N - Section E-ET Collar Azimuth 45"

*Borehole 6498 was discharging but ceased to flow as of 97/OS/7.

gas when sampted on

Figure 2.2g Section view of boreholes fiom drilling station 9N on L.6800 (See Fig 2.2b for caption and legend).

Chapter 2: Spafial Distribution and Temporal Evolution of Hydrocarbon Gases ... 62

DEEP A - Section F-F' Collar Azimuth 45"

F

Figure 2.2h Section view of boreholes corn drilling station Deep A on L.6800 (See Fig 2.2b for caption and legend).

C hapter 2: Spalial Distribution and Temporal Evolution of H)chcarbn Gases- - - 63

1 0 'This Study 1

Figure 2.3 Comparison of major component compositional data fiom L.6800 Kidd Creek Mine with previously obtained values fiom L.4600 to 6100. For boreholes that were sampled morz than once only the most recent sarnples are plotted.

Chapter 2: Spatial Dlshiburion and Temporal EvoZution of Hydrocarbon Gases ... 64

Red Lake Elliot Lake Thompson Yellowknife

O Matagami Norita

O Kidd Creek Mine

Fi-re 2.4 Comparison of major component compositions of gases h m Kidd Creek (this study) wth gases fiom other sites on the Canadian Shield (Shenvood Lollar, 1993a, 1993b)

Chapter 2: Spatial Distribution and Temporal Evolution of Hydrocarbun Gmes ...

0 Deep A A 9N H 6N A 2N O 1s

Figure 2.5 Plot of 6%,,, vs. borehole termination depth for boreholes frorn 5

drilling stations on the 6800 fi. level. Each point represents gas from a different borehole. For L.6800 as a whole, as well as within each station, the boreholes with the deepest termination depths have the most enriched ~ ~ ~ ~ , , , s i ~ a t u r e s .

C ha p ter 2: Spa f iaZ Distribution and Temporal Evolution of Hydrocarbon Gases ...

Sampling Event

Sampling Event

Figure 2.6 Plot of gas discharge rates Wmin.) for the 4 boreholes that were rneasured on different sampling dates. While 6298 shows no significant decrease in discharge rate with t h e , typically some decrease in discharge rate is observed over time.

C hap ter 2: Spatial Disrriburion and Temporal Evohtion of Hydrocarbon Gases ... 67

\ 1

Microbial Culture 1 (Doig, 1994)

Figure 2.7 Plot of CI/(C2+C3) vs. shouring gas samples from Kidd Creek. Solid lines are mixing lines between: 1) a Shield endmember and a conventional bacterial endmember; and 2) a mixing Iine between a Shield endmember and an endmember with a 6I3ccHa equal to that found in laboratory cultures of Kidd Creek methanogens (1) (Doig, 1994). Black circles (a) indicate gas samples fiom L.6800, Kidd Creek. Outlined circles (O) indicate gas samples fiom borehole # 6070.

Chap ter 2: Spatial Disrribution and Temporal Evohltion of Hydrocarbon Gases ... 68

2.9 LIST OF TABLES

Table 2.1: Compositions (vol%) and lower explosive limits(%LEL) of bulk gas sarnples

from Kidd Creek L. 6800.

Table 2.2: Lower expIosive limits of gases encountered at Kidd Creek (Lewis and Elbe,

1987).

Table 2.3: Isotopic results (%a) relative to PDB for sarnples from Kidd Creek L.6800.

Table 2.4: Gas discharge rate meanirements (Umin) from boreholes on L.6800, Kidd Creek.

C ha p ter 2: Spatial Diswibzrtion and Temporal Evolution of Hydocarbon Gases ... 69

Table 2.1 Compositions (vol%) and Lower Explosive Limits (%LEL) of b d k gas sam~les frorn Kidd Creek. L.6800.

A

Sampling Date: Borehole N Hz He CH, C1& C a s i-CaIo n-CaIo LEL

Chapter 2: Spatial Disrribution and Temporal Evolution of Hydrocarbon Guses ... 70

Table 2.2 Lower explosive limits* of gases encountered at Kidd Creek. (Values expressed as percentage-by -volume in air) Compound Formula LEL Methane CH4 5.3

Propane CSHR 2.2 neobutane C4Hlo 1.9

Hydrogen Hz 4

*Note: al1 values obtained fi-om Lewis and Elbe, L987

Chapter 2: Spatial Distribution and Temporal Evolution of Hydrocarbon Gases .-. 71

Table 2.3 Isotopic results ( %O) relative to PDB for samples from Kidd Creek L.6800.

Sampling Date: Borehole s ' ~ c ~ ~ ~ s " c c ~ H ~ 6 ' ' ~ ~ ~ ~ 6 " ~ ~ ~ ~ ~ ~ ~ 6 1 3 ~ n C a H I 0

Chapter 2: Spatial Disrriburion and Temporal Evolurion of Hydrocarbon Gmes ... 72

Table 2.4. Gas discharge rate measurements (L/rnin) fiom boreholes on L6800, Kidd Creek.

BorehoIe 7/ 1996 9/1996 91997 Average Total volurneE)*

11.9 - - - - -

*Estirnated total volume of =as discharged over a 10 month period calculated based on the average discharge nte.

Chapter 3 : Carbon isotopic and compositional evidence for inorganic synthesis ...

Chapter 3: Carbon isotopic and compositional evidence for inorganic synthesis of hydrocarbon gases in crystalline rock environrnents

3.1 ABSTRACT

Based on a unique set of isotopic signatures, volumetrically significant and

regionally extensive accumulations of methane gas in the crystalline rocks of the

Canadian Shield were suggested to originate through processes of abiogenic synthesis

(Sherwood Lollar et ai., 1993). In this study, compound specific isotope analysis has

facilitated the f ~ s t detailed characterization of carbon isotopic signatures for higher

molecular weight hydrocarbons associated with methane gas accumulations. Ci to Cq

akanes typically becorne isotopically lighter with increasing molecular weight. Such a

trend is opposite to that found in alkanes denved from the thermal decomposition of high

molecular weight organic matter. However, more depleted 613C values with increasing

molecular weight are consistent with hydrocarbon gases produced by polymerization of

methzne via inorganic synthesis (DesMarais et al., 1 9 8 1). Additionally, the relative

concentrations of Ci to C4 alkanes in Shield gases follow a Schulz-Flory distribution

which is also charactenstic of hydrocarbons produced by the polyrnenzation of lower

homologues. Together the distinct compositional and iso topic signatures indicate that

these gases are not formed by conventional thermogenic or bacteriogenic processes.

Rather, they support an origin via inorganic synthesis.

Chapter 3: Carbon isotopic and cornpositional evidence for inorganic synthesis. .. 74

3.2 CARBON ISOTOPE DISTRIBUTIONS IN CI-C2 HYDROCARBONS

Isotopes of carbon react at different rates during chernical reactions, and hence

isotope signatures of the light hydrocarbon molecules Cl to C4 can provide insight into

the processes responsible for their synthesis. Carbon isotope distrîbutions arnong light

alkanes from thermogenic processes, which involve breaking large hydrocarbon

molecules into smaller fragments, have a characteristic distribution whereby the

hydrocarbons become more enriched in "C with increasing molecular weight. This

orderly isotopic distribution is accounted for by kinetic hctionation effects that anse due

to differences in the physical chemistry of "C and I3c which cause "C - 12c bonds to be

weaker, and thus break more easily, than I3c - 12c bonds (Chung et al., 1988). When

alkyl groups separate from the source organic matter, cleavage preferentially occurs at

weaker "C - "C bonds. As a result, the terminal carbon atom of the released

hydrocarbon rnolecufe is subject to isotopic discrimination which favors "c, thus diluting

the I3c/"c ratio. Methane gas shows the greatest e ~ c h r n e n t in "C because no other

'isotopically heavy' carbon atoms are present to dilute this 'k e ~ c h t n e n t . Furthemore,

with increasing carbon number the hydrocarbon molecules become isotopically heavier as

the influence of the Light terminal carbon atom on the overall isotope signature of the

molecule becomes less significant. This isotopic distribution pattern is ubiquitous among

thermogenically derived gases throughout the world and is considered to be diagnostic of

gases produced by the thermal decomposition of high molecular weight organic matter

(DesMarais et al., 1988). This characteristic isotope distribution has been confirmed

experimentally via pyrolysis experiments (see Fig 1.2, Chapter 1; DesMarais et al., 198 1).

Chap ter 3 : Carbon Lrotopic and compositional evidence for inorgunic synthesis ... 75

Altematively, processes whereby hydrocarbons are produced by kinetically

controlled synthesis of higher molecular weight homologues frorn lower ones produce a

carbon isotope trend opposite to that discussed above. Such a trend cm be generated in

hydrocarbons produced by spark discharge in a methane atmosphere (see Fig.l.2;

DesMarais et al., 1981; Chang et al., 1983). These hydrocarbon molecules, which

becorne more depleted in 13c with increasing rnolecular weight, are also a result of

kinetic isotope effects whereby CH^ reacts faster than ''CI& to form chains so that "C

is more likely to be incorporated into longer hydrocarbon chains, resulting in lower 6l3c

values. Natural occurrences of hydrocarbon gases having similar isotopic depletion

trends are rare, and appear to be restricted to those proàuced via inorganic processes.

Reported occurrences include: CI-Cj hydrocarbons obtained from the Murchison

rnsteorite (Yuen et al., 1984); C ,-C3 hydrocarbons in 3 gas samples from fluid inclusions

associated with the Khibiny massif, on the Kola Peninsula, Russia (Khitarov et al., 1979)

and CI-C2 hydmcarbons from 7 gas samples from fluid inclusions associated with the

Ilimaussaq complex, South Greenland (Komerup-Madsen et al., 1988). While Gerling et

al. (1988) report 6I3cc2 signatures more isotopically depleted than 6 ' 3 ~ c i for gases

extracted from sait formations in North Germany, they anribute this to post-genetic

alteration of conventional thermogenically derived gas.

3.3 HYDROCARBON GASES AT KIDD CREEK MINE

Gas samples for the present study were obtained from a single location on the

Canadian Shield (Kidd Creek Mine, Timmins, Ontario). Gases in Shield rocks are stored

in pressurized pockets and fracture systems in association with saline groundwaters and

Chapter 3: Carbon isotopic and compositional evidence for inorganic synthesk-. 76

Ca-CI rich brines. Dunng the course of driiling, these systems are ruptured, tnggering

depressurization and gas release. Gas was collected from lreely discharging exploration

boreholes. Gases are predominantly composed of methane (70.5-78.1%), ethane (5.5-

1 1.6%) and nitrogen (3.9-1 1.9%) along with minor concentrations of helium (1.9-2.4%),

propane (0.8-2.31%) and butane (0.08-0.5%) (Table 3.1). Al1 samples contain free

hydrogen gas with concentrations f?om 0.3 to 12.7%. These gases were previously

attributed to inorganic synthesis (Sherwood Lollar et al., 1993), based on 6 ' ' ~ values (-

33.0 to -40.7%0) that are too high, and CII(Cr + Cs) ratios (5.3 to 11.89) that are too low

to be accounted for by bacteriogenic processes. Furthemore, previously reported 6 ' ~

values (-326 to -373%0 (Sherwood Lollar et al., 1993; Montgomery, 1994)) for methane

are significantly lower than those of conventional thermogenically derived gases.

The distribution of 13c among the Ci to Cd hydrocarbons in the gases from Kidd

Creek examined in this study is also inconsistent with a conventional origin via

bacteriogenic or thermogenic processes. Methane in the gas samples fiom Kidd Creek is

consis tent1 y more enriched in I3c than associated higher molecular weight alkanes.

Ethane and propane are typically depleted in I3c by 1-4 %O with respect to methane.

Butane is typicaliy depleted by 2-4%0 with respect to ethane and propane (Fig. 3.1). Gas

sarnples fiom 3 boreholes (6070, 6429, 6499) (see appendix 3.1) show the only exception

to the overall trend of increasing depletion, with methane and ethane 6 ' ' ~ signatures that

are the same within error. However, resampling of the gases issuing fiom borehole 6070

over a 19 month period indicates that the 613cCH4 values becarne lighter over time by

2.2960, likely due to the addition of a minor component of more isotopically depleted

bacterial Cl& (Chapter 2). Clearly, addition of a bacterial Cl& component to any of the

Chap ter 3 : Carbon iroropic and compositional evidence for inorganic spthesis- - - 77

other samples would also result o d y in iess isotopic e ~ c h m e n t between Ci and C2-

Neither mixing with an Isotopically more depleted bacterial CH4 gas, nor post-genetic

processes such as migration (Fuex, 1980; James, 1983; SchoelI, 1983) are known to

produce an isotopic enrichment trend between Ci-C4 homologues such as that seen in

these samples. Thus, the distribution of carbon isotopes in the gases from Kidd Creek

appear to result from primary processes of gas formation. WhiIe not entirely consistent

with isotopic depletion trends of Cl to C4 hydmcarbom produced via polymerization in a

spark discharge process, these 11 Shield gas samples represent the f i s t definitive set of

terrestrial samples to demonstrate isotopic depletion trends between Ci to C4 homologues

sirnilar to those produced by polymerization.

The relative concentrations of the Ci to C4 hydrocarbons in the gases from Kidd

Creek lend M e r support to synthesis via inorganic polymerizarion. Ci to Cq

concentrations follow a Schulz-Flory distribution with r' correlations between 0.98 and

0.99 for al1 samples (Fig. 3.2). The Schulz-Flory distribution of reaction products is

charactenstic of polymerization processes in which higher molecular weight homologues

are produced by the stepwise addition of individual monomers (Schulz, 1935; Flory,

1936). For example, reaction products of the Fischer-Tropsch synthesis, a well-known

industrial process by which higher hydrocarbons are produced through the addition of

single carbon atom species onto a growing chain, have been found to have a Schdz-Flory

distribution (He~ci-Olive and Olive, 1976; Satterfield and Huff, 1982). Expenmental

work indicates that hydrocarbons produced by serpentinization reactions related to the

alteration of ultramafic rocks also follow a Schulz-Flory distribution (Bemdt et al., 1996).

More recently it has been shown that Ci to C5 hydrocarbon gases in fluid inclusions

Chapter 3: Carbon isotopic and compositional evidence for inorganic synthesis--. 78

associated with peralkaline granites in Quebec, Canada follow a Schub-Flory distribution

and have been attributed to Fischer-Tropsch synthesis during sub-solidus alteration (Salvi

and Williams-Jones, 1997). It is possible that a Schulz-Flory distribution of reaction

products could be produced during the random breakage of C-C bonds of a higher

molecular weight source (Flory, 1936). Consequently this compositional distribution

alone c a ~ o t be used as definitive evidence for inorganic hydrocarbon synthesis. In this

study, the carbon isotope distributions in conjunction with the compositional evidence

together provide strong evidence for uiorganic synthesis of the hydrocarbons via

kinetically controlled polymenzation reactions. Potential mechanisms for inorganîc gas

synthesis in this environment include: the Fischer-Tropsch synthesis (Lancet and Anders,

1970); heating or metamorphism of graphite-carbonate-bearing rocks (Giardini md

Salotti, 1969; Holloway, 1954); and serpentinization-type reactions associated with the

alteration of mafic and ultramafic rocks (Apps, 1985; Berndt et al., 1996) and are the

subject of on-going investigation.

Chapter 3: Carbon isotopic and compositional evidence for inorganic synthes is... 79

Apps J. A. (1985) Methane formation during hydrolysis by mafic rock. University of California.

Bemdt M. E., Allen D. E., and Seyfiied W. E. J. (1996) Redÿction of CO2 during serpentinization of olivine at 300C and 500 bar. Geology 24(4), 351-354.

Chang S., Des Marais D. J., Mack R., Miller S. L., and Stratheam G. E. (1983) Prebiotic organic syntheses and the ongin of life. In Earth 5 Earliest Biosphere, Its Origin and Evolution (ed. J. W. Schopf), pp. 53-92. Princeton Univensity Press.

Chung H. M., Gormly J. R., and Squires R. M. (1988) Origin of gaseous hydrocarbons in subsurface environrnents: Theoretical onsiderations of carbon isotope distribution. Chemical Geology 71,97-103.

DesMarais D. J., Donchin J. H., Nehring N. L., and Tmesdell A. H. (198 1) Molecular carbon isotopic evidence for the origin of pothemal hydrocarbons. Nature 292,826- 828.

DesMarais D. J., Stallard M. L., Nehring N. L., and Truesdell A. H. (1 988) Carbon isotope geochemistry of hydrocarbons in the Cerro Prieto geothermal field, Baja California Norte, Mexico. Chemical Geology 71, 159- 167.

Flory P. J. (1 936) Molecular size distribution in linear condensation polymers. Journal of the American Chernical Society 58, 1 877- 1 885.

Fuex A. N. (1 980) Experimentai evidence against an appreciable isotopic fractionation of methane during migration. In Advances in Organic Geochemisby 1979 - Physics and Chernisv of the Earth, Vol. 12 (ed. A. G. Douglas and J. R. Maxwell), pp. 725-732. John Wiley and Sons.

Gerling P., Whiticar M. J., and Faber E. (1988) Extreme isotope fractionation of hydrocarbon gases in Pemiian salts. Organic Geochemimy l3(I -3), 335-341.

Giardini A. A. and Salotti C. A. (1969) Kinetics and relations in the calcite-hydrogen reaction and relations in the dolomite-hydrogen and sidente-hydrogen systems. American MineraZogzSt 54, 1 15 1 - 1 172.

He~ci -Ol ive G. and Olive S. (1976) The Fischer - Tropsch synthesis: molecular weight distribution of prirnary products and reaction mechanism. Angew. Chern. Int. Ed. Engl. 15(3), 136-141.

Holloway J. R. (1 984) Graphite-C&-H20-CO2 equilibria at low grade metamorphic conditions. Geology 12,455-458.

Chapter 3 : Carbon isotopic and compositional evidence for inorganic synrhesis-.. 80

James A. T. (1 983) Correlation of natural gas by use of carbon isotopic distribution between hydrocarbon components. American Rssocintion of Petdeum GeoZogists Bulletin 67, 1 176- 1 1 9 1.

Khitarov N. L, Kravtsov A. I., Voitov G. I., Fridman A. I., Ortenberg N. A., and Pavlov A. S. (1979) Free ernanation gases of the Khibiny massif ( in Russian). Sovetsknia geologiia 2,62-73.

Konnerup-Madsen J., Kreulen R., and Rose-Hansen J. (1 988) Stable isotope characteristics of hydrocarbon gases in the alkaline Ilimaussaq complex, south Greenland. Bull. Minerai. 106, 642-653.

Lancet H. S. and Anders E. (1 970) Carbon isotope fi-actionation in the Fischer-Tropsch synthesis of methane. Science 170,980-982.

Montgomery 1. (1 994) An isotopic study of C& and associated Nr and Hz gases in Canadian Shield rnuiing environrnents. MSc, University of Toronto.

Salvi S. and Williams-Jones A. E. (1997) Fischer-Tropsch synthesis of hydrocarbons during sub-solidus alteration of the Strange Lake perdkaline granite, QuebeciLabrador, Canada. Geochimica et Cosmochimica Acta.

Satter-field C. N. and Huff G. A. J. (1982) Carbon nurnber distribution of Fischer-Tropsch products formed on an iron catalyst in a s i u ~ ~ y reactor. Journal of Camlysis 73, 187- 197.

Schoell M. (1 983) Isotope techniques for tracing migration of gases in sedimentary basins. Journal of Geological Socîey London 140,4 15-422.

Schulz C. V. (1935) Ueber die beziehungen nvischen reakionsgschwinddigkeit and zusamrnenstrung des reaktionprodukts bei makropolyrnensations-vorgangen. Zeîtschrifr fur Physkalische Chemie B30,379-398.

Shenvood Lollar B., Frape S. K., Weise S. M., Fritz P., Macko S. A., and Welhan J. A. (1 993) Abiogenic methanogenesis in crystalline rocks. Geochimica et Cosmochimica Acta 57,5087-5097.

Yuen G., Blair N., DesMarais D. J., and Chang S. (1984) Carbon isotope composition of low molecular weight hydrocarbons and monocarboxlic acids fiom Murchison meteorite, Nature 307, 252-254.

C ha p ter 3 : Carbon isoiopic and compositional evidence for inorganic synthesis- .. 8 1

3.5 LIST OF FIGURES

Figure 3.1: Plots of 6

number for gas sample

" 3 ~ p D B values of individual hydrocarbon molecules against carbon

:s from Kidd Creek L- 6800.

Figure 3.2: Plots of the log (mole fraction) of individual hydrocarbon molecules against

carbon number for gas samples from Kidd Creek L. 6800.

C hapter 3 : Carbon isoropic and compositional evidence for inorganic synthesis ... 82

1 1 1 i

1 2 3 4

Carbon nurnber

Figure 3.1 Plots of 6 '3~ , , , values of individual hydrocarbon molecules against carbon number for gas sarnples from Kidd Creek. The gas samples shown are representative of 5 out of 1 1 samples.

Chapter 3 : Curbon isotopic and compositionul evidence for inorganic synthesis-.. 83

. a

2 3 Carbon number

Figure 3.2 Plots of the log (mole fraction) of individual hydrocarbon molecules against carbon nurnber for gas samples from Kidd Creek. Gas samples are representative of 5 out of I I sarnples and correspond to those shown in Fig. 1. The 2 correlation shown is for borehole 6080.

Chapter 3: Carbon isoropic and compositional evidence for inorganic sdvnrhesis ... 84

3.6 LIST OF TABLES

Table 3.1: Compositional and 6 ' ' ~ isotopic signatures of gas samples fiom Kidd Creek

L-6800.

Chapter 3: Carbon kotopic and compositiona2 evidence for Nlorganic synthesis. .- 85

Table 3.1 Gas compositions and isotopic signatures* Borehole 6''ccH4 6'kcHs 813cnHs 6 " ~ ~ ~ ~ ~ ~ ~ 613~nC-~~~ N2 Hz He C& Cd% i-C,Hia n-C4Hlo

6070 -39.5 -39.3 -38-1 32.6 -37.6 11.9 6.05 2.36 70.5 7.59 1-22 O 0.26 6080 -35.1 -38.4 -37.5 41.5 -37.7 5.25 4.78 2.02 77.1 8.86 1-51 0.13 0.32 6348 -33.0 -36.3 -35.3 -38.9 -35.9 6.38 0.40 1.86 77.5 11.1 2.16 0.18 0.36 6499 -38.0 -38.7 -36.8 41.2 -36.4 9.23 0.99 2.16 73.4 11.4 3-17 0.17 0.38 6500 -36.2 -38.0 -36.5 -40.3 -36.1 8.06 0.71 2.05 74.6 11-6 2-31 0.18 0.54 6447 6 -39.9 38-9 -44.1 40.1 4.03 8.70 1.9376-9 6.99 1.09 0.11 0.21 6500 -36.1 -39.1 -37.5 42.5 -37.8 7.19 3-01 2.10 76.5 9.14 1.58 0.13 0.33 6298 -36.8 40.4 -39.1 33.1 -38.6 3.88 12.7 1.96 73.9 5.55 0.75 0.08 0.13 6300 -35.7 -39.7 -39.1 -43.4 -39.7 4.96 5.34 1.93 77.9 8.08 1.33 0.12 0.26 6429 40.7 1 40.4 45.1 42.0 11.1 1.79 1.91 78.1 6.16 0.71 0.08 0.13 6299 -36.6 -39.5 -39.0 -42.5 -39.3 6.55 1.63 2.27 77.4 9.77 1.80 O 0.39

*Ga samples were a11 obtained from the 6800 ft. level of Kidd Creek Mine. Samples were collected from freely discharging boreholes in evacuated glass sample vessels- Gas compositions are conected for air contamination by assuming that al1 O2 (typically <2 voi. %) results from contamination and by subtracting a corresponding amount of N2. Resultant compositions are nonnalized to 100%. Cornpositional analyses were performed using a Vanan 3300 gas chrornatogmph. Hydrocarbons were separated using a Porapak Q" 80/100 mesh, 6fi.x1/8 inch colurnn with a FID derector. Inorganic cornponents were separated using a Molecular sieve3 5A 45/60 mesh, 16Ftx 118 inch coIumn with a TCD detector. Reproducibility of al1 analyses are within 20.5%. Carbon isotopes were rneasured by compound specific isotope analysis using a gas source Finnigan MAT 252 mass spectrometer interfaced with a Varian 3400 GC. Accuracy and reproducibility of 6% values are better than 0.5% widi respect m PDB standard.

Chapter 3 : Carbon isoropic and compositional evidence for inorganîc syn fhesis. .. 86

3-7 LIST OF APPENDICES

Appendix 3.1: 6 ' k vs. carbon number plots for Kidd Creek hydrocarbon gas sarnples

Appendix 3.2: Log (mole fraction) vs. carbon nurnber plots (Schulz-Flory plots) for Kidd

Creek hydrocarbon gas sarnples.

C ha pter 3 : Carbon isotopic and composiiional evidence for inorganic synthesis ... 87

Appendix 3.1 6 ' k vs. carbon number plots for Kidd Creek hydrocarbon gas samples

1 2 3 4

Carbon number

-44 1 1 2 3 4

Carbon number

-46 1 1 2 3 4

Carbon number

Carbon number

C hap ter 3 : Carbon isoropic and compositional evidence for inorganic synthesis.. . 88

Appendix 3.2 Log (mole fraction) vs. carbon number (Schulz-Flory) plots of Kidd Creek

hydrocarbon gas samples

2 3 Carbon number

1 3 Carbon nurnber

4

I 1 Carbon number 4

1 Carùan number 4

2 3 Carbon number

-25 1 1 2 3 4

Carbon number

Chapter 3: Carbon isotopic and compositional evidence for i n o r p i c synthes k.. 89

1 * Carbon number

Carbon num ber

2 3 Carbon number

1 2 3

Carbon number

Chapter 4: Szrmrnary and ConcZzrsions.

Chapter 4: Summary of Conclusions

4.1 SUMMARY OF CONCLUSIONS

1. Based on a unique set of isotopic signatures, regionally extensive and volumetrically

significant accumulations of methane gas in the crystalline rocks of the Canadian Shield

were suggested to originate through processes of inorganic synthesis (Sherwood Lollar et

al., 1993). In this study, hi& sensitivity compound specific isotope analysis has

facilitated the f ~ s t detailed characterization of the carbon isotopic signatures for more

complex organic compounds (ethane, propane, butane) for a set of I 1 gas samples from

the Kidd Creek Mine, Timmins, Ontario. Ci - C4 alkanes derived from thermal

decomposition of high molecular weight organic matter are typically isotopically

enriched in I3c with increasing molecular weight due to kinetic fiactionation associated

with bond breakage dunng thermogenic gas generation. In contrast, CI - CJ alkanes

from the Kidd Creek Mine show a consistent isotopic depletion trend. Ethane and

propane are typically depleted in "C by 1 - 4x0 with respect to methane. Butane is

typically depleted by 2 - 49/00 with respect to ethane and propane. While not entirely

consistent with isotopic depletion trends identified for CZ - C4 produced via

polymerization from methane in a spark discharge process (DesMarais et al., 1981;

Chang et al., l983), these Shield gases represent to Our knowledge the first definitive set

of terrestrial samples to demonstrate isotopic depletion trends between Ci - Cr

homologues similar to the spark discharge gases.

Compositionally the relative distribution of Ci - C4 alkanes follows a Schulz-Flory

distribution with R' correlations between 0.98 and 0.99 for al1 sarnples. This

distribution, whereby the log (weight fraction) of alkanes decreases linearly with

Chapter 4: Surnrnary and ConcZzrsions. 9 1

increasing carbon number is charactcristic of processes of polyrnerization whereby

heavier alkanes are fonned by the addition of Iower rnolecular weight homologues. The

Schulz - Flory distribution is not diagnostic of inorganic synthesis by itself since this

distribution of homologues can occur by random breakage of bonds as well as via

polyrnerization. However, taken together, the distinct isotopic and compositional

signatures of hydrocarbon gases from the Kidd Creek iMine indicate that they are not

produced by conventional biogenic processes. Rather, they suggest that these gases are

formed by processes of abiogenic synthesis.

2. Gas discharge rate measurements from the boreholes on L.6800 at Kidd Creek indicate

that large quantities of gas are stored in fractures and structural cavities in the crystalline

rock (Chapter 2). Based on averaged gas flow rates, an estimate of the total volume of

gas discharging onto L.6800 of 2 . 5 ~ 1 0 ~ ~ month was calculated. Gas discharge rates

from individual boreholes are variable ranging from no detectable flow up to >30L/min.

In general, discharge rates decreased over the two year study period; however, several

boreholes maintained uniformly high discharge rates and showed no indications of

abatement with time. This variability in discharge rates does not appear to be related to

borehole length, orientation, rock type intersected or fracture density. Nonetheless, such

a pattern does indicate that the system is characterized by fracture networks that contain

volumetrically large quantities of gas, as well as smaller isolated gas pockets. In addition,

the heterogeneity of the gas flow rates suggests that the fracture systems which host these

gases are not, in general, hydraulicall y connected. This has important implications for

mining operations since it cannot be assumed that the gases stored in deeper fracture

C hap ter 4: Sumrnary and Conclusions. 92

systems will be drained by boreholes drilled at higher elevations. This implies that new

pockets of gas will likely be encountered as the mine goes deeper or is developed

laterally. As a result, these gases present a long tenn hazard to mining operations in th is

environment-

3. In general, the compositions and 613cCH4 signatures of gases discharging from

individual boreholes do not change significantly over time (Chapter 2). SiJnificant

differences do exist, however, in cornpositional and carbon isotope signatures of gases

from different boreboles. From these data three conclusions c m be drawn: i) the

boreholes tap into a large number of hydraulically isolated fractures systems which host

geochemically distinct gases; ii) over periods of time of up to 2 yeas the compositions

and isotopic signatures of gases discharging from the boreholes do not change

appreciably from that of the 'pristine' Shield gas which flowed from the borehole

immediately afier cornpletion; and iii) the addition of bacteriogenic C& resulting from

the introduction of methanogenic rnicroorganisms by drillhg and mining operations is

negligible. Processes controlling spatial variations in gas compositions and isotopic

signatures are not clear, however. Cornparison of gas charactenstics with the

rnineralogical and structura1 features indicated in the borehole core log revealed no

significant associations.

4. In addition to Cb, the gases at Kidd Creek contain Ct+ hydrocarbons (Le., CrH6,

C3H8, C4Hio), Nz and free H2 gas. The presence of Cz+ hydrocarbons and Hz has

significant implications for the calculation of the lower explosive lirnit (LEL), a measure

C ha p ter 4: Surnrnary and Conclusions. 93

of the explosive hazard, since the presence of these other flamrnabie species results in a

lower calculated LEL value for the buik gas mixture as compared to a pure Cl& phase in

air. LEL values calculated for the Kidd Creek gas mixtures are typically in the 3.91 to

4.92% range. These values represent an average >17% reduction compared to LEL

values previously assumed by the Canadian Shield mining comrnunity (5.3%), which

were used based on the assumption that Shield gases were enùrely composed of CH4.

Sibgnificantly, this study also shows that concentrations of fkee Hz gas are consistently

higher on L.6800 as compared to shailower levels of the mine, resulting in lower

calculated LEL values and, therefore, an increased explosive hazard. This shows that

different LEL values may be appropriate for different levels of the mine. In addition, the

overail concentration of flammable species versus non-flammable species such as Nz is

greater in the gases at Kidd Creek compared to other sites in the Canadian Shield. For the

Canadian Shield mining cornmunity as a whole, assessment of the relative concentrations

of flammable species such as C a , C3+, and Hz, as we1l as charactenzation of non-

flammable species in the gas are clearly important to determine explosive hazard.

Chapter 4: Surnrnaty and Conclusions. 94

Chang S., Des Marais D. J., Mack R., MilIer S. L., and Strathearn G. E. (1983) Prebiotic organic syntheses and the ongin of life. In Earih S Earliest Biosphere, Iis Origin and Evoluiion (ed. J . W. Schopf), pp. 53-92. Princeton Univerisity Press.

DesMarais D. J., Donchin J- H., Nehring N. L-, and Truesdell A. H. (198 1 ) ~Molecular carbon isotopic evidence for the ongin of geothermal hydrocarbons. Nature 292,826- 828.

Shenvood Lollar B., Frape S . K., Weise S. M., Fritz P., Macko S- A., and Welhan J. A. (1993) Abiogenic methanogenesis in crystailine rocks. Geochimica er Cosrnochimica Acta 57,5087-5097.