interpreting fracture development from diagenetic

$: Interpreting fracture development from diagenetic$
www.elsevier.com/locate/tecto

Tectonophysics 400 (

Interpreting fracture development from diagenetic mineralogy and

thermoelastic contraction modeling

Renee J. PerezT, James R. Boles

University of California, Santa Barbara, Geological Sciences Department Room 1006, Webb Hall, Santa Barbara,

CA 93106-9630, USA

Received 1 March 2004; accepted 1 March 2005

Available online 9 April 2005

Abstract

Four sets of thin-section scale, Mode I (open mode), cemented microfractures are present in sandstone from the Eocene

Misoa Formation, Maracaibo basin, Venezuela. The first set of microfractures is intragranular (F1), formed early during

compaction and are filled with quartz cement precipitated at temperatures equal to or higher than 100 8C. The second set of

microfractures (F2) is cemented by bituminite–pyrite, formed at temperatures between 60 and 100 8C, and are associated with

kerogen maturation and hydrocarbon migration from underlying overpressured source rocks. The third set of microfractures

(F3) is fully cemented by either quartz cement or calcite cement. The former has fluid inclusion homogenization temperatures

between 149 and 175 8C. These temperatures are mostly higher than maximum burial temperatures (~160 8C), suggesting that

upward flow, caused by a pressure gradient, transported silica vertically which crystallized into the fractures. Upward

decompression may have also caused a PCO2drop, which, at constant temperature, allowed simultaneous carbonate precipitation

into the third microfracture set. The fourth set of thin-section scale microfractures (F4) is open or partially cemented by siderite–

hematite and other iron oxides. The presence of hematite and iron oxides in microfractures is evidence for oxidizing conditions

that may be associated with the uplift of the Misoa formation. In order to time and place constraints on the depth of formation of

the fourth set of microfractures, we have coupled published quartz cementation kinetic algorithms with uniaxial strain equations

and determined if, in fact, they could be associated with the uplift of the formation. Our results suggest that thermoelastic

contraction, caused by the formation’s uplift, erosion, and consequent cooling is a feasible mechanism for the origin of the last

fracture set. Hence, we infer that meteoric water invasion into the fractures, at the end of the uplift, cause the precipitation of

oxides and the transformation of siderite to hematite.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Sandstone diagenesis; Fracture formation; Veins; Thermoelastic contraction; Overpressure

0040-1951/$ - s

doi:10.1016/j.tec

T Correspondi

Northwest, Calg

E-mail addr

2005) 179–207

ee front matter D 2005 Elsevier B.V. All rights reserved.

to.2005.03.002

ng author. Current affiliation: University of Calgary, Department of Geology and Geophysics, 2500 University Drive,

ary, Alberta T2N 1N4 Canada.

esses: [email protected] (R.J. Perez), [email protected] (J.R. Boles).

R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207180

1. Introduction

Reservoir and thin-section scale partially cemented

fractures and veins represent examples of tensile

failure and fluid/fracture interactions. Extension

fractures exhibit simple power-law scaling across

3.4–4.9 orders of magnitude (Marret et al., 1999), but

regardless of their scale, hydraulic fractures decrease

pore fluid pressure in the adjacent rock. Furthermore,

depending on their connectivity, frequency, and

hydrologic regime they may facilitate mass transfer

and heat advection. Over time, some fractures are

cemented while others remain open, causing a host-

rock scale porosity and permeability heterogeneity

that ultimately affects fluid production. Microfrac-

tures are used to determine far field stresses (i.e.,

Laubach, 1989) and magnitude, direction, and scale

of fluid movement (e.g., Laubach, 1988; Eichhubl

and Boles, 1997). Geochemical studies of cemented

fractures in sandstone are typically performed in

tectonically deformed areas such as fault zones and

folds (e.g., Hippler, 1993; Macaulay et al., 1997;

Perez and Boles, 2004; Boles et al., 2004), but

microfractures and veins are not restricted to

deformed areas.

In the absence of tectonic stresses, compaction

disequilibrium and quartz cementation can result

fluid retention and pore pressure increase (Helset et

al., 2002; Swarbrick et al., 2002), which may lead, in

turn, to hydraulic fracturing (e.g., Laubach, 1988;

Engelder and Lacazette, 1990). Uplift, erosion, and

cooling can also cause significant pore fluid changes

and thermoelastic stresses that subsequently lead to

tensile failure (e.g., Suppe, 1985; Swarbrick and

Osborne, 1998). Tensile failure produces, in turn,

pore pressure changes, allowing fluid movement, and

leading, ultimately, to vein formation (Eichhubl and

Behl, 1998). In these cases, the formation and

cementation history of veins indirectly reflect the

host rocks stress history, fluid pressure, and fluid

composition.

The Misoa Formation, the subject of our study, is

present in the subsurface on the east side of the

Maracaibo Lake, Venezuela. The Misoa Formation is

one of the most prolific siliciclastic hydrocarbon

reservoirs of the world (Higgs, 1996), and, in this

paper we show microfractures provide avenues for

aqueous and hydrocarbon flow as well as vertical

connection between sandstone horizons. The Misoa

Formation was deposited during Eocene time in a

shallow marine-deltaic environment (Gonzalez et al.,

1980), buried to more than 4 km under high

sedimentation rates, subject to temperatures higher

than 180 8C (Rodriguez et al., 1997), and uplifted

more than 2/3 of its maximum burial depth. Because

of the wide range of pressure and burial regimes, the

formation is ideal for the recognition of compaction,

and the study of inversion related fractures (Law and

Spencer, 1998). Previous diagenetic studies (e.g.,

Ghosh et al., 1990; Perez et al., 1999b) and burial-

thermal reconstruction (Rodriguez et al., 1997; Perez

et al., 1999a) of the Misoa Formation provide input

for our semi-quantitative thermo-mechanical analysis

used to interpret fracture conditions during burial and

uplift.

Our study describes the diagenesis of the Misoa

Formation sandstone and the mechanisms of fracture

development and cementation during burial and

uplift. The timing of cementation and fracture

forming-filling events are interpreted from fluid

inclusion data and the application of the quartz

precipitation kinetic model of Walderhaug (1996).

Based on petrography and diagenetic cements we

distinguish microfractures that originated during

burial subsidence (fractures sets F1 to F3) from

others possibly originated during uplift (fracture set

F4). The origin of the fourth fracture set is difficult

to ascertain. Combining, however, petrologic infor-

mation and diagenetic-mechanical modeling we

infer they have been formed and cemented in recent

in time at shallow depth, and are associated with

uplift.

Samples were collected from the Lagunillas,

Bachaquero, and Motatan onshore oil fields and a

wildcat exploration well, BA-1, which is located

approximately 2 km to the northwest of the Bach-

aquero field (Fig. 1). No information on core or

sample orientation was available. These localities

provide, however, excellent chronology of fracture

events. All fractures cements are monomineralic,

which led us to group the structures in sets, based

strictly on their mineralogy, depth of occurrence,

orientation relative to bedding, and cross-cutting

relations. The data and results that we herein present

may characterize the general diagenesis, veining

conditions, paleofluid composition, pore pressure,

Venezuela

Caribbean Sea

MARACAIBO LAKE

MIS

OA RIV

ERVE

OA

MIS

Zulia Oriental Region

N

A

LAGUNILLAS FIELD

BACHAQUERO FIELD

A'

MOTATAN FIELD

0 10 20 30 km

MARACAIBO

Fig. 1. Schematic map of the Maracaibo Lake basin depicting the Zulia Oriental Region and locations of oil fields mentioned in text. Well

coordinates and locations are proprietary information of Petroleos De Venezuela (PDVSA).

R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207 181

and stress regimes present during the burial and uplift

of similar siliciclastic formations in other regions.

2. Geologic setting

The studied sections of the Lower to Middle

Eocene Misoa Formation are located in the eastern

onshore region of the Lake Maracaibo basin, western

Venezuela (Fig. 1). The basin represents a stable

intracratonic terrain surrounded by active plate boun-

daries and high relief areas (Gonzalez et al., 1980;

James, 2000. The basin overlies a Mesozoic crystal-

line basement and traps Jurassic volcaniclastic, Creta-

ceous marine, and Paleogene–Neogene delta

sediments (Fig. 2; Lugo and Mann, 1995), derived

from cratonic sources as well as Caribbean and

Andean orogenic belts (Castillo et al., 1996). Late

Eocene and Miocene inversions, resulting from the

continental collision between the northern South

American border and the Lara Napes, interrupted the

subsidence of the basin (Lugo, 1991). During the

initial stage of the collision (~54 Ma) the eastern

onshore region, also called the Zulia Oriental Region

(ZOR; Fig. 1), developed into a back-arc sub-basin,

that was quickly filled by turbidite, deltaic, and

shallow marine deposits (Gonzalez et al., 1980). The

Misoa Formation is a tidal-influenced deltaic deposit

Misoa Fm.Trujillo Fm.

S-SW

21 km

A

Fig. 3. Schematic northeast–southwest structural section through Tertiary str

Formations, Members, and Zones

Bachaquero

GUASARE

CR

ETA

CE

OU

SPA

LEO

CE

NE

EA

RLY

E

OC

EN

EM

IDD

LE

EO

CE

NE

UP

PE

R

EO

CE

NE

MIO

CE

NM

IO-P

LEIS

TO-

PLI

OC

EN

E

Series

LAG

UN

ILLA

MISOA

MENE GRANDE / JARRILLAL

PAUJI

LA R

OS

A

Laguna

La Rosa

Lagunillas

Santa Barbara

?

BETIJOQUE

ISNOTU

39.5-36 to

39.

42.

44

49.

51.

54

66

84

TRUJILLO

Age in Million of

Years Ago (Ma).

Regional unconformity

Regional Seal

Studied Formation

Regional Source RocksMITO JUAN/COLON LA LUNA

Fig. 2. Schematic stratigraphic column of the Zulia Oriental Region,

western part of the Maracaibo basin. Source rocks underlies the

Misoa Formation, whereas regional shale (seal) overlies (Gonzalez

et al., 1980).


(Van Veen, 1972; Maguregui and Tyler, 1991; Higgs,

1996). Based on spontaneous potential and resistivity

logs, the sandstone to shale ratio is ~1:6. Several

normal and reverse faults cut through the Misoa strata

into Cretaceous strata, at more than 4 km subsurface

depth (Fig. 3). Reverse faults were generated by the

reactivation of Cretaceous–Paleocene normal faults

(Roure et al., 1997).

The burial history of the Misoa Formation consists

of two subsidence periods, the first from 50 to 42 Ma

and the second from 17 to 5 Ma, both interrupted by

uplifts. Much of the basin was inverted during the first

uplift between ~42 and 38 Ma. In the study area, the

second uplift occurred from 7 to 4 Ma, and was

associated with the Miocene Orogeny. Vitrinite

reflectance data indicate a variable heat flow through-

out the history of the basin (Rodriguez et al., 1997).

The thermal data are interpreted to represent a heat

flow increase from 52 to 58 mW/m2, resulting in

thermal gradients between 30 and 35 8C/km from 54

to 49 Ma, followed by an exponential decay to ~48

mW/m2, from 52 Ma to present. The present day

thermal gradient varies from 25 to 28 8C/km(Gonzalez et al., 1980). The thermal interpretation

has been calibrated against apatite fission track,

present-day temperature data, and temperature logs

(Rodriguez et al., 1997). Erosion plays an important

role in our stress analysis and varies from 0.2 to ~3

km (Rodriguez et al., 1997). Based on measurements

from exploration and production wells the Misoa

N-NE

Well #LB-114

3.5 km

Pauji Fm.Undifferenciated

La Rosa-Lagunillas-Betijoque Fms.

0 km

A'

ata of the studied area. The cross section location is depicted in Fig. 1.


Formation is currently at hydrostatic or sub-hydro-

static pressure due to extensive oil production

(Gonzalez et al., 1980).

3. Methods

3.1. Analytic

We collected approximately 70 sandstone core-

plug samples along 300 m of core taken from seven

wells, distributed over three oil fields and a wildcat

well in the east side of the Maracaibo Lake basin,

western Venezuela (Fig. 1). The samples belong to

unfaulted segments of the Eocene Misoa Formation

and their present depths range from 0.2 to 4.2 km,

which covers the entire formation (Fig. 2). From the

70 core plugs we made 40 thin sections and stained

them for calcium carbonate with alizarine red and

counted 300 points per slide.

We observed and analyzed the fracture mineralogy

with a conventional polarized microscope and a JEOL

scanning electron microscope (SEM) model JSM-

6300v. The SEM was set to an acceleration voltage of

15 kV and a beam current of 160 AA. An energy

dispersive analyzer (EDA Tracor Northern TN2010)

was coupled with the SEM, and used a beam current

of 250 AA. Additionally, the amount of quartz

overgrowth was estimated in 29 quartz arenites and

in 5 fractured sandstone samples with a cathodolumi-

nescence (CL) unit attached to the SEM. We obtained

15 images per sample, covering an area of ~1 cm2,

and determined quartz overgrowth percentages from

digital analysis of CL images.

The elemental composition of calcite cements is

based on electron microprobe analysis using a

CAMECA SX50, set with an acceleration voltage of

15 kV, and beam current and beam diameter of 10 nA

and 10 Am, respectively. Results were normalized to 1

mol total of Ca+Fe+Mg+Mn.

Homogenization and melting temperatures of

fluid inclusions in quartz were determined using a

Fluid-Inc., modified USGS gas-flow stage. Fluid

inclusions were present in quartz overgrowth, quartz

grain-scale fracture cement, and in quartz veins in

sandstone away from fault zones. We only used

fluid inclusions assemblages with constant liquid/

vapor ratio to avoid necking-down effects, which

change original inclusion volumes, distorting tem-

perature interpretations (c.f., Goldstein and Rey-

nolds, 1994; Roedder, 1984). Methane inclusions are

found with H2O-rich inclusion assemblages. Thus,

we did not apply pressure corrections to the

homogenization temperatures (Th) because small

CH4 concentrations cause Th to approximate true

trapping temperatures (Hanor, 1980). We calculated

salinity values from melting temperatures (Tm)

assuming an H2O–NaCl system and using Bodnar’s

(1992) revised equation of state. We used to Th’s

(1) to delimit (in time) the precipitation of quartz

overgrowth in intragranular fractures, quartz over-

growth surrounding detrital grains, quartz filling the

set of microfractures F3 using burial histories and

assuming the quartz filling occurred during burial,

and (2) to verify the quartz cementation modeling

results.

4. Modeling methods with brief theoretical

background

After determining the distribution and quantity of

quartz overgrowths, we modeled in 1-D the precip-

itation of quartz overgrowth and the stress history of

the Misoa Formation. The quartz overgrowth model

yields the amount of quartz cement and porosity

decline curves through time.

Our stress analysis yields the effective horizontal

stress through time as a function of burial depth,

while changing the rock properties. The bridge

between the diagenetic and stress model is the

porosity evolution. Experiments and empirical obser-

vations suggest that mineralogy and porosity control

the thermo-mechanical properties of sandstones (i.e.,

see review by Giles, 1997). In the Misoa sandstone,

compaction and quartz cementation mainly control

the porosity (Perez et al., 1999b). Thus, if the porosity

is calculated through time, depth, and temperature,

the horizontal effective stress, assuming the uniaxial

stress model, is indirectly a function of time, depth,

and temperature.

4.1. Quartz cementation modeling

The quartz cementation model is based on numer-

ical algorithms, which consist of empirically derived


kinetics based on observations in the North Sea basin

(Walderhaug, 1994), surface area (Walderhaug, 1996)

and a compaction decline function (c.f. Lander and

Walderhaug, 1999). This model has been tested in

basins around the world, including the Maracaibo

basin (Awwiller and Summa, 1997; Perez et al.,

1999a; Chatellier and Perez, 2000). The results of

these tests indicate that the use of the kinetic

parameters published by Walderhaug (1994) and

presented in Table 1 yield errors smaller than 3%,

which is acceptable to us. The algorithms were

numerically solved in a Microsoft Excelk spread-

sheet. The basic concepts and mathematical formula-

tions of the model are described elsewhere

(Walderhaug, 1994, 1996; Walderhaug and Bjbrkum,

2003). Hence, we include here only a brief summary.

The volume of quartz cement Vq (in cm3) precipi-

tated in a cm3 of sandstone with quartz surface area

A (cm2), during time t (in s) at constant temperature

is:

Vq ¼ MrAt=q ð1Þ

whereM is the molar mass of quartz (60.09 g/mol), r is

the rate of quartz overgrowth precipitation in mol/cm2

s, and q is the density of quartz (2.65 g/cm3). The rate r

is expressed as a logarithmic function of temperature

(Walderhaug, 1994):

r ¼ a10bT ð2Þ

where T is temperature (8C) and a and b are constants

with units of mol/cm2 s and 1/8C, respectively (Table

Table 1

Selected list of samples with thin-section scale microfractures and microv

Well BA-1 Well LS-1387

Depth (ft) Depth (m) Fracture type Depth (ft) Depth (m

1336 404.8 F4 1023.8 310.2

2503 758.5 F2–F4 6359.4 1927.1

2506 759.4 F2 7340 2224.2

3327 1008.2 F2–F4 9093.3 2755.5

3587.6 1087.2 F2 9343.3 2831.3

4169 1263.3 F2–F4 9346.7 2832.3

4748 1438.8 F2–F3–F4 9842.8 2982.7

5274 1598.2 F3–F4

6345 1922.7 F2–F3

Under cathodoluminescence, all samples contain F1 intragranular microfr

F2: bituminite–pyrite microveins.

F3: quartz or calcite microveins.

F4: open uncemented microfractures or partially cemented by iron oxides

2). When the T(t) path of the sandstone is known, T can

be divided in small linear time steps. Thus, the total Vq

precipitated from time t0 to tn is the integration of Eq.

(1) along the T–t path, that is:

Vq ¼ M

q

Xni¼0

Ai

Z tiþ1

ti

10b tciþ1þdið Þdt ð3Þ

where c is the temperature rate change or slope (8C/s)and di the initial temperature within the time step. The

surface area A is estimated as the cumulative surface

area of spheres of diameter D with the total volume

equal to the quartz fraction f times a unit volume of the

sandstone v (Walderhaug, 1996). The surface area A is

readjusted after each time step and is considered

proportional to the porosity loss through time and

depth, expressed as:

Ai ¼ 1� coatð Þ 6f v

D

�/i

/0

�ð4Þ

where /i is the porosity for the present time step and

/0 is the initial porosity at the time of deposition. The

porosity /i is also readjusted each time step because it

is considered a function of the effective vertical stress–

through a compaction function (see Eqs. (5) and (6))–

and the amount of quartz cement. The effect of grain

coating (coat parameter in Eq. (4)), which inhibits

cementation (Lander and Walderhaug, 1999), was

considered zero at all times.

Different compaction decline functions have been

coupled with the quartz overgrowth precipitation

eins

Well LB-273

) Fracture type Depth (ft) Depth (m) Fracture type

F2–F5 6468.2 1960.1 F3

F2–F5 6878 2084.2 F3

F5 7504 2273.9 F3

F3–F4 7729 2342.1 F3

F3 7904.3 2395.2 F3–F4

F3–F4 8890 2693.9 F2–F3

F2–F3

actures.

.

Table 2

Parameters used in the quartz cementation kinetic model and thermo-mechanical parameters used in the stress analysis of sandstones

Symbol Variable Units Initial

value

Final

value

Source

a Pre-exponential parameter, Eq. (2) mol/cm2 s 1.98e�22 T Walderhaug (1994)

b Exponential parameter, Eqs. (2) and (3) 1/8C 2.2e�2 T Walderhaug (1994)

C21 Exponential constant, Eq. (5) 1/MPa 8.3e3 T Chuhan et al. (2002)



E4 Young’s modulus, Eq. (10) MPa4 �1.0e3 �16.5e3 Engelder (1985)

v4 Poisson’s ratio, Eqs. (7)–(10) D 1.5e�1 3.3e�1 Bachrach et al. (2002),

Engelder (1985)

a4 Coefficient of thermal expansion, Eqs. (8) and (10) 8C�1 10.0e�6 10.8e�6 Engelder (1985)

b Biot poroelastic parameter, Eq. (7) D 1 5.6e�1 Breckels and van Eekelen (1982)

T Constant value through time.1, 2, 3 Exponential constants for IGV reduction as a function of effective stress for fine, medium, and coarse-grained sandstones, see Eq. (5) in

text.4 Assembled from various sources by Engelder (1985).

D, dimensionless.


model (e.g., Lander and Walderhaug, 1999). We used

new experimental compaction data based on effective

stress, rock composition, and grain size (c.f. Chuhan

et al., 2002). The advantage of using the new data set

is two-fold. First, the data reveal a correlation between

intergranular volume decrease and grain size not

revealed in other compaction studies (i.e., Pittman

and Larese, 1991). Second, the mineralogy of the

sands used in these experiments is similar to the

mineralogy of the Misoa Formation; both are quartz

arenites. We numerically fit Chuhan’s et al. (2002)

results with curves of the form:

/ ¼ C1eC2rVv ð5Þ

where C1 and C2 are constants (Table 2) and rvV is theeffective vertical stress in MPa. The latter term, rvV, isthe total overburden stress rv minus the pore fluid

pressure Pf (Terzaghi and Peck, 1948). A cross plot of

predicted vs. measured amount of quartz cement for

this specific area using the kinetic parameters pre-

sented in Table 1 is shown elsewhere (in Fig. 3 from

Perez et al., 1999b).

With the purpose of determining the origin of the

fourth fracture set, which based on diagenetic cements

is inferred to have been created recently at shallow

depth, we use a simple thermoelastic model to

understand the possible stress evolution of the Misoa

Formation.

4.2. Stress history modeling

Cementation or mineralization change the thermo-

mechanical properties of sediments (Jizba and Nur,

1990; Giles, 1997). Experimental results demonstrate

that the Poisson’s ratio, Young modulus, poroelastic

coefficient, and the coefficient of thermal expansion

vary significantly with mineralogy, porosity, and

sandstone hardness (Engelder, 1985). Thus, because

sediments are unconsolidated when buried, diagenesis

ensures that the stress path during burial is different

from that during uplift.

Our model assumes that sandstone layers are

poroelastic, homogenous, and isotropic. Thermo-

mechanical properties are assumed to (1) vary linearly

and synchronously with the amount of quartz cement

and porosity throughout the burial; (2) attain their

maximum values as porosity decreases to zero; and

(3) remain constant after the minimum porosity is

approached and during uplift (Warpinski, 1989).

Moreover, we assume that (4) the horizontal stress is

the minimum principal stress; (5) the overburden is

the maximum principal stress; and (6) that all stresses

are governed by the uniaxial strain model. Burial

histories, described before, are basic input for the

stress analysis.

In reality, our assumptions are not strictly met for

several reasons. A sandstone body may not behave

homogeneously, isotropically, and elastically due to


compaction, diagenesis heterogeneities, and stress

concentration around flaws. Moreover, some micro-

fractures may reflect regional tectonic stresses (Lau-

bach, 1989), such as the late Eocene and Miocene

plate convergence. However, based on the correlation

of fracture formations and model predictions, our

oversimplifications do not alter first-order effects of

burial and uplift on the modeled state of stress (Narr

and Currie, 1982; Apotria et al., 1994). In the

Maracaibo basin, furthermore, the present day hori-

zontal stresses are low and vary significantly from one

oil field to another (Breckels and van Eekelen, 1982)

suggesting that some oil field sub-regions, particularly

those distant from major fault and folds, may behave

as tectonically relaxed areas.

The mathematical formulations for the uniaxial

strain model are described elsewhere (e.g., Engelder

and Lacazette, 1990; Twiss and Moores, 1992).

Therefore, we will only explain briefly the theory

and numerical treatment. Assuming poroelastic

behavior the effective vertical stress rvV in the uniaxial

strain model is:

rvV ¼ rv � bPf ð6Þ

where b is the Biot poroelastic parameter (Biot, 1941)

and Pf is the pore fluid pressure. The model is based

on the premise that the stress is partially accommo-

dated by elastic grain contacts. Empirical observations

suggest that exponential functions of the porosity

reproduce b (Giles, 1997), such as:

bi ¼�

/i

/0

�0:33

ð7Þ

where /0 is the initial porosity at the time of

deposition and /i the porosity for the present time

step. The horizontal stress (rh) is a function of the

overburden (Engelder and Lacazette, 1990). When the

effective horizontal stress includes the thermal effect,

rhV is calculated as:

rhV ¼v

1� vrv � bPfð Þ þ a

E

1� v

�DT

�ð8Þ

where v is Poisson’s ratio, a is the coefficient of

thermal expansion in 8C�1, DT is the change in

temperature, and E is Young’s modulus in MPa, the

horizontal effective stress rhV sometimes decreases

faster than rvV during uplift, and may become tensile,

to the point where microfractures occur. The temper-

ature change as a function of depth Z may be

expressed as:

DT ¼ dT

dzDz: ð9Þ

If the P–T–t path (burial history) of a sandstone

body is divided in small linear time steps, the change

in the horizontal effective stress DrhV can be calculated

numerically as a function of time. The sum can be

numerically performed along a P–T–t as:

Xni¼0

rhiþ1V ¼

Xni¼0

�vi

1� viðrvi � biPf iÞ

þ�

Ei

1� vi

�aiðTiþ1 � TiÞ þ rhi

V

�ð10Þ

where i is a time step and (i+1)� i is the linear

increment. As explained above, E, v, and a are

assumed to vary linearly and synchronously with

compaction, quartz cement, and ultimately the poros-

ity, whereas b varies with Eq. (7). Breckels and van

Eekelen (1982) measured values of b on rocks of the

Misoa Formation. The initial and final values of E, v,

and a were taken directly from the literature and were

obtained experimentally from sandstone of similar

composition, but different formations (see references

in Table 2). Additional stress terms in Eq. (10), for

example those that would account for tectonic

stresses, are not included because they are not defined

in the uniaxial strain model. The pore fluid pressure Pf

varies with time. In the following section the

diagenetic evolution of quartz cement in the sandstone

of the Misoa Formation is calculated, including its

effect on porosity.

5. Host rock’s paragenesis and thermo-mechanic

evolution

The Misoa sandstone is very fine, to medium-

grained, well to moderately sorted quartz arenite to

sublitharenite. The average composition is quartz86-feldspar5lithic9 (Table 3). Early diagenesis in these

sandstones includes less than 1% of tangential grain

coating of smectite clay minerals, intergranular

framboidal pyrite, siderite cement, feldspar over-

growths, and less than 2% to 5% of kaolinite.

Table 3

Selected petrographic results

Well Depth

(m)

Quartz Feldspar Met.

lithics

Sed.

lithics

Muscovite Kaolinite Calcite Siderite Pyrite S/I Chlorite QC Bituminite P. P S. P

BA-1 692.4 53 6 1 2 2 0 29 3 0 1 0 2 1 0 1

BA-1 878.3 59 4 5 1 2 0 7 1 0 13 0 4 0 0 3

BA-1 942.8 60 6 4 3 0 1 3 2 0 10 0 6 0 1 3

BA-1 1463.9 52 5 1 3 0 0 14 11 0 0 3 3 5 0 3

BA-1 1593.9 63 4 2 2 1 2 1 0 0 0 0 17 0 8 1

LB-114 2757.6 68 1 1 1 0 0 24 0 0 1 1 0 0 0 1

LB-1387 1392.4 60 2 2 1 1 0 28 0 1 0 0 4 0 0 0

LB-1387 2244.9 58 1 3 2 0 5 1 0 0 0 2 28 0 0 1

LB-1387 2412.4 60 2 11 3 1 1 2 1 0 0 2 15 0 0 3

LB-1387 2857.3 59 1 0 2 4 0 5 3 0 0 13 10 3 0 0

LB-1387 2858.3 57 2 3 1 1 0 2 6 0 0 12 15 0 0 1

LB-1387 3854.4 55 3 7 0 1 0 4 0 1 1 0 26 0 0 0

LB-273 1978.0 53 3 3 3 4 0 7 2 0 3 6 12 1 1 0

LB-273 2040.1 67 4 1 1 0 1 0 1 0 1 0 12 1 10 0

LB-273 2100.6 56 5 6 1 4 1 0 0 2 1 5 16 0 0 2

LB-273 2103.4 59 4 6 1 1 0 3 2 0 0 3 17 3 0 0

LB-273 2293.1 61 2 6 2 2 0 11 3 0 0 0 8 1 0 3

LB-273 2363.6 49 11 6 2 3 0 1 2 0 5 0 16 4 0 1

LB-273 2511.9 56 4 8 2 2 1 3 2 1 6 15 1 0 0 0

LB-273 2694.8 61 1 5 1 2 1 0 1 0 0 0 27 0 0 0

LB-273 2718.7 62 6 4 1 4 0 2 3 0 0 8 6 1 0 3

LS-837 496.1 61 8 2 4 1 0 21 0 0 0 0 3 0 0 0

The relative % is based on 300 point counts per thin section.

Met. lithics—metamorphic lithic fragments.

Sed. lithics—sedimentary lithic fragments.

S/I—smectite/illite clay.

QC—quartz cement as overgrowth.

P. P—primary porosity.

S. P—secondary porosity.


Petrographic observations suggest that kaolinite is

related to feldspar dissolution and predates quartz

cement.

Intermediate diagenesis is primarily characterized

by quartz overgrowth formation. Quartz overgrowths

postdate the aforementioned minerals and are volu-

metrically the most important authigenic phase in the

host rock. Overgrowths average 12% of the bulk rock,

but in fine-grained sandstone may reach 26% of the

bulk host rock (Table 3), based on cathodolumines-

cence (CL). Petrographic and CL observations indi-

cate the presence of two distinct generations of quartz

cement. The first quartz cement generation is volu-

metrically less than 2% and may be inherited (Ghosh

et al., 1985). The second generation of quartz cement

has a darker luminescence than the first, contains fluid

inclusion assemblages away from dust rims, and is the

main factor reducing the rock porosity. Homogeniza-

tion temperatures from two-phase liquid/vapor iso-

lated fluid inclusions, present within the last

overgrowth event, ranged from 100 to 175 8C,averaging 119 8C (Table 4, Fig. 4). Melting temper-

atures vary from �0.4 to �2.2 8C, yielding salinity

values between 0.8 and 2.7 wt.% NaCl, assuming an

H2O–NaCl system (Bodnar, 1992). Modeling results,

based on Walderhaug (1996) algorithms, indicate that

the precipitation of quartz overgrowth took place at

depths between 2.5 and 4 km. This range (2.5 to 4

km) corresponds to 10 and 18 my after deposition,

mostly during early Eocene to Oligocene time (i.e.,

Fig. 5). Based on fluid inclusion data solely it would

be tempting to presume that quartz cement also

precipitated during uplift, however, according to the

modeling results, quartz cement does not precipitate in

the sandstone beyond 18 my after deposition, even

though it is within the precipitation window, because

Table 4

Typology of fluid inclusion data analyzed on sandstones

Well Field Depth

(m)

P.D.T M.B.T. Occurrence Th range Th average Tm range Tm

average

wt.%

NaCl

LB-1182 Bachaquero 1180.3 64 117 Grain-scale quartz

cemented

microfracture (F1)

111 to 117 (n =11) 113.5F2 �1 to

�2.3 (n =2)

�1.6 2.74

MOT-4 Motatan 2545 63 N 63 Grain-scale quartz

cemented

microfracture (F1)

98–119 (n =11) 105.6F6.5 N.D. N.D. N.D.

LS-1257 Lagunillas 3819.1 131 170 Quartz microvein (F3) 149–175 (n =14) 167.6F6.5 �0.4 to

�1 (n =13)

�0.9 1.57

LS-1257 Lagunillas 3819.1 131 170 Quartz overgrowth 145–160 (n =4) 151.3F6.3 �0.4 to

�1 (n =2)

�0.7 1.23

MOT-4 Motatan 2545 63 N 63 Quartz overgrowth 107–129 (n =6) 116.3F7.5 �1.6 to

�2.2 (n =5)

�0.5 0.88

BA-1 Wildcat 1953 48 N 48 Quartz overgrowth 101–128 (n =5) 110.2F10.4 N.D. N.D. N.D.

MOT-2 Motatan 2565 68 N.D. Quartz overgrowth 106–139 (n =11) 113.7F9.3 �1 to

�1.8 (n =5)

�1.3 2.24

Temperatures in Celsius.

P.D.T.—present day temperature in Celsius.

M.B.T.—maximum burial temperature based on thermal modeling.

Th—homogenization temperature.

Tm—melting temperature.

wt.% NaCl—assuming water–salt system, and using Bodnar (1992) revised equation.

N.D.—not determined.


there is no intergranular space available for quartz

cement to grow.

Locally, post-quartz siderite and calcite cement

filled up to 18% of the intergranular volume, and

prevented further compaction and quartz cementation.

Based on microprobe analysis, the average composi-

tion of the late-stage calcite is Ca0.91Mg0.01(Fe+

Mn)0.08CO3. The precipitation P–T–t of the local

late-stage calcite may be inferred using results from

the quartz precipitation kinetic model (Fig. 5), specif-

ically, using intergranular volume (IGV) reduction

curves through geologic time. The method consists of

matching the available IGV left by compaction and

quartz overgrowth with the amount of late-stage calcite

cement quantified by microscopic analysis. Post-quartz

calcite cement is calculated to have precipitated

between 100 and 150 8C, at depths greater than 3 km,

between 12 and 14 my after deposition. These petro-

graphic results and cementation temperatures are

similar to other studies in the basin (e.g., Chatellier

and Perez, 2000). Other local late-stage cements

represent 2% to 6% of the host rock and consist of

siderite, chlorite, and chert (Table 3). However, the

majority of the sandstones, in which we apply the

quartz cement model, are clean, quartz-rich, and with

no other cement than quartz overgrowths.

The diagenetic model suggests that the porosity in

the clean quartz arenites decreased to almost zero

before the maximum burial depth was attained (Fig.

5C,D). Experimentally, it has been demonstrated that

the Poisson’s ratio v, Young’s modulus E, and

poroelastic parameter b of sandstone in quartz-rich

sands vary with the porosity and degree of cementa-

tion (see review by Giles, 1997). The mathematical

function describing how these properties vary with

porosity and time, i.e., either exponentially or linearly,

is to our knowledge unknown. Consequently, we

arbitrarily made v, E, and a of clean quartz arenites to

evolve linearly and synchronously with the porosity

and amount of quartz cement, from their initial values

at deposition to their final values when /c0 (Fig.

5A,B). For instance, the porosity of clean quartz

arenites in well LB-273 at 2488 m depth decreased–as

a function of quartz cementation and compaction–

from 47% to less than 2% from 0 my to 18 my after

deposition (Fig. 5C,D), respectively. Consequently,

the v, E, and b varied from 0.15 to 0.33, from

�1�103 to �16.3�103, and from 1 to 0.56,

<140 145 150 155 160 165 170 175 180 ≥1800

1

2

3

4

5

6

7

0

1

2

3

4

5

6

7

-1 -0.9 0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.10

1

2

3

4

5

6

7

8

0.0 -2.4,-2.2 -2.0-1.8 -1.6 -1.4 -1.2 -1.0-0.8 -0.6 -0.4 -0.20

1

2

3

4

5

6

7

8

0.0

90 95 100 105 110 115 120 125 135 140130 145 150

HOMOGENIZATION TEMPERATURES(in fractures and host rocks)

ICE MELTING TEMPERATURES(in fractures and host rocks)

Fre

quen

cyF

requ

ency

Fre

quen

cyF

requ

ency

Tm (oC) Tm (oC)

Th (oC)Th (oC)

LS-1257, 3819 m

Veins (F3)Overgrowth

Microfractures (F1)

MOT-4, 2545 m

Overgrowth

Microfractures (F1)

MOT-4, 2545 m

Overgrowth

LS-1257, 3819 m

Veins (F3)Overgrowth

n= 21 n= 26

n= 5n= 17

C D

BA

Fig. 4. A and B show fluid inclusion data from quartz-filled microfractures (set F1 and F3) from the Misoa Formation. Quartz precipitation

occurred over a wide temperature range, beginning at 100 8C. (C, D) Ice melting temperatures suggesting the presence of marine-diluted and

meteoric water influence in Misoa pore waters throughout the burial history.


respectively, during the same period of time (Table 2;

Fig. 6A–D). The maximum burial depth was reached

20 my after deposition (Fig. 5A), 2 my after all rock

properties had attained their maximum values.

In our model the rock properties vary with time,

temperature, and diagenesis, not with depth solely,

and after the sandstone is totally cemented, when

porosity reaches zero, the mechanical properties

remain constant. Understanding the evolution of the

aforementioned parameters is important because they

are part of the basic input for our stress analysis in

determining the host-rock’s thermo-mechanical con-

ditions during uplift.

6. Analysis of microfracture morphology

Four kinds of thin-section scale fractures occur in

Misoa sandstone. The lack of core orientation forced

us to group the fractures in sets strictly based on their

mineralogy, cross cutting relations, and orientation

relative to lamination. Intragranular fractures repre-

sent the earliest set (F1) and are filled with quartz

cement. The second set of thin-section scale micro-

fractures (F2) are filled with bituminite–pyrite and are

truncated by a third set of microfractures (F3) filled by

either quartz or calcite cement. The fourth set of

microfractures (F4) is either uncemented or partially

0

500

1000

1500

2000

2500

3000

3500

4000

45000.0 10.0 20.0 30.0 40.0 50.0

Time after deposition (MY)

0

20

40

60

80

100

120

140

160

180

2000.0 10.0 20.0 30.0 40.0 50.0


0

5

10

15

20

25

30

0.0 10.0 20.0 30.0 40.0 50.0


0

5

10

15

20

25

30

35

40

45

50

0.0 10.0 20.0 30.0 40.0 50.0

BURIAL - THERMAL HISTORY OF HOST ROCKSD

epth

(m

)

Tem

pera

ture

(C

)

Qua

rtz

cem

ent %

Prim

ary

poro

sity

%

Present day depth

Thin section porosity based on point countMeasured

amount of quartz overgrowth

MODEL RESULTS: QUARTZ CEMENTATION AND COMPACTION OF HOST ROCK

Present day temperature


A B

C D

Fig. 5. A and B represent burial and thermal history for fine-grained, clean, quartz-rich, and quartz cemented sandstone from well LB-273, at

2488 m depth (Rodriguez et al., 1997). (C) Quartz cementation kinetic model results, based on Walderhaug (1996) algorithms (Eqs. (1)–(5)),

representing the 26% of quartz cement precipitated through time. (D) Calculated porosity decreases through time as a result of compaction and

quartz cement.


cemented by hematite and other concurrent iron

oxides. The latter cement type and set of micro-

fractures (F4) are observed in sandstone strata uplifted

more than 1/3 from their maximum burial depth. All

microfractures are Mode 1 (open mode), extension

fractures, and confined to host rocks rich in inter-

granular quartz cement. Their morphology and aper-

tures are described in the following text.

6.1. Intragranular fractures (F1)

A summary of the characteristics of the micro-

fractures is presented in Table 5. Intragranular

fractures–fractures within or across grains–are the

most common type of fracture in the Misoa Formation

and represent post-depositional features associated

with compaction during burial (e.g., Milliken, 1994;

Milliken and Laubach, 2000; Chuhan et al., 2002). All

intragranular fractures are Mode I, formed by a

displacement normal to the fracture wall. However,

they vary randomly in pattern, shape, and size. These

fractures generally have straight or curvilinear traces

(Fig. 7), they cut grain boundaries, and in some cases

cut overgrowth cements. Displaced fractured walls

indicate normal dilation, followed by solid-volume

increase, and repacking. In some cases, microfractures

extend across two or three grains forming trans-

granular microfractures. Wedge-shaped fractures at

grain–grain contacts commonly display triangular

shapes in a radiating pattern, and are diagnostic of

stress concentrations (Laubach, 1997). The radiating

patterns are most likely produced by compaction with

or without tectonic stresses present (Milliken and

Laubach, 2000). Many quartz-filled intragranular

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

6.0 11.0 16.0 21.0 26.0 31.0 36.0 41.0 46.0 51.0


0

0.000002

0.000004

0.000006

0.000008

0.00001

0.000012

0.0 10.0 20.0 30.0 40.0 50.0

-18000

-16000

-14000

-12000

-10000

-8000

-6000

-4000

-2000

0

0.0 10.0 20.0 30.0 40.0 50.0 60.0

0

0.2

0.4

0.6

0.8

1

1.2

0.0 10.0 20.0 30.0 40.0 50.0

Bio

t Por

oela

stic

P

aram

eter

β

Poi

sson

's R

atio

V

Youn

g's

Mod

ulus

E

(MP

a)

Coe

ffici

ent o

f The

rmal

Exp

ansi

on α

(1/

C)


Time after deposition (MY) Time after deposition (MY)

A B

C D

Fig. 6. Synchronous and linear variation of v, E, and a with / through geologic time (see Eqs. (1)–(5)); b varies with Eq. (7). Example for

quartz arenites from well LB-273, presently at 2488 m depth; dashed line represents the approximate time of maximum burial depth. We assume

the parameters evolve from their initial values at the time of deposition, to the final values when the primary porosity reached zero /c0 (Table

1), which is before maximum burial depth. Results are basic input for thermoelastic contraction calculations. The final b value corresponds

specifically to sandstones from the Misoa Fm. The rest are theoretical values taken from the literature, see Table 2.


fractures are inherited, but these, however, have

distinctive bright luminescence under CL and blunt

terminations at grain boundaries, which distinguishes

them from quartz-filled fractures developed in situ—

in the present setting. Based on image analysis of 15

different CL photographs from three samples (five

photographs per sample) the quartz cement trapped in

microfractures averages less than 3% of the total

photograph. Qualitatively, there is a positive correla-

tion between the abundance of intragranular micro-

fractures and depth, similar to that observed in the

Gulf Coast basin (Makowitz and Milliken, 2003).

6.2. Cemented thin-section scale fractures (F2–F4)

Other microstructures present in Misoa sandstone

are microfractures that extend across thin sections.

They occur either isolated or in parallel sets. Again,

we grouped these microstructures in three different

sets based on their mineralogy and relative timing of

formation, not orientation as lack of oriented cores

prevented us from determining fracture direction and

from further testing our grouping method. Never-

theless, the three sets, in order of occurrence, consist

of (1) microfractures partially or fully cemented by

bituminite–pyrite (F2), (2) microfractures fully

cement by either quartz or calcite (F3), and (3)

microfractures either open or cemented by hematite–

iron oxides (F4). All fracture filling cements display

finer crystals than the host rock, except for fractures

filled by calcite cement. The following three sets of

microfractures can be described as separate events,

because their mineralogy suggests that they formed at

different geochemical conditions and geological time.

6.2.1. Second set of microfractures (F2)

The microfractures of the second set are filled by

bituminite (residual oil) and pyrite cement (Fig. 8A–

Table 5

Characteristics of vein generations including timing, fracture mechanisms, and possible temperature range

Basin event Fracture

generation

Microvein

cements

Microvein

secondary cement

Fracture

termination

Orientation

relative

to bedding

lamination

Host rock

intergranular

cement

Mechanism

that lead to

fracturing

Fluid flow, and

mass transport

mechanism

Temperature

(8C)

Subsidence F1 Quartz Quartz cement Triangular,

tabular,

radiating

pattern

Random Quartz arenites Compaction

Stress

Grain scale silica

diffusion

~ 100+

Oil generation–

migration-charge

F2 Bituminite Pyrite, siderite Triangular Sub-parallel Quartz cemented

quartz arenites

CD, kerogen

maturation

Kilometer scale from

underlying source rocks

50–100TT

Maximum burial F3 Quartz None Abrupt/blunt High angle Quartz cemented

quartz arenites

CD, kerogen

maturation

Local diffusion or less

than a kilometer scale

upward flow

140–170+

Maximum burial Calcite None Abrupt/blunt High angle Quartz cemented

quartz arenites

CD, kerogen

maturation

Local diffusion or less

han a kilometer scale

upward flow

140–170+

Uplift F4 Uncemented Pyrite, Hematite Triangular Random,

high angle

Quartz cemented

quartz arenites

Thermal

stresses

Small scale diffusion

and meteoric water influx

b 50T

Temperatures derive from fluid inclusion data.

T Hydrocarbon generation–migration from Rodriguez et al. (1997).+ Inferred temperature from IGV of host rock, petrology, and burial-thermal curves.

TT Inferred from petrology and thermoelastic contraction modeling.

R.J.

Perez,

J.R.Boles

/Tecto

nophysics

400(2005)179–207

192

QC

QG

QG

QGQC

F1

200 microns

Fig. 7. Sample from well LB-1182, 1172 m depth; F1 intragranular

quartz-filled microfractures; in black is quartz cement QC, in gray

the detrital quartz grain QG. The figure depicts the solid volume

increase, normal dilation, and stress concentration at grain contacts.

The triangular fracture shape suggests compaction origin but we

have no information on orientation.


D). These fractures are 0.2 to 0.5 mm wide (aperture)

and less than 0.5 cm long. They occur at a high

angle with respect to bedding lamination, in parallel

sets. They are sub-laminar and follow grain bounda-

ries (Fig. 8B). These observations suggest that

fracture propagation and bituminite infiltration were

both controlled by fluid percolation through a

compacted, but not fully cemented, host rock.

Fracture terminations are triangular tips, depicting

simple tapering. Fracture spacing is usually less than

0.5 cm. Bituminite-filled microfractures have shrink-

age cracks filled by quartz cement (Fig. 8B)

indicating quartz precipitation in the presence of

hydrocarbons. In the absence of quartz crystals,

bituminitic microfractures are also filled by concur-

rent siderite and pyrite.

Based on petrographic observations, siderite cor-

roded adjacent quartz grains and percolated in the

host rock adjacent to the vein. Based on SEM-EDX

pyrite, in contrast, precipitated within the bituminitic

filled space. The bituminite cement in microfractures

is in all cases less than 2% of the total rock. The

presence of bituminitic microfractures suggests that

the fracture event took place after hydrocarbon

generation-expulsion or synchronously with hydro-

carbon charge.

6.2.2. Third set of microfractures (F3)

The third set microfractures is filled by quartz and

calcite cement, they are orthogonal or cut at high

angle to the set filled by bituminite–pyrite (Fig.

8C,D). Quartz and calcite microfractures are present

in samples that are currently at the same depth range.

They are grouped in sub-parallel sets, at a high angle

with respect to bedding lamination. They also post-

date quartz cement or occur within the intergranular

quartz cementation window. The microfractures of the

third set are 0.3 to 0.5 mm wide (aperture) and up to 2

to 3 cm long. In several samples, F3 microfractures

overlap, adjoining different segments. Fracture termi-

nations are abrupt, and the fracture spacing is

relatively close, ranging from 1 to 3 per 2 cm2 of

thin section.

In core samples, calcite crystals in these micro-

fractures are euhedral. Under CL, the crystals are dull-

orange to non-luminescent and lack zonation. The

sandstone adjacent to the fracture-filling space com-

monly consists of irregular zones of calcite cement,

0.1 to 0.2 mm in width. These rims may have formed

by host rock displacement through filling by calcite,

indicating that part of the fracture cement grows

outward into the host rock matrix, as well as inward

into the vein center. The euhedral crystal terminations

are indicative of growth into open space, whereas the

cements diffuse contact with the host rock is

indicative of dissolution/precipitation or matrix

recrystallization.

Quartz-cemented microfractures are sub-perpen-

dicular to the set F2, which is filled with bituminite–

pyrite (Fig. 8D,E). Euhedral quartz crystal termina-

tions indicate growth into open fracture space. Quartz

cemented fractures occur along grain boundaries.

Under CL, the quartz cement in veins has dark

luminescence and lacks zonation (Fig. 8F), indicating

a diagenetic origin. We suggest that the mechanism

responsible for the F3 microfracture event took place

after hydrocarbon migration, based on the fact that the

F3 quartz and calcite set cut F2 bituminitic micro-

fractures.

6.2.3. Fourth set of microfractures (F4)

The microfractures from the fourth set consist of

transgranular, uncemented, and partially cemented

microfractures (Fig. 9A–C), that are isolated or cut

the previously described microfracture sets F2 and F3,

Bituminite microvein

F2

Bituminite filling microfractures

F2

Quartz-filled microfracture

F3

2mm0.1mm

A D


F3

Bituminite-filled microfracture

F2 Quartz bridge

2mm0.02mm

B E


F3


F2


F2

1mm 1mm

Calcite-filled microfracture

F3

Quartz overgrowth preceeding calcite

cement

C F

Fig. 8. Representative photomicrographs of microfracture sets F2–F4; (A) sample from well LS-1387, 2755.4 m depth with microfracture filled

by bituminite–pyrite (set F2); (B) detail of quartz filled cracks, same sample as above. (C) Sample from well BA-1, 1450 m depth, calcite

cemented microfracture (F3) postdating (cross cutting) bituminite filled microfractures (F2) intergranular quartz cement. D to F are from well

LB-1387, 2755.5 m depth, (D) F2 displaced by set F3; (E) conjugated microfractures filled by quartz cement (F3) in quartz arenite sandstone;

(F) cathodoluminescence reveals no zonation, possibly (but not conclusively) suggesting a single precipitation event in F3 quartz-filled

fractures.


and occur in sandstone that are presently buried to

depths less than 0.5 km deep. These sandstone bodies

have been uplifted more than 2.5 km. These micro-

fractures are Mode I in origin, as the rest of the sets,

and are 0.2 to 0.5 mm wide (aperture) and less than

0.5 cm long. Fracture terminations are blunt, triangu-

lar tips, and cut grains, suggesting a high degree of

cementation and cohesion of the host rock at the time

of fracturing. Irregular crystals of siderite, pyrite, and

patches of hematite line the fracture walls (Fig. 9B).

0.5mm

P P

Reopened Microfault

Iron Oxide filled microfracture

F4

P

P

2mm

Open uncemented microfractures

F4

2mm

A

B

C

Fig. 9. Three representative F4 photomicrographs; (A) Well BA-1,

1922 m depth uncemented tensile fracture; (B) well BA-1, 1008.2 m

depth, partially cemented fractures by iron oxides and pyrite; (C)

well LS-1387, 1925 m depth, microfaults are common features and

we interpret reopening associated to uplift, based on calculations in

text and cementation. P is pore space.


Furthermore, this set (F4) can also be associated with

the dilatancy and reopening of microfaults (Fig. 9C).

Microfaults, when present in uplifted intervals of the

Misoa Formation, reveal dilatancy and reopening, and

have small sub-euhedral quartz crystals growing

inward into the opened space, suggesting a contrac-

tion event and late-stage precipitation.

7. Interpretation of fracture cements from

petrologic, microthermometric, and probe data

7.1. Generation of the first set of microfractures

Fluid inclusions occur in quartz-filled, intragranu-

lar microfractures. The inclusions occur in small

clusters without preferred orientation and contain

two visible fluid phases at room temperature. In most

cases, the homogenization temperature (Th) in intra-

granular microfractures is lower than the Th in

overgrowths. These observations clearly suggest that

quartz grain fracturing and re-sealing preceded quartz

overgrowth (Fig. 10). Moreover, fluid inclusion data

suggest that quartz cement precipitated initially within

microfractures and subsequently surrounded grain

surfaces, as observed under CL in other studies

(e.g., Milliken, 1994; Laubach, 1997; Makowitz and

Milliken, 2003). The observations also indicate that

fluid salinity remained relatively constant during

quartz precipitation (Fig. 10).

7.2. Generation of the second set of microfractures

A compositional analysis of residual hydrocar-

bons present in microfractures is beyond the scope

of our research. Based, however, on burial-thermal

histories and the most current oil generation-migra-

tion model, the Misoa Formation was charged with

hydrocarbon at temperatures between 60 and 100

8C (Rodriguez et al., 1997). Thus, bituminite–pyrite

microveins represent a fracture event that occurred

within the same temperature range or later than the

oil generation-migration event. Furthermore, geo-

chemical experiments demonstrate that when resins

and asphaltenes are reduced between 60 and 100

8C, they yield H2S, CO2, and viscous black tar (see

review by Machel, 1987), similar to that observed

in pipelines.

-2.5

-2

-1.5

-1

-0.5

0

90 100 110 120 130 140 150 160 170 180

Homogenization Temperature Th (°C)

Quartz-filling microfractrures set F3

Quartz cement in fracture set F1

Quartz overgrowth in grains

Ice

Mel

ting

Tem

pera

ture

Tm

(°C

)

Legend Depth of occurrence

Fig. 10. Homogenization vs. ice melting temperatures from quartz cements in overgrowths, intragranular fractures, and quartz-filled

microfractures indicating the relative depth and sequence of occurrence.

CaCO3

(Fe+Mn)CO35%

100%MgCO3

5%

n=18

Fig. 11. Normalized molar compositions, based on microprobe

analysis, of calcite cements present in microfractures showing

relatively pure composition.


7.3. Generation of the third set of microfractures

The third set of microfractures is filled by either

quartz and or calcite. Calcite and quartz micro-

fractures are grouped together because they both cut

bituminite-filled fractures in an orthogonal direction

(Fig. 8C,D), but they do not occur in the sample nor at

the same depth. Homogenization temperatures in

quartz cements present in the third set of micro-

fractures F3 range from 149 to 175 8C suggesting a

late stage of fracturing, and quartz precipitation (Table

5). Matching these temperatures with burial histories,

it is possible that the cementation occurred after

hydrocarbon emplacement, consistent with F3 cross

cutting of F2. Melting temperatures range between �1

and �0.4 8C (Fig. 10), yielding salinity values

between 1.7 and 0.5 wt.% NaCl assuming an H2O–

NaCl system (Bodnar, 1992). The salinity range

indicates dilute marine and meteoric fluid sources

were present during F3 fracture cementation.

Based on microprobe analysis, calcite crystals in

calcite microfractures averages Ca98(Fe+Mn)1.6Mg0.4CO3, have a Fe/Mg ratio of about 3, and have

from 1 to 3 mol% Fe+Mg+Mn substitution for Ca

(Fig. 11). Deep late-stage carbonate precipitation has

been reported in the Misoa Formation (e.g., Perez et

al., 1997) as well as in other basins of the world (i.e.,

Boles 1998). In general, calcite becomes less soluble

with increasing temperature—i.e., at greater depths. In

addition, fracturing causes decompression of fluids

and gases, which decreases PCO2. The PCO2

drop at

constant temperature increases the carbonate satura-

tion, which can lead to carbonate precipitation. If the

Fe/Ca ratio of the pore fluid is relatively low (Boles

and Ramseyer, 1987; Eichhubl and Boles, 1998)

calcite would precipitate upon decompression, if high,

siderite would dominate. Petrographic study indicates

that F3 calcite microveins formed after oil emplace-

ment, cutting F2 bituminite–pyrite microveins (Fig.

8C). Thus calcite and quartz filled microfractures may


have occurred within the same temperature range, i.e.,

~150 to 175 8C, even though they do not coexist in

the same sample. It is important to denote that some,

but not all, fractures filled with calcite cement show

quartz lineaments or quartz grains growing into the

fracture, predating the calcite cement (see detail in

Fig. 8C). This type of relationship could be associated

to synkinematic quartz cement, and possibly, post-

kinematic calcite cement (Laubach, 2003).

7.4. Generation of the fourth set of microfractures

The fourth set of microfractures are transgranular,

extensional, Mode I, and are partially cemented by

pyrite–hematite and other concurrent iron oxides.

Pyrite is the main fracture cement, whereas hematite

is secondary but still fills the fracture space. The

presence of late-stage hematite and iron oxides

supports a diagenetic event associated with uplift

and exposure to meteoric fluids. Hematite is stable in

reducing environments with pHN4.6, however, its

free energy is greater under oxidizing conditions and

may result from oxidation of pyrite or siderite (Garrels

and Christ, 1965). Pyrite and siderite have small

stability fields, and should precipitate under strongly

reducing conditions at pHN7 (Garrels and Christ,

1965). Thus, dissolution of pyrite or siderite and

precipitation of hematite is a reaction that can take

place under oxidizing conditions during influx of

meteoric water associated with uplifting of the Misoa

Formation at temperatures less than 50 8C.

8. Microfracture and mass transfer mechanisms

Compaction and repeated fracturing in the Misoa

Formation led to four microfracture sets (F1 to F4;

Fig. 12; Table 5). Intragranular microfractures (set F1)

were the first set to occur and compaction stresses are

usually interpreted as the main fracture mechanism for

these (e.g., Milliken, 1994). Compaction experiments,

performed under hydrostatic pressures and using

sands of similar composition and grain size to

Misoa’s, demonstrate that intragranular fractures

could initiate at 6 MPa and increase in number and

size at stress levels up to 20–30 MPa (Chuhan et al.,

2002). Average Th’s in F1 fractures are 105.6F6.5

and 113F2 8C (Table 4) suggesting that cementation

took place after fracture formation (Fig. 10), at depths

~3 km corresponding to rvV from 25 to 30 MPa—

assuming a ~25 8C/km thermal gradient and hydro-

static pressure gradient, respectively.

Based on the shape, mode, fracture shape, and

relative orientation to bedding (Table 5; Figs. 8 and 9),

we interpret microfractures F2 to F4 to be extensional

and to have formed perpendicular to the minimum

confining stress r3. Based on their inferred depth of

formation and the absence of nearby faults, micro-

fractures F2, F3, and F4 may have formed when Pf

was higher than r3, so that the minimum effective

stress r3V became tensile and higher than the fracture

toughness To. In other words, generations F2 to F4

may have occurred hydraulically, meaning r3V=r3�b d PfbTo, where To is the tensile strength of

the rock (Fig. 13). The presence of Pf causes a rock to

behave as if the confining pressure were decreased by

an amount equal to Pf (Twiss and Moores, 1992). At

small differential stress, an increase in pore pressure

drives the effective stress toward the tension field

along the normal stress axis, where it meets the

criteria for tension failure (Fig. 13A). At large

differential stresses (large r1V–r3V), an increase of pore

pressure would shift the stress toward the failure

envelope in the Coulomb failure criteria, causing

shear.

Hydraulic fractures by themselves are, by no

means, indicative of overpressures. However, within

our geologic context, burial depths greater than 3 km,

the presence of an oil-gas source rock, the hydraulic

microfractures F3 strongly, but not conclusively,

suggest the presence of paleo-overpressures, meaning

(a) the pore fluid (Pf) pressures was higher than

hydrostatic, and (b) the r3V was higher than the

hydrostatic stress, higher than the fracture toughness,

and lower than the overburden stress.

Overpressure is created by a pore fluid volume

increase with minimal change in porosity and at a rate

that does not permit dissipation (Swarbrick et al.,

2002). Several mechanisms may have caused Pf to

increase above hydrostatic in the Misoa Formation.

The first to occur in the burial sequence and most

important is compaction disequilibrium, which is

caused by compaction exceeding rates of pore fluid

loss. Clay dehydration, smectite–illite transformation,

kerogen transformation, gas generation, hydrocarbon

buoyancy, quartz cementation, and osmosis are other

500.0

1000.0

1500.0

2000.0

2500.0

3000.0

3500.0

4000.0

4500.0

0.0 10.0 20.0 30.0 40.0 50.0

Time from deposition (MY)

D

epth

(m

)

A

?

Present day

0.0

FR

AC

TU

RE

PA

RA

GE

NE

SIS

(1) Intragranular microractures F1

(2) Quartz cement overgrowth within intragranular fractures

(3)Quartz cement overgrowth around detrital grains

(5)Bituminite filling fractures

(10)Quartz filling completely F3 microfractures

(8)Synkinematic quartz overgrowth into fractures

(7)Calcite cement filling microfractures set F3

(9)Postkinematic calcite cement filling fractures

(13)Iron Oxides fills set F4

(4)Occurrence of set F2



(12)Meteoric water invasion

Fig. 12. Paragenetic sequence of fracture events present in the Misoa Formation. *The occurrence of F4 contraction fractures varies from field to

field, depending upon the burial history. **Iron oxides precipitated in sandstones units that were uplifted to depths less than 0.5 km from the

surface. (1) Based on Chuhan et al. (2002) experiments; (2) based on fluid inclusions data (FI); (3) based on FI data and Walderhaug, 1996

model; (4) inferred from petrography; (5) based on timing of oil migration (Rodriguez et al., 1997); (67) inferred from petrography; (89) based

on quartz lineaments; (10) based on FI data and petrography; (11) based on thermoelastic contraction model; (1213) based on the presence of

iron oxides.


causes that occur at a relatively same temperature

window (see review by Swarbrick and Osborne,

1998). Several of the mechanisms above may have

occurred in isolation, simultaneously, or sequentially,

triggering overpressure and ultimately microfracture

in the Misoa Formation.

At depths shallower than 2 km there is generally

little potential for overpressure generation. However,

in basins characterized by sedimentation rates

between 0.5 and 1 km/my and restricted pathways

for fluid loss, such as the Central North Sea basin and

Nile Delta (Law and Spencer, 1998) as well as the

Coulomb fracture criterion

Tensile fracture criterion

σn−σn σ1 σ1 σ1 σ3σ3σ3 =Το

Normal stress

Shear stress

τ

−τ

^

Tension Compression

Applied stress

Overburden

Pf

Pf increase leads to tensile failure

Effective stressHydraulic fracture

Coulomb fracture criterion

Tensile fracture criterion

σn−σn σ1

σ1σ1σ1 σ3 σ3 σ3σ3 =Το

Shear stress

τ

−τ

^

^^

Tension Compression

Overburden

Hydraulic fracture

Initial state of stress

Final state of stress after cooling

Initial state of stress

Final state of stress after Pf

increase

Mohr Envelope

Mohr Envelope

Normal stress

A

B

Fig. 13. Mohr circle sketch diagram representing various possible stress stages of the Misoa Formation; (A) burial cycle. Pore fluid pressure Pf

reduces the consolidation. Tensile fractures will form if the minimum stress is equal of higher than the fracture toughness, i.e., r3zTo. During

burial, increasing pore pressure shifts the differential stress to the tensile field leading to hydraulic fracturing; (B) uplift cycle. During uplift the

initial state of stress, as a result of erosion and cooling, may increase the differential stress and shift it to the tensile field. As we have shown the

net effect of load decreased and thermal contraction could be extensional.



Maracaibo basin, pressure profiles reveal overpressure

at depths shallower than 1.3 km.

Clay dehydration also occurs in the Maracaibo

basin (Perez et al., 1997) and its effect may add to the

overpressure caused by compaction disequilibrium

(Swarbrick and Osborne, 1998). As explained,

increased pore fluid pressures shift the minimum

effective stress r3V to the tensile field at values equal

to, or higher than, the fracture toughness To (Fig. 13A).

Microfractures filled by bituminite–pyrite cement,

formed at depths greater than 2 km, based on oil

migration and generation models. We interpret these

microstructures to reflect hydraulic microfractures

associated with overpressure caused by kerogen

maturation, oil expulsion/gas generation, and gas to

oil cracking—in addition to disequilibrium compac-

tion. Kerogen maturation was most likely absent in

the Misoa Formation due to its low TOC, high O2

index, and low oil generation potential (Jaffe and

Gardinali, 1990). Thermobarometry and organic

maturation modeling, however, suggest the presence

of near-lithostatic pressures in the underlying source

rock La Luna Formation (Fig. 2) during oil gener-

ation/expulsion (Sweeney et al., 1995; Vrolijk et al.,

1996a and 1996b). These high pore pressure con-

ditions may have been transferred upward to lower

sections of the highly cemented Misoa. Upward

transfer of pressure is a common mechanism and is

a main control of overpressure distribution in basins

such as the central North Sea and Mahakam Delta

(Swarbrick and Osborne, 1996).

Upward transfer of excess pore fluid pressure may

have created a pressure gradient that could explain

hydrofracture, upward flow, and the addition of

dissolved carbonate and silica into microfractures.

Microfractures filled with quartz cement (F3) have

Th’szmaximum burial temperatures and contain

abundant oil fluid inclusions. Thus, they might

indicate upward transport of hydrocarbons and silica,

by decompressing fluids into fractures, from intervals

that were more advanced in burial diagenesis and

temperature. The quartz lineaments in some calcite-

filled microfractures (Fig. 8C) strongly suggest the

presence of synkinematic quartz overgrowth and post-

kinematic calcite precipitation.

Calcite filling microfractures (F3) could also reflect

decompression and upward fluid flow. Fracturing

increases the total pore volume locally, decreasing

the pore pressure. At constant temperature, conse-

quently, the PCO2drop after hydraulic fracturing

increases carbonate precipitation. Moreover, if the

temperature is decreased during upward flow, the

PCO2decrease caused by vertical fluid drainage could

outweigh the temperature effect, and also favors

carbonate precipitation into fractures (Boles and

Ramseyer, 1987; Eichhubl and Boles, 1998). Further-

more, hydraulic fracturing could occur under adiabatic

(constant heat) irreversible conditions–due to the low

thermal conductivity of rocks–leading to temperature

increases caused by the internal work generation of

the fluid phase during decompression (Wood and

Spera, 1984). The increase in temperature, accompa-

nied by PCO2drop, may decrease carbonate solubility

even further, leading to faster carbonate precipitation.

Additionally, geochemical experiments suggest

that shale in the Misoa and La Luna Formations

release cyclic and acyclic organic acids at %Ro 0.33–

0.77 and %Ro 0.5–1.8, respectively (Jaffe and

Gardinali, 1990). In general, the thermal breakdown

of kerogen yields directly CO2, CH4, H2S, and organic

acids (Lundegard and Land, 1986). At high concen-

trations (i.e., N 1.4 M acetic acid at 60 8C) organicacids act as a buffer to added CO2 resulting in

carbonate precipitation (Surdam and Crossey, 1985).

At lower organic acid concentrations, carbonate would

be precipitated with PCO2drops, presumably caused

during veining and its associated decompression and

upward fluid flow (Eichhubl and Boles, 1998).

Based on correlations between the amount of

quartz cement and concurrent fractures in rocks

lacking any other tectonic driver (Laubach, 1988), it

has been proposed that the rapid rate of porosity

reduction due to quartz overgrowth precipitation may

be another primary factor in generating overpressures

(Helset et al., 2002). Given the high cementation rates

in Misoa’s sandstones, it is not unreasonable to

assume quartz cementation was an important factor

in elevating fluid pressures. However, a much more

investigation is needed on this particular topic. For

example, Swarbrick and Osborne (1998) disregarded

the importance of quartz cementation as an over-

pressure generator based on Darcy’s Law.

The origin of open uncemented microfractures,

such as the F4 set, is difficult to ascertain. Open

microfractures may result from stress during drilling,

core handling, sample preparation, or unloading


during coring (Nelson, 1981; Laubach, 1997). We

interpret some F4 fractures, however, as the result of

basin unroofing, similar to those described by

Walderhaug (1992) based on the presence of hematite

cement, which suggest a late stage of oxidation event.

Precipitation of hematite and iron oxides takes place

under oxidizing conditions, possibly during meteoric

fluid invasion associated with uplift of the Misoa

Formation. Present fluids of non-treated reservoirs in

the Misoa Formation have an Na/ClN1, TDS ~1500

mg/l, and Ca/Mg ratio averaging 1.5, clearly suggest-

ing meteoric influence (Vasquez, 1998).

Moreover, a stress analysis of the formation during

uplift supports our argument. The net effect of uplift,

which implies erosion and cooling, is that the

horizontal stress becomes the minimum compressive

stress and the effective horizontal stress becomes

tensile even at depths greater than 1 km. We believe

that this mechanism, also called thermoelastic con-

traction, may have been an important factor during the

generation of the last microfracture event.

9. Modeling results in support of thermoelastic

contraction

To support our argument that late stage fractures

(F4) are associated with uplift, we analyzed the stress

conditions for fracturing during uplift assuming a

uniaxial strain model. In the uniaxial strain model, if

the rocks behave elastically, the Poisson effect

predicts that a decrease in the vertical load causes

expansion in the vertical direction, and contraction in

the horizontal direction. Thus, the horizontal stress at

any given depth may be quite different during burial

relative to uplift.

In our first simple approximation we calculated the

effect of burial and uplift on stress on a representative

P–T curve of a Misoa horizon (Fig. 5A,B; Eq. (9)).

From the assumption of tectonic stress being absent,

we assume the maximum and minimum horizontal

normal stresses are equal (DrH(max)=DrH(min)), a

function of the vertical stress, and temperature. For

simplicity, we chose a geobaric and thermal gradient

of 25 MPa/km and 25 8C/km, respectively. The

calculations were done at three points: at the surface

(0 m), at maximum burial (4 km), and at minimum

burial after uplift (1.5 km). In this calculation, the

initial values of v, E, and a (Table 2) are assumed

constant during burial. Subsequently, after the max-

imum burial depth is reached v, E, and a are at final

values shown in Table 2, and remain constant there-

after. Results indicate that the horizontal stress is

compressive at maximum burial (rh 40 MPa) and

tensile during uplift (�11 MPa). The fracture tough-

ness is generally between �5 and �20 MPa for

sandstone (Suppe, 1985), and is exceeded after ~1/2

of the thickness of the strata is gone (Fig 14A). Hence,

the appropriate conditions for fracturing formed

toward the end of the uplift. Subsequently, fractures

were filled with iron oxides at shallow subsurface

conditions, less than 0.5 km depth, and presumably at

hydrostatic fluid pressure conditions.

In our second approximation, equations for effec-

tive horizontal stress (Eqs. (3)–(10) are integrated

numerically throughout the time–depth path of the

Misoa sandstone (Fig. 5A,B). Even though F2 and F3

microveins strongly suggest an excess of fluid

pressures throughout the Misoa Formation during

deep burial, we assume minimum conditions for

fracturing, i.e., near-hydrostatic fluid pressures, where

k=Pf /rv varied from 0.45 to 0.51. Clearly this

assumption is an underestimation. Any increase in

Pf over hydrostatic shall accentuate the stress effect,

leading to a faster or deeper thermoelastic contraction.

We assumed a Poisson’s ratio of 0.15 for uncon-

solidated sand (Bachrach et al., 2002) and 0.33 for

well-cemented sandstone (Engelder, 1985). The bulk

overburden density increases from 2 (rock+pores) at

the time of deposition to 2.65 g/cm3 (highly quartz

cemented sandstone) at maximum burial depth. Hence

the total vertical stress is greater during uplift–due to

higher rock density–than at the corresponding depth

during burial. The poroelastic parameter decreases

from 1 to 0.56, following Eq. (6). As we explained, v,

E, and a are assumed to vary linearly and synchro-

nously with the porosity and degree of cementation

during burial (Fig. 6A–C). Because the porosity

approaches zero before the maximum burial depth,

the parameters remain constant at their final values

before maximum burial depth is achieved and during

uplift (Fig. 6A–D), although high fracture densities

may change the bulk density. Various rock properties

and burial history scenarios from wells MB-1, LB-

1387, LS-1257, LB-114, mostly around the Bach-

aquero field (Fig. 1) were incorporated into the stress

Stress History Analysis

0

500

1000

1500

2000

2500

3000

3500

4000

-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100

Effective Horizontal Stress (MPa)

Bur

ial D

epth

(m

)

Tension Field

CompressionField

0

500

1000

1500

2000

2500

3000

3500

4000

-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100

Effective Horizontal Stress (MPa)

Bur

ial D

epth

(m

)

CompressionField

Burial

Burial

Uplift

Uplift

Calculation in 3 time steps

Calculation in 30 time steps. Based on Equations 1 to 10.

(See text)

Tension Field

A

B

Fig. 14. Effective horizontal stress as a function of burial depth assuming uniaxial strain model. (A) Calculation performed in three steps

assuming thermo-mechanical parameters constant through time (Table 2), based on Eqs. (8)–(10). (B) Calculation performed in 30 time steps

assuming that thermo-mechanical parameters vary through time (Table 2), based on Eqs. (1)–(10). Results from both calculations show that the

effective horizontal stress rhV becomes lower than Pf at ~3.7 km depth and becomes tensile 3 Ma, at less than 2.5 km depth. Shaded area

represents typical values of fracture toughness To.



history calculation and consistently demonstrated that

uplift reduces rhV to the tensile field (Fig. 13B).

It is noteworthy to recall that the Biot coefficient

was originally defined to accommodate the elastic

deformation of mineral grains under pressure. How-

ever, at temperatures above 100 8C the rock’s volume

reduction is also partially controlled, among many

other parameters, by chemical compaction and not by

mechanical squeezing solely (Bbjrkum, 1996). Under

these circumstances the use of the Biot coefficient in

Eqs. (6)–(10) perhaps would not adequately describe

the decreased effect of the fluid pressure on the

effective vertical stress. Nonetheless, during burial,

chemical compaction, although inelastic and irrever-

sible, will also decrease the net effect of the vertical

stress (rock overburden load) decreasing consequently

the effective vertical stress. In view of the above

discussion, we decided to perform simulations in

which the Biot coefficient was considered constant

and equal to 1 at all times. In other words, neglecting

its effect. The results are illustrated with dashed lines

in Fig. 14B. All calculations seem to indicate that, if

the Biot coefficient is dropped or not considered, it

would be easier to obtain a tensile horizontal effective

stress at the end of the uplift period, supporting further

our interpretation.

Summarizing, under our assumptions, for increas-

ing values of rhV during burial, Pf exceeds rhV leadingto conditions favorable for hydraulic fractures only if

Pf is above hydrostatic. However, during uplift, even

under hydrostatic pressure and at depths greater or

equal to 1 km the effective horizontal stress is

ultimately reduced to the tensile field. The Poisson

ratio contributes to lower stresses, but the slope of rhVduring uplift is dominated by the Young’s modulus in

the thermal stress term. In other words, thermal

contraction of the rock associated with uplift over-

whelms the Poisson effect and leads to tensile failure.

In addition to the horizontal effective stress, the fluid

pressure must exceed the fracture toughness To in

order to propagate a fracture (Fig. 14B). For the

purpose of this discussion, this term is not included in

our stress modeling. It is noteworthy that sandstone

fracture toughness is typically between �5 and �20

MPa (Suppe, 1985), and rhV in our model reaches

between �5 and �25 at the end of the uplift,

depending upon the burial history analyzed, and the

inclusion or exclusion of the Biot coefficient.

10. Discussion

Models as simple as the one we are presenting

cannot express accurately the complex stress, burial or

pressure history of Misoa sandstone for various

reasons. For instance, observations from geologic

structures and in situ measurements suggest that a

state of stress in which r1=r3 is uncommon (e.g.,

Engelder and Geiser, 1980). Pressure-solution and

fracturing change the basic assumption of elasticity

(Narr and Currie, 1982). Several faults that are present

in the studied oil fields can accommodate radial

contraction of the strata during uplift, however there is

no correlation between the occurrence of micro-

fractures and these faults. And last, but not least, we

neglected tectonic stress because its identification and

quantification is difficult, subjective, and prone to

error (Narr and Currie, 1982).

Thermoelastic contraction modeling has been used

to explain fracture genesis in several sedimentary

basins. For instance, Narr and Currie (1982) devel-

oped a stress-history model to explain joints in Utah’s

Uinta basin. Similar to us, their elastic model included

effects of overburden, pore pressure, temperature, and

tectonic strains, and they suggested that high pore

fluid pressure was responsible for the joint system

during uplift. Engelder (1985) studied loading paths

and joint propagation of the Appalachian Plateau

during different tectonic cycles including uplift. The

study also concluded that failure propagated by

contraction during uplift. Warpinski (1989) developed

a mathematically complex model for estimating stress

states in reservoirs for the Piceance basin assuming

elastic and viscoelastic rock behavior. The model

included pore pressure, temperature gradients, and

consolidation, but lacking, however, diagenetic

effects. The model concluded that stresses in sand-

stone are fairly accurately represented with an elastic

analysis similar to ours. Apotria et al. (1994) studied

the fracturing and stress history of the Antrim Shale in

the Michigan basin, and concluded that unloading

provided an important mechanism for fractures con-

sistent with the contemporary stress field of the area.

We believe that the calculations, herein presented,

are useful in deciphering physical effects and geologic

processes that are important in controlling the state of

stress in quartz-rich sandstone during different stages

of burial and uplift. We also believe that, in spite of


the oversimplifications, the model reproduces first

order effects of burial and uplift. Finally, although we

cannot prove that open uncemented microfractures

were caused by tensile stresses during uplift, they are

consistent with our first order predictions of stress

conditions resulting from thermoelastic contraction

within the Misoa Formation.

11. Conclusions

Intragranular quartz-filled microfractures (set F1)

in the quartz-rich Misoa Formation are the most

common microstructures and represent post-deposi-

tional structures associated with compaction stresses.

Th’s in F1 fractures are lower than in overgrowths,

indicating that quartz grain fracturing and annealing

predates quartz overgrowth. Quartz overgrowth is

the most important authigenic phase in the fracture

host rocks, averaging 12%, but may locally reach up

to 26%. Quartz precipitated between 100 and 175 8Cduring burial. Modeling and fluid inclusion data

suggest that the porosity in quartz arenite decreased

to almost zero before the maximum burial depth.

Sequentially, four sets of microfractures formed in

the Misoa Formation. Bituminite–pyrite (set F2) as

well as quartz and calcite filled microfractures (set

F3) suggest periods of hydraulic extension that

could be associated with overpressure. Compaction

disequilibrium in the Misoa may have been the

primary overpressure mechanism, however, it may

have been augmented by an upward pressure

gradient generated by underlying oil source rocks

at the time of kerogen maturation and by the rate of

porosity reduction due to quartz cementation.

Homogenization temperatures in quartz-filling micro-

fractures (F3) range between 149 and 175 8C,whereas maximum burial temperatures are around

160 8C. The vertical upward transfer of pore fluid

pressure explains the formation and addition of silica

into F3 fractures from intervals more advanced in

diagenesis. Microfractures filled by calcite (set F3),

which postdate those filled by bituminite–pyrite, may

reflect hydrofracture, decompression, and a subse-

quent PCO2drop, which may have outweighed any

temperature effect on solubility, leading to carbonate

precipitation. Synkinematic quartz cement growing

into the fracture space predates calcite cement,

suggesting that calcite may have a post-kinematic

origin. As discussed previously, hydraulic F3 fractures

by themselves are not indicative of overpressures. As

we explained, however, within the Misoa’s geologic

context, they suggest the presence of pressures above

hydrostatic.

We interpreted open uncemented and pyrite–

hematite partially cemented microfractures (F4) to

have formed late in the diagenetic history as a result of

meteoric water incursion during the late stage of

uplift. Based on our calculations, we conclude that the

Misoa Formation had conditions favorable for tensile

failure during the uplift. We also conclude that a total

denudation, uplift, or surface exposure does not

constitute a requirement to drive the stress to the

tensile field. Contraction fractures can occur even at

several kilometers depth, if tectonic stresses are

absent. Thermal elastic contraction of sandstones

may have been the dominant, but not the only,

mechanism responsible for late stage, fourth set of

fracturing. Our analysis indicates that there is an

intimate interplay between host-rock diagenesis, over-

pressure, hydraulic fracturing, mass transport, and

veining mechanisms that should be examined as

coupled transformation processes.

Acknowledgments

We thank E. Perez and Dr. Arthur Sylvester for

their editorial and critical reviews. Early comments by

Drs. Jean-Yves Chatellier, Santosh K. Ghosh, and

Thomas L. Dunn were very useful. Suggestions by the

journal referees Drs. Kevin Furlong, Olav Walder-

haug, and Steven Laubach greatly improved the

content of the paper. PDVSA-Exploration provided

samples and the U.S. Department of Energy (DOE)

funded our research under Grant No. 444033-22433.

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