characterization of hydrocarbon gas within the stratigraphic interval of gas-hydrate stability on...

Applied Geochemistry, Vol. 5, pp. 279-287, 1990 0883-2927/90 $3.00+ .00 Printed in Great Britain Pergamon Press plc

Characterization of hydrocarbon gas within the stratigraphic interval of gas-hydrate stability on the North Slope of Alaska, U.S.A.

TIMOTHY S. COLLETT, KEITH A. KVENVOLDEN and LESLIE B. MAGOON U.S. Geological Survey, MS-999, Menlo Park, CA 94025, U.S.A.

(Received 7 March 1989; accepted in revised form 12 October 1989)

Abstract--In the Kuparuk River Unit 2D-15 well, on the North Slope of Alaska, a 60 m-thick stratigraphic interval that lies within the theoretical pressure-temperature field of gas-hydrate stability is inferred to contain methane hydrates. This inference is based on interpretations from well logs: (1) release of methane during drilling, as indicated by the mud log, (2) an increase in acoustic velocity on the sonic log, and (3) an increase of electrical resistivity on the electric logs. Our objective was to determine the composition and source of the gas within the shallow gas-hydrate-bearing interval based on analyses of cuttings gas. Headspace gas from canned drill cuttings collected from within the gas-hydrate-bearing interval of this well has an average methane to ethane plus propane [C1/(C2 + C3)] ratio of about 7000 and an average methane 613C value of -46%0 (relative to the PDB standard). These compositions are compared with those obtained at one well located to the north of 2D-15 along depositional strike and one down-dip well to the northeast. In the well located on depositional strike (Kuparuk River Unit 3K-9), gas compositions are similar to those found at 2D-15. At the down-dip well (Prudhoe Bay Unit R-I), the CI/(C2 + C3) ratios are lower (700) and the methane 613C is heavier (-33%0). We conclude that the methane within the stratigraphic interval of gas hydrate stability comes from two sources--in situ microbial gas and migrated thermogenic gas. The thermal component is greatest at Prudhoe Bay. Up-dip to the west, the thermogenic component decreases, and microbial gas assumes more importance.

INTRODUCTION

GAS HYDRATES are crystalline substances composed of water and gas in which a solid-water lattice accom- modates gas molecules in a cage-like structure. Gases such as methane (C~) and ethane (C2) are small enough to be included in the water lattice of structure I gas hydrate. Significant quantities of naturally occurring gas hydrates have been detected in many regions of the Arctic, including western Siberia, the Mackenzie Delta of Canada, and the North Slope of Alaska; gas hydrates may represent an unconven- tional source of natural gas (reviewed by KVENVOL- DEN and McMENAMIN, 1980). Direct evidence for gas hydrates on the North Slope comes from a core in the Northwest Eileen State-2 well (reviewed by COLLETF and KVENVOLDEN, 1987), Indirect evidence is provided by drilling information and geophysical well logs, which suggest the presence of numerous gas- hydrate-bearing layers in the area of the Prudhoe Bay and Kaparuk River oil fields (COLLETT, 1983; COL- LET/" et al., 1988).

Gas hydrates exist under a limited range of temperatures and pressures. The depth and thickness of the zone of potential gas-hydrate stability in permafrost regions can be calculated, if the geothermal gradient and gas chemistry are known (BtLY and DICK, 1974). In Fig. 1, the methane hydrate-stability curve and the depth to the base of the ice-bearing permafrost were used to determine the depth and thickness of the potential methane-hydrate zone at the Northwest Eileen State-2 well. In this example, the depth to the base of the ice-bearing permafrost,

as determined from industry well logs, is -530 m (COLLETT et al., 1988). Temperature surveys from surrounding wells (LACHENBRUCH et al., 1982) suggest that the mean-annual surface temperature at the Eileen well is -11.0°C, and the base of ice- bearing permafrost is at - 1.0°C. These temperature data have been used to project a geothermal gradient that intersects the methane-hydrate stability curve at 210 m and 950 m, delineating a 740-m zone in which methane hydrate would be stable. Results reported in COLLETr et al. (1988), indicate that methane hydrate would be stable beneath most of the coastal plain province of northern Alaska, thicknesses being greater than 1000 m in the Prudhoe Bay area. Ther- mal conditions, however, preclude the occurrence of gas hydrates in the north-central part of the National Petroleum Reserve in Alaska, located -300 km west of Prudhoe Bay, and in the foothills east of the Umiat oil field, which is located -150 km southwest of Prudhoe Bay. Also permafrost and gas hydrates will not be present under large lakes and major river systems due to the thermal disturbance of these unfrozen water bodies.

Within the methane-hydrate stability field on the North Slope, evidence for gas hydrates has been found in 34 wells by means of well-log responses calibrated to the logs within the confirmed gas hydrate occurrences in the Northwest Eileen State-2 well (Fig. 2). Most of the gas hydrates identified by well logs are geographically restricted to the eastern part of the Kuparuk River Drilling Unit and the western part of the Prudhoe Bay Drilling Unit (COL- LETT, 1983; COLLETT et al., 1988).

279

METERS

0 7

300

600

900

1200

1500

1800

DEPTH

m f t

610 -- 2000

FEET 0

1000

2000

3000

40O0

5000

B a s e of i ce -bear ing permafrost

/ : 6 taomty -" K

R

Geothermal "~. -- gradient

i I I I -10 0 10 20 30

TEMPERATURE, IN DEGREES CELSIUS

POUNDS PER

SQUARE MEGA- INCH PASCALS

0 0

- 500 4

- 1000 W

8 g W

1500

12

2000

16

~2500

FIG. 1. Gas hydrate phase diagram for the Northwest Eileen State-2 well on the North Slope of Alaska. showing the zone in which gas hydrates may occur. The system is assumed to be composed of pure methane and pure water. Geothermal gradient is 1.9°C/100 m above base of ice and 3.2°/100 m below it.

Location of well shown in Fig. 3.

N o r t h w e s t E i l e e n S t a t e - 2

msee/ft

670--2200-

730--2400-

Resistivity [ Transit Time Gamma Ray CiGas

12 2000 130 501100 010 400,0001 ohm-m

280 Timothy S. Collett et al.

API ppm

H y d r a t e

~ Core No. 4

D r i l l s t e m T e s t

FIc. 2. Selected well logs from an interval in the Northwest Eileen State-2 well and the location of a 2-m core (665-667 m) that was shown to contain gas hydrate. A drillstem test of the interval from

663-671 m flowed gas at a rate of 112 m3 (3960 ft 3) per day.

Rese rvo i r ed s t ra t igraphical ly be low the m a p p e d infe r red-gas-hydra te occur rences are several super- giant oil fields tha t conta in e n o r m o u s quant i t ies of gas in solut ion or as a gas cap. These accumula t ions include the P r u d h o e Bay and K u p a r u k River oil fields, and the heavy-oi l and ta r deposi ts wi th in the

West Sak and U g n u sands (WERNER, 1987). Wes t Sak and U g n u are informal t e rms used by A R C O to ident ify a series of shal low oi l -bear ing sands tone reservoirs in the K u p a r u k River area. Oil-source- rock corre la t ions (SEIFERT et a l . , 1979) indicate tha t the oil in the en t i re P rudhoe Bay area is f rom a

Hydrocarbon gas in gas-hydrate interval, North Slope, Alaska 281

c o m m o n source; however , little is k n o w n abou t the

his tory of oil or gas migra t ion . The object ives of this s tudy were to de t e rmine the

compos i t ion and source of the gas t r apped wi th in the s t ra t igraphic interval where gas hydra tes are expected , and to explain the history of gas-hydrate fo rmat ion on the Nor th Slope. To reach these objectives, we have analyzed cut t ings gas col lected f rom three dr i l l ing-product ion wells in the P r udhoe Bay and K u p a r u k River oil fields, and we have c o m p a r e d these data with in fo rmat ion f rom the Nor thwes t Ei leen State-2 well where gas hydra tes have b e e n conclusively identified. A descr ip t ion of the sampl ing p rocedures and analytical results follow.

GEOCHEMICAL SAMPLING AND ANALYTICAL PROCEDURES

of unopened cans not used in the CI-C7 analysis. Headspace-gas samples were analyzed by thermal- conductivity gas chromatography. During the chromato- graphic separation, the methane peak was diverted into a syringe for injection and subsequent oxidation of the methane to CO 2 in a Leco induction furnace. The oxidized methane was dehydrated and analyzed for 13C/12C enrich- ment on the isotope-ratio mass spectrometer (writt. commun., C. N. Threlkeld, U.S. Geological Survey, Denver, Colorado). The stable C isotopic composition of methane was measured on a Nier-McKinney mass spectrometer, and reported in the delta notation relative to the Peedee belem- nite (PDB) marine carbonate standard.

For this study, gas analyses of drill-cuttings were obtained from three wells drilled in the Prudhoe Bay and Kuparuk River oil fields (Table 1, Fig. 3). The drill cuttings were collected from the shaker table, washed with water to remove the drill fluid, and placed in quart cans. Water was added to submerge the cuttings, leaving a 0.5-in ( -1 .5 cm) air space or headspace at the top of the can. A bactericide (zephiran chloride) was added to the water to prevent biological activity in the samples. The can was sealed with a metal lid. Sample depth

At the laboratory (Geochem Research Incorporated, (ft) Houston, Texas), a silicone rubber septum was attached to the lid of the canned sample in preparation for the C1-C 7 1110 headspace-gas analysis. Prior to analysis, the can was 1170 shaken by hand for 1 min. A small hole was pierced through 1230 the septum, and degassed water was allowed to flow into the 1290 can until the pressure equilibrated to one atmosphere. A 1350 sample of gas was withdrawn with a 2-ml syringe after a 1470 positive pressure was created in the can by injection of 2 ml 1650 of degassed water. This 2-ml headspace gas sample was 2500 injected into a standard l-ml gas-sample loop attached to the gas chromatograph.

A Varian Aerograph 1400 isothermal gas chromatograph, equipped with a 1/8-in by 8-ft alumina-packed Sample depth column and a flame ionization detector, was used for (ft) analysis of headspace gas. This column resolves methane, 1140 ethane, propane, iso- and normal butane, and if present, the 1270 C 2, C 3 and C4 olefinic hydrocarbons. After the normal- 1350 butane peak eluted, the flow of carrier gas through the 1440 system was reversed with a back-flush valve, and the C5-C7 1560 hydrocarbons were eluted as a single composite chromato- 1680 graphic peak. The concentration of each hydrocarbon was 2985 computed from the peak area by means of an electronic 3030 integrator with baseline correction. Before a suite of samples was analyzed, a light-gas standard containing 100 ppm each of methane, ethane, propane, iso-butane and normal-butane was analyzed in triplicate. Analytical repro- Sample depth ducibility is consistently within 2-3% of the observed value. (ft)

After the can was opened, an aliquot of 10 ml of wet 420 cuttings was placed in a specially designed, sealed blender 1020 for the Ct--C7 cuttings-gas analysis. The sample was disag- 1620 gregated in a blender for 2-3 min. A 2-ml sample of 2220 degassed water was injected with a syringe into the 10-ml air 2820 space at the top of the blender and an equal amount of the 3420 gas sample withdrawn. This 2-ml gas sample was analyzed in 4020 the same manner as the headspace gas. The 10-ml aliquot of 4620 cuttings and the 10-ml volume of air space were used for 5250 calculation of the standard volumes of rock disaggregated. 5850

Gas samples for the determination of the stable C isotopic 6450 composition of methane were obtained from the headspace

GEOLOGICAL SETTING

The geology and pe t ro l eum geochemis t ry for rock uni ts on the Nor th Slope of Alaska are descr ibed in cons iderab le detail in a n u m b e r of publ ica t ions (MORGRIOGE and SMITH, 1972; LERANO, 1973; GRANTZ et al., 1975; JONES and SPEARS, 1976;

Table 1. C1/(C2 + C3) ratios and methane 613C compositions for Kuparuk River Unit 2D-15, Kuparuk River Unit

3K-9, and Prudhoe Bay Unit R-1 wells

Kuparuk River Unit 2D-15

Cd(C2 + C3) 6~3C (ratio) (%0)

2479 -39.3 3630 -49.6 4222 -48.9 5935 -50.0 5885 -49.1

14,579 -46.6 7060 -41.2 3381 -38.6

Kuparuk River Unit 3K-9

CI/(C2 + C3) (ratio)

~13 C

(%0)

15,487 73,111 16,103 51,051 82,340 88,789 15,870 16,014

Prudhoe Bay Unit R-I

Cd(C: + C3) (ratio)

-50.2 -50.7 -53.8 -50.7 -48.0 -48.8 -46.5 -48.0

613C (%°)

4684 273 250 213

80 420 366 114 356

15 5

-32.7 -34.2 -33.0 -36.0 -29.0 -34.5 -39.3 -30.1 -39.9 -27.7 -31.6


70030 .

150"00' 149"00' 148"00 ' I I I

t~likt~t Pnin¢

o K

%...

rxula=uur, ruvuf \ Prudhoe B a y oil f ield

West Sak a n d oil f ield Ugnu oil f ields

0 1o KILOMETERS I I 0 6 MILES

70000' I I I

FIG. 3. Distribution of in-situ gas hydrates and oil occurrences in Prudhoe Bay-Kuparuk River area. West Sak and Ugnu oil fields are shown as a single accumulation, enclosed by a dashed line. Major fault zones are indicated by bold lines; direction of dip shown by arrow. Oil well locations are shown by solid circles.

AHLBRANDT, 1979; SEIFERT et al. 1979; MAGOON and CLAYPOOL, 1982, 1985; CARMAN and HARDWlCK, 1983; MOLENAAR, 1983; BIRD, 1985; HUFFMAN, 1985; MACOON and BIRD, 1985; and HUBBARD et al . , 1987). The sedimentary rocks of the North Slope can be conveniently grouped into three sequences that indicate major episodes in the tectonic development of the region and, to a degree, its lithological character (Fig. 4). Defined on the basis of source area, these sequences, proposed by LEaAND (1973) and applied to northern Alaska by GRANTZ et al. (1975) are, in ascending order, the Franklinian (Cambrian through Devonian), the Ellesmerian (Mississippian through Jurassic), and the Brookian (Cretaceous to Holo- cene).

The petroleum-bearing stratigraphic section penetrated in the Prudhoe Bay-Kuparuk River area includes the Ellesmerian and Brookian sequences. The Ellesmerian Sequence consists of carbonate and siliciclastic rocks, whose provenance is to the north of the present coast line. The Prudhoe Bay oil field is the primary oil accumulation in the Ellesmerian Se- quence, with production from the Sadlerochit Group, which also includes a 0.74 trillion m 3 (26 trillion cubic feet) gas cap. The Lisburne and Kuparuk River oil fields also occur within the rocks of the Ellesmerian Sequence. The Brookian Sequence includes only siliciclastic rock whose provenance is the Brooks Range to the south. The principal oil accumulations of the Brookian Sequence, in the Prudhoe Bay-Kuparuk River area, are in the West Sak and Ugnu sands of the Sagavanirktok Forma-

tion. This formation is the site of the inferred occurrences of gas hydrates.

A review of all available data sources from 445 wells revealed that most of the inferred gas hydrates occur in six laterally continuous sandstone and conglomerate units and are geographically restricted to the east end of the Kuparuk River oil field and the west end of the Prudhoe Bay oil field (Fig. 3; COL- LETT, 1983; COLLETT et al. , 1988). Most of the gas hydrates occur below the base of ice-bearing permafrost; however, mud logs from wells in the Kuparuk River oil field indicate that several of the gas-hydrate occurrences extend up-dip into the permafrost layer. Well log and drill-cuttings analysis revealed the presence of numerous thick coal seams closely associated with several gas-hydrate-bearing units, and reser- voired in one of the gas hydrate intervals is a significant volume of oil (COLLE'rr et al . , 1988).

Petroleum geochemical information and interpretation of oil types (MAGOON and CLAYPOOL, 1981) and oil-source-rock correlations (SEIFERT et al. , 1979) indicate that the oil in the Prudhoe Bay-Kuparuk River area is from the same source rocks-- the Shub- lik Formation, Kingak Shale, and to a lesser extent, the pebble shale unit. In late Early Cretaceous time, these source rocks were mature in the western part of the North Slope area. Maturity progressed eastward toward the Barrow Arch, in the Prudhoe Bay area, which was a structural high relative to the source rocks to the west. Oil and gas migrated to the Prud- hoe Bay area and accumulated in the Prudhoe Bay, Lisburne and Endicott oil fields until mid-Tertiary


time. Sometime during the Tertiary this trap was tilted to the northeast, and oil and gas migrated up the faults at the west end of the Prudhoe Bay oil field into the Kuparuk River and West Sak-Ugnu reservoirs (CARMAN and HARDWlCK, 1983). The spatial relations among the major oil and gas accumulations, the inferred gas-hydrate occurrences, and major fault systems are shown on the map in Fig. 3 and the generalized cross section in Fig. 5.

COMPARISON OF RESULTS

To characterize the gas within the interval of gas- hydrate stability on the North Slope, three drilling production wells have been sampled and analyzed: Kuparuk River Unit 2D-15, Kuparuk River Unit 3K- 9, and Prudhoe Bay Unit R-1 (Table 1, Fig. 3).

AGE LITHOLOGY

SW NE

QUATERNARY

TERTIARY

65 LATE CRETACEOUS

975

EARLY CRETACEOUS

Ma

JURASSIC

TRIASSIC

PERMIAN

PENNSYLVANIAN

MISSISSIPPIAN

PRE-MISSISSIPPIAN

213

248 -

286

320

360

, : , , , , E _ f i I I I I

+÷

Results of these analyses have been graphically dis- played in the cross section of Fig. 6, which shows the distributions with depth of C isotopic compositions of the headspace gases and hydrocarbon gas ratios [C1/(C2 + C3)]. An important assumption in our work is that the gas compositions of drill-cuttings reflect the in-situ gas composition of the stratigraphic interval that was sampled. To interpret gas sources, we consider that gas with stable C isotopic compositions of -50%o and heavier is thermally generated; conversely, an isotopic composition of - 6 0 % or lighter suggests that the gas was formed by microbial processes. Similarly, a hydrocarbon gas ratio of C1/(Cz + C3) which is 100 or less indicates thermally generated gas; and a hydrocarbon gas ratio of 1000 or more suggests the presence of microbial gas. These isotopic and compositional limits have been adapted from BERNARD et al. (1976),

NORTH SLOPE STRATIGRAPHIC NOMENCLATURE OIL AND

GAS FIELDS

Gubik Formation and surficial deposits

sandstone ot' the Sagavanirktok Formation and Colvile Group

shale of the Sagavanirktok Formation and Colville Group

Nanushuk Group

Torok Formation

pebble shale unit and gamma ray zone ~_K.uparuk River Formation

Kingak Shale

Sail River Formation

Shublik Formation

Sadlerochit Group

Lisburno Group

Endicott Group

basement complex

EXPLANATION

U se.,,, . . . . . , , . . . , o a o , [ ] ..,...,..nt.. 0., conglomerate dolomite moltly xrgillite ~- Oil led gltt

~ ' ~ ShalesiltstoneOr [ ] Grlnitic rocks • Oil

Ugnu West Sak Flaxman Island Gubik Square Lake Simpson Fish Creek Umiat East Umiat Meade Wolf Creek

Point Thomson Walakpa Prudhoe Say

I Kuparuk River Milno Point Barrow Prudhoe Bay

I Prudhoe Bay Kemik

r Prudhoe Bay Kavik Seal Island North Prudhoo

.Gwydyr Say

Lisburne

Endicott

Point Thomson

FIG. 4. Generalized stratigraphic column for western North Slope, exclusive of the allochthonous rocks in the Brooks Range, showing stratigraphic locations of petroleum-bearing formations (from BIRD, 1985).

AG S:$-C


. . . . . . . . . . . . . . . . . . . . . . . . . 1 ° ° Z w ~ . . . . > , > , < ¢J IPF = - -Z ~

I .... •

Z <w ~o wZ

I KUPARUK RIVER I PRUDHOE BAY

OIL FIELD P OIL FIELD West East

EXPLANATION

Gas hydrate ~ Oil - - - Base o f marine shale sequence

~ F r e e gas ,~m Migration path

FIG. 5. Generalized west-to-east cross section through the Prudhoe Bay-Kuparuk River area illustrating possible gas migration routes and spatial relations between gas hydrates, the Eileen fault zone, oil, base of ice-bearing permafrost (BIPF), and gas-hydrate stability field (adapted from CARMAN and HARDWlCK,

1983, Fig. 13).

Information about the geochemical nature of the cored gas hydrates in the Northwest Ei leen State-2 well has been provided by A R C O (writt. commun. P. Barker , A R C O Alaska Inc., Anchorage , Alaska). For each well, a discussion of all analytical results and interpretat ions follows.

N o r t h w e s t E i leen State-2

The only confirmed natural gas hydrate on the North Slope was obtained in 1972, when A R C O and E X X O N successfully recovered two cores from the Northwest Ei leen State-2 well, both of which con-

Depth (ft) (m)

o. o

200O.

10OO.

4000.

15OO,

6000.

tOOO

Kuparuk River UnR 20-15 Kuparuk FIIver Unit 3K-9

. . . . . . . . . . ....

M ~ I~ T

i_,, ° i , t -~o 1ooeoa css c11c~,s

Key:.

L

Cl /C~*¢3 , \. o \

\ \

P r u d h o e B s y Lk'~t R - 1

, (

L . * ~ r , r.8~_M ' r I

' - e0 CS3 - ~ 1 1 s 0 ~ o o CSSC2~C3

Fro. 6. Cross section depicting geochemical and geological data from the Kuparuk River Unit 2D-15, Kuparuk River Unit 3K-9, and Prudhoe Bay Unit R-1 wells. Geochemical data includes stable methane C isotopic [613C] compositions and the ratio of amounts of methane to ethane-plus-propane [C1/(C2 + C3)]. Also shown is the zone in which methane hydrate would be stable (CoLLE~ et al., 1988) and the depth of a lithological contact between a predominant marine shale interval and an overlying marine-nonmarine

sandstone and conglomeratic section.


tained gas hydrates (reviewed by COLLETT and KVEN- VOLDEN, 1987). The two gas-hydrate beating cores were from depths of 577-580 m (core number 2) and 665-677 m (core number 4). Gas analyses of the cores indicate that methane is the primary hydrocarbon gas within the cored gas hydrates. The volume of methane within these samples ranged from 85-99%, and only trace amounts of ethane and propane were detected. Because methane was the only hydrocarbon gas detected in significant amounts, it can be assumed that the CJ(C2 + Ca) ratio of gases from the gas hydrate cores is ->1000, suggesting the presence of mainly microbial gas. No isotopic data are available, however, to support this interpretation of source.

Kuparuk River Unit 2D-15

The Kuparuk River Unit 2D-15 well was drilled as a development well for the Kuparuk River oil field and penetrated three oil-beating zones in the Brook- ian Sequence: The West Sak, Ugnu and 2150 Sands. The 2150 Sands is an informal term used by ARCO to identify a 30-60-m-thick heavy-oil-beating sandstone unit that occurs locally in the southeast corner of the Kuparuk River oil field. The hydrocarbon gas ratios [Cx/(C2 + C3)] within the upper 900 m of section indicate the presence of predominantly microbial gas having ratios > 1000. In addition, there is an abrupt change in the hydrocarbon-gas ratio at the depth of a distinct lithological contact (Fig. 6). Carbon isotopic compositions suggest that some thermogenic gas is present in the gas mixture, leading to an average 613C value of -45%0.

A 30-m-thick interval (at a depth of 375-405 m) is believed to contain gas hydrates in the Kuparuk River Unit 2D-15 well (Fig. 7). The occurrence of gas hydrates is suggested by the release of unusually large (up to 750 ppt) amounts of methane as indicated on the mud log (Fig. 7, column A), and an increase in transit-time velocity (Fig. 7, column B) and electrical resistivity (Fig. 7, column C) as shown on the open- hole well logs. Besides the Northwest Eileen State-2 well, this well is the only one in our study that contains evidence of gas hydrates. Vitrinite reflectance measurements (0.4% Ro) show that the near- surface, gas-hydrate-beating sedimentary rocks have never been subjected to temperatures within the thermogenic window. Thus, the thermogenic gas component likely migrated from greater depths and mixed with in-situ gas of microbial origin.

A B C M u d - M e t h a n e Transi t Time R e * i * t i v i t y

0 7 5 0 2 0 0 7 5 2 2 0 0 0

ppt m . * / f t ohm-m

3 5 0

' H y d r a t e

4 0 0 - - - - - - - - - J405m

e~

/ 4 5 0 -

Fro. 7. Well logs from the Kuparuk River Unit 2D-15 well, showing a response thought to indicate a gas-hydrate occur-

rence (adapted from COLLErr, 1987, Fig. 3).

(Fig. 6), are similar to the data from the Kuparuk River Unit 2D-15 well. The hydrocarbon gas ratios [C1/(C2 + C3)] indicate the presence of mainly microbial gas. The methane C isotopic compositions suggest the presence of a mixture of microbial and thermogenic gas with an average (~13C value of -49%0.

Prudhoe Bay Unit R-1

The Prudhoe Bay Unit R-1 well was drilled on the north flank of the Prudhoe Bay oil field just west of the gas cap. Within the Brookian Sequence, this well is down dip from the Northwest Eileen State-2, Kuparuk River Unit 2D-15, and 3K-9 wells. Vittinite reflectance data indicate that the entire section is immature (R 0 < 0.6%). Well-log responses indicate that no gas hydrates were penetrated in this well. Hydrocarbon ratios [C1/(C 2 + C3) ] averaging 700 and methane isotopic compositions averaging -33%o suggest a significant thermogenic gas component within the upper 1800 m of this well. An abrupt change in C1/(C 2 d- C3) ratios also marks the base of the predominant sandstone interval, similar to that observed in the Kuparuk River 2D-15 well (Fig. 6).

DISCUSSION

Kuparuk River Unit 3K-9

Kaparuk River Unit 3K-9 was drilled in the northeast portion of the Kuparuk River oil field and, based on well logs, did not penetrate any gas hydrates (Fig. 3). The geochemical data from the 3K-9 well

Methane C isotopic analyses of cuttings from the Prudhoe Bay Unit R-l , Kuparuk River Unit 2D-15, and 3K-9 wells reveal a possible trend from east to west (Fig. 6). The Prudhoe Bay Unit R-1 well exhibits relatively constant methane isotopic compositions with depth, suggesting homogenization of gas


throughout much of the stratigraphic section penetrated by the well. Compared to the Prudhoe Bay R-1 well, the C isotopic values for cuttings gas from the Kuparuk River Unit 2D-15 well are isotopically lighter, as are the values from the Kuparuk River Unit 3K-9. This difference in methane isotopic compositions may indicate mixing of microbial gas with gas that has migrated along major fault systems related to the Prudhoe Bay oil field. Although faults can serve as barriers to migration, in this case the faults are likely conduits. For example, CARMAN and HAROWXO< (1983) consider the Eileen fault zone (Fig. 3) to be a conduit for oil migration from the Sadlero- chit reservoir. If this idea that faults are migration conduits is correct, it is likely that the thermally derived free gas and dissolved gas associated with the oils would also have migrated along the faults into the overlying strata that contain in-situ microbially derived gas. The decrease in the isotopic composition to the west, as seen in the 2D-15 and 3K-9 wells, suggests that the proportion of thermogenic gas relative to microbial gas is less as distance from the source of thermogenic gas increases up-dip to the west. Changes in rock porosities and permeabilities limit the extent of the migration. In addition, any gas hydrates that are formed may act as traps for migrat- ing free gas. An additional observation, which sup- ports this idea of thermogenic gas migration along faults, can be seen in Fig. 3, where most of the known and inferred gas hydrates and shallow-heavy-oils (West Sak-Ugnu deposits) are shown as occurring either up-dip, or near to, major fault systems.

If the gas within the zone of gas-hydrate stability is a mixture of in-situ microbial gas and thermogenic gas which migrated from deeper structures, the shallow gases should have molecular and isotopic compositions reflecting the two sources. The gas cap of the Prudhoe Bay oil field is composed primarily of methane (75-80%), the isotopic composition of which is not known to us at present, along with small quantities of ethane (5-7%) and propane (2--4%) (O'- DO,NELL, 1976). However, no significant amounts of ethane or propane were detected within the stratigraphic interval of gas-hydrate stability. Figure 6 shows that the amount of ethane and propane relative to methane change abruptly at the contact between the shale section and the overlying sandstone and conglomerate. Hydrocarbon gas ratios [C1/(C 2 -1-C3) ] are distinctly lower in the shales. Within the sandstone and conglomerate section, ethane and propane are depleted relative to methane. Depletion of heavier hydrocarbons, such as ethane and propane, from gases by stripping during migration has been suggested by SCHOELL (1983) and JENDEN and KAPLAN (1986) to explain natural gases containing thermogenic methane but only minor amounts of heavier hydrocarbons. We believe that the thermogenic component of the gas within the stratigraphic interval of gas hydrate stability on the North Slope has been stripped of most of its heavier

hydrocarbons. This stripping of the heavier molecular weight gases does not appear to be caused by the formation of gas hydrates; however, the stripping may be caused by differential gas migration through a heterogeneous rock medium. Such a process could account for the molecular and isotopic compositions that we have observed.

Given our present understanding, possible expla- nations for the origin of the gas hydrates in the Prudhoe B ay-Kuparuk River area are as follows: (1) Migration of thermogenic solution gas and free gas from reservoirs of the Prudhoe Bay oil field upward, possibly along the Eileen and other fault zones, into the overlying shallow strata containing in-situ microbial gas, where the addition of thermogenic gas caused total gas concentrations to reach amounts that triggered gas-hydrate formation within the zone of gas-hydrate stability; or (2) the concentration of in- situ and migrated gas in shallow traps which initially were outside the zone of gas-hydrate stability but which moved into that zone in response, for example, to climatic cooling causing gas hydrates to form within and beneath permafrost. In either case, the natural gas that forms gas hydrates in the region of the North Slope discussed in this paper is probably composed of microbial and thermogenic methane.

CONCLUSION

The objectives of this study were to determine the composition and source of the gas within the stratigraphic interval where gas hydrates are stable, and to develop a possible scenario which explains the history of gas hydrate formation on the North Slope. Gas hydrates have been conclusively identified in the Northwest Eileen State-2 well, and inferred to be present in Kuparuk River Unit 2D-15. Headspace cuttings and core analysis suggest that methane is the principal hydrocarbon gas in the near-surface (0- 1500 m) strata of the North Slope, and that methane is virtually the only hydrocarbon gas to occur in the gas hydrates. Stable methane C isotopic analyses of samples from the gas-hydrate bearing unit in the Kuparuk River 2D-15 well indicates that the methane within the inferred gas-hydrate interval is from mixed sources, microbial and thermogenic. The thermogenic gas has probably migrated from below along faults. Gas hydrate formation may have been triggered by the addition of thermogenic methane to the in-situ microbial methane within the stratigraphic interval of gas-hydrate stability. The timing of the gas-hydrate formation is uncertain.

Acknowledgements--This study was funded by the U.S. Department of Energy under interagency agreement No. DEA121-83MC20422.

Editorial handling: Brian Hitchon.


REFERENCES

AHLBRANDT T. S. (1979) Preliminary geologic, petrologic, and paleontologic results of the study of Nanushuk Group rocks, North Slope, Alaska. U.S. Geol. Surv. Circ. 794, 1-163.

BERNARD, B., BROOKS J. M. and SACKETr W. M. (1976) Natural gas seepage in the Gulf of Mexico. Earth Planet. Sci. Lett. 31, 48--54.

BILY C. and DICK J. W. L. (1974) Naturally occurring gas hydrates in the Mackenzie Delta. Northwest Territories. Bull. Can. Petrol. Geol. 22. 340-352.

BIRD K. J. (1985) The framework geology of the North Slope of Alaska as related to oil-source rock correlations. In Alaska North Slope Oil~Rock Correlation Study (eds L. B. MAGOON and G. E. CLAYPOOL), pp. 3-29. Am. Ass. Petrol. Geol. Studies in Geology 20.

CARMAN G. J. and HARDWICK P. (1983) Geology and re- gional setting of the Kuparuk oil field, Alaska. Am. Ass. Petrol. Geol. Bull. 67, 1014--1031.

COLLETI" T. S. (1983) Detection and evaluation of natural gas hydrates from well logs, Prudhoe Bay, Alaska. M.Sc. Thesis, University of Alaska, Fairbanks, Alaska.

COELETT T. S. (1987) Geochemical and geological controls on the inferred occurrence of natural gas hydrate in the Kuparuk 2D-15 well, North slope, Alaska. In Geologic Studies in Alaska by the U.S. Geological Survey during 1986 (eds T. D. HAMILTON and J. P. GALLOWAY), pp. 24-- 26. U.S. Geol. Surv. Circ. 998.

COLLETT T. S. and KVENVOLDEN K. A. (1987) Evidence of naturally gas hydrates on the North Slope of Alaska. U.S. Geol. Surv. Open-File Rept. 87-255.

COLLE"ffr T. S., BIRD K. J., KVENVOLDEN K. A. and MAGOON L. B. (1988) Geologic interrelations relative to gas hydrates within the North Slope of Alaska. U.S. Geol. Surv. Open-File Rept. 88-389.

GRANTZ A., HOLMES M. L. and KososKi B. A. (1975) Geologic framework of the Alaskan continental terrace in the Chukchi and Beaufort Seas. In Canada's Continen- tal Margins and Offshore Petroleum Exploration (eds YORATH, PARKER and GLASS), pp. 669-700. Can. Soc. Petrol. Geol. Mere. 4.

HUBBARD R. J., EDR1CH S. P. and RATTEY R. P. (1987) Geologic evolution and hydrocarbon habitat of the Arctic Alaska microplate. Mar. Petrol. Geol. 4, 2-34.

HUFFMAN A. C. (1985) Geology of the Nanushuk Group and related rocks, North Slope, Alaska. U.S. Geol. Surv. Bull. 1614.

JENDEN P. D. and KAPLAN 1. R. (1986) Comparison of microbial gases from the Middle American Trench and Scripps Submarine Canyon--implications for the origin of natural gas. Appl. Geochem. 1,631-646.

JONES H. P. and SPEARS R. G. (1976) Permo-Triassic reser-

voirs of Prudhoe Bay field, North Slope, Alaska. In North American Oil and Gas Fields (ed. J. FRAUNSTEIN), pp. 23-- 50. Am. Ass. Petrol Geol. Mem. 24.

KVENVOLDEN K. A. and MCMENAMIN M. A. (1980) Hy- drates of natural gas: A review of their geologic occurrence. U.S. Geol. Surv. Circ. 825.

LACHENSRUCH A. H., SASS J. H., MARSHALL B. V. and MOSES T. H. JR. (1982) Permafrost, heat flow, and the geothermal regime at Prudhoe Bay, Alaska. J. geophys. Res. 87, 9301-9316.

LERAND M. (1973) Beaufort Sea. In Future Petroleum Provinces o f Canada--their Geology and Potential (ed. R. G. McCRoSSAN), pp. 315-386. Can. Soc. Petrol. Geol. Mem. 1.

MAGOON L. B. and BIRD K. J. (1985) Alaskan North Slope petroleum geochemistry for the Shublik Formation, Kingak Shale, pebble shale unit, and Torok formation. In Alaska North Slope Oil-Rock Correlation Study (eds L. B. MAGOON and G. E. CLAYPOOL), pp. 31-45. Am. Ass. Petrol. Geol. Studies in Geol.20.

MAGOON L. B. and CLAYPOOL G. E. (1981) Two oil types on the North Slope of Alaska--implications for exploration. Am. Ass. Petrol. Geol. Bull. 65,644-652.

MAGOON L. B. and CLAYPOOL G. E. (1982) Petroleum geology of the North Slope of Alaska--time and degree of thermal maturity. In Proceedings o f Tenth Inter- national Meeting on Organic Geochemistry (ed. M. BJOROY), pp. 28-38. Advances in Organic Geochemistry- 1981.

MAGOON L. B. and CLAYPOOL G. E. (1985) Alaska North Slope oil-rock correlation study. Am. Ass. Petrol. Geol. Special Studies in Geol. 20, 1-682.

MOLENAAR C. M. (1983) Depositional relations of Creta- ceous and Lower Tertiary rocks, Northeastern Alaska. Am. Ass. Petrol. Geol. Bull. 67, 1066-1080.

MORGRIDGE D. L. and SMITH W. B. JR. (1972) Geology and discovery of Prudhoe Bay field, eastern Arctic Slope, Alaska. In Stratigraphic Oil and Gas Fields (ed. R. E. KING), pp. 489--501. Am. Ass. Petrol. Geol. Mere. 16.

O'DONNELL J. P. (1976) Refrigeration is key to Arctic gas- pipeline system, Oil and Gas J., January 5, 88-96.

SCHOELL M. (1983) Genetic characterization of natural gases. Am. Ass. Petrol. Geol. Bull. 67, 2225-2238.

SEIFERT W. K., MOLDOWAN J. M. and JONES J. W. (1979) Application of biological chemistry to petroleum exploration. In Proceedings of the lOth World Petroleum Con- gress, 425-440.

WERNER M. R. (1987) Tertiary and Upper Cretaceous heavy oil sands, Kuparuk River area, Alaskan North Slope. In Alaskan North Slope Geology (eds I. TAILLEUR and P. WEIMER), pp. 109-118. Bakersfield California, Pacific Section, Society o f Economic Paleontologists and mineralogists and the Alaska Geological Society, Book 50.

characterization of hydrocarbon gas within the stratigraphic interval of gas-hydrate stability on...

Documents