Comparison of microbial* gases from the Middle America Trench and Scripps Submarine Canyon: implications for the origin of natural gas

Applwd Geochemistry. Vol. 1. pp. 631--64,5, 1986.

0883-2927/86 $3.00 + .t~l Pergamon Journals Ltd.

Printed in Great Britain

Comparison of microbial* gases from the Middle America Trench and Scripps Submarine Canyon: implications for the origin of natural gas P. D. JENDEN and I. R. KAPLAN Global Geochemistry Corporation, 6919 Eton Avenue, Canoga Park, CA 91303, U.S.A. (Received 19 September 1986: accepted in revised form 13 December 1986) Abstract--Chemical and isotopic compositions have been measured on 62 microbial gases from Tertiary hemipelagic sediments in the Middle America Trench off Guatemala and from decaying kelp and surf grass currently accumulating in Scripps Submarine Canyon off southern California. Gases from the Middle America Trench have been generated primarily by the reduction of carbon dioxide; methane 6 ~3C varies from -84%0 to -39%0. methane diD varies from -208%0 to - 145°~. and carbon dioxide b ~3C varies from -27%o to +28%0. Gases from Scripps Submarine Canyon have been generated primarily by acetate dissimilation: methane ~ 13Cvaries from -63%0 to -43%o, methane bD varies from -331%o to -280%. and carbon dioxide b ~3C varies from - 17%0to + 3%0. Methane 6 ~3C values as heavy as -40%0 appear to be uncommon for gases produced by carbon dioxide reduction and. in the Middle America Trench, are associated with unusually positive carbon dioxide 613C values. However. based on the 25%o intramolecular fractionation between acetate carboxyl carbon and methyl carbon estimated from the Scripps Submarine Canyon data, methane produced by acetate dissimilation may commonly. have heavy 6 13C values. The 6D of methane derived from acetate is more negative than natural methanes from other origins. Microbial methane bD values appear to be controlled primarily by interstitial water OD and by the relative proportions of methane derived from carbon dioxide and acetate. The chemical and isotopic compositions of microbial gas and thermo~enic gas overlap, making it difficult to determine the origins of many commercial natural gases from methane b~3C and C2+ hydrocarbon concentrations alone. Measurements of methane bD and carbon dioxide 6 ~3C can provide useful additional information, and together with ethane 6~3C data, help identify gases with mixed microbial and thermogenic origins.

INTRODUCTION MOST economic natural gas accumulations appear to have been derived from the thermal alteration of deeply-buried sedimentary organic matter. However, RiCE and CLAVI'OOL (1981) estimated that at least 20% of the world's gas reserves have been generated by relatively shallow microbial activity. The ability to distinguish between thermogenic and microbial methane is important in oil and gas exploration, particularly in the areas of source-rock identification (e.g. COLOMBOet al., 1969; STAHLand CAREY, 1975; JAMES, 1983) and surface prospecting (OREMLAND. 1981; PHILP and CRISP, 1982). It is also important ~n assessing human safety hazards in places where oil and gas are produced or where natural seeps are known (RICE and LADWlG, 1983; COBARRUBIASet al., 1985). C : ~ : t o t a l hydrocarbon ratios less •than 1% together with methane ~13C values more negative than -55%0 are considered to be good evidence that * We use the term "'microbial" rather than "'biogenic'" to avoid the conflict in terminology which has arisen between studies of methane derived from sedimentary organic matter and studies of inorganically-derived or "abiogenic'" methane (e.g. GOLD and SOTER, 1982). According to the latter usage, "biogenic" refers to any substance with biological origins including both microbial and thermogenic methane. 631 AG

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a gas was generated by microbial activity, especially when supported by other evidence such as shallow production depth and organically-immature source rocks ( C L A Y P O O I . and KAPLAN, 1974; STAHL, 1974; FUEX, 1977; BERNARD, 1978; RICE and C L A Y P O O L , 1981; SCHOELL, 1983). Based on these criteria, commercial microbial gas fields have been reported in Japan (NAKAI, 1960), China (ZHANG and CHEN, 1985), northwestern Siberia (YERMAKOVet al., 1970), northern Italy (MAIIAVELLIel al., 1983), southern Germany (ScrIOELL, 1977. 1980), the Gulf of Mexico (RICE, 1980), the U.S. Great Plains (RICE and CLAVPOOL, 1981; RICE, 1984), and elsewhere. Unfortunately, the concentration of (72+ hydrocarbons is not always a reliable indicator for distinguishing microbial and thermogenic gases. In the first place, little is known about the generation of C~÷ hydrocarbons during sediment diagenesis. Although the concentration of these components in naturallyoccurring microbial gases is generally less than 0.1% (BERNARD, 1978; SCHOELL, 1983), concentrations as high as 2% have been reported (RICE and C L A Y P O O L , 1981). Secondly, thermogenic gas generated from post-mature organic matter may also have very low C2÷ hydrocarbon contents (STAHL, 1974; STAHLet al., 1977). Thirdly, thermogenic gases may lose C2÷ hydrocarbons during near-surface migration due to adsorption on sedimentary organic matter and to chromatographic separation (COLEMAN, 1976, pp.

632

P.D. Jenden and I. R. Kaplan

70-71; BERNARD, 1978, pp. 29-36; SCHOELL, 1983). Finally, the concentration of C2- hydrocarbons in a thermogenic gas is relatively insensitive to the addition of large quantities of microbial methane (BERNARD, 1978). An additional problem is that the 6~3C boundary between microbial and thermogenic methane has been arbitrarily assigned (FuEx, 1977; RICE and CLAYPOOL, 1981; SCHOELL, 1983). Although a value of -55%0 to -60%0 is commonly cited, microbial methane less negative than -55%o (and occasionally less negative than -45%o) has been reported from a number of different environments. These include hemipelagic marine sediments (CLAYPOOL et al., 1973; CLAYPOOL et al., 1985), lacustrine sediments (WOLTEMATE e t al., 1984), marshes (LEBEDEV el al., 1969: FUEX, 1977), and landfills and sludge digesters (COLEMAN, 1976; GAMESand HAYES, 1976). In some cases, unusual isotopic compositions may be explained by methane oxidation (e.g. DoosE, 1980; OREMLANDand DESMARAIS,1983). However, experimental studies on the fractionation of carbon isotopes by methanogenic bacteria indicate that unoxidized microbial methane with 6~3C values heavier than - 5 5 % could be relatively common (GAMES et al., 1978; BELYAEVet al., 1983). The distribution of hydrogen isotopes in methane is not adequately established. SCHOELL (1980) suggested that 6D measurements could be used to distinguish between two different pathways for microbial methanogenesis, (1) carbon dioxide reduction: CO~ + 4H_, ~ CH4 + 2H_,O,

(1)

and (2) methyl group reduction. The most important reaction involving methyl group reduction is the dissimilation of acetate: C H ~ - C O O - + H* ~ CH4 + CO,.

al. (1984) and WHITICARet al. (1986) suggests that

many methane 6 ]SC values less negative than -55%o could be due to acetate dissimilation. In this paper, we present data on microbial gases from two different environments. Gases recovered from hemipelagic Tertiary sediments landward of the Middle America Trench off Guatemala are derived primarily from carbon dioxide reduction (CLAYPOOL et al., 1985). Based on the 6D model of WOLTEMATE et al. (1984), gases bubbling out of modern-day organic mats collecting at the head of Scripps Submarine Canyon off La Jolla, California, appear to be derived primarily from acetate dissimilation. These two environments aptly illustrate the wide range of chemical and stable isotopic compositions characteristic of microbial gases and demonstrate the potential overlap with thermogenic gas compositions. Analyses are presented for several commercial gases whose origins are particularly difficult to classify and which may have been produced by mixing of microbial and thermogenic components.

GEOLOGIC SET1 ING The locations of the sampling sites are shown in Fig. 1. Gases from the Middle America Trench off the Pacificcoast of Guatemala were sampled in February 1982during Leg 84 of t he Deep Sea Drilling Project. The purpose of Leg 84 was to investigate the geologic structure of the convergent margin of Central America and to determine the mode of occurrence and properties of the gas hydrates known to be present in the sedimentary cover (BALTUCKet al., 1985). At Site 568, located in 2010 m of water, drilling penetrated 418 m of Early Miocene and younger hemipelagic mudstones. Hydrates were recovered in fractured tufaceous mudstones from 391 m to 410 m. At Site 570, located in 1700m of water, drilling penetrated 374 m of Early Eocene and younger limestones, sandstones, and shales. Gas hydrate was recovered below 200 m, a particularly massive 3-4 m-thick layer

(2)

Other methyl-bearing substrates such as methanol and methyl amines can also be utilized by methanogens but methanol is not known to be a significant product of anaerobic decomposition (ZE[KUS, 1977) and the general importance of methyl amines has not been established (KING, 1984). The 6D values of methane produced by carbon dioxide reduction vary from - 2 5 0 % to - 150% and overlap the range observed for thermogenic gases (SCHOELL, 1980, 1983). The 6D values of methane produced by acetate dissimilation, on the other hand, are more negative than -300?/00 and are quite distinct (WOLTEMATE et al.. 1984; WmTtCAR et al., 1986). Carbon dioxide reduction and acetate dissimilation can occur simultaneously and their relative importance appears to be a function of the methanogenic species present as well as the organic substrate, the extent of its decomposition, and the depositional environment (JERIS and MCCARTY, 1965; GAMESand HAYES, 1976; MOUNTFORT and ASHER, 1978; WHIT1CARel al., 1986). Recent work by WOLTEMATEet

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Gases from the Middle America Trench and Scripps Submarine Canyon: implications for natural gas origin being encountered in fractured dolomite at 250 m. The hole bottomed in 28 m of serpentinized peridotite basement rock which outgassed significant quantities of C.,+ hydrocarbons. The presence of the C_,÷ hydrocarbons suggests that thermogenic gas has migrated into the basement rock, perhaps from marginally-mature sediments subducted along the Middle America Trench to the southwest (YONHUENE et al., 1985). In situ temperature measurements from Hole 568 indicate a local geothermal gradient of approximately 50°C/ km. Gases from Scripps Submarine Canyon were collected at water depths of 13-34 m from December 1983 to July 1986. Located off La Jolla, California, Scripps Submarine Canyon is one of over 60 submarine canyons which have been identified along the California coast (SHEPARO and EMERY, 1941 ). Scripps Submarine Canyon cuts through Eocene and Cretaceous mudstones and sandstones to within 250 m of the present shoreline, channelling littoral debris into the La Jolla Canyon and eventually into the San Diego Trough, 40 km distant and over 900 m deep. The origin and morphology of Scripps Submarine Canyon and the nature of the canyon fill have been described in detail by CHAMBERLAIN (1964). According to CHAMBERLAIN,the canyon transports an estimated 2 × 105 m 3 of sediment/a out of the littoral zone. This sediment consists of three distinct facies: a fine, quartzose beach-sand; a biotite-mica concentrate with minor amounts of organic fibers and shell fragments: and a fibrous organic facies consisting of subequal proportions of brown kelp (Macrocystis pyrifera) and surf grass (Phyllospadix torreyi) together with minor amounts of red algae and other organic debris. When fresh, the organic material, roughly 10% by volume, imparts great shear strength to the sediment allowing considerable quantities to accumulate in the canyon heads. Within a few weeks of deposition, however, anaerobic conditions develop within the sediments and fermentation occurs, producing large quantities of gas (including hydrogen sulfide). A sample of gas analyzed by LIMBAUGH and SHEPARD (1957) contained 74% methane, 24% carbon dioxide, and 2% trace components. During decomposition, the organic mats lose their shear strength and gas accumulating within the sediments increases their buoyancy. Loss of shear strength and increased buoyancy are thought to be responsible for the catastrophic slope failures which periodically sweep-clean the upper reaches of the canyon. SAMPLING AND METHODS

Forty samples of gas were obtained from the Deep Sea Drilling Project at sediment depths up to 41)0 m in the Middle America Trench. Most of these were withdrawn by syringe from gas expansion pockets in the clear plastic core liners immediately following core recovery and stored in 20 ml glass vacutainers. Four gases were obtained from decomposing blocks of gas hydrate and were stored in previouslyevacuated 100 ml aluminum containers. Twenty-two gases from Scripps Submarine Canyon were sampled by A. Rindell from California State University, San Diego, using SCUBA. Glass bottles (500 ml) with spring-loaded porcelain stoppers and rubber gaskets were inverted over bubbling gas vents and the gas collected by water displacement with the aid of a funnel. When possible, several volumes of gas were used to flush the bottles thoroughly. The bottles were cap(ged at a water depth of 3 m to maintain approximately 5 Ib/in- positive pressure. Sampling details and exact locations are described by RINDEI.E et al. (in preparation). Molecular composition of the canyon gases was measured by gas chromatography using flame ionization detection for the C_, to C5 hydrocarbons and thermal conductivity detection for methane and the non-hydrocarbon compounds. Methane and carbon dioxide were purified and prepared for

633

isotopic analysis using methods similar to those described by SCHOELL (1980). Carbon and hydrogen isotope ratios were measured on Nuclide 3-60 dual-collecting stable isotope ratio mass spectrometers and are reported relative to PDB and SMOW standards, respectively. Based on pooled data for sample duplicates, estimated 1~7precisions are _+{I.157~ for methane ~3C, _+3%ofor methane 6D, and _+0.5%, for carbon dioxide c~3C.

RESULTS M i d d l e A m e r i c a Trench D a t a for the Middle A m e r i c a T r e n c h gases are p r e s e n t e d in T a b l e 1. Insufficient sample was available for b o t h chemical and stable isotopic analyses so the c o n c e n t r a t i o n s of m e t h a n e and c a r b o n dioxide were i n t e r p o l a t e d (on an air-free basis) from u n p u b lished m e a s u r e m e n t s by G. Claypool and C. Threlkeld. N o a t t e m p t was m a d e to i n t e r p o l a t e data across the m a s s i v e - h y d r a t e zone at Site 570. C2- hydrocarb o n s were calculated from the relative c o n c e n t r a t i o n s of C l to C5 gases r e p o r t e d by the s h i p b o a r d scientists (voN HVENE et al., 1985, pp. 183 a n d 302). T h e gases in this study were sampled within 1 m of the gases selected for s h i p b o a r d analysis so that i n t e r p o l a t i o n of C_~+ : m e t h a n e ratios was not required. M e t h a n e , C2+ h y d r o c a r b o n s , a n d c a r b o n dioxide were normalized to 100%. T h e stable isotope m e a s u r e m e n t s in T a b l e 1 were previously r e p o r t e d by CLAYPOOL et al. (1985) a n d are r e p r o d u c e d here for c o n v e n i e n c e . In m a n y cases, c a r b o n dioxide was p r e s e n t at c o n c e n t r a t i o n s too low for reliable 613C m e a s u r e m e n t s . T h e r e f o r e , theoretical c a r b o n dioxide 6 ~3C values have b e e n calculated from 613C m e a s u r e m e n t s of total dissolved c a r b o n dioxide species r e p o r t e d by CLAVPOOI, et al. (1985). T h e calculations are described in detail in the footnotes to T a b l e 1. F o r samples with b o t h m e a s u r e d and calculated d a t a , the m e a s u r e d 6 ~3C values arc, on average a p p r o x i m a t e l y 3°/,,, more negative than the calculated values. JEFFREY et al. (1985) m e a s u r e d essentially the same c a r b o n dioxide t5 ~3C values for gases from Sites 568 and 570, which suggests that the calculated data arc slightly in error. H o w e v e r , the small a m o u n t s of c a r b o n dioxide present in the sampies could have easily b e e n f r a c t i o n a t e d d u r i n g core recovery or during the sampling and storage of the gases. W e believe that the m e a s u r e d and calculated c a r b o n dioxide ~ ~3C data are p r o b a b l y within a few per rail of the actual in situ values (with the exception of the m e a s u r e d data in samples 271) a n d 271 which are a n o m a l o u s l y low). M e t h a n e is by far the most i m p o r t a n t c o m p o n e n t of the Middle A m e r i c a T r e n c h gases a n d ranges from 93% to virtually 100%. T h e b a l a n c e gas is mostly c a r b o n dioxide because e t h a n e a n d higher hydrocarb o n s average less t h a n t000 p p m ( 0 . 1 % ) . A t Site 568, the c o n c e n t r a t i o n of e t h a n e and higher h y d r o c a r b o n s increases with d e p t h from less t h a n 10 p p m n e a t the

634

P.D. Jenden and I. R. Kaplan

T a b l e 1. Chemical composition and stable isotope ratios of gases from Middle America Trench Sediments. Deep Sea Drill[ ng Project Leg 84

Chemical composition* Sub-bottom depth(m)

CH 4

C2+

No.

Holecoresection

(%)

276 277 278 279 280 281 282 283 285 286 287 288 289 290 291 292 293 294 295

568-4-4 568-6-4 568-8-5 568-10-4 568-13-4 568-15-6 568-16-4 568-18-3 568-22-5 568-23-2 568-25-6 568-28-4 568-30-6 568-33-6 568-34-6 568-36.-6 568-38-6 568-40-3 568-42-6

28 47 68 85 114 135 143 1611 203 208 232 259 281 310 319 339 358 374 398

257 260 261 262 263 264 265 266 267 268 269 29711 29811 29611 270

570-2-5 570-7-4 570-8-3 570-10-2 570-14-5 570-16-6 570-18-1 570-20-1 570-22-3 570-24-3 570-26-4 570-27-1 570-27-1 570-28 570-29-4 570-29-6 570-30-4 570-32-4 570-34-1 570-35-2 570-36-1

11 61 69 88 130 151 164 183 205 224 245 250 250 264 274 277 284 302 318 329 336

Sample

29911 271 272 273 274 275

Stable isotope ratios Estimated R(CO_,)'I'§

(ppm)

6D(CH4) (%)

95.6 95.4 96.4 96.6 97.2 98.7 98.9 99.2 99.4 99.4 99.6 . 99.6 99.5 99.5 99.1 99.5 99.6 99.5

8 15 19 33 28 65 67 89 170 141) 1511 . . 2311 170 211} 22(} 30t1 75(I 11011

4.4 4,6 3.6 3.4 2.8 1.3 1.1 11.8 0.6 11.6 11.4 . (I.4 {).5 0.5 11.9 0,5 0.3 11.4

-71.3 -67.7 -66.2 -65.8 -63.8 -62.7 -61.6 -57.8 -51.7 -511.4 -49.1 47.3 -46.7 -46.6 -46.2 -44.7 -45.3 -44.5 -41.4

- 196 - 194 - 192 - 196 -194 - 193 -198 -191 -1811 - 177 -171 -171 -166 -164 -162 -161 -164 - 161 -145

-3.4 6.3 5.6 9.8 ---------------

-1.076 1,1178 1.1176 l.ll70 ---------------

4.7 6.9 8.1 8.~ 10.1 11.4 12. l 13.5 15.7 15.0 16.2 154 14.8 15.4 16.3 18.6 21.2 23.3 26.7

1.1t82 1.080 1.11811 1.1180 1.117O I.[179 1.1179 1.1176 1.1171 1.11711 1.1169 1.1t66 1.1165 I.II65 1.{166 1.1t66 1,070 1.1171 1.117t

. 95.5 94.7 93.6 97.1 96.2 95.0 94.4 94.9 94.5 . . . . 98.3 . 98.4 98.8 99.2 99.4 99.5

.

. 4.5 5.3 6.4 2.9 3.8 5.(1 5.6 5.1 5.5 . . . . 1.4 . 1.4 1.1 0.8 0.5 0.4

84.11 -72.1 -71.0 -70.8 -69.1 -68.8 -67.8 -65.6 -62.5 -59.0 54.9 44.0 40.6 39.9 -43.3 39.11 -43.2 -43.11 -42.5 -42.3 -42.{)

-208 -193 - 191 - 193 - 193 -191 -190 - 191 - 191 - 185 - 189 - 178 - 163 - 183 - 185 - 188 -188 -196 - 187 -188 - 185

--2.11 0.3 -0.4 -0.2 0.3 -0.2 1.2 4.0 5.3 . . . . 15.5 . 15.2 -----

-1.076 1.1177 1.076 1.074 1.{174 1,073 1.1171 1.1171 1.1168

-26.5 -11.2 -0.1 I).3 3.11 4.3 4.9 5.8 8.4 10.7 . . . . -. 27.6 24.4 21.8 19.9 18.5

1 .th53 1.[177 1.1176 1.1177 1.077 1.1179 1.078 1 .(}76 1.1170 1.1174

. . . . .

R(CO2)'t ~

Calculated bI~C(CH~) (%o)

. 27 28 24 39 59 99 120 17{I 42(1 . . . . 281111 . 1900 1100 530 850 10130

•13C(CO,) (%0)

bt3C{CO,)~: (%0)

CO_, (%)

. . . .

. . . . 1.061

.

. 1.061 -----

-1.1174 1.0711 1.[167 1.1165 1.1163

* Data normalized to 100%. Methane and carbon dioxide concentrations were interpolated from unpublished measurements by G. Claypool and C. T h r e l k e l d . C_,+ hydrocarbon concentrations were adapted from data reported in yon HUENE et al. (1985), p p 183 and 3112. + R = 13C/12C. :1:Calculated from d 13C values of total dissolved carbon dioxide species reported by CLAVr'OOt. et al. (1985). The fractionation factor between carbon dioxide gas and dissolved carbon dioxide species was taken from the equation a ( C O . , - b i c a r b o n a t e ) = exp 111.0241 - 9.55211/T)]. where Tis temperature in Kelvin (FRIEDMANand O'NEIL, 19771. Subsurface temperature was calculated using an ocean-bottom temperature of 3°C and a geothermal gradient of 50°C/km (VON HUENE et al.. 1985. p. 181 ). § Estimated from calculated b t 3 c ( c o 2 ) values.

IIDecomposed hydrate gas. - - Not measured.

sediment-water interface to about 1100 ppm (0.11%) at the bottom of the hole. Similar trends have been observed in numerous other marine sediment profiles and appear to be caused by t h e production of lowtemperature diagenetic hydrocarbons (CLAYPOOL and KVENVOLDEN,1983). At Site 570, C_,+ concentrations increase with depth to nearly 5000 ppm (0.5%) in the hydrated gas at 250 m (KvENVOLDErq and McDONALD, 19851 and then decrease to 500 ppm at about 320 m. Below 320 m the C,.~ hydrocarbons again increase and provide strong evidence for mixing with thermogenic gas derived more deeply-buried (subducted?) sediments. VON HUENE et al. (1985) report a C2+ : total hydrocarbon ratio of 2.9% in the

basement serpentinite at 394 m, well above the concentration normally observed for microbial gases. The C>. hydrocarbon maximum at 250 m is probably caused by the crystallization of gas hydrate in this interval because hydrates appear to concentrate ethane relative to methane (KVENVOLDEN and McDor~ALO, 1985). Methane carbon isotope compositions become heavier with increasing depth and rangc from - 7 1 . 3 % to - 4 1 . 4 % at Site 568 and from -84.0%0 to -39.0%0 at Site 570. Based upon the low C_,+ concentrations and upon the light methane bl3C values observed in the shallower samples, the gases at Site 568 and the gases at Site 570 down to about 250 m

Gases from the Middle America Trench and Scripps Submarine Canyon: implications for natural gas origin

635

30

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b13C CH((%0) FIG. 2. Plot of carbon dioxide 0 ~3Cvs methane b 13Cfor the Middle America Trench gases. The solid lines indicate carbon isotope fractionations of 1.050, 1.060, 1.070, 1.080, and 1.090 between carbon dioxide and methane. Solid symbols are used to indicate measured carbon dioxide b~3C values; open symbols are used to indicate calculated carbon dioxide 613C values. The dotted lines connect the calculated data in order of increasing depth from left to right. "Hydrate interval" indicates the relative position of the 3m-thick hydrate layer penetrated at 250 m sub-bottom depth at Site 570.

appear to be of microbial origin. Several lines of evidence suggest that the methane is produced primarily by carbon dioxide reduction. Firstly, carbon dioxide reduction produces ~3C-enriched carbon dioxide because methanogenic bacteria preferentially reduce 12CO2 (CLAvPOOL and KAPLAN, 1974; GAMES et al., 1978; BELVAEV et al.. 1983). Carbon dioxide t~13C values reach calculated values of +26.7%0 at 398 m in Site 568 and +27.6%0 at 284 m in Site 570, among the highest values determined for natural carbon dioxide. Secondly, carbon dioxide 6~3C and methane b~3C are strongly correlated as shown in Fig. 2. Although the methane bl3C valucs vary by over 45%Oand the carbon dioxide b ~3C values vary by over 50%o, the isotopic fractionation between the two species is relatively constant at 70 __. 10%o. This is essentially the same isotopic fractionation observed in marine sediments at other DSDP Sites and is close to the isotopic fractionation observed for microbial gases from glacial till in Illinois (CLAvPOOL and KAPLAN, 1974; COLEMAN, 1976: CLAVPOOL and THRELKELD, 1983). Lastly, the decrease in carbon dioxide concentration with increasing depth observed at both sites (Table 1) is consistent with carbon dioxide reduction, but not with acetate dissimilation. Figure 3 shows that carbon dioxide bl3C values become more positive as carbon dioxide concentration decreases. This relation can only occur in a system in which the rate of carbon dioxide removal by reduction to methane is greater than the rate of

I

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C02

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I

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6

(%)

FIG. 3. Plot of carbon dioxide 6~3C vs carbon dioxide concentration for the Middle America Trench gases. As in Fig. 2, solid symbols are used to indicate measured carbon dioxide 6 - C values, and open symbols to indicate calculated carbon dioxide ~13C values. The dotted lines connect the calculated data in order of increasing depth from bottom to top. "Hydrate interval" indicates the relative position of the 3 m-thick hydrate layer penetrated at Si:e 570.

carbon dioxide addition by anaerobic oxidation, acetate dissimilation, or thermal decarboxylation (CLAWOOL and THRELKELD 1983). Hydrogen isotope variations. The methane hydrogen isotope compositions reported in Table 1 vary by more than 60%o and the 6D trends at Site 568 are remarkably different from those at Site 570. At Site 568, methane 6D values become more positive with increasing depth, ranging from -196%o at 28 m to - 145%o at 398 m. Methane 6D values at Site 570 vary from -208%O to -163%o, but show no clearcut depth dependence. The differences between the sites are apparent in a plot of methane 6D vs methane 6~3C (Fig. 4). According to SCHOELL (1980) and WOLTEMATEet al. (1984), the hydrogen isotope composition of microbial methane is principally x~ontrolled by the methanogenic pathway employed by the bacteria and by the isotopic composition of the associated formation water. Following the approach of SCHOELL (1980) and WOLTEMArE et al. (1984), we have developed the following relationship between these parameters: f = (6D w - ~D m - 160)/(0.857 × 6Dw + 233).

(3)

Here, f is the fraction of methane derived from acetate dissimilation (and related methyl group reduction reactions), 6Dw is the isotopic composition of the formation water, and diDm is the isotopic composition of the methane. Details of the derivation are given in Appendix 1.

636

P.D. Jenden and I. R. Kaplan -140

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670

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t

blzC CH, (%a) FuG.4. Plot of methane 6D vs methane 613C for the Middle America Trench gases. The half-filledsquares indicate samples recovered from decomposed hydrates at Site 570. The dotted lines connect the data in order of increasing depth from left to right.

Unfortunately, the application of Eqn (3) to the Middle America Trench gases is hampered because, except for a few isolated measurements, interstitial water 6D data are not available. We can, however, use the approximation diD,,. = 05/ooto estimate f for the shallowest samples at Site 568 and 570. For sample 276 (28 m. Site 568), 6D m = - 196%0, and we calculate f = 0.15, indicating that approximately 15% of the methane has been derived from acetate dissimilation. For sample 257 (11 m, Site 570), diDm = - 2 0 8 % , and we calculate that approximately 20% of the methane has been derived from acetate dissimilation. According to these results, carbon dioxide reduction is the principal pathway for methane production at shallow depths in Sites 568 and 570. The more positive methane diD values encountered at greater depths at Site 568 may be due to the decreasing importance of acetate dissimilation. This is because (for diDw = 0%) as diD m varies from -1965/oo to -160%O, f varies from 0.20 to 0. Only sample 295 (398 m, Site 568) has a methane 6D value more positive than -160%o. Because there is no compelling evidence for the presence of thermogenic hydrocarbons at Site 568, the diD value of -1455/oo measured for sample 295 suggests that 6Dw is greater than 0%o at this depth. The isotopic composition of the interstitial water at depths significantly below the sediment-water interface is difficult to estimate. Studies of other DSDP sites indicate that interstitial water diD and b~so values generally become more negative with increasing depth (LAWRENCEand GIESKES, 1981; JENDEN and GmSKES, 1983). At Sites 568 and 570, however, b l 8 0 increases with depth (HEssE et al., 1985;

CLAYPOOL et al., 1985). This unusual behavior is probably related to the presence of gas hydrates, because hydrates concentrate ~sO relative to water (HEssE and HARRISON,1981; HESSE etal., 1985). The interstitial water di~O value of 3.3%o observed at the bottom of Hole 568 is similar to the isotopic composition predicted for gas hydrate (and ice) in equilibrium with normal sea water at about 0°C (DAVIDSONet al., 1983: O'NEIL, 1968). If we use ice as a model for the fractionation of hydrogen isotopes by gas hydrate, then gas hydrate in isotopic equilibrium with sea water should have a diD value of about 20%0 (O'NEIL, 1968). Taking diDw = 205/00for the interstitial water at the bottom of Hole 568 suggests that only 2% of the methane in sample 295 has been produced by acetate dissimilation. Figure 4 shows that in the upper half of Hole 570, methane diD becomes more positive with increasing depth, paralleling the trend for Site 568. The maximum methane 6D value, - 163%o, was measured for a sample of decomposed hydrate recovered from the massive hydrate layer at 250 m (sample 298). Below the massive hydrate layer, the diD values drop to more negative values, averaging -1885/oo. The drop in methane diD may be due to a negative shift in the isotopic composition of the interstitial water. Alternatively, it may reflect mixing with thermogenic methane as noted above in the discussion of the C2+ hydrocarbon data. Based on analyses of a variety of natural gases, SCHOELL (1980) estimated that thermogenic gases with high concentrations of C2÷ hydrocarbons have methane diD values from -260%0 to - 150%o. Moreover, as inferred from pyrolysis experiments (CHUNG, 1976; SACKETT, 1978; SCHOELL, 1984), the earliest-formed thermogenic methane has the most negative isotopic composition. If thermogenic methane is present below 250 m at Site 570, it could well be light enough to reverse the trend towards more positive diD with increasing depth.

Scripps Submarine Canyon Data for the Scripps Submarine Canyon gases are presented in Table 2. Methane and carbon dioxide are the dominant components, the latter reaching concentrations up to 2 7 ° , considerably higher than observed for the Middle America Trench gases. C2+ hydrocarbons average approximately 100 ppm (0.01%) and, unlike the Middle America Trench gases, show no apparent correlation with the isotopic composition of the methane. Nitrogen, oxygen, and argon range from less than 1% to nearly 15% and are derived from dissolved gases stripped from the seawater during collection and from air admitted during laboratory sampling. The wide range of concentrations observed for these components reflects the degree of difficulty encountered in collecting the samples. Gas emission rates less than 2 ml/s were encountered at most vents and collection occasionally

Gases from the Middle America Trench and Scripps Submarine Canyon: implications for natural gas origin

637

Table 2. Chemical composition and stable isotope ratios of gases from Scripps Submarine Canyon sediments Chemical composition* Water depth CH~ C_,- CO, H_,S N, O., Ar Sample Date No. sampled (m) (%) (%) (%) (%) (%) (%) (%) 727 728 729 897 899 900 903 905 979 980 1040 1041 1042 1050 11173 1104 1105 1182 t207 12118 1209 1210

12.83 12.83 12.83 1.84 1.84 1.84 1.84 1.84 " 10.84 1(I.84 11.84 11.84 12.84 1.85 4.85 6.85 9.85 1.86 3.86 6.86 6.86 7.86

19 14 16 22 13 17 17 18 25 20 23 23 13 25 29 16 15 23 31 19 24 34

79.8 87.9 86.3 86.0 78.0 80.9 80.9 69.9 89.9 84.8 75.2 89.9 90.6 90.6 85.6 89.9 82.6 75.4 87.8 82.6 88.2 93.2

0.02

0.04 0.01 0.01 0.02 0.01 0.01 ---0.01 0.03

tr. tr. 0.04 1/.03 tr. tr. 0.01 tr. tr. tr.

17.6 8.5 111.0 11.4 20.7 17.5 17.3 27.3 1.0 0.40 22.9 4.4 0.81 7.5 12.7 7.4 16.4 22.3 1(/.8 13.2 8.2 4.3

d. 2.0 0.47 d. 3.1 0 . 4 7 d. 3.3 0.35 d 2.3 0.13 d. 1.2 0. t2 d. 1.4 0 . 1 2 d. 1.7 11.04 0.35 2.1 0 . 3 8 n.d. 8.4 0.47 n.d. 12.5 2.1 n.d. 1.5 0.31 d. 5.1 0.41/ n.d. 7.7 0.76 n.d. 1.6 0.29 0.53 0.99 0.13 0.08 2.5 0.17 11.14 0.8(I 0.03 0.61 1.4 0.25 0.15 1.1 (I.19 0.(16 3.3 11.79 0.13 2.9 0 . 5 6 (I.1O 2.1 0 . 1 6

0.035 0.063 0.041 0.054 0.018 1/.035 0.(141 0.033 11.19 0.26 0.026 11.098 0.16 0.029 0.020 -(I.019 (I.025 0.020 0.050 0.048 0.043

Stable isotope ratios H(%)

(~I~C(CH~) OD(CH~) blOC(CO:) (%o) (%O) (%o)

---0.041 0.001

0.(X)2 11.015 tr. n.d. 0.(X)2 tr. 0.031 n.d. n.d. n.d. (/.002 n.d. n.d. n.d. n.d. n.d. n.d.

-58.4 -54.9 -59.1 -47.1 -43.4 -43.8 -44.7 -44.8 -61.2 -62.6 -47.1 -58.8 -59.5 -52.4 -50.6 -59.2 -48.5 -53.5 -47.0 -46.4 -48.0 -49.5

-307 -316 -318 -314 -311 -3116 -3fl9 -301 -315 -331 -28(I -317 -325 -308 -310 -312 -308 -317 -314 -3116 -314 -314

R(CO,)'~ f~R(CH~) (%)

-2.6 -8.6 -5.2 -0.2 2.4 1.2 2.1 1.5 --3.1 ---7.2 -6.2 -16.6 -8.6 -111.6 -3.5 11.8 -5.7 -11.8

1.1159

1.1149 1.057 1.049 1.1148 1.1147 1.114t) 1.(148 --1.1153 --1.048 1.047 1.1145 1.042 1.1145 1.1146 1.1149 1.1144 1.1/411

63 67 68 66 65 63 h4 61 67 73 52 67 71 64 64 65 64 67 66 63 66 66

* Normalized to 100%. Actual yields varied from 94 to 101%, reflecting both analytical precision (+_3% for major components) and the presence of water vapor. ";'R = 13C/t2C. :]:Estimated fraction of methane produced from acetate dissimilation (see text for discussion). Not measured. d. Detected by smell only. n.d. Not detected. tr. Trace quantity. -

-

r e q u i r e d 15 m i n o r m o r e p e r s a m p l e ( A . RtNDELL, p e r s . c o m m u n . ) . E m i s s i o n r a t e s w e r e so s l o w d u r i n g O c t o b e r 1984 t h a t o n l y 50 mt o f gas c o u l d be o b t a i n e d f o r s a m p l e s 979 a n d 980. W i t h t h e e x c e p t i o n o f t h e s e s a m p l e s , n i t r o g e n c o n c e n t r a t i o n s a v e r a g e less t h a n 3 % , i n d i c a t i n g t h a t o n l y small a m o u n t s o f o x y g e n c o u l d h a v e b e e n c o n s u m e d a n d t h a t a l t e r a t i o n o f the g a s e s d u r i n g s t o r a g e w a s p r o b a b l y insignificant. N o c o r r e l a t i o n is a p p a r e n t b e t w e e n t h e c o n c e n t r a t i o n o f nitrogen, oxygen, or argon and any of the other p a r a m e t e r s m e a s u r e d , l e n d i n g f u r t h e r s u p p o r t to this conclusion. M o s t o f the S c r i p p s S u b m a r i n e C a n y o n s a m p l e s c o n t a i n h y d r o g e n sulfide, p l a c i n g t h e m in close a s s o c i a t i o n w i t h the s u l f a t e r e d u c t i o n z o n e o f e a r l y d i a g e n e s i s . H y d r o g e n sulfide w a s n o t d e t e r m i n e d f o r the earliest samples, but Table 2 s h o w s that concent r a t i o n s r a n g e u p to at least 0 . 6 % . T h e M i d d l e A m e r i c a T r e n c h g a s e s w e r e all s a m p l e d b e l o w t h e s u l f a t e r e d u c t i o n z o n e (HESSE et al., .1985). T h e Scripps Submarine Canyon gases therefore represent an e a r l i e r s t a g e o f d i a g e n e s i s t h a n the M i d d l e A m e r i c a T r e n c h g a s e s . H y d r o g e n gas w a s d e t e c t e d in s o m e S c r i p p s S u b m a r i n e C a n y o n s a m p l e s at c o n c e n t r a t i o n s u p to 400 p p m . T h e p r e s e n c e o f h y d r o g e n i n d i c a t e s t h a t at least s o m e o f the S c r i p p s S u b m a r i n e C a n y o n m e t h a n e m a y h a v e b e e n p r o d u c e d by c a r b o n dioxide reduction. T h e isotopic composition of the Scripps S u b m a r i n e C a n y o n g a s e s differs f r o m t h a t o f t h e M i d d l e A m e r i c a

T r e n c h g a s e s in s e v e r a l i m p o r t a n t r e s p e c t s . F i g u r e 5 s h o w s t h a t c a r b o n d i o x i d e 613C a n d m e t h a n e 6~3C are p o s i t i v e l y c o r r e l a t e d , j u s t as w a s o b s e r v e d f o r the Middle A m e r i c a T r e n c h gases. H o w e v e r , carbon d i o x i d e 613C v a l u e s v a r y f r o m - 1 6 . 6 % o to 3.1%o a n d are m o r e n e g a t i v e t h a n m o s t o f the Middle A m e r i c a T r e n c h g a s e s , w h e r e a s m e t h a n e c~Z3C v a l u e s v a r y f r o m - 6 2 . 6 % 0 to - 4 3 . 4 % 0 a n d are less n e g a t i v e t h a n m o s t o f t h e Middle A m e r i c a T r e n c h gases. A s a result, the average carbon isotope difference b e t w e e n

5-

O-

./

•

//

c~ - 10 ,0 - 15

- 20

•b

- 60

-55

-50

- 45

I

-40

bloc CH4 (o/~) FIG. 5. Plot of carbon dioxide b ~3C vs methane (~~C for the Scripps Submarine Canyon gases. The lines indicate carbon isotope fractionations of I.(140, 1.1)50. and 1.060 between carbon dioxide and methane.

63~

P.D. Jenden and I. R. Kaplan S0 0

-280

0-290

v

6 o o

-5"

I 0

-300

I

-10

"

-15

•

g

o E3

,0

-310

,0

•

=**

j, -320

-20

I 0

I 10

I

I 20

I

I 30 -330

•

C02 (%) -60

FIG. 6, Plot of carbon dioxide b~C vs carbon dioxide concentration for the Scripps Submarine Canyon gases.

I

_510

~

I - 4 0

OlaC CH4 (%=) FIG. 7. Plot of methane OD vs methane 6 z3C for the Scripps Submarine Canyon gases.

carbon dioxide and methane is 5(1 _+ 10%o for the Scripps Submarine Canyon gases and 70 __ 10%ofor the Middle America Trench gases. Another important difference can be seen in a plot of carbon dioxide 613 C vs carbon dioxide concentration (Fig. 6). Unlike the Middle America Trench gases (Fig. 3), Scripps Submarine Canyon carbon dioxide bLoC values increase with increasing carbon dioxide concentration. This argues that the rate of carbon dioxide production by acetate dissimilation and anaerobic oxidation is greater than the rate of carbon dioxide removal bv carbon dioxide reduction (CLAYPOOL and THRELKELD, 1983). The change in carbon dioxide concentration from less than 5% to at least 27% is presumably related to the pH of the interstitial water, the buffering capacity of the sediments, and the extent of fermentation. Higher concentrations would be expected as carbon dioxide and organic acids are generated by fermentation, lowering the interstitial water pH. The increase in carbon dioxide bJ3C could be due to mixing between isotopically-light bicarbonate (-20%0?) derived from bacterial respiration and isotopically-heavy carbon dioxide (+3%0?) derived from fermentation. The heterogeneous nature of the organic matter deposited in the canyon suggests that the carbon dioxide ?~3C trend could also reflect changes in thc bLoC value of the organic substrate being fermented. Carbon isotope ratios werc measured for several samples of plant tissue and organic debris recovered from the canyon during the present study and vary from -24%0 to - 13%o. These data arc similar to thc rangc ofb]3C values for surf grass, giant kelp, and rcd algae, the retnains of which arc commonly obscrvcd in the canyon (CHAMBERLAIN. 1964: SMITH and EPSTEIN, 1971). The most distinctive characteristic of the Scripps Submarine Canyon gases is their unusually negative methane hydrogen isotope composition. The methane bD values reported in Table 2 vary from -3317/oo to -280?/o0 and arc at least 1007/oomore negative than the values observed for the Middle America

Trench gases. Like the gases from Site 568. a positive correlation exists between methane 6D and methane b 13C (Fig. 7). Unlike Site 568, however, the variation in 6D values cannot reflect significant changes in the isotopic composition of the interstitial water because even when the canyon tributaries are full, the sediments are, at most, only about 10 m thick (CHAMBERLAIN, 1964; DILL, 1964). It seems very unlikely that the interstitial water in the Scripps Submarine Canyon sediment differs much from normal sea water, about 0 + 5?/00 (HOLES, 1980, p. 109). The methane bD and 613C values for the Scripps Submarine Canyon gases are a function of a number of variables, including the proportion of methane derived from acetate dissimilation vs carbon dioxide reduction, the amount of carbon dioxide contributed by bacterial respiration, and the isotopic composition of the organic matter being fermented. As discussed above, the relative importance of the last two factors is not presently clear. The basis for the correlation between methane 6D and 6 ~3C must await resolution of this problem. The fraction of Scripps Submarinc Canyon methane derived from acetate dissimilation can be estimated from Eqn (3) using the methane bD data in Table 2 and the approximation bD,,. = 0%0. The estimated fraction varies from 52 to 73%, significantly higher than the inferred range of (I-20% for the Middle America Trench gases. It seems reasonable to conclude that most of the chcmical and isotopic differences between the Scripps Submarine Canyon and Middle America Trench gases are related to the fact that acetate dissimilation is the dominant pathway for methanogenesis in the former, whereas carbon dioxide reduction is the dominant pathway in the latter.

Distribution of carbon isotopes. The Middle America Trench gases represent the cumulative effects of methanogenesis and related sedimentary

Gases from the Middle America Trench and Scripps Submarine Canyon: implications for natural gas origin

FIG. 8. Steady-state model for the isotopic composition of carbon dioxide and methane in the Scripps Submarine Canyon gases. A is the rate of carbon dioxide production by anaerobic oxidation of sedimentary organic matter. B is the rate of acetate dissimilation, C is the rate of carbon dioxide reduction to methane, and D is the rate of dissolution of carbon dioxide (as bicarbonate) into the interstitial water and the overlying water column. Carbon dioxide bl3C and methane b :3C are controlled by the isotopic composition of the sedimentary organic matter, by the rela'.ive magnitudes of A, B, C. and D, and by the isotopic fractionation factors associated with each of these steps. Details of the model are presented in Appendix 2.

processes over a time interval of millions of years. The Scripps Submarine Canyon gases, in contrast, represent the composition of microbial gases produced at a single instant in time. Because of this difference, the Scripps Submarine Canyon gases can be investigated using relatively simple mathematical models based on steady-state assumptions. Figure 8 illustrates a model in which the carbon isotope compositions of carbon dioxide and methane are controlled by the isotopic composition of the different reservoirs shown, and by the rates of carbon transfer between the reservoirs. Specifically, carbon dioxide 6~3C values are a function of inputs from acetate dissimilation and anaerobic oxidation of organic matter, and outputs to carbon dioxide reduction and dissolved bicarbonate. Methane b'3C values are a function of inputs from acetate dissimilation and carbon dioxide reduction. The oxidation of methane within the sediments by bacteria utilizing nitrate, sulfate, or residual traces of oxygen (REEBURGHand HEGGIE, 1977: CLAYPOOL and THRELKELD, 1983; OREMLAND and DESMARAIS. 1983) is not considered in the model because the effects are probably small and no data arc available to estimate the rate at which oxidation might occur. The possible influence of methane oxidation is discus,,;ed separately below. The model in Fig. 8 is adapted from the approach of LAZERTE (1981) and uses many of the same assumptions. In particular, it assumes that the isotopic composition of carbon dioxide derived from anaerobic oxidation of organic matter is equal to the bulk composition of the organic matter itself, and that the gross ratio of carbon dioxide : methane produced during fermentation is equal to the theoretical production ratio of 1 (LAZERTE,1981). The latter assumption requires that significant amounts of carbon dioxide be lost as bicarbonate to the sediments

639

and to the overlying water column because the CO 2: CH 4 production ratio measured for the Scripps Submarine Canyon gases does not exceed 0.4 (Table 2). The model also assumes a fractionation factor for carbon dioxide dissolution, a (bicarbonateCO2) = 1.009, corresponding to equilibrium conditions at 15°C (FRIEDMANand O'NEIL, 1977), and an instantaneous fractionation factor for carbon dioxide reduction, a ( C O 2 - C H 4 ) = 1.07 (CLAYPOOL and KAPLAN, 1974; CLAYPOOL and THRELKELD, 1983). Finally, the model assumes that carbon dioxide reduction and acetate dissimilation are the only methanogenic pathways operating in Scripps Submarine Canyon. The application of steady-state approximations to the Scripps Submarine Canyon gases is justified because changes in gas composition appear to be slow compared to the rate of gas production. Nevertheless, due to the uncertain role of mixing between carbon dioxide derived from bacterial respiration and carbon dioxide derived from fermentation, we have only applied the model to gases with carbon dioxide 613C > 0%o, or close to the apparent upper limit of about 3%0 (Fig. 6). Under the assumptions outlined above, the concentration and isotopic composition of methane and carbon dioxide in these gases can be used to calculate an average b 13C value of - 1 8 _+ 1%o for Scripps Submarine Canyon organic matter and an average fractionation factor of 1.025 _+ 0.004 between acetate carboxyl carbon and methyl carbon. The details of these calculations are presented in Appendix 2. The calculated composition of the organic matter is very close to the 613C value of -17.5%o measured by SMITHand EPSTE!N(1971) for Macrocystis pyrifera, one of the major organic components in the sediment. It is also within the -24%o to -13%o range of 613C values measured for several samples of plant tissue and organic debris recovered from the canyon during the present study. The calculated intramolecular fractionation factor for acetate is a "net" value and includes effects produced by the assimilation of a small proportion of acetate into bacterial tissues, and by the possible synthesis of acetate from carbon dioxide and hydrogen by acetogenic bacteria (ZEIKUS et al., 1985). If a significant proportion of methane is oxidized within the sediment (REEBtJRGH and HEGGIE, 1977), the net intramolecular fractionation factor may be somewhat larger than 1.025. On the other hand, if the instantaneous fractionation factor for the reduction of carbon dioxide is assumed to be as large as 1.09 (RoSENFELDand SILVERMAN,1959), the net value may be as low as 0.998 _+ 0.009. Despite these uncertainties, the original estimate of 1.025 is consistent with the findings of ME!NSCHEIN et al. (1974) who measured a fractionation factor of 1.019 for acetate isolated from apple-cider vinegar. It is also similar to the values of 1.04-1.06 calculated by LAZERTE (1981) for anaerobic lake sediments and sewage sludge.

640

P.D. Jenden and I. R. Kaplan -50

DISCUSSION

-100

The stable isotope compositions observed in this study illustrate the enormous variability of microbial gases. In the Middle America Trench and Scripps Submarine Canyon, methane 613C values range from less than -80%0 to more than -40?'00, methane 6D values range from -330%o to -1457'oo, and carbon dioxide 613C values range from less than -15%0 to more than +25?'00. The heaviest methane 613C values fall well within the range commonly cited for thermogenic gases, namely more positive than -55%0 or -60?'00 (RICE and CLAYPOOL, 1981; SCHOELL, 1983). The heavy methane 613 C values noted for the Middle America Trench gases have not commonly been observed for gases produced by carbon dioxide reduction (CLAYPOOLand KAPLAN,1974; CLAYPOOL et al., 1985). The heavy 613C values observed for Scripps Submarine Canyon may, however, be typical of methane produced by acetate dissimilation because of the relatively small fractionation between acetate carboxyl carbon and methyl carbon. The results of this study suggest that carbon dioxide di13C and methane 6 D data may help distinguish microbial gases with heavy methane bl3C values from thermogenic gases with low C2~ hydrocarbon contents. In the Middle America Trench. gases with methane 613C values as heavy as -40?'00 are associated with carbon dioxide 613C values more positive than +25?'00. Natural gases with carbon dioxide di13C values more positive than the normal range for marine carbonates (0 _+ 4?'00)may also contain a significant amount of microbial methane. In a similar fashion, gases derived from acetate dissimilation are characterized by methane 6D values more negative than about -3000/o0.6D values more negative than -250?'00 are uncommon for thermogenic methane (SCHOELL, 1980, 1983). Therefore, natural gases with methane 6 ~3C more negative than -250%0 may contain a significant component of microbial methane.

Application to commercial natural gases

Figure 9 is a plot of methane bD vs methane 613C for unpublished Global Geochemistry Corporation (GGC) data on 379 commercial gases from the United States. The region marked "T'" indicates the compositional field for thermogenic gases determined by SCnOELL (1983) from published data. The areas marked "BI" and "'B2'" indicate the compositional fields for microbial gases from the Middle America Trench and Scripps Submarine Canyon, respectively. As discussed above, the most important variables governing the isotopic composition of microbial methane may be the relative rates of acetate dissimilation and carbon dioxide reduction, the hydrogen isotopic composition of the interstitial waters, and the carbon isotopic composition of the

-

-is0 :~

-200

~0

-250

,. -;- . "..

~"" . -"" . "• """:~" " '.~,:/ "-V" : r~t

-3oo -350

-~o

~:'" .2

B2 ~

~

~

-so

-70

-60 btaC

L

-so CH4

,

-40

-3o

-20

-lO

(%o)

FIG. 9. Plot o f methane 6 D vs methane 6=3C f o r 379 U.S.

natural gases analyzed by Global Geochemistry Corporation (unpublished data). The fields marked B1 and B2 represent the range of isotopic compositions observed for microbial gases from the Middle America Trench and the Scripps Submarine Canyon respectively. The field marked T (adapted from SCHOELL, 1983) represents the range of isotopic compositions observed for thermogenic gases based on published data.

organic matter. Variation of these parameters defines a general microbial gas field lying, for the most part, between B1 and B2 but extending to more negative 6D values (e.g. WOLTEMATEetal., 1984), and to more negative 6 t3C values (e.g. SACKE~ et al., 1977). The potential overlap between microbial and thermogenic gas compositions is obvious and noteworthy. Although few of the commercial gases plotted in Fig. 9 appear to be of pure microbial origin, the distribution of points is skewed towards the microbial gas field, especially towards B1. This suggests that a significant proportion of the commercial gases are mixtures of microbial and thermogenic components. The apparent predominance of B1 gases relative to B2 gases in Fig. 9 is consistent with statements in the literature that carbon dioxide reduction is the most important methanogenic pathway in sedimentary systems (CLAYPOOLand KAPLAN, 1974; GAMES et al., 1978: SCnOELL, 1980; RICE and CLAYPOOL,1981). The reason for the predominance of B1 gases is unclear. WmTICAR et al. (1986) have suggested that carbon dioxide reduction is the dominant pathway in salt marshes, estuaries, and marine environments, whereas acetate dissimilation is the dominant methanogenic pathway in freshwater lakes and swamps. Therefore, to the extent that marine sediments are more abundant than freshwater sediments, natural gases derived from carbon dioxide reduction should outnumber those derived from acetate dissimilation. The results of the present study argue that salinity is not the primary factor controlling the methanogenic pathway. As illustrated by the Scripps Submarine Canyon gases, acetate dissimilation can dominate marine as well as freshwater sediments. The apparent distinction noted by WHITICAR et al. (1986) may, however, reflect the type of organic matter present. For example, planktonic debris is an important component in most marine sediments

Gases from the Middle America Trench and Scripps Submarine Canyon: implications for natural gas origin whereas herbaceous and woody organic matter predominate in freshwater environments. The importance of acetate dissimilation in Scripps Submarine Canyon may, therefore, reflect the local abundance of herbaceous and sessile algal debris. The predominance of B1 gases in Fig. 9 could also be explained if acetate dissimilation occurs primarily in recently-deposited sediments and decreases with increasing time and ensuing burial. In this case, most methane produced by acetate dissimilation may be lost before suitable reservoir rocks and traps are available ( Z H A N G and CHEN, 1985). Many of the marine sediments cited by WHITICAR el al. (1986) were cored during the Deep Sea Drilling Project and are millions of years old. The fact that the methane in these sediments has been derived primarily from carbon dioxide reduction may reflect their age as well as their depositional environment. Tentative support for the aging hypothesis is provided by the results of the present study. In the Middle America Trench, the fraction of methane derived from acetate dissimilation apparently decreases from 20% near the sediment-water interface to practically 0% at 400 m. In Scripps Submarine Canyon, the fraction of methane derived from acetate dissimilation appears to decrease from 70 to 50% during fermentation of the canyon fill. Table 3 presents unpublished G G C data for five natural gases which illustrate many of the difficulties and pitfalls encountered in distinguishing between microbial and thermogenic methane. Samples 677 and 1085 are associated gases from oil fields in the Tertiary Ventura Basin of southern California and the Paleozoic Cherokee Basin of southeastern Kansas. Both gases are produced from reservoirs less than 1500 m deep, both have methane ~13C values more negative than -60%oo, and both have methane 5D values within the B1 field in Fig. 9, characteristics which argue for a microbial origin. Yet sample 677 contains 17.5% C2+ hydrocarbons and sample 1085 contains 6.2% Cz, hydrocarbons, characteristics which are expected for thermogenic gases. Other natural gases with similar properties have been

641

reported in the literature, suggesting that such occurrences are common (COLOMBO et al, 1969: SMITH et al., 1971: CLAYPOOL et al., 1980; BELVAEV et al., 19831. Pyrolysis experiments conducted by SACKErr (1978) indicate that cracking of crude oils could initially yield wet gases with methane 513C values as light as -65%0. Wet gases with methane 5 ~ C values more negative than -60%0 may also be derived from mixing between microbial and thermogenic components. The mixing hypothesis seems to be the best explanation for samples 677 and 1085 because the ethane 5~3C data are consistent with cogenetic methane values of -40%o rather than -60%0 (STAHL and CAREV, 1975: SCHOELL, 1983). Sample 1060 in Table 3 was collected from a surface tar seep associated with shallow Neogene oil production in the Los Angeles Basin. The concentration of C2+ hydrocarbons is only 0.93% but the methane 513C and 5D values, -42.4%0 and -181%o respectively, plot within the T field in Fig. 9 and the ethane 513C value of -29.1%0 also argues for a thermogenic origin. The carbon dioxide ~13C value of + 25.9%o, on the other hand, suggests that a significant proportion of the methane is of microbial origin. Mixing with microbial methane may account for the fact that the concentration of C,÷ hydrocarbons in this gas is significantly less than that observed in casinghead gases from local wells (unpublished G G C data). CAROTHERS and KHARAKA (1980) have measured bicarbonate 5(3C values up to + 2 8 % in formation waters from several shallow oil fields in the San Joaquin Basin of California and in the Texas Gulf Coast. The presence of isotopically-heavy bicarbonate in these areas suggests that microbial methane may be an unrecognized component in many shallow associated gases. Sample 808 in Table 3 is a non-associated gas produced from Late Cretaceous strata in the Sacramento Basin of northern California. Considered alone, the C2+ concentration of 1.3% and the methane di13C value of -54.1%o suggest that this gas is of early thermogenic origin. The methane bD value of - 188%0, however, places it well outside the T field

Table 3. Analyses of commercial natural gases which may be derived by mixing between microbial and thcrmogcnic gases

Sample No.

Basin

677 Ventura

Field

Ventura (oil) Kingston (oil) 1(160 Los Sail Angeles Lake (oil) 808 Sacramento Millar (gas) 1(125 Sacramento Corning (gas) 11185 Cherokee

Well

Production depth (m)

Chemical composition Geologic age

Stable isotope ratios

CH~ C.,. (%) (%)

CO2 (%)

Nz (%)

82.2 17.5

(I.31

Tr.

-61.{1

-2(12

-3(I.9

--

550 Mississippian

71.3

6.2

6.3

13.3"

-62.3

-2117

-34.5

-6.7

611(I Neogene

79.9

11.93 15.6

3.5

-42.4

-181

-29.1

25.9

6.(1

-54.1

-188

-26.6

--

24.1

-42.9

-161

--

--

Lloyd No.211 Aiken No, I Tar Seep

1460 Pliocene

DixonE. Unit No. 2 Saldubehere No. 2

15411 Cretaceous 4311 Pliocene

* Balance gas is 1.3% H~ S. II. I 1% Ar, 1.5% He.

92.6

1.3

7~;.9 (1.01

0.16 Tr.

OI~(CIi~) 0D(CH4) 6~C(C,H~) d'3C(CO_,) (%~) ('~) ('y~) ('~,)

642

P.D. Jenden and I. R. Kaplan

and within the B1 field of Fig. 9. Also, according to the work of STAHL and CAREY (1975) and SCHOELL (1983), ethane with t~I3C = -26.6%o is cogenetic with methane around -35%oo, rather than -54%0. The methane `5D and ethane ,5t3c data indicate that sample 808 is probably a mixture of microbial gas and dry, post-mature thermogenic gas. The presence of both of these components in the Sacramento Basin is verified by methane `513C values which range from less than -60%0 to more than -20%o and by low C,+ hydrocarbon concentrations which average less than 2% (JENDEN and KAPLAN,in preparation). The last sample in Table 3 is also from the Sacramento Basin and was produced from non-marine Pliocene strata at 430 m depth. The methane `513C and 6D values of -42.9%0 and - 161%oplot within the T field, suggesting that sample 1025 has a thermogenic origin. The concentration of C2+ hydrocarbons, however, is only i00 ppm, far less than expected for a thermogenic gas with such a negative `513C value (SCHOELL, 1983). By analogy with the previous example, sample 1025 may be a mixture of dry, post-mature thermogenic gas and microbial gas. It could also be a thermogenic gas which has been stripped of C_,+ hydrocarbons during migration (cf. "'Medesano'" gas, MATTAVELLIet al., 1983). Alternatively, sample 1025 could be a microbial gas which has undergone reservoir oxidation. Experimental studies indicate that both ,513C and `SD values become more positive during the oxidation of methane by aerobic bacteria (COLEMANe t al., 1981: BARKERand FRITZ, 1981). The extremely low concentrations of ethane and carbon dioxide make it difficult to determine which of these alternatives is correct.

Steady-state modelling of the Scripps Submarine Canyon gases indicates an intramolecular fractionation between acetate carboxyl carbon and methyl carbon of approximately 25%o. Based on this estimate, methane produced primarily by acetate dissimilation may commonly have `513C values more positive than -.55%o, well within the range for thermogenic gases. The methane `513C values around -40%o observed for the Middle America Trench gases appear to be uncommon for methane produced by carbon dioxide reduction and are associated with carbon dioxide `513Cvalues of up to +28%0. The results of this study suggest that unusually negative methane 6D values, or unusually positive carbon dioxide `513C values, may help to distinguish microbial gases with heavy methane 613C values from thermogenic gases with low C2+ concentrations. In some cases, ethane `5t3C measurements may be useful in recognizing commercial gases with mixed microbial and thermogenic origins. Acknowledgements--We wish to thank the Deep Sea Drilling Project for providing gas samples from Sites 568 and 570 in the Middle America Trench. G. Glaypool and C. Threlkeld kindly provided the data from which the chemical composition of the Middle America Trench gases were calculated. We are indebted to R. F. Dill for suggesting the Scripps Submarine Canyon study and to A. Rindell who provided samples for a period of over two years. L. Chu and M. Drut determined the chemical composition of gases from Scripps Submarine Canyon and prepared samples from Scripps Submarine Canyon and the Middle America Trench for stable isotope analysis. Support for this research was provided by the Gas Research Institute. Contract No. 5081-360-533. Editorial handling: J. Brooks

CONCLUSIONS

REFERENCES

Microbial gases from the Middle America Trench are generated primarily by carbon dioxide reduction and have methane `513Cvalues varying from -84%o to -39%o, methane `5D values varying from -208%o to - 145%o, and an average carbon isotope fractionation between carbon dioxide and methane of 70 _ 10%o. Gascs from Scripps Submarine Canyon are derived primarily from acetate dissimilation and have methanc `5'3C values ranging from -63%o to -43%0, methane bD values ranging from -331 to -280%o, and an average carbon isotope fractionation between carbon dioxide and mcthanc of 50 +_ 10%o. The methane bD values for the Middle America Trench gases and the Scripps Submarine Canyon gases appear to be a function of the isotopic composition of the interstitial water and of the relative proportions of mc;.hane produced by carbon dioxide reduction and acetate dissimilation. If interstitial water bD measurements are available, the fraction of methane derived from acetate dissimilation can be estimated from the methane `SD Value of any microbial gas.

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APPENDIX 1: EMPIRICAL MODEL FOR 8D VARIATIONS IN MICROBIAL M E T H A N E

The empirical model presented in Eqn (3) is adapted from the work of SCHOELL(1980) and WOLTEMATEet al. (1984). The model is based on the assumption that in natural systems, methanogenesis occurs either by carbon dioxide reduction or by acetate dissimilation and related pathways involving methyl group reduction. In the former case, 100% of the hydrogen in the methane is derived from the formation water (DANIELS et al., 1980). In the latter case, by analogy with acetate dissimilation, only 25 % of the methane hydrogen is derived from the formation water because three of the four methane hydrogen atoms are inherited from the methyl group (Pine and Barker, 1956). The derivation implicitly assumes that the hydrogen derived from the formation water is fractionated by a constant amount and that the isotopic concentration of the methyl groups is also roughly constant. By good approximation to mass balance, ~Dm = f6Dm~ + (1 - f)aDm,

(i)

where bD m is the isotopic composition of a microbial methane. 6Dma is the isotopic composition of the fraction f which is derived from acetate dissimilation and related pathways, and bDmc is the isotopic composition of the fraction 1 - fwhich is derived from carbon dioxide reduction. If bD,, is the isotopic composition of the formation water, then from the empirical work of SCHOELL(1980), the isotopic composition of methane produced by carbon dioxide reduction is given by bDm~ = 8D,,. - 160.

(ii)

Also from SCHOELL (1980), the isotopic composition of methane produced during the fermentation of sewage sludge is given by bD m = 0.46D,, - 323.

(iii)

Now studies indicate that approximately 70% of the

Gases from the Middle America Trench and Scripps Submarine Canyon: implications for natural gas origin m e t h a n e produced during sewage sludge digestion is derived from acetate (JERIS and McCARrV, 1965: SMITH and MAn. 1966). We can therefore substitute Eqns (ii) and (iii) into E q n (i). s e r f = 0.7, and solve for bD,,~, to get 6 D ~ = 0.143bD~ - 393.

(iv)

Equations (ii) and (iv) carl be substituted into E q n (i) to give bDm for any microbial gas as a function o f f and bD,, : bD m = (1 - 0.857fJ,bD~ - 233f - 160.

tive values o f A , B, C. and D are calculated by solving Eqns (vi)-(viii) in terms o f x and y to give: A / C = 2.

(v)

APPENDIX 2: STEADY-STATE MODEL FOR THE CARBON ISOTOPE COMPOSITION OF SCRIPPS SUBMARINE CANYON GASES

x = (A + B-

C-

D)/(B + C).

(vi)

If we assume that the gross ratio of carbon dioxide to m e t h a n e produced during fermentation is equal to the theoretical value of 1 (LAZER3"E. 1981), then we also have the relation (A + B -

C ) / ( B + C) = 1.

(ix)

Ro, R¢, and R m are defined to be the ~3C : ~zC ratios of the organic matter, carbon dioxide, and m e t h a n e : b ~3C,,, 613 C~,. and bl3C m are defined to be the corresponding ratios (expressed in b notation) relative to the PDB standard. 6t3C~ and b ~3Cm are measured quantities, t)t3C,, can be calculated from the model using data in Table 2. The ratio ~3C : ~-~Cin the carbon dioxide produced at any m o m e n t in time is given by the mass balance approximation

(vii) Re =-

In addition, we define .x'to be the ratio of m e t h a n e produced by acetate dissimilation to m e t h a n e produced by carbon dioxide reduction, so that v=

+y).

ct,, = Isotopic fractionation between carbon dioxide produced by anaerobic oxidation and organic matter in the sediments. After LAZERTE (1981). we take ct;, = 1.0(10. ctb = Isotopic fractionation factor between acetate carboxyl carbon and total acetate carbon. This quantity is not well-characterized in the literature and can be calculated from the model using data in Table 2. (z h is related to ctb'. the fractionation between acetate carboxyl carbon and methyl carbon, by Eqn (xv) below. a c = Instantaneous fractionation factor between m e t h a n e and carbon dioxide during carbon dioxide reduction. After CLAYPOOLand KAPLAN ( 19741 and CLAYPOOL and THRELKELD (1983), we take ctc = 0.93 (1/ct~ = 1.07). ct d = Equilibrium fractionation factor between dissolved bicarbonate and carbon dioxide gas. After FRIEDMAN and O'NEIL ( 19771, we take a u = 1.0(19. corresponding to a temperature of 15°C.

(3)

The model presented in this section is adapted from the approach of LAZERtE (1981) and is outlined in Fig. 8. Figure 8 shows that the isotopic compositions of carbon dioxide and m e t h a n e are a function of the rates of anaerobic oxidation of organic matter (A). acetate dissimilation (B), carbon dioxide reduction (C). and carbon dioxide dissolution as bicarbonate (D). Conservation of mass indicates that the net a m o u n t of carbon dioxide produced at any m o m e n t is equal to A + B - C - D and that the a m o u n t of m e t h a n e is equal to B + C. The net ratio of carbon dioxide to m e t h a n e , which we define as x, is therefore given by

B / C = s , and D / C = (1 - x ) ( 1

Knowledge of the isotopic fractionations encountered during processes A, B. C. and D is also required by the model. The effective fractionation factors for these processes are defined as follows.

R e a r r a n g e m e n t of Eqn (v) yields the desired result: f = (6D,, - c~Dm - 160)/(0.857bD~ + 233).

645

(viii)

B/C.

The value o f x can be calculated from the carbon dioxide and m e t h a n e concentrations listed in Table 2. The value o f y is equal to]7( 1 - f ) where f i s the fraction of m e t h a n e derived from acetate dissimilation, also listed in Table 2. T h e rcla-

(ARoa,, + BR,,cth - CR~c~, - DRcctd) (A+B-C-D)

(x)

where we have assumed that the isotopic composition of the acetate is approximately equal to the isotopic composition of the organic matter. The first two terms in the n u m e r a t o r reflect the addition of carbon dioxide from anaerobic oxidation of organic matter and acetate dissimilation, and the second two terms reflect the loss of carbon dioxide by reduction to m e t h a n e dissolution. After dividing the

Table 4. Calculations based on steady-state modelling of the carbon isotope distribution of the Scripps Submarine Canyon gases Input data

O u t p u t data

Sample No.

.~+:

v~'

10110 -~- blZCm~ I(X~I + bl+C~

bl3Cc§ (%0)

ab'l]

(~lZC,,~ (%0)

899 9till 9ll3 9(/5 10411 1208

0.265 0.216 0.214 0.391 0.305 11.160

1.84 1.68 1.77 1.53 1.06 1.68

(I.954 (I.955 11.953 0.954 0.95(I 0.953

2.4 1.2 2.1 1.5 3.1 0.8

1.027 1.023 1.028 1.021 1.019 1.028

- 17.2 -17.8 -17.8 - 18.9 -18.9 - 19.0

:~x = mole ratio of C O , to CH~. + ~' = ratio of C H , derived from acetate dissimilation to CH4 derived from carbon dioxide reduction. -~ ( I~C/IzC)c,n/( l~C/I'~C)¢,o . § Carbon isotope ratio of ca-rbon dioxide relative to the PDB standard. Hlntramolecular isotope fractionation factor between acetate carboxyl carbon and methyl carbon. Calculated from Eqns (xiv) and (xv), Appendix 2. using +~c = (/.930 and aa = 1.009. ¶:, Carbon isotope ratio of organic matter in Scripps Canyon sediments. Calculated from Eqn (xii), Appendix 2, using a¢ = 0.930 and a , = 1.009.

646

P . D . Jenden and I. R. Kaplan

numerator and and denominator by C, substituting Eqns (ix) for A/C, B/C, and D/C. and setting a~ = 1. Eqn (x) can be rearranged to give

R,, =

[x(1 + y) + a~ + (1 - x)(l + y) ao] [2 + yah]

(1000 + 61)Cm) ] ,_ x:. + < . + l l (lb =

- xly,

.,

.-----

+

l

{1000 A- ~}13Cm)]

Rc (xi) (xiv)

or, in customary d notation, d.3C., = [x(l + y) + ct,, + (1 - x)(1 + y)ad] [2 + yah] X (1000 + dt3Cc) - 1000.

(xii)

Equation (xii) shows that if the fractionation factors ah. cq,. and aa are known, then the isotopic composition of the organic matter can be calculated from the measured ratio of carbon dioxide to methane, the estimated proportion of methane produced by acetate dissimilation, and the measured d 13C value of the carbon dioxide. To derive an expression for a h, we begin with the mass balance approximation for the I3C : IzC ratio of the methane produced at any moment in time which is given by R~ = [B(2 - ah)Ro + Ca~R~]/[B + C].

(xiii)

The first term in the numerator reflects the methane contributed by acetate dissimilation, where ( 2 - cth) is the isotopic fractionation factor between acetate methyl carbon and total acetate carbon. The second term in the numerator reflects the methane contributed by carbon dioxide reduction. After dividing the numerator and denominator by C and substituting Eqn (ix) for B/C, Eqn (xi) for Ro, and the expression (1000 + 613C,,)/(1000 + d13C~) for Rm/R~. Eqn (xiii) can be rearranged to give

Equation (xiv) shows that if the fractionation factors a,, and ad are known, then the intramolecular fractionation factor for acetate can be calculated from the measured ratio of carbon dioxide to methane, the estimated proportion of methane derived from acetate dissimilation, and the isotopic composition of the methane and carbon dioxide. ab', the fractionation factor between acetate carboxyl carbon and acetate methyl carbon, can readily be calculated from ah using the equation ~b' = a~,/(2 -- ah).

(XV)

We have not applied Eqns (xii), (xiv), and (xv) to Scripps Submarine Canyon gases with 6 ~3C,. < 0%. This is because of the possible presence of isotopically-negative carbon dioxide produced by microbial respiration. Figure 6 suggests that the 6t3C value of carbon dioxide derived solely from fermentation reactions may fall between 0% and 3%0. Model calculations for samples with carbon dioxide bBC within this range are presented in Table 4. Calculated fractionation factors between acetate carboxyl carbon and methyl carbon vary from 1.019 to 1.028, in reasonable agreement with the few data reported in the literature (MEINSCHEINe t al.. 1974: LAZERTE, 1981). 613C values for Scripps Submarine Canyon organic matter vary from - 19.0%o to - 17.2%o, well within the actual range of values measured in this study.