Thermochemical Sulphate Reduction in Cambro±Ordovician Carbonates in Central Tarim

Marine and Petroleum Geology 18 02001) 729±741 www.elsevier.com/locate/marpetgeo

Thermochemical sulphate reduction in Cambro±Ordovician carbonates in Central Tarim

Chunfang Caia,*, Wangshui Hub, Richard H. Wordenc

aInstitute of Geology and Geophysics, CAS, P.O. Box 9825 Beijing 100029, People's Republic of China bJianghan Petroleum Institute, Jingzhou, Hubei 434102, People's Republic of China cJane Herdman Laboratories, Department of Earth Sciences, University of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP, UK Received 3 December 1999; received in revised form 24 May 2001; accepted 29 May 2001

Abstract

H2S and CO2 are found in elevated concentrations in Palaeozoic reservoirs in the Tarim Basin in China. We have carried out analyses on gas, petroleum, mineral cement and bulk rock compositions and isotope ratios together with petrography and ¯uid inclusion to assess the origin of the H2S. A deep crustal 0e.g. volcanic) origin of the H2S and CO2 is unlikely since the inert gases, N2 and He, have isotope ratios totally uncharacteristic of this source. Organic sources are also unlikely since the source rock has low a sulphur content and the sulphur isotope ratio of the petroleum correlates positively with the sulphur content, the opposite of what would be anticipated from petroleum- derived H2S. Bacterial sulphate reduction is unlikely because temperatures are too high for bacteria to have survived. Thermochemical sulphate reduction of petroleum ¯uids by anhydrite in Lower Ordovician and Cambrian carbonate reservoirs is the most likely source of both the H2S and the CO2 causing isotopically characteristic pyrite, CO2 gas and calcite cement. H2S, and possibly CO2, migrated into Silurian sandstone reservoirs by cross formational ¯ow. The H2S, with the same sulphur isotope ratio as Ordovician anhydrite, was partially lost from the ¯uid phase by extensive growth of late diagenetic pyrite. Similarly the CO2 was partially lost from the ¯uid phase by precipitation of late diagenetic calcite. The H2S that resulted from TSR underwent reaction with the remaining petroleum resulting in locally elevated organic sulphur concentrations in the petroleum and the progressive adoption of the Ordovician anhydrite sulphur isotope ratio. q 2001Elsevier Science Ltd. All rights reserved.

Keywords:H2S; CO2; Thermochemical sulphate reduction; Organic sulphur; Pyrite; Tarim basin

1. Introduction Couloume, & Magot, 1996), petroleum 0Sassen, 1988) and bituminous tar 0Powell & MacQueen, 1984) become altered

Elevated concentrations of dissolved H2S exist in oil®eld during TSR. Heydari 01997) and Heydari and Moore 01989) waters in Ordovician carbonate and Silurian sandstone demonstrated the effects of TSR on burial diagenesis and petroleum reservoirs in Central Tarim, Tarim Basin, China porosity evolution. Integrated approaches to distinguish 0Cai & Hu, 1997). The origin, spatial distribution and rela- TSR from bacterial sulphate reduction 0BSR) were tionship of the H2S with mineral diagenesis, porosity evolu- suggested by Connan et al. 01996), Machel, Krouse, and tion and hydrocarbon alteration are of importance to Sassen 01995) and Worden et al. 01995). The lowest petroleum exploration and development. Previous work temperature at which TSR can occur is a matter of debate, has shown that high concentrations of H2S in petroleum but it is now clear that the kinetics of TSR depend upon a ¯uids may be generated by thermochemical sulphate reduc- variety of factors including petroleum type, rock fabric and tion 0FTSR; e.g. Orr, 1977; Worden, Smalley, & Oxtoby, the amounts of pre-TSR water and reduced sulphur species 1995). Correspondingly, chemistry and the isotopic compo- 0Worden, Smalley, & Cross, 2000). Most recent studies sitions of light hydrocarbon gases 0Connan, Lacrampe- have paid much more attention to the origin of cements

and the H2S gas than to the related CO2 gas. It has been suggested that CO2 may be one of the products of TSR as * Corresponding author. Present address: Jane Herdman Laboratories, 13 shown by CO2 that increasingly adopts reduced d C values Department of Earth Sciences, University of Liverpool, 4 Brownlow Street, as TSR proceeds, and the CO concentration in the gas phase Liverpool, L69 3GP, UK. Tel.: 144-0151-794-5200; fax: 144-0151-794- 2 5170. seems to increase in some TSR systems 0Krouse, Viau, E-mail address: [email protected] 0C. Cai). Eliuk, Ueda, & Halas, 1988; Worden & Smalley, 1996).

Fig. 1. Map showing geological tectonics and location of major petroleum exploration wells 0a) and West±East cross section AB of Central Tarim 0b).

Elevated concentrations of dissolved H2S in oil®eld Tarim Basin, northwest of China 0Fig. 1a). It contains a waters in the Tarim Basin have been reported and it has large, NW trending reverse fault with a throw of 2200 m been suggested that they might result from TSR 0Cai & developed during the Caledonian orogeny and crossing the Hu, 1997). Based upon biomarker parameters, Zhang et al. Ordovician and Silurian strata. An E±W cross section is 02000) and Hanson, Zhang, Moldowan, Liang, and Zhang shown in Fig. 1b. The faults and fractures are thought to 02000) concluded that Central Tarim Paleozoic oils were be the main conduit for petroleum migration. The deepest derived from the Middle and Upper Ordovician source exploration well 0Tc1) in Central Tarim reached a depth of rock. Xiao, Song, Liu, Liu, and Fu 02000) demonstrated more than 7200 m and penetrated a complete section of vertical secondary migration of oil and natural gases from the Cambrian. Cambrian strata do not outcrop and are Lower Palaeozoic source rock into overlying reservoirs considered to represent broadly continuous sedimenta- in the Tarim Basin. Cai, Franks, and Aagaard 02001a) tion. The Cambrian section is composed of tidal, plat- suggested that Ordovician oil®eld waters have migrated form and platform-marginal carbonate and evaporate up to Carboniferous and Silurian reservoirs based on rocks. The Middle Cambrian is a suite of supratidal water geochemistry and 87Sr/86Sr ratios. This paper presents anhydrite-bearing dolomite interbedded with dolomitic new data from cement, oil, gas and water samples, and anhydrite or dolomite with thin anhydrite interlayers. attempts to account for the origin of H2S and CO2 and to According to seismic data, anhydrite and salt beds prove the occurrence of TSR and in the Tarim Basin, north- extendnorthtotheBachuUplift0Fig.1a)andtothe west China. west of North Tarim with an area of 200,000 km2 and a thickness of 400±1400 m. The burial and geothermal history of Central Tarim shows that rapid sedimentation 2. Geological setting took place at the phase of the passive continental margin during the middle Cambrian to early Ordovician. Central Tarim is located in the center of Tazhong Uplift, Present-day and maximum temperatures of Cambrian C. Cai et al. / Marine and Petroleum Geology 18 92001) 729±741 731 in Cambrian, Lower Ordovician and Lower Carboniferous strata but not in Silurian rocks.

3. Sample collection and analysis

Up to 150 mg samples of disaggregated sandstone were reacted overnight with 100% phosphoric acid at 258C under

vacuum to release CO2 from calcite. The sample was then allowed to continue to react for 15 h at 758C to release CO2 from dolomite. Both calcite and dolomite sources of CO2 were analyzed for d13C on a Finnigan MAT251mass spectrometer standardized with NBS-18. Calcite cements in Ordovician limestone are present as vug-®lling, coarse and white crystals and contain numerous ¯uid inclusion. These cements were extracted from core using a dentist's drill and subject to stable isotope analysis using the same mass spectrometer as the bulk dissolved Fig. 2. Diagram showing a burial history constructed from well Tc1. Isotherms are constrained by ®ssion track data and vitrinite re¯ectance samples. All isotope data are reported relative to the PDB measurements. About 125±1758C occur in the Lower Ordovician and standard with a precision of ^0.1½. Cambrian. The Central Tarim oil®eld water samples that contain

dissolved H2S from Ordovician carbonate and Silurian sandstone reservoirs were collected in 500 ml glass jars contain- and lower Ordovician strata range from 125 to 1758C ing excess cadmium acetate 01.5 g) to precipitate dissolved based on bottom hole temperatures and thermal history sulphide as CdS. The SO2 gas for S isotope analyses was analysis 0Fig. 2: Zhang et al., 1999). The burial history produced by combustion of a mixture of sulphide and Cu2O was re-built by running ThermMod software using the in a 1:10 at 11008C under vacuum. SO2 gas was chemical kinetic model of Burnham and Sweeney collected in a sample tube by freezing. To transform

01989). sulphate to SO2 gas, barium chloride reagent was The Ordovician carbonate rocks show deepening- added to precipitate sulphate as BaSO4. The BaSO4 was upward and then shoaling-upward sequences from mixed with V2O5 and SiO2 in a proportion of 1:3.5:3.5, bioclastic grainstone, packstone, and mudstone to sand- and the mixture was placed in porcelain bottle and covered stone. The Silurian sandstone sequence consists of tidal with a layer of copper wires. Sulphur isotope ratios were sandstone with extensive bitumen occurrence, and measured on a MAT 251mass spectrometer and results are brown or red mudstone. Carboniferous sandstone strata reported in the standard d notation relative to Canyon consist of marine clastics while the Mesozoic and Ceno- Diablo troilite 0CDT). Reproducibility for d34Sis^0.1± zoic are mainly composed of terrestrial sandstones and 0.3½. 3He/4He and d15N were measured at VG-5400 and mudstones0Caietal.,2001a). MAT271mass spectrometer, respectively. The reproduc- Table 1summarizes the source rocks, petroleum reser- ibilities of the 3He/4He and d15N are ^0.6% and ^1½, voirs, and the occurrence of various sulphur compounds in respectively. the Central Tarim Basin. Sulphate minerals were only found Microthermometry of ¯uid inclusions in the vuggy

Table 1 Main petrology, petroleum source and reservoir, and sulphur species in the sampled strata

Strata Main petrology Genetic relation Sulphur species

Carboniferous Sandstone and mudstone with Petroleum reservoir Plenty of anhydrite, but little

anhydrite-bearing dolostone pyrite and low H2S Silurian Sandstone with extensive Petroleum reservoir No anhydrite, but widespread

bitumen and mudstone pyrite, and high dissolved H2Sin oil®eld water Middle and Upper Ordovician Mudstone, shale and limestone Petroleum source rock and local No anhydrite, little pyrite and no

reservoir H2S Cambrian and Lower Ordovician Dolostone, shale, limestone and Petroleum source and reservoir Plenty of anhydrite, pyrite and

anhydrite beds high dissolved H2S in oil®eld water 732 C. Cai et al. / Marine and Petroleum Geology 18 92001) 729±741

Table 2

Chemistry and isotopic composition of natural gases and H2S concentration in oil®eld water

Well Age Deptha 0m) Natural gas Co-produced water

mCO b mN b mCH b d13C d15N d13C H S 0ppm) 2 2 4 co2 CH4 2

Tz101 CIII 3730 0.65 6.47 89.24 211.57 10.22 246.3 ± Tz11 S 4425 0.15 7.11 87.01 220.04 1.63 ± ± Tz12 S 4382 2.80 7.06 88.88 ± ± ± 600

Tz12 S 4412 1.61 6.83 90.69 ± ± ± Intense smell of H2S Tz12 O 4707 1.25 2.15 95.45 210.94 4.23 242.8 780

Tz16 CIII 3800 1.21 49.06 44.88 218.65 2.53 ± ± c Tz161 S 4180 2.33 2.96 93.13 ± ± 241.8 Intense smell of H2S Tz161c O 4293 0.77 18.97 71.88 ± ± 241.8 ± Tz162c O 5059 3.55 19.97 72.06 ± ± ± 986 Tz162 O 5991 7.84d 0.6188.72 ± ± ± 37500 d Tz30 O 5011 15.51 1.51 82.25 1.21 3.51 244.9 ± Tz4 C 3632 1.42 21.55 75.29 213.6 ± 243.2 ± c TZ401 CIII 3685 0.25 14.08 81.43 ± ± 243.3 ± c Tz421 CI 3259 0.15 19.02 77.63 217.59 4.04 ± ± Tz421C II 3486 0.17 13.89 82.17 ± 3.89 ± ± Tz421C III 3573 0.19 11.59 80.6 28.45 2.42 ± ± c Tz422 CII 3544 0.99 39.42 55.56 220.88 1.05 ± ± Tz44 O112 4827 1.95 3.42 92.91 29.57 2.97 ± 1150 Tz44 O 48714.42 3.93 90.28 ± ± ± 376

Tz6 CIII 3726 0.77 6.26 91.51 212.01 2.38 242.3 ± Tc1O 1 6004 ± ± ± ± ± ± Intense smell of H2S Tz162 O122 5984 ± ± ± ± ± ± 684 Tz43 O 5410 ± ± ± ± ± ± 1175

Tz43 O 5697 ± ± ± ± ± ± Intense smell of H2S Tz49 O 6196 ± ± ± ± ± ± Intense smell of H2S

a Depth is set as the middle point between perforations in meters. b Gas composition in mol%. c Wells Tz161, 162 are close to wells Tz16. Tz421, 422, Tz401 are close to well Tz4. d After acidi®cation. calcite cement in the Ordovician limestones was carried Basic oil ®eld data and sulphur contents of petroleum and out using a Linkam THM600 heating±cooling stage bitumen were collected from annually published reports on with a precision of 0.18C. Measurements were made in production from the Tarim Basin. Two bitumen extracted strictly increasing temperature order so as to minimize from sandstone were measured for sulphur content in Leeds inclusion re-equilibration. University.

13 Fig. 3. Variations of the concentration of H2S dissolved in oil®eld waters, CO2 molar volume and d C value in natural gas vs. depth for wells from the Central 13 Tarim. Note that high CO2 values occur at the depths of 4050±5100 m, and that most of the CO2 have d C values less than 210½, indicating an organic origin. C. Cai et al. / Marine and Petroleum Geology 18 92001) 729±741 733

values. Conversely water samples with low H2S concentrations tend to have less associated gas 0low gas-oil ratios).

Of 96 analysed gas samples, about 60% have a CO2 concentration less than 1.2 mol% 0Fig. 3, Table 2). There

is no simple correlative relationship between CO2 and H2S concentrations. Another characteristic of Central Tarim

natural gas accumulations is that CO2 concentration does not increase with depth. The highest CO2 concentrations occur between depths of 4050 and 5100 m 0Fig. 3). Addi- 13 tionally, the CO2 d C values range from 28.5 to 220.9½ 13 Fig. 4. Relationship between d34S and sulphur content in petroleum and except for one sample which has a d C value of 11.2½ bitumen from Ordovician, Silurian and Carboniferous. No signi®cant d34S 0Table 2). The relative volume of N2 in Paleozoic gas difference among the formation, and the sulphur-enriched samples have compositions is high, up to 57 vol% 049 mol%) but with a 34 15 elevated d S values, indicating the same source of H2S has been incorpo- wide range 0Table 2). The d N varies over a small range rated into the organic matter. 011.0± 1 4.2½; Table 2). The 3He/4He ratio of four gas samples ranges from 4.0 to 4.6 £ 1028. Ten petroleum samples and bitumen from Carboni- 4. Results ferous, Silurian and Ordovician reservoirs have sulphur contents that vary from 0.015 to 1.7%. The petroleum 4.1. Reservoir ¯uid chemistry and isotopic composition and bitumen samples with the highest sulphur contents occur mainly in Silurian reservoirs. The oils and bitu- Oil®eld waters range from being colorless, through to men have d34Svaluesfrom113.6 to 126.5½ with a being yellow, or even dark brown. Most of the dark water mean value of 121.3 ^ 3.9½ n 10: There is a good samples are characterized by elevated dissolved H2S correlation between the sulphur content of the petro- concentrations, ranging up to 1175 ppm 0Table 2), whilst leum and the sulphur isotope ratio. Those samples colorless and pale yellow water samples contain little H2S. with elevated sulphur contents tend to have elevated 34 The water samples with the highest H2S concentrations are d S values 0Fig. 4). There is no signi®cant pattern associated with petroleum ®elds that have a gas cap or at between the age of the reservoir and the sulphur isotope 13 least have high gas-oil ratios and have CO2 with low d C ratio.

Fig. 5. Synthetic paragenetic sequences showing main stages in the diagenetic evolution of, 0a) Cambrian and Ordovician carbonates 0modi®ed after Chen, Sheng, Wang and Zhu 01994) and Ye 01994)), and 0b) Silurian sandstones 0modi®ed after Cai et al., 2001b). 734 C. Cai et al. / Marine and Petroleum Geology 18 92001) 729±741

Fig. 6. Photomicrographs showing calcite cement occluding in vug of Ordovician grainstone of well Tz54 under plane light 0a) and under cathodoluminesence 0CL) 0b), 0scale bar 0.2 mm). The feature suggests that cementation took place during at least two periods, and that late-stage calcite characterized by orange or dark orange CL has low Fe/Mn ratios.

4.2. Paragenetic sequences, diagenetic minerals and well Tz54, and in anhydritic dolomite at 5800 m in well mineral isotopic ratios Tz1. The initial depths for replacement of anhydrite and calcite correspond to more than 1258C according to data 4.2.1. Cambrian and Ordovician limestones from either bottom hole temperatures or from calculated The main diagenetic events in the Cambrian and Ordovi- geothermal gradients 0Fig. 2). cian grainstones during burial include gypsum dehydration Drilled out calcite cements from Ordovician limestone to form anhydrite, early diagenetic calcite cementation, have d13C values from 26.0 to 29.4½. The d13Cof18 saddle dolomite cementation, late calcite cementation and bulk limestone samples ranges from 10.6 to 12.9½, anhydrite replacement by pyrite and calcite 0Fig. 5a). Saddle averaging 11.8 ^ 0.7½ 0Fig. 8). Therefore the d13Cof dolomite was found to occur in Lower Ordovician limestone the cement in the limestones is much lighter than d13Cof in well Tz2 and in well Tc1in Cambrian dolomites and the bulk limestone. Ordovician and Cambrian anhydrite coexists with bitumen and heavy petroleum. More than from the Bachu Uplift, to the west of Central Tarim, have two generations of calcite with different crystal shapes d34S values of 126.1and 133.7½, respectively, which are were identi®ed by cathodoluminesence, suggesting that close to the contemporary seawater sulphate d34S values calcite cementation took place during at least two episodes reported by Claypool, Holser, Kaplan, Sakai, and Zak 0Fig. 6a and b). Late stage calcite cement has orange and 01980). dark orange cathodoluminescence, indicating that the pore Late stage euhedral authigenic pyrite was found in small ¯uid had a relatively low Fe/Mn ratio. quantities in the Cambrian and Ordovician limestones. Bitu- Calcite and pyrite have, at some localities, completely men was found within vugs 0intercrystalline dissolution replaced anhydrite in the Middle Cambrian and Lower pores) after biosparite between 5757 and 5765 m in well Ordovician gypsum-bearing carbonate rock or anhydrite Tz54, in dolomitized limestone in 5978.48 m in well beds. Calcite and pyrite occur as pseudomorphs after anhy- Tz162, at .5800 m in dolomite of well Tz1, and in dissolu- drite in Ordovician micritic limestone at 4720 and 4723 m tion pores along stylolites and fractures in the Cambrian and in well Tz12 0Fig. 7), in bioclastic grainstone at 5944 m in Ordovician 0Ye, 1994).

Fig. 7. Photomicrographs showing calci®cation of anhydrite in Lower Ordovician limestone 0scale bar 0.2 mm): 0a) TSR calcite occupying pre-anhydrite location under plane light; 0b) anhydrite original con®guration under CL. C. Cai et al. / Marine and Petroleum Geology 18 92001) 729±741 735

Table 3 d34S values of authigenic pyrite and anhydrite

Well Age Depth 0m) Mineral d34S

Tz12 S 4378 FeS2 34 Tz12 S 4384 FeS2 34 Tz12 S 4388 FeS2 20 Tz11 S 4390 FeS2 9.5 Tz11 S 4403 FeS2 25 Tz11 S 4450 FeS2 13 Tz11 S 4432 FeS2 20 Tz161 O213 4243 FeS2 18 Ma401O 1 2042 CaSO4 26 Fang 1 [ 4602 CaSO4 34

from heavy oils 0Cai et al., 1997; Cai, Gu, & Cai, 2001b; Zhang et al., 1999). The heavy oils were residues from the biodegradation of early emplaced petroleum 0Xiao, Zhang, Fig. 8. Distribution of d13C of Ordovician bulk limestone and carbonate cements of Silurian sandstones and Ordovician limestone. Note that much Zhao, & Zheng et al., 1997; Zhang et al., 1999) when the lighter carbon occurs in cements than in bulk rock, indicating an organic- Silurian was uplifted and exposed to the surface and in®l- derived carbon. trated by paleometeoric water 0Cai et al., 2001a).

4.2.2. Silurian sandstones 4.3. Fluid inclusions in late calcite in Cambrian and The paragenetic sequence in the Silurian sandstones Ordovician limestones 0Fig. 5b) shows that there is no evidence for the existence of anhydrite 0or other sulphate) cements in these sandstones. 105 ¯uid inclusions in calcite cement in Cambrian and The paragenesis is dominated by early diagenetic calcite, Ordovician limestone have a varied size of 1.5±60 mm, but quartz cement and late stage ferroan dolomite, as well as are mostly smaller than 5 mm. The inclusions occur as minor kaolinite cement and albitisation of K-feldspar. The single inclusions or in planar groups. All of the ¯uid inclu- later diagenetic carbonate cements in the Silurian sand- sions are two-phase liquid-gas inclusions, with gas/liquid 13 stones are characterized by depleted d C values ranging ratios from 2 to 40. They can be categorized as primary from 23.8 to 221.5½ with an average of 29.9 ^ 4.8½. and secondary. The primary inclusions occur as a single The last diagenetic mineral to form was pyrite which occurs inclusion in an otherwise inclusion-free crystal, or isolated as large, euhedral crystals 0up to 2 mm, Fig. 9). The cubic habit of pyrite 0and absence of framboidal pyrite) may indi- cate a non-bacterial origin at a relatively slow crystal growth rate during burial diagenesis at relatively high temperatures. Pyrite replaced detrital grains and calcite cement and represents up to 3% rock volume. The pyrite has a range of d34S values from 117.7 to 134.4½ with an average of 125.2 ^ 7.4½ n 6 0Table 3, Fig. 10). Solid bitumen was found within the intergranular porosity in Silurian sandstones, and was considered to precipitate during late-stage diagenesis due to an in¯ux of gas, increasing the gas oil ratio and leading to asphaltene exsolution

Fig. 10. Distribution of d34S of anhydrite, pyrite and petroleum and bitumen from the Cambrian±Ordovician, Silurian and Carboniferous. No signi®cant Fig. 9. Photograph showing cubic pyrite in bedding plane of Silurian sand- difference among the values, indicating the sulphur in the pyrite and organic stones. The pyrite has sizes up to 3 mm. matter could have been derived from Cambrian±Ordovician anhydrite. 736 C. Cai et al. / Marine and Petroleum Geology 18 92001) 729±741 from bacterial and thermal degradation of organic matter, decomposition of carbonates, and degassing of the mantle and atmosphere 0e.g. Connan et al., 1996; Dai et al., 1989, 1996; Wycherley, Fleet, & Shaw, 1999). The d13C values of 2 HCO3 0aq) from dissolved atmospheric CO20g) in subsurface shallow waters are close to 0½ 0Carothers & Kharaka,

1980). Atmospheric CO2 exerts an in¯uence upon carbonate precipitation within the zone of meteoric water activity during early diagenesis and close to unconformities or fault zones. Consequently, meteoric water carrying atmospheric

CO2 is unlikely to be a source of the CO2 in the Tarim Basin reservoirs. 13 d C values of mantle-derived CO2 are in the range from 24to27½ 0Dai et al., 1989, 1996; Thrasher & Fleet,

Fig. 11. Histogram showing homogenization temperatures in ¯uid inclu- 1995). If CO2 is mainly mantle-derived, then the gas should sions in the cements in the Cambrian and Ordovician carbonate rock with also have entrained He and N2 from the same 0mantle) two peak values of 75±998C and 125±1508C. source. The isotopes of He and N2 can be used to de®ne their origin 0mantle, atmospheric, etc.) so that the isotopes away from other inclusions. Primary inclusions have a of these gases can also be used to de®ne the source of bimodal distribution of gas/liquid ratios with the two ranges CO2 0e.g. Dai et al., 1996; Sherwood-Lollar, Ballentine, falling between 4 and 8, and 15 and 40. The homogenization & O'Nions, 1997; Wycherley et al., 1999; Zhu, Shi, & temperatures of the primary inclusions in calcite cement in Fang, 2000). the interval of 5074±5079 m of the Cambrian in well Tz1 3He/4He ratio ranges in Central Tarim from 4.0 to can be divided into three groups: 80±908C, 140±1508C and 4.6 £ 1028. This ratio is much lower than both the atmos- up to 1808C. Calcite cement homogenization temperatures pheric helium isotope ratio 01.4 £ 1026) and the mantle- in the Ordovician are 75 ±1008C and 112±1498C in Tz24, derived helium isotope ratio 03 £ 1025) 0Mamyrin & and 135±1548C in Tz25. A histogram of homogenization Tolstikhin, 1984). The helium isotope ratio may, instead, temperatures shows that about 22% data 0the second peak originate from a-decay of radioactive elements in organic value) have a temperature range of 125±1508C 0Fig. 11). matter 0Xu et al., 1998) or the crystalline basement 0Hiya- The temperature is close to present-day reservoir tempera- gon & Kennedy, 1992). ture 0125±1758C) so that resetting of homogenization The N2 concentrations of gases in the Tarim Basin reach temperatures, which may occur in calcite, is considered to 57 vol%. This is thought to be typical for N2 generated from have been negligible. Twenty two percent of the calcite sedimentary organic matter at high temperatures in sedi- cement growth therefore is interpreted to have occurred at mentary basins 0e.g. Littke, Krooss, Idiz, & Frielingsdorf, temperatures greater than 1258C. 1995; Zhu et al., 2000). The d15N values from 11.0 to 14.2½ and are comfortably within the range for N2 derived from sedimentary organic matter 023to113½; Rigby & 5. Discussion Batts, 1986). Together the N concentrations, the d15N value and the The association of relatively high molar volumes of CO 2 2 3He/4He ratio, supported by the relatively low d13C values in gases and high concentrations of H S in associated waters 2 of CO , suggest little contribution from mantle-derived suggest that they may be genetically related. However, there 2 sources to the natural gases. Instead, the inert gas data is no simple linear correlation between the two. The two suggest that the N and He had a predominantly organic gases are not inert to water-rock systems so that the lack of a 2 source. In conclusion, the mantle 0or any other very deep simple relationship may be due to different degrees of loss earth source) is discarded as a possible source of the CO . of H S and CO from the ¯uid phases 0due, for example, to 2 2 2 Detrital limestone can be an important source of gas precipitation as carbonate and sulphide cements). phase CO2 and carbonate in later minerals cements. Ordo- vician limestone has an average bulk rock d13C value of 5.1. Origin of gas phase CO2 and carbonate cements 11.8½. If CO2 was mainly produced from the limestone- Various criteria have been proposed to account for the water interaction 0as suggested by, for example, Coudrain- 13 generation of CO2 in oil®elds in the eastern China and the Ribstein, Gouze, & Marsily, 1998), then the d C value of South China Sea 0e.g. Dai, Ji, & Hao, 1989; Dai, Song, Dai, CO2 would be expected to be close to that of limestone since & Wang, 1996). It is thus possible to account for the origin there is minimal fractionation for the transformation of of CO2 in the Tarim Basin where recent studies on petro- carbonate to CO2 at high temperatures. The gas phase CO2 leum source rock and ¯uid ¯ow of Palaeozoic reservoirs has a range of d13C values but they are almost exclusively have been performed. In general, CO2 may be derived much less than 11.8½. Thus, it seems to be very unlikely C. Cai et al. / Marine and Petroleum Geology 18 92001) 729±741 737

13 that CO2 0as well as calcite cement with the light d C bottom-bole temperatures and are possibly indicative of values), could be derived solely from inorganic minerals. upward migration of relatively hot, CO2-bearing gas or Moreover, Coudrain-Ribstein et al. 01998) and Smith and oil®eld water. This vertical migration might have released

Ehrenberg 01989) have suggested that partial pressures of CO2 due to the consequent decrease in temperature and CO2 0Pco2, where Pco2 CO2 mol% £ subsurface pressure) pressure. Cross formational ¯ow 0Worden & Matray, may be controlled by chemical equilibration between carbo- 1995), upward migration and mixing of ¯uids are supported nate minerals and associated water. Over the temperature by water chemistry and 87Sr/86Sr ratios 0Cai et al., 2001a). range from 10 to 2008C, Pco2 values in some basins have been shown to increase with increasing depth of burial and 5.3. Mechanism of H S generation in the Tarim Basin thus temperature. However, in this study of the Tarim Basin, 2 such a relationship does not feature. Pco2 and CO2 molar Three mechanisms for H2S generation have been volumes do not increase with increasing depth of burial. proposed: 01) thermal decomposition of organic sulphur- Note that at depths greater than 5100 m, Pco2 values are containing petroleum and kerogen; 02) bacterial sulphate less than those between 4050 and 5100 m. That is, Pco2 reduction 0BSR) and 03) thermochemical sulphate reduction does not increase consistently with increasing temperature by hydrocarbons 0TSR) 0e.g. Aplin and Coleman, 1995; Orr, or depth, suggesting that temperature is not the only control 1977). These mechanisms can be distinguished on the basis on CO2 content. Thus an inorganic carbon contribution from of temperature data, the characteristics of the diagenetic 34 the decomposition of carbonate or reactions between miner- systems, ranges of H2S concentrations and d SofH2S als and solutions controlled by temperature 0Coudrain- and other reduced sulphur species 0e.g. Cai et al., 1997; Ribstein et al., 1998; Hutcheon & Abercrombie, 1990), is Machel et al., 1995). not signi®cant in the Tarim Basin. CO can be generated by oxidation, sulphate reduction or 2 5.3.1. Organic sources of H S thermal decarboxylation of organic matter. These sources 2 Temperatures of more than 1758C are thought to be result in CO d13C values of generally less than 220½ 2 required to cause the decomposition of organic matter to 0Irwin, Curtis, & Coleman, 1977). Petroleum in Cambrian, generate a signi®cant amount of H S 0e.g. Aplin & Cole- Ordovician and Silurian reservoirs and kerogen in the 2 man, 1995). Although a plot of burial history shows that Cambrian and Ordovician source rocks have an average temperatures of Cambrian strata can be up to 1758C, d13C value of 231.8½ 0Zhao & Huang, 1996). If the CO 2 Lower Paleozoic petroleum has been considered to be derived from organic matter has a similar isotopic composi- derived from Middle and Upper Ordovician non-evaporite tion as the parent organic matter, then associated dissolved source rock 0Zhang et al., 2000) with temperatures less than HCO2 would be about 10½ heavier at 508C and about 5½ 3 1258C. Therefore, it is unlikely that a signi®cant quantity of heavier at 1008C 0Wood & Boles, 1991). Therefore, the d13C reduced sulphur 0total pyrite and dissolved H S in the value of dissolved HCO2 from oxidised Cambrian and 2 3 oil®eld waters) could be derived from the decomposition Ordovician organic matter would be anticipated to be of organic matter. This is supported by the similar d34S about 230½ at temperatures of greater than 1258C. Since values of the pyrite, petroleum and bitumen. The explana- calcite cement in the Silurian sandstones has d13C values as tion is as follows. low as 220½, then this suggests an important contribution In general, if sulphur in oil has been derived from the from the oxidation of organic matter to the overall CO and 2 parent kerogen, then they typically have very similar carbonate cement budget. sulphur stable isotope ratios. Similarly, organically-derived 34 H2S is thought to have a d S value very close to parent 5.2. Fluid migration S-enriched kerogen and oil 0Orr, 1977). Marine kerogen contains sulphur that has a d34S value that is typically The geochemistry of petroleum source rocks and hydro- about 15½ lower than the contemporary seawater. Thus, carbon inclusions 0Xiao et al., 1997) has been used to show Ordovician kerogen 0and resulting sulphur in oil and orga- that the source of high maturity, light oil in the Silurian nically-derived H S) should have a d34S of about 111½ bitumen-bearing sandstone was the Middle and Upper 2 0Claypool et al., 1980). However, petroleum and bitumen Ordovician source rocks 0Hanson et al., 2000; Zhang et in the Tarim Basin have a d34S range mainly between 118.4 al., 2000). Organic geochemical data have also shown that and 125.6½. The sulphur isotope data thus discount the generation and emplacement took place during the Late possibility of the reduced sulphur 0in sulphide compounds, Yanshan and Early Himalayan Orogeny 0late Cretaceous e.g. pyrite) predominantly having an organic source. to early Eocene) 0Xiao et al., 1997, 2000). Emplacement of dry gas from Ordovician source rocks with R0 up to 1.45% in the Upper Paleozoic reservoir has also been 5.3.2. Sulphate reduction sources suggested by Huang et al. 01996). The high 0.1208C) Since we have argued against an organic source for the homogenization temperatures in calcite cement in Silurian sulphur in the H2S, aqueous phase sulphate reduction is the sandstones of well A 0Xiao et al., 1997) are higher than likely origin of H2S in the Tarim Basin 0e.g. Machel et al., 738 C. Cai et al. / Marine and Petroleum Geology 18 92001) 729±741 1995; Worden and Smalley, 1996): be too high to come from a SRB source. SRB typically lead to a large sulphur isotope fractionation effect between CaSO 1 petroleum fluid ! CaCO 4 3 sulphate and sulphide 0e.g. Machel et al., 1995). The 1 H S 1 altered petroleum or bitumen 1 H O sulphur isotope data for anhydrite and pyrite in the Tarim 2 2 Basin 0Table 3) are within the same range and do not Anhydrite is typically replaced by calcite during sulphate support the occurrence of BSR-induced isotope reduction while the petroleum reactant will be altered since fractionation. Thus the data from temperatures, the H2S some compounds will be more reactive than others to concentrations and sulphur isotopes do not seem to sulphate. Organically-sourced CO2 and inorganically- support the occurrence of BSR in Central Tarim. sourced H2S are the gas phase products of sulphate reduction reactions. However, at least some of the CO2 is likely to 5.3.2.2. Thermochemical sulphate reduction. TSR occurs in react with aqueous calcium from the anhydrite source to systems that undergo progressive burial 0Cai et al., 1997). produce solid phase calcite. Similarly H2S will react with The temperature range for the initiation of TSR varies any ferric mineral that are present in the system to precipi- according to factors such as the presence of catalysts, the tate pyrite. Fe may be derived from the clay minerals such as availability of anhydrite, the rock fabric 0anhydrite crystal illite within carbonates in the Cambro±Ordovician 0Worden size; Worden et al., 2000), but is typically greater than 120± et al., 2000), in Upper Ordovician mudstone, or from Fe- 1408C 0Machel et al., 1995; Worden et al., 1995). TSR bearing minerals in the Silurian sandstones. One of the generally produces large volumes of H2S. However, the consequnces of ¯ooding the system with H2S is that aqueous concentration of H2S is limited by the availability of Fe Fe becomes depleted 0especially relative to Mn), resulting in 0or other transition metals) in the rock or formation waters low Fe/Mn ratios in formation water. Note that Fe-poor as these metals lead to base metal sulphide precipitation and calcite typically is brightly luminescent, while calcite with loss of ¯uid phase sulphide. Consequently, the total amount more than 2 wt% siderite is non luminescent 0e.g. SpoÈtl, of sulphide 0reduced sulphur) generated by TSR or by BSR Longstaff, Ramseyer, Kunk, & Wiesheu, 1998; Tucker & is the best parameter to assess how much H2S has been Wright, 1990). The late stage calcite in the Ordovician produced and thus to distinguish between BSR and TSR. limestones is typically bright orange in a cathodolumines- The proposal is supported by a case-study of North Sea oil cence microscope 0CL) 0Fig. 6b) so that this stage of calcite ®elds, where the concentrations of H2S, thought to have is most likely to have been formed in Fe-poor formation been generated by TSR, are less than 1% by volume of water. The absence of Fe in the contemporary formation the associated gas but late-stage pyrite is common water is likely to have been due to the presence of H2S 0Worden & Smalley, 2001). In Central Tarim, the total and the formation of pyrite. sulphide quantity is high, with dissolved H2S ranging up Although some evidence seems to support sulphate to 1175 ppm in Ordovician and Silurian oil®eld waters, reduction in the Central Tarim, both BSR and TSR share and as much as 3% late-stage cubic pyrite within Silurian a similar overall reaction, and it is necessary to distinguish sandstones. It is unlikely for such a large quantity of reduced these mechanisms. sulphur to be generated in-situ in the Silurian sandstone during burial diagenesis, as no sulphate minerals have yet

5.3.2.1. Bacterial sulphate reduction. The growth and been found in these rocks. Together with the CO2,H2S must propagation of sulphate reducing bacteria 0SRB) depend have undergone secondary migration 0cross formational on the temperature, salinity, oxygen content and nutrient ¯ow: Worden & Matray, 1995) from Lower Paleozoic levels remaining favourable to the bacteria. Although it is strata, similar to the case-study reported by Moldovanyi, possible for different kinds of SRB to grow in different Walter, and Land 01993). environments, most SRB are active under restricted During TSR, there is commonly no signi®cant sulphur conditions: temperatures less than 60±808C, low salinities, isotope fractionation in the process of transformation of strictly anoxic conditions and the maintenance of aqueous sulphate to sulphide 0e.g. Orr, 1977). Anhydrite can be sulphide concentration below the threshold at which used to constrain the seawater sulphur isotope ratio at the the SRB can survive. SRB are thought to consume time of gypsum deposition 0Worden, Smalley, & Fallick, hydrocarbons directly 0Kirkland & Denison, 1995) or to 1997). This is supported by similarity between the measured consume byproducts of petroleum degradation by aerobic Cambrian and Ordovician anhydrite and the contemporary bacteria 0e.g. Cai, Mei, Ma, Chen, & Liu, 1996; Jobson, seawater sulphate 0Claypool et al., 1980). The six pyrite

Cook, & Westlake, 1979). The content of H2S produced samples with a mean value of about 125½, thus probably by BSR is generally less than 3% of the natural gas result from TSR. However, two of the samples have rela- 34 composition. For example, dissolved H2S from BSR in tively low d S values 019.5 and 113.4½, respectively), Devonian oil®eld waters in the Alberta Basin ranges from and could have a complex origin involving both eogenetic 136 to 415 ppm 0Connolly, Walter, Baadsgaard, & BSR-related pyrite and later TSR. For example, Riciputi,

Longstaffe, 1990). The H2S concentration data from the Cole, and Machel 01996) showed the pyrite in Devonian Tarim Basin 0up to 1175 ppm dissolved in water) seem to Nisku Formation has d34S values ranging between 235 C. Cai et al. / Marine and Petroleum Geology 18 92001) 729±741 739 and 120½, and were thought to be originated from both isotopes in the Silurian-hosted gas phase CO2 and late BSR and TSR. stage carbonate cements cannot have been locally generated. According to temperatures for the initiation of the replacement of anhydrite and bottom-hole temperatures, in 5.6. TSR, H2S and sulphur in petroleum conjunction with homogenization temperatures of ¯uid 34 inclusions, sulphate reduction is considered to have The petroleum and bitumen have similar d S values occurred at a temperature greater than about 1258C. 0mainly from 118.4 to 126.5½) and are similar for reservoirs of different ages 0Fig. 10). This suggests that the 5.4. Organic reactants involved in TSR organic sulphur has a common source. This is consistent with the result obtained by analysis of the petroleum system During sulphate reduction processes, the organic reac- 0Xiao et al., 2000), source rock 0Zhang et al., 2000) and tants can be petroleum, kerogen or methane. Petroleum oil®eld water 0Cai et al., 2001a). 13 and kerogen 0with d C values of about 231½ in the If the sulphur isotope enrichment in petroleum was due to 13 Tarim Basin) have d C values that are closer to the lowest fractional loss 0of 34S-depleted sulphur) during cracking, 13 CO2 or calcite cement d C values 0about 221½) rather then there would be a negative correlation between the than methane 0with an average of 243.3 ^ 1.6½, n 8; sulphur weight percent and the d34S 0as sulphur is lost Table 2). However, the involvement of methane in TSR is from the oil, the remainder would become progressively well documented 0Krouse et al., 1988; Worden & Smalley, enriched in remaining 34S). However, there is a positive 1996; Worden, et al., 1995), and CO2 carbon isotope values correlation between the sulphur content of petroleum and may become blurred if there has been mixing of different its d34S 0Fig. 4), suggesting that sulphur enriched in 34S has sources of CO2. Gas isotope and geochemistry data reveal been added to the petroleum. 13 that there is no relationship between the methane d C ratio The isotopically enriched sulphur is likely to have come and the CO d13C ratio 0Table 2). Since there is typically an 2 from the H2S given the relative similarity of the petroleum inverse relationship between methane and CO2 carbon sulphur and the sulphide. It is likely that there has been a isotopes in systems that have undergone methane-sulphate reaction between the TSR-derived H2S and the petroleum in reduction 0Worden et al., 2000), it is unlikely that methane the reservoir. Thus there has been back-incorporation of was the main reactant. It thus seems to be reasonable to some of the TSR H2S into the organic matter 0e.g. Orr, assume that liquid phase petroleum was the reactant rather 1974; Powell & MacQueen, 1984; Worden & Smalley, than the methane. 2001). A reaction may be written to account for the process: 5.5. Location of TSR in the petroleum system and migration of H2S Petroleum fluid 1 H2S ! sulphur-rich

It is possible to conclude that TSR is responsible for the altered petroleum and=or bitumen H2S and the CO2 in the reservoir ¯uids. It is thus pertinent to assess where TSR has occurred. Thus the petroleum system ®rst undergoes TSR causing The Ordovician and Cambrian marine limestones contain conversion of the sulphate into sulphide and consuming variable quantities of anhydrite cement. This has been some of the petroleum itself. Subsequently, at least some partially replaced by calcite that is: 01) characterized by of the H2S has then reacted with the remaining 0or heavy) isotopically depleted carbon, 02) luminsecent 0suggesting petroleum resulting in an increase in the sulphur content of minimal aqueous iron and possibly enriched sulphide), 03) the petroleum and bitumen and the progressive adoption of de®ned by ¯uid inclusion homogenisation temperatures the sulphide 0and thus anhydrite) sulphur isotope ratio. typically greater than 1258C. They also contain late stage pyrite cement. It is thus very likely that TSR has occurred 6. Conclusions within these limestones involving anhydrite replacement by calcite and that once the local aqueous iron was exhausted due to the formation of pyrite, H2S could accumulate in the 1.CO2,N2 and He all have a mainly organic source in both ¯uid phase. Cambrian and Ordovician carbonate and Silurian sand- TSR is unlikely to have occurred in the Silurian sandstone reservoirs in the Tarim Basin. Meteoric water, deep stones because, seemingly, they contained no sulphate crustal or mantle sources and inorganic water-rock inter- minerals capable of causing TSR to occur. It is thus likely action cannot explain the isotope ratios and distribution that TSR occurred in other formations and migrated into the patterns of these gases

Silurian. The resulting elevated H2S concentrations in the 2. The sulphur in petroleum in both Cambrian and Ordo- Silurian reservoirs may have been due to cross formational vician carbonate and Silurian sandstone reservoirs in the ¯ow from Ordovician and Cambrian limestones, in which Tarim Basin is at least partly derived from other than TSR was occurring. Cross formational ¯ow of gas phase organic rich petroleum source rocks, as shown by the

CO2 must also have occurred since the depleted carbon positive correlation between the relationship between 740 C. Cai et al. / Marine and Petroleum Geology 18 92001) 729±741

the sulphur content and the sulphur isotope ratio of the rock interaction in the diagenetically altered system nearby the uncon- petroleum. formities of Tarim Basin. Chinese Sciences Bulletin, 41, 1631±1635. 3. The Tarim Basin system has too much sulphide and the Cai, C., Mei, B., Ma, T., Chen, C., Li, W., & Liu, C. 01997). Approach to Fluid-Rock Interaction in Tarim Basin, Beijing: Geological Publishing reservoir temperatures are too hot for BSR to be the House, pp. 1±155, 0in Chinese). origin of the sulphide. Cai, C., Franks, S., & Aagaard, P. 02001a). Origin and migration of brines 4. Thermochemical sulphate reduction occurred in anhydrite- from Paleozoic strata in Central Tarim, China: constraints from bearing Cambrian and Ordovician carbonate reservoirs, 87Sr/86Sr, dD, d18O and water chemistry. Applied Geochemistry, 16, producing H S, as shown by the occurrence of anhydrite 1269±1284. 2 Cai, C., Gu, J., & Cai, H. 02001b). Effect of hydrocarbon emplacement on pseudomorphed by calcite and pyrite, the high tempera- diagenesis of Silurian sandstones of Central Tarim. Acta Sedimento- ture of the replacive calcite growth and the sulphur logical Sinica, 19, 60±65 0in Chinese). isotope ratio of pyrite. TSR may also have produced Carothers, W. W., & Kharaka, Y. K. 01980). Stable carbon isotopes of HCO2 in oil-®eld waters Ð Implications for the origin of CO . Geo- the signi®cant volumes of CO2 seen in the Tarim Basin 3 2 limestone reservoirs. chimica et Cosmochimica Acta, 44, 323±332. Chen, J., Sheng, Z., Wang, Z., Zhu, S., 01994). Analysis and prediction of 5. Cross formational ¯ow of TSR-derived H2S and CO2 Cambrian and Ordovician carbonate reservoirs in the Tarim Basin. occurred from the Cambrian and Ordovician carbonate China's National Key Project Report 0Grant No. 85-101-01-05-06), reservoirs to the Silurian sandstones as shown by the 0pp. 1±142). Xinan Petroleum Institute 0in Chinese). sulphur isotopes of diagenetically late abundant euhedral Claypool, G. E., Holser, W. T., Kaplan, I. R., Sakai, K., & Zak, I. 01980). pyrite and the stable isotopes of gas phase CO and dia- The age curves of sulfur and oxygen isotopes in marine sulfate and their 2 mutual interpretation. Chemical Geology, 28, 199±260. genetically late calcite cement. TSR could not have Connan, J., Lacrampe-Couloume, G., Magot, M., 01996). Origin of gases in occurred in the Silurian sandstones since there were no reservoirs. In D.A. Dolec 0Ed.), Proceedings of the 1995 International sulphate mineral in these sandstones. Gas Research Conference, 0pp. 21±61). Government Institutes, Inc. No. 1. 6. TSR-derived H2S back-reacted with the remaining petroleum resulting in the addition of sulphur to the petroleum Connolly, C., Walter, L. M., Baadsgaard, H., & Longstaffe, F. J. 01990). Origin and evolution of formation waters, Alberta Basin, Western and the progressive adoption of the initial Cambrian and Canada Sedimentary Basin: I. Chemistry. Applied Geochemistry, 5, Ordovician anhydrite sulphur isotope ratio. 75±395. Coudrain-Ribstein, A., Gouze, P., & Marsily, G. 01998). Temperature- carbon dioxide partial pressure trends in con®ned aquifers. Chemical Acknowledgements Geology, 145, 73±89. Dai, J., Ji, H., & Hao, S. 01989). A general review on natural gas geology, Beijing: Petroleum Industry Publishing House pp. 10±30, 0in Chinese). The research is ®nancially supported by China's National Dai, J., Song, Y., Dai, C., & Wang, D. 01996). Geochemistry and accumu- Natural Sciences Foundation 049773198) and Royal Fellow- lations of carbon dioxide gases in China. American Association of ship of the Royal Society, U.K. Dr S.C. George from Petroleum Geologists Bulletin, 80, 1615±1626. Australia is thanked for valuable contributions through his Hanson, A. D., Zhang, S., Moldowan, J. M., Liang, D., & Zhang, B. 02000). 13 34 Molecular organic geochemistry of the Tarim basin, northwest China. review of the original manuscript. d C and d S measure- American Association of Petroleum Geologists Bulletin, 84, 1109± ment was performed in Open Laboratory for isotopic 1128. compositions of Geology and Mineral Agency, Land and Heydari, E. 01997). The role of burial diagenesis in hydrocarbon destruction Resource Ministry of China in Yichang city, Hubei and H2S accumulation, Upper Jurassic Smackover Formation, Black province. 3He/4He and d15N were measured at Open Labora- Creek ®eld, Mississippi. American Association of Petroleum Geologists tory for gas isotopic compositions of CAS, Lonzhou city, Bulletin, 81, 26±45. Heydari, E., & Moore, C. H. 01989). Burial diagenesis and thermochemical 34 China. d S of petroleum and bitumen was measured in sulfate reduction, Smackover Formation, Southeast Mississippi salt Leeds University, U.K. Keith Spence and Dave Hat®eld basin. Geology, 17, 1080±1084. are thanked for their help with the sulphur isotope analyses. Hiyagon, H., & Kennedy, B. M. 01992). Noble gases in CH4-rich gas ®elds, Alberta, Canada. Geochimica et Cosmochimica Acta, 26, 1569±1589. Huang, D., Liu, B., Wang, T., Xu, Y., Chen, S. J., & Zhao, M. 01996). Genetic types of natural gases and their maturity discrimination in the References east of Tarim Basin. Sciences in China Bulletin. 9D), 26, 365±372 0in Chinese). Aplin, A. C., & Coleman, M. L. 01995). Sour gas and water chemistry of the Hutcheon, I., & Abercrombie, H. 01990). Carbon dioxide in clastic rocks Bridport sands reservoir Wytch Farm, UK. In J. M. Cubit & W. A. and silicate hydrolysis. Geology, 18, 541±544. England, The Geochemistry of Reservoirs 0pp. 303±314), Vol. 86. Irwin, H., Curtis, C., & Coleman, M. 01977). Isotopic evidence for source of London: Special Publication of the Geological Society of London. diagenetic carbonates formed during burial of organic-rich sediments. Burnham, A. K., & Sweeney, J. 01989). A chemical kinetic model of vitri- Nature, 69, 209±213. nite maturation and re¯ectance. Geochimica et Cosmochimica Acta, 53, Jobson, A. M., Cook, F. D., & Westlake, D. W. S. 01979). Interaction of 2649±2657. aerobic and anaerobic bacteria in petroleum biodegradation. Chemical Cai, C., Hu, W., 01997). Bacterial and thermochemical sulfate reduction in Geology, 24, 355±465. Ordos, Sichuan, Tarim basins, western China. In Abstracts of Eight- Kirkland, D. W., & Denison, R. E. 01995). Diagenetic alteration of Permian eenth International Conference on Organic Geochemistry, 0pp. 33±34). strata at oil ®elds of south central Oklahoma, USA. Marine and Petro- The Netherlands. leum Geology, 12, 629±644. Cai, C., Mei, B., Ma, T., Chen, C., & Liu, C. 01996). Hydrocarbons-water- Krouse, H. R., Viau, C. A., Eliuk, L. S., Ueda, A., & Halas, S. 01988). C. Cai et al. / Marine and Petroleum Geology 18 92001) 729±741 741

Chemical and isotopic evidence of thermochemical sulfate reduction by Wood, J. R., & Boles, J. R. 01991). Evidence for episodic cementation and light hydrocarbon gases in deep carbonate reservoirs. Nature, 33,415± diagenetic recording of seismic pumping events, North Coles Levee, 419. California USA. Applied Geochemistry, 6, 509±521. Littke, R., Krooss, B., Idiz, E., & Frielingsdorf, J. 01995). Molecular nitro- Worden, R. H., & Matray, J. M. 01995). Cross formational ¯ow in the Paris gen in natural gas accumulations: generation from sedimentary organic basin. Basin Research, 7, 53±66.

matter at high temperature. American Association of Petroleum Geo- Worden, R. H., & Smalley, P. C. 01996). H2S-producing reactions in deep logists Bulletin, 79, 410±430. carbonate gas reservoirs: Khuff Formation Abu Dhabi. Chemical Geo- Machel, H. G., Krouse, H. R., & Sassen, R. 01995). Products and distin- logy, 133, 157±171.

guishing criteria of bacterial and thermochemical sulfate reduction. Worden, R. H., & Smalley, P. C. 02001). H2S in North Sea oil ®elds: Applied Geochemistry, 8, 373±389. importance of thermochemical sulphate reduction in clastic Mamyrin, B. A., & Tolstikhin, I. N. 01984). Helium isotopes in nature, New reservoirs. In R. Cidu, Proceedings of the 10th International York: Elsevier. Symposium on Water±Rock Interaction, Volume 2.Lisse:A.A. Moldovanyi, E. P., Walter, L. M., & Land, L. S. 01993). Strontium, boron, Balkema 0pp. 659±662). oxygen and hydrogen isotope geochemistry of brines from basal strata Worden, R. H., Smalley, P. C., & Oxtoby, N. H. 01995). Gas souring by of the Gulf Coast Sedimentary basin, USA. Geochimica et Cosmo- thermochemical sulfate reduction at 1408C. American Association of chimica Acta, 57, 2083±2099. Petroleum Geologists Bulletin, 79, 854±863. Orr, W. L. 01974). Changes in the sulfur content and isotopic ratios of sulfur Worden, R. H., Smalley, P. C., & Fallick, A. E. 01997). Sulfur cycle in during petroleum maturation-study of Big Horn Basin Paleozoic oil. buried evaporates. Geology, 25, 643±646. American Association of Petroleum Geologists Bulletin, 58, 2295± Worden, R. H., Smalley, P. C., & Cross, M. M. 02000). The in¯uence of 2318. rock fabric and mineralogy on thermochemical sulfate reduction: Khuff Orr, W. L. 01977). Geologic and geochemical controls on the distribution of Formation, Abu Dhabi. Journal of Sedimentary Research, 70,1210± hydrogen sul®de in natural gas. In R. Campos & J. Goni, Advances in 1221. Organic Geochemistry, 1975, 0pp. 571±597). Oxford: Pergamon Press. Wycherley, H., Fleet, A., & Shaw, H. 01999). Some observations on the Powell, T. G., & MacQueen, R. W. 01984). Precipitation of sul®de ores and origins of large volumes of carbon dioxide accumulations in sedimen- organic matter: sul®de reactions at Pine Point, Canada. Science, 224, tary basins. Marine and Petroleum Geology, 16, 489±494. 63±66. Xiao, Z., Zhang, S., Zhao, M., & Zeng, Q. 01997). A brief analysis on Riciputi, L. R., Cole, D. R., & Machel, H. G. 01996). Sul®de formation in forming periods of Silurian pools in the Tazhong A well. Acta Sedi- reservoir carbonates of the Devonian Nisku Formation, Alberta, mentological Sinica, 15, 150±153 0in Chinese). Canada: an ion microprobe study. Geochimica et Cosmochimica Xiao, X., Song, Z., Liu, D., Liu, Z., & Fu, J. 02000). The Tazhong hybrid Acta, 60, 325±336. petroleum system, Tarim Basin, China. Marine and Petroleum Geo- Rigby, D., & Batts, B. D. 01986). The isotopic composition of nitrogen in logy, 17,1±12. coals and oil shales. Chemical Geology, 58, 273±282. Xu, Y., Shen, P., Liu, W., Tao, M., Sun, M., & Du, J. 01998). Noble gas Sassen, R. 01988). Geochemical and carbon isotopic studies of crude oil geochemistry, Beijing: Publishing House of Sciences, pp. 1±231, 0in destruction, bitumen precipitation and sulfate reduction in the deep Chinese). Smackover Formation. Organic Geochemistry, 12, 351±361. Ye, D. 01994). Deep dissolution of Cambrian±Ordovician carbonates in the Sherwood-Lollar, B., Ballentine, C. J., & O'Nions, R. K. 01997). The fate of Northern Tarim Basin. Acta Sedimentological Sinica, 12, 66±710in mantle-derived carbon in a continental sedimentary basin: Integration Chinese). of C/He relationships and stable isotopic signatures. Geochimica et Zhang, S., Liang, D., Wang, F., Zhang, B., Pian, L., Zhou, F., Zhang, M., Cosmochimica Acta, 61, 2295±2307. Xie, Q., & Liu, L. 01999). A study on source rock and oil-rock Smith, T. M., & Ehrenberg, S. N. 01989). Correlation of carbon dioxide correlation in Tarim Basin, China's National Key Project Report abundance with temperature in clastic hydrocarbon reservoirs: relation- 9Grant No. 96-111-01-03), Beijing: Institute of Petroleum Exploration ship to inorganic chemical equilibrium. Marine and Petroleum Geo- and Development, pp. 80±158, 0in Chinese). logy, 6, 129±135. Zhang, S., Hanson, A. D., Moldowan, J. M., Graham, S. A., Liang, D., SpoÈtl, C., Longstaff, F. L., Ramseyer, K., Kunk, M. L., & Wiesheu, R. Chang, E., & Fago, F. 02000). Paleozoic oil-source rock correlations 01998). Fluid-rock reactions in an evaporitic meÂlange, Permian in the Tarim basin, NW China. Organic Geochemistry, 31, 273±286. Haselgebirge, Austrian Alps. Sedimentology, 45, 1019±1044. Zhao, M., & Huang, D. 01996). Oil-source correlation of Paleozoic in Tarim Thrasher, J., & Fleet, A. J. 01995). Predicting the risk of carbon dioxide Basin. In Q. Tong, D. Liang & C. Jia, Advance on Petroleum Geology in pollution in petroleum reservoirs. In J. O. Girmalt, Organic Geochem- Tarim Basin, 0pp. 300±310). Beijing: Publishing House of Sciences, 0in istry: Developments and Applications to Energy, Climate, Environment Chinese). and Human History, 0pp. 1086±1088). San Sebastian: A.I.G.O.A. Zhu, Y., Shi, B., & Fang, C. 02000). The isotopic compositions of molecular Tucker, M. E., & Wright, V. P. 01990). Carbonate Sedimentology, Oxford: nitrogen: implications on their origins in natural gas accumulations. Blackwell, pp. 1±482. Chemical Geology, 164, 321±330.