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Fluid inclusion and stable isotopic studies ofthermochemical sulfate reduction: Upper permian andlower triassic gasfields, northeast Sichuan Basin, China

Kaikai Li a,b,c,⇑, Simon C. George c, Chunfang Cai d,e,f,⇑, Se Gong g, Stephen Sestak g

Stephane Armand g, Xuefeng Zhang h

aSchool of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, ChinabKey Laboratory of Marine Reservoir Evolution and Hydrocarbon Enrichment Mechanism, Ministry of Education, China

cDepartment of Earth and Planetary Sciences and MQMarine Research Centre, Macquarie University, NSW 2109, AustraliadKey Lab of Petroleum Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

eCollege of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, ChinafKey Laboratory of Oil and Gas Resources and Exploration Technology, Yangtze University, Wuhan 430100, China

gCSIRO Energy Business Unit, North Ryde, NSW 2113, Australiah Institute of Oil and Gas, Peking University, Beijing 100871, China

Received 1 July 2018; accepted in revised form 21 November 2018; available online 26 November 2018

Abstract

Fluid inclusions hosted in different stages of TSR-derived diagenetic minerals are expected to record compositions and iso-topes of paleo-fluids at the time of trapping during different TSR extents. Here we report the first set of data on carbon iso-topes of CH4 and CO2 and hydrogen isotopes of H2O trapped in fluid inclusions in TSR calcites. We find that the NE Sichuansour dolostones have initially experienced oil- and wet gas-dominated TSR, as recorded in H2S-bearing oil inclusions withlower homogenization temperatures (Th) values (e.g., �137 �C) and the coexistence of C2+ hydrocarbon gas and H2S in fluidinclusions. The subsequent dry gas-dominated TSR occurred in higher reservoir temperatures (> about 161.5 �C) when mostC2+ hydrocarbons were exhausted. The three-stage TSR resulted in CH4 d13C values becoming progressively heavier from�46.7‰ to �29.6‰, H2O d2H values shifting negatively from �36.4‰ to �67.8‰ and salinities decreasing to as low as0.9 wt% NaCl. The dry gas-dominated TSR reaction seems to be the most efficient at water production, which, however,was limited by available reactive sulfate, and shows significant differences within the reef and shoal reservoirs along the plat-form margin, and the anhydrite-bearing reservoirs in the paleo-lagoon area. The TSR reaction within the porous shelf-marginreservoirs is capable of causing carbonate dissolution owing to high porosity and good connectivity of the micropore networkand the resulting mass transport away from TSR sites. This resulted in CO2 d

13C positive shift from �9.3‰ to +6.3‰, and apositive correlation of this parameter with Th. In contrast, in the tight anhydrite-bearing reservoirs, slow mass transport andquick saturation of calcium and dissolved CO2 in the pore waters is expected to precipitate TSR calcite near the anhydritecrystals, resulting in calcite crystals having more depleted d13C values (�1.4‰ to �18.9‰). This study shows that thereare essential differences in the process and effects of TSR reaction due to geological differences in the settings of TSR sites.� 2019 Elsevier Ltd. All rights reserved.

https://doi.org/10.1016/j.gca.2018.11.032

0016-7037/� 2019 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors at: School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China (K. Li). KeyLab of Petroleum Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China (C. Cai).

E-mail addresses: [email protected] (K. Li), [email protected] (C. Cai).

www.elsevier.com/locate/gca

Available online at www.sciencedirect.com

ScienceDirect

Geochimica et Cosmochimica Acta 246 (2019) 86–108

Keywords: Fluid inclusion; TSR; Carbonate dissolution; Carbon isotope; Hydrogen isotope; Sichuan Basin

1. INTRODUCTION

Thermochemical sulfate reduction (TSR) is a well-documented process that occurs at elevated temperatures(�>120 �C) and involves the reaction between sulfate andhydrocarbons to produce hydrogen sulfide, elemental sul-fur, calcite, carbon dioxide and organic sulfur compounds(e.g., Machel et al., 1995; Worden and Smalley, 1996;Krouse et al., 1988; Cai et al., 2003; 2016). The overall pro-cess can be summarized as the simple reaction:

SO42� þ hydrocarbons ! H2S HS�ð Þ þ CO2 HCO3

�ð Þ�H2O� S

� altered hydrocarbons ð1ÞThe variations of chemical and stable isotopic composi-

tions in hydrocarbons and minerals have been extensivelyused in experiments to understand the role of the organicand inorganic reactants (e.g., Manzano et al., 1997;Zhang et al., 2008), hydrocarbon alteration (e.g., Kelemenet al., 2010; Cai et al., 2016), rate-controlling factors duringTSR (e.g., Worden et al., 2000; Bildstein et al., 2001) andthe effect of TSR on rock and fluid properties (e.g.,Worden et al., 1996; Machel, 2001). However, there is stillconsiderable confusion on explanation of some chemicaland isotopic features and understanding of some basicissues about TSR.

Thermocatalytic experiments on medium-molecular-weight hydrocarbons, e.g., n-octadecane, have shown thatmethane increases in 13C with extent of cracking (Frankand Sackett, 1969; Sackett et al., 1978). As TSR alterationproceeds, methane d13C values seem to respond more com-plex to alteration of hydrocarbons. Claypool and Mancini(1989) claimed that methane d13C value did not increasewith increasing amount of H2S in the oil pools withinSmackover and Norphlet formations in southwestern Ala-bama. In contrast, a positive relationship between methaned13C value and gas souring index [GSI = H2S/(H2S + C1-6)]has been well documented in light hydrocarbon gas reser-voirs, e.g., the Devonian and Mississippian formations inwestern Canada (Krouse et al., 1988), the Permian KhuffFormation of Abu Dhabi and Saudi Arabia (Wordenet al, 1996; Jenden et al., 2015) and the Triassic Jialingjiangand Feixianguan Formations (Cai et al., 2003; 2004; 2013).A similar trend was also reported in the experiments ofTSR alteration on C1-5 hydrocarbons (Pan et al., 2006).However, Hao et al. (2008) and Liu et al. (2013) claimedthat methane d13C values remain constant throughout wetgas-dominated TSR stage, and increase at methane-dominated TSR stage. Further work is therefore requiredto trace the behaviour of methane d13C values throughoutTSR alteration, including the involving process of oil-,wet gas- and dry gas-dominated TSR.

The possible occurrence of post-TSR migration or alter-ation of natural gas in the hydrocarbon-water-rock system(e.g., Worden et al., 1995) would increase uncertainties of

data interpretation. A positive shift in d13C of CO2 in thepresent-day gas reservoirs was regarded as compellingevidence for the occurrence of TSR-induced dissolution ofdolomite in the NE Sichuan Basin (Cai et al., 2014;Liu et al., 2014a). The shift was either ascribed tore-equilibration of CO2 with the 13C-enriched water-rocksystem in the Mobile Bay gas field (Mankiewicz et al.,2009), or to preferential precipitation of 12C-rich CO2 asTSR associated calcite in the NE Sichuan Basin (Haoet al., 2015), in which no significant carbonate dissolutionand porosity change was proposed during TSR.

Considering that typical TSR settings are generallyhydrodynamically closed (Machel, 2001), some informationrelated to mass transfer and isotopic variation would berecorded in the closed system. Fluid inclusions in diageneticminerals formed by TSR are examples of tiny closed systemtime-capsules and are representative of the parent mineral-izing fluids (Roedder, 1984; Karlsen et al., 1993). Oil-, wetgas- and methane- stages of TSR are proposed to occur inthe NE Sichuan Basin based on methane and ethane d13Cvalues (Hao et al., 2008; Liu et al., 2013; Jiang et al.,2015), however, the lines of evidence are not conclusive.New data are required to determine if multi-stage TSRoccurred in NE Sichuan, which will help explain othermethane-dominated TSR cases worldwide. Informationabout chemical and isotopic composition of fluid duringdifferent TSR stages is possible to have been recorded influid inclusions. However, very few studies have focusedon the compositions and stable isotopes of fluid inclusiongases related to TSR (Yang et al., 2001).

Reduced fluid inclusion salinity and water d18O valueswith the extent of TSR reaction was documented in the Per-mian Khuff Formation in Abu Dhabi and invoked to sup-port the water generation during TSR (Worden et al.,1996). The balanced reactions for methane and ethanecan be written as:

CaSO4 þ CH4 ! CaCO3 þH2SþH2O ð2Þ2CaSO4 þ C2H6 ! 2CaCO3 þH2Sþ Sþ 2H2O ð3Þ

Light d18O value of water produced by TSR was consid-ered to be derived from the TSR-induced breakdown ofanhydrite and oxygen fractionation of anhydrite, despitethat the water from anhydrite breakdown in the DevonianWabamun Group of southwestern Albera was reported tohave heavy d18O values (Yang et al., 2001). A similar neg-ative shift in d18O values of water was calculated in theT1f Formation of the NE Sichuan Basin and attributed towater production during TSR (Jiang et al., 2015). In con-trast, Machel (1987, 1998) claimed that the depleted d18Ovalues of the saddle dolomite from the Devonian Nisku reeftrend in the Alberta Basin resulted from transfer of oxygenfrom the SO4

2� into the CO32� groups rather than the forma-

tion of TSR water. Machel (1998, 2001) also claimed thatvolumetrically minor to negligible water was released dur-ing TSR and some sub-reactions even consume water.

K. Li et al. /Geochimica et Cosmochimica Acta 246 (2019) 86–108 87

Reactions (1), (2) and (3) suggest that hydrogen inhydrocarbon reactant would go into water if water isreleased during TSR. The water released during TSR ismore likely to have significantly lighter d2H values derivedfrom hydrocarbon reactants (mainly from �250‰ to�85‰; Schoell, 1980; Yeh and Epstein, 1981) comparedwith many H2S-free saline formation waters (mainly from0‰ to �26‰, Moldovanyi et al., 1993; Clayton et al.,1966). This will pull down d2H of the water trapped in fluidinclusions, which can be used to assess whether water isproduced during TSR or not.

The accumulation of high H2S concentrations in theUpper Permian Changxing Formation (P3ch) and LowerTriassic Feixianguan Formation (T1f) in the NE SichuanBasin in China have been demonstrated to be due to TSRbased on multiple lines of evidence (Cai et al., 2004; Zhuet al., 2005; Ma et al., 2008; Liu et al., 2013). The alterna-tion of potential reactants for TSR from oil to wet hydro-carbon gases (C2-C5), and lastly to methane might haveresulted in great diversity in methane d13C values followingTSR, and in the amounts of TSR-derived water (Cai et al.,2003, 2004, 2013; Pan et al., 2006; Hao et al., 2008). CH4,H2S, CO2 and H2O have been detected in high levels withinthe fluid inclusions of diagenetic minerals using laserRaman microprobe spectroscopy (e.g., Xie et al., 2006;Zhu et al., 2006). Data from these fluid inclusions areexpected to provide valuable information about TSR inthe NE Sichuan Basin.

This paper presents the first data on the carbon isotopesof CH4 and CO2, and hydrogen isotopes of H2O, releasedfrom fluid inclusions hosted in TSR-derived calcite crystals.Carbon and oxygen isotope analyses were also performedon both TSR and non-TSR calcite samples. The aims areto (1) assess the role of methane in TSR and thedissolution-precipitation process in the hydrocarbon-water-rock system, and (2) to determine if water is newlygenerated and what is the amount during TSR. The resultsprovide some insights into the TSR mechanism, and haveapplication for predicting the porosity of deep-buried car-bonate reservoirs, so have global theoretical and practicalsignificance.

2. GEOLOGICAL SETTING

The Sichuan Basin is a rhombic basin with an area of2.3 � 105 km2 in southwestern China. The basin has a com-plex tectonic and sedimentary history and has experiencedfive orogenies, including the Caledonian-Hercynian move-ment (Ordovician-Permian), the Indosinian movement(Upper Permian-Upper Triassic), the Yanshan movement(Jurassic-Cretaceous) and the Himalayan movement(Paleogene-Quaternary). The NE-trending structuresstarted developing during the late Indosinian orogeny, thenformed their basic styles during the middle Yanshan oro-geny, and finalized adjustment during the Himalayan oro-geny. The NW-trending structures started evolving duringthe late Yanshan orogeny, and reached their final shapeduring the Himalayan orogeny (Ma et al., 2007).

Potential source rocks for the NE Sichuan Basin gasesare mainly marine rocks including Lower Cambrian shales

and mudstones, Lower Silurian shales, and Upper Permiancoals, mudstones, and muddy limestones (Ma et al., 2008;Fig. 2). Geochemical signatures show that the Upper Per-mian source rocks mainly contributed hydrocarbons tothe P3ch and T1f reservoirs in the NE Sichuan Basin (Caiet al., 2017). The distribution of the P3ch and T1f reservoirsis controlled by depositional facies and the effective reser-voirs are mainly developed in platform-margin reef andshoal environments (Ma et al., 2007). The platform-margin reef facies, mainly developing in the P3ch forma-tion, is made up of gray limestones and the dolomites ofa sponge reef, and gray limestones and dolomites of aframework sponge reef (Fig. 2). The platform-margin bankfacies, chiefly developed in the P3ch and T1f formations, ismade up of thickly-bedded to massive oolitic and bioclasticdolomites and dolarenites (Ma et al., 2007; Fig. 2).

The evaporite beds of the lagoon-tidal flat facies in theLeikoupo (T2l) and Jialingjiang (T1j) formations, as wellas in the upper part of the Feixianguan Formation (T1f

4

member and the top of the T1f3 member), may serve as

regional caprocks for the underlying T1f1-2 and P3ch car-

bonate reservoirs (Ma et al., 2008; Fig. 2). Massive evapor-ite sediments in the T1f

1-2 Formation also formed inlagoon-tidal flats around the areas of the Jz1 and Y1 wells(Fig. 3), and show a trend of decreasing thickness withincreasing distance to the evaporated lagoon (Jiang et al.,2002; Chen, 2005).

The P3ch-T1f reefs and shoals were subjected to occa-sional exposure related to high-frequency sea-level fluctua-tions during early diagenesis. Then the reservoirsexperienced continuous and rapid burial and reached theirmaximum burial depths of over 8000 m and temperaturesof approximately 220 �C at 80 Ma at the end of the Creta-ceous. The reservoirs were then uplifted to the presentdepths of 4000–6000 m and cooled to <120 �C during theNeogene (Hao et al., 2008; Li et al., 2012). There are nogreat differences in the burial and geothermal historybetween the sour and sweet gas reservoirs, except that thelatter have experienced deeper maximum burial (Caiet al., 2014). The major oil generation period for the UpperPermian source rocks was between 210 Ma and 190 Ma(T3-J1, Wang et al., 2010). The cracking of oil to naturalgas was proposed to have occurred during the MiddleJurassic (Hao et al., 2008; Wang et al., 2010). Large H2S-rich gas accumulations have been discovered recently inthe Upper Permian Changxing Formation (P3ch) andLower Triassic Feixianguan Formation (T1f) in the NESichuan Basin, located in the Puguang, Luojiazhai, Duk-ouhe, Tieshanpo, Maobachang, Longgang and Yuanbastructures (Fig. 1a, b).

3. SAMPLES AND METHODS

Rock samples were taken from several wells in thenortheast Sichuan Basin (Tables 1, 2; Fig. 1B), and fromthe outcrop field sections HH, PLD and YGD (Fig. 1A).230 thin sections were made from the core and outcropsamples and were half stained with Alizarin Red S to distin-guish dolomite from calcite. Based on preliminary core andthin section examination, 34 post-bitumen calcites showing

88 K. Li et al. /Geochimica et Cosmochimica Acta 246 (2019) 86–108

Fig. 1. (a) General map showing the location of important gas fields in the NE Sichuan Basin and the paleoenvironment during the EarlyTriassic. (b) Detailed map showing the geological structures on the platform on the east side of the Kaijiang-Liangping (K-L) Trough, whichis the focus area of this study. The trend of gypsum thickness in (b) is based on Jiang et al. (2002) and Chen (2005), who used core observationand logging analysis techniques. PLD = the Panlongdong field section; YGD = the Yanggudong field section; HH = the Honghua fieldsection.


coarse vug/fracture-filling spar, with solid bitumen or otherhydrocarbon inclusions associated, are related texturally toTSR (Machel, 1987, 2001; Worden et al., 1995; Worden andSmalley, 1996; Cai et al., 2004; Li et al., 2012). Four pre-bitumen calcite samples with vug/fracture-filling texturesand without solid bitumen or other hydrocarbon inclusionsare typical of non-TSR diagenetic minerals (Li et al., 2012).These calcites were sub-sampled using a dentist’s drill, andwere then crushed to powder for C and O isotope measure-ments. The powdered samples were dissolved in anhydrousH3PO4 to release CO2 gas, which was analyzed on aFinnigan MAT 251 mass spectrometer. d13C and d18O arereported as ‰ relative to the Pee Dee Belemnite (VPDB)standard, with a precision of ±0.1‰.

Fluid inclusions were observed in double polished thicksections from sour gas reservoirs, using a calibrated Linkam

THM600 heating-cooling stage fitted with a ultraviolet(UV) lamp to determine whether they contained oil, gasor water. For microthermometry, only aqueous two-phaseprimary inclusions with a small size (<15 lm), regularshape and low vapor to liquid ratios (<15%) were measuredfor homogenization temperatures (Th). The results arereported with a precision of ±1 �C. Salinities were calcu-lated from the final ice melting temperatures using theequation of Bodnar (1993) for the H2O-NaCl system.

Carbon isotope analysis of fluid inclusion gases fromcrushed TSR-derived calcite samples were performed atboth the Commonwealth Scientific and Industrial ResearchOrganization (CSIRO) in Australia and the Research Insti-tute of Petroleum Exploration and Development (LangfangBranch), PetroChina Ltd. For the CSIRO analyses, eightsingle-crystal calcite samples containing abundant

Fig. 2. Synthetic stratigraphy for the NE Sichuan Basin and comparison of lithology between the Yuanba and Puguang gas fields and theJinzhuping gas-bearing structure.


hydrocarbon-bearing fluid inclusions were selected from thepaleo-TSR regions where high concentrations of H2S,S-rich bitumen and/or elemental sulfur were detected. Thesurfaces of the calcite grains were successively cleaned bymethanol and dichloromethane. Carbon isotope analysesof the fluid inclusion gases were conducted on an onlinecrushing–trapping system comprising a gas-tight crusherand a concentrator with micro-trap connected to a gaschromatograph-combustion-isotope ratio mass spectrome-ter (GC-C-IRMS). Fluid inclusion gases were released byvertical vibration of the gas-tight crusher with the

pre-cleaned calcite grains loaded, and cyro-trapped in themicro-trap on the concentrator for 5 minutes using liquidnitrogen. The gases were then released by heating the trapto 250 �C to the GC-C-IRMS, via the GC injector andd13C was measured with a precision of ±0.5‰. A systemblank was run before each crush to ensure that there wasno contamination from the system. Duplicate analyses wereperformed on each sample. Carbon isotopes of fluid inclu-sion gases also were analyzed on four similar calcite sam-ples at the Research Institute of Petroleum Explorationand Development (Langfang Branch), PetroChina Ltd.

Fig. 3. Photomicrographs showing different types of fluid inclusions in void-filling calcite cements. (a) Plane-polarized light (PL), well PG6,5350.5 m, P3ch. (b) View taken under UV light, well PG6, 5350.5 m, P3ch. (c) Plane-polarized light (PL), well Yb101, 6904 m, P3ch. (d) Viewtaken under UV light, well Yb101, 6904 m, P3ch. (e) Plane-polarized light (PL), well PG9, 5738.6 m, T1f

3. (f) View taken under UV light, wellPG9, 5738.6 m, T1f

3.


Tab

le1

Carbonisotopic

compositionsofCH

4an

dCO

2trap

ped

influid

inclusions(d

13CCH4;d1

3CCO2),carbonan

doxygenisotopic

compositionsofthecrushed

TSR-derived

calcitecrystals(d

13CCaCO3;

d18O

CaCO3),thedifference

ind1

3C

betweenCO

2trap

ped

influid

inclusionsan

dthehost

calcite,

andthehomogenizationtemperaturesofthefluid

inclusions(T

h).Theindex

H2S/(H

2S+C1-6)is

calculatedusingthemolarconcentrationsofnaturalgasin

theassociated

gasreservoirs.

Subject

Sam

ple

number

Form

ation

Depth

(m)

Mineral

d13CCO2

(‰PDB)

d13CCH4

(‰PDB)

d13CCaCO3

(‰PDB)

d18O

CaCO3

(‰PDB)

Dd1

3C(C

O2-C

aCO3)

(‰PDB)

Th

(�C)

H2S/(H

2S

+C1-6)

Fluid

inclusions

andtheirhost

minerals

Dw3-1

T1f2

4735

Post-bitumen

Calcite

7.20

�30.1

1.43

�6.67to

6.67

3.97

185.0

15.8

Dw3-1*

T1f2

4735

Post-bitumen

Calcite

5.40

�31.3

1.70

�6.56

5.5

––

Dw102-1

T1f2

4900.8

Post-bitumen

Calcite

�6.30

�34.7

�1.41

�9.24

�4.89

145.8

2.3

Dw102-1*

T1f2

4900.8

Post-bitumen

Calcite

�6.80

�36.3

�1.08

�9.52

�5.72

––

HH-1

P3ch

Outcrop

Post-bitumen

Calcite

6.30

�30.0

1.10

�6.60

5.2

165.0

–HH-1

*P3ch

Outcrop

Post-bitumen

Calcite

8.40

�30.6

2.11

�6.14

6.29

––

PLD-1

P3ch

Outcrop

Post-bitumen

Calcite

�1.90

�37.2

�11.9

�3.30

10126.0

–PLD-2

P3ch

Outcrop

Post-bitumen

Calcite

�2.10

�36.9

�11.8

�7.10

9.7

137.0

–Yb11-1

P3ch

6917

Post-bitumen

Calcite

�9.30

�29.6

�9.50

�6.00

0.2

170.1

7.0

Yb102-3

P3ch

6724

Post-bitumen

Calcite

2.40

�30.1

�3.70

�13.6

6.1

176.2

4.9

Yb224-1

P3ch

–Post-bitumen

Calcite

4.48

�29.8

0.04

�5.76

4.44

173.8

11.2

Yb224-1*

P3ch

–Post-bitumen

Calcite

5.50

�30.3

1.70

�5.50

3.8

––

Yb204-2

P3ch

6549

Post-bitumen

Calcite

�1.50

�33.1

3.64

�6.65

�5.14

161.5

–Yb204-2*

P3ch

6549

Post-bitumen

Calcite

�7.00

�31.1

3.10

�6.20

�10.1

––

YGD-1

P3ch

Outcrop

Post-bitumen

Calcite

�0.75

�46.7

�8.56

�4.55

7.81

133.1

–YGD-1

*P3ch

Outcrop

Post-bitumen

Calcite

0.35

�45.0

�8.10

�4.20

8.45

––

YGD-2

P3ch

Outcrop

Post-bitumen

Calcite

1.38

�23.3

�5.56

�5.76

6.94

133.1

–YGD-2

*P3ch

Outcrop

Post-bitumen

Calcite

3.05

�23.0

�3.80

�5.70

6.85

––

YGD-3

P3ch

Outcrop

Post-bitumen

Calcite

�5.89

�43.1

�14.0

�6.38

8.14

137.4

–YGD-3

*P3ch

Outcrop

Post-bitumen

Calcite

�3.07

�42.7

––

––

–

Void-fillingcalcite

D1

T1f2

–Post-bitumen

Calcite

––

�2.60

�6.30

––

16.4

Jz1

T1f1

2978

Post-bitumen

Calcite

––

�13.4

�6.50

––

6.9

Mb3

T1f

43876

Post-bitumen

Calcite

––

�7.76

�6.29

––

–Pg3

T1f2

4953.7

Post-bitumen

Calcite

––

1.45

�7.24

––

–Mb3

P3ch

4391.07

Post-bitumen

Calcite

––

�2.80

�7.09

––

–Mb3

P3ch

4382.8

Post-bitumen

Calcite

––

�1.64

�5.25

––

–Mb3

P3ch

4356.45

Post-bitumen

Calcite

––

0.28

�5.95

––

–Mb3

P3ch

4349.26

Post-bitumen

Calcite

––

�1.36

�7.96

––

–Mb3

P3ch

4415.24

Post-bitumen

Calcite

––

�5.46

�4.62

––

9.9

Pg2

P3ch

5279.77

Post-bitumen

Calcite

––

1.01

�7.95

––

17.3

Pg2

P3ch

5292.31

Post-bitumen

Calcite

––

1.96

�7.38

––

–Pg5

P3ch

5295

Post-bitumen

Calcite

––

2.37

�7.83

––

–Pg6

P3ch

5280.8

Post-bitumen

Calcite

––

0.64

�5.79

––

–Pg6

P3ch

5297.7

Post-bitumen

Calcite

––

2.02

�5.04

––

–Pg6

P3ch

5338.65

Post-bitumen

Calcite

––

1.96

�5.36

–176.2

16.2

Pg6

P3ch

5247.1

Post-bitumen

Calcite

––

�0.65

�6.85

–150.6

–Pg6

P3ch

5350.5

Post-bitumen

Calcite

––

2.21

�4.58

–178.9

16.2

Pg6

P3ch

5323

Post-bitumen

Calcite

––

1.13

�8.09

––

–Pg6

P3ch

5246.5

Post-bitumen

Calcite

––

�0.26

�7.20

–146.0

–


Pg8

P3ch

5644

Post-bitumen

Calcite

––

2.12

�5.21

––

–Pg8

P3ch

5640.5

Post-bitumen

Calcite

––

1.82

�7.80

––

–Pg8

P3ch

5525

Post-bitumen

Calcite

––

�0.08

�9.42

––

–Void-fillingcalcite

Lj1

T1f2

–Pre-bitumen

Calcite

––

0.61

�7.20

–99.8

–Lj1

T1f2

3471

Pre-bitumen

Calcite

––

1.24

�6.20

–112.3

–Mb2

T1f2

4344

Pre-bitumen

Calcite

––

2.16

�6.63

–109.7

–Pg6

T1f3

4875.5

Pre-bitumen

Calcite

––

2.61

�5.18

–104.6

–

–Nomeasurementordataavailable.

*Repeatedsamples.

Tab

le2

Hyd

rogenisotopic

compositionsoffluid

inclusionwaters,

andthecorrespondinghomogenizationtemperaturesofthefluid

inclusions(T

h).Thed1

8O

values

ofthefluid

inclusionwater

were

calculatedbased

ontheoxygenisotopeequilibrium

fractionationequationbetweencalcitean

dpure

water:1000

lna=

2.78

�10

6/T

2-2.89(O

’Neilet

al.,1969).Theindex

H2S/(H

2S+C1-6)is

calculatedusingthemolarconcentrationsofnaturalgasin

theassociated

gasreservoirs.

Sam

ple

number

Form

ation

Depth

(m)

Host

mineral

d13D

(‰SMOW)

Measuredd1

3O

calcite

(‰PDB)

Calculatedd1

3O

water

(‰SMOW)

Th(�C)

H2S/(H

2S+

C1-6)

Dw3-2

T1f2

4720

Calcite

�39.0

�6.67

10.14

134.0

–Dw102-1

T1f2

4900.8

Calcite

�36.4

�9.24

8.42

145.8

–Dw102-2

T1f2

4824

Calcite

�58.9

�7.81

11.04

161.7

–HH-1

P3ch

Outcrop

Calcite

�50.7

�6.14

12.98

165.0

–HH-2

P3ch

Outcrop

Calcite

�67.8

�5.00

15.52

187.3

–Yb11-1

P3ch

6917

Calcite

�63.4

�6.0

13.45

170.1

7.0

Yb204-2

P3ch

6549

Calcite

�41.0

�6.65

12.22

161.5

2.5


Calcite samples (15 g) were cleaned as above, and crushedoff-line in a stainless steel container with ball bearings tocollect fluid inclusion gases (Yang et al., 1995; Gonget al., 2007), which were then measured for d13C on aThermo Finnigan Delta Plus IRMS.

The method of Yang et al. (1995) was adopted to deter-mine the hydrogen isotope compositions of the fluid inclu-sion water. 15 g of pure giant coarse milky white calcitecrystals from 7 samples (Table 2) were soaked in acetonefor about 3 hours and then ethanol overnight, dried andthen crushed in a stainless steel container with ball bearings.The released water was cryogenically transferred into vac-uum tubes immersed in liquid N2 and then reacted with zincto produce H2. The d2H values were analyzed on a Finni-gan MAT 252 mass spectrometer. Results are reported as‰ relative to the VSMOW standard, with a precision of±0.2‰.

Laser Raman Microspectrometry (LRM) was used tocharacterize the gas compositions inside the fluid inclusionsin the void-filling calcite crystals. The Raman system usedwas a JY/Horiba Lab-Ram HR Raman spectrometer, usinga He/Ne laser kit 632.8 nm/17 mW, a 40� Olympus objec-tive with 0.5 numerical aperture, and a 600 grooves/mmgrating with a spectral resolution of about 1 cm�1.

Formation water samples were collected from the P3chand T1f formations in various sour gas fields (e.g. Puguang,Dukouhe, Luojiazhai and Maobachan) and (Table 3;Fig. 1). All the water samples were filtered through glasswool to remove any solids and oil droplets. Then the majorionic compositions of the samples were analyzed using ionchromatography. The total dissolved solids (TDS) were cal-culated by summing the cation and anion concentrations.Details of the measurement procedure conducted isdescribed in Connolly et al. (1990).

4. RESULTS

4.1. Fluid inclusion petrography, microthermometry and gas

composition

Abundant diverse fluid inclusions were observed in void-filling calcite cements (Fig. 3), including 2-phase aqueousinclusions and single-, two- and three-phase hydrocarboninclusions, e.g., solid bitumen inclusions, pure gas inclu-sions and a few oil inclusions. The aqueous inclusions dis-play achromatic transparent characteristics and widevapor to liquid ratios from 2% to 25% and do not fluoresce(Fig. 3a, b). The gas inclusions have greyish black colorswith a brighter centre under polarized light, and mostlydo not fluoresce (Fig. 3a, b, c, d). Some of them exhibitweak white fluorescence due to the presence of minor oil.These are readily distinguished from the solid bitumeninclusions that are black under polarized light and do notfluoresce (Fig. 3d, e, f). The oil inclusions commonly exhibityellow and light yellow colors under polarized light and flu-oresce yellow–white under UV light (Fig. 3e, f). Within theTSR-derived calcite crystals, greatly varying proportions ofoil and gas inclusions were observed.

The Th and salinities of 2-phase aqueous fluid inclusionsin non-TSR calcite crystals range from 95.5 �C to 116 �C

(n = 22) and from 7.3 to 21 wt% eq. NaCl (n = 22), respec-tively. The values are slightly higher than those of the bulkdolomites reported previously (Jiang et al., 2014, Fig. 4)The Th and salinities of TSR-derived calcite crystals fromsour gas reservoirs range from 120 �C to 215.7 �C(n = 98) and from 0.9 to 21 wt% eq. NaCl (n = 98), respec-tively. There is a weak inverse relationship between salinityand temperature for the sour gas reservoirs (R2 = 0.31,Fig. 4), showing that salinity decreases as the temperatureof calcite increases.

Typical Raman spectra of gas compositions in fluidinclusions are reported in Fig. 5 for several samples. H2Speaks are present in each spectrum at a very low Ramanshift value of 2600 cm�1 (usually �2609 cm�1). CO2 peakscan be identified in HH-1, YGD-2 and YGD-3 at�1282 cm�1 and/or �1383 cm�1. Strong CH4 peaks arepresent in D5 and HH-1 at 2909 cm�1 and a weak C6H6

peak is observed in HH-1 at 3062 cm�1. The exclusive pres-ence of C6H6 peak without peaks of other C2+ hydrocarbongas has been also reported previously (Xia et al., 2012) andcan be attributed to that aromatic compounds are morestable at pyrolysis conditions relative to saturated hydro-carbons (Behar et al., 2002). Two peaks that are presentin YGD-3 at 2958 cm�1 and 3067 cm�1 are assigned toC2H6 and C6H6, respectively. CH4 and other hydrocarbongas are almost absent from the spectra or below detectionlimit in PLD-1 and YGD-2. The gas wetness was obtainedusing the semi-quantitative data of fluid inclusion gas com-positions, being from numerical estimation of relative peakareas (Fig. 6; Xie et al., 2006; Zhu et al., 2006; Xia et al.,2012). There is a first increasing trend of wetness as Th

increases from 119 �C to 137 �C and a subsequent decreas-ing trend with increasing Th. The wetness approaches avalue of less than 10% at temperatures above 165 �C.

4.2. d13C and d18O of CH4 and CO2 trapped in fluid

inclusions and void-filling calcite

The carbon isotopic compositions of CH4 and CO2

trapped in fluid inclusions and their host calcite samplesare listed in Table 1. Reproducibility of the measurementswas determined for eight samples by repeat analyses, andthese show reasonable replication (±1‰ for CH4 d

13C val-ues and ±1.8‰ for CO2 d13C values, Table 1). The CH4

from fluid inclusions in the crushed calcite crystals haved13C ratios mainly ranging from �46.7‰ to �29.8‰ withone abnormally positive d13C value (�23.3‰, n = 12, with7 replicates). Five samples have more depleted fluid inclu-sion CH4 d

13C values (�46.7‰ to �34.7‰) compared withthose for methane and ethane in the P3ch and T1f gasreservoirs in the NE Sichuan Basin (�34.5‰ to �27.5‰,Liu et al., 2013; Cai et al., 2004, 2013; Hao et al., 2015;Fig. 7a). The plot of the fluid inclusion CH4 d13C valuesand Th shows a wide scatter of d13C values at Th of126–137.4 �C, a less scatter at Th of 145.8–161.5 �Cand almost no change at Th of 165–185 �C (Fig. 7b).The Th values show positive correlative relationships toH2S/(H2S + C1-6) (Fig. 7c).

The CO2 in the fluid inclusions in the crushed calcitecrystals has a wide range of d13C values from �9.3‰ to


Tab

le3

Comparisonofform

ationwater

chem

istryassociated

withsourgasbetweenin

reef

andshoal

reservoirsan

devap

orite-bearingreservoir.

Reservo

irtype

Well

Depth

(m)

Strata

Na+

+K

+(g/

L)

Mg2

+(g/

L)

Ca2

+(g/

L)

Cl�

(g/

L)

SO

42�(g/

L)

HCO

3�(g/

L)

TDS(g/

L)

Water

Typ

eH

2S

(%)

Sourgasin

reef

andshoal

reservoir

Pg3

b5448–5469

T1f2

16.5

0.03

0.7

21.9

0.2

–39.8

NaH

CO

362.2

Pg6

5423.6–5432.5

P3ch

22.3

0.1

24.0

158.4

0.4

1.5

92.3

CaC

l 2–

Pg10b

6250–6270

T1f1-2

22.6

0.1

0.6

31.3

1.5

6.2

62.2

NaH

CO

3–

Pg10b

6193–6202

T1f1-2

19.1

0.1

0.6

27.3

0.2

1.1

50.8

NaH

CO

3–

Pg11

5705.0–5715.5

T1f2

10.3

0.01

0.1

12.1

3.6

2.5

28.7

NaH

CO

3–

D5a

4784.5–4823

T1f1

43.0

0.4

0.8

66.4

0.8

3.0

114.4

MgC

l 215.9

Pg8

b5614–5625

P3ch

26.3

0.2

1.3

40.3

0.6

2.4

72.2

Na 2SO

4–

Pg8

b5634–5643

P3ch

–0.03

3.1

47.8

0.000

2.1

83.2

CaC

l 26.9

Pg8

6110.0–6130.0

P3ch

–6.5

15.6

54.4

0.000

3.9

87.0

CaC

l 2–

Pg9

b6151–6175

P3ch

18.8

0.03

0.6

26.1

0.9

3.1

51.0

NaH

CO

314.7

Mb3

4609–4630

P3ch

16.5

0.1

0.6

25.5

0.3

–42.9

NaH

CO

334.7

Mb3

4340–4420

P3ch

14.4

0.05

0.6

23.1

0.3

–38.4

NaH

CO

3–

Yb123

6978–6986

P3ch

18.7

0.05

–28.2

––

47.6

CaC

l 225.7

Yb123b

6904–6918

P3ch

18.8

1.0

5.4

39.7

03.0

67.8

CaC

l 24.1

Yb224b

6625–6636

P3ch

18.7

0.1

0.6

27.5

––

46.9

MgC

l210.6

Yb9b

6836–6857

P3ch

12.1

2.1

–37.6

––

58.1

CaC

l 212.1

Yb9b

7000–7020

P3ch

23.9

0.1

1.2

36.2

1.4

3.5

66.3

Na 2SO

4–

Yb16

b6950–6974

P3ch

19.6

0.2

0.6

31.3

0.6

–52.2

CaC

l 212.2

Sourgasin

evap

orite-bearing

reservoir

D3c

3899

T1j

13.4

0.06

0.3

9.9

15.0

0.8

39.4

Na 2SO

4–

D4c

3848

T1j

12.6

0.03

0.5

12.1

11.0

0.3

36.5

Na 2SO

4–

Lj4

c3066

T1j

15.2

0.03

0.4

13.6

27.5

1.0

57.7

Na 2SO

4–

Mb2

4145–4427.5

T1f3

33.3

0.002

0.3

32.1

11.0

–77.1

Na 2SO

4–

Pg5

4830.00–

4868.00

T1f3

1.3

0.6

16.1

31.4

1.0

0.2

50.5

CaC

l 211.3

Po1a

3527.4–3579.5

T1f1

9.2

0.2

0.9

10.6

6.2

2.0

29.0

Na 2SO

414.2

Z1a

3411–3430

T1f1

8.6

01.2

11.2

5.2

0.5

26.6

Na 2SO

4–

Zj1

a5576.3–5615

T1f1

23.6

00.6

34.9

2.6

1.1

62.9

Na 2SO

4–

–Nomeasurementordataavailable.TDS=

totaldissolved

solids.

aDatafrom

Shen

(2005).

bDatafrom

Liet

al.(2016).

cDatafrom

Zhao

etal.(2014).


+8.4‰ (n = 12, with 7 replicates) (Table 1). The range ofvalues is similar to the reported data for CO2 in the reser-voirs (�12.9‰ to +3.3‰, Zhu et al., 2008; Liu et al.,2013; Cai et al., 2014) and is significantly heavier than

TSR-derived CO2 based on laboratory experiments(�37.1‰ to �34.9‰, Pan et al., 2006). Two samples(DW3-1 and HH-1; both repeated) have slightly higherd13C values for CO2 in the fluid inclusions (+5.4‰ to+8.4‰; Table 1) compared with those of the bulk carbon-ates (�0.8‰ to +4.9‰, Huang, 1994). Similarly, heavy car-bon isotopes of CO2 beyond the range of the carbonate hostrocks have been previously reported (Mankiewicz et al.,2009). There is no significant correlation between the d13Cvalues of CO2 and CH4 trapped in the fluid inclusions(Fig. 7d). The d13C values of CO2 trapped in the fluid inclu-sions has a very weak positive correlation with the Th val-ues (Fig. 7e).

The non-TSR calcite samples have a narrow range ofd13C values (�1.4‰ to +2.6‰; n = 15, Fig. 8a), which isclose to those of the contemporary seawater and bulk car-bonates. In contrast, the TSR calcite samples from thisstudy (Table 1) and from the literature have a wide rangeof d13C values from �18.9‰ to +3.6‰ (n = 112, Fig. 8a),

Fig. 4. Relationship between homogenization temperatures andsalinities of the fluid inclusions from bulk dolomite, non-TSRcalcite and TSR calcite samples from sour gas reservoirs.

Fig. 5. Laser Raman spectra of gas compositions for the D5, HH-1, YGD-3, PLD-1 and YGD-1 samples. Note the difference in size andpresence of the peaks of the hydrocarbon gases. The temperature data shown for each spectra are the measured Th values.


with an average of �4.3‰. Most of the calcite samples thatwere selected for the crush experiment have lighter d13C val-ues than the released CO2, with the differences being mainlybetween �0.2‰ and �10‰ (Table 1). There are weak pos-itive correlations between the d13C values of the calcite andthe CO2 (Fig. 8b) trapped in the fluid inclusions. There is nocorrelation between the d13C values of diagenetic calcitecements and Th (Fig. 8c). However, for the calcites withTh � 119.7 �C, the d13C values have a positive correlation(R2 = 0.46) with the Th (Fig. 8c). A similar trend is also pre-sent based on the data from others (Jiang, 2009; Jiang et al.,2014, 2015). Interestingly, the replacive calcite crystals afteranhydrite have lighter d13C values (�1.4‰ to �18.9‰)than the TSR calcites in the reef and shoal facies (Fig. 9).

4.3. d2H and d18O values of the water in the fluid inclusions

Measured d2H of the water in the fluid inclusions arefrom �67.8‰ to �36.4‰ (with an average of �51‰,n = 7, Table 2). d18O values of the fluid inclusion waterwere calculated based on the oxygen isotope equilibriumfractionation equation between calcite and pure water(O’Neil et al., 1969) to have from +8.4‰ to + 15.5‰(n = 7, Table 2). Most of the d2H values are significantlylighter than those of the contemporary seawater (close to0; Craig, 1961; Hagemann, 1970). There is no significantchange or a slight decrease in the d2H values of the waterin the fluid inclusions as the Th increases from 134 �C to161.5 �C, followed by a first sharp increasing and then agradual increasing trend from 161.5 �C to 170.1 �C andfrom 170.1 �C to 187.3 �C, respectively (Fig. 10).

4.4. Formation water chemistry

The chemical compositions of the P3ch-T1j formationwater from the sour gas reservoirs are shown in Table 3.There are significant vertical and regional heterogeneitiesin formation water chemistry. Generally, most of the for-mation water samples from the P3ch and T1f

1-2 strata (reefand shoal facies) in NE Sichuan Basin have SO4

2� concen-trations from 0 to 0.9 g/L, significantly lower than theP3ch (mean 2.2 g/L; Lowenstein et al., 2005) and T1f (mean2.3 g/L; Horita et al., 2002) formation waters from other

basins, respectively. In contrast, the formation water sam-ples of the T1j Formation, which contains evaporite layers,have much higher SO4

2� concentrations (11.0–27.5 g/L).The formation water samples from the T1f

3 member havevariable SO4

2� concentrations from 1.0 g/L to 11.0 g/L.The T1f formation water samples collected from thelagoonal facies, e.g., from the Z1 and Zj1 wells, have higherSO4

2� concentrations (2.6–6.2 g/L) compared with thosefrom the reef and shoal facies. In addition, along theplatform margin, the formation water samples associatedwith sour gas reservoirs commonly have lower SO4

2� con-centrations (0–0.9 g/L, Table 3) than those with sweet gas(1.4–3.2 g/L, data from Shen (2005) and Li et al. (2016)).

There are no significant differences in the total dissolvedsolid (TDS) concentrations throughout the sedimentaryfacies (Table 3). The formation water samples from the sourgas reservoirs have a wide TDS range from 26.6 g/L to114.4 g/L, equal to 2.7 to 11.4 wt% NaCl. The values areclose to most of the low-salinity values of the TSR-derived calcites.

5. DISCUSSION

5.1. Process for variation of fluid inclusion CH4 d13C values

5.1.1. Oil-dominated TSR.

TSR is most likely to have been initiated by liquidhydrocarbons at the relatively low Th of 133.1 �C and137 �C, as is supported by the presence of oil inclusions(Fig. 3a, b, e, f) and H2S inclusions but not associated withhydrocarbon gas inclusions (Fig. 5). During this period,CH4 is likely present in volumetrically minor amounts asoil-dissolved gas, since an initial slow and non-autocatalytic stage is usually associated with the TSR reac-tion involving long-chain hydrocarbons (Xia et al., 2014).Similar small amounts of CH4 were also detected in reac-tion of liquid hydrocarbons with elemental sulfur(Kowalewski et al., 2010). Compared with the naturalgas, CH4 released from fluid inclusions has a much widerspread of d13C values from �46.7‰ to �23.0‰ (Fig. 7a)and shows significant variation as Th increases. The largeshift can be attributed to multiple sources with different car-bon isotope signatures within the oil precursor, as proposedby Smith et al. (1985) and Tang et al. (2000).

5.1.2. Wet gas-dominated TSR

As temperature gradually rose, TSR proceeded into wetgas-dominated stage, as indicated by the coexistence of C2+

hydrocarbon gas and H2S in the fluid inclusions with Th of156.3 �C (Fig. 5). The detection of a wide concentrationrange of C2-6 gases with wetness from �5% to 55.9% in fluidinclusions in TSR-derived calcite crystals provided furtherevidence (Fig. 6; Xie et al., 2006; Zhu et al., 2006; Xiaet al., 2012). The roughly decreasing trend of wetness andmethane shifts to a trend of 13C enrichment at Th approxi-mately >137 �C (Fig. 7b) hints at extensive oxidation ofC2-6 gases and new generation and accumulation ofmethane. Similar positive shift in methane d13C is alsoreported in oxidization of C2-4 gases during TSR simulation

Fig. 6. Fluid inclusion gas wetness as a function of Th (the wetnessrefers to the ratio of C2-6/C1-6).


reactions with MgSO4 (Pan et al., 2006; Lu et al., 2010) andduring non-TSR reactions with CuO (Kiyosu andImaizumi, 1996).

5.1.3. Dry gas-dominated TSR

CH4 was detected as the predominant gaseous organiccompound and coexists with H2S in fluid inclusions withTh of 165 �C (Fig. 5). The minimum temperature of drygas-dominated TSR is somewhat blurred based on avail-able data. The variation in wetness with Th (Fig. 6) cannotbe used to constrain the lower thermal limit because of theinvolving semi-quantitative data of fluid inclusion gas com-positions, stemming from estimation of relative laserRaman peak areas (Xie et al., 2006; Zhu et al., 2006;

Xia et al., 2012) However, the abrupt change of d13CCH4

with temperature at 161.5 �C yields instructive cluesregarding the corresponding threshold temperature of drygas-dominated TSR. This proposal is consistent with theacceleration of water production rate when the Th is over161.5 �C, as discussed in the following sections.

Assuming that dry gas-dominated TSR commenced at�161.5 �C, CH4 d13C values show an increase from�33.1‰ to �29.6‰ throughout the stage (Fig. 7b) andthe variation (3.5‰) is closed to the calculated value(2.9‰) based on the data of present-day gas compositions(Cai et al., 2004). This, together with the Th shows positivecorrelative relationships to H2S/(H2S + C1-6) or TSR extent(Worden et al., 1995; Cai et al., 2003; 2004) (Fig. 7c),

Fig. 7. Scatter plots showing (a) the difference in the d13C values of CH4 in fluid inclusions, sour gas reservoirs and sweet gas reservoirs. Theshift in methane d13C in the sweet reservoirs is supposed to result from hydrocarbon pyrolysis, while the shift in the sour reservoirs resulteither from hydrocarbon pyrolysis or TSR, and (b) the relationship of the d13C values of CH4 in fluid inclusions to Th, reflecting differenteffects of various stage of TSR on CH4 d

13C values and (c) the relationship of H2S/(H2S + C1-6) to Th, and the relationships of d13C values ofCO2 in fluid inclusions to the d13C values of CH4 in fluid inclusions (d), and to Th (e). The data for sour and sweet gas reservoirs are from Caiet al. (2004, 2013), Hao et al. (2008, 2015), Zhu et al. (2008) and Liu et al. (2013). Note that one data point has an abnormally positive d13Cvalue of �23.3‰ in (a, b and d) and is an outlier, because it is significantly heavier than the reported methane d13C values in the reservoirs,which should be the end-products of continuous oxidation and therefore isotopically heaviest.


indicates that CH4 becomes isotopically heavier as TSRproceeds during the stage of dry gas-dominated TSR. Thisis because the isotopically lighter methane reacts rapidlywith dissolved sulfate, leaving the remaining methane iso-topically heavier (Worden and Smalley, 1996; Cai et al.,2003; 2004; 2013; Jenden et al., 2015). The effects are alsoreflected in the perceptible increase of methane d13C withGSI and gas dryness (CH4/RCnH2n+2) (Cai et al., 2004;2013; Hao et al., 2008, Liu et al., 2013). We have to pointout that, with the present data, we cannot clarify exactlythe effects of dry gas-dominated TSR on C-isotope fraction-ation effect, because of the uncertainty on the thresholdtemperature of dry gas-dominated TSR. However, the vari-ation of CH4 d13C value in the NE Sichuan Basin is obvi-ously not as vigorous as those reported in other methane-dominated TSR cases worldwide (e.g., Worden et al.,1996) and in experimental work (Pan et al., 2006).

At Th � 165 �C, CH4 d13C values range from �30.1‰ to

�29.6‰ and show no obvious increasing trend with tem-perature (Fig. 7b), implying dwindling dry gas-dominatedTSR reaction. This is consistent with the proposal that afinal, or late-stage TSR reaction continues at a slower rate(Orr, 1990; Machel, 2001; Pan et al., 2006; Mankiewiczet al., 2009; Xia et al., 2014). It is noteworthy that the extentof gas souring (GSI < 20%) in the T1f reservoirs in the NESichuan Basin is much lower than that in the Khuff Forma-tion of Abu Dhabi (GSI = 50%; Worden and Smalley,1996), although the former reservoirs experienced similar

high temperatures (up to 220 �C) and longer duration oftemperatures >150 �C (>140 Ma vs. <60 Ma, Gumati,1993; Li et al., 2012). It seems that the extent of the slowdry gas-dominated TSR reaction has, to a large extent, beenlimited at an advanced stage in the NE Sichuan Basin.

As shown in Table 3, the SO42� concentrations of the for-

mation water from the sour gas reservoirs in reef and shoalfacies are generally pretty low (mostly lower than 0.9 g/L)compared with those from the contemporary seawater(�2.2 g/L, Lowenstein et al., 2005) and those from thesweet gas reservoirs (1.4–3.2 g/L, Shen, 2005; Li et al.,2016), indicating that the depleted SO4

2� in the formationwater is attributed to its consumption by TSR. Unlike inthe evaporite-bearing strata, where the dissolution of gyp-sum or anhydrite might have constantly released SO4

2� intothe pore waters, in the high-energy shelf-margin reservoirs,the slow transport rate of aqueous sulfate from evaporativeunits to the remote reaction sites would have greatly limitedthe TSR reaction, as suggested by Worden et al. (2000).One more possible limiting factor is the long duration offormation temperatures >200 �C (�110 Ma; Li et al.,2012) in the NE Sichuan Basin. It might have greatlyobstructed formation of MgSO4 contact ion-pairs (CIP)in the pore waters (pH � 6.5–8.5) because a magnesium-hydroxide-sulfate-hydrate complex will be formed insteadat temperatures in excess of �200 �C (Ma et al., 2008).The [MgSO4]CIP, however, is proposed as a dominatedreactive sulfate species for TSR in the typical petroleum

Fig. 8. Cross plots showing relationships of d13C values of void-filling calcite samples with (a) their d18O values, (b) the d13C values of CO2 inhosted fluid inclusions, (c) the d13C values of CH4 in hosted fluid inclusions, and (d) the Th values. Note that the black circle in (d) representsone anomalous value. The trend line with R2 = 0.46 is only for the data from this study, not including the literature values.


reservoir formation waters (Ma et al., 2008; Zhang et al.,2008, 2012).

The effects of the heterogeneities in SO42� concentrations

on dry gas-dominated TSR were also observed in a gener-ally decreasing trend of the methane d13C values for thegas reservoirs from the paleo-lagoon areas (�30.1‰ to�28.9‰) to the reef and shoal margin (�33.7‰ to�29.8‰, data were from Cai et al. (2004, 2013), Haoet al. (2008, 2015), Zhu et al. (2008) and Liu et al. (2013),Fig. 11). A maturity effect on the d13C values can be ruledout because the reservoirs experienced similar maximumburial and temperatures (Huang et al., 2010; Cai et al.,2014) and there is poor relationship between the methaned13C values and the present depths (Fig. 12). The shifttoward heavier d13C values in methane may signify largerisotopic fractionation and more intense oxidation ofmethane by sufficient sulfate, while the methane with rela-tively depleted d13C values reflects lower consumption ofthe methane due to the unsustainable migration of sulfate.

A significant S-isotope fractionation (10–20‰) amongS0, H2S and sulfate has been well demonstrated duringexperimental simulation of TSR with excessive dissolvedsulfate (e.g., Na2SO4 solution, Kiyosu et al., 1990;Meshoulam et al., 2016), being correlated with initial cleav-age of the S–O bond of sulfate (Goldstein and Aizenshtat,1994) or equilibrium effects (Meshoulam et al., 2016). Sim-ilarly, in the paleo-lagoon areas, a negative shift in the d34Svalue (2.0–7.9‰, Fig. 13a) of the elemental sulfur can also

be attributed to the excessive sulfate relative to dissolvedhydrocarbons. The accumulation of elemental S, showingbright yellow colours and extremely fine crystal sizes in coreand cloudy appearance in thin section (Fig. 13b, c, d),might result from low availability or low rate of supply oforganic compounds (Machel et al., 1995, 2001; Alonso-Azcarate et al., 2001). Similar substantial kineticS-isotope effect has been previously documented in some

Fig. 9. Map of the regional variation in d13C values of TSR-related calcite crystals on the east side of the Kaijiang-Liangping (K-L) Trough inthe NE Sichuan Basin. The available data are from Table 1 and Li et al. (2012, 2014), Jiang et al. (2014, 2015) and Zhu et al. (2005), and arepresented as average values.

Fig. 10. Cross plot showing the relationship between d2H values ofwater extracted from fluid inclusions in the calcite crystals and theTh values of the fluid inclusions. The figure shows that isotopically-distinct (low d2H) water was mostly produced during methane-dominated TSR.


case studies in the Cameros Basin, the Bongara mine, theTarim Basin and the Sichuan Basin (Alonso-Azcarateet al., 2001; Basuki et al., 2008; Cai et al., 2008; 2010). Incomparison, in the grainstone reservoirs away from theevaporative area, the rate of TSR is limited by unsustain-able supply and paucity of reactive sulfate, reduction ofwhich went to completion. Then elemental sulfur shows

d34S values (9.7–19.1‰, Fig. 13a) close to the parent sulfatebecause no kinetic sulfur isotope fractionation is realizedduring TSR in such case (Machel, 1995).

5.2. Water generation during TSR?

5.2.1. Evidence from fluid inclusion salinity and formation

water TDS

Formation of abundant low-salinity aqueous fluid inclu-sions and the regular decrease in salinity with the extent ofthe TSR reaction were first observed and invoked to con-firm water production during TSR by Worden et al.(1996). For the NE Sichuan Basin, similar fluid inclusiondata from the sour gas reservoirs were obtained in thisstudy (Fig. 4) and have been also documented in our previ-ous work (Li et al., 2012) and by others (e.g., Jiang et al.,2015).

The low-salinity water might have been inherited fromearly diagenetic water, e.g., the eogenetic meteoric water(Li et al., 2012). However, the meteoric water in thepores was proposed to have subsequently been replacedby semi-saline to saline dolomitizing fluids from back-reef lagoons (Jiang et al., 2014; Li et al., 2014). The mod-erate to high salinities of fluid inclusions detected in dolo-mite crystals by Jiang et al. (2014) imply a significantchange in the diagenetic fluids (Fig. 4). This means that

Fig. 11. Map of the regional variation in d13C values of methane in the present-day gas reservoirs on the east side of the Kaijiang-Liangping(K-L) Trough in the NE Sichuan Basin. Data were collected from Cai et al. (2004, 2013), Hao et al. (2008, 2015), Zhu et al. (2008) and Liuet al. (2013).

Fig. 12. Relationship between d13C values of methane in thepresent-day gas reservoirs on the east side of the K-L Trough andthe present-day burial depths. Data were collected from Cai et al.(2004, 2013), Hao et al. (2008, 2015), Zhu et al. (2008) and Liu et al.(2013).


the TSR reaction was initiated in a brackish or salinepore-fluid environment, which is further supported bythe medium to high salinities of water trapped in fluidinclusions in non-TSR calcite (Fig. 4). The salinity of thisinitial water likely depended on the mixing ratio of theearly pore water and the more saline dolomitizing fluid.The water was then heterogeneously diluted by the newTSR-derived water to form the wider spread in the salin-ity of the fluid inclusions (mostly from 1 to 20 wt%NaCl, Fig. 4).

In the sour gas reservoirs, the formation water with thelowest TDS value (26.6 g/L) is considered to have under-gone the most extreme dilution by early meteoric waterand/or new water derived from TSR (Fig. 4). Consideringthat the dilution effect of the meteoric water might have beenremoved by displacement of the subsequent saline dolomi-tizing fluids, as mentioned above, the TSR-derived water

is very likely to exert a profound influence on formation-water salinity in some locations. There seem to be significantheterogeneities in the occurrence of TSR and imperfect mix-ing between TSR-derived water and the pre-existing porewater, as proposed by Worden et al. (1996).

5.2.2. Evidence from d2H values of fluid inclusion water

Most of the fluid inclusion water samples collected fromthe TSR-derived calcite crystals have lighter d2H values(Table 2) compared with contemporary seawater. Sincethe latitude of the Sichuan Basin during the Late Permianand Early Triassic was close to the equator (Renne et al.,1995), meteoric water is speculated to have had a d2H closeto zero based on the proposed atmospheric Rayleigh pro-cess (Craig, 1961), as vapor is removed from poleward-moving tropospheric air. It is unlikely that much of thiswater with isotopically heavy hydrogen was trapped in

Fig. 13. (a) Map of the regional variation in d34S values of elemental sulfur (green values) on the east side of the Kaijiang-Liangping (K-L)Trough. The data were collected from Cai et al. (2004), Zhu et al. (2005) and Zhu et al. (2008). The photographs show the distribution ofelemental sulfur. (b) Core specimen, D5 well, 4775.82 m, T1f

1; (c) Cross-polarized light, Lj5 well, 2940.5 m, T1f1; (d) Core specimen, Jz1,

2990.8 m, T1f1; (e) Scanning electron microscope image showing sulfur-rich pellets wrapped in solid bitumen, which has been previously

reported (Jiang et al., 2002; Zhu et al., 2008; Li et al., 2014), Pg2 well, 4977 m, T1f2; (f) Energy-dispersive X-ray spectrum of the pellet

highlighted in red in (e). Note the two d34S data points in the Mb3 and Pg6 wells (10.1‰ and 9.8‰, respectively) were determined forelemental sulfur extracted from solid bitumen (Zhu et al., 2008). (For interpretation of the references to colour in this figure legend, the readeris referred to the web version of this article.)


the analysed fluid inclusions. Instead, the addition of TSR-derived water with significantly lighter d2H values derivedfrom hydrocarbon reactants is likely to be responsible forthe negative shift in d2H of the fluid inclusion water.

The published d2H data of methane and ethane in theP3ch-T1f gas reservoirs in the NE Sichuan Basin have a nar-row range from �89‰ to �131‰ (Liu et al., 2014b). Sincethe hydrogen in TSR generated water is coming from thehydrocarbons, the relatively heavier d2H values of the fluidinclusion water samples compared with those of the reactivehydrocarbons can be ascribed to mixing of TSR-derivedwater and formation water. If the average d2H value(�51‰) of the fluid inclusion water samples is used, and ifthe d2H values of the pre-TSR formation water are presumedto range from �37.5‰ (i.e., the most depleted d2H value ofthe formation water) to 0‰ (i.e., d2H of the contemporaryseawater), then the mixing ratio of TSR-derived water andformation water is calculated to be from 1:2.9 to 1:0.8. Itseems that water generated during TSR is of volumetricimportance, at least in relatively localized TSR reaction sites.

The overall decrease of the d2H values of the fluid inclu-sion waters with increase of Th (Fig. 10) provides anotherclue regarding TSR as a water-generation reaction. Therelationships between the two parameters as Th increasesfrom 134 �C to 161.5 �C suggest that, in general, there isa very slight decrease in the d2H values (Fig. 10), probablyindicating volumetrically minor water was produced duringthe oil- and wet gas-dominated TSR stage, as reported pre-viously from Tarim basin (Li et al., 2017). The subsequentsharp decrease in d2H when Th increases from 161.5 �C to170 �C hints at an acceleration of water production as earlydry gas-dominated TSR proceeds (Fig. 10). At higher Th,the d2H values continue to decrease, but with a lower slope(Fig. 10), possibly indicating dwindling water productionduring late dry gas-dominated TSR stage.

Despite some uncertainties, it seems that the change ofwater production rate during TSR is consistent with threestages of TSR, i.e., liquid hydrocarbons, C2+ hydrocarbongases and dry gas stages. The occurrence of the dry gas-dominated TSR reaction (>161.5 �C) seems to be the mostefficient for water production and for negative shift in waterhydrogen isotopes. This observation is consistent with theproposal for net mass balance reactions for multiple typesof hydrocarbon gases and sulfate (Pan et al., 2006):

SO42� þMg2þ þ CH4 ! MgCO3 þH2SþH2O ð4Þ

3SO42� þ 3Mg2þ þ 4C2H6 ! 3MgCO3 þ 4CH4

þ3H2Sþ CO2 þH2O ð5Þ3SO4

2� þ 3Mg2þ þ 2C3H8 ! 3MgCO3 þ 2CH4

þ3H2Sþ CO2 þH2O ð6Þ5SO4

2� þ 5Mg2þ þ 4n-C4H10 þH2O

! 5MgCO3 þ 8CH4 þ 5H2Sþ 3CO2 ð7ÞIt can also be speculated that different hydrocarbon reac-

tants result in variation in the amount of water producedduring TSR, thus resulting in the controversy on water gen-eration during TSR. This proposal is supported by the factthat the sour gas provinces, where water production during

TSR was documented (e.g., Worden et al., 1996; Yang et al.,2001; Vandeginste et al., 2009; Jiang et al., 2015), withoutexception, experienced light hydrocarbon gas-dominatedor methane-dominated TSR. As for the Devonian NiskuFormation of western Canada and Ordovician carbonatereservoir in the Tarim Basin, where heavy hydrocarbonsinduced the TSR reaction, volumetrically minor to negligi-ble water was released during TSR (Manzano et al., 1997;Machel, 2001; Li et al., 2017). Especially in the NE SichuanBasin, the combined effects of a decrease in TSR rate as thehydrocarbon phase becomes methane rich, a shortage ofreactive sulfate and/or limited [MgSO4]CIP across a longduration of formation temperatures >200 �C likely retardedwater production during the advanced TSR stage (Fig. 10).

5.3. Dissolution of carbonate minerals during TSR?

Theoretically, as the TSR reaction proceeds in the watersystem being dominated initially by dissolved matrix car-bonate, the production of TSR-derived CO2 with an iso-topically light carbon source (i.e., hydrocarbons, Machel,1995) will result in lighter d13C values of total CO2 in reser-voirs. In this study, no correlation is found to occurbetween the fluid inclusion CO2 and CH4 d13C values(Fig. 7d). This means that the TSR-derived CO2 mightnot be the only carbon source in the system. As an alterna-tive, inorganic CO2 may have been added to lead to enrich-ment in 13C and thus a change in the trend of isotopicallylighter CO2. The significantly positive d13C values of theCO2 (Table 1), compared with the TSR-derived CO2 onthe TSR reaction of gaseous hydrocarbons with MgSO4-�7H2O at 350 �C for a duration of 288 h (Pan et al.,2006), indicates a substantial contribution of inorganic car-bon. This might be ascribed to either re-equilibration ofCO2 with the dissolved carbonate of heavy carbon isotopesignature in the water film (Mankiewicz et al., 2009; Haoet al., 2015), or bulk dolomite dissolution. However, a weakpositive relationship between the d13C values of CO2 in thefluid inclusions and the corresponding Th (Fig. 7e) indicatesthat the positive shift in the CO2 d

13C yields as a function ofincreasing thermal stress, which would promote the ther-modynamically favourable TSR reaction (Machel, 2001).That is, the constant enrichment in 13CCO2 is more likelyto be correlated with the TSR process and the resulting dis-solution of carbonates.

Most of the crushed calcite crystals and other selectedcalcite samples have d13C values �5‰ � �10‰ lighter thanthe CO2 trapped in fluid inclusions in each sample (Table 1,Fig. 7b). This, together with the positive correlationbetween their d13C values (Fig. 8b), hints at a preferentialsequestration of 13C-depleted CO2 into the calcite cements,with a positive shift in d13C for the residual CO2. Similarcases have been previously documented (e.g., Hao et al.,2015, and references therein). Another plausible explana-tion is that the calcite cements might have precipitatedrapidly in the 12C-rich diagenetic solutions, where theTSR reaction occurred, and may not have reached isotopicequilibrium with the free CO2 gas. As the temperatureincreases from below to above 119.7 �C, there is a dramaticshift from heavy to light d13C values of the calcite (Fig. 8c),


suggesting that 119.7 �C is the minimum temperature forTSR in the NE Sichuan Basin. For the TSR-derived cal-cites, the d13C values show a positive correlation(R2 = 0.46) with Th (Fig. 8c), which is consistent with datafrom Jiang et al. (2014, 2015) with slightly lower R2 of 0.26.

However, this trend is completely opposite to that of theKhuff Formation in Abu Dhabi (Worden and Smalley,1996). A progressive increase in the mixing ratio of TSR-derived CO2 and 13C-rich CO2 in the residual pore watersmay explain this occurrence. Worden et al. (2000) claimedthat within these anhydrite-bearing strata, TSR-derived cal-cite tended to grow on the surface of anhydrite crystals asreplacive masses due to the low porosity and permeabilityof anhydrite crystals (Fig. 14a). That is, the aqueous cal-cium (from dissolved anhydrite) and newly formed 12C-rich bicarbonate (from TSR) did not travel far from thesites of TSR and precipitated as calcite crystals directlyadjacent to anhydrite. The TSR reaction became transportcontrolled as the calcite began to isolate remnant anhydritefrom dissolved hydrocarbon gases.

In contrast, in the NE Sichuan Basin, there seems to be adistinctive setting of TSR sites, probably resulting in differ-ent dissolution-precipitation processes. The reactive sulfatefor the TSR reaction in oolitic shoal and reef reservoirs wasdemonstrated to have been derived from the early refluxingof evaporative brines from the back-reef restricted lagoons(Li et al., 2014), and may have reacted with the availablehydrocarbons during TSR. Assuming that calcium releasedfrom anhydrite were precipitated close to the sites of TSR,then the calcite cements should be distributed uniformly inthe pores. However, the cements are present as occasionalintergrown aggregations in the thin sections (Fig. 14c, d).One possible scenario is that, unlike the anhydrite-bearingstrata, the better petrophysical properties and greateramounts of preserved pore waters (Fig. 14c, d) offered thepossibility of short distance transport of the solutes awayfrom the TSR sites within the system. As TSR-derived cal-cite precipitated in open void spaces elsewhere, Ca2+ in theinitial water was quickly consumed and Ca2+ concentra-tions decreased rapidly because of no continuous calcium

Fig. 14. Schematic representation of the essential differences in TSR settings, rate-controlling steps for the TSR reaction, and relateddissolution-precipitation processes during TSR for anhydrite-bearing reservoirs and high-energy grainstone reservoirs. The simplified versionof rate-controlling steps for TSR is revised from Worden et al. (2000). The way in which reactive sulfate is supplied to the site of TSR (step 2)and precipitation of calcite (step 5) has a profound influence on carbonate alteration. The photographs are used for showing the location ofthe TSR reaction zone and replacive calcite: (a) Thin section photomicrograph, plane-polarized light (PL), Khuff Formation, Abu Dhabi(Worden et al., 2000); (b)-(d) from NE Sichuan Basin; (b) Core specimen of dolomite with vugs filling calcite, anhydrite and elemental sulfur,Po1 Well, 3468.7 m, T1f

2; (c) Thin section photomicrograph, with abundant pores stained blue and calcite stained pink, D4 well, 4235.6 m,T1f

2; (d) Thin section photomicrograph, plane-polarized light (PL), D4 well, 4235.6 m, T1f2. (For interpretation of the references to colour in

this figure legend, the reader is referred to the web version of this article.)


supply from anhydrite dissolution. This might have pro-vided an environment conducive to significant dissolutionof dolomite minerals to release Mg2+, which is thermody-namically favourable to form [MgSO4]CIP and promotesthe TSR reaction (Ma et al., 2008; Cai et al., 2014). The car-bonate dissolution led to sustained release of 13C-rich CO2,which was captured in the fluid inclusions and incorporatedinto the TSR-derived calcite.

Clearly, the magnitude of carbonate dissolution duringTSR seems to be influenced by the ability to transport reac-tants to the surface of the minerals, and move the dissolvedions away from the site. The reported deep carbonate disso-lution and porosity alteration in the TSR regions with theco-occurrence of hydrothermal activity (Biehl et al., 2016)also highlights the importance of fluid movement and solutetransport.

In the NE Sichuan Basin, the settings of TSR sites in theback-reef lagoon areas with gypsum associated (Fig. 14b)are similar to those of the Khuff Formation in Abu Dhabi(Fig. 14a, Worden and Smalley, 1996) but different fromthose of the reef and shoal margin. Slow mass transportand a quick saturation of calcium in the pore watersresulted in the growth of replacive calcite at the very edgesof anhydrite nodules (Fig. 14b). These regional differencesmay have exerted a profound influence on the d13C ofTSR-derived calcites, which become heavier from thepaleo-lagoon area to the platform margin (Fig. 9). Thelighter d13C values suggest a substantial contribution oforganic carbon from the oxidation of hydrocarbons byTSR, whereas the heavier values reflect incorporation ofmore inorganic carbon released from carbonate dissolution.This, in turn, gives compelling evidence for the close rela-tionship between the carbonate dissolution-precipitationprocess and the different settings of the TSR sites.

6. CONCLUSIONS

(1) TSR was initiated by liquid hydrocarbons in the NESichuan Basin to form H2S-bearing oil inclusionswith lower Th values (e.g., �137 �C) in TSR-derivedcalcite crystals. Subsequently, wet gas-dominatedTSR proceeded and led to the coexistence of C2+

hydrocarbon gas and H2S in fluid inclusions. Drygas-dominated TSR occurred in higher reservoir tem-perature (> about 161.5 �C) when most C2+ hydro-carbons were exhausted.

(2) Extremely depleted CH4 d13C values (as low as�46.7‰) were first reported in fluid inclusions inthe NE Sichuan Basin. The positive shift in CH4

d13C values (from �46.7‰ to �33.1‰) was initiatedby oil- and wet gas-dominated TSR, and then pro-moted by dry gas-dominated TSR to reach the finald13C value (�29.6‰).

(3) The extent of dry gas-dominated TSR in the NESichuan Basin was limited by the availability of dis-solved sulfate. Within the high-energy shelf-marginreservoirs, the sulfate has been almost completelyconsumed by TSR, resulting in more depletedmethane d13C, lesser amounts of elemental sulfur,

and d34S values close to the parent sulfate. In con-trast, within the anhydrite-bearing reservoirs in thepaleo-lagoon area, the reactions of excessive dis-solved sulfate with hydrocarbons resulted in 13Cenrichment in the methane, accumulation of elemen-tal sulfur, and significant fractionation of the sulfurisotopes.

(4) The d2H and salinity data of fluid inclusion watersuggest water generation during TSR. Dry gas-dominated TSR seems to be much more efficient atwater production compared with oil- and wetgas-dominated TSR. The process resulted in thed2H values of fluid inclusion water shifting from�36.4‰ to �67.8‰ and salinities decreasing to aslow as 0.9 wt% NaCl. The final decline of water pro-duction rate is consistent with the slower rate of thedry gas-dominated TSR reaction due to the shortageof reactive sulphate.

(5) The occurrence of TSR in the high-energy shelf-margin reservoirs led to intense dolomite dissolution.The addition of the released inorganic CO2 into thesystem promoted the positive shift in d13C of theCO2 trapped in the fluid inclusions from �9.3‰ to+6.3‰ as well as that of the TSR-derived calcite.In contrast, in the paleo-lagoon area, extensive car-bonate precipitation was the predominant process,resulting in more depleted calcite d13C values(�1.4‰ to �18.9‰).

(6) These results show that differences in the geologicalsettings for the occurrence of TSR worldwide (e.g.,sedimentary settings, reservoir temperature andTSR extent) could result in a great diversity in theappearance and effects of the TSR reaction. In theNE Sichuan Basin, the combined occurrence of longduration of high reservoir temperatures, alteration ofmultiple hydrocarbon reactants and different settingsof TSR sites have resulted in great diversity inmethane d13C values following TSR, the amountsof TSR-derived water and in carbonate dissolution-precipitation processes.

ACKNOWLEDGMENTS

We would like to thank three anonymous reviewers for givingus constructive suggestions, which were helpful in improving thequality of the manuscript. This work was financially supportedby the National Natural Science Foundation of China (GrantNos. 41572129 and 41730424) and the Special Major Project onPetroleum Study (2017ZX05008003-040). This paper was writtenwhilst Dr. Kaikai Li was a visiting scholar at the Department ofEarth and Planetary Sciences, Macquarie University, Australia,the support of which is gratefully acknowledged.

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