Thermochemical sulfate reduction and fluid evolution of ... - CiteSeerX

Thermochemical sulfate reduction and fluid evolution of the Lower Triassic Feixianguan Formation sour gas reservoirs, northeast Sichuan Basin, China Lei Jiang, Richard H. Worden, and Chun Fang Cai

ABSTRACT The dolomite-hosted, Lower Triassic Feixianguan Formation from the northeast Sichuan Basin, China, is an economically important reservoir that contains sour natural gas. These reservoirs were initially filled with oil, later replaced by gas during burial to 7000 m (22,965 ft) followed by uplift to about 4000 m (13,123 ft). We have studied the souring process (thermochemical sulfate reduction [TSR]) and diagenetic evolution of the Feixianguan Formation using detailed petrology, fluid-inclusion studies, and stable-isotope data from carbonate minerals. PreTSR diagenesis included (in time order) the eodiagenetic main stage of dolomitization by a reflux mechanism; fracture-related calcite cementation; barite, quartz, celestite, and fluorite mineralization; and a dolomite recrystallization stage. Thermochemical sulfate reduction resulted in anhydrite replacement by calcite, petroleum destruction, formation of sulfur-rich pyrobitumen and elemental sulfur, and generation of large volumes of H2 S and CO2 . Diagenesis during TSR can be subdivided into oil-stage TSR and gas-stage TSR, with oil-stage TSR defined by the presence of primary oil and bitumen inclusions in the TSR calcite. Based on aqueous inclusion homogenization temperatures, oilstage TSR commenced at a temperature of 116°C, with a mode between 130°C and 140°C. Gas-stage TSR started at a temperature of 135°C and continued to maximum burial temperatures of about 220°C. Trace amounts of pyrite, barite, quartz, and celestite grew during TSR. Post-TSR diagenesis was dominated by fracture-related calcite precipitation as well as celestite and anhydrite crystallization. Formation water salinity increased from

Copyright ©2014. The American Association of Petroleum Geologists. All rights reserved. Manuscript received December 9, 2012; revised manuscript received February 21, 2013; revised manuscript received June 8, 2013; accepted October 17, 2013. DOI: 10.1306/10171312220

AAPG Bulletin, v. 98, no. 5 (May 2014), pp. 947–973

947

AUTHORS

Lei Jiang ∼ Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; University of Chinese Academy of Sciences, Beijing 100049, China; Department of Earth and Ocean Sciences, School of Environmental Sciences, Liverpool University, Liverpool L69 3GP, United Kingdom; jary1224@hotmail. com Lei Jiang is a Ph.D. student at the Institute of Geology and Geophysics, Chinese Academy of Sciences, and an Honorary Research Fellow at the University of Liverpool. He gained his B.S. degree in sedimentary geology from the Chengdu University of Technology. His research interests mainly focus on carbonate diagenesis, including dolomitization and bacterial and thermochemical sulfate reduction. Richard H. Worden ∼ Department of Earth and Ocean Sciences, School of Environmental Sciences, Liverpool University, Liverpool L69 3GP, United Kingdom; [email protected] Richard Worden is a professor of petroleum geology and geochemistry at Liverpool University, United Kingdom. He gained his B. Sc. degree and his Ph.D. from Manchester University in the 1980s. Following a postdoctoral with Ian Parsons at Aberdeen and Edinburgh in Scotland, he worked for BP at their Sunbury-on-Thames site. He then took a lectureship at Queens University in Belfast followed by a move to Liverpool. His research interests include sandstone, mudstone, and carbonate petrology; diagenesis; reservoir quality; petrophysics and geochemistry; water-rock interaction; petroleum-rock interaction; thermochemical sulfate reduction; and the geology of CO2 subsurface disposal. Chun Fang Cai ∼ Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; [email protected] Chun Fang Cai is a professor at the Institute of Geology and Geophysics, Chinese

Academy of Sciences. He gained his B.S. and M.S. degrees from Jianghan Petroleum Institute (Yangtze University) and his Ph.D. from the Institute of Geology, Chinese Academy of Sciences. His research interests include water-rock interaction, bacterial and thermochemical sulfate reduction and application of biomarkers, organic sulfur and nitrogen isotopes to source rockhydrocarbon correlation, and marine paleoenvironmental reconstruction.

depositional values (3.5 wt. %) up to 24 wt. % during pre-TSR dolomite recrystallization probably because of an influx of evaporite-associated water from the overlying Jialingjiang Formation, although pre-TSR barite, quartz, celestite, and fluorite mineralization was associated with a transient decrease in water salinity. During TSR, formation water salinity decreased from 26 wt. % to as low as 4 wt. % as a result of water being produced during TSR reactions. INTRODUCTION

ACKNOWLEDGEMENTS

We thank AAPG Editor Stephen E. Laubach and Julia Gale, Richard Koepnick, and one anonymous reviewer for their thorough and constructive reviews of the manuscript. We also thank Mohamed Bukar, Carmel Pinnington, and Utley James of the Department of Earth and Ocean Sciences, Liverpool University, for their support during the laboratory analyses. This work was financially supported by the United Foundation of the National Natural Science Foundation of China and China’s Petroleum Chemical Industry (Grant 40839906), China National Funds for Distinguished Young Scientists (41125009), and an award from the Chinese Scholarship Council for a joint Ph.D. project in the United Kingdom with R. H. Worden. This work was also supported by the 12th Five-Year National Key Petroleum Project (2011ZX05008-003). The AAPG Editor thanks the following reviewers for their work on this paper: Julia F. Gate, Richard B. Koepnick, and an anonymous reviewer.

Many of the natural gas fields in the Feixianguan Formation carbonate reservoirs of northeast Sichuan Basin are sour because of the presence of a significant concentration of H2 S (>10% on average by volume in the produced fluids) (Cai et al., 2003, 2004; Liu et al., 2013). Hydrogen sulfide is an undesirable constituent of petroleum and, at concentrations greater than a few percent, is derived from thermochemical sulfate reduction (TSR) (Orr, 1977; Worden et al., 1995; Cai et al., 2003; Hao et al., 2008). Analysis of the origin, distribution, and controls on H2 S and TSR will help in the prediction and mitigation of sour gas and help in the design of production equipment and gas-processing facilities. Thermochemical sulfate reduction occurs when petroleum compounds react with aqueous sulfate, i.e., marine connate water sulfate or, more importantly, from the dissolution of sulfate minerals (mainly anhydrite but also celestite and barite) at elevated temperatures (approximately 100°C to 140°C for oil and greater than 140°C for gas) (Heydari and Moore, 1989; Worden et al., 1995; Cai et al., 2004; Worden and Smalley, 2004). Thermochemical sulfate reduction leads to a significant alteration of petroleum and generates a variety of reduced forms of sulfur (S and H2 S) and oxidized forms of carbon (carbonate minerals and CO2 ) as well as a combination of water, sulfides, organosulfur compounds, and bitumen (Machel, 1987; Krouse et al., 1988; Heydari and Moore, 1989; Machel et al., 1995; Worden et al., 1995, 1996, 2000; Worden and Smalley, 1996; Heydari, 1997; Bildstein et al., 2001; Cai et al., 2003, 2004, 2010). A general reaction can simply be written as follows: sulfate þ petroleum → calcite þ H2 S H2 O CO2 S altered petroleum

(1)

Many studies have been published on the fluid-related aspects of TSR including its effect on gas and oil composition, its effect on the carbon and sulfur isotopes of fluids, and its link to bitumen and reservoir quality. Detailed studies of rock-based evidence of 948

Gas Souring in the Feixianguan Formation, China

TSR in the Permian of the Arabian Basin (Worden et al., 2000) and the Devonian of the Western Canada Sedimentary Basin (Yang et al., 2001) have successfully defined links between rock fabric and the reaction rate of TSR and its controlling factors. The Feixianguan Formation in the Sichuan Basin has undergone much higher temperatures, and the gas is much drier than the Arabian and Western Canada Sedimentary Basin. Most TSR-related studies in the Feixianguan Formation carbonate reservoirs in northeast Sichuan Basin have focused on gas compound carbon isotopes and geochemistry and the carbon, oxygen, and sulfur isotope ratios of minerals in the reservoir (Cai et al., 2003, 2004, 2010, 2013; Li et al., 2005; Liu et al., 2013). There have also been reports of the effect of TSR on Feixianguan Formation reservoir quality (X. Z. Wang et al., 2002; Zhu et al., 2005b). No previous attempt has been made to document fluid inclusion and stable-isotope data to distinguish TSR-related cements from earlier cements in the Feixianguan Formation. In contrast to previous studies of TSR in the Feixianguan Formation, this study focused on the rock-based evidence of TSR and other diagenetic processes. The overall objectives were to identify the role of TSR, define the temperature and time at which TSR occurred, and help identify other controls on the extent of TSR in the Lower Triassic Feixianguan Formation carbonate reservoirs in northeast Sichuan Basin, China. Specifically, we have addressed the following questions: 1. What is the whole sequence of events in the dolomite-hosted reservoirs in the Feixianguan Formation? 2. How did the formation water evolve during diagenesis? 3. What are the controlling factors and role of TSR in the Feixianguan Formation? GEOLOGICAL SETTING The Sichuan Basin is located in the east of Sichuan Province, southwest China. It is a large intracratonic basin with an area of about 230; 000 km2 (8880 mi2 ) (Figure 1A) and is surrounded by several

mountain belts. The Sichuan Basin is tectonically bounded by the Longmenshan fold belt in the northwest, the Micangshan uplift in the north, the Dabashan fold belt in the northeast, the HubeiHunan-Guizhou fold belt in the southeast, and the Emeishan-Liangshan fold belt in the southwest. The basin has undergone several major tectonic events since the Proterozoic, including the Caledonian (320 Ma), the Indo-China (205–195 Ma), the Yanshan (140 Ma), and the Himalayan (80–3 Ma) movements. The Sichuan Basin underwent uplift prior to the Cretaceous Yanshan movement. Folding and deformation at the edge of Sichuan Basin occurred at approximately 140 Ma during the Yanshan movement. During the Cenozoic Himalayan tectonic event, uplift of the Sichuan Basin occurred because of the extrusion of the Pacific plate. The Sichuan Basin underwent open-marine sediment deposition from the pre-Sinian to middle Lower Triassic and then evolved to continental deposition during the Indo-China tectonic event. A transient marginal-marine phase occurred during the Early Permian. The Early Silurian transgression caused extensive black shale deposition (Y. S. Ma et al., 2008b). The upper Lower Triassic Xujiahe (T3 x) Formation consists of continental sediments, predominantly lacustrine-alluvial sandstones, thin shales, and locally distributed coal seams. Jurassic and Cretaceous sediments consist of continental red sandstones, mudstones, and black shales, with a present thickness of 2000–5000 m (6560–16,400 ft) (Cai et al., 2003). The gas fields are currently at a range of depths but had similar burial histories, with rapid burial in the Triassic and Jurassic reaching depths of about 7000 m (22,965 ft) and temperatures of 220°C in the Jurassic to the earliest Cretaceous, followed by somewhat variable degrees of uplift bringing the Triassic reservoirs to between 100°C and 140°C at the present day (Y. S. Ma et al., 2008b). Temperatures in the Lower Triassic reservoirs thus reached 100°C at about 200 Ma, 150°C at 175 Ma, and a maximum temperature of about 225°C at 120 Ma before inversion occurred from the Cretaceous onward (Figure 2). Two types of regional Triassic caprocks create the traps in the Feixianguan Formation in the JIANG ET AL.

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Figure 2. A typical burial and paleotemperature history constructed of well PG2 from the east Sichuan Basin (modified from Y. S. Ma et al., 2008a). Isotherms are constrained by vitrinite reflectance (Ro ) and fluid-inclusion measurements (data from Y. S. Ma et al., 2008c).

Sichuan Basin: thick anhydrite layers in the Lower Triassic Jialingjiang Formation and the middle Lower Triassic Leikoupo Formation. Four types of marine petroleum source rocks have been proposed for the eastern Sichuan Basin: Lower Cambrian, Lower Silurian, Upper Permian, and Lower Permian (Y. S. Ma et al., 2008b). A Lower Cambrian source rock has not been encountered in the northeast Sichuan and so remains speculative. The Silurian Longmaxi Formation black shale has high total organic carbon and is a mixed type I and II kerogen. Natural gas in the eastern Sichuan Basin may have been derived mainly from the Upper Permian Longtan Formation mudstone and marlstone source rocks, based on biomarkers and carbon and sulfur isotopes (Cai et al., 2010). The thickness of the Upper Permian source rock with a total organic carbon (TOC) greater than 0.5% is about 120 m (394 ft), and the mean TOC value of this potential source interval is 4.32%, with a vitrinite reflectance maturity of

1.8%–2.6% (Y. S. Ma et al., 2008b). By the Late Permian, large-scale faulting led to extensive basalt eruption and dolerite intrusion in the eastern and western parts of the basin. During the time of deposition of the Feixianguan Formation (Early Triassic), an arid climate persisted in the northeast Sichuan Basin and a semi-isolated evaporate carbonate platform formed on the east side of the Kaijiang-Liangping trough (Figure 1A). Oolitic banks were present on the margins of the trough, which were intensely dolomitized and now form good-quality gas reservoirs (Figure 1B). In the study area, the Feixianguan Formation has a total thickness ranging between 300 and 500 m (1000 and 1640 ft) and has been subdivided into four members in stratigraphic order: T1 f 1 , T1 f 2 , T1 f 3 , and T1 f 4 using core and wireline-log analysis (Cai et al., 2004). Multiple gypsiferous layers in the Feixianguan Formation exist because of sea level fluctuations and the persistently arid climate (Y. S. Ma, 2008). Purple shales and some anhydrite beds are interlayered with thin-bedded micritic limestones in the upper part of the Feixianguan Formation; these make up a good regional seal for the underlying T1 f carbonate reservoir (Zhao et al., 2005) (Figure 3). METHODS A total of 55 carbonate reservoir dolostone samples were collected from Puguang, Maoba, Luojiazhai, and Dukouhe sour gas fields in the lower unit of the Triassic Feixianguan Formation. All samples were used for petrology and studied for fluid-inclusion analyses. Finely polished and etched slabs and thin sections were stained with alizarin red S and potassium ferricyanide to distinguish calcite from dolomite and their ferroan equivalents. Table 1 lists the samples selected for detailed petrological and geochemical investigation. All core samples were examined as hand specimens to help focus sampling. The sampling was biased toward core that contained TSR calcite and

Figure 1. Location of the study area of the Feixianguan Formation in northeast Sichuan Basin. (A) A semi-isolated evaporite-carbonate platform occupied the east of Kaijiang-Liangping trough. (B) The location of gas fields in Feixianguan Formation reservoirs in the northeast Sichuan Basin. Modified from Zhao et al. (2005). JIANG ET AL.

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Figure 3. Lithology and sedimentary evolution of the Changxing to Feixianguan Formations in the Puguang region of the Sichuan Basin (modified from Y. S. Ma et al., 2008c).

anhydrite, celestite and barite nodules or fractures. The occurrence of yellow-green elemental sulfur in core samples was also noted. Calcite samples that showed late, vug-filling textures typical of TSR (Machel, 1987; Krouse et al., 1988; Machel et al., 1995; Worden et al., 1995, 2000; Worden and Smalley, 1996; Cai et al., 2004; Zhu et al., 2005a; Li et al., 2012) were selected from drill core. All samples that showed clear signs of TSR were examined by cathodoluminesence (CL) and a scanning electron microscope (SEM) in backscattered electron imaging mode (BSEM). The BSEM has a higher resolution than optical and CL microscopy; it can resolve minerals and grains to about 1–2 μm. Minerals composed of high-atomic-mass elements are brighter in the BSEM than those composed of 952


low-atomic-mass elements. Thus, in the order of increasing brightness, are elemental sulfur, dolomite, fluorite, calcite, anhydrite, celestite, pyrite, and barite. Fluid inclusions in double-polished wafers were studied using a Linkam THMSG 600/TS90 heatingcooling stage connected to a Nikon petrographic microscope to obtain temperature data. Ultraviolet fluorescence was performed on these doubly polished wafers to determine whether they were oil or aqueous inclusions and to identify inclusion types for further analysis and to determine their relationship to the host minerals. Instrumental precision is 0.1°C. For each fragment, the inclusions were heated first to measure homogenization temperatures (Th ) and then cooled to record melting temperatures to avoid stretching the fluid inclusions from expansion during the formation of ice. Ice-melting temperatures were converted to salinity using standard equations (Oakes et al., 1990; Bodnar, 2003). Rock samples were broken into chips and subsequently picked to identify rock fragments with identifiable TSR calcite free from visible contamination and to identify pure dolomite matrix. Fine powder samples of calcite and dolomite were then extracted using a dentist’s drill and subjected to carbon and oxygen isotope analyses. Samples were roasted at 60°C for 12 hr and then 110°C for 3 hr to remove any petroleum compounds. About 30–50 mg of drilled out sample was reacted overnight with 100% phosphoric acid at 25°C under vacuum to release CO2 from the carbonate minerals. The CO2 was then analyzed for carbon and oxygen isotopes on a Finnigan MAT251 mass spectrometer calibrated using standard with NBS-18. All carbon and oxygen data are reported in units per mil relative to the Vienna Peedee belemnite (VPDB) standard, respectively. The precision for both δ13 C and δ18 O measurements is 0.1‰.

RESULTS Dolomite Host Rocks The dolomite-hosted Feixianguan Formation reservoir was originally shallow-water limestone that was dolomitized during shallow diagenesis. The initial

dolomitization of host rocks in the Feixianguan Formation has been discussed (Jiang et al., 2013); the main dolomitization stage must have commenced at a temperature lower than about 50°C and a depth less than 500 m (1640 ft) before the formation of stylolites. Some recrystallized dolomites, however, developed at temperatures in the range 80°C to 140°C, locally formed during intermediate burial diagenetic setting at depths between 1500 and 3000 m (4920 and 9840 ft). No evidence of oil and bitumen inclusions exists in the early diagenetic or burial diagenetic dolomite, suggesting that these dolomites formed before oil charging and bitumen formation. The best reservoir is in sucrosic dolomitized oolitic units that have been extensively recrystallized (J. X. Wang, 1996). Dolomicrite is also common in the Feixianguan Formation and typically contains anhydrite nodules. Dolomicrite was formed by reflux dolomitization in a tidal-flat environment ( Y. S. Ma et al., 2007; Jiang et al., 2013). Dolomicrite tends to have relatively lower porosity and permeability than the dolomitized oolitic facies but still retains potential as a gas reservoir. Diagenetic Fabrics and Minerals Anhydrite (CaSO4 ) is present in three forms in the Feixianguan Formation. The first is bedded anhydrite layers, which are pervasive in the Feixianguan ( Zhao et al., 2005; Y. S. Ma et al., 2008c). Isolated anhydrite nodules occur in a matrix of fine-grained dolomicrite. These anhydrite nodules are plainly visible in core and have a diameter of 1 to 5 cm (0.4 to 2 in.) (Figure 4A). They display typical evaporate chicken-wire textures (Tucker and Wright, 1990). Where present in clusters, anhydrite nodules are locally associated with elemental sulfur. The third type of anhydrite was only visible in thin section and is present as poikilotopic cements in the oolitic host rock dolostone reservoirs; such anhydrite totally fills primary pore spaces between ooids (Figure 4B, D). These anhydrite cements are composed of lathshaped crystals of less than 100 μm in length (Figure 4C). From previous studies, reflux dolomitization has been associated with a significant amount of anhydrite growth (Jones and Xiao, 2005; Al-Helal et al., 2012). Therefore, the anhydrite in the studied

Feixianguan Formation was most likely formed because of the reflux of saline water in the Feixianguan Formation during relatively early-burial diagenesis (Jiang et al., 2013). The ubiquity and different forms of anhydrites indicate that no shortage of one of the prime reactants for TSR (sulfate) occurred in the Feixianguan Formation in northeast Sichuan Basin. Finally, rare veins of anhydrite are present, which appear to crosscut all depositional fabrics and crosscut all later diagenetic fabrics (to be described). Vein anhydrite reportedly has distinctly higher sulfur isotope values than those from the anhydrite nodule and bedded anhydrite (Cai et al., 2010). Calcite cement and fracture-filling calcite (CaCO3 ) postdate the main, early diagenetic, reflux dolomitization event. This early-burial diagenetic calcite does not have oil or bitumen primary inclusions and is locally coated with bitumen. This calcite contains secondary inclusions of bitumen in fractures (Figure 5A). These observations indicate that this phase of calcite precipitation was prior to oil emplacement and bitumen formation. Barite (BaSO4 ) locally fills pores and is intimately grown with fluorite and calcite formed before bitumen emplacement. Barite is present as crystals from less than 20 μm to centimeters in length. It does not have oil or bitumen primary inclusions (Figure 6A, B), indicating precipitation prior to oil emplacement and bitumen formation. Celestite (SrSO4 ) fills vugs and fracture predominantly in dolomicrite. The vugs are from 1 to 5 cm (0.4 to 2 in.) in diameter; vug-filling celestite is commonly associated with elemental sulfur and is intimately intergrown with fluorite and calcite formed before bitumen emplacement. Vug-filling celestite does not have oil or bitumen primary inclusions (Figure 6C, D), suggesting that precipitation occurred prior to oil emplacement and bitumen formation. In contrast, fractures that are several centimeters wide and longer than the width of the core are locally filled with celestite crystals. These are as much as 10 cm (4 in.) in length and are commonly associated with primary oil and bitumen inclusions. In contrast to the vug-filling celestite, fracture-filling celestite precipitated during, or after, the emplacement of oil and formation of bitumen. JIANG ET AL.

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Table 1. List of Samples and the Various Techniques Applied to Them* Sample No. D-1 D-2 D-2-1 D-2-2 D-4 D-5-1 D-5-2 D-5-3 D-5-4 D-5-5 D-5-6 DW-3-1 DW-3-2 DW-102-1 DW-102-2 DW-102-3 HL-4 JZ-1-1 JZ-1-2 JZ-1-3 JZ-1-4 JZ-1-5 JZ-1-6 JZ-1-7 LJ-1-1 LJ-1-2 LJ-2-1 LJ-2-2 LJ-2-3 LJ-2-4 LJ-2-5 LJ-2-6 LJ-2-7 LJ-2-8 LJ-6 MB-2-1 MB-2-2 P-2-1 P-2-2 P-2-3 PG-1 PG-2-1 PG-2-2 PG-2-3 PG-2-4 PG-2-5 954

Present Depth (m) – – – – 4235.6 4793 4793 4793 4790.8 4738.1 4758.1 4735 4735.1 4822 4822 4900.7 – 2978.8 2978.8 2978.8 2978.8 2978.8 2978.8 2978.8 3470.4 3513.2 – – 3267.4 3256.5 3252.8 3233 3232.2 3215.1 3936 4135 4135 – – – 5156 4958.5 4977.1 4977.4 4696.6 4984.9

Gas Field Dukouhe Dukouhe Dukouhe Dukouhe Dukouhe Dukouhe Dukouhe Dukouhe Dukouhe Dukouhe Dukouhe Dawan Dawan Dawan Dawan Dawan Huanglong Jinzhu Jinzhu Jinzhu Jinzhu Jinzhu Jinzhu Jinzhu Luojiazai Luojiazai Luojiazai Luojiazai Luojiazai Luojiazai Luojiazai Luojiazai Luojiazai Luojiazai Luojiazai Maoba Maoba Tieshanpo Tieshanpo Tieshanpo Puguang Puguang Puguang Puguang Puguang Puguang

Facies

Host Mineral

Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Tidal flat Tidal flat Tidal flat Tidal flat Oolitic shoal Tidal flat Lagoon Lagoon Lagoon Lagoon Lagoon Lagoon Lagoon Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Tidal flat Tidal flat Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal

Dolomite Dolomite Dolomite Pre-TSR calcite Dolomite Dolomite Quartz TSR calcite Dolomite Dolomite Dolomite Post-TSR calcite Post-TSR calcite Post-TSR calcite Dolomite TSR calcite Dolomicrite Dolomicrite Pre-TSR calcite Quartz Fluorite Vug celestite Fracture celestite Dolomite Pre-TSR calcite Dolomite Dolomite Dolomite TSR calcite Dolomite Dolomite Dolomite Dolomite Dolomite TSR calcite Dolomite Post-TSR calcite Pre-TSR calcite Barite TSR calcite TSR calcite Dolomite Dolomite Dolomite Dolomite Dolomite


Techniques Thin-section petrography Thin-section petrography Thin-section petrography, UV, FI thermometry Thin-section petrography, UV, FI thermometry Thin-section petrography Thin-section petrography, UV, FI thermometry SEM, BSEM, UV, FI thermometry Thin-section petrography, UV, FI thermometry SEM, BSEM, UV, FI thermometry Thin-section petrography Thin-section petrography SEM, BSEM, UV, FI thermometry, δ13 C, δ18 O Thin-section petrography Thin-section petrography Thin-section petrography SEM, BSEM, UV, FI thermometry, δ13 C, δ18 O Thin-section petrography Thin-section petrography, SEM, BSEM, δ13 C, δ18 O SEM, BSEM, UV, FI thermometry, δ13 C, δ18 O SEM, BSEM, UV, FI thermometry SEM, BSEM, UV, FI thermometry SEM, BSEM, UV, FI thermometry SEM, BSEM, UV, FI thermometry Thin-section petrography Thin-section petrography, UV, FI thermometry Thin-section petrography Thin-section petrography Thin-section petrography Thin-section petrography, SEM, BSEM Thin-section petrography Thin-section petrography Thin-section petrography, SEM, BSEM Thin-section petrography Thin-section petrography, SEM, BSEM Thin-section petrography, UV, FI thermometry Thin-section petrography Thin-section petrography, SEM, BSEM SEM, BSEM, UV, FI thermometry, δ13 C, δ18 O SEM, BSEM, UV, FI thermometry Thin-section petrography Thin-section petrography Thin-section petrography Thin-section petrography Thin-section petrography, UV, FI thermometry Thin-section petrography Thin-section petrography

Table 1. Continued Sample No.

Present Depth (m)

Gas Field

Facies

Host Mineral

PG-2-6 PG-2-7 PG-2-8 PG-2-9 PG-2-10 PG-2-11 PG-2-12 PG-6 PG-11

4994.6 5074.6 5081.6 5082.5 5082.2 5185 5186.6 – 5818

Puguang Puguang Puguang Puguang Puguang Puguang Puguang Puguang Puguang

Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal Oolitic shoal

Dolomite Dolomite Dolomite Dolomite Dolomite Dolomite Dolomite Post-TSR calcite Anhydrite cement

*

Techniques Thin-section petrography Thin-section petrography, UV, FI thermometry Thin-section petrography Thin-section petrography Thin-section petrography Thin-section petrography Thin-section petrography Thin-section petrography, UV, FI thermometry Thin-section petrography

UV ¼ ultraviolet fluorescence; FI ¼ fluid inclusion; SEM ¼ scanning electron microscope; BSEM ¼ backscattered electron imaging mode; – = data unavailable.

Fluorite (CaF2 ) is locally present in vugs and is intergrown with celestite and barite. Fluorite crystals in vugs typically are 50–200 μm in length. Fluorite does not contain primary oil or bitumen inclusions, but it does contain secondary bitumen inclusions in cemented fractures (Figure 6B, D), suggesting initial precipitation prior to oil emplacement and bitumen formation. Solid bitumen (now pyrobitumen) is a dark, hard organic substance that is abundant in large quantities

in pore spaces in the dolomite reservoirs of the Feixianguan Formation and appears as rod-shaped and sheetlike aggregates when viewed under SEM (Figure 7A, B). The rod-shaped aggregates are very similar to the pyrobitumen reported from the Upper Jurassic Smackover Formation sour carbonate reservoirs in Mississippi (Heydari, 1997). Two types of anhydrite dissolution and calcite replacement texture are present in the Feixianguan Formation. In dolomicrite reservoirs, anhydrite Figure 4. Photographs showing the presence of anhydrite and the way in which calcite replaces anhydrite in the Feixianguan Formation. (A) Core photograph showing elemental sulfur-bearing anhydrite nodules partially replaced by calcite in dolomicrite, well P1, 3467.80 m (11,374.38 ft), modified from Y. G. Wang et al. (2002). (A ¼ anhydrite; C ¼ calcite; D ¼ dolomicrite; S ¼ sulfur). (B) Photomicrograph of anhydrite cement emplaced in pores in oolitic dolomite, well PG11, 5818.00 m (19,083.04 ft). (C) Photomicrograph of calcite totally replacing anhydrite (yellow arrow) in dolomitized oolitic sample, well D2. (D) Photomicrograph of anhydrite partly replaced by calcite (yellow arrow) in dolomitized oolitic sample, well PG1, 5156.00 m (16,911.68 ft). JIANG ET AL.

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Figure 5. Photomicrographs of calcite types and their fluid inclusions in the Feixianguan Formation. (A) Pre-TSR calcite (yellow arrow) with bitumen filling the microfracture in calcite (blue arrows), implying that the calcite formed before the formation of fracture and oil charging, well LJ1, 3470.40 m (11,382.91 ft). (B) Twophase aqueous fluid inclusions (yellow arrows) present in pre-TSR calcite, well LJ1, 3470.40 m (11,382.91 ft). Note that no petroleum inclusions were found in preTSR calcite. (C) Oil-stage TSR calcite (yellow arrow) with bitumen and oil inclusions, well LJ6, 3936.00 m (12,910.08 ft). (D) Twophase aqueous fluid inclusions (yellow arrow), bitumen, and oil inclusions (red arrows) are commonly present within the calcite, implying that the calcite formed during the time that oil reacted with anhydrite, well LJ6, 3936.00 m (12,910.08 ft). (E) Gasstage TSR calcite (yellow arrow), with no evidence of bitumen or oil inclusions found in this type of calcite, suggesting that calcite was formed by gas reacting with anhydrite, well D4, 4793.00 m (15,721.04 ft). (F) Two-phase aqueous fluid inclusions (yellow arrows) commonly present in calcite. Note that no petroleum inclusions were found in gas-stage TSR calcite, well D4, 4793.00 m (15,721.04 ft).

nodules have been partially or fully replaced by calcite crystals and elemental sulfur. In some cases, remnant anhydrite has been armored by calcite (see Bildstein et al., 2001) (Figure 4A). This may serve to render the remaining anhydrite isolated from the reactive petroleum, similar to the situation in the Khuff Formation in Abu Dhabi (Worden et al., 2000). In dolomite petroleum accumulations (oolitic dolomites and sucrosic dolomite), anhydrite has been completely dissolved, leaving well-developed, orthorhombic molds in these dolomites. Less commonly, 956


these molds have been filled by later blocky calcite crystal (Figure 4C). Anhydrite cement has also been shown that was partly replaced by calcite (Figure 4D). Note that anhydrite cements are well preserved in petroleum-free parts of the Feixianguan Formation (Figure 4B). Two forms of elemental sulfur are present in core. One can be directly observed in core samples in dolomicrite host rocks and is present as patches of yellow-green sulfur, as much as a few centimeters in length and commonly associated with anhydrite or

Figure 6. Photographs of noncarbonate minerals present in the Feixianguan Formation. (A) Photomicrograph showing a mineral assemblage of barite, fluorite, plus calcite in the fracture in dolomicrite, well P2. (B) A BSEM image of (A). (C) Photomicrograph showing a mineral assemblage of celestite, fluorite, and calcite in the vug and fracture in dolomicrite, well JZ1, 2978.80 m (9770.46 ft). (D) A BSEM image of (C). (E) and (F) Photomicrographs showing quartz (yellow arrow) present within calcite and occurring as quartz cement in both the dolomicrite ([E], well D4, 4793.0 m [15,721.04 ft]), and dolomitized oolite ([F], well D4, 4790.8 m [15,713.82 ft]).

celestite nodules as well as replacive calcite (Figure 4A). Another form of elemental sulfur was revealed by petrographic analysis and BSEM analysis in the SEM and is commonly present in the intergranular and intragranular pores in dolostone reservoirs and is associated with calcite and bitumen and less commonly associated with pyrite (Figure 7C, D). This kind of sulfur, with length in a range from 10 μm to more than 100 μm, looks similar to bitumen (opaque and brown) in the optical microscope and may not have been identified in previous studies (Cai et al., 2004, 2010; Hao et al., 2008). Two distinct types of TSR calcite were observed in the Feixianguan Formation. One type of TSR calcite has a vug and pore-filling texture and was found in sucrosic, coarse crystalline dolostone reservoirs

(Figure 5C, E). These large calcite crystals in pores are typically 50–200 μm in length and in vugs locally reach up to 10 cm (4 in.) in length. Pore-filling TSR calcite has a negative δ13 C values (Figure 8) (Cai et al., 2004; Zhu et al., 2005a; Li et al., 2012). The other type of TSR calcite is visible in hand specimen and occurs as replacive rinds around the surfaces of anhydrite nodules in dolomicrite (Figure 4A). Anhydrite-rimming TSR calcite is very similar to the classical TSR calcite present in the Khuff Formation in Abu Dhabi (Worden et al., 1995, 2000, 2004). Thermochemical sulfate reduction calcite rimming the anhydrite also has a negative δ13 C value (Y. G. Wang et al., 2002). Thermochemical sulfate reduction calcite that contains oil or bitumen inclusion is hereby called oil-stage TSR calcite. Thermochemical sulfate JIANG ET AL.

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Figure 7. (A) An SEM photomicrograph showing ball-shaped pyrobitumen aggregates, well LJ2, 3215.10 m (10,545.53 ft). (B) An SEM photomicrograph showing sheetlike pyrobitumen aggregates, well LJ2, 3244.50 m (10,641.96 ft). (C) An SEM photomicrograph showing pyrite (black arrows) present at the edge of the pore and associated with elemental sulfur (white arrows), well D2. (D) An SEM photomicrograph showing dolomite partly replaced by pyrite (black arrow) and associated with elemental sulfur (white arrows), well LJ2, 3256.45 m (10,681.16 ft).

reduction calcite with no evidence of oil or bitumen inclusions is hereby called gas-stage TSR. Calcite cement is present in late-stage veins that have widths up to several centimeters and lengths up to meters and can easily be discriminated in both thin

section and core. The calcite veins crosscut all the other minerals and do not contain oil or bitumen inclusions. This suggests that the veins must have precipitated after all the other diagenetic minerals. Quartz is commonly present in pores closely associated with TSR calcite; this suggests that quartz growth was synchronous with TSR calcite (Figure 6E, F). Quartz crystals are typically euhedral and are in a range of 50–200 μm in size. Pyrite was revealed by thin-section analysis and is locally present in pores (Figure 7C) or as partial replacements of matrix dolomite crystals (Figure 7D). Pyrite is locally present along stylolite surfaces. The overall mount of pyrite in the study area is less than 0.5 wt. % by point counting. Pyrite crystals are as much as 50–100 μm. Stable-Isotope Analysis

Figure 8. The carbon and oxygen isotopic compositions of Feixianguan Formation carbonates in the northeast Sichuan Basin. Open symbols are new data, closed data have been collated from the literature from nearby gas wells with the same facies associations and with similar burial and thermal histories: (a) Zhu et al. (2005a); (b) Zhao et al. (2005); (c) Jiang et al. (2013); (d) Y. G. Wang et al. (2007); (e) Li et al. (2012). TSR ¼ thermochemical sulfate reduction. 958


Stable-isotope values (most of which are collected from published data, with six new data points from this study) and other results are summarized in Table 2. The matrix dolomite displays a narrow range of δ13 C, between 0‰ and 4‰ VPDB (Figure 8). Dolomite δ18 O ranges from −3‰ to −6‰ VPDB. These are similar to previously reported values

Table 2. Synthesis of the Results of Carbon and Oxygen Isotopes and Average Fluid-Inclusion Homogenization Temperatures and Salinities from Different Samples Well D-2-1 D-2-2 D-5-1 D-5-2 Du-5-3 DW-3-1 DW-102-3 JZ-1-1 JZ-1-2 JZ-1-3 JZ-1-4 JZ-1-5 JZ-1-6 JZ-1** LJ-1-1 LJ-5† LJ-6 P-1† P-1† P-1†† P-2-1 P-2-2 P-3† P-4† PG-2‡ PG-2‡ PG-3‡ PG-6 PG-6‡ PG-6‡ PG-6‡ PG-6‡ PG-6‡ PG-6‡

Depth (m)

Gas Field

– – 4793 4793 4793 4735 4900.7 2978.8 2978.8 2978.8 2978.8 2978.8 2978.8 3012 3470.4 3002.9 3936 3461.5 3464.7 3467.8 – – 3536 3238 4784.5 4775.2 5894.4 – 4876 5246.3 5238.9 5356.8 5285.6 5248.3

Dukouhe Dukouhe Dukouhe Dukouhe Dukouhe Dawan Dawan Jinzhu Jinzhu Jinzhu Jinzhu Jinzhu Jinzhu Jinzhu Luojiazai Luojiazai Luojiazai Tieshanpo Tieshanpo Tieshanpo Tieshanpo Tieshanpo Tieshanpo Tieshanpo Puguang Puguang Puguang Puguang Puguang Puguang Puguang Puguang Puguang Puguang

Host Mineral Recrystallized dolomite Pre-TSR calcite Recrystallized dolomite Quartz TSR calcite Post-TSR calcite TSR calcite Dolomicrite Pre-TSR calcite Quartz Fluorite Vug celestite Fracture Celestite Anhydrite Pre-TSR calcite TSR calcite TSR calcite TSR calcite TSR calcite TSR calcite Pre-TSR calcite Barite TSR calcite TSR calcite TSR calcite TSR calcite TSR calcite Post-TSR calcite TSR calcite TSR calcite TSR calcite TSR calcite TSR calcite TSR calcite

δ13 C (‰) VPDB

δ18 O (‰) VSMOW

Th (°C)

Salinity (% NaCl)

– – – – −16.48* 1.43 −1.08 −0.04 −0.52 – – – – – – −6.1 – −13.8 −18.2 −7.26 −2.28 – −17 −16.3 – – – 0.61 – – – – – –

– – – – −8.59* −6.67 −9.52 −2.98 −4.91 – – – – – – −5.7 – −6.6 −6.3 −6.41 −9.08 – −5.9 −6 – – – −5.83 – – – – – –

104.8 119.7 113.7 168 167.4 185 145.8 – 101.1 105.1 124.6 121.8 175.6 114 104 – 149.2 – – 125 69.5 104.6 – – 179.9 182 154.5 125.6 114.6 146.2 144.8 183.5 172.5 180.3

15 21.5 21.6 7.6 10 4.2 18.9 – 16.6 5.6 9 6.3 12.5 9.2 14.4 – 19.2 – – – 16.5 10.5 – – 6.8 3.4 6.4 6.8 21 6.8 10.5 5.7 6.6 7.2

*

From Li et al. (2012). From Liu et al. (2006). † From Zhu et al. (2005a). †† From Y. G. Wang et al. (2002). ‡ From Hao et al. (2009). **

(Zhao et al., 2005; Zhu et al., 2005a). Thermochemical sulfate reduction–related calcite in secondary vugs after anhydrite dissolution and as a rind around remaining anhydrite nodules has a range of δ13 C values from about −3‰ to as low as −19‰

VPDB and δ18 O values between −4‰ and −9‰ VPDB (Figure 8). For reference, non-TSR-related calcite cements from the Feixianguan Formation have been reported to have a restricted range of δ13 C values (approximately 0‰–2‰ VPDB) and δ18 O JIANG ET AL.

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values between −6‰ and −8‰ VPDB (Zhu et al., 2005a; Li et al., 2012). Fluid-Inclusion Homogenization Temperature and Salinity Data Doubly polished detachable wafers (11 in total) of celestite, barite, fluorite, quartz, fracture calcite, and TSR calcite were prepared from subsurface core samples. Petrography using transmitted light and UV fluorescence was performed on doubly polished wafers to identify inclusion types for further analysis and to determine their relationship to the host minerals. All minerals contain primary, two-phase aqueous inclusions big enough for the study of thermometry (Figure 5B, D, F), and it is possible to find more than one mineral with usable fluid inclusions in one wafer. Single-phase aqueous inclusions were observed in dolomite and fracture-filling calcite. Complex multiphase inclusions are present in celestite and fluorite. Microthermometric analyses of the inclusions in individual minerals can be viewed as simple subsets of the overall range; No evidence exists to suggest that different wells or samples underwent different histories of minerals growth. These fluid inclusions showed no signs of deformation or leakage, suggesting that the homogenization temperature data are not an artifact of postformational processes. The dominance of methane in the petroleum system means that the homogenization temperature will be close to the true trapping temperature, and no pressure correction will be needed (Hanor, 1980). Dolomite cement had homogenization temperatures ranging from 80°C to about 140°C (Figure 9A). Salinity data show that the water present

at the time of dolomite growth was saline; the values ranging from 6.0 to about 25 wt. % (Figures 9B, 10A). Barite samples had homogenization temperatures mainly ranging from 70°C to 170°C (Figure 9C). The minimum recorded temperature of growth of barite was 67.5°C, and the maximum was 170.0°C. Salinity data show that the water present at the time of barite growth was saline; the values ranging from 18.0 to 7.0 wt. % (Figures 9D, 10B). The celestite samples had homogenization temperatures typically ranging from about 90°C to 200°C (Figure 9E). The minimum recorded temperature of growth of celestite was 89.9°C, and the maximum was 205.3°C. Low temperature data were mostly derived from vug-filling celestite with the histogram mean value of 125°C. The higher temperatures from celestite, ranging of 150°C to 210°C, were measured from the fracture-filling celestite. Salinity data, derived from the observed last ice-melting temperature measurements, show that the water present at the time of celestite growth was saline; the values range from 13.2 to 3.3 wt. % (Figures 9F, 10C). Fluorite samples intimately grown with celestite had narrow homogenization temperatures ranging from 120°C to 130°C (Figure 9G). The minimum recorded temperature of growth of fluorite was 119.7°C, and the maximum was 129.9°C. Salinity data show that the water present at the time of fluorite growth ranged from 21.3 to 6.2 wt. % (Figures 9H, 10D). Quartz cement crystals had homogenization temperatures ranging from 95°C to 190°C (Figure 9I). The minimum recorded temperature of growth of quartz was 94.8°C, and the maximum was 215.6°C.

Figure 9. Fluid-inclusion data from different minerals in the 11 new wafers that contained measurable inclusions in Feixianguan Formation, northeast Sichuan Basin. (A) Fluid-inclusion homogenization temperatures from aqueous inclusions from recrystallized dolomite. (B) Salinity of aqueous inclusions from recrystallized dolomite. (C) Fluid-inclusion homogenization temperatures from aqueous inclusions from barite. (D) Salinity of aqueous inclusions from barite. (E) Fluid-inclusion homogenization temperatures from aqueous inclusions from celestite. (F) Salinity of aqueous inclusions from celestite. (G) Fluid-inclusion homogenization temperatures from aqueous inclusions from fluorite. (H) Salinity of aqueous inclusions from fluorite. (I) Fluid-inclusion homogenization temperatures from aqueous inclusions from quartz. (J) Salinity of aqueous inclusions from quartz. (K) Fluid-inclusion homogenization temperatures from aqueous inclusions from nonreplacive calcite with populations split into pre-TSR (thermochemical sulfate reduction) and post-TSR fracture-related calcite. (L) Salinity of aqueous inclusions from nonreplacive calcite. (M) Fluid-inclusion homogenization temperatures from two-phase aqueous inclusions from TSR calcite with populations split depending on whether primary oil or bitumen inclusions are present in calcite (those with oil or bitumen inclusions are called oil-stage TSR; those without are called gas-stage TSR). (N) Salinity of aqueous inclusions from oil-stage and gas-stage TSR calcite. JIANG ET AL.

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Figure 10. The relationships between the salinities of two-phase aqueous fluid inclusions in different minerals in the 11 new wafers that contained measurable inclusions and their growth temperatures. (A) Salinity of recrystallized dolomite increases with increasing temperature. (B) Salinity of pre- and syn-TSR (thermochemical sulfate reduction) barite showing increasing salinity and temperature. (C) Salinity of pre- and syn-TSR celestite showing increasing salinity and temperature. (D) Salinity of fluorite increase with increasing temperature. (E) Salinity of pre- and syn-TSR quartz showing increasing salinity and temperature. (F) Salinity of nonreplacive calcite with two populations; pre-TSR mostly fracture-related calcite and post-TSR fracture-related calcite. (G) Contrasting salinity and temperature of oil-stage and gas-stage TSR calcite.

Salinity data show that the water present at the time of quartz growth ranged from 3.9 to 9.9 wt. % (Figures 9J, 10E). For dolomite, barite, celestite, fluorite, and quartz, salinity values seem to increase with the increasing temperature of trapping (Figure 10A, E). Fracture-filling calcite fluid-inclusion data can be subdivided into two populations reflecting two stages of cement precipitation. Early-stage fracture calcite prior to oil emplacement is one type, whereas the second was at a later stage after oil emplacement. The 962


measured fluid inclusions from non-TSR fracture-filling calcite have a wide range of homogenization temperatures from less than 60°C to more than 200°C; a well-defined mode at 90°C exists and then a few much higher values between 160°C and just greater than 210°C (Figure 9K). The lower temperature mode shows that the first stage of fracture-filling calcite (at just greater than 60°C) happened before dolomite recrystallization (at 80°C to 130°C). Salinity data show that the water present at the time of the first

phase of calcite growth in fractures (before TSR) ranged from about 22 wt. % NaCl for the lowest temperature of growth to about 10 wt. % NaCl at the higher temperatures for the second stage (Figures 9L, 10F). Ultraviolet-fluorescent oil inclusions are found in some TSR calcite crystals. Based on visual estimate, these oil inclusions have approximately uniform liquid–vapor ratios (approximately 70/30 by estimated volume). Coeval aqueous fluid inclusions in oil-stage TSR calcite have homogenization temperatures ranging from about 120°C with a modal value of 140°C (Figure 9M). Based on aqueous fluidinclusion homogenization, the minimum recorded temperature of growth of oil-stage TSR calcite was 116°C. Salinity data show that the water present at the time of oil-stage TSR calcite growth was saline with values ranging between 8.4 and 26.0 wt. % (Figures 9N, 10G). Salinity values seem to decrease with the increasing temperature of trapping. By contrast, the gas-stage TSR calcite contains aqueous fluid inclusions with homogenization temperatures ranging from about 130°C to more than 220°C, with a modal value of 155°C (Figure 9M). Inclusions in gas-stage TSR have been shown to be dominated by CH4 and H2 S with minor amounts of other alkane gases (Liu et al., 2006). The minimum recorded temperature of growth of gas-phase calcite was 130.1°C, and the maximum was 223.2°C. Salinity data show that the water present at the time of gas-phase TSR was generally less saline than the water present at the time of oil-stage TSR, with salinity values ranging typically less than 10 wt. % (Figures 9N, 10G). The key attributes of the three types of calcite are summarized in Table 3. Clear differences exist in the pre-TSR, TSR, and post-TSR calcite in terms of position within the pore network, isotopes, and fluidinclusion temperatures and salinities.

DISCUSSION Paragenetic Sequence Diagenesis of the dolomite-hosted reservoirs of the Feixianguan Formation in the northeast Sichuan Basin in China encompassed a wide spectrum of

burial and uplift processes. Thermochemical sulfate reduction has been reported widely in these dolostone reservoirs (Cai et al., 2004, 2010, 2012; Li et al., 2005, 2012; Zhu et al., 2005b; Hao et al., 2008; Liu et al., 2013). Following established methods (Heydari, 1997) and referring to the burial and temperature history of the Feixianguan Formation (Figure 2), diagenesis has been divided into three stages defined by timing relative to TSR (Figure 11). These three phases are (1) pre-TSR diagenesis (< 3500-m [< 11480-ft] burial, < 130°C), (2) TSR diagenesis (∼3500–7000 m [∼11,480– 22,960 ft], 120°C–220°C followed by inversion and uplift) and (3) post-TSR diagenesis (following uplift to ∼4500 m [∼14,760 ft], 115°C–130°C). Diagenesis before Thermochemical Sulfate Reduction Diagenetic processes before TSR were influenced by progressively increasing temperature and consisted of two stages of dolomitization; precipitation of preTSR calcite, quartz, barite, celestite, and fluorite; as well as oil emplacement. Pre-TSR diagenesis began soon after burial and ended with the onset of TSR; pre-TSR diagenesis thus occurred over the temperature range 35°C to about 120°C and burial depths from near-surface to somewhat less than approximately 4500 m (14,760 ft) (Figure 2). The two stages of dolomitization have been discussed previously (Jiang et al., 2013); the dominant stage commenced in the near-surface environment because of a reflux dolomitization process. Anhydrite growth accompanied the main (early) reflux dolomitization stage. A second stage of dolomitization occurred when the temperature was in the range 80°C to 140°C possibly overlapping with the onset of oil-stage TSR. Pre-TSR calcite precipitation (in fractures) overlapped with the second stage of dolomitization (50°C to 130°C) and seems to have coincided with mineralization by barite, quartz, celestite, and fluorite (60°C to 130°C; Figure 11). The sources of barium, strontium, silica, and fluorine are obscure but are possibly related to hydrothermal mineralization. Further work is warranted to determine the source of these species and the carrier hydrothermal fluid. JIANG ET AL.

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Post-TSR calcite

TSR calcite

Pre-TSR calcite

Calcite Type

Crystal Form

Relationship to Fractures

Presence of Bitumen or Petroleum Inclusions

vug fill or blocky fabric microfracture fill (submillimeterscale fractures) localized subsequent refracturing and healing

enclosed or 5–10 μm no bitumen or overgrown by petroleum barite, celestite, inclusions fluorite, and burial dolomite vug-filling or either blocky or localized grew after 5–20 μm contains replaces anhyrite-nodule subsequent burial-related bitumen anhydrite replacive refracturing and dolomite, and and petroleum nodules healing overgrown by inclusions quartz, barite, and celestite centimeter-scale centimeter-scale not refractured overgrown by 5–20 μm contains gas macrofracture euhedral and celestite and inclusions, fill (crystals in crystals healing anhydrite no bitumen or centimeter-scale oil fractures)

Occurrence in Rock

Relationship to Other Size of Diagenetic Aqueous Minerals Inclusions

Table 3. Summary of the Key Attributes of the Different Types of Calcite Found in the Feixianguan Formation

VPDB

mode at 90°C

from −0‰ to −2‰

mode at 190°C

from −3‰ to spread from 110°C to as low as 220°C; oil-related −19‰ mode at 130°C–140°C, gas-related mode at 170°C–180°C

from −0‰ to −2‰

δ13 C

Aqueous Fluid-Inclusion Homogenization Temperature

spread from 0 to 24 wt. % (oil-related mode at 20–22 wt. %, gasrelated mode at 10 wt. %) mode at 4 wt. %

mode at 20 wt. %

Fluid-inclusion– Derived Salinity

Figure 11. Paragenetic sequence of dolomite-hosted Feixianguan Formation in the northeast Sichuan Basin summarizing major products of pre-TSR diagenesis, TSR diagenesis, post-TSR diagenesis, and the temperature and salinity in each diagenetic realm. Temperature and salinity data have revealed fluid-inclusion analysis. The bar represents the range of salinity values, the black dot represents the average salinity. Because of the absence of fluid-inclusion data from reflux-related dolomite, we here assume that the original formation water was normal seawater with a salinity of 3.5 wt. % (NaCl).

The salinity of pre-TSR calcite ranges from 10 to 24 wt. % (Figures 9–11), thus approaching halite saturation; the salinity of the pre-TSR calcite decreases with increasing temperature. By contrast, pre-TSR barite, quartz, celestite, and fluorite have somewhat lower salinities ranging from 3.5 to 14 wt. % NaCl; the salinity of these minerals all increase with increasing temperature. The later dolomite recrystallization occurred in high-salinity water. The high-salinity water may have been derived from the stratigraphically overlying Jialingjiang Formation, which contains halite. This suggests that although salinity broadly increased with time during progressive burial, a transient influx of lower salinity

water associated with the mineralization event occurred. Thermochemical Sulfate Reduction Related to Diagenesis Reactions in the TSR diagenetic realm were caused by temperatures between more than 116°C and 220°C. The highest temperatures resulted from the great burial depths achieved in the basin (as much as 7000 m [22,960 ft] at maximum burial), although inversion led to some degree of uplift and cooling (Figure 2). Thermochemical sulfate reduction was also caused by the presence of sulfate minerals JIANG ET AL.

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in the Feixianguan Formation in the presence of petroleum liquid and gas. A result of TSR was the production of large volumes of H2 S (Cai et al., 2003; Liu et al., 2013). Thermochemical sulfate reduction processes were influenced by the specific petroleum phase. The most important processes and products of this stage of diagenesis were (1) dissolution of anhydrite and subsequent growth of calcite induced by (2) oil-related TSR and (3) gas-related TSR, (4) formation of pyrobitumen, (5) postbitumen calcite precipitation, and (6) the creation of relatively late-stage element sulfur, celestite, anhydrite, barite, and quartz. Calcite Growth and Anhydrite Dissolution Caused by Thermochemical Sulfate Reduction Molds interpreted to represent former anhydrite laths occur predominantly in sucrosic dolostone reservoirs. After anhydrite dissolution, the molds were locally filled by large single crystals of calcite cement (Figure 4C). The sulfate-sulfur in anhydrite undergoes reduction and is the primary source of the sulfide in H2 S. The chemical reduction is caused by carbon in the petroleum fluids that subsequently become oxidized. Simplified stoichiometric reactions between anhydrite and the two simplest hydrocarbons can be written as follows: CaSO4 þ CH4 → CaCO3 þ H2 S þ H2 O 2CaSO4 þ C2 H6 → 2CaCO3 þ H2 S þ S þ 2H2 O

(2) (3)

7CaSO4 þ 4C2 H6 → 7CaCO3 þ 7H2 S þ CO2 þ 5H2 O (4)

These reactions indicate that TSR can produce sulfur as well as calcite, H2 S, CO2 , and water (Worden et al., 1995, 2000; Worden and Smalley, 1996; Cai et al., 2003, 2004, 2010; Hao et al., 2008). Reaction 2 shows that 1 mol of anhydrite is replaced by 1 mol of calcite as well as the reaction of methane producing 1 mol each of H2 S and water. The carbon stable-isotope ratios of the TSR calcite demonstrate that the carbon was derived from the oxidation of isotopically light hydrocarbon gases (Figure 8). Thermochemical sulfate reduction calcite typically has a range of δ13 C values from as high as that of the marine host rock to as low as about 966


−20‰ VPDB but seldom reaching values as low as the δ13 C values of the initial hydrocarbons (Worden et al., 1996). The third reaction represents the incomplete reduction of sulfate in the anhydrite by ethane because some elemental sulfur is produced. Complete reduction of sulfur in sulfate by ethane is represented in reaction 4, which leads to the generation of CO2 as well as H2 S (albeit CO2 in relatively small volumes). Anhydrite dissolution is a prerequisite of TSR, and calcite growth is a natural consequence of releasing calcium into solution and producing oxidized carbon. Sulfur isotope data from previous publications show that the H2 S has elevated δ34 S relative to pre-existing sulfide minerals as is typical of TSR, but uncommonly, this value (mean about 12‰ Vienna Canyon Diablo troilite standard [VCDT]) is about 10‰ lower than the mean of the associated anhydrite (22‰ VCDT) (Cai et al., 2004, 2010; Zhu et al., 2005a; Y. S. Ma, 2008). The paradox of the uncommonly low δ34 S values of H2 S relative to anhydrite remains unsolved. The lowest temperature fluid inclusions in replacive calcite in the Feixianguan Formation are 116°C (Figure 9), and therefore, sulfate reduction cannot be any consequence of bacterially mediated processes (Orr, 1977). The interpretation of TSR in the Feixianguan Formation is also supported by the occurrence of bitumen and elemental sulfur, the δ34 S ratios of the bitumen and sulfur (Y. M. Wang et al., 2002; Cai et al., 2004, 2010; Zhu et al., 2005a), and the carbon isotope data from TSR calcite (Figure 8). However, the following aspects of the TSR process remain controversial: (1) the minimum temperature for initiation of TSR, (2) the timing of TSR, (3) the rate-controlling steps during TSR, and (4) the role of formation water during TSR. Minimum Initiation Temperature of Thermochemical Sulfate Reduction Temperatures in the Lower Triassic reservoirs reached 100°C at about 200 Ma, 150°C at 175 Ma and a maximum temperature of about 225°C at 120 Ma before inversion occurred from the Cretaceous onward (Figure 2). The present-day gas reservoirs in the Feixianguan Formation reportedly evolved from a previous oil fill (Hao et al., 2008, 2009). Hence, anhydrite in the Feixianguan Formation likely reacted

with both liquid- and gas-phase petroleum during progressive heating. From the petrological observation, two stages of TSR have been recognized; namely, oil-stage TSR and gas-stage TSR. Oil-stage TSR resulted in TSR calcite that contains bitumen; gas-stage TSR resulted in bitumen-free TSR calcite. Homogenization temperature analysis of aqueous fluid inclusions associated with the two types of TSR calcite have revealed the temperature range over which oil-stage and gas-stage TSR occurred. The minimum recorded temperature of growth of oil-stage TSR calcite was 116°C, with a mode at about 130°C. The minimum temperature for gas-stage TSR calcite was about 130°C, the maximum is more than 220°C, and the modal value is 155°C. Oil-related TSR occurred at rather lower temperatures than gas-related TSR. Overlap exists between oil- and gas-stage TSR, although oil-stage TSR seems to be mostly complete by about 180°C (Figures 9, 10). Previous fluid-inclusion studies where data have been demonstrably collected from calcite that has replaced anhydrite have shown that the minimum temperature for TSR in a given basin can be as low as 110°C or as high as 140°C (Machel, 1987; Videtich, 1994; Worden et al., 1995, 2000; Worden and Smalley, 1996; Yang et al., 2001). The lower minimum TSR temperatures (110°C) tend to come from reservoirs that contained oil at the time of TSR. In contrast, the higher TSR temperatures (140° C) are from reservoirs that contained gas at the time of TSR. No studies before this work on the Feixianguan Formation have clearly documented that different types of petroleum have different onset temperatures for TSR in the same formation. Three main reasons exist why these different but overlapping temperature regimes have been found. The first possible reason is that oil is simply more reactive with sulfate than gas, and the initiation temperature for TSR was consequently lower. A second possible reason is that the initial oil charge to the Feixianguan Formation was mostly replaced by gas when the formation was at a temperature greater than about 130°C. The replacement of oil by gas could have been caused by thermal breakdown of the oil or dilution by incoming gas and transient breaching of the trap during structural movements with loss of initial (oil) charge and the subsequent charge being

gas. A third possible reason is that TSR preferentially destroyed the more reactive liquid-phase higher hydrocarbons in the early stages of TSR, leaving the petroleum progressively richer in gas-phase compounds at higher temperatures. Previous fluid-inclusion studies have shown that TSR can occur at temperatures much below 200°C (Machel, 1987; Videtich, 1994; Worden et al., 1995; Yang et al., 2001) despite earlier assertions (Trudinger et al., 1985). The data reported here also show that TSR is common much below 200°C with a minimum temperature of 116°C for oil-stage TSR. Furthermore, the critical role of phase of the petroleum reactant seems to have been illustrated here by the fluid-inclusion data (Figures 9, 10), suggesting that fluids with different gas-oil ratios result in different degrees of TSR reactivity at a given temperature. In mechanistic terms, this suggests that different families of compounds undergo TSR in different ways with different degrees of ease. Timing of Thermochemical Sulfate Reduction Comparison of the modeled thermal history (Figure 2) and the aqueous inclusion homogenization temperature data from TSR calcite (Figure 9) reveal the timing for the oil-stage and gas-stage TSR. By reference to the thermal history model (Figure 2), oil-stage TSR must have occurred between about 180 and 160 Ma, during the Middle Jurassic. Gasstage TSR must have occurred from about 160 to about 30 Ma (from the Middle Jurassic to the late Eocene). Possibly, TSR is ongoing in some of the deeper Feixianguan Formation reservoirs given that some are hotter than 150°C (Liu et al., 2013). Rate-Controlling Steps during Thermochemical Sulfate Reduction In theory, TSR can be controlled by any of the following steps (Worden et al., 2000; Bildstein et al., 2001): 1. The dissolution of anhydrite; 2. The redox step (sulfate into sulfide and the commensurate petroleum into carbonate); 3. The transport step carrying the petroleum and/or the aqueous sulfate to the site of the redox reaction; and 4. The calcite precipitation step. JIANG ET AL.

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The overall rate of TSR will be controlled by the slowest of the processes listed above. Possibly, the rate-controlling step changed during the course of TSR as the physical and geochemical properties of the petroleum-rock system evolved. Given that oilphase TSR occurred at a lower initial temperature than gas-phase TSR, it can be surmised that oilrelated TSR had more favorable kinetics than gasrelated TSR. Experimental assessment of the rate of TSR has shown that the redox step should be a geologically rapid process that is complete within a few million years at temperatures of about 150°C (Cross et al., 2004; Q. S. Ma et al., 2008a; Tang et al., 2009). However, anhydrite remains in the Feixianguan Formation, especially the nodular form in the dolomicrite parts of the unit (Figure 4A), despite being associated with petroleum fluids for as long as 150 m.y. (Figure 2). This implies that the rate of the redox step has not controlled the overall rate of TSR in this case so that it would be inappropriate to model TSR simply using the rate constants of the redox step. The growth of replacive TSR calcite at the edge of the anhydrite nodule has likely physically isolated the remaining anhydrite from the reactive petroleum fluids (oil or gas). This suggests that the overall rate of TSR has been controlled by the transport rate of petroleum and/or aqueous sulfate through the crystalline calcite rim (Worden et al., 2000; Bildstein et al., 2001). Note that little or no anhydrite remains in the coarser crystalline dolomite facies, suggesting that either (1) anhydrite could dissolve rapidly in the higher permeability matrix or (2) anhydrite that is not nodular dissolves easily and allows transport of the aqueous sulfate to the site of the redox step. Calcite rims tend not to be found on larger single crystals of anhydrite in the crystalline dolomite facies. Thermochemical sulfate reduction in coarser dolomite may be able to proceed more efficiently than TSR in finely crystalline dolomite matrix that also contains finely crystalline anhydrite nodules. Sulfur-Enriched Bitumen and Elemental Sulfur Generation In the northeastern Sichuan Basin, bitumen in the Feixianguan Formation is sulfur enriched with high sulfur:carbon ratios (Hao et al., 2008; Cai et al., 2010). Bitumen in the Feixianguan Formation also has a sulfur isotope ratio as high as the associated 968


H2 S (Cai et al., 2010). The sulfur in the bitumen was therefore most likely derived ultimately from TSR (possibly via H2 S and a backreaction between the H2 S and petroleum). The high sulfur contentbitumen may be related to oil-stage TSR. Elemental sulfur occurs as a replacement of anhydrite (Figure 4A) (Y. M. Wang et al., 2002) in the low-porosity dolomicrite reservoir and as a pore-filling cement (Figure 7C, D) in high-porosity dolostone reservoir section in the same formation. Elemental sulfur typically occurs in parts of the reservoir that are devoid of bitumen; hence, it seems possible that it is related to gas-stage TSR. Unlike the presence of large, bubble-shaped cavities in sulfur and features resembling curled-up rope in the Smackover Formation, Black Creek field, Mississippi (Heydari, 1997), the elemental sulfur in the Feixianguan Formation seems to be orthorhombic sulfur, suggesting that the elemental sulfur was crystalline in the subsurface, possibly because the whole section was geologically uplifted from deepest burial depth (>7 km [22,960 ft], >200°C) to the present depth (4.5 km [14,760 ft], 110°C), whereas the Smackover example contained liquid sulfur in the subsurface, which was quenched upon retrieval of the core. Hydrogen Sulfide Scrubbing in the Feixianguan Formation Pyrite in the Feixianguan Formation is locally present in pores associated with TSR calcite and dolomite crystals (Figure 7C, D). Pyrite is a byproduct of TSR based on sulfur isotope studies (Cai et al., 2010). Growth of pyrite requires the presence of nonsulfide Fe minerals in the reservoir, which could have been either ferrous carbonate minerals or Fe-bearing clay minerals. The lack of clay minerals in the Feixianguan Formation has been revealed here, as well as in previous studies, by petrological analysis (Cai et al., 2004, 2010, 2012; Zhu et al., 2005b; Li et al., 2012). Pre-TSR calcite cement has been reported to be ferrous from the Feixianguan Formation (Li et al., 2012) and is a potential source of iron for the late-stage pyrite. It is the siderite component in ferrous calcite that supplies the iron, so the reaction will be of the following type (Liu et al., 2013): FeCO3 þ 2H2 S → FeS2 þ CO2 þ H2 þ H2 O

(5)

Water Generated by Thermochemical Sulfate Reduction A simple balanced reaction between anhydrite and methane (reaction 2) theoretically creates 1 mol of H2 O for every mole of H2 S. This pure water will dilute the salinity of the initial (pre-TSR) formation water and lead to (at least in the immediate vicinity of the site of TSR) reduced salinity values. This has been previously demonstrated from fluid-inclusion studies of TSR calcite, where salinities have been reduced to one-fifth of the original salinity, suggesting that locally significant volumes of water have been created (Worden et al., 1996; Yang et al., 2001; Worden and Carrigan, 2005; Du et al., 2009). The suggestion of water generation caused by TSR has been the subject of debate (Machel, 1998; Worden et al., 1998). In the Feixianguan Formation, aqueous fluidinclusion (last ice-melting temperature) data (Figures 9, 10) show that the formation water before TSR was highly saline, probably because of mixing with highly saline water derived from the overlying, evaporite-bearing, Triassic Jialingjiang Formation (Zhao et al., 2005). The salinity of water associated with TSR calcite decreased with increasing temperature (Figures 9, 10). The salinity of water at the start of TSR, when the temperature was 116°C, was more than 24 wt. % NaCl. The salinity decreased to about 4 wt. % NaCl as the temperature increased to greater than 200°C as TSR proceeded. The average salinity of other diagenetic minerals that grew during TSR (barite, calcite, and quartz) also had relatively low salinity (Figures 9, 10). Therefore, the water generated by TSR in the Feixianguan Formation must have diluted the residual water salinity from as high as 24 wt. % NaCl to as low as 4 wt. % NaCl. This suggests that, at least at the reaction site where the water was created and then trapped within newly grown TSR calcite (and other minerals), TSR has produced about five times the volume of water that was present prior to TSR. The present-day formation water from the Feixianguan Formation is about 4 wt. % NaCl, suggesting that the water created by TSR has led to widespread dilution of formation water (Zhu et al., 2007).

Diagenesis after Thermochemical Sulfate Reduction Diagenetic processes after TSR diagenesis were strongly influenced by the tectonic uplift; these are dominated by the bitumen-free, fracture-filling calcite, and less commonly by growth of celestite and anhydrite (Liu et al., 2006; Li et al., 2012). Vein anhydrite has distinctly higher sulfur isotope values than those from the earlier diagenetic anhydrite nodule and bedded anhydrite (Cai et al., 2010) and, together with its crosscutting texture, can be concluded to be the result of a much later event. Thus, post-TSR, uplift-related diagenesis occurred within the temperature range of 110°C to 140°C and a burial depth of 4500 to less than 5500 m (14,760 to less than 18,040 ft) (Figure 2). Formation Water Salinity Evolution Using petrological observations, aqueous fluid-inclusion data analysis, and thermal-history modeling, the paragenetic sequence as well as the evolution of the paleowater of the dolomite-hosted reservoir in the northeastern Sichuan Basin has been produced (Figure 11). The initial (connate) water was Feixianguan-age seawater (3.5 wt. % NaCl) from which the limestone accumulated. The dominant dolomitization stage of the Feixianguan Formation happened soon after deposition because of a reflux-seepage process in the presence of Feixianguan seawater. Fluid inclusions in this type of dolomite are very small and single phase; thus, no inclusion salinity data could be produced. However, the salinity of the formation water must have increased because of the mixing of the original seawater with evaporated seawater; the growth of diagenetic anhydrite nodules testifies to the elevated salinity. The salinity of formation water increased to 24 wt. % NaCl (approaching halite saturation) as indicated by inclusion analysis of pre-TSR calcite (Figure 9). Overlapping in time but toward the final stages of pre-TSR calcite, a lower salinity water (mean 7.5 wt. % NaCl) locally mixed with the high-salinity formation water as recorded in fluid inclusions in pre-TSR barite, quartz, celestite, and fluorite. The salinity of inclusions in pre-TSR barite, JIANG ET AL.

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quartz, celestite, and fluorite all increased with increasing temperature (Figure 10), whereas the salinity of inclusions in pre-TSR calcite decreased with increasing temperature. This suggests that the salinity was toward its initial peak (mean of 17.4 wt. % NaCl) when pre-TSR calcite started to grow, the formation water was subsequently diluted by lower salinity water that led to pre-TSR barite, quartz, celestite, and fluorite growth (to a local minimum mean of 7.5 wt. % NaCl), and then salinity started to rise again. The second stage of dolomite growth occurred after the mineralization event. The salinity data revealed by dolomite fluid inclusion suggest that another influx of highly saline water occurred progressively during burial, reaching a peak of about 24 wt. % NaCl. Note that the stratigraphically younger (and thus, overlying) Lower Triassic Jialingjiang Formation contains halite. Possibly, halite dissolved in the local formation water, which then migrated into the underlying reservoir unit, e.g., caused by compaction or overpressure buildup (O’Connor et al., 2011). Oil-stage TSR calcite records progressively reduced salinity with increasing temperature, whereas all the gas-stage TSR calcite inclusions are low salinity (Figure 10), suggesting that most of the diluting was done relatively early and by the oil-related TSR event. In addition, fracture-filling calcite cement that is free of bitumen, with temperatures ranging 160°C to 220°C, could be part of the gas-stage TSR calcite generation. This type of calcite has a salinity range from 3 to 6 wt. % NaCl, with an average value of 4.2 wt. % NaCl. Therefore, the salinity of gas-stage TSR calcite is close to the salinity reported for present-day formation waters (Li et al., 2012), suggesting that the salinity of formation water may be influenced by the water generated by TSR in a closed diagenetic environment during burial. In summary, the formation water salinity has been affected by seawater evaporation, an influx of highly saline water, possibly from overlying evaporite-rich Jialingjiang Formation Lower Triassic rocks, an influx of lower salinity water possibly associated with faulting and mineralization by exotic minerals (quartz, fluorite, etc.), another influx of highly saline water during the second dolomitization event, and then progressive dilution, probably in a closed system, by water generated by TSR. This final 970


diagenetically altered water seems to be in the Feixianguan Formation at the present time.

CONCLUSIONS 1. Thermochemical sulfate reduction in the Lower Triassic Feixianguan Formation, Sichuan Basin, China, has led to anhydrite dissolution and replacement by calcite, petroleum destruction, bitumen and elemental sulfur formation, and the generation of large volumes of H2 S and CO2 . 2. Thermochemical sulfate reduction happened in two phases. The first was caused by an oilanhydrite reaction that commenced at 116°C with a mode between 130°C and 140°C. The second was caused by a gas-anhydrite reaction with a minimum temperature of 135°C, continuing up to maximum burial at 220°C and possibly during cooling at structural sites where the Feixianguan Formation has remained hotter than 140°C. 3. Oil-stage TSR occurred relatively early in the burial history (approximately from 180 to 160 Ma) because of the rapid burial of the Lower Triassic reservoir. Gas-stage TSR occurred subsequently until the Tertiary uplift finally cooled the Feixianguan Formation to less than 140°C and, thus, out of the TSR window. Gas-stage TSR was thus approximately between 160 and 30 Ma. 4. Thermochemical sulfate reduction was preceded by the main eogenetic stage of reflux dolomitization; fracture-related calcite cementation; dolomite recrystallization; and mineralization by barite, quartz, celestite, and fluorite. Mineralization by quartz, barite, and celestite appears to have been episodic and was also partly coincident with TSR. After uplift and cooling, TSR was followed by fracture-related calcite cementation. 5. Formation water salinity increased from the interpreted marine depositional value (∼3.5 wt: % NaCl) to high salinity (more than 24 wt. % NaCl) during dolomite recrystallization and the initial stages of oil-related TSR. The salinity increase was punctuated by a transient decrease during the pre-TSR mineralization event. Formation water salinity decreased during TSR to a minimum value of about 7 wt. % NaCl. During the post-TSR calcite cementation of fractures, water salinity was as low as 2 wt. % NaCl.

6. The initial increase in salinity, during dolomite recrystallization, may have involved an influx of evaporite-related water from the overlying Triassic Jialingjiang Formation. The transient decrease in salinity during mineralization, before TSR, may have been caused by large-scale, opensystem movement of relatively low-salinity water in the basin. 7. Thermochemical sulfate reduction caused by closed-system oil-anhydrite and then gasanhydrite reaction must have generated a significant volume of water to dilute the highly saline pre-TSR formation water.

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