Coupled alteration of hydrothermal fluids and thermal sulfate reduction (TSR) in ancient dolomite reservoirs – An example from Sinian Dengying Formation in Sichuan Basin, southern China

Precambrian Research 285 (2016) 39–57

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Coupled alteration of hydrothermal fluids and thermal sulfate reduction (TSR) in ancient dolomite reservoirs – An example from Sinian Dengying Formation in Sichuan Basin, southern China Quanyou Liu ⇑, Dongya Zhu, Zhijun Jin, Chunyan Liu, Dianwei Zhang, Zhiliang He State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Beijing 100083, China Petroleum Exploration and Production Research Institute of SINOPEC, Beijing 100083, China

a r t i c l e

i n f o

Article history: Received 17 May 2016 Revised 19 August 2016 Accepted 11 September 2016 Available online 15 September 2016 Keywords: Sinian Dengying Formation Dolomite reservoir Hydrothermal TSR Rare earth element

a b s t r a c t To better understand the fluid activities and their roles in the formation of petroleum reservoirs, dolomite was thoroughly studied in the ancient Sinian Dengying Formation in Sichuan Basin, southern China. The high homogenization temperatures (above 190 °C) of fluid inclusions, positive Eu anomaly, and O and Sr isotopic values of coarse crystalline pore-filling dolomite (FD) in dissolution pore spaces have suggested that the FDs are of hydrothermal origin. There are also other minerals (pyrite), organic compounds (pyrobitumen) and gas components (H2S, CO2 and CH4) in pore spaces. The sulfur isotope compositions of pyrite and H2S, along with the carbon isotopes of CO2 have shown the involvement of thermochemical sulfate reduction (TSR). Hydrothermal activities during burial diagenesis of the Dengying Formation provided high temperature environment necessary for TSR to proceed. The presence of CO2 and H2S in both processes enhanced fluid acidities and consequently porosity and permeability of the Dengying Formation. Emplacement of petroleum and acidic fluids helped to maintain pre-existing reservoir spaces during deep burial stage. Overall, the coupled hydrothermal activities and TSR provided favorable environments to allow for maintaining high quality reservoirs. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction Hydrocarbon resources from ancient Precambrian sedimentary strata are in large amounts all over the world (Craig et al., 2013; Melezhik et al., 1999; Schröder et al., 2005). The detailed hydrocarbon accumulation process has long been studied (Craig et al., 2013; Melezhik et al., 1999; Schröder et al., 2005). In Sichuan Basin, southern China, the newly found giant Anyue gas field (Zhu et al., 2015a; Zou et al., 2014) and Weiyuan gas field (Jin, 2005; Wei et al., 2008) are examples of this type (Fig. 1). The widely distributed high-quality dolomite reservoir, the Sinian Dengying Formation, is believed to be one of the most important factors contributing to high gas production. The Sinian period (also called as Ediacaran) is the last one in the Neoproterozoic Era, from 680 to 543 Ma. The Dengying Formation is the rock unit formed from 630 to 543 Ma during the late Sinian period. The Tongwan movement at the end of Dengying period caused uplift and exposure of the Precambrian strata in almost entire ⇑ Corresponding author at: State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Beijing 100083, China. E-mail addresses: [email protected], [email protected] (Q. Liu). http://dx.doi.org/10.1016/j.precamres.2016.09.006 0301-9268/Ó 2016 Elsevier B.V. All rights reserved.

Sichuan Basin (Wang et al., 2014). The dolomite in Denying Formation was subsequently dissolved and karstificated by meteoric water to form large amount of dissolution vugs and caves (Zou et al., 2014). However, most secondary vugs and caves, along with primary pores, were gradually destructed due to compaction and mineral cementation during a long burial diagenesis process at depths (Schmoker and Hally, 1982). Thus, lack of high quality reservoirs poses potential high risk for hydrocarbon exploration in the ancient deep Precambrian strata in the Sichuan Basin. Previous studies have shown that there are two major types of activities involving interaction with dolomite: hydrothermal fluid movement (Davies and Smith, 2006; Jin et al., 2006a; Lavoie et al., 2010), and thermochemical sulfate reduction (TSR) with fluids enriched in H2S and CO2 (Cai et al., 2001, 2013, 2014; Hao et al., 2008, 2015; Liu et al., 2013; Machel, 1998; Worden and Smalley, 1996; Worden et al., 1995). Preliminary analyses of natural gas and observation of the drilling cores in the Sinian Dengying Formation dolomite reservoir may have shown that the hydrothermal activities and TSR were not separated processes but coexisted. Both processes may be crucial factors controlling the development and ultimate formation of the dolomite reservoirs during deep burial diagenetic stage. To our knowledge, however, little is

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Q. Liu et al. / Precambrian Research 285 (2016) 39–57

Fig. 1. Tectonic units and locations of wells, field outcrops (A) and gasa reservoir profile (B) in the Sichuan Basin, southern China.

known about the relationship between hydrothermal activities and TSR. The detailed mechanism of this coupled processes and its impact on reservoir development has not been systematically investigated. The objectives of this study are to (1) provide geochemical constraints on the hydrothermal activities and TSR during diagenesis in the deep Dengying Formation dolomite reservoirs, (2) discuss the mechanism of the coupled hydrothermal-TSR alteration processes, and (3) reveal impacts of both hydrothermal and TSR on the development of dolomite reservoirs.

2. Geological background After decades of oil and gas exploration in Sichuan Basin, many drillings and field outcrops (Fig. 1) have revealed high quality deep dolomite reservoirs in Sinian Dengying Formation (Z2dn), which is the main production layer in the giant Anyue gas field (Zhu et al., 2015a; Zou et al., 2014) and Weiyuan gas field (Jin et al., 2006a; Wei et al., 2008). The Sinian Dengying Formation is divided into four members (Fig. 2). From bottom to top, the first member (Z2dn1) is mainly

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Fig. 2. Stratigraphy column of the Cambrian and Sinian systems and characteristics of pore-filling minerals in the Sichuan Basin.

mudstone and sandstone with dolomite interlayers. It is only present in limited areas. The second member (Z2dn2) is predominantly dolomite with typical stromatolitic and crust-shaped structure. The third member (Z2dn3) is mainly gray-brown or brown-yellow sandstone and black mudstone, whereas the fourth (Z2dn4) is completely dolomite, including algal, granular and siliceous dolomites (Fig. 2). Underneath the Dengying Formation is the Doushantuo Formation (Z1ds), which consists of epimetamorphic clastic and carbonate rocks. The Lower Cambrian Mingxinsi Formation (21m)

and Qiongzhusi Formation (21q) are mainly mudstone and silt sandstone. The Lower Cambrian Canglangpu Formation (21c) and the Middle Cambrian Douposi Formation (22d) are predominately sandstone. The Lower Cambrian Longwangmiao Formation (21l) and the Upper Cambrian Xixiangchi Formation (23x) are mainly dolomite (Fig. 2). In the Sinian and Lower Paleozoic strata, the main hydrocarbon source rocks consist of the Lower Cambrian black shale and the Z2dn3 black mudstone (Fig. 1). A large amount of hydrocarbons

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were generated during the middle and late Caledonian orogeny (Li et al., 2011; Zhao et al., 2003). The major gas reservoirs in the Weiyuan and Anyue gas fields are in the Sinian Dengying (Z2dn), Lower Cambrian Longwangmiao (21l) and Upper Cambrian Xixiangchi (23x) Formations (Fig. 1). The uplifting during Tongwan movement in the Late Sinian of Sichuan Basin and its peripheral areas caused dolomite in the Dengying Formation to be widely exposed to the surface (Wang et al., 2014) (Fig. 3). The karst topography was also formed by meteoric water. The generation of vugs and caves was the important feature contributing to this major reservoir. The Dengying Formation was buried from Paleozoic to Mesozoic and uplifted again

during Himalaya orogeny in the Middle Cretaceous (Li et al., 2011; Zhao et al., 2003). There are two major hydrothermal events caused by tectonic movement, i.e., the Xingkai movement and Emei movement in Sinian- Paleozoic period in Sichuan Basin and adjacent Upper Yangtze area (Liu et al., 2008). They occurred in the early Cambrian (21) and early Permo- middle Triassic (P1-T2) period, respectively (Fig. 3). Hydrothermal events involving siliceous fluids have been reported in the early Cambrian period (Chen et al., 2009). Indeed, in Sichuan Basin and adjacent Sichuan-Yunnan-Guizhou areas on the southwestern margin of the Yangtze Platform, the formation of a series of Mississippi Valley Type (MVT) and Sedimentary

Fig. 3. Burial history, diagenetic sequence and porosity development of the deep Sinian Dengying Formation dolomite reservoirs.

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exhalative (SEDEX) type of Pb and Zn deposits has shown the extensive occurring of hydrothermal activities (Liu et al., 2008).

3. Samples and methods Dolomite, pyrite and natural gases were obtained from field outcrops and wells. Outcrop samples were collected from 5 profiles, including Zhengyuan, Qiaoting, Liaojiacao, Yankong and Baima (Fig. 1). Core samples were obtained from wells Jinshi 1 and Lin 1, whereas natural gas samples were retrieved from wells in Weiyuan and Anyue gas fields (Fig. 1). Coarse crystalline dolomite and pyrite grains filling in dissolution pores in the Dengying Formation were systematically sampled. For comparison, matrix dolomite (MD) crystals near the pore-filling dolomites (FD) were also collected. Thin sections were studied to understand mineralogical relationship between dolomites, and the role of hydrothermal fluids on porosity as well. Fluid inclusions within dolomite grains were analyzed for their salinities and formation temperatures. The analyses of rare earth elemental abundances, and stable carbon, oxygen and sulfur isotopes were also conducted to constrain origins of dolomite and pyrite. All these analyses were completed at the State Key Laboratory of Ore Deposit at Nanjing University. The samples were polished on both sides to about 0.03 mm thick for thin section observation, and to about 0.2 mm thick for temperature measurement of fluid inclusions. Microscopic observations of thin sections for petrology, mineralogy and pore structures are carried out using a Leica DM4500 microscope. The measurement of fluid inclusions was performed on a LinkamTH600 heating-cooling stage. Following temperature calibration, the measurement started with an increasing temperature rate of 15 °C/min. The rate was then decreased to 1 °C/min when reaching homogenization. The analytical error of temperature measurement is ±1 °C. For geochemical analyses, the samples (pore-filling dolomite and pyrite) were grinded into 20–40 mesh in grain size, ultrasonically cleaned and air dried. Pure particles were selected under a binocular stereo microscope. The selected particles were then grinded to powder of less than 200 mesh for elemental and stable isotope analyses. The abundances of rare earth elements were obtained by ICPMS (Yokogava PMS-200). The powder sample in amount of 40 mg was mixed with 3 mL of HNO3. The mixture of sample and HNO3 was kept on an electrothermal plate at 120 °C for 24 h, and 150 °C for another 24 h. The solution was then mixed with 2 mL HNO3 after being concentrated, and kept at 150 °C for 2 h. After being concentrated one more time into 1 mL, the solution was mixed with deionized water for analyses. Carbon and oxygen isotope analysis was carried out on a Gas Bench II apparatus connected to a MAT 253 isotope ratio mass spectrometer. Each powdered sample in 200 lg or so was placed in a vial that was then flushed with pure He gas. Following the addition of 100% H3PO4, the vial was kept at 72 °C for 1 h before analysis. All C and O isotopes were reported relative to PDB and calibrated against NBS-18 and NBS-19 standards. Reproducibility of replicate analyses for NBS-18 and NBS-19 standards was better than ±0.1‰. The strontium isotope analysis was performed using a Finnigan MAT Triton TI. Approximately 100 mg of the powdered sample was placed in a jar, and 2 mL of 6 M HCl was added. The sample was dissolved for 24 h at a temperature of 100–110 °C. The ion chromatography technique, pioneered by Aldrich et al. (Aldrich et al., 1953), was used to separate the strontium isotopes. Using HCl as the eluant, the AG 50W-X12 200–400 mesh ion resin produced by Bio-Rad Corp. (USA) was used to separate and enrich the stron-

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tium isotopes. The measured 87Sr/86Sr values were adjusted according to the mass fractionation standard 87Sr/86Sr = 0.1194. The analyzed 87Sr/86Sr values of the NBS987 standard sample averaged 0.710273 ± 0.000012. The sulfur isotopic compositions of pyrite were analyzed on Thermo Finnigan Delta S mass spectrometer. Sulfur in pyrite was oxidized directly into SO2, and the values of d34S of SO2 were measured by the mass spectrometer. The d34S results were expressed using the international standard CDT. Reproducibility as determined through replicate analyses of reference materials was better than ±0.3‰ (1r). 4. Results 4.1. Petrographic characteristics 4.1.1. Pore-filling dolomite (FD) The Z2dn2 and Z2dn4 dolomites in the Sinian Dengying Formation are the main hydrocarbon reservoir layers (Figs. 1 and 2). The occurrence of crust-shaped dolomite is common in the Z2dn2 (Fig. 4A). Under microscope, textures like algal grains and algal bounded clots can be observed frequently (Fig. 5A). In the Z2dn4, algal laminar, sucrosic and breccia dolomites are abundant (Fig. 4B). Due to the uplifting and subsequent exposure above ground during the Tongwan movement in the late Dengying period, dolomite was dissolved by meteoric water and a large number of dissolution pores were formed (Zou et al., 2014). The size of pore ranges from a few millimeters to several centimeters (Fig. 4). Most dissolution pores in MD were filled by coarse crystalline dolomite (Fig. 4). Those FD crystals are usually a few millimeters to almost one centimeter in size, and mostly occur as white rhombohedral crystals (Fig. 4A, C, D, and E). Under microscope, the FDs present curved crystal plane and wavy extinction (Fig. 5B and C). 4.1.2. Pyrite Along with FD, pyrite is often observed in the pore spaces caused by hydrothermal dissolution. The pyrite grains generally are present as individual fine granules, coarse granules (Fig. 4C, D and E), or clumps (Fig. 4E). Microscope (Fig. 5D, E and F) and scanning electron microscope (Fig. 5I) observations have shown that pyrite is in cubic shape. In most samples, pyrite coexists with FD crystals (Fig. 4C, D and E; Fig. 5D, E, F and I). 4.1.3. Pyrobitumen Pyrobitumen, solid and amorphous organic compounds, can be frequently observed in dissolution pores along with dolomite and pyrite (Fig. 4C, D, E and F; Fig. 5F, G and H). Under microscope, dolomite crystals can be seen dissolved and then filled by pyrobitumen (Fig. 5H), suggesting that the formation of pyrobitumen is later than that of FD. Under scanning electron microscope, pyrobitumen is often scaly or sphere-shaped (Fig. 5J, K and L). It is common that there are numerous micropores of about a few micrometers in diameter in pyrobitumen (Fig. 5K and L). 4.2. Fluid inclusions A large number of fluid inclusions are present in coarse crystalline FDs. The fluid inclusions are generally small, ranging from 10 to 15 lm in size (Fig. 6). Under the irradiation of ultraviolet light, no fluorescence was observed, suggesting that their compositions are pure brines without hydrocarbons. The analytical results of microthermometry have shown that the homogenization temperatures of the inclusions from Well Jinshi 1 are between 150.3 °C and 187.9 °C, with the average of 173.3 °C (Table 1). They have high salinity of 16.7–21.3 eq. wt%

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Fig. 4. Photographs of pore-filling coarse crystalline dolomite, pyrite and pyrobitumen in the Sinian Dengying Formation dolomite. FD - Pore-filling coarse crystalline dolomite; Bn - Pyrobitumen; Py - Pyrite. A - Coarse crystalline dolomite filling in dissolution pore in crust-shaped dolomite, Well Jinshi 1, 4019.85 m; B- Algal laminar and brecciated dolomites with pyrobitumen filling in inter-breccia pores, Well Lin 1, 2657.71 m; C- Coarse crystalline dolomite, pyrite and pyrobitumen filling in dissolution pores, Well Lin 1, 2825.70 m; D- Coarse crystalline dolomite, pyrite and pyrobitumen filling in dissolution pores, well Lin 1, 2826.79 m; E- Coarse crystalline dolomite, pyrite and pyrobitumen filling in dissolution pores, Well Jinshi 1, 4030.31 m; F- Coarse crystalline dolomite, pyrite and pyrobitumen filling in dissolution pores, Well Lin 1, 2657.93 m.

NaCl, with the average value of 19.5 wt% NaCl (Table 1). The homogenization temperatures of the inclusions from Well Lin 1 show two major ranges (Table 1). One is between 180 °C and 227.5 °C, with the average of 192.6 °C and the main peak between 190 and 200 °C (Fig. 7). The other is between 222 °C and 258 °C, with the average of 228.9 °C and the main peak between 220 °C and 240 °C (Fig. 7). Both types of inclusions have high salinities (Table 1). The former is between 22.7 and 23.1 eq. wt% NaCl with the average of 22.9 eq. wt% NaCl, and the latter between 16.2 and 16.7 eq. wt% NaCl with the average of 16.4 eq. wt% NaCl. 4.3. Elemental and isotopic characteristics of dolomite and pyrite 4.3.1. REE The overall abundances of REEs in MD are generally low. The P total concentration ( REE) ranges from 1.06 lg/g to 3.76 lg/g with the average of 2.27 lg/g (Table 2). The PAAS-normalized abundances show a flat distribution pattern with slightly light REE depletion (Fig. 8A). The MD has negative Ce anomaly, and pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi the dCe (dCe = Ce/ La  Pr N; N: normalized value by PAAS) ranges from 0.44 to 0.74 with an average of 0.61 (Table 2; Fig. 8A). The total REE concentration in FD is ranged from 1.25 lg/g to

11.77 lg/g with the average of 4.81 lg/g, slightly higher than MD. Similar to MD, the PAAS-normalized values show a flat pattern with trivial light REE depletion (Fig. 8B). A significant feature of FD is that most samples have remarkable positive Eu anomalies pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (Fig. 8B). The overall range of dEu (dEu = Eu/ Sm  Gd N; N: normalized value by PAAS) varies from 1.59 to 9.79, with the average value being 3.24 (Table 2). 4.3.2. Carbon and oxygen isotopes There is a remarkable difference in carbon and oxygen isotope compositions between MD and FD (Table 3, Fig. 9). The d13CPDB values of MD are between 0.4‰ and 3.5‰ with the average of 2.5‰, and d18OPDB values are between 7.8‰ and 3.5‰ with the average of 5.8‰. Compared with MD, FD is relatively light in both carbon and oxygen isotope compositions. The d13CPDB values of FD are between 7.6‰ and 3.1‰ with the average value of 0.5‰. The d18OPDB values are between 13.5‰ and 8.1‰ with the average of 11.3‰ (Table 3, Fig. 9). 4.3.3. Strontium isotope The 87Sr/86Sr ratio of MD varies from 0.708502 to 0.709289 (0.708887 on average), while from 0.709113 to 0.709828

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Fig. 5. Microphotographs of pore-filling coarse crystalline dolomite, pyrite and pyrobitumen in the Sinian Dengying Formation. FD - Pore-filling coarse crystalline dolomite; Bn - Pyrobitumen; Py – Pyrite. A- Dolomite with texture of algal grains and algal bounded clots, Well Jinshi 1, 4019.98 m, polarized light, 25; B- Coarse crystalline dolomite crystals with wavy extinction filling in dissolution pores, Yankong outcrop, cross polarized light, 200; C- Coarse crystalline dolomite crystals with wavy extinction filling in dissolution pores, Yankong outcrop, cross polarized light, 200; D- Coarse crystalline dolomite and pyrite filling in dissolution pores, well Lin 1, 2619.31 m, reflected light, 200; E- Coarse crystalline dolomite and pyrite filling in dissolution pores, Well Lin 1, 2625.72 m, polarized light, 100; F- Coarse crystalline dolomite, pyrite and pyrobitumen filling in dissolution pores, well Lin 1, 2825.72 m, reflected light, 100; G- Coarse crystalline dolomite, pyrite and pyrobitumen in dolomite pores, Well Lin 1, 2825.72 m, polarized light, 50; H- Coarse crystalline dolomite was dissolved and then filled with pyrobitumen, well Lin 1, 2619.22 m, polarized light, 25; I- Pore-filling coarse crystalline dolomite and pyrite in dissolution pores, Well Lin 1, 2661.57 m, SEM image, 50; J-Scaly and sphere-shaped pyrobitumen in dissolution pore, Well Jinshi 1, 4032.89 m, SEM image, 200; K- Sphere-shaped pyrobitumen in dissolution pore and miropores in the pyrobitumen, Yankong outcrop, SEM image, 2400; L- Scaly and sphere-shaped pyrobitumen in dissolution pore and miropores in the pyrobitumen, Yankong outcrop, SEM image, 8000; M- Dissolution pores in fine-middle crystalline dolomites, Well Jinshi 1, 4033.05 m, polarized light, 25; N- Pyrobitumen in the central of and separated from dissolution pores in fine crystalline dolomites, Well Jinshi 1, 4033.25 m, polarized light, 25; O- Pyrobitumen in the central of and separated from dissolution pores in dolomites, and coarse crystalline dolomite crystals with harborshaped dissolution, Yangba ourcrop, polarized light, 25.

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Fig. 6. Microphotographs of gas-liquid fluid inclusions in the pore-filling coarse crystalline dolomite (FD) in the Sinian Dengying Formation dolomite reservoirs. A, B4030.31 m, well Jinshi 1, polarized light; C, D- 2649.83 m, well Lin 1, polarized light; E, F- 2825.73 m, well Lin 1, polarized light.

Table 1 Microthermometry results of fluid inclusions in pore-filling coarse crystalline dolomite (FD) in the Sinian Dengying Formation reservoirs. Well

Formation

Depth (m)

Jinshi 1 Lin 1 Lin 1 a

Z2dn Z2dn Z2dn

4030.31 2649.83 2825.73

Formation temperature (°C)a

Size of fluid inclusion (lm)

Gas/liquid

Homogenization temperature (°C)

Salinity (wt% NaCl eq.)

Ratio

Range

Average

Main peak

Range

Average

95.6 68.0 71.5

8-12 10-15 10-15

0.1 0.1 0.1

150.3–187.9 180.0–227.5 222.0–258.0

173.3 192.6 228.9

185.0–187.9 190.0–200.0 220.0–230.0

16.7–21.3 22.7–23.1 16.2–16.7

19.5 22.9 16.4

The present formation temperature is calculated based on the ground surface temperature of 15 °C and geothermal gradient of 20 °C/km (Ma et al., 2008).

Fig. 7. Histogram for the homogenization temperatures of the fluid inclusions in pore-filling coarse crystalline dolomite (FD) in the Dengying Formation dolomite reservoirs.

(0.709579 on average) for FD (Table 3). Compared with MD, FD contains relatively higher concentration of 87Sr.

value of 8.9‰. This may be due to the contamination during sampling or sample preparation processes.

4.3.4. Sulfur isotope Pyrite samples from wells Lin 1 and Jinshi 1 were analyzed for sulfur isotope compositions (Table 4). The 34S isotope is enriched in pyrite. Most d34SCDT values are consistent and in the range of 21.8–23.4‰, except only one sample from Well Jinshi 1 with a

4.3.5. Sulfur concentration in pyrobitumen The elemental compositions of pyrobitumen in dolomite were analyzed using energy-dispersive X-ray spectroscopy (EDS) when the SEM images were observed. The pyrobitumen generally contains C, O and S elements (Table 5). The elemental percentage of

Table 2 Concentrations of rare earth elements in the matrix dolomite (MD) and pore-filling coarse crystalline dolomite (FD) in the Sinian Dengying Formation dolomite reservoirs. Sample

dolomite (FD) Lin 1 Lin 1 Lin 1 Lin 1 Lin 1 Jinshi 1 Jinshi 1 Jinshi 1 Zhengyuan Zhengyuan Baima Baima Yankong Yankong Yankong

Matrix dolomite (MD) JS1-6-4 Jinshi 1 JS1-5-2 Jinshi 1 L1-13-2 Lin 1 L1-12-2 Lin 1 L1-12-1 Lin 1 L1-11-3 Lin 1 L1-11-2 Lin 1 L1-9-1 Lin 1 YK-5 Yankong YK-6 Yankong BM-5 Baima BM-6 Baima BM-7 Baima Average PAASa

P REE

LREE/HREE

La/Yb

dEu

dCe

0.032 0.028 0.005 0.016 0.025 0.005 0.037 0.008 0.005 0.004 0.04 0.013 0.013 0.004 0.047 0.019

4.402 3.712 3.498 6.945 4.416 7.515 7.345 5.251 1.721 2.525 5.609 2.610 3.602 1.250 11.767 4.811

2.591 2.184 2.436 3.376 2.095 4.596 4.191 5.828 4.137 7.252 2.482 4.553 5.848 7.681 4.585 4.256

2.631 3.673 2.495 4.000 2.690 12.475 8.212 10.923 4.177 15.607 2.217 9.273 12.661 11.750 8.359 7.410

2.705 1.586 3.623 2.058 1.687 1.762 1.271 2.158 9.787 1.815 3.104 5.096 7.461 2.490 2.003 3.240

0.762 0.680 0.563 0.726 0.552 0.652 0.656 0.920 0.696 1.147 0.953 0.646 0.689 0.959 0.709 0.754

0.013 0.037 0.079 0.036 0.054 0.014 0.026 0.015 0.062 0.023 0.059 0.035 0.045 0.038

0.002 0.004 0.008 0.004 0.007 0.002 0.007 0.002 0.012 0.003 0.009 0.005 0.011 0.006

1.063 2.387 3.763 1.706 2.904 1.180 2.775 1.279 2.844 1.641 3.624 2.110 2.246 2.271

8.164 7.092 4.099 4.783 5.676 8.365 4.584 8.915 4.238 7.044 7.350 7.941 5.975 6.479

23.923 18.811 9.810 11.111 13.500 21.357 12.385 20.867 10.452 20.739 18.644 14.857 11.911 16.028

0.971 0.989 1.050 0.991 0.979 0.930 1.060 0.982 1.012 1.055 1.107 1.281 1.304 1.055

0.529 0.439 0.473 0.555 0.553 0.731 0.734 0.695 0.599 0.612 0.587 0.740 0.726 0.613

2.82

0.43

Depth

Rare earth elements (lg/g)

(m)

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

2619.31 2649.83 2664.82 2767.66 2825.73 4029.85 4030.31 4032.89 / / / / / / /

0.421 0.404 0.464 0.836 0.565 1.26 1.24 0.568 0.401 0.437 0.501 0.612 0.785 0.188 2.14 0.721

0.965 0.804 0.691 1.8 0.872 2.04 2.01 1.76 0.523 1.1 1.43 0.733 1.11 0.487 3.51 1.322

0.203 0.184 0.173 0.391 0.235 0.413 0.403 0.343 0.075 0.112 0.239 0.112 0.176 0.073 0.61 0.249

1.235 0.886 0.867 1.88 1.00 1.94 1.83 1.48 0.249 0.471 1.27 0.491 0.659 0.281 2.59 1.142

0.189 0.176 0.16 0.314 0.235 0.365 0.325 0.212 0.03 0.066 0.322 0.082 0.126 0.052 0.563 0.214

0.163 0.092 0.125 0.137 0.082 0.154 0.122 0.119 0.108 0.033 0.236 0.11 0.22 0.025 0.247 0.132

0.426 0.424 0.165 0.313 0.223 0.464 0.629 0.318 0.09 0.111 0.398 0.126 0.153 0.043 0.599 0.299

0.063 0.059 0.036 0.085 0.063 0.037 0.039 0.042 0.01 0.012 0.077 0.02 0.022 0.01 0.105 0.045

0.289 0.301 0.264 0.606 0.491 0.538 0.297 0.211 0.081 0.098 0.463 0.134 0.16 0.038 0.59 0.304

0.039 0.026 0.059 0.073 0.071 0.019 0.083 0.036 0.014 0.011 0.09 0.026 0.028 0.006 0.109 0.046

0.185 0.197 0.283 0.253 0.321 0.161 0.15 0.084 0.036 0.04 0.278 0.076 0.079 0.024 0.357 0.168

0.032 0.021 0.02 0.032 0.023 0.018 0.029 0.018 0.003 0.002 0.039 0.009 0.009 0.003 0.044 0.020

0.16 0.11 0.186 0.209 0.21 0.101 0.151 0.052 0.096 0.028 0.226 0.066 0.062 0.016 0.256 0.129

4029.85 4019.93 2824.64 2767.66 2766.38 2664.82 2664.13 2618.14 / / / / /

0.311 0.696 0.775 0.401 0.729 0.299 0.322 0.313 0.648 0.477 1.102 0.52 0.536 0.548

0.3 0.574 0.785 0.441 0.795 0.41 0.727 0.433 0.762 0.505 1.11 0.72 0.778 0.642

0.055 0.131 0.189 0.084 0.151 0.056 0.162 0.066 0.133 0.076 0.173 0.097 0.114 0.114

0.222 0.555 1.02 0.395 0.651 0.24 0.832 0.278 0.614 0.31 0.677 0.449 0.37 0.509

0.049 0.115 0.211 0.075 0.119 0.041 0.195 0.05 0.117 0.055 0.102 0.067 0.099 0.100

0.01 0.021 0.045 0.016 0.024 0.008 0.04 0.01 0.027 0.014 0.028 0.021 0.027 0.022

0.048 0.087 0.193 0.077 0.112 0.04 0.162 0.046 0.135 0.071 0.139 0.089 0.096 0.100

0.006 0.017 0.039 0.015 0.025 0.007 0.03 0.008 0.024 0.01 0.021 0.011 0.014 0.017

0.023 0.08 0.234 0.091 0.127 0.038 0.151 0.03 0.162 0.047 0.123 0.039 0.08 0.094

0.006 0.016 0.051 0.018 0.026 0.006 0.03 0.007 0.034 0.008 0.022 0.015 0.019 0.020

0.016 0.047 0.118 0.047 0.073 0.016 0.081 0.019 0.103 0.037 0.054 0.037 0.049 0.054

0.002 0.007 0.016 0.007 0.011 0.003 0.01 0.002 0.011 0.005 0.007 0.005 0.008 0.007

38.2

79.6

8.83

33.9

5.55

1.08

4.66

0.77

4.68

0.99

2.85

0.41

Q. Liu et al. / Precambrian Research 285 (2016) 39–57

Pore-filling L1-9-2 L1-10-1 L1-11-3 L1-12-2 L1-13-3 JS1-6-3 JS1-6-6 JS1-6-7 ZY-1 ZY-2 BM-1 BM-2 YK-1 YK-2 YK-3 Average

Well/Outcrop

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P PAAS: Taylor and pffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi McLennan (1985). REE: Total concentration of rare earth elements; LREE/HREE: Concentration of light rare earth element/concentration of heavy rare earth element; La/Yb = La(N)/Yb(N); dEu = Eu/ Sm  Gd N; dCe = Ce/ La  PrN; N: normalized value by PAAS. a

47

48

Q. Liu et al. / Precambrian Research 285 (2016) 39–57 Table 3 Carbon and oxygen isotope compositions in the matrix dolomite (MD) and pore-filling coarse crystalline dolomite (FD) in the Sinian Dengying Formation dolomite reservoirs. d18OPDB (‰)

87

Pore-filling coarse crystalline dolomite (FD) L1-9-2 Lin 1 2619.35 0.4 L1-10-1 Lin 1 2649.83 2.2 L1-11-3 Lin 1 2664.82 2.1 L1-11-2 Lin 1 2663.08 2.2 L1-12-2 Lin 1 2767.66 2.6 L1-13-3 Lin 1 2825.73 1.9 L1-13-4 Lin 1 2827.64 2.8 JS1-6-3 Jinshi 1 4029.85 3.1 JS1-6-6 Jinshi 1 4030.31 1.2 JS1-6-7 Jinshi 1 4032.89 2.6 JS1-6-8 Jinshi 1 4032.18 2.8 ZY-1 Zhengyuan / 7.6 ZY-2 Zhengyuan / 0.3 YK-1 Yankong / 0.3 YK-2 Yankong / 2.2 QT-1 Qiaoting / 4.7 QT-2 Qiaoting / 3.6 QT-3 Qiaoting / 1.1 Average 0.5

10.3 9.2 12.8 13.2 10.4 12 8.1 10.2 11.7 11.4 13.2 11.3 8.6 12.2 13.2 13.5 9.8 12.4 11.3

0.713251 0.712591 0.710989 0.711023 0.712062 / / 0.712050 0.709447 0.709846 0.710023 0.711674 0.710835 0.711387 0.712501 / / / 0.711360

Matrix dolomite (MD) L1-13-6 Lin 1 L1-13-1 Lin 1 L1-12-3 Lin 1 L1-12-1 Lin 1 L1-11-1 Lin 1 L1-11-3 Lin 1 L1-10-1 Lin 1 JS1-6-6 Jinshi 1 JS1-6-4 Jinshi 1 JS1-6-2 Jinshi 1 JS1-5-2 Jinshi 1 YK-5 Yankong YK-6 Yankong BM-5 Baima BM-6 Baima QT-4 Qiaoting QT-5 Qiaoting ZY-7 Zhengyuan ZY-8 Zhengyuan ZY-9 Zhengyuan Average

7.2 6.5 7.2 6.4 6.1 7.1 3.2 5.6 4.9 6.5 6.7 7.8 4.6 4.2 4.1 3.5 6.4 6 6.2 5.3 5.8

0.708751 0.708884 0.709201 0.708338 0.709113 0.708897 0.708926 0.708949 0.708524 0.709145

Sample

Fig. 8. Rare earth element distribution patterns for pore-filling coarse crystalline dolomite (FD) and matrix dolomite (MD) in the Sinian Dengying Formation dolomite reservoirs. A- Matrix dolomite (MD); B-Pore-filling dolomite (FD).

C in three samples is 91.55%, 89.0% and 94.3%, while S is 1.8%, 4.3% and 3.7%. The S/C atomic ratio ranges from 0.02 to 0.05. 5. Discussion 5.1. Geochemical constraints on hydrothermal activities 5.1.1. REE and positive Eu anomaly The formation environment of dolomite may be variable, but massive dolomitization is believed to take place in marine environment (Warren, 2000). Dolomite precipitated from seawater is usually very-fine or fine in grain size. The matrix dolomite in the Dengying Formation represents the original dolomite formed in such environment: very fine to fine crystalline structure with sedimentary textures such as algae laminar and algal bound clot (Fig. 4B; Fig. 5A). Marine carbonates obtain REE from seawater and show signatures of seawater with relatively low REE concentration, LREE depletion and negative Ce anomaly (Hu et al., 2010; Nothdurft et al., 2004; Wang et al., 2009; Webb and Balz, 2000). The matrix dolomite analyzed in this study has low REE concentration (2.14 lg/g on average) and negative Ce anomaly (dCe, 0.59 on average), suggesting its seawater origin. The REE concentrations may be varied significantly in different hydrothermal systems (Klinkhammer et al., 1994; Michard, 1989). On a global basis, however, the REE distribution patterns are very similar, commonly presenting positive Eu anomalies (James and Henry, 1996; Mills and Henry, 1995). The variation of Eu concentration is related to changes in temperature and redox conditions. In reducing environment at higher temperatures, Eu3+ is reduced to Eu2+ (Cai et al., 2008). The Eu2+/Eu3+ ratio in fluids is controlled by temperature while both ions reach chemical equilibrium at 250 °C (Bau and Möller, 1992; Sverjensky, 1984). The ion radius of Eu2+ is bigger than that of Eu3+ (0.117 and 0.095 nm, respectively). Therefore, Eu2+ is not only more difficult to be adsorbed than Eu3+, but also more difficult to enter the structure of rock-forming minerals (Cai et al., 2008). As a result, Eu can be enriched in the form of Eu2+ in hydrothermal fluids at high temperatures. As the tempera-

Well/ outcrop

Depth (m)

2828.12 2824.79 2767.95 2766.38 2654.02 2664.82 2649.83 4030.31 4019.93 4029.51 2824.64 / / / / / / / / /

d13CPDB (‰)

3.5 3.2 3.3 3.5 2.4 2.8 2.4 3.2 3.3 3.3 1.2 1.7 2.5 2.3 2.7 2.4 1.4 0.4 1.3 2.2 2.5

Sr/86Sr

0.709289 0.709081 0.709203 0.708521 0.708857 0.708531 0.708887

Fig. 9. Carbon and oxygen isotope compositions of pore-filling coarse crystalline dolomite (FD) and matrix dolomite (MD) in the Sinian Dengying Formation dolomite reservoirs. The FDs have lighter oxygen isotope composition due to oxygen fractionation between dolomite and hydrothermal. Some of the FDs have lighter carbon isotope composition due to contribution of CO2/CO2 3 related to TSR.

49

Q. Liu et al. / Precambrian Research 285 (2016) 39–57 Table 4 Sulfur isotope compositions of the pyrite in the Sinian Dengying Formation dolomite reservoirs. Sample

Well

Depth (m)

Occurrence

d34SV-CDT (‰)

L1-13-2 L1-13-3 L1-13-5 L1-13 6 JS1-6-2 JS1-6-4 Average

Lin 1 Lin 1 Lin 1 Lin 1 Jinshi 1 Jinshi 1

2825.46 2825.73 2826.79 2828.12 4029.51 4020.31

Fine grain Fine grain Coarse grain Fine grain Fine grain Clumps

23.0 21.8 23.4 23.1 8.9 22.6 20.5

Table 5 EDSa element compositions of the pyrobitumen in the Sinian Dengying Formation dolomite reservoirs. Elements

C O S S/C Total a

Sample 1

Sample 2

Sample 3

Weight (%)

Atom (%)

Weight (%)

Atom (%)

Weight (%)

Atom (%)

35.1 3.4 2.1

91.6 6.6 1.8 0.02 100

9.9 1.00 1.4

89.0 6.7 4.3 0.05 100

19.4 0.9 2.8

94.3 2.0 3.7 0.04 100

40.6

12.3

23.1

EDS: Energy-dispersive X-ray spectroscopy, mounted on Scanning Electron Microscopy.

ture gradually decreases, the enriched Eu2+ is transformed into Eu3+. The positive Eu anomaly in hydrothermal fluid tends to be inherited by minerals that are precipitated from the same fluid. The homogenization temperatures of the fluid inclusions in FD from well Lin 1 are mostly varied from 220 °C to 240 °C, with some of them higher than the maximal burial temperature of the Dengying Formation (<230 °C, Fig. 3). Much higher homogenization temperatures, 270–290 °C, have been reported in previous studies (Wang et al., 2010). Experiments revealed the fluid inclusions in some minerals (e.g. calcite) were subject to alteration due to stretching or leakage and consequently the temperature, pressure and components of the fluid inclusions tended to re-equilibrate with surrounding conditions (Bourdet et al., 2008). However, the fluid inclusions in the dolomite may unlikely attain full reequilibration with the surrounding Dengying Formation in natural burial diagenetic processes, because the homogenization temperatures are not similar to but much higher than the present surrounding formation temperatures (Table 1). This high temperature condition is the reason of the positive Eu anomalies present in FD crystals. For example, the dEu value of the ZY-1 sample is up to 9.8 (Table 2). All of the above results confirmed the hydrothermal origin of FD (Davies and Smith, 2006). 5.1.2. Oxygen, carbon, and strontium isotopes The average d18OPDB and d13CPDB values of MD are 5.8‰ and 2.5‰, respectively. These values are consistent with the dolomite of seawater origin in the same period (Veizer et al., 1997). The 87 Sr/86Sr values of MD are between 0.708502 and 0.709289 (Table 3), in a similar range reported previously (Sawaki et al., 2010), suggesting matrix dolomites were less likely altered during the burial diagenetic stage. The oxygen isotopic composition of dolomite is controlled by temperature and the original oxygen isotopic composition of fluids where dolomite is precipitated. The oxygen isotope fractionation between dolomite and fluid is a function of temperature following the equation: 1000 ln adolomite-water = 2.73  106 T2 + 0.26 (Vasconcelos et al., 2005). If dolomite crystalized from high temperature fluids, it would be depleted in 18O. The d18O values of precipitated dolomite would also be lower if the source fluid is depleted in 18O, such as meteoric water. The coarse crystalline FD in this study is much more depleted in 18 O than MD. The average d18OPDB value of FD is 11.3‰ with the

lowest of 13.5‰ (QT-1) (Table 3). Similar to our results, the d18OPDB values of FD reported by Wang et al. (2010) are in the range of 14.5‰9.1‰. The homogenization temperatures of the fluid inclusions in FD suggest FD was precipitated from a fluid with relatively high temperature (up to 258 °C). This high temperature excludes the possibility that the origin of fluids was meteoric water. Therefore, depletion of the heavy 18O isotope in FD is mainly attributed to temperature-dependent fractionation (Davies and Smith, 2006). The temperature of seawater surrounding the Yangtze craton was in the range of 20–25 °C in the Sinian epoch

Fig. 10. Oxygen isotope composition of the hydrothermal fluids precipitating coarse crystalline dolomite in the Sinian Dengying Formation dolomite reservoirs. The date points (red triangle) representing the oxygen isotope composition of the hydrothermal fluids were based on the oxygen isotope composition of the coarse crystalline dolomite filling in the vugs or fractures in the Sinian Dengying Formation dolomite reservoir and the homogenization temperature. The d18OSMOW values of the hydrothermal fluids are +5‰ to +10‰. The lines of the fluid d18OSMOW is determined according to oxygen isotope fractionation factor between dolomite and fluid: 1000 ln adolomite-fluid = (3.2  106/T2)  3.3) (Land, 1983). The d18OSMOW values of the Sinian seawater are estimated to be 9‰4‰ according to temperature (20–25 °C, Meng et al., 2011) of the Sinian seawater and d18OPDB values (115‰, Jacobsen and Kaufman, 1999) of the Sinian marine carbonate and oxygen isotope fractionation factor between calcite and fluid: 1000 ln acalcite-fluid = (2.78  106/T2)  2.89) (Land, 1983). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

50

Q. Liu et al. / Precambrian Research 285 (2016) 39–57

(Meng et al., 2011). The d18OPDB values of the limestones with the same age are between 11‰ and 5‰ (Jacobsen and Kaufman, 1999). According to the oxygen isotope fractionation factor between limestone and seawater (1000 ln acalcitewater = (2.78  106/T2)  2.89) (Land, 1983), the d18OSMOW value of seawater in the Sinian epoch was estimated to be 9‰4‰ (Fig. 10). Based on the d18O values and homogenization temperatures of FD and oxygen isotope fractionation factors between dolomite and fluid, however, the d18OSMOW value of the source fluid of FD was predicted to be in the range of 5–10‰ (Fig. 10). Furthermore, this source fluid also has relatively high salinities, from 16.4 to 22.9 eq. wt% NaCl (Table 1), suggesting the fluid responsible for the FD formation is of hydrothermal origin, i.e., concentrated hot brine. The high 87Sr/86Sr ratios of FD suggest high concentrations of radiogenic 87Sr, possibly from the underlying sandstones, mudstones, shales or metamorphic basement rocks (Davies and Smith, 2006). During upward migration along faults and fractures, hydrothermal fluids obtained additional radiogenic 87Sr by interacting with those clastic rocks, leading to high concentrations of 87 Sr in FD grains that are subsequently precipitated due to temperature drop. Overall, this set of evidence, including high homogenization temperatures, positive Eu anomalies, light oxygen isotope compositions and high concentrations of radiogenic 87Sr, along with petrographic features, such as curved crystal face and wavy extinction, supports that the coarse crystalline pore-filling dolomite was precipitated from high temperature hydrothermal fluids. 5.2. Geochemical constraints on TSR Natural gases in the Dengying Formation and the Cambrian reservoirs are commonly considered to be formed by the cracking (decomposition) process of oil (Zhu et al., 2015a; Zou et al., 2014). During the middle and late Caledonian orogeny (O2-S) (Fig. 3), the Lower Cambrian source rocks generated large amounts of hydrocarbons that accumulated as oil in the Dengying and Cambrian reservoirs. In Jurassic to early Cretaceous, most oil was thermally cracked into hydrocarbon gases (Li et al., 2011; Zhao et al., 2003). Natural gas samples in this study were obtained from the Cambrian and Dengying Formation reservoirs in the Weiyuan and Anyue gas fields. The chemical compositions and carbon isotope values of natural gases have been reported (Zhu et al., 2015a). The concentrations of H2S and CO2 in the gases are 0–2.75% and 1.08–14.19%, respectively. The alkane components are dominated by methane (CH4), and RC2H6+ accounts for less than 0.5% of the total alkanes. The d13C values of CH4 are varied from 35.1‰ to

34

Fig. 11. Diagram of d SH2S vs concentration of H2S for the natural gases in the Sinian Dengying Formation and Cambrian reservoirs in the Sichuan Basin. The data were published in Zhu et al. (2015a).

31.8‰, C2H6 from 27.5‰ to 34.9‰, and CO2 from 14.6‰ to 0.6‰. The sulfur isotope (d34SCDT) values of H2S vary from 11.5‰ to 16.9‰. The data are re-plotted in this study for following discussions (Figs. 11–13). 5.2.1. Sulfur isotopes of H2S and pyrite The possible sources of H2S or S2 in pyrite in marine carbonates include deep magma-volcano activities, bacterial sulfate reduction (BSR), and thermal sulfate reduction (TSR). The average d34SCDT value of ultramafic rock and basalt is 1.2‰ and 2.7‰, respectively, while in meteorite, d34SCDT varies from 5.6‰ to 2.6‰ (Meng et al., 2006). The d34SCDT value of bulk sulfur in volcanic gases is +2.2‰ on average (Wei and Wang, 1988); and d34SCDT of H2S in volcano gases is 1.2‰ to 5.5‰ (Nakai and Jensen, 1967). In the process of BSR, bacteria tend to reduce light sulfur isotope (32S) in sulfate (Pierre et al., 2000), causing a strong sulfur isotope fractionation between SO2 and S2 (Machel et al., 1995). As a 4 result, sulfides are depleted in 34S and have negative d34S values. In modern deep sea and euxinic environments, the d34S value of H2S and sulfides produced via BSR ranges from 40‰ to 19‰; in shallow water, d34S is approximately 5‰ (Zheng and Chen, 2000). The d34S of pyrite formed via BSR in the hydrate zone of Blake Ridge off the east coast of the U.S. is between 26.2‰ and 42.7‰ (Pierre et al., 2000). The sulfur isotope compositions of H2S and pyrite in the Dengying Formation are relatively heavy, with d34SCDT values varying from 14.9‰ to 16.9‰ and 8.9‰ to 23.4‰, respectively (Table 4; Fig. 11). Those values are significantly different than expected from BSR or volcanic-magmatic systems. The TSR commonly occurs at relatively high temperatures. By consuming organic matters (gas2 eous or liquid hydrocarbons), SO2 (Cai et al., 4 is reduced into S 2001; Cai and Li, 2005; Worden et al., 1995) following the reaction:

P Fig. 12. Diagram of d13CC2H6 vs d13CCH4 (A) and d13CCH4 vs H2S/(H2S + CnH2n+2) (gas souring index) (B) for the natural gases in the Sinian Dengying Formation and Cambrian reservoirs in the Sichuan Basin. The data were published in Zhu et al. (2015a).

Q. Liu et al. / Precambrian Research 285 (2016) 39–57

51

are similar (Fig. 12A). However, the d13C values of ethane from the Dengying Formation are higher than the Lower Cambrian, which in turn are higher than the Upper Cambrian. This increase of d13CC2H6 values with burial depths mainly is the reflection of the impact of thermal evolution. In order to evaluate the level/extent of TSR, the gas souring P index, H2S/(H2S + CnH2n+2), which suggests the relative H2S concentration in natural gases, has been proposed and used effectively as a geochemical proxy (Worden et al., 1995). The gas souring index and TSR level always have a positive relationship: the H2S/ P (H2S + CnH2n+2) value increases with increasing level of TSR (Liu et al., 2013; Worden and Smalley, 1996). In the Dengying Formation, the gas souring indexes reach as high as about 0.03 and are much higher than the gases from the Cambrian, indicating CH4 in the Dengying Formation was significantly impacted by TSR (Fig. 12B).

P Fig. 13. Diagram of concentration of CO2 vs H2S/(H2S + CnH2n+2) (gas souring P 13 index) (A) and d CCO2 vs H2S/(H2S + CnH2n+2) (B) for the natural gases in the Sinian Dengying Formation and Cambrian reservoirs in the Sichuan Basin. The data were published in Zhu et al. (2015a).

CaSO4 þ Cn Hm ðhydrocarbonÞ ! CaCO3 þ H2 S þ CO2 þ H2 O

ð1Þ

During TSR, sulfur isotope fractionation takes place between the 2 SO2 product, leading to 34S depletion in 4 reactant and reduced S S2 to different extents (Orr, 1977). However, the fractionation is likely to be relatively small, resulting in similar sulfur isotopic compositions of S2 and SO2 4 (Cross and Bottrell, 2000). Globally, sulfur isotope compositions in anhydrite deposited in seawater during the Sinian period are similar. For example, d34S of anhydrite in the junction of Neoproterozoic and Cambrian period is about 20–30‰ (Claypool et al., 1980). It has been reported that d34S of anhydrite in carbonate rocks of Dengying Formation in Yangtze area is approximately 20–37.8‰ (Zhang et al., 2004), while the d34SSO4 of seawater during the same period is around 20‰ (Goldberg et al., 2005). The d34S values of anhydrite in the Dengying Formation dolomite in the Anyue and Weiyuan gas fields are between 20.8‰ and 22.5‰ (Zhu et al., 2015a, 2007). The sulfur isotope results from this study are in line with previous reported values. The sulfur isotope compositions of H2S are lighter than anhydrite, while pyrite compositions are in a narrow range within the reported values of anhydrite. Weak fractionations between sulfate and S2 during TSR may be the reason for lower values of H2S (Zhu et al., 2015a). 5.2.2. Carbon isotopes of alkanes Previous studies have shown that carbon isotope compositions of alkanes change differently during TSR and oil cracking processes. The d13C values of methane remain constant, while the d13C values of ethane increase continuously with increasing thermal maturity (Liu et al., 2013). The d13C values of methane from the Dengying Formation, the Lower Cambrian and the Upper Cambrian reservoirs

5.2.3. Abundance and carbon isotopes of CO2 The concentration of CO2 increases with the gas souring index (Fig. 13A). Most gas samples from the Dengying Formation have higher CO2 concentrations than gases from the Cambrian due to higher level of TSR. However, the increased amount of CO2 becomes small at higher levels. It reaches a maximum value, and decreases when the gas souring index is higher than 0.01. The reason for such trend is that CO2 gradually gets oversaturated in the reservoir with increasing level of TSR. It then combines with metal ions to precipitate as carbonate minerals, leading to decreased concentrations of gaseous CO2 (Liu et al., 2013, 2014a). In theory, the carbon isotope composition of CO2 that is related to TSR is relatively light because the carbon atoms in CO2 comes from hydrocarbons (Mougin et al., 2007). Laboratory experiments have also demonstrated the d13C value of CO2 is less than 30.0‰ (Pan et al., 2006). Worden et al. (Worden et al., 1995) reported that the d13CCO2 value decreased from 9‰ to 15‰ with increasing level of TSR. Similar to theoretical and experimental results, the carbon isotope composition of CO2 in this study follows a similar trend. It decreases (to as low as 14.6‰) with increasing gas souring index (Fig. 13B), implying contribution of CO2 from TSR. However, some CO2 samples with d13C values of 0‰ (Fig. 13B) are much more enriched in 13C than alkane gases (<27.5‰). The d13CCO2 value does not decrease all along with increasing H2S/ P P (H2S + CnH2n+2) ratio but increases when the H2S/(H2S + CnH2n+2) ratio reaches around 0.03 (Fig. 13B), suggesting other processes may contribute to CO2 in reservoirs. Hydrothermal activities may lead to input of CO2 from underlying rocks, that are ultimately derived from magmatic activities or decomposition/reaction of carbonate rocks. The carbon isotope value of CO2 of this origin is commonly high (Zhang et al., 2008). Dissolution of carbonate host rocks can release CO2 with heavy carbon isotope composition to the reservoirs due to increasing acidity of formation fluids with increasing level of TSR (Hao et al., 2008; Liu et al., 2013). Therefore, hydrothermal fluids may be another source of CO2. Overall, the source of gaseous CO2 in the Dengying Formation dolomite reservoirs may be the combination of TSR and hydrothermal activities (Giuliani et al., 2000; Krouse et al., 1988). 5.3. Coupled hydrothermal activities and TSR 5.3.1. Hydrothermal dolomite and TSR pyrite Previous experimental results have shown that TSR occurs at temperatures above 175 °C, and no reaction was observed below this threshold under laboratory conditions (Goldhaber and Orr, 1995). In fact, most TSR experiments were conducted at temperatures higher than 220 °C (Cross et al., 2004; Goldhaber and Orr, 1995; Kiyosu et al., 1990; Pan et al., 2006). Due to other controlling

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factors, including availability of Cu, Fe and other metal catalysts (Goldhaber and Orr, 1995; Kiyosu et al., 1990), and water (Seewald, 2003; Worden et al., 1996), TSR may take place at lower temperatures in geological processes (Cai et al., 2001; Seewald, 2003; Worden et al., 1995). According to well core and microscope observations, pyrite is found to be formed in coarse crystalline dolomite crystals (Fig. 4; Fig. 5D–I). The dolomite and pyrite are of hydrothermal and TSR origins, respectively. This coexistence relationship suggests they are genetically related, and the hydrothermal and TSR processes share common features as well. The homogenization temperatures of the fluid inclusions in the hydrothermal dolomite are mainly between 180 °C and 200 °C or 220 °C and 230 °C, and some of them are much higher than 230 °C. The hydrothermal fluids not only provide high temperature environment that was necessary for TSR, but also accelerate heat convection of formation water facilitating anhydrite dissolution and reaction product migration. 5.3.2. Carbon isotopes of hydrothermal dolomite Carbon isotope compositions of hydrothermal dolomite mainly reflect the potential carbon sources, including the contribution of inorganic carbon derived from carbonate matrix and organic carbon derived from microbial activities and thermal breakdown of organic material (Davies and Smith, 2006). If influenced only by the Dengying Formation carbonate matrix, the carbon isotope compositions of dolomite should be similar to the matrix. However, the d13CPDB values of the hydrothermal dolomite in the Dengying Formation dolomite are between 7.6‰ and 3.1‰ (Table 3), most of which are lighter than the matrix dolomite, reflecting the influence of organic carbon source (Fig. 9). Owing to the presence of TSR, the Dengying Formation reservoirs contain CO2 with relatively light carbon isotope composition (d13CPDB, 14.6‰0.6‰). The CO2 became saturated in the formation fluids as more CO2 was created with increasing level of TSR. Consequently, CO2 combined with metal ions, such as Mg2+ and Ca2+, to form carbonate minerals, which are light in carbon isotope compositions (Hao et al., 2015; Liu et al., 2014b). The Mg2+ and Ca2+ may come from hydrothermal fluids or dissolution of dolomite matrix. With contribution of organic carbon, the precipitated hydrothermal dolomites demonstrate light carbon isotope compositions (Fig. 9). 5.3.3. Sulfur contents in pyrobitumen Under abnormally high heat stress conditions, such as magmatic intrusion (Zhu et al., 2008) and hydrothermal activities (Glikson et al., 2000; Wilson, 2000), petroleum can be transformed into highly evolved pyrobitumen. In this study, pyrobitumen in the dissolution pores appears scaly or globose (Fig. 5J–L) with its reflectance as high as 5.33% (corresponding to the vitrinite reflectance of 3.91%, Zhu et al., 2013). According to burial history analysis (Fig. 3), the maximal burial temperature of the Dengying Formation is about 230 °C. The reflectance of vitrinite (Ro) experiencing such a burial temperature is supposed to be around 3.0% (Hao et al., 2015). The much higher reflectance values, up to 5.33%, imply extremely high heat flows imposed on pyrobitmen. This kind of high heat flows may be released from the hydrothermal fluid that has a temperature higher than maximal burial temperature (230 °C). The pyrobitumen with high reflectance commonly contains high levels of sulfur with the S/C ratio being 0.02–0.05 (Table 5). The high sulfur content indicates the effect of TSR on pyrobitumen. As observed in previous studies, solid bitumen associated with TSR is chemically distinct from reservoir bitumen arising from asphaltene precipitation. It is insoluble, and has high reflectivity (>1.5% Ro) (Stasiuk, 1997), and high concentrations of sulfur with S/C atomic ratios of 0.04–0.1 (Kelemen et al., 2010; Walters et al.,

2015). The generation of sulfur-rich bitumen have been confirmed by TSR experiments in laboratory (Zhang et al., 2008). The occurrence of pyrobitumen with hydrothermal dolomite and pyrite suggested that the evolution of pyrobitumen, hydrothermal activities and TSR are closely related. Not only did the hydrothermal fluids accelerate TSR processes in the deep Dengying Formation, but also converted previously formed petroleum into high reflectance pyrobitumen gradually (Fig. 3). 5.3.4. Hypothesized Scenario of coupled hydrothermal and TSR processes The coexistence of hydrothermal dolomite, pyrite, and pyrobitumen with gaseous compositions (H2S, CO2 and CH4) and their geochemical and isotopic features in dolomite reservoirs in the Dengying Formation indicate both hydrothermal fluids and TSR have influenced the dolomite reservoirs. A scenario describing the coupled hydrothermal and TSR processes is proposed as follows (Fig. 14). At the end of Sinian, the Dengying Formation dolomites were tectonically uplifted due to the Tongwan movement (Wang et al., 2014) and then subjected to dissolution by meteoric water to create numerous pores and vugs (Fig. 3; Fig. 14A). After surface karstification, the Dengying Formation dolomites underwent burial (Fig. 3). Since the middle and late Caledonian orogeny (O2-S), petroleum generated from the Lower Cambrian source rocks migrated into the Dengying Formation reservoirs (Fig. 3; Fig. 14B) (Li et al., 2011; Zhao et al., 2003). A regional high heat flow during and after the Emei magmatic and volcanic activities starting from the Permian (Ali et al., 2005) period facilitated migration of fluids from deep to shallow strata, namely hydrothermal activities (Fig. 3). By providing adequate heat, the hydrothermal activities initiated TSR and enhanced TSR levels in the reservoirs (Fig. 14C). In Permian, the TSR should not have taken place due to low burial temperatures; nevertheless, the hydrothermal activities triggered and accelerated the process of TSR. Large amounts of CO2 and H2S were released from hydrothermal activities and TSR. The formation fluids became acidic due to the presence of CO2 and H2S, and consequently changed the Dengying Formation dolomites. The coupled hydrothermal and TSR processes (Fig. 14C) altered the development of the dolomite reservoirs by precipitating hydrothermal dolomite, pyrite and pyrobitumen as well as dissolving the dolomite reservoirs (Fig. 4C–F; Fig. 5G–J). In the Middle Cretaceous, the Dengying Formation was substantially uplifted which signaled the termination of TSR. However, the gaseous components (CO2 and H2S) from coupled hydrothermal and TSR processes continuously created dissolution pores in dolomites (Fig. 14D). The porosity created by co-alteration of hydrothermal and TSR significantly contributed the natural gases accumulation in the current Dengying Formation reservoirs (Fig. 14E). 5.4. Influence of coupled hydrothermal and TSR processes on dolomite reservoirs 5.4.1. Dissolution associated with coupled hydrothermal and TSR During deep burial diagenesis processes, dissolution and alternation of carbonate rocks by hydrothermal fluids play an important role in the development of hydrocarbon reservoirs. Hydrothermal fluids can not only dissolve carbonate rocks but also make limestone dolomitized, which enhances quality of carbonate reservoirs (Davies and Smith, 2006; Jin et al., 2006b; Lavoie et al., 2010). Petrography, fluid inclusion and other geochemical results have shown that the dolomites in the Dengying Formation experienced hydrothermal alteration, and the dolomite reservoir quality was enhanced.

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Fig. 14. Development of deep dolomite reservoir under influence of coupled alteration of hydrothermal fluids and TSR.

The hydrothermal activities facilitated TSR in the Dengying Formation (Fig. 14C). Both processes introduced large amount of CO2 and H2S, and the CO2 and H2S concentrations are up to 14.19% and 2.75%, respectively. The presence of CO2 and H2S generated acidity in pore water. It has been proposed that acidic pore water may cause carbonate dissolution, producing secondary porosity (Gluyas and Coleman, 1992). Several case studies confirmed that charge of H2S- and CO2-rich fluids into carbonate reservoirs resulted in carbonate dissolution and secondary porosity generation (Beavington-Penney et al., 2008; Heward et al., 2000). Petroleum migration into carbonate reservoirs and alteration of petroleum into bitumen lead to the formation of bitumen coating over carbonate mineral surfaces in pore spaces. The hydrothermal and TSR processes in the Dengying Formation were believed to occur after petroleum accumulation in reservoirs. The CO2 and H2S from hydrothermal and TSR dissolved carbonates, leading to disappearance of bitumen coating and separation of pore bitumen from surrounding carbonate minerals (Fig. 5M, N and O; Fig. 14C). In addition, hydrothermal dolomite minerals were found partially dissolved, presenting tiny harbor-shaped pores (Fig. 5H and O). The lack of bitumen coating surrounding those dolomite minerals is another evidence supporting the dissolution was later than the formation of bitumen and may be caused by CO2 and H2S from hydrothermal and TSR processes. Indeed, lack of solid bitumen coating, occurrence of solid bitumen in the center of pores, and unevenly dissolved surfaces of hydrothermal dolomites were proposed as evidence for latestage, TSR-induced dissolution (Cai et al., 2014; Chen et al., 2007; Wang et al., 2007). In addition to acidic pore waters, high temperature and fluid convection can also promote secondary dissolution. Anhydrite dissolution, yielding SO2 4 necessary for TRS process, could create porosity in carbonates. Machel (2001) proposed that if TSR reaction is balanced in such a way that the ratios of CaSO4: CaCO3:H2S = 1:1:1 and no sulfides are formed, the net porosity development as a result of TSR is positive. For every mole of CaSO4 that is converted to CaCO3 and H2S, there is a loss in rock volume of about 10 cm3. Expressed in volumes, 1 m3 of CaSO4 results in 0.78 m3 of CaCO3 and 0.22 m3 (22%) of additional porosity (Hao et al., 2015). However, the contribution of CaSO4 dissolution to porosity increase in the Sinian Dengying Formation dolomite reservoir may be insignificant because the SO2 in formation water 4 might not be always available but was derived from anhydrite dissolution elsewhere and migrate into the reservoir (King et al., 2014). The anhydrite is rare to be found and the abundance of anhydrite appears quite low in the Dengying Formation.

5.4.2. Dissolution during uplift The formation fluids were gradually saturated with respective to CO2 as more CO2 was generated from hydrothermal and TSR processes. The reaction of CO2 with Ca2+ and Mg2+ led to precipitation of dolomite minerals in the reservoirs (Fig. 14C). As a result, the dissolution became gradually weak and then ceased. The light carbon isotope composition of FD (Table 3) in the Dengying Formation implies the precipitation of FD was affected by TSR-related CO2 (Fig. 9). In the uplift stage since the Late Cretaceous (Fig. 3), concentration of CO2 in the Dengying Formation reservoirs stopped increasing due to cessation of TSR. The formation fluids became undersaturated with respective to CO2 because of decreasing formation temperature and pressure (Huang et al., 2010). Consequently, more CO2 could be dissolved into the formation fluids and the dissolution of dolomites by formation fluids was resumed (Fig. 14D) (Liu et al., 2014c). 5.4.3. Re-equilibrium between CO2 and dolomite The role of CO2 derived from organic thermal alteration on the porosity of carbonate reservoirs has long been debated. Increased concentration of CO2 and H2S from hydrothermal and TSR processes can inevitably dissolve carbonate rocks. The observation of features, such as non-structure selective dissolution of dolomite matrix and hydrothermal dolomite (Fig. 5H and O), lack of bitumen coating surrounding dissolved carbonate minerals (Fig. 5M–O), and bitumen present in pores (Fig. 5N and O), suggest the dissolution are related to hydrothermal and TSR processes (Cai et al., 2014; Chen et al., 2007; Wang et al., 2007). However, the dolomite minerals filling in the pores in reservoirs are relatively light in carbon isotope compositions (Table 3, Fig. 9), implying cementation of CO2 that is related to TSR. Ehrenberg et al. (Ehrenberg et al., 2012) proposed the contribution of pore fluids to creation of porosity in carbonate reservoirs was limited. Calculations have shown that an increase of 1% porosity in a 100 m thick limestone requires 27,000 m3 of water per square meter of the limestone surface (Ehrenberg et al., 2012). Similarly, carbonate cementation from pore fluids may also be insignificant. A re-equilibration model between dissolution and precipitation can be used to explain the contradiction. With increasing concentration of CO2, carbonate rocks are gradually dissolved into pore fluids; nevertheless, carbonate minerals will be precipitated when the pore fluids are saturated with more CO2 released from TSR process. The 12C-rich CO2 is preferentially involved in re-equilibration reactions (Hao et al., 2008), forming coarse crystalline dolomite

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with relatively low d13C values (Fig. 9). The remaining CO2 reequilibrated with the 13C-enriched system leading to its higher d13C values, reaching as high as 0.6‰ (Fig. 13). In a TSR-dominated close system, the balance between dissolution and precipitation or recrystallization of carbonate minerals (Dong et al., 2013; Zhu et al., 2010) has a significant impact on diagenetic processes. It leads to the formation of coarse crystalline dolomite minerals in pore spaces of the Dengying Formation reservoirs. The influence of the equilibration on porosity is insignificant due to limited amount of pore fluids. However, the dissolution of carbonates should be enhanced when the system is open. For example, input of hydrothermal fluids along faults and fractures into the dolomite reservoirs and then output of the fluids with dissolved carbonate components constituted an open system (Fig. 14C). The amount of hydrothermal fluids was extremely larger than the pore fluids. It could carry dissolved carbonate efficiently (Fig. 14C). Consequently, the coupled hydrothermal and TSR processes may effectively enhance carbonate porosity. 5.4.4. Effect of Pyrobitumen formation on porosity The pore spaces in the Dengying Formation are commonly filled with pyrobitumen that was the result of alteration of petroleum related to hydrothermal activities and TSR. The pyrobitumen usually occupies partial areas of pore spaces. However, the presence of

petroleum can inhibit cementation of diagenetic minerals and consequently is favorable for preservation of porosity in the long burial stage (Gluyas et al., 1993; Marchand et al., 2001). According to observation of well cores (Fig. 15), the dolomites above paleo oilwater contact in the Dengying Formation contain pyrobitumen, with high porosities. However, the dolomites below the paleo oil-water contact contain scarce pyrobitumen, and most of the pores were cemented by calcite, dolomite or quartz. There are numerous tiny pores, like desiccation cracks, in pyrobitumen (Fig. 5K and L). The pores generally occur as subrounded or striped shape and are several micrometers in size (Fig. 5K and L). They are the products of thermal cracking and TSR. During thermal cracking and TSR, the petroleum was partially altered as well as thermally transformed into pyrobitumen, and pores were created in pyrobitumen due to decrease in amount and shrinkage in volume. 5.4.5. Influence of coupled alteration on reservoir development As a result of coupled alteration of hydrothermal fluids and TSR, porosity of the Dengying Formaton dolomite reservoir was significantly enhanced due to dissolution of dolomite and anhydrite as well as development of micropores in pyrobitumen. Furthermore, the acidic formation water due to dissolution of CO2 and H2S derived from hydrothermal activities and TSR inhibited cementa-

Fig. 15. Porosity development in the Sinian Dengying Formation dolomite reservoirs.

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tion of carbonate minerals and hence ensured pre-existing pores to be well preserved during deep burial diagenesis (Zhu et al., 2015b) (Fig. 14). With enhancement and preservation of porosity, the Dengying Formation dolomites currently have large amount of pore spaces and are high-quality hydrocarbon reservoirs. The dolomite reservoirs above paleo oil-water contact generally have much higher porosity than the reservoirs below the paleo oil-water contact (Fig. 15) because the dolomites above the contact contain large amount petroleum that is the necessary substance for TSR to proceed. Above the paleo oil-water contact, the dolomites were influenced by the co-alteration of hydrothermal activities and TSR, and consequently the reservoir quality is relatively high. The porosity is 4.02% on average and up to 10.63%, and the permeability is 1.79  103 lm2 on average and up to 18.68  103 lm2. In contrast, the dolomites below the paleo oil-water contact were less likely influenced by the co-alteration and consequently the reservoir quality is relatively poor. The porosity is less than 3.0% and 1.95% on average, and the permeability is 0.08  103 lm2 on average. 5.4.6. Potential contribution of hydrothermal CH4 and H2 Large amount of abiogenic CH4 and H2 are commonly found in various hydrothermal systems, e.g. along the Mid-Atlantic ridge (Charlou et al., 2010; Schmidt et al., 2011), within the Del Puerto Ophiolite in California (Blank et al., 2009) and Canadian Precambrian shield (Sherwood Lollar et al., 2008; McCollom, 2013). The H2 production rate from the Precambrian continental lithosphere is estimated to be up to 2.27  1011 moles per year (Sherwood Lollar et al., 2014). The CH4 may be released directly from the mantle or can be generated via well-known Fischer-Tropsch synthesis reactions. The major process for H2 generation is believed to be serpentinization of olivine under hydrothermal conditions at the Moho’s surface as well as in the deep crust (McCollom and Bach, 2009; Seyfried et al., 2007; Sleep et al., 2004). The presence of H2 can increase the rate of hydrocarbon generation (up to 147%) from kerogen through hydrogenation reaction (Jin et al., 2004). Especially for the high-evolved hydrocarbon source rocks in the Precambrian sequences, external H2 may reactivate kerogen in the source rocks to further generate alkane gases. The contribution of CH4 and presence of H2 released from hydrothermal systems will consequently increase gas potential in the old deep Dengying Formation dolomite reservoirs that have high porosity due to coupled alteration of hydrothermal and TSR. 6. Conclusions 1. Results of fluid inclusions, REE, oxygen and strontium isotopes have shown the coarse crystalline pore-filling dolomite in the deep Sinian Dengying Formation in Sichuan Basin was formed as a result of hydrothermal activities. The sulfur isotope compositions of the granular pyrite and H2S and the carbon isotope compositions of the CH4 and C2H6 suggest the occurrence of TSR. 2. The dolomite of hydrothermal origin is enriched in 12C and the carbon isotope composition of the CO2 varies in a wide range, suggesting coupled alteration of hydrothermal and TSR processes. The S-rich pyrobitumen with high level of thermal evolution also indicates this coupled process. The coexistence relationships among pore-filling dolomite, pyrite, pyrobitumen, CO2 and H2S in the Dengying Formation indicate that TSR was closely related to hydrothermal activities: the hydrothermal activities facilitated and accelerated TSR.

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3. The H2S and CO2 generated from coupled hydrothermal fluids and TSR not only created secondary dissolution pores but also helped to maintain the pre-existing pore spaces in the dolomite reservoir during deep burial processes. 4. CH4 and H2 released from hydrothermal systems may be accumulated in the high-quality Dengying Formation dolomite reservoirs. Consequently study of hydrothermal events should be focused on for further understanding of hydrocarbon exploration potential in the ancient deep Precambrian dolomite reservoirs.

Acknowledgements We would like to express our gratitude to SINOPEC Petroleum Exploration Company for technical support and gas sampling. We would also like to acknowledge the State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development and Laboratory of Structural and Sedimentological Reservoir Geology of PEPRIS, SINOPEC, for measurement of samples. This work was financially supported by the National Natural Science Foundation of China (Grant No. 41230312, 41372149 & 41322016), as well as the Key Project of China National Program for Fundamental Research and Development (Grant No. 2012CB214800). We appreciate improvement of the English and constructive comments from Dr. Qi Fu and Professor Guochun Zhao. The constructive comments and suggestions by two anonymous reviewers are sincerely appreciated. References Aldrich, L.T., Doak, J.B., Davis, G.L., 1953. The use of ion exchange columns in mineral analysis for age determination. Am. J. Sci. 251, 377–387. Ali, J.R., Thompson, G.M., Zhou, M.F., Song, X., 2005. Emeishan large igneous province, SW China. Lithos 79, 475–489. Bau, M., Möller, P., 1992. Rare earth element fractionation in metamorphogenic hydrothermal calcite, magnesite and siderite. Mineral. Petrol. 45, 231–456. Beavington-Penney, S.J., Nadin, P., Wright, V.P., Clarke, E., McQuilken, J., Bailey, H. W., 2008. Reservoir quality variation in an Eocene carbonate ramp, El Garia Formation, offshore Tunisia: structural control of burial corrosion and dolomitisation. Sed. Geol. 209, 42–57. Blank, J.G., Green, S.J., Blake, D., Valley, J.W., Kita, N.T., Treiman, A., Dobson, P.F., 2009. An alkaline spring system within the Del Puerto Ophiolite (California, USA): a Mars analog site. Planet. Space Sci. 57, 533–540. Bourdet, J., Pironon, J., Levresse, G., Tritlla, J., 2008. Petroleum type determination through homogenization temperature and vapour volume fraction measurements in fluid inclusions. Geofluids 8, 46–59. Cai, C., Hu, W., Worden, R.H., 2001. Thermochemical sulphate reduction in CambroOrdovician carbonates in Central Tarim. Mar. Pet. Geol. 18, 729–741. Cai, C., Zhang, C., He, H., Tang, Y., 2013. Carbon isotope fractionation during methane-dominated TSR in East Sichuan Basin gasfields, China: a review. Mar. Pet. Geol. 48, 100–110. Cai, C.F., He, W.X., Jiang, L., Li, K.K., Xiang, L., Jia, L.Q., 2014. Petrological and geochemical constraints on porosity difference between Lower Triassic sourand sweet-gas carbonate reservoirs in the Sichuan Basin. Mar. Pet. Geol. 56, 34– 50. Cai, C.F., Li, H.T., 2005. Thermochemical sulfate reduction in sedimentary basin: a review. Adv. Earth Sci. 20, 14–19. Cai, C.F., Li, K.K., Li, H.T., Zhang, B.S., 2008. Evidence for cross formational hot brine flow from integrated 87Sr/86Sr, REE and fluid inclusions of the Ordovician veins in Central Tarim, China. Appl. Geochem. 23, 2226–2235. Charlou, J.L., Donval, J.P., Konn, C., Ondréas, H., Fouquet, Y., Jean-Baptiste, P., Fourré, E., 2010. High production and fluxes of H2 and CH4 and evidence of abiotic hydrocarbon synthesis by serpentinization in ultramafic-hosted hydrothermal systems on the Mid-Atlantic Ridge. In: Rona, P.A., Devey, C.W., Dyment, J., Murton, B.J. (Eds.), Diversity of Hydrothermal Systems on Slow Spreading Ocean Ridges. Wiley Online Library, Washington, pp. 265–296. Chen, D.Z., Wang, J.G., Qing, H.R., 2007. Mechanism and accumulation effect of thermochemical sulfate reduction in northeast area of Sichuan Basin. Natl. Gas Geosci. 18, 743–749. Chen, D.Z., Wang, J.G., Qing, H.R., 2009. Hydrothermal venting activities in the early Cambrian, south China, petrological, Geochronological and stable isotopic constraints. Chem. Geol. 258, 168–181.

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