Origin and distribution of hydrogen sulfide in the Yuanba gas field,Sichuan Basin,Southwest China |
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| Institution: | 1. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, Changping, Beijing 102249, China;2. Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan, Hubei 430074, China;3. SINOPEC Exploration Company, Chengdu, Sichuan 610041, China;4. Department of Earth Sciences, Zhejiang University, Hangzhou, Zhejiang 310027, China;1. Water, Energy and Environment Laboratory, Sfax Faculty of Sciences, POB 1171, Soukra Road km 3.5, Sfax 3000, Tunisia;2. Sedimentary Petrology, B20, Liège University, Sart-Tilman, Liège 4000, Belgium;3. Laboratory of Georessources, Materials, Environment and Global Changes, Sfax Faculty of Sciences, POB 1171, Soukra Road km 3.5, Sfax 3000, Tunisia;4. Water, Energy and Environment Laboratory, Sfax National Engineering School, POB 1178, Soukra Road km 4, Sfax 3038, Tunisia;1. Key Laboratory of Karst Dynamics, MLR/GZAR, Institute of Karst Geology, CAGS, Guilin, 541004, China;2. State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing, 210023, China;3. Research Institute of Experiment and Testing, PetroChina Xinjiang Oilfield Company, Karamay, 834000, China;4. Research Institute of Petroleum Exploration and Development, PetroChina Xinjiang Oilfield Company, Karamay, 834000, China;1. Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China;2. Research Institute of Petroleum Exploration and Development, Tarim Oilfield Company, PetroChina, Korla 841000, China;1. State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510640, PR China;2. University of Chinese Academy of Sciences, Yuquan Road, Beijing 100049, PR China;1. PetroChina Researth Institute of Petroleum Exploration & Development, Beijing 100083, China;2. Peking University, Beijing 100871, China |
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| Abstract: | The Yuanba gas field in the Permian Changxing Formation (P2c), which exhibits wide variations in its hydrogen sulfide (H2S) concentration (1.20–12.16%), is a typical sour gas field in the northern Sichuan Basin. The sulfur-rich reservoir's solid bitumen (atomic S/C ratios are 0.032–0.142), and late calcite cement δ13C values, which are smaller than the δ13C values of the host dolostone, indicate that the H2S originated from thermal sulfate reduction (TSR) and oil was involved in TSR. The gas souring index (GSI) of P2c's gases is generally lower than 0.1. The ethane δ13C values increase as the GSI increases, although no obvious increase was observed in the methane δ13C values. The calcite cements' δ13C values (−15.36 to +4.56‰) in dolostone are heavier than the typical reported values, which implies that only limited heavy hydrocarbon gases were involved in TSR. No anhydrites developed in P2c's reservoirs, and dissolved sulfate anions (SO42−) were mainly enriched during dolomitization. Insufficient dissolved SO42− most likely caused the lower H2S concentrations in the Permian to Triassic reservoirs in the northeastern Sichuan Basin compared to the Permian Khuff Formation in Saudi Arabia and the Jurassic Smackover Formation in Mississippi. Except for the SO42− in residual water in paleo-oil zones, SO42− from bottom water may also be involved in TSR; therefore, oil reservoirs with bottom water have more SO42− and can produce more H2S than pure oil reservoirs. This phenomenon may be the main cause of the great difference in the H2S concentrations between reservoirs, while gravitational differentiation during late uplift most likely creates differences in H2S concentrations in a single reservoir. Carbon dioxide (CO2), which has a relatively heavy δ13C value (−3.9 to −0.3‰), may be the combined result of TSR, the balance between CO2 and inorganic fluid systems, and carbonate decomposition. |
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| Keywords: | Hydrogen sulfide Thermal sulfate reduction Carbon dioxide Stable carbon isotopes Yuanba gas field Sichuan Basin |
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