AAPG Bulletin; May 2008; v. 92; no. 5;
p. 611-637; DOI: 10.1306/01210807090
© 2008 American Association of Petroleum Geologists (AAPG)
Evidence for multiple stages of oil cracking and thermochemical sulfate reduction in the Puguang gas field, Sichuan Basin, China
Fang Hao1,
Tonglou Guo2,
Yangming Zhu3,
Xunyu Cai4,
Huayao Zou5 and
Pingping Li6
1 State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Changping, Beijing 102249, China; present address: State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China; haofang{at}cup.edu.cn
2 Southern Exploration and Development Company, Sinopec, Kunming, Yunnan 650023, China
3 Department of Earth Sciences, Zhejiang University, Hangzhou, Zhejiang 310027, China
4 Southern Exploration and Development Company, Sinopec, Kunming, Yunnan 650023, China
5 State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Changping, Beijing 102249, China
6 State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China
Fang Hao received his Ph.D. from China University of Geosciences in 1995. He is now a professor of geology at China University of Petroleum and Director of the State Key Laboratory of Petroleum Resource and Prospecting. He has conducted petroleum geology and geochemistry studies in several Chinese basins. His interests include petroleum generation, migration, and accumulation in overpressured sedimentary basins.
Tonglou Guo received his Ph.D. from Tongji University in 1996. He is now the chief scientist of the Southern Exploration and Development Company (SEDC) of Sinopec, and a professor in geology at China University of Geosciences. He has conducted petroleum geology studies in several Chinese basins. His interest is now in the study of petroleum accumulation and preservation in marine carbonates in South China.
Yangming Zhu received his Ph.D. from Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (CAS) in 1993. He is a professor of geochemistry at Zhejiang University. He has conducted petroleum geochemistry studies in several Chinese basins. His interest is in the study of sedimentation and evolution of overmature source rocks in marine carbonate sequences in China.
Xunyu Cai received his Ph.D. from Chengdu University of Technology in 2003. He is now a senior geologist at Exploration Company of Sinopec. He held various technical and managerial positions in the Southern Exploration and Development Company (SEDC). His interest is in the study petroleum accumulation, preservation, and occurrence in marine sequences in China.
Huayao Zou received his Ph.D. from China University of Geosciences, and is now an associate professor at China University of Petroleum. He spent two years as a postdoctorate research fellow studying petroleum generation and accumulation in the Bohai Bay basin before he joined the university. His recent interest is in the study of petroleum accumulation mechanisms in lacustrine fault basins.
Pingping Li received his M.S. degree from China University of Geosciences in 2006. He is now a doctoral student at the China University of Petroleum in Beijing. His interest is in the study of petroleum migration, accumulation, and alteration in the Sichuan Basin.
The Puguang gas field is the largest gas field found in marine carbonates in China. Marine carbonate reservoirs in this field were buried to a depth of about 7000 m (22,966 ft) and experienced maximum temperature up to 220°C before uplift to the present-day depth of 5000–5500 m (16,404–18,045 ft), with present-day thermal maturity between 2.0 and 3.0% equivalent vitrinite reflectance (Ro). Sulfur-rich pyrobitumens with reflectance up to 3.5% are widespread in the reservoirs and resulted from thermal cracking of crude oils most likely generated from Upper Permian source rocks and thermochemical sulfate reduction (TSR). Natural gases in the Puguang gas field have wide variations in nonhydrocarbon gas contents, with H2S contents between 5.1 and 58.3% and CO2 contents between 7.9 and 18.0%. The hydrocarbon gases originated mainly from thermal cracking of accumulated oil but were altered by TSR. Thermochemical sulfate reduction in the study area exerted different effects on the isotope compositions of different hydrocarbon gas components at different TSR stages. The differential increase of 
13C values for different gas components reflects transformation from a heavy-hydrocarbon-gas–dominated TSR stage to a methane-dominated TSR stage. This caused a decrease of 13Cmethane–13Cethane values and a corresponding conversion from reversed to normal isotope distributions. Thermochemical sulfate reduction in the study area appears to have been limited by sulfate concentrations in the reservoirs. A successive, three-stage TSR series, namely, liquid-hydrocarbon–involved TSR, heavy-hydrocarbon-gas–dominated TSR, and methane-dominated TSR, occurred in reservoirs with sufficient sulfate concentration. Methane can be the dominant organic reactant for TSR, but only at elevated temperature and after most C2+ hydrocarbons are exhausted.
Copyright © 2008 by American Association of Petroleum Geologists (AAPG)
