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AAPG Bulletin
Abstract
AAPG Bulletin, V.
2004. The American Association of Petroleum Geologists. All rights reserved.
Geochemistry, origin, and accumulation of CO2 in natural gases of the Yinggehai Basin, offshore South China Sea
Baojia Huang,1 Xianming Xiao,2 Weilin Zhu3
1State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China; China National Offshore Oil Corporation, Nanhai West Research Institute, Zhanjiang 524057, China
2State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China; [email protected]
3China National Offshore Oil Corporation Limited, Zhanjiang 524057, China
AUTHORS
Baojia Huang is a senior geologist in the Nanhai West Research Institute of China National Offshore Oil Cooperation (CNOOC). He received his Ph.D. in petroleum geology from Guangzhou Institute of Geochemistry, Chinese Academy of Sciences in 2002. His main research interests are oil and gas geochemistry, reservoir geochemistry, and petroleum systems.
Xianming Xiao is a professor in Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. He received his Ph.D. in geology from the Beijing University of Mining and Technology in 1989. His current research interests include the dynamics system of petroleum accumulation, with particular attention to hydrocarbon generation kinetics and paleopressure reconstruction using fluid-inclusion information.
Weilin Zhu is the general manager of CNOOC (China) of Zhanjiang Limited. He received his Ph.D. in 2001. His main research interests include petroleum system, basin analysis, and risk evaluation of petroleum exploration.
ACKNOWLEDGMENTS
The authors are indebted to J. Lorenz, P. K. Mukhopadhyay, B. Katz, and R. Clark for their constructive comments and suggestions on the original manuscript. We thank R. W. T. Wilkins for improving the English of the manuscript and CNOOC-Nanhai West Corporation for making available the data and interpretations included in several internal reports prepared by Huang et al. (1995) and Luo et al. (1998). This study was supported by the State 973 Project (G1999043308, 2003CB214607), the Chinese Academy of Sciences (GIGCX-04-08), and the Chinese Foundation of Natural Sciences (Grant No. 40072043).
ABSTRACT
Geochemical and isotopic data indicate the presence of CO2 of both organic and inorganic origin in the natural gas reservoirs of the Yinggehai Basin. The natural gases with inorganic CO2 commonly show a high content of CO2, ranging from 15 to 85%; a heavier carbon 
isotope
value of CO2, from 
0.56 to 8.16
; and a lower 3He/4He ratio, ranging from 0.20 to 6.79 
107, indicating a crustal origin. These gases occur locally, commonly related to diapir structures. Natural gases rich in hydrocarbons occur widely and are characterized by a low CO2 content, from 0.1 to 5.0%, and a lighter C1 carbon isotope value from 10.59 to 20.7, indicating an organic origin. Geological background and geochemical data indicate that the Sanya and Meishan formations are the main source of hydrocarbon gases and the organic CO2. Pyrolysis experiments on Tertiary calcareous shales and thermal history modeling both suggest that the calcareous shales occurring in the lower Miocene strata are the main source of the inorganic CO2 gas, whereas thermal contact metamorphism of the Paleozoic carbonates and/or magmatic CO2 may have made only a small contribution. Abnormally high paleogeothermal gradients (4.25–4.56
C/100 m; 12.09–12.26F/100 ft) and a rapid heating rate caused the lower Miocene calcareous shales to reach the threshold temperature (about 300C [570F]) of their thermal decomposition at the burial depth of about 6500 m (21,300 ft) and to generate great volumes of inorganic CO2 gas. Diapir faults acted as the main pathways for the upward migration of deep inorganic CO2 gases into reservoirs connected with shale diapirism along the central Yinggehai Basin. The heavier carbon isotope
values of associated methanes and a strong thermal anomaly in the CO2-rich gas reservoirs provide evidence that the inorganic CO2 gas migrated into the reservoirs later than their associated hydrocarbon-rich gases. This suggests that the earlier formed traps and sandstone reservoirs distant from shale diapir structures may have greater potential in the exploration for hydrocarbon-rich gases.
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