Stable carbon isotopes of alkane gases from the Xujiahe coal measures and implication for gas-source correlation in the Sichuan Basin, SW China
Introduction
This study focuses on the gas geochemistry of the Upper Triassic Xujiahe Formation in the Sichuan Basin, SW China. This Phanerozoic basin has the most stable tectonics in China, delimited by the terrestrial Xujiahe Formation, with an area of 180,000 km2 (Wang et al., 2002). The cumulative recoverable reserves in this basin have reached 842 billion m3 by the end of 2005 (Wang et al., 2008). This basin contains the largest number of gas fields in China (127), and had an annual gas production about 17 billion m3 in 2007 (Yang et al., 2008).
Section snippets
Geological setting
The Sichuan Basin can be divided into east, south (including south and southwest), west and central Sichuan gas–oil accumulation zones (Fig. 1). Gases are produced from 21 stratigraphic horizons, ranging from Sinian to Jurassic in age, and the dominant pay zones are in the Sinian Dengying Formation (Z2d), Carboniferous Huanglong Formation (C2h), Permian Maokou (P1m) and Changxing formations (P2ch), Triassic Feixianguan (T1f), Jialingjiang (T1j), Leikoupo (T2l) and Xujiahe (T3x) formations and
Analytical methods
The molecular composition of gas samples was determined using an Agilent 6890N gas chromatograph equipped with a flame ionization detector and a thermal conductivity detector, at the PetroChina Research Institute of Petroleum Exploration and Development-Langfang (RIPED-Langfang). Individual hydrocarbon gas components (C1–C5) were separated using a capillary column (PLOT Al2O3 50 m × 0.53 mm). Noble gases were separated using two capillary columns (PLOT Molsieve 5 Å 30 m × 0.53 mm, PLOT Q 30 m × 0.53 mm). GC
Results and discussion
Empirical studies of the gas geochemistry in the Sichuan Basin (Wang, 1994) and in many other Chinese sedimentary basins (Dai et al., 2005) suggested a cut off point in the δ13C2 value (−29‰) for differentiating the origin of thermogenic gases in China, with greater values indicating coal derived gases and lower values for oil associated gases. Utilizing this criterion, two types of gases produced from the Xujiahe Formation reservoirs in the Sichuan Basin can be recognized: (a) self-sourced
Conclusions
The Upper Triassic Xujiahe coal measures in the Sichuan Basin contain Types II and III gas prone dark mudstones and coals in the first, third and fifth members, with the sandstone reservoirs in the second, fourth and sixth members forming three separate gas payzones. Most of these gases were derived within the coal measures, leading to the formation of Guang’an, the second largest gas field in the basin. These coal derived gases display a normal stable carbon isotope trend for C1–C4 alkanes,
Acknowledgements
We thank Dr. Maowen Li of Geological Survey of Canada for constructive suggestions and extensive editing of an earlier version of this manuscript and Wei Li for generously providing us some analytical data.
References (32)
- et al.
Thermochemical sulphate reduction and the generation of hydrogen sulphide and thiols (mercaptans) in Triassic carbonate reservoirs from the Sichuan Basin, China
Chemical Geology
(2003) - et al.
Origins of partially reserved alkane δ13C values for biogenic gases in China
Organic Geochemistry
(2004) - et al.
Significances of studies on natural gas geology and geochemistry for natural gas industry in China
Petroleum Exploration and Development
(2008) Isotope organic geochemistry
Organic Geochemistry
(2006)Where did coal all the coal gas go?
Organic Geochemistry
(2003)- et al.
Geochemical features of natural gas and the progress of Upper Triassic hydrocarbon accumulation in Middle Sichuan area
Natural Gas Geoscience
(2007) - et al.
Source rock conditions and resource potential analysis of reef flat gas reservoirs in Northern Sichuan Basin
Natural Gas Exploration and Development
(2007) Natural gas and oil formation during coal formed
Petroleum Exploration and Development
(1979)Distribution, classification and origin of natural gas with hydrogen sulphide in China
Acta Sedimentologica Sinica
(1985)- et al.
Carbon isotope characteristics of natural gas in the Sichuan Basin, China
Petroleum Geology and Experiment
(2001)
The problems in natural gas exploration and development in China
Causation of partly reversed orders of δ13C in biogenic alkane gas in China
Oil and Gas Geology
Development trends of natural gas industry and the significant progress on natural gas geological theories in China
Natural Gas Geoscience
Carbon isotopic geochemistry and origin of natural gases in the southern part of the western Sichuan depression
Acta Geoscientica Sinica
Evidence for multiple stages of oil cracking and thermochemical sulfate reduction in the Pugang gas field, Sichuan Basin, China
American Association of Petroleum Geologists Bulletin
Carboniferous Gas Fields in High Steep Structures of Eastern Sichuan, China
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