Geochemical features and origin of natural gas in heavy oil area of the Western Slope, Songliao Basin, China
摘要
The Western Slope of the Songliao Basin is rich in heavy oil resources (>70 脳 108 bbl), around which there are shallow gas reservoirs (鈭?.0 脳 1012 m3). The gas is dominated by methane with a dryness over 0.99, and the non-hydrocarbon component being overwelmingly nitrogen. Carbon isotope composition of methane and its homologs is depleted in 13C, with 未13C1 values being in the range of 鈭?5鈥?to 鈭?5鈥? 未13C2 being in the range of 鈭?0鈥?to 鈭?3鈥?and 未13C3 being in the range of 鈭?0鈥?to 鈭?2鈥? respectively. These values differ significantly from those solution gases source in the Daqing oilfield. This study concludes that heavy oils along the Western Slope were derived from mature source rocks in the Qijia-Gulong Depression, that were biodegraded. The low reservoir temperature (30-50 掳C) and low salinity of formation water with neutral to alkaline pH (NaHCO3) appeared ideal for microbial activity and thus biodegradation. Natural gas along the Western Slope appears mainly to have originated from biodegradation and the formation of heavy oil. This origin is suggested by the heavy 未13C of CO2 (鈭?8.78鈥?to 0.95鈥? which suggests that the methane was produced via fermentation as the terminal decomposition stage of the oil.