Origin of natural gas in terrestrial reservoirs in Yuanba and Heba regions in northeastern Sichuan Basin, China

The origin of natural gas in terrestrial reservoirs of Yuanba and Heba regions is investigated by analyzing the geochemical characteristics of natural gases and surveying the regional background of petroleum geology in the northeastern Sichuan Basin, China. Results show that: 1) In Yuanba region, natural gases from the Jurassic clastic rock reservoirs contain high levels of heavy hydrocarbon gases. The ��13C and ��D values of methane respectively range from -42.2�� to -34.4�� and -208�� to -168��, whereas the ��13C values of ethane range from -31.4�� to -21.4�� in most gas samples. There is a strong correlation between the ��13C and ��D values of methane in gas samples tested, and the gases are mainly originated from terrestrial source rocks of the Jurassic Ziliujing and Qianfoya Formation. Differently, natural gases from the Xujiahe Formation contain low levels of heavy hydrocarbon gases; the ��13C and ��D values of methane in natural gas samples respectively range from -31.7�� to -29.2�� and -170�� to -148��, whereas the ��13C values of ethane range from -27.7�� to -26.5��. The gases are mainly originated from high-to-over-maturity humic terrestrial source rocks of the Upper Triassic Xujiahe Formation. The origin of natural gases is mainly controlled by the development of terrestrial source rocks and their maturity. 2) In Heba region, natural gases from the terrestrial reservoirs contain varying levels of heavy hydrocarbon gases. The ��13C values of methane are mostly greater than -32.0�� and the corresponding ��D values widely range from -186�� to -122��; the ��13C values of ethane range from -33.2�� to -29.6��. There is a poor correlation between the ��13C and ��D values of methane in gas samples. The ��13C values of methane and ethane are reversed in a few cases, suggesting the complex origin of the gases that include those from terrestrial source rocks of the Xujiahe Formation and marine source rocks, as well as mixed gases from the above two origins. The development of marine and terrestrial source rocks as well as fractures of various levels is the main factor that controls the origin of natural gases in this region. 3) The ��13C values of carbon dioxide in natural gas samples are mostly less than -12��, indicating that the gases are mainly generated through organic matter decomposition. Noble gas isotopic ratios of helium (3He/4He, 0.0033Ra~0.018 1Ra) are distributed in the range of values for deep crust or radiogenic decay, demonstrating a lack of mantle-derived noble gas in the samples tested.