Geochemical characteristics and genetic types of natural gas in the Changxing-Feixianguan Formations from the Yuanba Gas Field in the Sichuan Basin, China

The Yuanba Gas Field is a large gas field in the marine strata with the largest burial depth in China up to date. The studies performed on the mentioned gas field have achieved significant progress made possible by determining the characteristics of reef flat reservoirs in the marine strata and the main controlling factors of hydrocarbon accumulation. However, there is no consensus on the origin of natural gases stored in the reservoir in the Changxing-Feixianguan Formations in the Yuanba and its adjacent area. The study on geochemical characteristics indicates that natural gases in the reservoir in the Changxing-Feixianguan Formations in the Yuanba Gas Field are mainly composed of alkane gases which are dominated by methane with dryness coefficients generally higher than 0.995. The CO2 and H2S display an average content of 8.55% and 6.47%, respectively. The δ13C1 and δ13C2 values are from −31.2‰ to −27.9‰, and from −29.9‰ to −25.0‰, respectively, displaying the positive carbon isotopic series. The δ13CCO2 values are generally higher than −8‰, and the δD1 values are from −156‰ to −107‰. The identification of gas origin and gas-source correlation indicate that the natural gases reservoired in the Changxing-Feixianguan Formations in the Yuanba Gas Field have been altered by thermochemical sulfate reduction, and they are mainly composed of oil cracking gases derived from the secondary cracking of oil generated by the sapropelic-prone source rocks in the Permian Longtan Formation. The CO2 in the gas pools are mainly inorganic and were derived from the interaction between the acidic fluid and carbonate reservoirs.