The geochemical differences and origin of multilayered natural gases in marine strata, the northwestern Sichuan Basin, China

The gas-source correlation of highly mature natural gas in marine strata has always been a contentious issue due to the complexities of the petroleum systems, which involve multiple source-reservoir-cap assemblages. To shed light on this matter, a study was conducted using data from five typical gas fields and two hot exploration blocks in the northwestern Sichuan Basin. Thirty-seven samples from marine strata were collected, and their natural gas molecular compositions and stable carbon isotopes were examined to investigate the gas genetic types and origins. Based on detailed gas geochemistry information, combined with source rocks and reservoir characteristics, hydrocarbon accumulation history, and tectonic evolution, the formation of natural gas was studied. The results suggest that the natural gases in the area are dominated by hydrocarbon gases with extremely high dryness coefficients and a trace of CO2 and N2. These natural gases can be separated into three groups based on the development of structures and different source rocks, according to the identification of the gas-source correlation. The presence of oil cracking gas in natural gas increases with the amount of fracture development in the gas-bearing area and the amount of input from sapropelic organic matter in the Qiongzhusi Formation (Fm). Conversely, the contribution from mixing sapropelic-humic organic matter in the Middle-Upper Permian increases noticeably. The different geochemical characteristics of natural gas in the northwestern Sichuan Basin are the result of three-factor coupling, involving different source rocks, diversified fracture evolution, and various secondary alterations (thermochemical sulfate reduction alteration, thermal maturity effect, and mixing of gas). The development of fracture determines the contribution proportion of different source rocks, while secondary alteration further transforms the geochemical characteristics of natural gas.