Origin and source of natural gas in Paleozoic strata of the southwestern Ordos Basin, China

The discovery of the Qingyang gas field in the southwestern Ordos Basin, following more than 50 yr of petroleum exploration, has generated significant scientific interest in its origins, gas-generation mechanisms, and sources. This paper delves into gas genetic type, thermal maturity, source, and the influence of secondary alteration by analyzing the geochemical characteristics of the gas and comparing it with tight sandstone gas found in other parts of the Ordos Basin. The study reveals that the natural gas in the Qingyang gas field is dry. When compared to the Sulige and Dongsheng gas fields, the δ13C and δD of methane are relatively heavier, whereas the δ13CC2+ (ethane, propane, etc.) are lighter. This suggests that the thermal maturity of the gas in this region is relatively high. The high thermal maturity is attributed to the over-mature thermal cracking of a Permian coal measure source rock. Additionally, the gas contains components from the primary cracking of an Ordovician source rock and the secondary cracking of oil from a Cambrian source rock. This mixing of different gas sources contributes to the unique geochemical characteristics of the gas in the southwestern Ordos Basin. Specifically, the lighter carbon isotope value of heavy hydrocarbons in the gas can be explained by the mixing of sapropelic kerogen-cracking gas and oil-cracking gas, which are rich in 12C. This study further substantiates the validity of multiple marine hydrocarbon source rocks in the Paleozoic of the Ordos Basin, offering significant insights and guidance for future natural gas exploration endeavors.