Origin, maturity and geochemistry of natural gas in the Yancheng Sag, Subei Basin, China: Insights from pyrolysis experiments and basin modeling

Natural gas-sources correlation is crucial for determining the target of natural gas accumulation. The Zhujiadun gas field discovered in the Cretaceous Taizhou-Paleogene Funing (K2t-E1f) formations is one of the most important gas fields in the Subei Basin. However, the origin of natural gas in the Zhujiadun gas field is controversial, seriously restricting future exploration. In this study, organic geochemistry, gold-tube hydrous pyrolysis experiments, and basin modeling were performed to comprehensively investigate the origin, maturity and geochemistry of natural gas in the Zhujiadun gas field. The results show that: (1) the natural gas in this gas field is sapropelic-sourced thermogenic gas, which is predominately from the secondary cracking of crude oil. (2) New empirical equations for calculating the maturity of natural gas in the Yancheng sag are established based on the relationship between the carbon isotopic composition of methane and the thermal maturity of kerogen. The calculated average Ro value of natural gas is 2.2 %. (3) The Yancheng Sag has evolved through three subsidence cycles. Potential Paleozoic source rocks are currently in the post-maturity period, while the Paleogene source rocks are still in the oil generation window. (4) The natural gas in the Zhujiadun gas field predominantly originates from the source rocks of the Ordovician Wufeng-Silurian Gaojianbian formations. These results provide insights into the source of natural gas in the Zhujiadun gas field and a basis for natural gas exploration in the future.