Natural gas accumulation from carbonate source rock with low total organic carbon content and high maturity: a case study of the Leikoupo Formation in the western Sichuan Depression

Abstract Carbonate source rocks with low total organic carbon content (LTOC) are generally considered to be challenging to form large-scale oil and gas fields independently and are even regarded as ineffective source rocks. Based on the chemical and isotopic composition of natural gas and kerogen carbon isotope data of source rocks, this study indicates that the natural gas in the Middle Triassic Leikoupo Formation (T 2 l) in the western Sichuan Depression primarily originates from the LTOC carbonate source rocks of the T 2 l itself. Combining the geochemical characteristics, lithological characteristics, macroscopic distribution of the T 2 l source rocks in the western Sichuan Depression and seismic data, a mechanism of hydrocarbon expulsion through structural fractures in the LTOC carbonate source rocks in the foreland area is proposed: the fractures formed by intense structural compression can promote the expulsion, migration and accumulation of hydrocarbons from the LTOC carbonate source rocks. In structurally active areas, thick, highly mature LTOC carbonate source rocks have the potential to form large-scale commercial oil and gas fields. The results of this study are of great significance for expanding the exploration field of carbonate oil and gas.