Natural gas accumulation in the basin-mountain transition zone, northwestern Sichuan Basin, China

The basin-mountain transition zone is an important area for hydrocarbon exploration. To better understand the process and mechanism of hydrocarbon accumulation in this area, the middle Permian Qixia Formation at the piedmont of the Longmenshan Mountains, northwestern Sichuan Basin, China, was investigated. It is showed that natural gas in the Qixia Formation in the thrust belt (i.e., the Kuangshanliang and Hewanchang areas) was derived from lower Cambrian strata by oil cracking. The reservoirs are filled with amounts of bitumen, which reflects strong oil charging. However, the gas accumulation is not rich, which is due to the later destruction of hydrocarbon accumulations by the invasion of meteoric waters. In contrast, in the thrust front (i.e., the Shuangyushi area) and forebulge belt (i.e., the Jiulongshan area), the reservoirs contain few bitumens and large amounts of natural gas, which is indicative of good preservation for hydrocarbon accumulation. The natural gas was mainly derived from kerogen cracking in middle Permian source rocks, although it was mixed with a small amount of lower Cambrian-derived oil-cracking gases. The thrust front and forebulge belts occur in areas of lesser deformation, and thus are favorable for gas accumulation, due to the strata containing both sapropel- and humictype source rocks and reservoirs. It is generalized that hydrocarbon resources in (ultra-)deep strata in the basin-mountain transition zone may be an important target for exploration worldwide.