The origin and source of natural gas in the Lower Permian in Hewanchang area, Northwest Sichuan

The Lower Permian is rich in natural gas resources in the northwestern Sichuan Basin of China. However, the cause and source of natural gas are still unclear. In this paper, core observation and thin slice identification under the microscope. The geochemical characteristics of the study area are combined. The biomarker characteristics of the reservoir and source rock in the study area are combined to determine the genesis of Lower Permian natural gas. The results show that (1) according to the Hewanchang natural gas He and CO 2 content combined with ln ( C 1 / C 2 ) and ln ( C 2 / C 3 ) intersection diagrams. By comparison, it is found that the natural gas in the Hewanchang structure is a typical crude oil cracked gas; (2) only a small amount of asphalt was found in the Lower Permian in the Hewanchang structure. It shows that the scale of crude oil cracking is not large. Less crude oil cracked gas is produced. However, large-scale pitch veins have been discovered in the Cambrian in northwestern Sichuan. It shows that large-scale crude oil cracking occurred in Cambrian reservoirs; (3) the biomarker characteristics of the extracts from the Lower Permian reservoirs are similar to those of the Lower Permian source rocks. It shows that the bitumen in the Lower Permian reservoir is formed by the cracking of the Lower Permian crude oil. The late structural adjustment resulted in the Hewanchang natural gas reservoir being formed by a large amount of Cambrian formation crude oil cracked gas and a small amount of Lower Permian crude oil cracked gas.