Carbon isotopic composition and genetic types of natural gas in the Sichuan Basin, China

Abstract

The origin and genetic types of natural gas in the Sichuan Basin are still disputed. To classify the origin and genetic types in different areas, the paper analyzes the carbon isotopic composition of gases and geologic features in the Sichuan Basin. The results showed that the gas sourced from terrestrial layers is typically characterized by terrestrial origin and was mainly accumulated nearby to form reservoir. The carbon isotopic composition of gas showed a normal combination sequence distribution, suggesting that natural gas in continental strata is not affected by secondary alteration or that this deformation is very weak. The gas source is singular, and only gas from the southern and northern Sichuan Basin shows the characteristic of mixed sources. However, marine gas presents the characteristics of an oil-formed gas. The carbon isotopic composition of natural gas in the western and central part of the basin mostly distributes in a normal combination sequence, and few of them showed an inversion, indicating that the gas perhaps had not experienced secondary alteration. The carbon isotopic composition of marine-origin gas in the southern, northern and eastern Sichuan Basin displays a completely different distribution pattern, which is probably caused by different mixing ratio of gas with multi-source and multi-period.