Differences and constraints of varying gas dryness coefficients in the Cainan oil-gas field, Junggar Basin, NW China

Abstract

There are obvious differences in the gas dryness coefficients of different oil and gas reservoirs in the Cainan oil-gas field in the eastern part of Junggar Basin, which is uncommon in petroliferous basins. Based on an analysis of the natural gas components, isotopes, light hydrocarbons, biomarkers, and the gas accumulation process, it is proposed that the mixing of gases from multiple sources and the diffusion process during accumulation are the major constraints on dryness coefficients, influencing them as follows: 1) highly mature coal-type gas dominates in Cainan area, mixed with some oil-type gas, some relatively low-maturity coal-type gas and some shallow secondary microbial gases, resulting in the vertical change in the dryness coefficients and the association of drier gas with crude oil. 2) Part of gases undergo long-distance migration, and the diffusion of gas during this process enriches methane along the migration direction, resulting in the horizontal change in the dryness coefficients; 3) the dryness coefficient changes gradually and is controlled by hydrocarbon generation events and structural changes, reflecting the complex source and accumulation process of natural gas. In this study, we determine why the gas dryness coefficient changes so markedly in the Cainan oil field, obtain a relatively clear understanding of how the three source rocks (Carboniferous, Permian, and Jurassic) contribute to the accumulation of oil/gas, and emphasize the application of dryness coefficients in the study of natural gas with complex origins.