Research paperGeochemistry and origin of natural gas in the eastern Junggar Basin, NW China

- Geochemistry and origin of natural gas in the eastern Junggar Basin were studied.
- Four genetic types mainly sourced from Carboniferous and Permian rocks were identified.
- Sapropelic- and humic-types gases with mixing were identified.
- Microbial gas sourced from Permian rocks with Type II-III kerogen was identified.
- The gas occurrence is “source-controlled”.

The eastern Junggar Basin in NW China is an important region for natural gas exploration with the giant Kelameili Gas Field having been discovered. However, the origin of gas in the region is unclear, which limits the understanding of gas formation and accumulation, as well as the planning of exploration strategies. Here, we address the origin of gas in this region through a comprehensive investigation of gas geochemistry in the context of geological setting, including gas chemical compositions, carbon isotopes and light hydrocarbons and biomarkers of retrograde condensates. Results show that four main types of natural gases were identified: (1) highly mature to post-mature humic-type gas sourced from Carboniferous rocks with Type III kerogen, (2) mature to highly mature humic-type gas sourced from Permian rocks with Type III kerogen, (3) mature to highly mature sapropelic-type gas sourced from Permian rocks with Type I-II kerogen, and (4) microbial gas sourced from Permian rocks with Type II-III kerogen. These gases vary significantly in geochemistry, and thus can be easily identified. The distribution of the gases follows the “source-controlled” principle in that it is controlled by the distribution and maturity of hydrocarbon source rocks. As such, the gases sourced from the Carboniferous and Permian rocks are the most viable exploration targets, with the areas in and surrounding the hydrocarbon-generation centers being favorable for exploration. These results also provide guidance to natural gas exploration and research in other basins with similar geological settings.