Geochemistry and origins of natural gas in the Hong-Che fault zone of the Junggar Basin, NW China

Abstract

The uplift zone adjacent to the sag is the dominant direction of natural gas migration and accumulation, and the multi-layer source rocks developed in the sag have laid a material foundation for natural gas accumulation. However, owing to the deep burial and high drilling cost of the sag, drilling the source rocks in the sag is challenging, which severely restricts the fine gas-source correlation study of natural gas in the uplift zone. The Hong-Che fault zone is adjacent to the Shawan sag, the natural gas is of various types, and the origin and source of natural gas are unclear. The referential contrast research methods on multi-source and multi-period natural gas are absent, which hinders their subsequent exploration and development. In view of these problems, we analysed the pyrolysis gas of source rocks, along with its composition, including the light hydrocarbon and the carbon isotope of natural gas. The experimental results show that: (1) There are significant differences in the carbon isotopes of ethane in pyrolysis gas from different source rocks, which is one of the research bases of natural gas source analyses. (2) The natural gas in the region is mainly present as kerogen cracking gas. (3) The natural gas in the study area is divided into five types. Type-Ⅰ and type-Ⅱ gases derived from the humic source rocks of the Carboniferous stratum and the Jiamuhe Formation, respectively. Type-Ⅲgas was sourced from the sapropelic source rocks of the Fengcheng Formation. Type-Ⅳ gas was originated from mixed source rocks of the Wuerhe Formation. Type-Ⅴ gas was derived from the mixing of multiple sets of source rocks. (4) The south section of the Hong-Che fault zone has high gas production, and is a favourable exploration target for humic pure gas reservoirs. The slope area near the Shawan sag has better preservation conditions and is a favourable target area for subsequent natural gas exploration activities. These results lay the theoretical foundation for the evaluation and identification of the origins of multi-source and multi-period natural gas in complex areas.