Natural gas in southern Junggar Basin in northwest China: Geochemistry and origin

The southern Junggar Basin has been an important area for natural gas exploration and research in China for decades, where gas has long been believed to be sourced mainly from Jurassic coal-bearing sequence. However, as multiple sets of source sequences besides the Jurassic strata were found to occur in the study area, the gas may have other sources. Therefore the previous study of gas origin limits the understanding of gas formation and accumulation and the formulation of exploration strategy. Here we address the issue of gas origin via relatively comprehensive investigation of gas geochemistry and gas geology, based on chemical compositions, carbon isotopes and light hydrocarbons. Our analytical results yield new understandings. Two other types of gases, besides the commonly-known Jurassic-derived gas, were newly found, including(1) highly to post mature coal-type gas sourced from Triassic and/or Permian source rocks, and(2) lowly to moderate mature oil-type gas sourced from Paleogene source rock. In contrast, the Cretaceous-sourced gas has not been revealed, which is likely due to a relatively low concentration in the mixed gases. These three main types of gases vary apparently in geochemistry and thus can be easily identified. The distribution of these multiple types of gases follows the theory of source-control, i.e., being controlled by the distribution and maturity of hydrocarbon source rocks. As such, natural gases in the study area with variable origins provide the material base for possible large-scale gas accumulation, and the gases sourced from Jurassic sequence are the most realistic exploration targets, while the Huo-Ma-Tu anticline belt is the key target area. These results can also be referred in the natural gas exploration and research in foreland basins in Central-West China and in other basins with similar geological settings.