Deep hydrocarbons in the northwestern Junggar Basin (NW China): Geochemistry, origin, and implications for the oil vs. gas generation potential of post-mature saline lacustrine source rocks

Deep ( > 4500 m) sedimentary units in petroliferous basins are a frontier field for petroleum exploration and exploitation worldwide. This study focuses on the northwestern Junggar Basin of NW China and presents the results of a comprehensive geochemical analysis of the source rocks and hydrocarbons hosted by deep reservoirs in this region, including data obtained from Rock-Eval pyrolysis, carbon isotope, biomarker, and light hydrocarbon analyses. These data provide insights into the exploration potential of deep hydrocarbons in the study area and in other areas with similar geologic settings (i.e., saline lacustrine petroleum systems). Results indicate that the deep hydrocarbons in the northwestern Junggar Basin are dominated by oil, with both the oil and natural gas being derived mainly from the lower Permian Fengcheng Formation. This formation was deposited in a heterogeneous saline lacustrine environment, which led to the generation of geochemically diverse oil compositions. Consequently, Fengcheng-derived oils can be divided into subcategories based on source rock lithology; i.e., hypersaline alkaline muddy dolomites, saline (dolomitic) mudstones, and brackish mudstones. A small number of deep oils in the study area were derived from the Carboniferous-lower Permian Jiamuhe Formation. The deep hydrocarbons in the study area generally accumulated in near-source environments, indicating that prospective areas are present in the form of Fengcheng-derived oils in the depocenter of the Mahu sag, especially oils derived from hypersaline alkaline muddy dolomites and saline (dolomitic) mudstones. These source rocks are likely to have generated oil even under the highly mature conditions associated with deep burial. This might explain why discoveries within saline lacustrine basins are generally dominated by oil, implying that these conditions might also exist within the deeper parts of saline lacustrine basins worldwide.