Geochemistry and source of crude oils in the Wensu uplift, Tarim Basin, NW China

Abstract The Wensu uplift, to the northwest of the Tarim Basin, is located at the intersection of the marine Awati basin and the continental Kuche foreland basin. The origin and accumulation of hydrocarbons are complex processes in the Wensu uplift. The XWD 1 and 2 wells, drilled in the Wensu uplift in 2017, produced high-yield commercial oil flows from Neogene strata, thus realizing the first major breakthrough in the region's 50 years of oil and gas exploration. This discovery invited significant attention from the research community for shallow hydrocarbon accumulations along the margins of the Tarim Basin. This study focuses on the Neogene hydrocarbons of the Wensu uplift along with their geochemical characteristics and origin. We propose a hydrocarbon accumulation model, reconstructed by combining geology and geochemistry. The results show that crude oils from the Wensu uplift display typical lacustrine characteristics: 1) higher Pr/Ph values (1.68–2.04, average 2.33) that are indicative of an oxidizing sedimentary environment for the source rock; 2) higher content of rearranged steranes, C29Ts, and C30 diasteranes and lower DBT/P value (0.09–0.11), indicating that the source rock is shale with a high content of clay minerals; and 3) lower sterane/hopane ratios (0.14–0.15) indicating high inputs of bacteria and microorganisms. The above characteristics are distinct from the adjacent marine crude oils which feature relatively lower ratios of Pr/Ph (1.27–1.37) and higher ratios of DBT/P (0.62–0.82) and steranes/hopanes (0.87–0.90). The oil-oil and oil-rock correlations conducted using hydrocarbon biomarkers and carbon isotope ratios show that the oils in Wensu uplift contain contributions from the lacustrine source rocks of the Jurassic Qiakemake (J2q) and Triassic Huangshanjie (T3h) formations of the eastern Wushi–Baicheng sag, suggesting that the Wensu uplift oils have a mixed origin. The age of stratigraphic traps in the Wensu uplift coincides with that of hydrocarbon expulsion peak and the hydrocarbons accumulated along the slope by long-distance migration. The results of this study aim to serve as an important guide for further studies on petroleum accumulation mechanisms and shallow hydrocarbon explorations in basin margins devoid of local hydrocarbon source rocks.