Geochemistry of High-Maturity Crude Oil and Gas from Deep-Ultradeep Reservoirs and Their Geological Importance in a Foreland Basin: A Case Study of the Southern Thrust Belt, Junggar Basin, Northwest China

In the southern thrust belt (STB) of Junggar Basin, hydrocarbon resources are abundant at ultradeep depths. The genesis of highly mature oil and gas is complex, with considerable geological implications, because of the burial depth of these resources and multiphase tectonic evolution. The deep-ultradeep oil and gas reservoir accumulation processes and the contribution of Permian source rocks are still unclear in the STB. In this study, using the ultradeep reservoirs of the middle section of the STB (MSTB) as an example, systematic evaluations were conducted on the hydrocarbon geochemical characteristics of the area. Additionally, this study detailed the accumulation and evolution processes of the reservoir in the lower play of the MSTB using various comprehensive technical analyses, such as those for the geochemical characteristics of crude oil, the genesis and source of crude oil and gas, inclusion observations, homogenization temperature, quantitative particle fluorescence, and basin simulation. The integrated analysis results showed that the oil and gas from Well HT-1 in Huxi Anticline (HXA) in the lower play was characterized by mixed sources from Permian argillaceous source rock and Jurassic coal-bearing source rocks. The pristane/phytane ratio (Pr/Ph) and biomarkers of the crude oil indicated the presence of Permian source crude oil. The natural gas drying coefficient and the carbon isotope of methane conveyed a typical coal-related gas from the Lower-Middle Jurassic epoch. In addition, three types of hydrocarbon inclusions (yellow-green fluorescence, blue-white fluorescence, and gaseous hydrocarbons) were developed in the MSTB. Oil and gas accumulation periods included Early Cretaceous, Eocene, and Miocene. Finally, substantial deviations were found for accumulation periods from different anticlines in the study area, which were controlled by the ancient structural background and tectonic evolution. The early anticlines, formed by Yanshanian tectonic movement (YSTM), controlled the early hydrocarbon migration as well as reservoir formation, and the newer anticlines, formed by Himalayan tectonic movement (HMTM), controlled the redistribution of hydrocarbons from ancient reservoirs. The oil and gas reservoir formed in Qingshuihe Formation (K1q) in HXA occurred during the Miocene. The ancient oil reservoirs to the south of HXA were damaged, and hydrocarbons were redistributed; the hydrocarbons migrated and accumulated in HXA and mixed with highly mature gas from Jurassic source rocks. The Permian hydrocarbons made a substantial contribution to the deep-ultradeep resources of MSTB and provided extremely rich hydrocarbon resources together with the Jurassic source rocks. Additionally, within MSTB, large anticlines were thoroughly developed in ultradeep strata, which provided superior trapping conditions for large-scale hydrocarbon accumulation.