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The Tahe-Lunnan hydrocarbon province is China's largest region with oil-and-gas-producing marine carbonate rocks. However, in terms of multi-source hydrocarbon generation, multi-episode reservoir adjustment and reconstruction, it remains unsettled how to determine the geological period of primary hydrocarbon filling of the Ordovician reservoir in this region. Based on the analysis of distribution and properties of reservoir, hydrocarbon filling of the Cambrian source rocks in the Late Caledon stage has largely been destroyed. However, hydrocarbon filling of the Middle Ordovician source rocks in the Early Carboniferous resulted in the major body of crude oil. During the charging process, the hydrocarbons were oxidatively degraded to heavy oil due to the poor closure conditions, which is corroborated by homogenization temperature of inclusions. Moreover, the capturing of hydrocarbon inclusions with high-temperature does not represent the filling of mature petroleum in the later period, but represents the result of the natural gas containing light fraction. Therefore, the Tahe-Lunnan area underwent two hydrocarbon filling processes, and the invasion of excessive dry gas led to a gas-washing fractionation upon the original Ordovician reservoirs. [ABSTRACT FROM AUTHOR]

Reservoired liquid petroleum has limited potential to be preserved under high temperature conditions due to thermal cracking and potentially thermochemical sulfate reduction, and thus exploration targets in ultradeep strata are mainly gas and condensate gas. However, a giant ultradeep liquid petroleum accumulation, with an average depth of >6500 m, was discovered recently in the Hadexun area, Tarim Basin. In this study, integrated geochemical analyses were conducted on the black oil samples from the Hadexun area. Intact terpanes and steranes and low concentration of diamondoids were detected, indicating that these oils were generated at peak oil window and were barely altered by secondary geochemical process (thermal cracking or others) despite their depth. The associated wet gas with light isotopic profiles was classified as mature oil-derived gas co-generated with the oil. Petroleum accumulation analyses suggested that the favorable carbonate reservoir bodies in the Middle Ordovician Yijianfang Formation were attributed to karstification and weathering, and mainly distributed along the major strike-slip fault belts. Together with the thick mudstones in the overlying Upper Ordovician Sangtamu Formation, they formed a favorable reservoir-seal assemblage in the Hadexun area. Massive volumes of liquid petroleum were generated and expelled from the Lower Cambrian black shales in the Late Hercynian and migrated into the Ordovician carbonate reservoirs through the strike-slip fault system, and subsequently were exposed to stable tectonic conditions. Due to the rapid subsidence since the Neogene, insufficient temperature-time exposure eliminated the impact of thermal cracking; therefore, the petroleum accumulation was preserved in the liquid phase, and further indicates that huge petroleum resource potential remains in ultradeep strata in the Tarim Basin. • Oil/gas sourced from the Cambrian shales were barely altered by geochemical process. • Insufficient compensation of temperature and time led to the preservation of the ultradeep oil. • Oil/gas was preferentially enriched in favorable carbonate reservoir bodies along faults. • Liquid petroleum and condensate/gas are the main targets in Ordovician and Cambrian assemblages, respectively. [ABSTRACT FROM AUTHOR]

As a clean and environmentally friendly energy source, deep oil and gas has always been the focus of the oil and gas industry. The study of hydrocarbon accumulation in deep strike-slip fault zones is a challenging and important area of research in the oil and gas industry. In particular, accurately modeling oil and gas accumulation in the Yuemenxi area of the Tarim Basin presents significant difficulties due to the varying physical properties and gas composition of the Ordovician reservoirs, as well as the complex origin of oil and gas in the area. However, by calculating biomarker parameter maturity on oil samples from strike-slip faults, researchers have discovered that the light oil in the area is sourced from high maturity source rocks in the Later Caledonian, with vitrinite reflectance ranging from 0.79% to 1.11%. The complete distribution of n-alkanes and high concentration of low-carbon n-alkanes in the crude oil suggest that the fluid in the reservoir has not undergone any secondary alteration since its initial accumulation. The carbon isotope and component ratio analysis of natural gas in the Yuemanxi area indicates that the Ordovician natural gas is predominantly kerogen cracking gas. Comprehensive hydrocarbon genesis and accumulation conditions, this paper presents a differential accumulation model for the Ordovician reservoirs in the region, which were controlled by strike-slip faults and source rocks. Based on these findings, it can be inferred that there is significant potential for oil and gas exploration and development in the deeper layers of these strike-slip fault zones. • The paper presents a differential model for the accumulation of oil & gas. • The Ordovician crude oil was sourced from highly mature Late Caledonian source rocks. • The Ordovician natural gas is mainly composed of gas generated by kerogen degradation. • The Ordovician crude oil didn't experience secondary alteration after accumulation. [ABSTRACT FROM AUTHOR]