Genesis and preservation of the giant ultradeep Hadexun petroleum accumulation in the Tarim Basin, China

Abstract

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.