Analysis of controlling factors of tight oil accumulation in Middle Jurassic in Qaidam Basin——A case study of Yuqia Sag in the northern edge of Qaidam Basin

The Middle Jurassic strata in the Qaidam Basin exhibit favorable geological conditions for tight oil accumulation, but the main controlling factors for large- scale exploration and development remain unclear. Methods: Through analyses of organic geochemistry, reservoir physical properties, and homogenization temperature tests of fluid inclusions from core/cuttings of six wells (e.g., the Well Long- 2 and Chaiye- 1 in the Yuqia Sag, northern Qaidam Basin), this study investigates the characteristics of hydrocarbon source rocks and tight reservoirs in the Middle Jurassic Dameigou Formation, as well as the source—reservoir coupling relationships and their control on hydrocarbon accumulation.Result: The results indicate that the main hydrocarbon source rocks of the Dameigou Formation are primarily developed in the J2d7 segment. Tight delta—front distributary channel siltstone reservoirs interbedded with these source rocks exhibit excellent source—reservoir configurations. The J2d7 segment serves as the primary accumulation layer for tight oil in the Middle Jurassic of the Yuqia Sag. The reservoir densification occurred approximately 30 Ma ago, while hydrocarbon charging and accumulation took place about 3.29 Ma ago, representing a single- phase charging and one- stage accumulation process. Conclusion: The thermal maturity of the source rocks controls the distribution of Middle Jurassic tight oil in the northern Qaidam Basin. Late Jurassic—Cretaceous compressional inversion and intense uplift during the late Neogene led to significantly lower thermal evolution degrees of Middle Jurassic source rocks in the Yuqia Sag compared to other hydrocarbon- generating sags in northern Qaidam. Prospective exploration areas for Middle Jurassic tight oil include the Qianxi region, Hongshan Sag, and southwestern Yuqia Sag in the northern Qaidam Basin.