关键词: 玛湖凹陷, 百口泉组, 下乌尔禾组, 白碱滩组, 八道湾组, 成藏模式

Abstract: Industrial oil flow has been discovered in hydrocarhon exploration from the Permian to the Jurassic in the central Mahu sag, showing promising indications of oil and gas. However, the level of overall exploration in the study area remains relatively low. In addition, there is a notable absence of systematic research on the geological conditions for hydrocarbon enrichment, accumulation mode and primary controlling factors. A deep understanding of accumulation conditions and the distribution of favorable zones within the central Mahu sag is crucial to improving the efficiency of hydrocarbon exploration efforts. Based on recent data from core, logging, well tie and seismic profiles, and microscopic rock thin section observation, this paper investigates the characteristics of source rocks in the central Mahu sag, as well as the sedimentary characteristics and reservoir-controlling factors of key target strata, including the Permian Xiawuerhe Formation, Triassic Baikouquan Formation and Baijiantan Formation, and Jurassic Badaowan Formation. On this basis, this study determines the locations of multi-layered oil-gas target strata and identifies three-dimensional accumulation modes, thus providing a forward-looking prediction for the next phase of exploration. The study indicates that a widespread strike-slip fault system facilitates communication with the underlying Permian source rocks. Thus, oil and gas can be transported vertically and laterally to the Permian Xiawuerhe Formation and Triassic Bakouquan Formation through faults and unconformities, and further to shallow formations through faults for oil-gas accumulations. The extensive hydrocarbon accumulation in Mahu sag lies in the good reservoir-cap assemblage of widespread fan delta front facies mud-poor gravel reservoirs and retrogradational fan delta facies sediments with the late massive set of mudstone in Xiawuerhe Formation and Bakouquan Formation. The shallow oil-gas accumulation is associated with the favorable reservoir-cap assemblage of the thin-bedded glutenite reservoirs with massive mudstone in Baijiantan Formation and Badaowan Formation. A comprehensive analysis suggests that the fan delta front facies sandbodies of the Xiawuerhe Formation and the upper submember of Member 1 of the Baikouquan Formation in the central Mahu sag can develop high-quality reservoirs, and possess conditions for the large-scale development of stratigraphic traps. Moreover, braided fluvial delta front facies thin-bedded glutenites of the Member 2 of Baijiantan Formation and the Member 1 of Badaowan Formation in the central Mahu sag can develop high-quality reservoirs and provide favorable conditions for the formation of lithological traps. All the above are key strata for further oil-gas exploration in the central Mahu sag.

Key words: Mahu sag, Baikouquan Formation, Xiawuerhe Formation, Baijiantan Formation, Badaowan Formation, accumulation mode