Sedimentary Characteristics of the Permian Zhesi Formation in Eastern Inner Mongolia, China: Implications for Sedimentary Background and Shale Gas Resource Potential

Eastern Inner Mongolia of China is located between the Siberian plate, the North China plate, and the Pacific plate and has a complex history of tectonic and sedimentary evolution. The sedimentary strata of the Zhesi Formation in the Middle Permian recorded rich environmental, structural, and petroleum geological information, which has great significance to the Paleozoic geological research in Northeast China. Through field outcrop observation and profile measurement, combined with geochemistry, mineralogy, and the reservoir physical property test, the sedimentary environment, tectonic setting, and shale gas resource potential of the Middle Permian Zhesi Formation are analyzed. The sedimentary facies of the Zhesi Formation are distributed in strips in the northeast direction, mainly developing littoral, shallow marine, and bathyal sedimentary environments. Clastic rock deposits are mainly developed in the littoral facies, carbonate platform and tempestite deposits are mainly developed in the shallow marine facies, and mudstone mixed with turbidite deposits are mainly developed in the bathyal facies. The sedimentary environment and chronological characteristics show that the Paleo-Asian Ocean was not completely closed in the Middle Permian, and its complete closure time should be later. The characteristics of source rocks and the shale gas resource potential in the Solon area are discussed. Controlled by the sedimentary environment, the Solon area mainly deposited thick dark shale mixed with turbidite sandstone, the accumulated thickness of the dark shale is more than 200 m, with an average vitrinite reflectance (Ro) of 2.44%, and the average residual total organic carbon (TOC) content is 0.85%. The average content of brittle minerals is 60.2% and the shale foliation fracture is developed, which is easy to form natural fractures and induced fractures, so the shale has good hydrocarbon generation potential, and the generated shale gas can exist in shale in an adsorbed state and a free state. In addition, the shale gas generated in the shale can migrate to the lenticular turbidite sand body in a short distance to form free shale gas. Therefore, there is a certain shale gas resource potential in the Solon area, and finding a favorable area with high TOC is the key to future exploration of Upper Paleozoic shale gas.