Formation and distribution of large low-permeability lithologic oil regions in Huaqing, Ordos Basin

Through in-depth analysis of sedimentary source, lake basin shape and tectonic events as well as careful study of high quality source rocks, reservoir-forming force and history in the Huaqing area, Ordos Basin, the accumulation mode of large low-permeability lithologic oil regions in the middle of the lake basin was established. The Ordos Basin is a typical inland depressed basin during the depositional period of the Upper Triassic Yanchang Formation. Rich-nutrient lake-basin sediments are developed in the Chang-7 Member during the maximum flooding period. High-quality source rocks of the Chang-7 Member have the characteristics of high abundance of organic matter, strong ability of hydrocarbon generation and expulsion, and wide distribution in Huaqing in the central basin. Delta is developed extensively in the depositional period of Chang-6 Member. Influenced by rich provenance and the bottom shape of slope break belts, thick layers of reservoir sands originated from the delta front and gravity flow are developed in the Chang-6 Member in Huaqing, and the favorable sedimentary facies and diagenetic facies control the extensive low-permeability reservoirs. The Chang-6 low-permeability reservoirs and the Chang-7 high-quality source rocks have a good association and form an accumulation mode characterized by “hydrocarbon-generating pressurization, proximal migration and accumulation, large-area charging, and continuous oil layers”.