Characteristics, Types, and Prospects of Geological Sweet Sections in Giant Continental Shale Oil Provinces in China

“Sweet sections” in giant shale oil provinces are preferential fields that primarily support China to increase the reserves and production of continental shale oil. Based on the study of the geological conditions of shale oil in the continental basins in China, it was found that the shale stratum in major oil generation windows generally has higher degrees of oil and gas accumulation, and mostly contains oil. Hydrocarbon generation and reservoir capacities are the two key parameters for evaluating and optimizing favorable shale oil provinces. The evaluation index (volume of shale stratum multiplied by total organic carbon (TOC) multiplied by total porosity) for the giant continental shale oil provinces is also proposed. It is optimized that the Upper Triassic Chang 7 Member in the south-central Ordos Basin, Lower Cretaceous Qing 1 Member in the Gulong-Changling Sag in the Songliao Basin, Middle — Lower Permian in the Junggar Basin, Da’anzhai Member of the Ziliujing Formation of Lower Jurassic in the central and northern Sichuan Basin, and Paleogene oil-rich sag in the Bohai Bay Basin are the five giant continental shale oil provinces. The word “geological sweet sections” in continental shale oil provinces of China refers to favorable shale intervals which are relatively rich in oil, with superior physical properties, and more easily modified and developed commercially under applicable economic and technological conditions. After evaluation, there are mainly two types of “geological sweet sections” of giant continental shale oil developed onshore in China. One type of “geological sweet sections” is generally mudstone with optimal physical properties or a thin tight reservoir, to which the shale oil migrates a short distance. They are medium-to-high-mature zones with a thin sandy shale stratum in the Chang 7 Member in the Ordos Basin, mixed shale stratum in the medium-mature Lucaogou Formation in the Jimsar Sag, and multi-layered mixed Paleogene shale stratum in the Bohai Bay Basin. The other type of “geological sweet sections” is generally shale oil residing in various shale reservoir spaces. This type was developed in the Qing 1 Member in the Gulong Sag and Da’anzhai Member in the north-central Sichuan Basin. Free shale oil mainly occurs in shale, sandy-carbonate lamina, micro-lamella structure, and micro-fractures. Layers with lamina, lamination, and micro-fractures are generally shale oil “geological sweet sections.” Starting from field tests and the construction of the “geological sweet sections” in giant continental shale oil provinces, the shale oil industry has been rapidly developing and will become an important supplement to domestic oil production in China.