Hydrocarbon generation and evolution characteristics of Triassic Zhangjiatan oil shale in southern Ordos Basin

The Zhangjiatan oil shale from the Upper Triassic Yanchang Formation in the southern Ordos Basin is an important solid mineral resource, it can also be considered as the main source rock of Triassic oil and gas in the basin. Results of analytical experiments such as TOC, bitumen extraction and group composition, oil content etc. of shale samples from key typical outcrop sections and drilling cores showed that the Zhangjiatan oil shale has a high organic matter abundance. The TOC content ranges 5.11% to 36.47%, with an average value of 16.15%. The S0+S1+S2 value distributes between 16.58-230.98 mg/g, with an average value of 94.20 mg/g. The extracted bitumen abundance is between 0.42%-2.22%, with an average value of 1.25%. Based on these analytical results, Zhangjiatan oil shale can be assigned to be high-quality source rock with an excellent hydrocarbon generation potential. The oil content of oil shale is between 3.52% and 14.6%, with an average value of 8.16%, which belongs to medium- and high-quality oil shale. The kerogen type is mainly Ⅰ-Ⅱ1, and the Ro value generally ranges from 0.43% to 1.09%. Based on field outcrop and drilling core samples, a thermal evolution profile of the Zhangjiatan oil shale in the southern Ordos Basin was established. Moreover, the oil contents of oil shale before and after solvent extraction were compared, and the variations of oil content and pyrolysis hydrocarbon products during the thermal evolution of oil shale were analyzed. There are two types of resources such as oil shale and shale oil in the Zhangjiatan oil shale with a great potential in the southern Ordos Basin. Moreover, some exploration and development countermeasures of oil shale and shale oil were put forward.