Restoration of palaeogeomorphology and its petroleum-geological significance: Transitional-facies deposition during the early Permian in the southeastern Ordos Basin, North China

The identification of favourable shale-gas areas in marine-continental transitional shales are challenging because of the multiple thin producing layers and abrupt lateral pinch-outs. Using data obtained from drilling, logging, and 3D seismic and natural gamma spectroscopic logging, this paper investigates the control of early Permian palaeogeomorphology on the distribution of organic-rich shale in the southeastern Ordos Basin. Using the impression method and de-compaction correction of strata thickness based on seismic velocity reconstruction, we restore the karst palaeogeomorphology at the top of the Taiyuan Formation for the early Permian. Three geomorphic units including karst highland, karst slope, and karst depression are identified, and constitute a geomorphological pattern of interjacent depressions and highlands. In addition, based on the multi-lithological eigenvalue method, the spatial distribution of transitional shales in the Shan 2 lower sub-member of the lower Permian Shanxi Formation was inferred using post-stack seismic parameter inversion. Our results show that organic-rich shale is distributed mainly in the low section of Shan 2 lower sub-member, which is controlled by karst palaeogeomorphology mainly distributed in karst depressions. Organic-rich shale distributed in karst depressions are characterised by large cumulative thickness, high TOC, moderate thermal maturity, strong hydrocarbon generation intensity, and relatively high gas content, indicating that shale-gas sweet spots should be distributed mainly in karst depressions rather than in highlands or slope areas. Meanwhile, Th/U and Th/K ratios, which indicate paleo-environment during the period of shale deposition, show that the strength of reducing conditions was greatest in karst depressions, moderate in karst slopes, and lowest in karst highlands. Therefore, it is inferred that the distribution of shale-gas sweet spots may be related to the weakly reducing environment, which favoured the preservation of organic-rich sediments during sedimentation. The upper section of Shan 2 lower sub-member was dominated by a high-energy transitional sedimentary environment, and the paleo-redox conditions tended to be oxic, disfavouring the preservation of shale organic matter, and the organic-rich shale in the upper section is relatively poorly developed. This study shows that palaeogeomorphology and paleo-redox conditions affect the deposition and preservation of organic-rich sediments in lower section, the relationship between palaeogeomorphology and the distribution of organic-rich shale can provide reference for shale gas sweet spots prediction and promote the rapid exploitation of shale gas in eastern Ordos Basin.