Geophysical identification and distribution of Upper Paleozoic coals in southeastern Ordos Basin

Based on core, logging, and seismic data, a comprehensive analysis was conducted on the identification methods of coal seams, thickness changes, and their relationship with paleogeomorphology and sedimentary facies in the southeastern part of the Ordos Basin. The lower part of Shanxi Fm and the upper part of Taiyuan Fm in the southeastern part of the basin characterize low natural gamma ray, low density, high resistivity and high sonic logging values. As the coal velocity is very low and alternates with high-velocity Taiyuan Fm and Ordovician limestones, the two sets of coalsbelow and above Taiyuan limestonesform an extremely-strong amplitude reflection on the seismic profile. The two sets of coals are 1~10 m thick cumulatively, average 2.4 m, with 4 m as the maximum induvial thickness. The coal seam with an individual layer thicker than 2 m covers an area of about 2500 km², and the buried depth of the coal seam range from 2500 to 3000 m. Research has shown that the thick coal seams of Taiyuan Fm are mainly distributed proximally in the Ordovician paleokarst depression with incised origin, while the Ordovician paleokarst depression without incised origin was deposited by distal coeval limestone rocks. There is a complementary relationship between the coal seams and the thickening zone of distributary channel sandstone for the Shanxi Fm.