Lithofacies-paleogeographic characteristics of early Cretaceous in Wulangai basin of Inner Mongolia and their oil-gas geological significance

Three sedimentary facies in lower Cretaceous Damoguaihe Formation of Wulangai Basin could be recognized, i.e., alluvial fan facies, lakes facies, and sublacustrine fan facies. With the coal seam as the key bed of isochronal correlation (in K1d2 section)，the authors compiled the stratigraphic thickness contour diagram and sand/mud ratio contour diagram of K1d2—K1d4 member of Lower Cretaceous Damoguaihe Formation in Wulangai basin. The results show that, in the central part of Wulangai basin, there existed three deep lakes in Early Cretaceous where there were widely distributed dark mudstone, which gradually changed into transitional facies to the north as marked by celadon mudstone, with gentle terrain slope. On the contrary, the slope on the southwest side changed drastically, and shallow lakes were developed which were rolling northeastward, parallel to the basin margin. Between the shallow lakes there was shallow lake beach area, which extended southward and northward to change into ancient land erosion areas. In combination with the characteristics of lithofacies-paleogeography and the organic geochemical indexes of hydrocarbon source rock samples，the authors hold that the hydrocarbon-generating depressions were located mainly in the lower Cretaceous Bayanhua Formation K1d half deep-deep lake deposits of Baqibei depression and Gaolihan depression in the study area.