The sandstone at the base of the Upper Pennsylvanian Taiyuan Formation (Ct2) is a prominent gas reservoir in the Daniudi gas field, northeastern Ordos Basin, China. Logs, cores, thin sections, outcrops, scanning electron microscopy, and three-dimensional seismic data are used to delineate sedimentary facies and to divide the succession into sequence-stratigraphic units.
The gas-bearing coarse-grained sandstone is interpreted as a fluvial-dominated incised-valley fill. A sedimentary hiatus documented for the first time in the study area forms the sequence boundary between the Lower Pennsylvanian Taiyuan Formation (Ct1) and Upper Pennsylvanian (Ct2). The hiatus and overlying incised-valley fill can be observed at the Heidaigou outcrop, where the presence of bauxite indicates a period of subaerial weathering. The incised-valley fill and hiatus can also be found through the outcrop correlation of Xipo, Chengjiazhuang, Qiaotou, and Heidaigou at the eastern margin of Ordos Basin. Two types of basinward and two types of landward facies shifts are identified between Ct1 and Ct2 based on core observations and cross-well profiles analysis to demonstrate abrupt facies change and punctuation. Further evidence for a hiatus is provided by an onlap at this surface on the seismic data.