Sedimentary Facies and Sedimentary Evolution from Permian Shanxi Formation and the 8th Member of Shihezi Formation in the Southeast Margin of Ordos Basin

The Ordos Basin, a major oil-bearing basin in China, features the Permian Shanxi Formation–He 8 Formation in its southeastern region. With substantial stratigraphic thickness and superior reservoir conditions, this area demonstrates exceptional hydrocarbon exploration potential. Research on sedimentary environments and facies in this region provides critical guidance for resource exploration. Although the Shanxi Formation-Hex8 Formation exhibits significant hydrocarbon potential, further clarification is required regarding the evolutionary patterns of sedimentary environments and the spatial distribution of sedimentary systems. Through field observations and descriptions of geological profiles at various locations within the basin, petrological and elemental geochemical analyses were conducted on samples from each stratigraphic section. By integrating regional tectonic backgrounds, this study investigates the sedimentary environment, sedimentary facies characteristics, sedimentary evolution, and depositional patterns in the study area. Based on field profile observations, petrological and elemental geochemical analyses, combined with regional tectonic context, the research examines the evolutionary features of sedimentary environments, as well as the distribution patterns and facies types within the study area. The study reveals significant evolutionary characteristics of the sedimentary environment in the research area during the Shan2-Box8 sedimentation period: continuous decline in water salinity, gradual transition from warm humid paleoclimate to arid hot conditions, increasing redox oxidizing capacity, and overall trend of shallowing paleo-depth—with the Box8 section showing the shallowest water body. From the Shan2 to Box8 sedimentation periods, paleoclimate shifted from warm humid to semi-arid-dry conditions, accompanied by sustained reduction in water salinity, enhanced oxidation activity, and progressive shallowing of paleo-depth. Analysis identified 12 lithofacies types, 4 sedimentary systems, and 14 sedimentary microfacies. During the Shan2 sedimentation period, influenced by the overall uplift of the North China Platform, the basin transitioned from a continental surface sea environment to a nearshore lacustrine basin, initiating a transcontinental sedimentary system that formed a coexisting sedimentary pattern of northern marginal shallow lakes and southern meandering river deltas. The Shan1 sedimentation period continued previous depositional features with further shrinkage of lacustrine basins and expansion of delta systems advancing northward. In the Box8 sedimentation period, intensified tectonic activity in the northern margin orogenic belt led to complete seawater withdrawal. Under bidirectional north-south material supply and seasonal flooding, terrestrial sedimentary systems dominated, primarily consisting of braided river-braided river deltas. The Box8 braided river delta sand bodies exhibited extensive distribution and substantial thickness, constituting the most favorable reservoir facies zone in the study area. These findings provide geological foundations for subsequent oil and gas exploration deployment in the region.