Abstract: In the western part of the Ordos Basin, the risk exploration well Huan Gu-1, located on the western side of the central paleo-uplift of the Lower Paleozoic, encountered weathered crust reservoirs at the top of the Ordovician System, showing favorable gas shows. This confirms that the western side of the paleo-uplift is a significant prospective area with promising natural gas exploration potential. Focusing on the western side of the central paleo-uplift and based on regional logging, analytical testing, 2D and 3D seismic data, a comprehensive analysis of the natural gas accumulation conditions in the Ordovician System was conducted. It is concluded that the western side of the central paleo-uplift is controlled by Fault No. 1, with relatively well-preserved Ordovician strata. The Majiagou Formation (Member 3 and Member 4) corresponds to the main target layers in the eastern part of the basin. The weathered crust primarily develops at the top of the Majiagou Formation of the Lower Paleozoic, where karst fractures and caves are easily formed by weathering and leaching. The upper Taiyuan Formation in the weathered crust is influenced by paleokarst topography, facilitating the formation of bauxite reservoirs. Near the weathered crust, two sets of source rocks are developed: the Upper Paleozoic coal measures and the dolomitic mudstone of the Lower Paleozoic Majiagou Formation. Among these, the Upper Paleozoic coal measures exhibit favorable hydrocarbon generation conditions. To the west of Fault No. 1, fractures are more developed, forming a “ridge-trough alternating” structural pattern. These fractures serve as favorable conduits between the Upper Paleozoic coal measures and the Lower Paleozoic marine source rocks, resulting in dual hydrocarbon supply conditions for the paleo-uplift’s western side. Three favorable accumulation models were established for the western side of the paleo-uplift: the fault-rift residual platform type, the fault-rift residual accumulation type, and the isolated residual mound type. It is pointed out that under the dual hydrocarbon supply background, the structural high ridges matching the weathered crust reservoirs, as well as the updip sections of the rifts and moundlike uplifts, represent promising exploration directions for future efforts.

Key words: Ordos Basin, Ordovician system, Weathered crust reservoir, Natural gas, Accumulation mode, exploration direction