Bauxite series in the Upper Paleozoic weathering crusts in the southwestern Ordos Basin: Development and distribution of dominant reservoirs

In the Longdong area-a gas exploration area in the southwestern Ordos Basin, breakthroughs have been made in gas exploration in the bauxite series of the Upper Paleozoic Permian Taiyuan Formation, offering a new exploration target. However, the distribution of the bauxite series and reservoir developmental characteristics within remain unclear, hindering exploration progress. Using data from core and thin-section observations, logs, and seismic data, we analyze the provenance, sedimentary environment, and formation process of the bauxite series and explore the characteristics and formation mechanisms of reservoirs within the bauxite series. Based on these effects, we establish the formation and distribution patterns of the bauxite series, as well as reservoir development model. The results are as follows. (1) The bauxite series in the Longdong area primarily originate from carbonate rocks in the Lower Paleozoic Majiagou Formation. (2) The bauxite series is developed in a warm and rainy sedimentary environment under marine regression. In the vertical direction, the rock series consists of funnel-shaped, lenticular, and stratoid lithologic assemblages. The lithologic assemblages of different origins are controlled by factors such as volumetric space of karsts and leaching degree. Regarding the planar distribution, the bauxite series is dictated by paleogeomorphology. For instance, karst-related sinkholes in the highlands house funnel-shaped bauxite series with considerable vertical thickness; karst-related terraces in the slope zone predominantly contain lenticular bauxite series with a broad and continuous lateral distribution, and the depression zones show the development of stratoid bauxite series. (3) The karstrelated negative landforms serve as a key factor controlling the development of bauxite series, while drainage channels are critical to the development of dominant reservoirs. Furthermore, as micro-paleogeomorphic units, including sinkholes in the highlands and terraces in the slope zone, provide favorable conditions for the development of dominant reservoirs within the bauxite series. These findings provide a basis for seeking high-quality reservoirs within the bauxite series in the Ordos Basin and can be used as a reference for exploring new types of natural gas.