Based on the sedimentary background, this study combines petrology and XRD analysis to investigate the types and characteristics of Ordovician dolomite in North China, and discusses their origins, revealing the dolomitization mechanisms of the Ordovician. The results show: ① According to the crystal morphology and grain size of dolomite, five types of dolomite (LD1 ~ LD5) have been identified in the Lower Ordovician Liangjiashan Formation, while three types of dolomite (MD1~ MD3) have been recognized in the Middle Ordovician Majiagou Formation; ② The larger the dolomite crystal grain size, the stronger the degree of dolomitization and the higher the structural order. Meanwhile, a larger cell parameter (c/Å) value suggests a more disordered lattice and a faster dolomite crystallization rate; ③ LD1 and MD1 formed through supratidal evaporative pump dolomitization, while LD2 and MD2 resulted from reflux dolomitization. LD3 through LD5, along with MD3, formed during various stages of burial dolomitization; ④ During the sedimentation stage, a continuous sea-level decline influenced the initial distribution of the Liangjiashan Formation dolomite. Additionally, three regression stages in the Middle Ordovician were the primary periods for the formation of the Majiagou Formation dolomite, which was subsequently altered primarily by burial dolomitization. This study provides a new perspective on the dolomitization process of the Ordovician in North China.

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.