Low-permeability sandstone reservoirs containing low resistivity pays (LRPs) are widely encountered within the Triassic Yanchang Formation in the Jiyuan area, Ordos Basin, China. The resistivity of these reservoirs can be as low as 2·Ω m, leading to misinterpretation by conventional logging methods. Previous studies have primarily focused on resultant factors such as conductive minerals and clay minerals. However, how different diagenetic processes couple and control low resistivity remains unclear. This study focuses on the Chang 2 interval of the Yanchang Formation, integrating petrology, hydrochemistry, and machine learning (ML) to address these challenges. Results reveal that compaction-induced concentration of formation water and feldspar dissolution are critical controls on LRP formation. Dissolution processes not only release ions such as Na⁺ and Cl⁻, increasing formation water salinity, but also produce authigenic clay minerals like kaolinite, which obstruct pore throats and trap highly saline irreducible water, significantly reducing reservoir resistivity. Additionally, dissolution generates larger pores (diameter >10 μm), enabling more stable conductive networks and further lowering resistivity. In terms of fluid identification, the two newly constructed fluid-sensitive parameters exhibit strong explanatory capability, and the RF-PFN model outperforms other ML methods in distinguishing fluid properties. Prediction case studies show that LRPs preferentially develop in areas characterized by higher sandstone content, high porosity, and intensive dissolution effects. These findings provide critical insights and practical frameworks for evaluating and predicting similar low-permeability sandstone reservoirs worldwide.

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.