Objective and Method The exploration of the Luohe Formation, located on the roof of the Huanglong Jurassic coalfield, faces challenges such as a lack of consensus on the sedimentary environments of sandstone aquifers and unclear lithologies with water yield properties in the aquifers. To address these challenges, this study investigated sandstones in the Luohe Formation in the Gaojiapu Coal Mine, Shaanxi Province. Specifically, a comprehensive analysis of core samples with different lithologies was conducted, including colors, lithologies, lithologic assemblages, the surface morphology of quartz grains, mineral components, and sedimentary structures and textures. Based on analytical results, as well as the grain size parameters, this study comprehensively analyzed the sedimentary environments of the samples. Through gas measurements and weighing under water saturation, this study measured the permeability, porosity, and saturated water content of sandstones with different lithologies and sedimentary environments throughout the Luohe Formation. Using nuclear magnetic resonance (NMR) and computed tomography (CT) scanning, this study investigated the microscopic pore structures of six types of sandstones, including gravelly and coarse-grained sandstones, in the Luohe Formation. By combining the characterization results of sedimentary environments, porosity, and permeability, this study comprehensively explored the microscopic water control patterns of various intervals of the Luohe Formation.

Results and Conclusions The upper part of the Luohe Formation is purplish-red or brownish-red, interbedded with grayish-white gravelly sandstones. In contrast, its middle part is purplish-red or brownish-red, interbedded with white gravelly sandstones, while its lower part is brownish-red, interbedded with thin layers of grayish-white gravelly sandstones at the bottom. The detrital grains of the sandstones exhibit saucer-shaped impact pits, and large planar cross-bedding is observed in the sandstones, indicating an aeolian sedimentary environment. Primary minerals in the sandstones include quartz, feldspar, calcite, and clay minerals. Besides, at the bottom of the Luohe Formation in the study area, brownish-red mudstones are found to contain substantial analcimes. Therefore, the lower part of the Luohe Formation exhibits typical lacustrine sedimentary facies, and a sedimentary system consisting of alternating aeolian and aquatic deposits occurs throughout this formation. The sandstones in the formation exhibit saturated water content ranging from 2.87% to 11.84%, porosity from 6.87% to 23.91%, and effective porosity from 2.32% to 12.79%. They primarily show large pores, followed by mesopores, with micro-fractures observed in fine-grained and coarser sandstones. Overall, the sandstones have pore sizes primarily ranging from 34.57 μm to 180.00 μm. Primary lithologies with water yield properties include coarse- to fine-grained sandstones. In contrast, silty sandstones exhibit weak water yield properties, while the aqueous gravelly sandstones show the weakest water yield properties. The upper part of the formation exhibits a lower proportion of gravelly sandstones than the middle part. This part consists primarily of coarse- to fine-grained sandstones, with silty sandstones and mudstone roughly lacking. Therefore, the upper part features strong water yield properties. The upper and middle parts contain thick layers of the alluvial facies as markers, exhibiting the lowest porosity, effective porosity, and saturated water content. Furthermore, the middle part is dominated by medium- and fine-grained sandstones. In this part, the aqueous medium-grained mudstones show low porosity and effective porosity, and the proportions of mudstones and silty sandstones increase significantly. Consequently, the middle part shows weaker water-yield properties than the upper part. The lower part is dominated by thick layers of mudstones, interbedded with silty sandstones, also featuring weak water-yield properties. The results of this study provide a theoretical basis for identifying the water yield properties of the Luohe Formation while also serving as a guide for water hazard prevention and control in mines.