Optimal selection of favorable production areas for low-permeability sandstone gas reservoirs in the Benxi Formation of the Upper Carboniferous in Ordos Basin

In recent years, as natural gas exploration and development have intensified, several high-product gas reservoirs of the Upper Carboniferous Benxi Formation have been discovered in the central and eastern parts of the Ordos Basin. Multiple gas wells have recorded open flow capacity exceeding 1.0 × 10 6 m 3 , indicating significant development potential. However, the proportion of high-product gas wells (daily gas production >5.0 × 10 4 m 3 ) remains generally low, with an uneven geographical distribution. Therefore, identifying the enrichment factors contributing to high product in the Benxi Formation gas reservoirs and enhancing targeted development are critical issues that need to be urgently addressed. The article focus on the Benxi Formation gas reservoir in the Hengshan Zizhou area, located in the central-eastern part of the basin. By enhancing the understanding of the formation conditions and migration patterns of the Benxi Formation gas reservoir, it systematically analyzes the sedimentary environment, source rock characteristics, reservoir conditions, and cap rock types. Evaluation criteria for favorable areas within the Benxi Formation reservoir are established based on the natural gas accumulation process, providing guidance for selecting optimal target areas for production. Research has shown that: ① The coal seam source rocks of the Benxi Formation in the central and eastern parts of the Ordos Basin are widely developed, providing sufficient gas source for the Benxi Formation and even the entire Paleozoic reservoir. ② The Benxi Formation belongs to a self-generating and self-storing natural gas reservoir, where gas generated from the coal seam source rocks at the top of the formation preferentially diffuses and migrates into the underlying low-permeability sandstone, accumulating to form the reservoir. ③ The average porosity of the Benxi Formation sandstone reservoir is 6.27 %, with an average permeability of 1.5mD. It is primarily composed of Class I and II reservoirs, exhibiting good reservoir performance. ④ The overlying direct cover layer forms three reservoir combinations with the Benxi Formation coal seam and reservoir: mud-coal-sand, lime-coal-sand, and sand-coal-sand. Limestone provides the best sealing conditions, followed by mudstone. This research has clarified the direction for natural gas exploration and development in the Benxi Formation of the central and eastern Ordos Basin, and has optimized and evaluated target areas for production, offering significant guidance and reference for the next stage of natural gas scale development. • The Upper Carboniferous Benxi Formation in the Hengshan Zizhou area consists of various lithologies, including sandstone, shale, limestone, bauxite, and coal seam, reflecting the frequent shifts in sedimentary environments from shallow sea to coastal rivers during the Benxi period. • Benxi Formation has good natural gas accumulation conditions. The No. 8 coal seam, which is widely distributed and thick within the study area, possesses high organic matter content and maturity, providing an ample gas source for gas accumulation. Despite the relatively small scale and lack of continuity of the sand bodies, they are influenced by the combined effects of rivers and tides, offering favorable porosity and permeability conditions, thus laying a solid foundation for natural gas accumulation. Notably, there are significant differences in the sealing capabilities of various rock layers in the upper Taiyuan Formation. Among them, limestone and mudstone exhibit strong sealing capabilities, serving as effective barriers for natural gas accumulation. • From the perspective of natural gas reservoir formation, multiple evaluation parameters—such as source rocks, reservoirs, and cap rocks—were selected. Using the comprehensive evaluation index (REI), the Benxi Formation in the study area is classified into four categories: high production areas, medium production areas, low production areas, and non-production areas. Additionally, 993 km 2 of favorable production areas, with a resource volume of 4.27 × 10 10 m 3 , have been identified to ensure sufficient preparation for future large-scale production.