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 × 10⁶ m³, indicating significant development potential. However, the proportion of high-product gas wells (daily gas production >5.0 × 10⁴ m³) 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.