Geological characteristics and favorable area evaluation of deep coalbed methane in Wangfu fault depression, southern Songliao Basin

The Wangfu fault depression in the southern Songliao Basin is rich in coalbed methane (CBM) resources. However, CBM exploration in this area has been restricted by the low-level exploration of deep CBM and a lack of systematic studies on the geological characteristics of CBM, primary factors controlling CBM enrichment and accumulation, and the selection of favorable targets. Therefore, based on the systematical analysis of the geological characteristics of coal-bearing strata, this study identified the major factors controlling deep CBM accumulation, established the index system for favorable area selection, and predicted the favorable areas of CBM enrichment. The results show that: (1) Macroscopically, coals in the Huoshiling Formation in the Wangfu fault depression can be primarily categorized into bright and semi-bright coals, possessing the characteristics of low moisture, low volatile constituents, and low ash content. They exhibit vitrinite reflectance > 1.8%, average porosity of 5.01%, and average gas content of 21.80 m³/t (calculated based on well CS38). (2) The sedimentary environment, filled and leveled through volcanic eruption, of the Huoshiling Formation in the Wangfu fault depression is conducive to the formation of coal seams. Specifically, coal seams primarily occur between the double steep slope zones and in the shallow water areas of the central and eastern Wangfu fault depression, which were formed by the filling and leveling effects of volcanic eruption. In this manner, the coal formation mode featuring filling and leveling is formed. Additionally, the presence of large-area thick mudstone roofs and closed faults allow for CBM enrichment and preservation in the fault depression. (3) Class I favorable areas in the Wangfu fault depression predominantly occur in the northern and central parts of the fault depression, with coal seam thicknesses > 5m, fault throw <50 m, cover thicknesses > 80 m, and gas logging-derived total hydrocarbon values > 15%. These suggest great potential for exploration and exploitation. Classes Ⅱ and Ⅲ favorable areas are mainly found in the northern and southern parts of the fault depression, with coal seam thicknesses < 5m, cover thicknesses < 80 m, and gas logging-derived total hydrocarbon values < 15%. Weak-sealing or open faults and greatly different cover thicknesses pose risks to CBM exploration and exploration in these areas.