Coalbed Methane Enrichment Characteristics and Exploration Target Selection in the Zhuozishan Coalfield of the Western Ordos Basin, China.

The Zhuozishan coalfield at the western margin of the Ordos Basin is one of the main coal-mining areas in China, and recent explorations have revealed the great potential for coalbed methane (CBM) resources in its Carboniferous and Permian strata. In this paper, the controlling factors of CBM enrichment of the major coals are studied in this coalfield and the CBM resources are estimated based on the analysis of the coal petrology and compilation of literature data on the gas content. The result of the coal petrology analysis of 10 samples shows that the vitrinite content of No. 16 coal (71.9-77.3%) is higher than that of No. 9 coal (59.1-65.1%), and the inertinite content of No. 16 coal (18.9-23.5%) is lower than that of No. 9 coal (30.1-34.9%). The R o,max value of No. 16 coal (1.18-1.35%) is higher than that of No. 9 coal (1.04-1.13%), and both coals are of medium rank. Due to greater thickness, deeper burial depth, and better coal petrology characteristics, the No. 16 coal seam of the Taiyuan Formation is selected as the major coal seam for CBM resource estimation, which has a thickness of 1-6 m and a present-day burial depth of 200-1100 m. The gas content of this coal seam varies mostly between 4 and 10 m 3 /t. Positive correlation between the coal seam thickness as well as present-day burial depth and the gas content suggests that the thick and deeply buried coal seams are favorable for CBM preservation. The ash yield shows an insignificant negative correlation with the gas content, indicating that ash yield is not an important factor for CBM enrichment. The syncline hinges located below the thrust zones show higher gas content due to greater burial depths. In contrast, the anticline hinges at shallower depths tend to have lower gas contents. Based on the combined information about sedimentary environments, structural patterns, and hydrogeology, two CBM accumulation models are put forward in the study area that include syncline-hydraulic plugging below thrust nappe and fault-confined aquifer plugging. The volumetric method is used to estimate the CBM resources, and results indicate that the CBM resource in the whole coalfield is 428.78 × 10 8 m 3 , and the total resource abundance is 0.74 × 10 8 m 3 /km 2 . Two favorable areas for the CBM exploration are optimized based on the resource amount and resource abundance. One of the favorable areas is the Kabuqi area in the northern part of the coalfield, and another is the Baiyunwusu area in the central part of the coalfield. These two areas will be the CBM priority exploration areas at the western margin of the Ordos Basin.