Geological controls and coalbed methane production potential evaluation: A case study in Liulin area, eastern Ordos Basin, China

The Liulin area in the eastern Ordos Basin is one of the optimum areas for medium-rank coalbed methane (CBM) exploration and development in China. This work investigated the CBM geology and accumulation characteristics of No.4 coal in the Permian Shanxi Formation in the Liulin area based on data from 29 coal mining wells, 25 CBM wells and 14 coal samples. The results show that coal rank (medium volatile bituminous and low volatile bituminous) varies laterally, with the maximum vitrinite reflectance (Ro, max) ranges from 1.23 to 1.90%. Coals are dominated by vitrinite (47.9-90.5%); followed by inertinite (7.7-50.5%) and liptinite (0-0.6%). Minerals account for only a small proportion (0.4%-16.5%, avg. 4.7%). The results of well tests show that the coal permeability ranges from 0.02 to 3.44 mD, and the reservoir pressure ranges from 2.58 to 9.26 MPa. Most coal pores are less than 100 nm in diameter, and are favorable for gas adsorption but unfavorable for gas flow. Microfractures of coals are characterized by dendritic and step-shaped structures, relatively good connectivity, directionality and infrequent mineralization. The dominated types of microfractures are type C (less than 5 μm wide and more than 300 μm long) and type D (less than 5 μm wide and 300 μm long), with a large span of frequency ranging from 18 to 78 per 9 cm2. Methane adsorption isothermal measurements reveal that their maximum adsorption capacity (dry ash-free) vary from 21.36 to 26.38 m3/t. The in-place gas content is generally 4.93-14.96 m3/t. In combination with the geological information, the data reveals that structure properties, coal roof lithology and hydrogeology conditions have important influences on gas accumulation, preservation and enrichment. Furthermore, integrated geographical information system (GIS) and analytical hierarchy fuzzy prediction method (AHP) are used to evaluate the CBM production potential in the Liulin area. The best prospective target area for CBM production is forecasted to be located in the central slope belt (near southern Beilijiyuan and Yangjiayu areas) of the Liulin area.