Evaluation of geological features for deep coalbed methane reservoirs in the Dacheng Salient, Jizhong Depression, China

• The coal rank is controlled by both the plutonic and magmatic metamorphism.
• The clay minerals in coal can add the BET surface area of coal.
• The deep CBM content increase first and then decrease from shallow to deep.
• The accumulation model of the deep CBM is explained.

The Dacheng Salient of the northeast Jizhong Depression is one of the most promising areas for deep coalbed methane (CBM) development in China. This work investigates the Permo-Carboniferous (C-P) Taiyuan and Shanxi Formation coals and CBM geology and gas accumulation characteristics in the Dacheng Salient based on field data from 24 wells and laboratory measurements of eight coal samples and eight mudstone and sandstones from exploration well D1-1.The results show that the coal rank in the Dacheng Salient is a high-volatile C bituminous coal to low-volatile bituminous coal (0.54–1.85% Ro) controlled by the magmatism in the region. The coals vary from 2.94 to 8.58% in porosity and 0.263 m2/g to 2.6 m2/g in BET surface area. The in-situ permeability is low, normally less than 0.5 × 10− 3 μm2.Methane isothermal adsorption measurements of five coal samples reveal that their maximum adsorption capacity (on an as-received basis) varies from 17.13 to 25.98 m3/t, with an apparent relationship with ash yield. The roof and floor of coal seams are mainly mudstone, with minor quantities of sandy mudstone and carbonate; all have the ability to be suitable cap rocks with adequate sealing capacity and large breakthrough pressure. The in-place gas content range from 10.23 to 1.41 m3/t with a decreasing trend from No. 3 to No. 6 coal seam group in the D1-1 well due to effects by the active ground water near the east Jinghai-Dacheng fault. However, the gas content and the gas saturation increase with burial depth and ground water salinity, and decrease with coal rank. Finally, based on a comprehensive analysis of coal thickness, gas content, hydrogeology conditions, roof, floor, and depth properties, this study reveals that the best prospective target areas for CBM production are the south-central parts of the Dacheng Salient at1200 ~ 1600 m in depth