Preliminary research on CBM enrichment models of low-rank coal and its geological controls: A case study in the middle of the southern Junggar Basin, NW China

To date, prospecting work on low-rank coalbed methane (CBM) resources in the middle of the southern Junggar Basin is still in the primary stage, and only a few CBM exploration wells or pilot wells have been deployed in local regions. Systemic understanding of CBM reservoir-forming conditions and geological controlling factors is lacking in the study area, resulting in the mismatch between CBM well deployment and actual geological conditions, as well as poor exploration efficiency. In this paper, the geological controlling effects of the structure, sedimentation, and hydrogeology on CBM enrichment are systematically discussed for the first time, based on the early CBM exploration achievements. The results show that the Xishanyao coal and the Badaowan coal are developed in the upper and lower part of the neutral surface of a fold, respectively. The reservoir-forming conditions of the Badaowan coal are not discussed in this paper due to its poor development. The Xishanyao coal that developed in the axial part of the syncline is most beneficial to CBM enrichment with concentrated extrusion stress and great methane adsorption capacity, while the axial part of the anticline is not favorable for CBM preservation with large tensional stress. The gas content of the Xishanyao thick seams developed in the syncline is higher (average of 4.63-6.34 m3/t) than that in the monocline (average of 2.84-4.56 m3/t). Reverse faulting is more beneficial to CBM enrichment than normal faulting, due to the better sealing capability. The gas content of the Xishanyao coal is obviously influenced by the coal thickness and its roof lithology. The hydrodynamic conditions and total dissolved solids (TDS) values of coalbed water range greatly on regional scale, which leads to a deeper methane weathering zone in the middle-west areas (>1119.62 m) than the eastern Liu-huanggou areas (<501.71 m) and have an important influence on exploration target optimization of CBM exploration wells. Combined with the geological characteristics of the structure, sedimentation and hydrogeology, three CBM enrichment models are proposed in this paper (i.e., broad fold model, northward monocline model and overlying composite model). The reservoir-forming processes and development positions of these CBM enrichment models are discussed systematically to provide a scientific basis for selecting CBM exploration target zones.