PRELIMINARY RESEARCH ON CBM ACCUMULATION 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 parameter wells or production-test wells have been deployed in local regions. Preliminary analysis suggests that systemic understanding of CBM reservoir-forming conditions and geological controlling factors is obviously 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 accumulation in the study area are systematically discussed for the first time based on coalfield prospecting reports and the early CBM exploration achievements. The results show that Xishanyao coal and Badaowan coal are developed in the upper and lower part of the neutral surface of a fold, respectively; the reservoir-forming conditions of Badaowan coal are not discussed in this paper due to its poor development. Xishanyao coal that developed in the axial part of the syncline is most beneficial to CBM enrichment with concentrated extrusion stress and great methane adsorbability, while the axial part of the anticline is bad for CBM preservation with large tensional stress. Reverse faulting is more beneficial to CBM enrichment than normal faulting, due to the better sealing capability. The main coal seam of the Xishanyao Formation is mainly developed in transgressive system tract (TST), followed by low-stand system tract (LST), and little in high-stand system tract (HST). 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 parameter-wells. Combined with the geological characteristics of the structure, sedimentation and hydrogeology, three CBM accumulation models are proposed in this paper (i.e., broad fold, northward monocline, and imbricated fan-shaped structure model), and the reservoir-forming processes and development positions of these CBM accumulation models are discussed systematically.