Geological controlling mode of coalbed methane enrichment controlled by vortex compression-torsion tectonic：Taking the Weijiadi Coal Mine in the eastern part of the Hexi Corridor belt as an example

The small faulted coal basins are extensively developed in the eastern part of the Hexi Corridor belt in the northwest region of China，and the coal seams exhibit large thicknesses and rapid variations. Affected by the superimposition of the Longxi vortex tectonic activity in the Mesozoic and Cenozoic eras, the coalbed methane (CBM) distribution is uneven and difficult to manage. This research takes the Weijiadi Coal Mine in the Jingyuan Coalfield as the research area, based on drilling and logging data of CBM, as well as tectonical and hydrological data obtained during exploration and production, to systematically analyze the occurrence characteristics of CBM and the tectonic control effect of it in the research area. The following main understandings are obtained: Controlled by the “vortex extrusion effect” and “fault blocking effect”, the study area forms high-angle reverse faults and anticline-syncline axes with east-west echel arrangement between them. Divided by the F3 fault, the study area consists of two sets of tectonic systems: the southwestern compressive tectonic area and the northeastern compressive-torsion tectonic area. The CBM content in all areas is gerenally low, with significant variations among them. The enrichment of CBM is mainly controlled by the positive factors such as the mudstone smearing and sealing of the fault plane, the effective sealing of the roof, the strong adsorption and sealing properties of the tectonically deformed coal with strong deformation. Three CBM enrichment models have been constructed: the CBM enrichment mode in the closed fracture zone of the transition section of the vortex tectonic, the CBM enrichment model in the high position of the reverse fault’s footwall, and the CBM enrichment model in the core of the syncline. Among them, the strata near the fault location show significant deviation, and there is a transitional section of twisting in the middle position, the direction of the current maximum horizontal principal stress is perpendicular to the fracture within the vortex tectonic structure, determines the high degree of fracture closure in the torsion transition section, resulting in the enrichment of CBM. The understandings obtained in this study provide theoretical support for gas control and CBM development in the vortex compressive-torsional faulted basins in the eastern part of the Hexi Corridor belt.