Structural controls on joint development in the southeastern margin of the Ordos basin

Abstract

The permeability of a coal reservoir fundamentally depends on joint development which is primarily structurally-controlled. However, it is difficult to obtain joint data of underground coal seams. Therefore, in order to provide a theoretical basis for predictions of joint development and the location of high-permeability zones of underground coal seam, joint orientation and intensity in beds outcropping above the coal seams were measured. The study area is located on the southeastern margin of the Ordos basin of central China. Field work shows that five sub-vertical joint sets are developed in the study area. Sets III (striking 145°) and V (striking 45°) are the most dominant. It is found that joint characteristics, i.e., orientation and intensity, are controlled by the structure. For any particular lithology, joint intensity not only shows a negative power function relationship with bed thickness but also is largely affected by structure and varies from high–low–high–low when traced from east to west across the area, reflecting the location of the deformation belts, i.e., fold and fault belts. The Yanshan and Himalayan tectonic events which occurred after the Middle Jurassic are critical periods for joint formation. Sets I and II are induced by an early, E-W oriented Yanshan compression; Sets III and IV are the outcome of a later, NW-oriented Yanshan compression; set V the result of a NE-oriented compression linked to the Himalayan event. The permeability of the No. 5 coal seam of the Shanxi Formation is found to increase with increasing joint intensity in the overlying sandstone beds.