Study on impact of multi-period tectonic movement on deep coal reservoir physical properties

The physical properties of coal reservoirs,i.e.,porosity,permeability,fractures,and coal structures,restrict the exploration and development of coalbed methane. The developmental characteristics of physical properties are controlled by the tectonic evolution process. Based on the occurrence statistics of field joints and natural fractures from loggings,using staging and matching of joints (cracks) and wave superposition principle,the tectonic stress field and structural features of Yanshanian period and Himalayan period are inversed in the Shizhuang area of southern Qinshui Basin. The deformation degree of the coal seam in geological history period was characterized through the largest tectonic curvature (r),and the influence on coal reservoir physical properties of multi period tectonic activities was discussed. Research has shown that the joint (fracture) trend has five advantageous directions,which are NNE (10～25 degrees),NEE (45～85 degrees),NWW (85°～110°),and NW (135°～145°),NNW direction (160°～175°).The inclination angle is large,mainly distributed between 50～90 degrees,and it is 76% greater than 70 degrees. Since the late Paleozoic,the NWW-SEE trend in the Yanshanian and NWW-SEE-direction tensile stress in the early Xishantectonic movement has caused the complex shape of the coal seam floor. The physical properties of the coal seam were quite different. The maximum deformation degree of the earth's history controls the actual damage degree of the coal body. With the increase of history's largest deformation degree,fractures and the broken degree of coal increased. When r<48×10-6 m-1,primary structural coals were developed; when 48×10-6 m-1150×10-6 m-1,granulated coals were developed; mylonite coals were barely developed. With the increase of the historical maximum deformation degree in a certain range,coal reservoir permeability shows a trend of increasing first and then decreasing. Its boundary value is at 110×10-6 m-1.The porosity of coal seams is mostly controlled by burial depth and stress characteristics,so the influence of deformation degree of the coal on the porosity was not obvious. This presents an applicable method for evaluating the influence of tectonic movement on the physical properties of coal during different times and would benefit the evaluation of coalbed methane reservoirs in Shizhuang area as well as other coal basins in the world.