Application of seismic curvature attributes in the delineation of coal texture and deformation in Zhengzhuang field, southern Qinshui Basin

Abstract

Coal texture refers to the in situ particle size, morphological characteristics, and structural components of coal beds, and it determines the permeability and producibility of a coalbed methane (CBM) reservoir. Coal texture is commonly classified into undeformed coal (with primary coal texture) and deformed coal (with increasing degree of deformation, including cataclastic coal, granulated coal, and mylonitized coal). Geophysical logging has recently been proven effective for detecting coal texture in boreholes, but it is inefficient for establishing a regional distribution where there have been only limited amounts of drilling. In contrast, seismic attributes have high lateral extent and an advantage in terms of characterizing fine structures (even minor faults with throws of 2–3 m) and other strata deformations. By comparing seismic curvature data to the results of 221 core samples from the Zhengzhuang field in the southern Qinshui Basin, we provide a seismic curvature attribute-based coal texture evaluation method. Evaluation of the results shows that the coal texture types of the no. 3 coal seam include undeformed coal, cataclastic coal, and some granulated coal. The undeformed coal is mainly located in the western and northwestern parts of the study area. The cataclastic and granulated coals are mainly distributed in the southern and eastern parts of the study areas. The permeability of no. 3 coal seam is weakly correlated with undeformed coal, and it has a high negative correlation with deformed coal. Permeability has a strong negative correlation with the maximum curvature attribute of the no. 3 coal seam. It is suggested that the in situ stress distribution combined with the curvature controlled the local development of the coal texture types and thus resulted in regional differences in permeability. Finally, the distribution characteristics of coal textures of no. 3 coal seam in the study area are divided into four types, in which the undeformed-cataclastic coal-dominated area (located in the midwestern part of the study area) is the most favorable area for CBM production.