Evaluation of structured coal evolution and distribution by geophysical logging methods in the Gujiao Block, northwest Qinshui basin, China

Structured coal is characterized by tectonically caused features (e.g. a cataclastic or mylonitic structure with small particle sizes), which is crucial for coal exploitation and coalbed methane (CBM) development. To quantitatively identify high-resolution coal structures, a destruction F-Index that determined through multiple geophysical logging with principal component analysis (PCA), was proposed to divide coal structure into five types: I-undeformed, II-transitional, III-cataclastic, IV-granulated and V-mylonitized coals. The undamaged coals (including types I and II) that dominate the target area are normally concentrated in low-angle and gentle strata, while the structured coals (including types III, IV and V) are only distributed along syncline axial parts and fault zones. Furthermore, a deformation D-Index, to quantify the coal deformation degree in single well, was set from 1 to 5, which indicates that the larger the number is, the higher the degree is. The results showed that a negative relation between parting (normally shales or mudstones) content and D-Index, which suggests that the parting provides the resistance for coal seam deformation. Furthermore, coal deformation degree increases with the burial depth and thickness of coal seam. From the Hercynian orogeny to Himalayan orogeny, coal seams of the research area have experienced three phases of deformation. Most structured coals that distribute along syncline axial parts and fault zones formed during this process, and the undamaged coals are distributed in the internals of extensional tectonic belts (e.g. graben and horst). In addition, for a local area, the regional tectonic styles may also determine the structured coal distribution.