Coal seam porosity and fracture heterogeneity of marcolithotypes in the Fanzhuang Block, southern Qinshui Basin, China

Abstract

Marcolithotypes play a significant role in coal seam porosity and fracture heterogeneity. The optical microscopy, CO2 and N2 adsorption, and X-ray CT were used for reconstructing the pore-fracture system of different marcolithotypes. The results show that the microfractures are well developed in the bright coal with the characteristics of high fracture density, good connectivity, and strongest fractal dimension, corresponding to the highest permeability and the best seepage capacity. The adsorption capacity measured with N2, CO2, and CH4 indicates that the specific surface area of micropores (<2 nm) increases linearly from the bright to the dull coal, and the bright coal is favorable for gas adsorption by providing more available surface sites. The difference in physical properties of various macrolithotypes causes different logging response characteristics, including the compensated density (DEN), the gamma ray (GR), and the acoustic time (AC). Based on that, the index (N = AC/(GR × DEN)) was put forward to forecast the distribution of coal macrolithotypes quantitatively (N ≤ 1.3, Partings; 1.3 < N ≤ 3, Dull coal; 3 < N ≤ 5, Semi-dull coal; 5 < N ≤ 8, Semi-bright coal; N > 8, Bright coal), and its accuracy was verified by coal cores from different areas, i.e. Fanzhuang, Zhengzhuang, and Hancheng blocks.