Pore structure and adsorption capability of marcolithotypes in the Weizhou mining area, western margin of Ordos Basin

Differences in the pore and fracture systems of four marcolithotypes (bright，semi-bright, semi-dull and dull) in coal reservoirs affect the quality of coalbed methane (CBM) and constrain exploration and development of CBM. In this paper, vitrinite reflectance (R _O), maceral composition, proximate analysis, scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), low-field nuclear magnetic resonance (NMR) and high-pressure isothermal adsorption experiments and the mathematical method of fractal dimension were carried out on high-rank coal samples of different marcolithotypes from the Weizhou mining area in the western margin of Ordos Basin. The results show that from bright coal to dull coal, the R _O and vitrinite content gradually decrease, while the mineral content and A _ad gradually increase. SEM images show that the bright and semi-bright coals develop open fractures, whereas semi-dull and dull coals develop closed fractures. According to pore size distribution established by MIP and NMR, the proportion of seepage pores gradually increased from bright coal to dull coal, and MIP fractal dimension of seepage pores (D _M2; 2.3352, 2.3532, 2.3755 and 2.4727, respectively) and NMR fractal dimension of seepage pores (D _N2; 2.8767, 2.9142, 2.9297 and 2.9981, respectively) show that the structure of the seepage pore is progressively more complex from bright coal to dull coal. In addition, percentage of NMR adsorption pores progressively increases from bright coal to dull coal (64.96%, 76.64%, 82.04% and 89.67%, respectively), but total porosity of bright coal (6.74%) is much greater than that of dull coal (1.34%), which results in that bright coal also has the strongest CH₄ adsorption capability (Langmuir volume is 22.898 cm³/g).