Petrographic controls on pore and fissure characteristics of coals from the southern junggar coalfield, northwest China

The productive potential of coalbed methane projects is controlled by pore and fissure characteristics, which are intrinsically related to coal petrology. This work attempts to identify the influence of petrographic factors on the development of pore and fissure systems in the southern Junggar Coalfield, Northwest China. Here, Middle Jurassic coal (lignite and subbituminous) petrology in coal seam No. 45 of the southern Junggar Coalfield (SJC) is studied with respect to the characteristics of pore and fissure structure with the aid of optical microscopes, scanning electron microscopy, mercury intrusion porosimetry, and nuclear magnetic resonance analysis. Maceral analysis shows coals at the SJC are dominated by vitrinite (38–87 vol %), with moderate quantities of inertinite (1–28 vol %) and liptinite (0.5–30 vol %). Decomposition of plants occurs under slightly oxic–anoxic conditions, with good tissue retention. Four types of coal facies are classified using petrographic indices, comprising (1) lower delta plain marsh, (2) lower delta plain fen, (3) upper delta plain wet forest swamp; and (4) piedmont plain moor. Pores and fissures are generally observed in telinite, collotelinite, fusinite, and semifusinite in SJC coals, indicating that the generation of pores and fissures is strongly influenced by coal macerals. Pore and fissure structures of coals in coal facies (1) appear weakly connected, whereas those in coal facies (2) reveal good connectivity. Coals in coal facies (3) and (4) show moderate connectivity between pore and fissure structure. Therefore, pore and fissure structures are significantly controlled by coal facies. This work provides practical recommendations and implementation methods for petrological studies in future coalbed methane exploration/development in the SJC. This study also serves to predict the physical properties of pores and fissures and interpret the control mechanism of coalbed methane production using coal petrology.