Occurrence and genesis of minerals and their influences on pores and fractures in the high-rank coals

Abstract

Pores and fractures constitute connected networks for fluid migration and production in or from coal reservoirs. The existence of minerals in coal not only increases the heterogeneity of the reservoir but also influences the development and connectivity of pores and fractures in the coal. By applying X-ray diffraction environmental scanning electron microscopy and field emission scanning electron microscopy with an energy dispersive X-ray spectrometer type, the occurrence modes and geneses of minerals were investigated in representative high-rank coals from the Qinshui basin, North China. Additionally, their impact on the development and connectivity of pores and fractures was explored in the high-rank coal. The results show that illite and kaolinite are the dominant minerals, comprising 57.74% of the mineral volume on average. The minerals fall into three genetic types, including detrital, syngenetic, and epigenetic, of which syngenetic minerals are the most common. The occurrence modes of the minerals depend on their geneses. The detrital minerals are mostly granular; the syngenetic minerals are agglomerates, stripped aggregates, and disseminated fine particles, and the epigenetic minerals are fracture fillings. At the micro-scale, the influences of the minerals on pores and fractures in the high-rank coals are mainly denoted by micro-fracture fillings, development of mineral pores, and generation of differential deformation pores and fractures. Differential deformation pores and fractures appear at the boundaries between clay and brittle minerals or clay and organic matter due to their physical and chemical property differences related to the regional thermal metamorphism of the coal. The coal samples are always connected by thermogenic pores and mineral pores, favoring connectivity of the pores and fractures in the coal. Moreover, the dissolved pores and intercrystalline pores are largely interconnected. This study focused on the effects of minerals on the pore and fracture structure of the high-rank coal reservoir. The results provide a better understanding of the migration networks and fluid mechanisms related to complex pores and fractures, particularly in high-rank coal reservoirs.