Occurrence, Origin, and Infill Modification Effects of Minerals in Deep Coals in the Ordos Basin, China

Abstract

Minerals in deep coals contribute to coal reservoir heterogeneity and influence coalbed methane enrichment by modifying the physical properties of the pore-fracture system. This study has used multiple qualitative and quantitative analytical techniques to characterize pore-fracture system and occurrences of minerals in deep coals in the Daji block, Ordos Basin. The deep coals with high rank and medium-low ash yield show a complex dual pore-fracture structure with obvious cross-scale effects. The pore-fracture system in deep coals is of complex genesis and diverse morphology, and open fractures of varying scales may communicate pores to form complex pore-fracture networks. Mineral phases in deep coals consist mainly of kaolinite, calcite, and pyrite, with minor occurrences of other minerals (quartz, Illite, siderite, collophanite, bauxite minerals, and halite). The modes of mineral occurrence, including syngenetic and epigenetic origins, depended on the mineralogical genesis associated with coal-forming process. Different mineral phases and infilling modes modified the pore-fracture system to varying extents. Overall, mineral occurrences in the deep coal seams have reduced porosity but increased permeability to some extent. Higher clay and carbonate mineral contents is collectively associated with lower porosity; however, higher clay and sulfide mineral contents is collectively associated with higher permeability.