Characteristics and genesis of vertical heterogeneity in a coal seam of the Carboniferous Benxi Formation, eastern Ordos Basin: A case study of well M172

The No.8 coal seam of the Carboniferous Benxi Formation in the Ordos Basin exhibits great thickness and extensive, continuous distribution, establishing itself as a favorable interval for the exploration of deep coal-rock gas in this region. In this study, we examine the characteristics and genesis of the vertical heterogeneity in the No.8 coal seam by conducting tests and analysis of the macroscopic characteristics, properties, reservoir characteristics, and gas content of coal samples collected from key well M172 in the eastern Ordos Basin. The results indicate that the No.8 coal seam exhibits pronounced vertical heterogeneity with dominated semi-bright coals, high ash content and high inertinite content in the lower section, indicating a slightly oxidizing coal-forming environment. In contrast, its middle and upper parts contain bright and semi-bright coals and feature low ash content and high vitrinite content, reflecting a reducing coal-forming environment with limited terrestrial input. The coal rock properties and reservoir development characteristics of the No.8 coal seam are controlled by changes in the sedimentary environment. The reservoir heterogeneity, to be specific, is influenced by both the input of terrigenous minerals to fill primary pores and the formation of abundant pores during hydrocarbon generation with high vitrinite content. For the sake of the stable sedimentary environment, the upper section of the No.8 coal seam exhibits favorable coal-forming conditions and low ash content. Furthermore, the pores and fractures of the coal reservoirs in this part are minimally filled with detrital minerals, resulting in improvement in reservoir properties and gas adsorption and preservation. In contrast, the middle and upper sections demonstrate high vitrinite content and a significant gas generation capacity, providing more favorable material conditions for the generation of coal-rock gas and resulting in higher coal-rock gas content. Therefore, the middle and upper sections of the No.8 coal seam represent favorable intervals for natural gas exploration and production in the Ordos Basin.