Abstract: The deep coal rocks are widely distributed in Ordos Basin, showing a major replacement field for natural gas exploration and development. However, with the increasing burial depth, the temperature, field stress and hydrogeological conditions of deep formations vary greatly, resulting in great differences in gas accumulation law between deep coal rocks and shallow coal rocks. Regarding the particularity of gas accumulation in deep coal rocks, there are insufficient studies on pore structural characteristics and genesis, gas accumulation types and enrichment characteristics. The experimental analysis on coal rock marcels and scanning electron microscopy have been conducted. The results show that No.8 coal seam in Benxi Formation was mainly deposited in lagoon–tidal flat sedimentary environments, with a thickness of 8–22 m in the northeast and 2–6 m in the southwest, a high vitrinite content of 78.5% on an average, and an increasing maturity from northeast to southwest. The porosity of deep coal reservoir ranges in 3.65%–5.84%. The formation of vesicles was controlled the type of coal swamps, and it was affected by formation water vaporization and rapid hydrocarbon generation of coal rocks. There are multiple types of coal rock gas reservoirs in deep formations, such as microstructural gas reservoir and anticlinal gas reservoir. The gas content of coal rocks increases from northeast to southwest, with a critical point at a depth of about 2000 m. The gas content distribution of coal rocks was affected by coal-forming swamp facies zones. The gas content of coal rocks with a silty mudstone roof is better than those with limestone and sandstone roofs. In addition, it is proposed that a geological evaluation system for deep coal rock gas with the core of pore development, a geophysical prediction technology with the core of pore development zone and gas enrichment zone, and a horizontal well fracturing design and targeted production technology for vesicle developed reservoir should be researched and developed, so as to accelerate the beneficial development of deep coal rock gas.

Key words: Ordos Basin, deep coal rock gas, vesicle, genesis mechanism, reservoir pattern, beneficial development