Abstract: Shaerhu sub-sag is an isolated coal-bearing sub-sag in southern margin of Tuha basin and a peculiar coal-enriched sub-sag in China, with the thickness of single coal seam of 150 m. The coal step belongs to lignite. Based on the existing seismic, geologic, drilling and various analytical testing data, the basic geologic features of its accumulation into gas reservoir are fully discussed. The macro and micro composition, pore growth properties and adsorption of coal seam gas of Xisnanyao formation (J2x) in this sub-sag are identified. It is concluded that this coal seam in huge thickness possesses the geologic conditions for forming coal seam gas reservoir originated from biologic origin; the coal seam gas reservoir with high saturation is easily formed in the lower position of the sub-sag's slope, being a favorable area for exploration.

Key words: Tuha basin, Shaerhu, gas reservoir, geologic characteristic, low step coal seam gas