Abstract: The reservoir permeability of the Upper Shaximiao Fo rmation of M iddle Jurassic in the Xinchang gas field, western Sichuan is highly heterogeneous and controlled directly by reservoir pore structures. The main geological facto rs giving rise to the differences of pore structures are generalized as follow s. (1) The compositions and structures and textures of the sandstones determined the development of original pore spaces. (2) The anti-compression of the authigenic chlorite cement effectively protected prim ary intergranular pore spaces and early solution pore spaces. (3) The dissolution facilitates the enhancement of reservoir permeability. In addition, a variety of bedding structures formed in different environments has also exerted an effective influence on reservoir permeability.