Abstract: Micro-pore structure is an important factor controlling the preservation properties and development results of unconventional gas reservoir. In this paper, the micro-pore structure of the Xu 5 reservoir in the western Sichuan depression was studied using SEM (scanning electron microscopy), low-temperature nitrogen adsorption, laser scanning confocal microscopy and micro-nano CT scanning techniques. The following results were obtained. First, the pore space in the Xu 5 sandstone reservoir is composed of solution pores, intercrystal pores and micro fractures, with the solution pores in dominance, and the pore space in the shale reservoir is composed of mineral matrix pores (e.g. intergranular, intragranular and intercrystal pores), organic pores, and micro fractures, of which the most developed types are intergranular and intercrystal pores. Second, the pore structure of the Xu 5 reservoir is complex, primarily consisting of mesopores and macropores which show in the shapes of wide body and narrow neck, and flat and narrow seam, with a specific surface area between 1.44 m²/g and 11.7 m²/g, and an average pore diameter ranging from 5.75 nm to 15.6 nm; the specific surface area of the sandstone is smaller than that of the mudstone, but the pore size of the sandstone is larger. Third, since the unconventional gas reservoir is very complex, single technique may only provide incomplete results, and several modern high-precision techniques can be combined together for characterizing the micro-pore structure. On this basis, the factors that control the micro-pore structure and the relationship between the production and pore types in the Xu 5 unconventional gas reservoir were analyzed, in order to provide the geological base for subsequent "sweet spot" prediction.

Key words: western Sichuan depression, the fifth member of Xujiahe Formation (Xu 5), pore type, micro-pore structure, controlling factor