Abstract: Through methods such as core observation, cast thin section identification, and scanning electron microscopy, the reservoir space characteristics of the tight oil reservoir of the Lower Jurassic Daanzhai Member in the Middle Sichuan region were studied. It is found that there are numerous microfractures in the tight oil reservoir, and the reservoir space mainly comprises of secondary pores, such as dissolution pores, intergranular dissolution pores, intragranular dissolution pores, intercrystalline micropores, and organic pores. Almost all primary pores are micro/nano pores. A three-dimensional microscopic model of the reservoir space in the dense limestone of the Daanzhai Member was established using computed tomography (CT) scanning technology. Plenty of reservoir space that can store oil and gas is noted inside the dense limestone matrix. Further, it is believed that, in general, the pore-throat system of the Daanzhai Member can be classified into two types:isolated-porous and fractured-porous. According to the reservoir space characteristics and pore/fracture configuration relationship, the reservoir of the Daanzhai Member is divided into three types:porous, fractured-porous, and fractured reservoirs. Among them, the fractured-porous reservoirs are more widely distributed. The early-stage productivity of the fractured reservoirs is the highest. The porous reservoirs are characterized by their long-term steady productivity and have good potential for exploitation.

Key words: tight oil, reservoir space, pore structure, carbonate rocks, Sichuan Basin