Abstract:

The types of micro-pores, pore structures and fractal characteristics of shale have been analyzed for the shale reservoir of Wufeng-Longmaxi Formation in Southeastern Chongqing by field emission scanning electron microscope（FESEM）, low temperature liquid nitrogen adsorption and overpressure permeability porosity test. It is found that the macroscopic fracture types are mainly foliated fractures and structural fractures, and most of the fractures in the samples along the basin are arranged in an echelon series, while the fractures in the atmospheric pressure shale gas well samples outside the basin are relatively straight. The positions where the macroscopic fractures develop most are also the places where the microfractures develop well. Usually, it has high gas content and permeability. There are obvious differences between the occurrence characteristics of pore fractures in deep shale and relatively shallow shale. The fractures in the deep samples are less developed compared with those in the shallow samples, and the microfractures are mainly shrinkage and dissolution fractures. The pore type, shape and structure of the shallow samples have similar longitudinal distribution characteristics. The average pore diameter and overburden porosity of shallow samples are obviously larger than that of deep samples. Quartz and clay have a certain influence on the development of micropores and mesopores. The microscopic type is an important influence factor on the development of organic pores. When the burial depth is greater than 3 500 m, depth is the main influence factor on pore morphology and pore size distribution. The deep shale undergoes more complex diagenesis, the pores undergo intense late transformation, and the pore structure is no longer dominated by organic matter. It is difficult to form fracture network when fracturing the deep shale with such pore fracture occurrence characteristics, so further research on the formation mechanism of deep shale fracture network is needed. Through the observation of fractures with different scales, the layer easy to fracture can be quickly determined. Even though such fractures account for a very small proportion of in-situ pore volume and have no effect on the permeability of the reservoir, such pore groups can be stimulated because of fracturing and serve as the main seepage channels of shale reservoir.

Key words: shale, Wufeng-Longmaxi Formation, pore, fracture, normal pressure