Pore structure characteristics and controlling factors of 2nd and 4th member reservoirs in Upper Triassic Xujiahe Formation of Penglai area, central Sichuan Basin

Abstract：Petrologic characteristics, physical properties and pore structure characteristics of Xu 2 (2nd member of Upper Triassic Xujiahe Formation) and Xu 4 reservoirs were studied by means of core observation, normal thin section and casting thin section observation, scanning electron microscope analysis, mercury injection test and nuclear magnetic resonance experiment. The results show that the lithology of Xu 2 and Xu 4 reservoirs is dominated by feldspar lithic sandstone and lithic sandstone, the main reservoir space consists of intragranular corrosion pores and intergranular corrosion pores, and the reservoirs are characterized by low porosity and low permeability with strong heterogeneity. The pore structure is overall characterized by small pores, tiny throats, fine skewness, and poor sorting and connectivity on the mercury injection curves. The reservoir pore structure can be divided into 4 types based on the parameters obtained by the mercury injection experiment together with the nuclear magnetic resonance experiment, and the typical characteristics of the four different pore structure types are summarized separately in this paper. Lastly the formation mechanism and controlling factors of various pore structure types are discussed in three aspects of tectonism, sedimentation, and diagenesis. It is demonstrated that the tectonism and sedimentation are the prerequisite conditions affecting the reservoir pore structure, while the types and intensity of diagenesis and its evolution sequence are the key factors for determining the pore structure characteristics. The above research results can provide guidance for the classified evaluation of Xu 2 and Xu 4 reservoirs and for the further gas exploration in the study area.