Comprehensive characterization of micro-pore structures in different lithologies of the lacustrine fine-grained mixed sedimentary reservoir of the Shahejie Formation in the Jiyang Depression

Abstract

Several methods, such as field-emission environmental scanning electron microscope (FE-SEM), low-temperature nitrogen adsorption-desorption, high-pressure mercury injection tests, focused ion beam scanning electron microscope (FIB-SEM), and high-magnification and large-scale quantitative analytical methods involving argon ion polishing-FE-SEM, have been carried out on lacustrine fine-grained mixed sedimentary reservoirs of the upper fourth-lower third members of the Shahejie Formation in the Jiyang Depression, with the aim of clarifying their microstructural characteristics. Five types of reservoir spaces of fine-grained mixed sedimentary reservoirs were recognized in this study, including intergranular, dissolution, intercrystalline and intracrystalline pores, and fractures. The favorable reservoir spaces are pores with diameters ranging from 40 nm to 300 nm, which are dominated by intergranular pores, grain edge seams, and dissolution pores. The high-pressure mercury injection curves can be divided into three types. Types I and II, with good pore structures, occur mainly in mudstone and dolomite facies, whereas type III, with poor pore structures, is present in limestone facies. The low-temperature nitrogen adsorption-desorption showed that the adsorption volume of dolomite facies is the highest, and the pore morphologies are plate-like. The adsorption amount of mudstone facies is higher than limestone facies, and the pore morphologies are dominated by ink bottle-shaped and parallel plate-like. FIB-SEM analyses reveal that the samples display moderate connectivity. These findings suggest that the different types of lithologic reservoirs of fine-grained mixed sedimentary rocks in the Jiyang Depression have different microstructures, with the argillaceous dolomite, calcareous mudstone, calciferous mudstone, and argillaceous limestone changing gradually from good to poor.