Reservoir spaces in tight sandstones: Classification, fractal characters, and heterogeneity

Abstract Evaluation of tight sandstone reservoir, including space types, storage capacity, and heterogeneity, provides a basis for the selection of favorable areas for tight gas and oil. Therefore, a variety of research methods, such as rate-controlled mercury injection, fractal theory, and observation of thin sections and SEM (scanning electron microscope) images, were integrated to explore these parameters. With the decrease of porosity and permeability, tight sandstones can be classified into three types: pore-dominated, pore-throat, and throat-dominated. The reservoir spaces of the three types of tight sandstones gradually evolve from micro-to nano-scale. The micro-scale spaces in pore-dominated and pore-throat type samples are primarily made up of intergranular and mixed pores and are characterized by a two-segment fractal feature, while the throat-dominated samples are mainly composed of intergranular pores with a one-segment fractal feature. The nano-scale reservoir spaces in the three types of tight sandstones also display similar two- and one-segment fractal characteristics, and are mainly composed of intragranular honeycombed dissolution pores, authigenic mineral intercrystalline pores, and throats in pore-dominated and pore-throat tight sandstones. However, the authigenic mineral intercrystalline pores constitute the main nano-scale spaces in throat-dominated tight sandstones. With the tight sandstones from pore-dominated through pore-throat to throat-dominated types, the fractal dimensions of micro-scale spaces increase gradually, but in nano-scale spaces the fractal dimensions decrease, which indicating that the heterogeneity of micro-scale spaces is gradually enhanced whereas that of the nano-scale spaces is gradually weakened. Because of differences in reservoir space types and heterogeneity, the enrichment and distribution of tight gas and oil changes with the distance between the reservoir layers and the source rocks. The tight reservoirs dominated by nano-scale spaces are also incorporated into the exploration and development goals in areas adjacent to source rocks. However, in areas located far from source rocks, the tight reservoirs dominated by micro-scale spaces should be the preferred targets for tight gas and oil.