Impact of rock type on the pore structures and physical properties within a tight sandstone reservoir in the Ordos Basin, NW China

The pore-throat systems and physical properties of tight sandstone reservoirs are complex,and deposition is thought to be a fundamental control for them.In this study,the impacts of the full ranges of rock types(from pebbly coarse sandstone to fine sandstone) on the pore structures and physical properties of the Permian tight sandstone reservoir in the eastern Ordos Basin were investigated comprehensively through a series of experiments including conventional physical testing,thin-section analysis,scanning electron microscopy,nuclear magnetic resonance analysis and high-pressure mercury injection tests.The results showed that the coarser-grained sandstones tend to have higher feldspar content and lower percentage of cements,leading to strong dissolution,weak cementation and improved porosity and permeability.The medium sandstone has the highest level of quartz and the lowest average content of feldspar,resulting in strong heterogeneity of physical properties.Only those medium sandstone reservoirs with relatively high content of feldspars have better physical properties.Additionally,the coarser-grained sandstones contain relatively large dissolution pores(nearly 200 μm),whereas the finer-grained sandstones have more intercrystalline pores with a relatively more homogeneous pore structure.The pebbly coarse sandstone and coarse sandstone reservoirs are favorable targets with best physical properties.