Micro‐ and nanoscale pore‐throat structure, fractal characteristics, and permeability estimation models in deep sandstones: Palaeogene Shahejie Formation, Nanpu Sag, NE China

Abstract

Long‐term burial and strong diagenesis modification result in highly heterogeneous pore structures in deep clastic reservoirs; these heterogeneities exert a controlling influence over the development of these deep petroleum systems. A clear understanding of the pore structure simplifies the tasks of identifying and modelling reservoirs for the purposes of hydrocarbon exploration. This study characterizes the mineral compositions, physical properties, and pore systems of the first member of the Palaeogene Shahejie Formation (Es1) in the Nanpu Sag using thin section and scanning electron microscope observations, X‐ray diffraction measurements, mercury intrusion capillary pressure, and nuclear magnetic resonance data. Our results show that the Es1 sandstones typically fall into one of three different pore‐throat sizes: mainly 10 μm (type‐I), 1 μm (type‐II), and less than 1 μm (type‐III). Between type‐I and type‐III reservoirs, the pore system changes from residual intergranular pore (InterP) and secondary InterP to secondary intragranular pores (IntraP) and intercrystalline pores. The fractal theory is selected to quantify the heterogeneity of the pore‐throat structure, due to the existence of an irregular and complex pore system (IntraP and intercrystalline pores), the type‐III reservoir has the highest fractal dimension, which indicates that the complex pore‐throat structure is not conducive to the formation of high‐quality deep sandstone reservoirs. Using the fractal dimension and displacement pressure, a novel permeability prediction is proposed, compared with the previous method, the new model is simpler and more effective. Our research can deepen the geological understanding of deep‐burial sandstone reservoirs and further guide petroleum exploration.