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.

Controlling effects of pore‐throat structure and fractal characteristics on the physical properties of ultra‐low permeability sandstone reservoirs: A case study of the sixth member of the Yanchang Formation in the Xiaojiahe area, Ordos Basin

“…Since the fractal theory was introduced into the field of petroleum geology, it has been frequently employed to describe the pore structures and assesses the petrophysical properties of sedimentary rocks (Hao et al, 2017; Huang et al, 2018; Mandelbrot, 1982; Wang et al, 2022). Numerous researchers have used scanning electron microscope (SEM) (Katz & Thompson, 1985), microscopic thin sections (Hansen & Skjeltorp, 1988), HPMI (Angulo et al, 1992; Dou et al, 2021; Li, 2010; Su et al, 2018), and other methods to study the fractal characteristics of the pore structure.…”

Pore‐throat structure characteristics and fluid mobility analysis of tight sandstone reservoirs in Shaximiao Formation, Central Sichuan

“…At present, the commonly used methods to characterize pore‐throat structure mainly include thin‐section observation, scanning electron microscopy, CT scanning, mercury intrusion technique, gas adsorption and nuclear magnetic resonance, which characterize the pore‐throat structure by direct observation or indirect measurement (Gui et al, 2021; H. Li et al, 2021; Su et al, 2022; E. Z. Wang et al, 2022; Xin et al, 2022; S. G. Yang, Li, et al, 2022; Y. B. Yang, Xiao, et al, 2022).…”

“…The method of mathematical statistics cannot effectively describe the pore‐throat structure of the reservoir. It is necessary to calculate the fractal dimension with the help of fractal theory, so as to describe the complexity of pore‐throat structure, the roughness of pore surface and the heterogeneity of the reservoir (Dong et al, 2022; Guo et al, 2019; H. Li, Zhou, et al, 2022; X. M. Li, Wang, et al, 2022; Qu et al, 2020; E. Z. Wang et al, 2022; Zang et al, 2022; K. X. Zhang et al, 2019).…”