Abstract
The study of pore characteristics is of great importance in reservoir evaluation, especially in deeply buried sandstone. It controls the storage mechanism and reservoir fluid properties of the permeable horizons. The first member of Eocene Shahejie Formation (Es1) sandstone is classified as feldspathic litharenite and lithic arkose. The present research investigates the pore characteristics and reservoir features of the deeply buried sandstone reservoir of Es1 member of Shahejie Formation. The techniques including thin-section petrography, mercury injection capillary pressure (MICP), scanning electron microscopy and laser scanning confocal microscope images were used to demarcate the pores including primary intergranular pores and secondary intergranular, intragranular, dissolution and fracture pores. Mercury injection test and routine core analysis were led to demarcate the pore network characteristics of the studied reservoir. Pore size and pore throat size distribution are acquired from mercury injection test. Porosity values range from 0.5% to 30%, and permeability ranges 0.006–7000 mD. Pore radii of coarse-grained sandstone and fine-grained sandstone range from 0.2 to > 4 µm and 1 nm to 1.60 µm, respectively, by MICP analysis. The mineral composition also plays an important role in protecting the pores with pressure from failure. Fractured sandstone and coarse-grained sandstone consist of large and interconnected pores that enhance the reservoir porosity and permeability, whereas fine-grained sandstone and siltstone consist of numerous pores but not well interconnected, and so they consist of high porosity with low permeability.
Highlights
The movement of different reservoir fluids via different geological systems is mainly controlled by capillary pressure within the rock unit
The pore volume and pore throat size that relate to displacement pressure are the main components that control the reservoir characteristics of reservoir rock, and they can be evaluated by mercury
Micropores from the matrix and authigenic minerals occupy a significant proportion of the total porosity; in addition to the pore size, the permeability of reservoir sandstone is much affected by the pore throat connectivity, i.e. the pore throat radius, geometry and structure (Zou et al 2012)
Summary
The movement of different reservoir fluids via different geological systems is mainly controlled by capillary pressure within the rock unit. Micropores from the matrix and authigenic minerals occupy a significant proportion of the total porosity; in addition to the pore size, the permeability of reservoir sandstone is much affected by the pore throat connectivity, i.e. the pore throat radius, geometry and structure (Zou et al 2012). Authigenic clay minerals play a significant role in decreasing the pore volume and concluding the pore apertures (Lai et al 2018a, b, c; Yue et al 2018) Because of their hair-like and honeycomb-like morphology, crystal habit and fibrous nature, they significantly affect the hydraulic and petrophysical properties of sandstone, thereby shaping the pore geometry (Lai et al 2018a, b, c; Samakinde et al 2016). Dissolution of detrital grains and cement is the primary process which enhances the porosity and permeability by increasing pores and pore throats (Lai et al 2018a, b, c; Mozley et al 2016)
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