Characterization of micropore structure of a middle to deep clastic reservoir: The oligocene reservoir in the Nanpu Sag, Bohai Bay Basin, China

Abstract

In this paper, the Oligocene reservoirs in the Nanpu Sag of the Bohai Bay Basin are studied by scanning electron microscopy (SEM), thin section observation, XRD, nuclear magnetic resonance (NMR), and mercury injection. The purpose is to judge the microscopic pore structure of the reservoir in the study area through: 1) Combining nuclear magnetic resonance technology with fractal dimension to analyze the pore structure characteristics of sandstone; 2) using mercury injection experiments to judge the connectivity of pore throats; and 3) combining petrophysical properties-NMR-fractal dimension-mercury injection to predict reservoir quality. The results show that the sandstones of the Ed3 and Es1 members in the Nanpu Sag of the Bohai Bay Basin have good physical properties, medium porosity (5.2%–18.7%, average 11.70%) and permeability (0.08mD-111mD, average 22.68mD). Using the T2 spectrum, the fractal dimension of the bound fluid pore and the fractal dimension of the movable fluid pore are determined. The movable fluid pore is more complicated than the bound fluid pore. There is a connection between fractal dimension and mineral composition. High clay mineral content leads to a complex pore network, and the larger the fractal dimension, the more complex the pore shape and the lower the permeability.