Abstract: This paper selected the Eocene low-permeability sandstones in the G oilfield, Bohai Bay basin as an example, and the influences of pore structure on reservoir quality and fluid occurrence were investigated by cast thin sections, scanning electron microscope (SEM), mercury injection capillary pressure (MICP), and nuclear magnetic resonance (NMR). Due to a highly variable pore size distribution, micro-scale and nano-scale pore throats coexist. The pore-throat size distributions generally present bimodal behaviors. The large pore systems are mainly composed of residual intergranular pores and enhanced intergranular dissolution pores，while the micropore systems are dominated by intragranular dissolution pores and intergranular pores. The results indicate that pore connectivity is the key factor affecting the physical properties and fluid occurrence state of the low permeability sandstone reservoirs. The sandstones with better petrophysical properties have a high proportion of macroscopic pore throats (right peak) with good connectivity, and the permeability capacity of the sandstones is strong. In contrast, the pore systems of the sandstones with poor petrophysical properties are dominated by the microscopic pore throats (left peak) with poor pore connectivity, and the permeability capacity of the sandstones is weak. Bound fluids mainly exist in small pore systems with poor pore connectivity and large fractal dimensions, while large pore systems have good pore connectivity and small fractal dimensions and mainly contain movable fluids. The study also clarified that the permeability and fluid occurrence state of low-permeability sandstone reservoirs is mainly controlled by the relatively large pore throats in the pore network. It is revealed that the permeability prediction model based on MICP data and porosity derived from the “effective movable fluid method” provides a better estimation effect compared with the classic models (e.g., Coates and SDR).

Key words: Bohai Bay basin, low-permeability sandstones, pore structure, reservoir quality, movable fluids