Characterizing the pore structure of low permeability Eocene Liushagang Formation reservoir rocks from Beibuwan Basin in northern South China Sea

The hydrocarbon exploration from low permeability (< 50mD) reservoirs in Beibuwan Basin has been on the agenda because of the production decline of old wells and the growing demand of fossil fuel with the rapid economic development of China. However, these low permeability reservoir rocks usually exhibit strong heterogeneity and the complicated pore structure of these rocks, which significantly affects the reservoir quality, is still unclear. This study attempts to reveal the pore structure of low permeability Eocene Liushagang Formation reservoir rocks from Weixinan and Wushi subbasins in Beibuwan Basin quantitatively and qualitatively using high pressure mercury intrusion porosimetry (HPMIP), N-2 adsorption and field emission-scanning electronic microscopy (FE-SEM). Our results show that macropores (> 50 nm) mainly contribute to the total pore spaces for most samples with lower shale content; mesopores(2-50 nm) dominate the pore spaces of samples with higher shale content, which leads to the relatively large surface area of these samples; permeability of these low permeability rocks is positively related with average pore-throat radius and negatively related with relative sorting coefficient while no such a close relationship exists between permeability and porosity; fractal dimension has a negative relationship with average pore-throat radius and permeability. In summary, this work comprehensively investigates the pore structure characteristics of low permeability Liushagang Formation reservoir rocks from Beibuwan Basin using multiple complementary approaches, which helps to further understand the hydrocarbon migration and accumulation mechanism of these low permeability reservoirs.