Shale Oil Potential and Mobility of Low-Maturity Lacustrine Shales: Implications from NMR Analysis in the Bohai Bay Basin

A vital factor influencing shale oil exploration in lacustrine shale reservoirs is oil mobility, which is closely associated with the shale pore structure and fluid properties, especially for the low-maturity lacustrine shales in China. In this study, the oil mobility and shale oil potential in the Middle Eocene Shahejie Formation lacustrine shales (MES shales) of the Nanpu Sag in the Bohai Bay Basin are evaluated by using nuclear magnetic resonance (NMR) experiments. The low-maturity MES shales have low porosity with NMR porosity ranging from 4.29–7.41%, and the oil saturation ranges from 9.35–36.09%. The pore types are various including intergranular and dissolution pores and fractures. The pore space size spans the range from nano- to microscale, and they are predominantly mesopres. The pore structure for fluid flow is complex and has good self-similarity with high fractal dimensions. The abundant brittle minerals with a relatively high brittleness index value benefit the fracturing of MES shales. Due to the high viscosity and heavy oil in low-maturity shales, bulk relaxation is proposed to analyze the oil properties in this study. The oil viscosity of MES shales mainly ranges from 2 to 70 cP. The movable oil with a viscosity lower than 10 cP accounts for 53.66% of the total oil-filling pore space. For the black mud-shales dominating MES shales, the thermal maturity influences the porosity, viscosity, free hydrocarbon content, and oil saturation in the rocks. Higher thermal maturity would facilitate pore space development with higher porosity, enhance the free hydrocarbon content and oil saturation, and reduce the oil viscosity to some extent. Moreover, MES shales have geological conditions similar to and better brittleness than those of other shale oil producing areas, which further supports the considerable and promising shale oil potential in this formation, especially for deposits located in deeper positions of the Nanpu Sag. The technologies of in situ conversion process and hydraulic fracturing make the resource potential of shale oil in the Nanpu Sag more attractive.