Distribution and occurrence of pore water and retained oil in nanopores of marine-terrestrial transitional shales during oil generation and expulsion: Implications from a thermal simulation experiment on shale plug samples

The distribution and occurrence of pore water and retained oil in shale nanopores is a key scientific issue confronting the evaluation, exploration and development of shale hydrocarbon resources. At present, however, relevant researches are scarce, especially on marine-terrestrial transitional (MTT) shales. In this study, a thermal simulation experiment was performed on a set of water-saturated shale plug samples prepared from a low-maturity MTT shale. The pore water and retained oil contents of the artificially matured samples were quantified, and the pore structures under moist, dried, and solvent extracted conditions were investigated. The results show that the pore water content of the shale decreases with Ro increasing from 0.75% to 0.97% and then increases with Ro further increasing to 1.53%, which exhibits a negative correlation with the retained oil content during the oil generation and expulsion. The pore water occurs mainly in filling, adsorption and water-cluster states in micropores (<2 nm), small non-micropores (2-10 nm), and large non-micropores (>10 nm), respectively. The retained oil is mainly in a filling or dissolved state in the micropores and small non-micropores and in an oil-cluster state in the large non-micropores. The pore water and retained oil in shale micropores are stored in the inorganic matter (IM) and organic matter (OM) micropores, respectively. During oil generation and expulsion, the IM micropores are fully saturated by the pore water that is barely expelled, and most of the pore water is stored in non-micropores, especially in large non-micropores. With increasing maturity, the distribution of retained oil in nanopores changes obviously. The retained oil in non-micropores tends to be stored in the large non-micropores at low-maturity stages, occurs uniformly in both the small and large non-micropores at mature stages, and is dominantly reserved in the small non-micropores and micropores at high-maturity stages. These results have significances for understanding the accumulation mechanism of shale oil and gas resources under geological conditions and evaluating their resource potential.