Oil occurrence mechanism in nanoporous shales: A theoretical and experimental study

In tight shales, occurrence characteristics limit oil migration and recovery. This paper proposes an innovative approach based on experimental and theoretical models to investigate the oil occurrence mechanism in nanoporous shales. Thermogravimetry (TG) and nuclear magnetic resonance (NMR) tests were conducted to investigate the migrations and distributions of adsorbed, irreducible, and movable oils. Furthermore, the oil micro-occurrence characteristics within shales were quantitatively clarified using the theoretical equation for the adsorption ratio. Lacustrine shales with different lithofacies from Dongying Sag, Bohai Bay Basin, China, were evaluated as a case study. The results showed that adsorbed, irreducible, and movable oils occur in different pore sizes. The adsorbed oil is primarily saturated in micropores (<0.1 μm) and mesopores (0.1–1 μm) but hardly in macropores (>1 μm), characterized by an average density of around 1.4974 g/cm3 and a mean adsorption thickness of approximately 0.723 nm at a temperature of 60 °C. The irreducible oil predominantly occurs in mesopores, whereas the movable oil primarily exists in mesopores and macropores, and the weight ratio rapidly increases with increased pore size. Additionally, the NMR T1–T2 patterns of the adsorbed, irreducible, and movable oils were reported, and these patterns can directly reveal the states of oil micro-occurrence in shale pore-fracture systems. This study gives substantial insights into the oil occurrence mechanisms in nanoporous shales.