Oil occurrence state and quantity in alkaline lacustrine shale using a high-frequency NMR technique

Complex fluid occurrence states and structures of shale reservoirs greatly affect shale oil productivity. For lacustrine shales with strong heterogeneity, it is important to identify oil movability, occurrence states and pore structures in different lithofacies in order to efficiently develop shale oil. This study combines conventional core analysis, X-ray diffraction (XRD), argon ion milled-scanning electron microscopy (AIM-SEM), multiple isothermal stages (MIS) pyrolysis and high-frequency nuclear magnetic resonance (HF-NMR) to create maps for samples in native, saturated, centrifuged, and dried states. A new T1-T2 map interpretation scheme is proposed, and fluid occurrence models for different shale lithofacies are established. Four oil species can be identified on the T1-T2 map. Free oil and adsorbed oil in macropores are located in the centers and edges of intergranular/ intercrystalline pores, respectively. Oil in mesopores is located in intragranular/intracrystalline dissolution pores. Adsorbed oil exists in the pores of dispersed organic matter and in the marginal fractures of organic matter laminae. NMR and pyrolysis correlate well in the oil contents that they define. The occurrence state of shale oil in the Fengcheng Fm is closely related to minerals and organic matter. Siliceous minerals are beneficial to the occurrence of free oil that is usually a highly movable. The fluid-holding capacity of calcareous shale is limited, especially for adsorbed oil and restricted oil. Clay minerals (>10%) are not conducive to shale oil enrichment and flow, and mixed shale almost exclusively contains bound water. Organic matter is conducive to increasing oil content and to the movable oil saturation of siliceous shale and mixed shale.