Risk assessment of high maturity lacustrine shale oil reservoir based on pore-fracture connectivity and decane accessibility, Ordos Basin (China)

Abstract

Risk assessment in the exploitation of lacustrine shale oil reservoir requires a more comprehensive approach. This task was accomplished through a wide range of complementary tests, including (ultra) small-angle neutron scattering (USANS/SANS), mercury intrusion capillary pressure (MICP), and scanning electron microscopy imaging after Wood's Metal (WM) impregnation. As a result, two key petrophysical parameters—pore-fracture connectivity and fluid accessibility—were determined. These parameters would help us to predict shale oil reservoir quality and significantly reduce the risks associated with exploitation. Results indicated that the Hg/WM only slightly fills the matrix pores due to elastic deformation under high pressure, particularly in clays and organic matter. The presence of pores in various sizes, determined by USANS/SANS and MICP made us to classify them into non-permeable, potentially permeable and permeable pores, among which permeable pores accounted for the highest proportion of the pores (58.98–79.03%). However, due to the restrictions of the throats, fractures (>10 μm) cannot effectively connect an important percentage of matrix pores, resulting in limited fluid flow. Moreover, the radius of gyration (Rg) obtained by contrast matching-SANS technique showed that oil molecules in the pore spaces of 2–110 nm are mainly in the adsorbed state. Collectively, such observations suggest that evaluating shale oil reservoirs, characterized by the above attributes, presents significant obstacles to efficient stimulation in production, making investment endeavors considerably challenging and risky.