Shale oil reservoir of the Palaeogene Shahejie Formation in the Dongpu Sag: Petrology and pore microstructural characteristics

Shale petrology and pore microstructures are key performance evaluation indicators for shale oil and gas reservoirs. In this study, the petrological and oil compositional characteristics of shale oil reservoir and reservoir lithofacies types, pore/fracture types and pore-throat distribution in the Shahejie Formation in the Dongpu Sag are investigated by core observation, thin section analysis, scanning electron microscopy, mercury injection-liquid nitrogen adsorption and microCT-guided 3D reconstruction methods. Results show that the shale oil reservoir contains mainly clay and carbonate minerals and some felsic minerals. The main lithofacies includes laminated limestone/clay stone/calcareous fine-grained mixed rocks/argillaceous fine-grained mixed rocks, and massive clay stone/dolomite. The reservoir pore space is mainly provided by a combination of intergranular/intragranular dissolution/clay mineral intergranular/organic pores. The microfractures can be divides into diagenetic and tectonic fractures, and the laminated limestone/calcareous fine-grained mixed rocks developed various high porosity pore spaces. Lithofacies and bedding structure are important factors influencing pore development and pore quality. Laminated rocks, except clay stone, possess large pore space and show good pore connectivity, relatively developed microfractures and high oil content and mobility, and laminated limestone is the most favorable lithofacies. These rocks are key shale oil exploration targets in the Dongpu Sag. © 2023 Science Frontiers editorial department. All rights reserved.