Pore Structure Characterization and Reservoir Quality Evaluation of Analcite-Rich Shale Oil Reservoir from the Bohai Bay Basin

Abstract

Pore characteristics and shale oil reservoir quality strongly influences the petroleum occurrence, enrichment, and economic production. In this work, to characterize the pore system and evaluate the reservoir quality of analcite-rich shale oil reservoir, a total of 33 shale samples from the second member of the Paleogcene Kongdian Formation (Ek₂) of Bohai Bay Basin were examined using a suite of techniques, including total organic carbon (TOC) content, X-ray diffraction (XRD), field emission SEM, gas (CO₂ and N₂) physisorption, and mercury injection porosimetry (MIP). Six typical shale lithofacies are identified, and their pore characteristics are compared to reveal the factors affecting shale reservoir quality. The mineral matrix-related pore (e.g., intergranular and intragranular pores) is the dominant pore type in different shale lithofacies, with few organic matter (OM)-hosted pores being observed. Analcite-rich siliceous shale exhibits the largest specific surface area (SSA) and micropore volume, while the smallest is observed in analcite-rich mixed shale lithofacies. In contrast, the volume and SSA values of mesopores for siliceous shale are the largest, while mixed shale has the smallest values. Multipermeabilities of five connected pore networks were determined using the Katz-Thompson equation, and the permeability of microfractures or interlayer bedding cracks is at the micro-Darcy scale, as reflected by the first two inflection points. Permeability values for the other three inflection points reflect the matrix pore system within a nano-Darcy range. Siliceous shale has the high matrix permeability and good pore connectivity, which is favorable for fluid flow. Authigenic analcite mineral has different effects on pore development and reservoir quality depending on the shale mineral composition and diagenesis processes. Some primary pores were lost during the filling of analcite mineral in the early diagenesis period, but some intergranular pores were also protected by the rigid framework formed by analcite minerals. In addition, some secondary dissolution pores are generated during the reaction between analcite and organic acids discharged from the source rocks within the oil generation window, and the reservoir porosity and permeability can be greatly improved. Except for analcite, the TOC content is also an important factor controlling reservoir quality.