Storage space development and hydrocarbon occurrence model controlled by lithofacies in the Eocene Jiyang Sub-basin, East China: Significance for shale oil reservoir formation

Abstract

Complex mineral composition, multiple depositional processes, and strong reservoir heterogeneity cause the complexity and uniqueness of a lacustrine shale oil reservoir. Understanding the storage space development and hydrocarbon occurrence model should facilitate the analysis of the shale oil reservoir and hydrocarbon accumulation in lacustrine shales. In this study, the storage space development model for different lithofacies and the modes of hydrocarbon occurrence in different storage space were analyzed in the Es4s–Es3x shale in the Jiyang Sub-basin, East China, based on thin-section and field emission scanning electron microscopy (FE-SEM) observations, X-ray diffraction (XRD) analysis, physical property testing, and geochemical analysis. Inorganic pores, particularly recrystallization intercrystalline pores in calcite, are the key matrix pores for lacustrine shale oil. However, the types and abundance of storage space are noticeably different in various lithofacies. The hydrocarbon occurrence is mainly manifested in three states: (A) free state in interlaminar fractures, structural fractures, and abnormal pressure fractures; (B) adsorbed state in organic pores, intercrystalline pores in pyrite, and floccule pores; and (C) free state in large pore spaces, including dissolution pores and recrystallization intercrystalline pores, which can form a continuous hydrocarbon accumulation. The hydrocarbon generation potential and thermal maturity are closely associated with the lithofacies and mineral composition. The matching mechanisms of pore formation as well as hydrocarbon generation, migration, and accumulation are favorable. Among the various lithofacies, the organic-rich calcareous shale has abundant storage space, high porosity, total organic carbon content, and hydrocarbon potential, making it a “sweet spot” for shale oil exploration.