Effect of shale diagenesis on pores and storage capacity in the Paleogene Shahejie Formation, Dongying Depression, Bohai Bay Basin, east China

Abstract

Abstract Compared to other marine-shale gas reservoirs in South China, the shale that developed in faulted basins in East China possesses peculiar characteristics regarding the mineral types and their evolution. The diagenesis of inorganic minerals, generation of hydrocarbons, and evolution of organic matter control the final features and quality of these shale reservoirs. Therefore, we must analyze this diagenesis based on the essential characteristics of the representative reservoir and discuss the influence of the diagenesis evolution stage on the shale reservoir. This topic is also of practical value for contemporary shale-oil exploration and development. In this paper, we observe diagenesis, study its essential characteristics (such as the structure of the reservoir and pores), determine the key factors that control the reservoir s storage capacity, and analyze the role of diagenesis (organic and inorganic) in the evolution of pores and the reservoir s storage capacity. The mineral types are diverse, and the content of calcite and other carbonate minerals is generally very high. Organic matter is abundant, thermal evolution is low, and the hydrocarbon generation capacity is strong. Because of their high variability, some pores are hard to differentiate from microfractures. Dissolved pores, which are generated by intercrystalline pores and the dissolution of calcite, dolomite, and other carbonate mineral grains, are the major component of a macroporous matrix. The large proportion of hydrocarbons that are generated by solid organic matter leads to the accumulation of pressure. The shale generates a high number of microfractures, which form a reservoir space network system that increases the porosity of the reservoir. Dolomite has a unique supporting role for the matrix pores, leading to the development of a matrix pore-seam system. The reservoir s mineral composition, pore diameter, porosity, and organic-matter content determine the reservoir properties and oil saturation. Organic-rich laminated shale has the best physical properties, dominated by macro-pores and easily connected and merged pore seams. The massive shale has the poorest reservoir property. The development and evolution of pores are mainly controlled by the transformation of clay minerals, the content of carbonate minerals, the sedimentary structure and the intensity of diagenesis. Dissolution depends on the intensity of hydrocarbon generation, the expulsion of acid by organic matter, the state of the carbonate minerals and organic matter, and the smooth depression of the fluid environment. Organic diagenesis is closely related to the diagenesis and evolution of inorganic matter and is typically reflected in the recrystallization of calcite, abundance of organic matter, and hydrocarbon generation by organic matter. A local alkaline diagenetic fluid environment and reduction in the degree of hydrocarbon generation transform incremental porosity to pore reduction. Favorable lithofacies and advantageous layer sections should be selected by considering the stage of diagenesis evolution.