Diagenetic evolution and formation mechanisms of high-quality shale reservoirs for mixed shales in the Jiyang Depression, Bohai Bay Basin: A case study of shales in the Chunshang interval of the upper sub-member of the 4th member of the Shahejie Formation, Minfeng Sub-sag

For mixed shales occurring in the Jiyang Depression, Bohai Bay Basin, the controlling mechanisms of their diagenetic evolution over pore development remain poorly understood, and their diagenetic differentiation and corresponding pore response mechanisms have yet to be revealed. In this study, we investigate the mixed shales in the Minfeng Sub-sag, Jiyang Depression. Using multiple experimental methods, including petrographic and mineralogical analysis, scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), and low-temperature nitrogen adsorption, we investigate the diagenetic and reservoir-formation processes of carbonate, felsic, and clay minerals. The results indicate that primary pores in the mixed shales include intergranular pores of quartz, intercrystalline pores of calcites and dolomites, and clay interlayer pores. Four evolutionary stages for the formation of high-quality reservoirs are proposed: framework building, authigenic mineral-supported pore preservation, dissolution-induced porosity enhancement, and overpressure-preserved porosity. Specifically, during the early diagenetic stages A₁-A₂, the weak dissolution and recrystallization of micritic calcites contribute to the formation of millimeter-scale lamellar frameworks, providing a structural basis for pore preservation. Subsequently, from the early diagenetic stage A₂ to the middle diagenetic stage A₁, K⁺ released from the dissolution of K-feldspars facilitates the transformation of mixed illite-montmorillonite into illite, leading to the release of SiO₂ and the precipitation of authigenic quartz. Consequently, rigid frameworks supported by authigenic quartz are formed locally. During the middle diagenetic stages A₁-A₂, calcites and feldspars are dissolved by organic acids, contributing to increased pore sizes and improved reservoir properties. Afterward, during the middle diagenetic stages B₁-B₂, overpressured fluids sustain pores and induce microfractures, thereby connecting isolated pore throats. These four progressive stages, namely rigid framework building, pore support and enhancement, dissolution-induced porosity enhancement, and overpressure-preserved porosity, completely reveal the mechanisms behind the diagenetic evolution of reservoirs, pore development, and reservoir property enhancement for shales in the study area.