Regulation mechanisms of material transfer within continental shale on reservoir properties: a case study of shale from upper submember of fourth member of Paleogene Shahejie Formation, Niuzhuang Subsag, Bohai Bay Basin

Continental shale oil and gas are important areas for increasing oil and gas reserves and production in China. However, traditional macroscopic-to-core scale studies are insufficient to reveal the relationship between material migration within laminae and pore evolution. Taking the continental shale from the upper submember of the fourth member of the Paleogene Shahejie Formation in the Niuzhuang Subsag, Dongying Sag, Jiyang Depression, Bohai Bay Basin as the study object, this study employed microscopic analytical methods such as X-ray diffraction, argon ion polishing-field emission scanning electron microscopy-energy spectrum spectroscopy, and rock thin-section microscopic observation to reveal the diagenetic evolution characteristics and the regulatory mechanisms of material transfer on reservoir properties. The study found that the formation of reservoir spaces in the strongly overpressured shale of the upper Es4 submember in the Niuzhuang Subsag was synergistically controlled by "overpressured fluid-diagenetic minerals". During the formation period of cone-in-cone and columnar-fibrous calcite in the early to middle diagenetic stage, a negative shift of δ18O by 3.56%-5.47‰ was observed, showing a distinct response to the overpressure increase of 52.14-76.17 MPa. The homogenization temperatures of yellow-blue-white fluorescent hydrocarbon inclusions and associated brine inclusions indicated that hydrocarbon entrapment occurred earlier than the overpressured brine. The phenomenon of salinity inversion reflected the process of salt supply from feldspar dissolution and salt consumption by carbonate cementation, confirming the pressurizing effect of hydrocarbon generation on pore fluids. Overpressured fluids constructed a nano-micron pore-fracture network through the dual mechanisms of "calcite dissolution-recrystallization throat expansion" and "clay mineral transformation-quartz cementation support". In the overpressured fluid environment, four types of calcite with different origins exhibited a δ18O shift path of "syngenetic micrite → diagenetic recrystallization → diagenetic cone-in-cone and columnar-fibrous calcite → fracture filling", leading to the preferential exsolution of 12C and 16O within the self-enclosed system and synchronous negative shifts in δ13C and δ18O. Organic acids preferentially dissolved micritic calcite to form pores smaller than 10 nm, and the recrystallization of calcite into a columnar-fibrous sparry structure expanded pore diameters to 20-50 nm. Hydrocarbon fluids in shale tended to accumulate at the edges of grains such as calcite and dolomite, forming micron-scale intergranular fractures. Silica derived from clay mineral transformation preferentially nucleated and precipitated in these areas where microfractures and pores developed, crystallizing into authigenic quartz crystals with particle sizes of 5-30 μm and euhedral-subhedral morphologies, thus exerting a synergistic effect of supporting, stiffening, and pore preservation. Different from the traditional model of silica filling pores, this mechanism enables authigenic quartz precipitation from clay-derived silica in intergranular fractures, thereby enhancing the stability of microscopic pores and fractures in the reservoir.