Controls on Development and Classification of Gypsum Mold-Vuggy Reservoirs: A Case Study from the Majiagou Formation, Jingbian Gas Field

This study focuses on the gypsum mold pore-type reservoir of the Ma541 sublayer in the Majiagou Formation, northern Jingbian Gas Field. Based on core experiments and comprehensive geological analysis, it systematically reveals the reservoir development mechanism synergistically controlled by sedimentary microfacies and multistage diagenesis, and conducts reservoir classification and evaluation. The research findings show that the fine-to-medium crystalline dolomite containing anhydrite nodules/columnar crystals developed in the gypsum-bearing dolomite flat microfacies of the restricted platform facies constitutes the main body of the reservoir. During the sedimentary period, the intercrystalline pores of dolomite developed to form a basic reservoir-permeability network; during the epigenetic stage, meteoric water selectively dissolved anhydrite nodules, and the water absorption and expansion during nodule dissolution were accompanied by a large number of fracture seams, forming a superimposed reservoir-permeability network of ″intercrystalline (dissolved) pores, gypsum mold pores, and fracture seams″; alternating hydrothermal dissolution–precipitation during the burial period complicated the early reservoir-permeability network; the temporal and spatial matching between Indosinian tectonic fractures and the Yanshanian peak gas generation period formed vertical migration channels. This paper proposes the ″sedimentary basement construction-epigenetic dissolution and expansion-effective fracture improvement″ three-element reservoir-controlling mechanism, which systematically couples the key roles of the three stages (sedimentary basement, epigenetic dissolution, and tectonic fractures) and combines quantitative physical properties and pore structure parameters, breaking through the limitations of previous single-factor genesis analysis and qualitative classification evaluation for such reservoirs. A reservoir classification and evaluation standard is established based on pore type, pore structure, and physical property parameters. Among them, Class I high-quality reservoirs with a porosity of >8%, a gypsum mold pore proportion of >20%, and a filling index of <40% are concentrated in the superimposed area of the gypsum-bearing dolomite flat microfacies during the sedimentary period and the epigenetic paleokarst slope. This achievement provides a key theoretical basis for the prediction of high-quality reservoirs in similar evaporitic marine settings.