Formation and preservation mechanisms of matrix pores in deep limestones: A case study of the Maokou Formation, central Sichuan Basin, China

Presently, large-scale oil and gas fields discovered in deep carbonate rocks are predominantly distributed within porous dolomite reservoirs and fractured-vuggy karst limestone reservoirs. However, recent discoveries in wells Pengshen 12 and Nanchong 2 in the Sichuan Basin confirm the presence of porous limestone reservoirs in the 2nd member of the deep Maokou Formation (also referred to as the Mao 2 Member). This finding challenges the traditional geological understanding of reservoirs, establishing the porous limestone reservoirs as a hot research topic. In this study, geological insights are gained using core and thin section observations, geochemical analysis of reservoirs, and reservoir identification and tracking through well-tied seismic interpretation. The results indicate that porous bioclastic limestone reservoirs are present in the Mao 2 Member. The dominant storage spaces include intergranular pores, biological cavity pores, moldic pores, and dissolved pores, which are formed in sedimentary and supergene environments. Specifically, the reservoir encountered in drilling well Pengshen 12 exhibits a cumulative thickness of 24.80 m, an average measured porosity of 5.1%, and an average measured permeability of 0.05 × 10^-3 μm². Early hydrocarbon charging and the presence of anomalous overpressure (overpressured compartments) are identified as key factors contributing to the deep preservation of pores formed during the sedimentary and early supergene stages. The intermittently distributed porous bioclastic limestones are wrapped by relatively tight micritic limestones, resulting in the formation of local anomalous overpressure within bioclastic shoals under the influence of ultra-high temperatures at great depths. Reservoirs in the Mao 2 Member are sandwiched by the floor of tight micritic limestones of the Mao 1 Member and the roof of tight mudstones interbedded with limestones from the Wujiaping Formation. This configuration facilitates the formation of regional anomalous overpressure in the Mao 2 Member. The mechanisms underlying pore formation and preservation suggest that the large-scale porous limestone reservoirs in the Mao 2 Member are developed under a combination of favorable conditions: intermittently distributed porous bioclastic shoals, early hydrocarbon charging, local anomalous overpressure within individual shoals, and regional anomalous overpressure. These reservoirs exhibit a laterally intermittent distribution. Based on the identification of bioclastic shoals, the roof and floor of the member, it is predicted through well-tied seismic interpretation that favorable bioclastic limestone shoal reservoirs cover an area of 9.5 × 10⁴ km². These insights provide a theoretical foundation for the occurrence of porous limestone reservoirs in deep parts, expanding the exploration targets of deep limestone reservoirs in the Sichuan Basin. Additionally, this study offers a valuable reference for the exploration of deep limestone reservoirs in other basins.