Abstract: The Changxing Formation on the Mianyang-Guang′an shallow-water shelf in central Sichuan is rich in oil and gas resources and can serve as an important successor exploration area.The Changxing Formation primarily develops pore-type limestone reef-shoal reservoirs,which is a new type of deep carbonate reservoir.In view of the insufficient understanding of the reservoir characteristics, the lower limits of physical property,and main controlling factors of the Changxing Formation limestone reservoir,a study was carried out based on drilling data and a variety of analytical methods including core and thin-section observation,capillary pressure analysis,gas-water infiltration tests,and production capacity simulation.The basic reservoir characteristics,the lower limits of physical properties,the pore preservation mechanism of the limestone reservoir,and the main factors controlling reservoir development were investigated.The results show that reservoir rocks in the study area are mainly sponge-framework reef limestone and bioclastic limestone,with pore space do minated by organism cavity pores,intergranular dissolution pores,and intragranular dissolution pores.Overall,the reservoir exhibits moderate porosity and low permeability,with localized zones of high porosity and permeability.Multiple experimental methods cross-validated that the effective porosity lower cutoff for the Changxing Formation limestone reservoir in central Sichuan is 2.10%,the water saturation upper limit is 48.9%,and the permeability lower cutoff is 0.010 mD.The reef-shoal body provides the material basis for reservoir development,and a relatively closed overpressure diagenetic system is key to preserving pores in the deep Changxing Formation reef-shoal complex limestones.Furthermore,during early hydrocarbon charging,the emplacement of hydrocarbons reduces the nucleation sites for cement growth,which is one of the important conditions for pore preservation.This study provides a theoretical basis for the development of deep pore-type limestone reservoirs in the Sichuan Basin.It also offers reference for the identification of effective reservoirs and the estimation of natural gas geological reserves in the deep Changxing Formation reef-shoal gas reservoirs of central Sichuan.

Key words: Mianyang-Guang′an shallow-water shelf, deep pore-type limestone reservoir, the lower limits of physical properties, pore preservation mechanism, main controlling factors, Changxing Formation