Carbonate diagenetic products and processes from various diagenetic environments in Permian paleokarst reservoirs: a case study of the limestone strata of Maokou formation in Sichuan Basin, South China

Understanding the processes and products of carbonate diagenesis is crucial to the exploration and optimum development of palaeokarst hydrocarbon reservoirs in Permian carbonate rocks. Several well-documented outcrops and well drills in the Permian marine carbonate strata around the Sichuan Basin, South China, encompassing well-developed paleokarst landforms in the Middle-Upper Maokou Formation have been investigated. The sections and cores recording paleokarst reservoirs exhibit very similar carbonate diagenetic products and processes from various diagenetic environments by much most cementation and formation of secondary porosity in carbonates. The major diagenetic environments experience the zones of seawater, surface or near-surface atmospheric water, and subsurface mixed water between marine and meteoric. Characteristic diagenetic textures produced in each environment are described and different generations of calcite cements related to the process are responsible for their formation by sedimentary petrology and stable isotope geochemistry. Paleokarst under the constrained condition of surface and near-surface atmospheric water is the most favorable for oil–gas accumulation and presents some karst-related pores-caves-fissures with less or no fillings. Dissolution and corrosion of carbonate by atmospheric freshwater can alter the accumulation spaces and develop better fractures-caves system. These characteristic could exhibit some substantial data for better predicting porosity trends in Permian carbonate rocks and also provide important geological information for further exploring oil–gas resource in marine carbonate strata.