Formation Mechanism of Reef and Bank Limestone Reservoirs With Deep Burial Depth:A Case Study from the Mianyang–Guang'an Trough, Sichuan Basin

Key words: 
• Deep-buried limestone reservoirs /
• Changhsing Formation /
• Sedimentary-diagenetic facies associations /
• Mianyang-Guang’an Trough /
• Sichuan Basin

Abstract: Abstract：[Objective] The Upper Permian Changhsing Formation in the Mianyang–Guang'an Trough, Sichuan Basin, is characterized by deeply buried limestone reservoirs. Clarifying the depositional and diagenetic evolution processes can provide new geological insights for deep carbonate exploration in the Sichuan Basin. [Methods] Based on core and drilling data, petrological and geochemical investigations were conducted on the reef- and shoal-bearing limestone reservoirs within Member 2 of the Changhsing Formation in the Mianyang–Guang'an Trough. [Results] Reservoirs in the study area are developed in the 2nd Member of the Changhsing Formation. The lithofacies, including sponge framestone, sponge dolomitic framesone, sponge doloframestone, foraminifer/crinoid bioclastic grainstone, packstone and wackestone, indicate sedimentary environments of reef, bioclast shoal, and inter-shoal settings. Fabric-selective dissolution along fusulinid foraminifer shells is the dominant mechanism for porosity formation, which occurred under meteoric leaching conditions associated with short-term subaerial exposure. In terms of δ13C compositions, calcites from various lithofacies show values consistent with coeval seawater, reflecting formation in an early seawater-dominated diagenetic environment. In contrast, the relationships between δ18O values in calcite and dolomite contents, and between stratigraphic distribution of dolomite and sea-level variations, suggest penecontemporaneous dolomitization, which occurred under intensified evaporative conditions during sea-level lowstands. These limestone reservoirs evolved into relatively closed diagenetic systems during the Induan. Based on the integration of sedimentary and diagenetic processes, three sedimentary-diagenetic facies associations (FA1, FA2, and FA3) can be identified, with FA2 being the best limestone reservoir type. [Conclusions] The formation of effective limestone reservoirs with a deeply burial depth can be attributed to the interplay between deposition and penecontemporaneous diagenesis, which are controlled by sea-level fluctuations. Therefore, “sedimentary-diagenetic facies associations” are of importance with aspects of finding deeply buried limestone reservoirs.