Relationship Between Shoals Distribution And Karst Reservoir In Middle Permian Maokou Formation,Southern Sichuan Basin

The Middle Permian Maokou formation in southern Sichuan Basin developed karst reservoir,and the distribution of karst reservoir is controlled by the denudation paleogeomorphic highland and paleofault.This paper focuses on the influence of the development of shoals in Maokou formation on the karst reservoir,and reveals the control mechanism of shoals on the development and distribution of layered karst.In this paper,based on outcrop,drill-core,well log and petrographic thin-section data,the sedimentary and diagenesis characteristics of Maokou formation in southern Sichuan Basin are studied,especially the influence of pore differential evolution of carbonate rocks on karstification after shallow burial.There are eight rock types can be identified from the Maokou Formation in the study area.Open platform facies can be divided into three subfacies:shoals(including high-/low-energy shoals),intershoal sea and open subtidal.The distribution of vertical shoals is controlled by the sea-level changed,and high stand system tract of sequences(P₂m² and P₂m³)are developed.The planar distribution of shoals is controlled by the sedimentary paleogeomorphologic changes formed by the basement uplift,and developed in the Luzhou paleo-uplift area.The Middle Permian Maokou formation in southern Sichuan Basin was uplifted and exposed for 7.5Ma only after early shallow burial.The carbonate rocks in shoals have preserved good primary pores and become the best strata for the development of groundwater seepage channels and dissolution pores.Bioclastic grainstone in high-energy shoals have high porosity and permeability after their shallow burial,which are conducive to karstification influence and show large-scale layered caves.Porosity and permeability of bioclastic packstone in low-energy shoals are slightly low than those of bioclastic grainstone,and the scale of dissolution fractures and caves are smaller,which is mostly filled with bioclasts,siliceous materials and calcite cement.Bioclastic wackstone and argillaceous bioclastic limestone in the intershoal sea and open subtidal environments are compacted before their shallow burial,and karst is not developed.Shoals deposition is the basement for karst reservoir,and the palaeokarst geomorphic highland is the key for karst reservoir.Shoals developed in karst platform and karst steep slope is most beneficial to form the high-quality karst reservoir.