Karst Characteristics And Developmental Patterns Of The Middle Permian Maokou Formation In The Eastern Sichuan Basin

The karstification characteristics,regularity in Maokou Formation of middle Permian in the East Sichuan Basin were studied by means of outcrop observation and laboratory analysis.The study area of Maokou Formation belongs to carbonate open sea platform facies.Bioclastic limestones of grain shoal facies and micrite limestones of interbank sea facies deposited in Mao 2 and Mao 3 member are overlapped vertically and horizontally.Then,Dongwu movement made seawater out of study area in the end of Middle Permian,developingl～3 Ma sedimentary discontinuity and exposed karst.Based on the study of outcrop and production data,a special karst phenomenon is identified:the experience differences in early diagenetic stage of diagenesis resulted in the differences of dissolution characteristics.the high energy granular rock of coarse grain forms a high permeable layer because of the retention of matrix pores and the dissolution of karst water.On the contrast,the pores of the denser argillaceous rock matrix are cemented to form the aquiclude,which is not conducive to the infiltration of karst water.The karst is mostly developed in the grain limestones which has high porosity and permeability.The interface between internal mixed fillings and bedrock is blur.At the same time,the lower karst dissolution scale in high permeability parts is lager than the upper part,which is filled by the underground river sedimentary.Compared with the grain limestones,the karst development in micrite limestones is less,only small vertical dissolution can be seen.Therefore,after the transformation of karst,the high permeability layer forms a layered karst system,and the compact layer forms small vertical grooves.When the thickness of the sedimentary rock is large,and the sealing effect of the lower dense layer is weak,the karst water is preferred to be dissolved in the larger intergranular pores.Once the water flows into a set,it will continue to select intergranular pores around the dissolution,with the accumulation of time,eventually forming a vertical seepage zone with the development of variegated karst.According to the observed characteristics of the karst,karst the vertical zonation is established,from surface to underground,can be roughly divided into surface karst zone,vertical vadose zone,horizontal subsurface flow zone and deep slow flow zone.The results indicates,after experienced eogenetic different diagenesis,polycylic superimposed permeability layer and dense water-resisting layer,crack and fracture system formed by tensile fracture activities jointly control conducting system of karst water during karstification,thus forming "same period multilayer" karst system controlled by grain shoal facies,in addition,karst reservoir can be developed both in karst platform and karst slope equally,which is diferent from the characteristics of tranditional karst apparently.The found of early diagenetic stage of stratabound type weathering crust karst may can provide new reference and added a new case in the further study.