Reservoir Characteristics And Main Controlling Factors Of The Reef Flats In The Changxing Formation, Wolonghe-Bodong Area, Eastern Sichuan

In this paper,the previous research results,field outcrops,logging data,core,thin section,physical property data and geochemistry data were used,and based on the theory of sequence stratigraphy,sedimentary petrology and carbonate reservoir geology to systematically study the reservoir characteristics,diagenesis types and reservoir formation mechanism of reef reservoirs of the interior of the carbonate platform in Changxing formation of Late Permian in Bandong-Wolonghe Area,eastern Sichuan Basin.This paper has achived several conclusions:(1)The weathering crust was found and confirmed at the top of Changxing Formation in eastern Sichuan Basin,and the vertical sequence of weathering crust was constructed.(2)The field outcrops,cores and thin section of the study area show that the reef-beach facies and non-reef facies are both affected by epigenic karstification.This paper summarizes the tpyical features of epigenic karstification in Changxing Formation,including "piebald"karst system,solution grooves and bed-parallel karst caves,which are filled by breccias and/or mixture of porous carbonate sands or silty sands.The remanent bioclastic grain fine-medium crystal dolomite in reef base/reef cap microfacies developed typical "piebald" karst system,mainly composed of residual intergranular(dissolved)pores,intercrystal(dissolved)pores,small caverns,and solution fracture to form pore type reservoir and fractured-porous reservoir,which is the best reservoir rocks in the area(3)This paper reviews the progress of global sea level rise and fall events around the Permian-Triassic boundary(PTB)by a large number of literature research which show that there was a long period of large-scale sea level fall in the late period of Late Changxing Age,and there was a long-term exposure in study area of upper Yangtze Block carbonate platform.The results show that the eogenetic karstification is the root cause of the formation of high quality reservoirs in the Changxing Formation,while the dolomitization is not significantly changed the pore structure of the reservoir,and moderate dolomitization is an important factor in reservoir pore storage due to it's supporting framwork.The burial dissolution and tectonic disruption have little contribution to the reef-besch reservoir in the study area,mainly used to adjust and reform the pre-reservoir.