Research On The Reservoir Characteristics Of The Middle Permian In The South Central Sichuan

This paper takes the Qixia and Maokou Formation of Middle Permian in the south of central Sichuan basin as the research objects,guided by multi-disciplinary theories of classical sedimentology and reservoir geology,adequately applying with field outcrops,drilling section,well logging and experimental data of geochemistry,to study sedimentary facies and characteristics of reservoirs systematacially.The south of central Sichuan basin deposited huge thick marine carbonate sediment in Middle Permian period.Based on petrologic characters,biological features and electric characters,the Qixia Formation is divided into the first subsegment and the second subsegment.Meanwhile,the Maokou Formation is divided into four subsegments.The lithology of Qixia formation are mainly charcoal-grey,grey,thick to massive micrite bioclastic limestone and bioclastic limestone,and locally flint-bearing nodules,which are marked by mottled micrite dolomitic limestones in the second subsegment;In the Maokou formation,it is mainly developing eye-like limestone in its lower part,light gray thick-bedded micrite bioclastic limestone and sparry bioclastic limestone in the upper,locally flint-bearing nodules or silica thin layers and dark grey limestone in the top,containing small amounts of shale and carbonaceous rocks.Through the district sedimentary facies marks,combined with regional sedimentary background research,the sedimentary facies of researched area can be divided into four subfacies,they are the open sea and inner platform bank subfacies,interbank subfacies,restricted lagoons.And the characteristics of each subfacies and microfacies are taken a deeper research.On the basis of the comparison of wells and profiles,the temporal and spatial distribution of the sedimentary facies and vertical evolution characteristics of paleogeograhpy have been analysed systematacially.The results show that bank subfacies are much developed in the second subsegment of Qixia Formation and the third subsegment of Maokou Formation than other facies,and most of the central area are covered by bank subfacies.According to the lithology characteristics and kinds of tests,it indicates the characteristics of reservoir rocks and reservoir spaces of Middle Permian in research area.Resevoir rocks mainly include powder-macrograin crystal dolostone,dolomitic limestone,limy dolostone and grain limestone;Reservoir spaces are mainly comprised of intercrystalline(dissolved)pore,intergranular(dissolved)corroded pore,secondly the dissolved fracture and cave.Through the relationship of these pores and caves,the most important reservoir is slit-cave style,which is low porosity and low permeability.Meanwhile,on the basis of stratigraphy,sedimentary facies,combining logging and drilling test results,the reservoirs of Middle Permian in the research area are mainly distributed in the second subsegment of Qixia Formation,the B second subsegment and the third subsegment of Maokou Formation.Among them,the reservoirs of the second subsegment of Qixia Formation mainly develop in the middle of the study area,its gross thickness is 2-18m;the reservoirs of the B second subsegment of Maokou Formation mainly develop in the western region of Guangan-Huaying,its gross thickness is 8-20m;the reservoir of the third subsegment of Maokou Formation pretend a tendency of strong heterogeneity,its gross thickness is 3?32m.Combined with previous results and analysis of sedimentary facies and reservoir characteristics,it is discovered that there are four factors,which include favourable sedimentary facies,constructive diagenesis,paleogeomorphology and fracturing action,affect the quality of reservoirs.The inner platform bank subfacies is the most important factor to decide the temporal and spatial distribution of earlier reservoir;constructive dissolution and dolomitization decide the quality of the late stage of the transformation of its reservoirs;the steep of paleogeomorphology affects the development of dissolution,which is the favorable reservoir region;superposition of tectonism improves the permeability ability of the reservoir,which lays a foundation for the quality of the high-quality reservoirs.