Research On Buried Hill Carbonate Karst Reservoir In Bozhong 21-22 Structural Area

As an important oil and gas exploration block in the southwest of Bozhong Sag,the buried hill at Bozhong 21-22 tectonic area is a new exploration field with low degree of exploration.Several oil and gas reservoirs of buried hill,including Bozhong 19-6 Archean metamorphic granite buried hill and Bonzhong 28 Paleozoic carbonate buried hill,have been discovered around Bozhong 21-22 tectonic area.The Paleozoic carbonate reservoir of the buried hill is developed with great potential of oil and gas exploration in Bozhong 21-22 tectonic area.Therefore,it is urgent to study the carbonate reservoir of buried hill in this area.Paper with carbonate reservoir at Bozhong 21-22 tectonic area as the research object,with petrology,sedimentology,imaging logging,carbonate reservoir geology theories as the instruction.On the basis of predecessors' research results,and cores,thin section observation,testing analysis,conventional logging and imaging logging data of scientific exploration well 1 and 2 in study area.This paper studies the rock type,sedimentary facies,reservoir space,diagenesis,karst type,structure and the factors that control reservoir development,and explores the development rules of reservoir.The Paleozoic carbonate strata of buried hills have been drilled in the research area,which are the Ordovician Yili formation,Liangjiashan formation and Xiamajiagou formation.The rock types are mainly dolomite,limestone,carbonate transition rock and interstitium.Further identificate micritic limestone,granular limestone,dolomitic limestone,crystalline dolomite,granular dolomite,limestone dolomite and carbonate silty mudstone.For the restricted platform facies and open platform facies,further divided into seven microfacies: restricted platform cloud(ash)mud flat,cloud lagoon,cloud gray lagoon,grey cloud lagoon,platform inner beach,open platform inner beach and interbank sea.The reservoir space is divided into three categories: pore,karst cave and fracture.Dissolved pores,dissolved caves and dissolved cracks are main types of reservoir space.Reservoir porosity ranged from 1.1?16.7%,with an average porosity of 4.3%.The reservoir in the study area have undergone a complex diagenetic process,and 10 diagenesis can be identified according to the diagenesis,the dissolution in the epigenetic rock stage is critical to the formation of the carbonate reservoir in buried hills.Through the core and imaging logging data.The beading karst cave,high Angle dissolution joint,low Angle dissolution joint,interlayer joint,layer dissolution hole,karst cave,muddy strip,dispersed dissolution hole and other karst elements can be identified in scientific exploration well 1 and 2.According to the development characteristics of each supergene karst zone,the karst structures in Wells 1 and 2 were classified:it is believed that the depth of the epigenetic karst in well 1 reaches nearly 150 m below the buried mountain surface,there are three vertical seepage belts-horizontal undercurrent belts and one strata dense belts,the total thickness of vertical seepage belts is 36 m,and the total thickness of horizontal undercurrent belts is 77 m.The epigenetic karst in well 2 reaches 180 m below the buried mountain,there are four vertical seepage belts-horizontal subsurface flow belts,the total thickness of vertical seepage belts is 38 m,and the total thickness of horizontal subsurface flow belts is 112 m.According to the comprehensive study,the main factors controlling the reservoir development in the study area are sedimentary facies,tectonic activity and karstification.The sedimentary microfacies affect the degree of dissolution.The development scale of microfacies in grey cloudy lagoon,cloud grey lagoon and cloud grey lagoon is large,and the development of intergranular pores in those microfacies is conducive to the enrichment of karst reservoirs.The paleogeomorphology generated by tectonic activity is the prerequisite for the occurrence of karstification.The resulting faults and fractures increasing the reservoir space and expand the dissolution range as an early channel of fluid.Multi-stage karstification is the key to reservoir formation.The present karstification structure in the study area is the result of the superposition of karstification in Caledonian epoch,Indo-Chinese epoch and Yanshanian epoch.The vertical seepage zone in the upper part of the buried hill is affected by cementation during burial period,resulting in poor physical properties.Large development scale of the horizontal subsurface zone in the middle and lower part with good storage space preservation,which is the main area for high-quality reservoir development.