Study On Characteristics And Main Controlling Factors Of Karst Reservoir In Ma₅¹-Ma₅⁴ Submember In Northeast Part,Ordos Basin

The Majiagou formation of Ordovician in Ordos Basin was strongly affected by Caledonian movement and subjected to long-term weathering and denudation,and finally formed unique weathering crust karst reservoirs.Deep understanding of the karst reservoir characteristics and its main control factors can correct evaluate and scientifically predict the distribution of the reservoir.Then petroleum geologists can look for the favorable reservoir development zone and achieve a major breakthrough in the natural gas exploration and development.Based on the theories of sedimentary petrology,petroleum geology and carbonate reservoir geology,according to regional geological data,through core observation and description,using casting thin section identification,cathode luminescence analysis and inclusion analysis,and combined with logging data,this paper choose the northeast part of Ordos Basin as the research region,and in turn study the sedimentary facies characteristics,karst reservoir characteristics,diagenesis and karst paleogeomorphology.Finally,the main controlling factors of karst reservoir in Yulin area were determined.This study mainly obtains the following conclusions.The sedimentary facies of the submember Ma₅¹-M₅⁴ in northeast part of Ordos Basin belongs to the tidal flat subfacies within the restricted platform facies,which can be further divided into five types of microfacies:gypseous dolomitic flat,anhydrite nodules dolomitic flat,muddy dolomitic flat,limy dolomitic flat and dolomitic flat.The Ma₅⁴ period was dominated by gypseous dolomitic flat.The Ma₅³ period was dominated by gypseous dolomitic flat.The Ma₅² period was dominated by gypseous dolomitic flat.The Ma₅¹ period was dominated by anhydrite nodules dolomitic flat.The reservoir rock types of the submember Ma₅¹-M₅⁴ in northeast part of Ordos Basin mainly include anhydrite nodules dolomite,powder crystal dolomite,micritic dolomite and so on.The reservoir space is mainly composed f intergranular pores,intergranular dissolved pores and fractures.In the longitudinal direction,the Ma₅¹ submember has the best physical properties,and the Ma₅² and Ma₅⁴ submembers were better,followed by the Ma₅³ submember.The porosity of the submember Ma₅¹-M₅⁴ karst reservoir is mainly distributed in the range of lower than 1%,and the frequency of low value porosity is obviously greater than that of high value porosity.The permeability was mainly distributed in the range of?0.001?0.05?×10^-3um²,and the frequency of low value permeability was significantly higher than that of high value permeability.The correlation between porosity and permeability is poor.Constructive diagenesis in the submember Ma₅¹-M₅⁴ in northeast part of Ordos Basin includes dolomitization,dissolution,degypsification and destructive diagenesis includes cementation,compaction-pressolution,gypsification and dedolomitization.The pore evolution of the karst reservoir in the submember Ma₅¹-M₅⁴ in Yulin area experienced the formation and reduction of primary pores in the early stage,the development and filling of secondary pores and fractures in the middle stage,and the development and filling of secondary pores and fractures in the late stage.The pre-carboniferous secondary paleogeomorphology in northeast part of Ordos Basin belongs to karst slope and karst basin,and can be further divided into several tertiary paleogeomorphology.On the plane,the karst reservoir in monadnock is the best reservoir.In the vertical,the karst reservoir in the vertical percolation zone and horizontal subsurface flow zone are the best reservoirs.The occurrence of karst in the syngenetic stage laid the foundation of the pore and hole system.The supergene karstification made the pores formed in the syngenetic stage continue to dissolve and erode.And because the underground hot water further dissolved the karst space along the fissures formed before,the karst in the burial stage gradually expanded.The main controlling factors of karst reservoir in northeast part of Ordos Basin include sedimentary facies,diagenesis and karst paleogeomorphology.The control of sedimentary relative reservoir is to affect reservoir physical properties through lithology and lithofacies.The physical properties of different lithologic reservoirs from good to bad are:anhydrite nodules dolomite>powder crystal dolomite>micritic dolomite>gypseous dolomite>limy dolomite>muddy dolomite.The reservoir physical properties of different sedimentary microfacies from good to bad are:anhydrite nodules dolomitic flat>dolomitic flat>gypseous dolomitic flat>limy dolomitic flat>muddy dolomitic flat.The control of diagenesis on reservoir is that dolomitization,dissolution and degypsification are favorable to the pore formation,while cementation,compaction-pressolution,gypsification and dedolomitization are detrimental to pore formation.In different tertiary paleogeomorphic units,the physical properties of the karst reservoirs in the submember Ma₅¹-M₅⁴ in the study area from good to bad are:monadnock>terrace>trough..Most of the favorable reservoir areas are distributed in the western part of the study area.During the Ma₅⁴-M₅¹ sedimentary period,these areas are not only in the dominant sedimentary microfacies zone,but also in the karst slope area,which is located on both sides of the trough with strong hydrodynamics and strong karstification,which is very conducive to the development of reservoirs.At the same time,some karst monadnock areas in the eastern karst basin are also favorable reservoir development areas.