Study On Reservoir Characterics And Distribution Law Of The Middle Majiagou Fomation In Daniudi Gas Field,Ordos Basin

There are many wells drilled in the Ordos Basin under the weathered crust,which shows that the potential of natural gas exploration in the internal dolomite reservoir of the middle combination is very great.In this paper,based on the theory of petroleum geology,reservoir geology and carbonate sedimentology,through core observation,casting sheet,logging interpretation and other data,the formation characteristics,karst development and physical characteristics of the reservoir are clarified,and the main controlling factors of the middle combination reservoir are determined,and the favorable reservoirs in the study area are determined..At last,the favourable distributions of main producing layer was forecasted.From the previous study of the Ordovician Majiagou group,we know that the sedimentary environment was in the tidal flat at that time,and the six subsections of the middle middle Majiagou fomation corresponded to three marine and three regression sedimentary environments respectively.On this basis,the stratigraphic characteristics of the study area were divided and the stratigraphic development characteristics were described in detail.The sedimentary microfacies are further divided by lithofacies combination and sedimentary facies.In the study area,the M₅⁶ submember-M₅¹00 submember is a karst development area.In this paper,the karst breccia is identified and described by Loucks?1999?classification scheme of karst breccia,to identify and describe the karst breccia.According to the relationship between the breccia and the difference of the filling,the karst breccia is subdivided into five lithofacies,and further from the single well facies and typical cases.The development of karst facies is analyzed by continuous wells.Through core observation and data analysis for the casting thin sections,what can be reckoned is that reservoir space in the study area is mainly intercrystal pore,intercrystal dissolved pore,denudation vugs,cracks and micropore.The types of the reservoir pore space are porous type,corrosion pores type,fractured-porous-hole type and fractured type.According to the correlation analysis,the reservoir belongs to low porosity,low permeability reservoir.Based on the observation of cores,conventional thin section and casting thin section identification,cathodoluminescence,carbon and oxygen isotope analysis etc.,together with physical properties,systematic studied the diagenesis and the diagenetic environment of the middle Majiagou formation in Daniudi area,and establish diagenetic evolution sequence and porosity evolution mode.Based on the reservoir characteristics combined with the characteristics of the sedimentary environment and the comprehensive evaluation of the karst facies,the author conducted a detailed analysis of the main controlling factors of the reservoirs of the M₅⁵ submember and the M₅⁶ submember,and the reservoir distribution of the M₅⁵ submember is affected by the dolomitization,the distribution of reservoirs in the M₅⁶ submember is controlled by the karst facies;and the flat distribution characteristics of the M₅⁵ and M₅⁶ reservoirs in the study area are described and It points out the direction for the next exploration work in the research area.