Study On Sedimentation Patterns And Favorable Reservoirs In The Majiagou Formation In The Southeastern Ordos Basin

Ordos Basin is a multi-tectonic,multi-cycle and multi-type basin.The carbonate in Majiagou Formation is thick and widely distributed,besides,dolomization and gypsum are also frequently occur within the formation,thus provide a basic reservoir forming conditions for petroleum.In the southeast part of the basin,Majiagou formation was greatly affected by Qinqi trough,Helan aulacogen and ancient uplift.And because of the degree of exploration in this area,The sedimentary microfacies and sedimentary model in this area have great controversy.Therefore,it is of great significance to further study the sedimentary system of majiagou formation in southeastern Ordos basin.Guided by the theory of sedimentology,this study through the field core observation,logging data analysis,rock slice polarizing microscope,major elements and trace elements analysis method,established the sequence stratigraphic framework in the study area,clear the sedimentary system in the study area,sedimentary microfacies combination,sedimentary environment evolution and sedimentary facies model,and draw the following conclusion:(1)According to the thin section and the records of drill core,the rock type in Majiagou formation in research area include argillaceous limestone,sub-argillaceous dolomite,dolomitic gypsum,gypsum micrite dolomite,sub-gypsum micrite dolomite,pelletic dolomite,argillaceous dolomite,argillaceous lime-dolomite and argillaceous limestone.In different part of research area,the type of carbonate is significant different.In the east part of research area,the main carbonate type of M5₄and M5₃are argillaceous dolomite and argillaceous dolomite which indicate a sub-tidal sedimentary environment,since M5₂and M5₁ period,argillaceous dolomite can be observed,it suggest that the water body was gradually became shallower.In the middle part of research area,the rock type contain dolomitic gypsum,gypsum micrite dolomite and sub-gypsum micrite dolomite,clearly,it suggests a evaporitic environment.In the west part,the rock type are mainly in M5₄and M5₃are argillaceous limestone with horizontal bedding,it reflect a relative deeper water body with weak hydrodynamic environment.The color of carbonate is dark,which means the sedimentary environment was under a reducing condition.And in M5₂and M5₁period,gypsum can be observed,the color of carbonate got lighter,which indicate a shallower tend of water body and the sedimentary environment was getting evaporation.(2)Based on the well logging information and the drill core character,the microfacies is clear.The research area contains lagoon,sub-gypsiferous dolomite tidal flat,micrite dolomite tidal flat,gypsum depression,gypsiferous dolomite tidal flat,tidal channel.On the basis of single well and well correlation with sequence profile,the following questions are clear:there are one transgression and three regression during M5₁-M5₄,within M5₄ period,open sea and lagoon was connected by tidal channel,the microfacies included lagoon,upper tidal zone and inter-tidal zone,within M5₃ period,because of the regression,upper tidal zone expended,which lead a shrink of tidal channel,the interaction of open sea and lagoon decreased.During M5₂ and M5₁ period,because of the decrease of the sea level,the tidal channel disappeared,normal lagoon turned to evaporating lagoon,the upper tidal zone got more larger,and developed more gypsum dolomite which indicate the evaporitic environment.(3)In the most part of the research area,the sedimentary environment was mainly evaporitic environment,dolomite is mainly dolomicrite,the order degree of dolomite concentrate in 0.52 to 0.7,the order degree is relative low,the ratio of Sr/Ba is lager than 1 which indicate the dolomite formed in salty water,the carbonate petrologic features,sedimentary environment,order degree and trace elements character suggested the dolomite in research area formed within pene-sedimentary period,the genesis mechanism is the sabuha genesis model.The research area was affected by multi-tectonic and multi-diagenesis which lead a complex character of diagenesis of 7 types of diagenesis and diagentic evolution.By the studying of sedimentary microfacies and diagenesis,the main reservoir space is gypsum mold pore.The rest of space such as fissure,stylolite and other structures can be a favorable migration channels for petroleum.(4)There are four types of reservoir types in the study area.The lithology of type I is dolomite with dissolved pores,type II is the dolomite with gypsum moldic pore,type III is rauhwacke and micrite dolomite with microfissure,type IV is micrite dolomite,and can not be the reservoir.The physical property of M5₁ and M5₂formation are relatively better,porosity can reach to 11.45%,mean permeability is0.42md,but the total bulk of type I and II reservoir are only 7.47%,the main type is type III.The lithology of M5₃ and M5₄ formation are mainly argillaceous dolomite and micrite dolomite,the influence of diagenesis is relatively small with poor physical property.