Genesis And Main Controlling Factors Of The Middle And Lower Dolomite Of The Majiagou Formation In The Wuqi-Ganquan Ordovician,Yishan Slope,Ordos Basin

In recent years,many large gas fields have discovered in the lower Paleozoic Ordovician in the southern part of the Yishan slope in Ordos basin are mainly concentrated in weathering crust.With the continuous deepening of exploration work,many wells have made breakthroughs in the middle-lower assemblage of Ordovician Majiagou Formation.For example,Su203 and Su345 in middle assemblage have both obtained high-yield industrial gas flow with a daily production of over one million cubic meters and Yan 671 and Yan 1097 in lower assemblage have both achieved industrial gas flow with a daily production of 10000cubic meters.These indicate that the dolomite of the middle-lower assemblage in the area has great exploration potential and can be used as a favorable replacement horizon for natural gas reserves.However,previous studies have mostly focused on the karst weathering crust,and few studies have been conducted on the lower-middle assemblage of Majiagou Formation.Therefore,it is necessary to conduct a comprehensive and systematic study on the origin and main controlling factors of dolomite in the middle-lower assemblage in the study area.Based on the new theories of petroleum and natural gas geology,sedimentary petrology,carbonate reservoir geology,geophysics and so on,fully absorbing the latest research results of predecessors,using the method of logging technique,through the geological data such as core,common slices,casting slices,cathodoluminescence,scanning electron microscopy,mercury penetration and major and trace elements,rare earth elements,carbon and oxygen isotopes,this paper makes an in-depth study and detailed analysis on the lower and middle assemblage strata,sedimentation,diagenesis,reservoir characteristics,reservoir control factors and other aspects of the Wuqi-Ganquan Majiagou Formation in the southern part of the Yishan slope in Ordos Basin.The formation mechanism and development control factors of the lower and middle assemblage reservoirs in Majiagou Formation in the study area are clarified,the favorable areas are predicted,and specific exploration deployment opinions are put forward.The specific understanding is as follows.In the lower and middle Ordovician Majiagou Formation in Wuqi-Ganquan area in the southern part of the Yishan slope in Ordos basin,there are mainly 4 subfacies named open platform,restricted platform,evaporation platform and platform margin,and 6 microfacies which are dolomitic flat,Ointment dolomitic phase,cream dolomitic phase,cream lake phase,beach bar and shoal,among which dolomitic flat and Ointment dolomitic microfacies are favorable sedimentary facies for reservoir development in the study area;There are three types developed in the middle-lower assemblage,which are Mud microcrystalline dolomite,Powder fine crystal dolomite and fine medium crystal dolomite.Dolomitization fluids are all marine fluids,which exist two kinds of dolomitization named quasi-syngenetic dolomitization and seepage-reflux dolomitization in shallow burial period.Mud microcrystalline dolomite and fine grain dolomite are quasi-syngenesis stages with relatively open,low oxidation and low temperature,while fine medium crystal dolomite is in shallow burial reflux period with relatively closed,low reduction and relatively high temperature;The reservoir has undergone various diagenesis,such as dolomitization,filling,dissolution,cementation,recrystallization,and gypsum removal,among which dissolution,dolomitization,and recrystallization have constructive effects on the formation of the reservoir.Four diagenetic phases,namely,seepage reflux cloud facies,quasi-syngenetic cloud phase,dolomite gypsum removal,and karst breccia facies,are favorable diagenetic phases for the development of the reservoir in the study area;The reservoir types are mainly of Class ?-medium type.The pore structure is mainly of 4 types:large-coarse pore throat structure,large-medium pore throat structure,medium-fine pore throat structure,micro pore throat structure and micropore-micro throat structure.The neutron and density responded best to the physical properties of the target layer.It was found that the Ma₅^5Ma₅⁶ sub-segment has relatively large pore permeability and wide distribution area,which is a favorable reservoir development interval.The effective reservoirs are mainly distributed in Jingbian,Wuqi and Zhidan areas.The accumulated effective reservoir thickness of the single layer of the sub-member Ma₅^6Ma₅⁵ is about 4m on average,with the maximum thickness reaching 10m.It is the main development section of the effective reservoirs in the study area.The formation and development of the middle and lower reservoirs of the Ordovician Majiagou Formation are controlled by various factors such as paleogeomorphology,sedimentary facies and diagenesis.Paleogeomorphology is a prerequisite for the formation of different types of sedimentary environments,sedimentary environments are the basis for the formation of dolomite reservoirs,diagenesis is an important guarantee for the formation of various types of reservoir spaces,and paleotectonics is an important assistant for the formation of high-quality reservoirs.However,paleogeomorphology,sedimentary facies and diagenesis are the key factors for the formation and development of reservoirs;The south of Jingbian-northeast of Wuqi-south central of Zhidan?Ma4-Ma₅⁷ sub-sections?,the south of Jingbian-northeast of Wuqi-southwest of Zhidan-southwest of Ganquan and northeast of Fuxian?Ma₅⁵ and Ma₅⁶ sub-sections?are favorable zones for natural gas enrichment in the study area.