鄂尔多斯盆地奥陶系马家沟组四段优质白云岩储层发育特征及成因机制OA北大核心CSTPCD

Well Mitan 1 in the Ordos Basin achieves a new discovery of natural gas in the dolomite section of the 4th Member of the Ordovician Majiagou Formation(hereinafter referred to as the Ma 4 Member),showing good natural gas exploration prospect,but the development characteristics and distribution laws of the dolomite reservoirs of Ma 4 Member are not understood clearly,which restricts their exploration deployment in the next step.Based on the data such as core,thin section,industrial CT,U-Pb dating,physical property analysis and geochemical characteristics,this paper systematically studies the development characteristics and genetic mechanisms of the dolomite reservoirs in the Ma 4 Member,and predicts the distribution of dolomite reservoirs and the favorable natural gas exploration zones in the Ordos Basin.And the following research results are obtained.First,in the Ma 4 Member,mound-beach facies dolomite reservoir and porphyry dolomite reservoir are mainly developed,among which,the former is lithologically composed of psammitic-psephitic dolomite,fine to medium crystalline dolomite,powder to fine crystalline dolomite and stromatolitic dolomite,and the latter is lithologically composed of porphyry powder to fine crystalline dolomite,porphyry powder crystalline dolomite and porphyry limy dolomite.Second,the reservoir space in the mound-beach facies dolomite reservoir is dominated by intercrystalline(dissolution)pores with a small number of dissolution cavities,residual intergranular pores and microfractures,with an average porosity of 5.06%.The reservoir space in the porphyry dolomite reservoir is dominated by intercrystalline pores,with a small number of intercrystalline dissolution pores and microfractures,with an average porosity of 2.46%.Third,the development of mound-beach facies dolomite reservoir is mainly controlled by microbial mounds and granular beaches,and has undergone penecontemporaneous dolomitization and burial dolomitization.The development of porphyry dolomite reservoir is mainly controlled by biological disturbance,penecontemporaneous dolomitization and burial dolomitization.Fourth,the development of mound-beach bodies,the degree of biological disturbance and the formation of dolomitized fluids are under the joint control of the tectonic paleogeographic pattern and sea level fluctuation.In conclusion,the high-quality dolomite reservoirs of Ma 4 Member are distributed in a large scale along the central paleo-uplift and the Shenmu-Zhidan low uplift.In addition,the dolomite reservoirs of Ma 4 Member in the Shenmu-zhidan low uplift are located in the lithological facies change zone,where reservoir-caprock configuration is good and natural gas is enriched with a predicted favorable exploration area of 18000 km2.