Genesis And Evaluation Of Dolomite Reservoir In Lower Ordovician Ma-5₅ Submember In Eastern Ordos Basin

The Ordos Basin is one of the important energy supply bases in China,where the dolomite has a large number of layers,diverse types and a wide range of distribution.It is the most important gas reservoir in the Lower Paleozoic.The submember of Ma5₅ is one of the most important gas-bearing formations of the Eastern Lower Ordovician,showing great exploration potential and becoming the most realistic carbonate oil and gas exploration area in the basin after the paleo-karst reservoirs.Therefore,further research is needed to explore the issue between dolomite and reservoir development in this area.In this study,we focused on the dolomite reservoir of Majiagou Formation of the Lower Ordovician in eastern Ordos Basin.The study is guided by theories of sedimentology,lithofacies and palaeogeography,reservoir geology with previous research results,a comprehensive evaluation of basic characteristics of reservoirs,diagenetic evolution and formation mechanisms of Ma 5₅are studied.The main conclusions are as follows,?1?The types of dolomite reservoir rocks in Ma 5₅ are dominated by intraclastic dolostones,residual intraclastic dolostones,silt-crystalline and fine-crystal dolostones,dolomite limestones,and mud-microcrystalline limestones;the types of reservoir space are dominated by intercrystalline pore,intergranular pore,intergranular dissolved pore,dissolved pore and micro-fracture.Correlation between porosity and permeability is poor.The reservoir is characterized by low porosity and permeability.The pore structure is relatively complex.It is a low-porosity tight reservoir and the thickness of the reservoir is gradually thinning to the east.?2?The dolomite is mainly crystal-powder dolomite and fine-crystalline dolomite,the value of?¹⁸O is negative,the values of Sr,Mn and Na are relatively low,the values of Fe are relatively high,There is no obvious anomaly or weak negative anomaly in Ce and Eu is a weak negative anomaly,it is considered the dolomites in study area are cause of shallow burial dolomitization.?3?The distribution of dolomite reservoirs are controlled by sedimentary microfacies,dolomization,and dissolution.The tidal flat has relatively good fluid seepage capacity,which is conducive to later dolomitization.It is the most favorable type of microfacies for natural gas exploration for Ma 5₅ submember.The dolomitization is the main diagenesis of the reservoir in the study area,which has a favorable influence on the pores and seepage.The dissolution is the main reason for the further formation of dissolved pores,which is beneficial to increase the porosity and connectivity of carbonate reservoirs.?4?A comprehensive evaluation of the dolomite reservoirs in Ma 5₅.Two reservoir areas are speculated including one type I reservoir area,two type II reservoir areas,and one type III reservoir area.Among which,Type I is the most favorable exploration target,whereas type II is relatively favorable exploration target.