Bioactivitiescontrol Paleogeographic Reconstruction And Dolomite Reservoirs

In the Ordos Basin,the carbonate reservoir of the Ordovician Majiagou Formation of the Lower Paleozoic strata contains extensive oil and gas resources.Specifically,the Ma5~5 sub-member is a typical dolomite reservoir.According to previous studies,the sedimentary facies of dolomite determines the state of organic materials.Therefore,sedimentary environment is a major determinant of the development of carbonate reservoir.Furthermore,the sedimentary environment of carbonate is usually inseparable from biological activities and fossils can provide important information about the former.Hence,research into body and trace fossils can provide guidance for the recovery of the sedimentary environment.Moreover,biological activities can play a constructive role in the formation of reservoir space.Thus,research into biological activities of the carbonate of the Ma5~5 sub-member can provide geological evidence for the recovery of the sedimentary environment and the distribution of high-quality dolomite.Based on systematic identification of the target trace fossils through field geological observations and analysis of rock cores,trace fossils of 4 genera and 17species are found in the Ma5~5 sub-member.According to their features,these fossils can be divided into the combination of Thalassinoides and Arenicolites,Thalassinoides,Bergaueria and Planolites,and Thalassinoides,Bergaueria and Palaeophycus.The combination of Thalassinoides and Arenicolites is mainly distributed at the east of the central paleo-uplift with intertidal zone as its sedimentary environment.The combination of Thalassinoides,Bergaueria and Planolites is located mainly in the eastern margin and its sedimentary environment is also identified to be intertidal zone.Lastly,the combination of Thalassinoides,Bergaueria and Palaeophycus is scattered across most of the middle-east area of the Ordos Basin with its sedimentary environment recognized as subtidal zone.Based on the basic study of petrologic characteristics,the trace fossils,palaeoecology of stromatolites and sedimentary geochemical information indicate that tidal-flat environment forms during the deposition of the Ma5~5 sub-member in the middle-east of the basin.According to the sedimentary environment indicated by the fossils,the intertidal zone can be divided into upper and lower intertidal zones.According to the distribution of fossils and combinations of lithic facies,it can be further classified into 9 microfacies,namely mud dolomite flat,gypsum dolomite flat,intertidal flat,tidal boundary flat,stromatolite dolomite flat,bioturbation(lime)dolomite flat,bioturbation lime flat and mud-crystallite lime flat.According to the distribution features of sedimentary facies,the influence of ancient landform on the Ma5~5 sub-member causes the differentiation of its sedimentary environment.Consequently,intertidal zone originates in the hollow sub-upland in the east.Meanwhile,because of the change of sea levels,the intertidal and subtidal zones develop alternatively.Through research into the basic features of reservoirs such as lithologic characteristics and physical properties,2 types of dolomite reservoirs related to biological activities in the Ma5~5 sub-member are discovered.One is stromatolite dolomite reservoir and the other is bioturbation dolomite reservoir.And the main reservoir space is lattice pore space in the stromatolite and the intercrystalline pores in the bioturbation dolomite.Through the comparison of the physical properties of the two,it is found that these two types of reservoirs both have good physical properties.The features of the structures of the pores discovered through the adoption of Computed Tomography indicate that the physical properties of bioturbation dolomite is controlled by the intensity of bioturbation and the dolomite with intensive bioturbation have higher porosity and permeability.Research into diagenesis shows that dolomitization is the primary diagenetic process.Through analyses of degree of order and geochemical information,it is believed that the primary dolomitization includes evaporative dolomitization in the syngenetic-penecontemporaneous period and seepage reflux dolomitization in the penecontemporaneous-shallow burial period.Specifically,the scanning electron microscope hasn’t detected any diagenetic features of relevant microorganisms in the stromatolite.Furthermore,its stable isotope is found the same as that of mud-crystallite limestone and presents itself as marine source fluid.And the proportion of rare earth elements is lower than that of limestone.Moreover,no aggregation of microorganisms is detected.Thus,it is limestone transformed by seepage reflux dolomitization.Besides,buried dissolution improves the state of the pores,while cementation,filling effect,compaction and pressure solution have destructive effect on the pores of the reservoir.In combination of the research into sedimentary facies and diagenesis,this essay analyzes the dominant effects on the dolomite reservoir and it is concluded that the target reservoir is controlled by the sedimentary facies and dolomitization.Both stromatolite and bioturbation dolomite reservoirs are controlled by sedimentary facies.Furthermore,through the analyses of degree of order and physical properties of dolomite with different degrees of bioturbation,it is found that the intensity of bioturbation influence positively the process of dolomitization and the physical properties of the reservoir.Therefore,biological activities strengthen dolomitization,have a constructive effect on dolomite reservoir and can promote the formation of high-quality dolomite reservoir.