Origin Of Dolomite And Controlling Factor Of Dolomite Reservoir In Ma5₅ Sub-member Of Ordovician Majiagou Formation In Ordos Basin

On the basis of the achievements of various previous research results,making full use of drilling core,conventional slice,SEM and geochemical test and other data,under the guidance of multi-disciplinary theories,this essay studied deeply and systemically the main controlling factors of dolomite genesis and reservoir development of Ma5₅sub-member of Ordovician Majiagou Formation in the Ordos Basin.The achievements and understanding of the paper are as follows:1.The type and distribution characteristics of dolomite of Ma5₅ in the study area are clearly defined.The dolomite of Ma5₅ in Ordos Basin can be divided into dolomicrite,muddy to powder dolomite and powder crystal dolomite,and that the dolomite are mainly distributed in the west margin of the dolomite flat,with the"mass"distribution of dolomite in the east-central depression.There may be a strait connecting the Qilian sea to the eastern depression in the northwest corner of Wushengqi in the period of Ma5₅,from the surface of which the normal sea water in the Qilian sea flows into the eastern depression,while through the bottom of which the high salinity water of the eastern depression flows into the Qilian sea.The seawater of Qilian forms a coastal flow at the edge of the depression,leading to the distribution of limestone between the dolomite flat of the west margin and the"mass"dolomite of the depression.2.The genetic mechanism of different types of dolomite is clarified,and the concept of carbon and oxygen isotope correlation is firstly put forward,and three kinds of correlation combinations are determined.Combined with geochemistry test data of different types of dolomite,such as carbon and oxygen isotopes,strontium isotopes,macro and trace elements and rare earth elements and so on,it is considered that micritic dolomite of dolomite flat is characterized by original color,fine structure,low order degree,"negative correlation"carbon and oxygen isotopes,high strontium isotope values,linear positive correlation of MgO-CaO,high content of Na and Sr as well as the low content of Fe and Mn,which indicates that the micritic dolomite are caused by syngenetic and quasi syngenetic dolomitization.The environment of micritic dolomite of the depression is similar to that of the Lagoa Vermelha Saline Lake,which has the possibility of microbial dolomite.The muddy to powder dolomite has a mud crystal-powder crystal structure,with a homogeneous distribution or with micrite dolomite distributed in the intergranular of fine powder crystal dolomite.Meanwhile,the muddy to powder dolomite has deep primary color,and the crystals of its fine powder crystal dolomite are euhedral or subhedral.The degree of order of the muddy to powder dolomite is middle-high,and that of the dolomite in depression is higher than that in platform.The carbon and oxygen isotopes are"normal".Compared with the micritic dolomite,the muddy to powder dolomite are characterized by a large decrease in Mn and a slight increase in Zn,indicating that it is formed by the reflux permeates dolomitization.Due to the close distance to the west edge of tidal flat,the dolomitization fluid of dolomite flat is the magnesium rich salt water which comes from the tidal flat.On the contrary,due to the long distance to the tidal flat,the dolomitization fluid of the depressions are magnesium rich salt water which comes from the upper Ma5₄.The coarse-grained crystalline structure of the powder crystal dolomite has a high order degree,and the carbon and oxygen isotopes have different combination characteristics of"normal"and"negative correlation".The strontium isotopes are higher,Fe,Na,K are reduced,and the trace elements are increased,indicating that the burial dolomitization is formed.The malformed dolomite filled in the cave are with"positive correlation"carbon and oxygen isotopes,as well as the high homogenization temperature and salinity.These characteristics indicate that there is no large-scale formation of tectonic hydrothermal dolomite in this area,only the dolomite filled in the cave are with tectonic hydrothermal origin.3.The main controlling factors of the development of dolomite reservoir are analyzed,and the division scheme of dolomitization phase is put forward for the first time,and the influlopence of dolomitization microfacies and dolomitization mode on the devement of the reservoir is discussed.The dolomite reservoir is obviously controlled by the sedimentary microfacies.The dolomite is developed in dolomite flat microfacies,and the dolomite in the depression distribute in a"mass".Dolomitization is the key to dolomite reservoir formation,thereinto,center microfacies of dolomitization phase forms mainly powder crystal dolomite.The dolomitization is fully and completely,with sufficient fluid and strong motivation,and the dolomite is given priority to anhedral form,contacting with mosaic texture,with poor physical property that there is very little or almost no obvious intercrystalline pore.The dolomite developed in transition microphase is mainly powder crystal dolomite as well,and the dolomitization is complete but not sufficient.The dolomite shows mainly idiomorphic or hypidiomorphic granular structure,with point and the line contact,developing intercrystalline pores,seams so that the physical properties are good.The marginal microfacies include homogeneous dolomitization and differential dolomite,in which the homogeneous dolomitization forms homogeneous dolomite or calcitic dolomite,and the metasomatism is inadequate and the porosity is not good,while in which the different dolomitization develops porphyritic dolomite,and most of the speckles are coarse structures,and the physical properties are better with intercrystalline pores and even intercrystalline dissolved pores.4?The history and characteristics of dolomite pore evolution of Ma5₅ in study area are clarified.In the study area,the formation of pores in dolomite reservoir of Ma5₅ is mainly affected by dolomitization and dissolution.Dolomitization controls the formation of pore in dolomite,and retains a small amount of intercrystalline micropores in the syngenetic and quasi syngenetic dolomitization,and the intercrystalline pores and intercrystalline dissolved pores are developed by the infiltration reflux dolomitization after the penecontemporaneous,and the intercrystalline dissolved pores were formed by burial dolomitization.In the west margin of the platform,some plaster mold holes are produced in the dolomite due to the affection of epigenetic dissolution;Epigenetic dissolution produces a large number of dissolution pores,holes as well as seams in dolomite,but the later filling affects the preservation of the pores.Among them,the karst caves related to epigenetic karstification are mainly distributed in the platform edge where is higher and more easily to get weathered and denuded.The intercrystalline pores and intercrystalline dissolved pores,which are related to infiltration reflux dolomitization after the penecontemporaneous,are mainly distributed in the platform near the tidal flat where the high salinity fluid promotes the occurrence of infiltration reflux dolomitization.The solution pores related to the burial solution are filled with giant crystal calcite,remaining a small amount of dissolved pores,which is related to the deep hydrothermal solutions provided by the basement fault.