Study On Genesis And Reservoir Distribution Regulation Of Lacustrine Dolomite In The Fourth Member Of Shahejie(Es4) Formation,Shaojia Sag

The formation mechanism of dolomite has always been one of the focuses in sedimentology.This thesis takes lacustrine dolomite of upper subsection of Es4 in Shaojia sub-sag,Zhanhua Depression as the research object,based on the research of previous related research results,combined with methods of core observation,thin section analysis,geochemical data and the measured physical properties and production data.The formation environment,spatial distribution and formation mechanism of dolomite are studied in terms of petrology,mineralogy and geochemical characteristics.In addition,through the study of macro characteristics and microstructure of the reservoir,the types and physical characteristics of dolomite reservoir space are summarized,and the control factors of dolomite reservoir development are defined in the study area.The main achievements and conclusions of this thesis are as follows:?1?During the deposition period of Es4 Shaojia sub-sag is relatively closed basin,with high temperature,strong evaporation,high salinity water.The marginal glutenite facies,the slope carbonate facies and the central gypsum mudstone facies are successively developed from the edge to slope to the center of the basin.The upper subsection of Es4 is mainly composed of carbonate rocks,including reef,nearshore limestone beach and far shore limestone beach facies.The limestone beach is controlled by high frequency meter-scale cycle,and in the single cycle,clastic marl and granular dolomite?or algal dolomite?are developed in sequence from bottom to top.In the slope shoal sub-facies there develops granular dolomite composed of mud-microcrystalline dolomite in size of 2⁵um.Yidong fault zone slope reef facies develops micro-crystal powder dolomite composition of algal reef dolomite in size of 8⁴0um,while the dissolution of dolomite in the slot are visible by buried thermal fluid dissolution regrowth euhedral fine crystal dolomite.Granular dolomite and algal reef dolomite are dark red from cathodoluminescence image.And distribution of carbon-oxygen isotope and carbon-oxygen isotope of bioclastic limestone have a homology,indicating that its formation is early of diagenesis.Characteristics of interbedded laminated dolomite in different stages can be speculated that it is formed in surface evaporation environment.In the slot euhedral dolomite has a nonluminous core and cathode luminescence characteristics of dark red-fog center with bright side,with rare earth elements revealing the positive E_U anomalies may bring with the hydrothermal fluid of igneous rock,and part of wells in the fluorite,barite,celestite and undulate extinction quartz and other typical hydrothermal minerals more evidence being closely related with the buried thermal fluid activity.?2?In the upper Paleozoic Ordovician strata in the study area,baked carbonate was found to directly contact with igneous rocks?lamprophyre?.Dolomitization sparry calcarenite and marl occurs,which according to their occurrences can be divided into plaque micro-powder dolomite,banded micro-powde dolomite,dolomite of grain shaped,saddle dolomite and residue psammitic dolomite.Thermal degradation phenomena and zebra-shape structure is visible in cores,and cracks and pores in dolomite and symbiotic metamorphic quartz and patchy pyrite are visible in thin section,and the combination of residual dolarenite dolomite intercrystalline pore filling and cracks of quartz crystal in fluorapatite and hydrothermal minerals is also visible in scanning electron microscope.The dolomite of cathode coring section has characteristics of showing dark red light in luminescence.Dolomite powder and oxygen isotope relative to the Ordovician brachiopod have obvious negative drift and less than-10‰,the carbon isotope did not significantly drift.Microcrystalline limestone,residual dolarenite dolomite and powder crystal dolomite have similar patterns of distribution of rare earth elements,which causes fluid sources of the recrystallization or dolomitization are similar.It is speculated that the original Mg²⁺in seawater is hydrothermal baked,and high temperature reduced kinetic barrier transformation to calcite dolomite,which makes Mg²⁺get into the lattice substitution of Ca²⁺.The result of the direct crystallization of magmatic hydrothermal solution in the process of filling the cracks with dolomite and middle crystalline dolomite?3?The types of dolomite reservoir space in the upper four member of Shahejie Formation in the study area include intergranular dissolution pore,intragranular dissolved pore,framework pore,intergranular pore,cement dissolution pore,cave and structure,and diagenetic fracture.From the physical property,the grained dolomite and algal reef dolomite reservoir space are good reservoir in the study area.The intergranular pores formed by dolomite associated with hydrothermal roasting in Ordovician formation are obviously filled by two calcite vein fluids,which seriously reduce the quality of the reservoir.The reservoir in the study area is mainly controlled by the reef flat sedimentary environment,quasi contemporaneous supergene karstification,buried dissolution and hydrothermal dolomitization.