The Mechanism Of Carbonate Cements In Sandstone Of Yanchang8 Formation In Ordos Basin And The Effects On Reservoir Quality

The ordos basin of Triassic Yanchang formation is the most important oil-producing formation and oil–reservoir in China.It is rich in oil resources,and the exploration potential is tremendous.With 50 years of exploiting,Show that the Yanchang formation is the typical low permeability and porosity.And the carbonate cementation is the main cause of the dense sandstone.By use of the polarizing microscope,fluid inclusions?Stable Isotope of Carbon and oxygen and scanning electron microprobe and so on,we study mineralogy.petrology and geochemistry of carbonate cements in sandstone reservoir in Yanchang8 formation of the Ordos basin Discuss the mechanism of the carbonate cements and the sources of the material and the effects on reservoir quality.The rock types of the Basin in Yanchang8 formation are mainly lithic arkose?feldspathic lithic sand?lithic sandstone and a small amount of arkosic sandstone The types of rock fragments are mainly metamorphic lithic contents,The magmatic lithic and the sedimentary lithic are after.The interstitial material in researched area are calcite?dolomite?clay mineral?silica?gypsum?quartz?feldspar?organic mud etc.calcite is one of the most abundant minerals in the raw mineral.The lithology of reservoir is chief fine grained sandstone with good sorting and low psephicity.The intergranular contact type is mainly a point line contact,The form of cement is mainly porous cement.The statistical results of the granularity are shown,Lithology of the Chang-8 oil reservoirs is mainly fine sandstone.The skew statistics are mostly positive and positive,Modal statistics show that peak states are mostly intermediate? very narrow.There are two main types of carbonate cements in Ordos basin of Yanchang8 formation,The early microcrystalline calcite and the late sparite calcite The early microcrystalline calcite in the reservoir sandstone are enriched in heavier oxygen and carbon isotopes(¹³C‰-6‰?2‰,¹⁸O‰-16‰?-10‰)with low temperature of 80?by fluid inclusion,formed in early stage of diagensis,directly precipitation from the saturated alkaline lake.The sparite calcite are mainly ferro-carbonate with lighter carbon and oxygen isotopes(¹³C‰-3‰?-17‰,¹⁸O‰-25‰?-15‰).The fluid inclusion are indicated that the temperature are generaly higher than 80?,The main temperature range between 80?110?.It is formed in the period of early diagensis related to the decarboxylation of organic acids.in addition to the cations released by the conversion of organic acid decarboxylation and atmosphereric fresh water associated with feldspar and magmatic lithic.Fragments dissolution provide important sources Ca²⁺?Fe²⁺for such carbonate cementation.a part of sparry calcite are related to the interbedded mudstones dehydration and the conversion of clay minerals.pore fluid from nearby mudstones migrated from the edge to the centre of sandbodies,causing strong calcite cementation along the sandbody boundaries and forming tight cementation zones.The content of carbonate cement has an obvious effect on porosity and permeability of reservoir.Early microcrystalline calcite cement filling pore of pore jams,but the tight sand reservoir structure effectively increase the body's ability to fight further compaction,the sandbody in the middle of the preservation of abundant pore,have positive significance for reservoir construction.The large amount of iron calcite cement in the latter period severely reduces the porosity and permeability of the reservoir,which is a kind of damage to the reservoir.