Research On Formation Mechanism Of Tight Dolomitic Reservoir In Fengcheng Formation,Junggar Basin

The tight reservoir of the Fengcheng Formation in the northwestern margin of the Junggar basin is a crucial research point in recent years.So far,however,there are different understandings of the genesis of the dolomitic rocks,the formation mechanism of the dolomitic rocks reservoir and the main controlling factors.In this paper,according to collection of core and logging data,thin sections analysis,electron probe and isotopic analysis,the genesis of the dolomitic rocks,the characteristics of the dolomitic rocks reservoir,the type of diagenesis,the formation mechanism and the main controlling factors have been synthetically studied to reveal the formation mechanism of the high-quality dolomitic rocks reservoir.The study of tight reservoir,including petrology,mineralogy and geochemistry,shows it is mainly composed of dolomitic mudstone,dolomitic siltstone,dolomitic siliceous rock,argillaceous dolomite,silty dolomite,siliceous dolomite and dolomite,which are chiefly formed in alkali(salty)lake and hot water sedimentary environment.In the period of hydrothermal exhalation,the main manifestation of the dolomitic rocks is the mixture of hot water and alkali(salty)lake water.The lithology is characterized by lamellar interbeds siliceous rocks,dolomitic siliceous rock and dolomite with rich salt minerals.While during the interval of hydrothermal exhalation,the dolomitic rocks show still water deposits in alkali(salty)lake,and the lithology represents dolomitic siltstone,dolomitic mudstone and microcrystalline dolomite in the deep-semi and deep lake.The special sedimentary environment makes the development and enrichment of soluble basic salt minerals providing a good material basis for later dissolution.Studies show that oil and gas are mainly reserved in dolomitic mudstones,dolomitic siliceous rocks and siliceous dolomites,which contain rich salt minerals and/ or fractures.The reservoir space in Fengcheng Formation is mainly composed of intercrystalline,transgranular pores and fractures.It is typical fracture-pore-type or pore-fracture-type reservoir.Intercrystalline and transgranular pores are mainly developed in porphyritic dolomitic mudstone and fractured calcite veins.Fractures are mainly developed in siliceous rocks and dolomitic siliceous rocks.The diagenesis can be divided into:(1)the constructive diagenesis:dolomitization,deep corrosion,structural fracture and pressure dissolution;(2)the destructive diagenesis: compaction,cementing filling,silicification,dolomitization and recrystallization,according to whose influences on the reservoir of the Fengcheng Formation.The comprehensive investigation shows that the formation of high quality dolomitic rock reservoir in Fengcheng Formation is controlled by two factors(sedimentary environment,lithology)and epigenetic(tectonic,diagenesis),and structure,lithology and diagenesis(dolomitization and deep buried erosion)are the key control factors for controlling the formation of high-quality dolomitic rocks reservoir.