A Study On Development Regularity Of Lithofacies Of Fine-Grained Sedimentary Rocks In Lucaogou Formation,Jimsar Sag

Fine-grained sedimentary rocks in Permian lucaogou formation in Jimusar sag are important replace areas of the tight oil and gas exploration in China.It has complex mineral components in reservoir of these fine-grained sedimentary,such as dolomite,clastic,volcanic,clay minerals and organic matter composition of gibberish mixed,arranged and laminated combinations,and with strong heterogeneity in vertical direction that affect tight oil enrichment.This research used well log information,core and analysis under cathodoluminescence microscope,fluorescent microscope,SEM,XRD and geochemical data to descrise microscopic fabric,sum up lithofacies,study the relationship of lithofacies associate in different environment and development law.Established lithofacies development mode of fine grain sedimentary rocks and discuss the forming process of lithofacies in lacustrine fine grain sedimentary rocks through material source and deposition dynamics.The result of the thesis enriching lacustrine sedimentology theory,and provide basic geological basis for tight oil exploration.(1)Established lithofacies classification scheme.The study area has diversified rock types that grayish carbonate and sandstone mainly developped on lake edge.Carbonate rocks include clastic particles in dolomite,oolitic dolomite and micritic dolomite sand particles.And sandstone including pebbly sandstone and siltstone.Dark grey to black fine-grained mixed sedimentary rocks mainly developed in central basin,which compose dolomitic,tuff,silt and organic matter.This research used "Three end-member and four-component unit" to classify fine grain mixed sedimentary rock.At first,divided the fine mixed rock into organic matter organic matter medium and organic lean by 1.5% and 4% of TOC value.Then drawing a triangle diagram associated the value of dolomite(chemical composition),quartzs and feldspars(siliciclastic components)and tuff(volcanic component)content.Divided fine-grained mixed sedimentary rocks into carbonate rocks(dolomite>75%),siltstone(quartz and feldspar>75%),tuff(volcanic components>75%)and mixed sedimentary rocks(each components <75%)by 75% of each component.Which component is higher than 25% using it in naming as a prefix,and the highest component as the lasr name.At last established lithofacies classification scheme combined rock classification,sedimentary structural,color and thickness.This study area is dividded into 8 lithofacies:sand – siltstone,siltstone,arenaceous dolomite,biological limestone,silvery dolomite,dolomitic dolomite,tuff and rich algae tuff.(2)Define lithofacies associations development regular pattern.The fine-grained sedimentary strata in lucaogou formation made by multiple high frequency sedimentary cycle which upward becomes shallow.Lithofacies association of high frequency cycle has the characteristics of cyclicity and diversity in Lucaogou formation.The swirling cycle of lithofacies association is dominated by climate change.And the diversity is controlled by the source and the depth of water.The four lithofacies associations is different because of different sedimentary environment.There are four types of lithofacies associations in the research area: sand – siltstone and tuff,siltstone and rich algae tuff,arenaceous dolomite and tuff,silvery dolomite and rich algae tuff.When the clastic is abundant,siltstone and rich algae tuff developed along the coastal,the arenaceous dolomite and tuff develops in trasiton belt.When the clastic is little,sand – siltstone and tuff developed along the coastal,silvery dolomite and rich algae tuff in trasiton belt,only tuff developed in deep water.(3)Establish lithofacies depositional model in the study area.The research area is divided into three stratigraphic sequence.The first stratigraphic sequence developped TST and HST.The second stratigraphic sequence developped TST and HST.During the period of the LST,the lake plane was lower and the deltaic developed.The shoreshore belt mainly developed siltstone and silvery dolomite.And the deep lake developed tuff.The lake level rose during TST period,and there are silvery dolomite and little siltstone developed along the coast.In the deep lake,there are gravity flow sedimentary and tuff.And the gravity flow sedimentary and tuff turn into silvery dolomite and rich algae tuff during the late of the TST.During HST period,lake level began to drop,and many lithofacies develop: When the clastic is abundant,siltstone and rich algae tuff turned into arenaceous dolomite and tuff with the deeper water.When the clastic is little,sand – siltstone and tuff turned into silvery dolomite and rich algae tuff with the deeper water.