Organic Matter Accumulation Conditions Of Coal-oil Shale Series Of Paleogene Dalianhe Formation In The Yilan Basin

In this paper,the coal-oil shale rock series of Eocene Dalianhe Formation in the Yilan Basin is taken as the main research target bed.Based on field profile measurements,industrial quality analysis,organic geochemical analysis,and microscopic observations,the organic matter accumulation conditions of the coal-oil shale series are studied in detail,and a vertical evolution model of organic matter accumulation is established.The target zone can be divided into coal-bearing section and oil shale section.Among them,the coal-bearing section is limno-telmatic facies,mainly developing coal,oil shale,mudstone and carbonaceous mudstone;the lower part of the coal-bearing section is developed with thick coal and thick oil shale interbedded;mudstone and carbonaceous mudstone are mainly developed in the upper part of the coal-bearing section.The oil shale section is deep and semi-deep lake facies,mainly developed mudstone and oil shale,and developed multiple sets of fine and fine sandstone due to gravity flow sedimentary.The mineral composition of the target layer is mainly clay minerals?35.76%⁹7.39%?,and the content of kaolinite in clay minerals is high?51.68%⁸1.22%?.According to industrial analysis,organic carbon analysis and rock pyrolysis analysis,the quality of oil shale in the coal-bearing section of the study area is generally higher than that in the oil-shale section.The organic matter abundance of the oil shale in the coal-bearing section is generally high?TOC:8.90%³0.10%?,the organic matter types are mainly I-II₁,the oil content is high?3.85%¹8.57%?,and it has the characteristics of low-high ash content?56.94%⁷7.41%?and low calorific value?3.93¹5.39MJ/kg?.In the oil shale section,the organic matter abundance is relatively low?TOC:7.87¹0.60%?,the type of organic matter is mainly II₁,the oil content is low?3.55%⁴.18%?,the ash content is high?73.83%⁷7.70%?,and the calorific value is low?3.47⁴.55MJ/kg?.The abundance of organic matter in coal bearing section is very high?TOC:41.30%⁷3.70%?,and the organic matter type is mainly II₁ and II₂,with high oil content?10.62%¹8.38%?,which is a typical"oil bearing coal",with the characteristics of ultra-low-high ash content?1.52%³5.93%?,medium low-high calorific value?18.51³0.81 MJ/kg?.According to the characteristics of organic macerals and biomarkers,the source of organic matter in the target layer of the study area is mainly terrestrial higher plants,followed by the internal bacteria and algae in the lake,as well as the submerged,floating and emergent aquatic organisms.Among them,the coal bearing period of coal is given priority to with terrestrial higher plant organic matter type,mixed type coal plants mainly for vegetation type,the oxidation of coal deposit in the ancient water conditions,main and coal deposit on the nutrient-rich peat bog down low,the pH of water medium is higher,shows that there exist certain bacteria organic matter input,the change of the GI value indicates coal bearing lower part of peat bog water and high pH value relative to the upper.The high-quality oil shale in the coal-bearing section is mainly the input of organic matter of algae body from the lake and deposited in the oxygen-deficient fresh water environment with relatively weak oxidation conditions.The low-medium quality oil shale in the coal-bearing section was dominated by organic matter input from terrestrial higher plants,followed by aquatic organisms such as bacteria and algae,which were deposited in the fresh water environment containing oxygen.The organic matter of oil shale in the oil shale section was imported as a mixed source of terrestrial higher plants and aquatic organisms such as bacteria and algae,and was deposited in the oxygen-poor freshwater environment.Based on the paleotectonic-sedimentary evolution background,paleoclimate,source of organic matter and preservation conditions,the vertical evolution model of organic matter accumulation in the coal-oil shale rock series in the study area?stage A-E?was established.A stage of stable sedimentary stage,the slant warm,humid,and plentiful rainfall of subtropical climate background,bacteria,algae and other aquatic organisms,terrestrial higher plant iwabuchi,overall for the lean oxygen-containing oxygen water of fresh water eutrophication fen-shallow lake environment.Through Lake flooding and swampization,oil shale and coal seam with good quality are deposited;B-C stage of tectonic activities,is still a peat bog-shallow lake environment,but the input of terrigenous organic matter and detrital minerals increases,which cause water to oxygen water environment,ultimately form the relatively low quality coal and oil shale,and C stage not deposit oil shale;stage D-E is the maximum rifting stage,the lake basin expands rapidly,and the sedimentary environment changes from limnetic facies to semi deep lake deep lake facies.In the warm and humid paleoclimate with more precipitation,the input of organic matter of aquatic organisms such as bacteria and algae is relatively increased compared with B-C stage.However,the destruction of organic matter preservation conditions by gravity flow sedimentation makes D stage deposit only low-quality oil shale,and E stage not deposit oil shale.Based on the above analysis,under the comprehensive control of paleostructure,paleoclimate and sedimentary environment,the change of source and preservation conditions of organic matter leads to the vertical enrichment of organic matter in the study area.