Oil Shale Characteristics And Metallogenic Conditions Of Lijiaya Formation Of Paleogene In Huangxian Basin

In this paper,the Paleogene Lijiaya Formation in Huangxian Basin is taken as the main research target bed,which is characterized by abundant oil shale and coal resources.Based on borehole data,industrial quality classification,geochemical analysis,microscopic observation and sedimentary facies analysis,this paper makes a detailed study on the differences of mineralization conditions and enrichment patterns of organic matter between oil shale and coal in the study area.The target strata in the study area can be divided into upper coal-bearing section and lower coal-bearing section.Oil 4 and coal 4 are developed in lower coal-bearing section.Oil 3,coal 3,coal 2,oil 2,coal 1,oil 1,coal 1,coal 2,coal 3 and oil 3 are mainly developed in upper coal-bearing section from bottom to top.Among them,the thickness of oil 4 is 0-5.5 m and the buried depth is 100-400 m;the thickness of oil 3 is 0-0.8 m and the buried depth is 100-500 m;the thickness of oil 2 is 0-22 m and the buried depth is 100-1100 m;the thickness of oil 1 is 0-4 m and the buried depth is 100-400 m;the thickness of the upper three layers is 0.4-1.0m and the buried depth is between 200-700 m,which shows that oil 4 has a relatively shallow thickness and a relatively high production value;although the buried depth of oil 3 is relatively shallow.Shallow,but because the thickness is too thin,it has no exploitation value;Oil 2 is a stable recoverable layer in the study area;Oil 1is of poor quality and low exploitation value;oil 3 is thinner and its exploitation value is low.Oil shale of Lijiaya Formation in Huangxian Basin is usually interbedded with coal,which can be divided into high-quality oil shale and low-quality oil shale.High quality oil shale?oil content 12.07-24.38%?has high organic matter abundance,organic carbon content is 20.05-40.95%.The organic matter type is mainly mixed type?I-II₁?,which is in the immature thermal evolution stage.It has low-medium ash content?38.68-64.41%?,medium-high calorific content?6.49-18.85 MJ/kg?and ultra-low-low sulphur content?0.31-1.77%?.The organic carbon content of low quality oil shale?oil content 3.69-4.56%?is 9.76-15.40%.The organic matter type is mainly mixed type?II₂?,which is in the immature thermal evolution stage.It has the characteristics of Medium Ash?70.71-74.41%?,low-medium-high calorific value?3.93-6.49MJ/kg?and ultra-low sulfur?0.29-0.32%?.Coal has very high organic matter abundance,organic carbon content is 58.6%,organic matter type is humic type?III?,oil-bearing coal with medium and high oil content?11.73%?,low ash content?8.80%?,high calorific value?24.89MJ/kg?and ultra-low sulfur content?0.25%?.According to the analysis of light film and biomarkers,the organic matter of high quality oil shale in Lijiaya Formation of Huangxian Basin mainly comes from endogenous bacteria and algae?layered algae?,followed by exogenous vitrinite from higher terrestrial plants,while the content of crust component is less;the organic matter of low quality oil shale mainly comes from terrestrial vitrinite,secondly from endogenous algae,and the organic matter of low quality oil shale mainly comes from terrestrial vitrinite,secondly from endogenous algae.The content of exogenous crust is also low.Coal organic matter is dominated by terrestrial higher plant vitrinite,followed by exogenous crust and no algae.The analysis results of CIA index,Th/U,Rb/Sr and Fe/Mn ratios of elemental geochemical parameters show that the paleoclimate of the formation period of high-quality oil shale in the Lijiaya Formation in the study area is warmer and wetter than that of low-quality oil shale,while coal develops in the stage of warmer and wetter climate with lower weathering abundance.Biomarker compounds(Pr/Ph,Pr/n-C₁₇,Ph/n-C₁₈)and Sr/Ba ratios indicate that the high-quality oil shale of Lijiaya Formation is mainly formed under the condition of weak reductive brackish water-saline water.The increase of salinity may be related to transgression events.Low-quality oil shale is formed in weak oxidized freshwater environment,while coal is formed in oxidized freshwater environment.Based on the comprehensive study of paleoclimate,paleo-water body condition and organic matter source,the organic matter enrichment model?A-E stage?for the vertical evolution of oil shale and coal in the study area was established.Under the warm and humid paleoclimate background,high-quality oil shale was formed in stage A and C.At this time,the deeper shallow lake environment of lake water body and the deeper water body formed oxygen-poor environment.The transgression resulted in a large number of injections of sea water,which increased the salinity of water body,formed salinity stratification,and carried a large number of nutrients,which made aquatic organisms flourish,high lake productivity and good presence.Under the condition of organic matter preservation,high-quality oil shale is formed eventually;low-quality oil shale is formed in B and E stages,when the transgression ends,the lake water body is shallower,the aquatic biomass of freshwater environment of shallow lake decreases,the input of terrestrial organic matter and clastic minerals increases,and the water body of Lake carries more oxygen,which leads to the transformation of water body to weak oxidation condition.The low quality oil shale is formed in the weak oxidation environment of fresh water,which is not conducive to the preservation of organic matter.Coal is formed in the D stage,when the lake basin is swamped and the shallow lake is deposited to form a swamp environment.The plants in the basin are flourishing,which provides rich organic matter sources for the formation of coal.It is found that the lake water environment and the source of organic matter are the main factors controlling the formation of high and low quality oil shale in the study area.