Paleogene Paleoenvironmental Evolution And Organic Matter Accumulation Mechanisms,Fushun Basin

The Paleocene Eocene Guchengzi Formation,Jijuntun Formation and Xiloutian Formation in the Fushun Basin are a continuous and stable set of terrestrial fine-grained sediments,which are important vehicles for delineating astronomical time scales and reconstructing palaeoenvironmental evolution.The Liaofudi-1 well(LFD-1)in the Fushun Basin was studied for its chronology,stratigraphic stratigraphy,geostratigraphy,organic petrology,isotope geochemistry,elemental geochemistry and organic geochemistry,to build a high-precision astronomical age frame,and to analyse the stratigraphic stratigraphy and sedimentary phases,palaeoclimatic and extreme climatic features,palaeolake water conditions and biological productivity,and other environmental factors.The research will be carried out on the mechanisms of organic matter enrichment,and eventually the organic matter enrichment model will be established.The petrological,mineralogical and geochemical analysis of various types of organic-rich fine-grained rocks in LFD-1 well shows that the total organic carbon(TOC)of coal in the study area is the highest(66.2 wt.%),followed by carbonaceous mudstone,oil shale and mudstone.The vitrinite content in coal and carbonaceous mudstone is the highest,up to 78%,and that in oil shale and mudstone is the highest,up to 72%.The Tmax values were between 406°C and 442°C,with an average of 427°C,indicating that the samples with different lithology were in the immature stage.The differences of pristane carbon isotope(?¹³C_Pr)and phytane carbon isotope(?¹³C_Ph)of coal,carbonaceous mudstone and mudstone in the study area are small,while the of?¹³C_Pr and?¹³C_Ph of oil shale vary considerably(-27.41?-28.19‰and-26.60?-28.23‰,respectively).All of the above parameters indicate differences in palaeoclimatic and palaeoenvironmental studies from Guchengzi Formation to Xiloutian Formation.The zircon U-Pb isotope dating of the upper part of the Lizigou Formation from well LFD-1indicates that the upper part of the Lizigou Formation is 54.72±0.20 Ma.Further,the stratigraphic age frame of the Fushun Basin was established by combining the results of previous studies on the Paleogene sporulation and palaeomagnetic age of the Fushun Basin,in which the depositional time frames of the upper part of the Lizigou Formation-Guchengzi Formation,Jijuntun Formation and Xiloutian Formation are 54.72-47.8 Ma,47.8-41.2 Ma and 41.2-37.8 Ma,respectively.On the basis of determining the stratigraphic chronostratigraphic framework,the cyclic stratigraphy analysis was carried out on the GR data sequence(sampling interval 0.07 m)of Well LFD-1.Through cycle analysis,17,16 and 9 long eccentricity cycles(405 kyr)and 67,64 and 35 short eccentricity cycles(100 kyr)were identified in the upper Lizigou Formation-Guchengzi Formation,Jijuntun Formation and Xiloutian Formation,respectively.The theoretical variation curve of 50°N summer sunshine eccentricity provided by La2010a scheme is selected as the target for astronomical tuning,and the high-precision astronomical chronology of Eocene in the study area is obtained.Based on the detailed core observation description,logging curve and rock thin section identification analysis of LFD-1 well,and combined with the artificial profile of the western open pit area of Fushun basin,the sedimentary facies of Fushun basin is studied.It is considered that the Eocene of Fushun basin is lacustrine sedimentary.The lacustrine facies can be further divided into swamp,shallow lake and semi-deep lake-deep lake subfacies.Based on the principle of sequence stratigraphy and sedimentary facies identification,the sequence stratigraphic framework of the Eocene in Fushun Basin was established,and 3 third-order sequences were identified.The thick coal was mainly developed in the lake-marsh environment of the sequence I period.The thick oil shale was formed in the semi-deep lake-deep lake environment of the sequence ? period,and the thin oil shale was formed in the high-frequency variation environment of the shallow lake and semi-deep lake-deep lake of the sequence ? period.Based on astronomical dating scale and fine organic carbon isotope(?¹³C_TOC)data in Fushun Basin,the isotope comparison framework of marine and continental strata in Paleogene was established.ETM2(Eocene Thermal Maximum 2,Guchengzi Formation),ETM3(Guchengzi Formation),EECO(Early Eocene Climatic Optimum,Guchengzi Formation and Jijuntun Formation),MECO(Middle Eocene Climatic Optimum,Xiloutian Formation)and“Doubthouse”climate(Jijuntun Formation)are the main events of Paleogene extreme climate in Fushun Basin.The short-term extreme heat events within the EECO event were finely identified,and 22 short-term extreme heat climates were identified.The negative offset of?¹³C_TOC in the study area was fitted with the corresponding negative offset of?¹³C_bulk carbon isotope of marine foraminifera.The results showed that?¹³C_TOC(Fushun Basin)was significantly correlated with the negative offset of?¹³C_bulk carbon isotope(R²=0.68).It is further proved that the extreme climate events in continental and marine systems originate from the same carbon source and are closely related to the increase of carbon content in the atmospheric system.Based on the qualitative and quantitative analysis of palynological fossils and elemental geochemistry,the paleoclimate evolution of Eocene was restored.The paleoclimate of Guchengzi Formation(Climate zone I),Jijuntun Formation(Climate zone ?)and Xiloutian Formation(Climate zone ?-?)in the study area experienced the evolution process of subtropical warm moist,warm temperate warm moist,warm temperate warm semi-arid and warm temperate warm semi-humid-semi-arid.Further quantitative analysis of paleoclimate in the Fushun Basin during extreme weather events The results showed that the paleoclimate during the extremely hot event period had the characteristics of increased weathering intensity,increased temperature and increased precipitation.Under the influence of ETM2event,the paleotemperature and precipitation increased the most.The paleotemperature increased by about 3-5°C,and the precipitation increased by about 600mm,followed by EECO and ETM3.Under the influence of MECO event,the paleotemperature and precipitation increased least,about 2-3°C and 200mm,respectively.The paleotemperature and precipitation decreased by 2-3°C and 200mm,respectively,during the“Doubthouse”climate event.The paleoatmosphere CO₂ concentration(p CO₂)of Eocene was reconstructed by using organic carbon isotope and single carbon isotope.The p CO₂ of Eocene was between 258 and 2475 ppmv,and the background p CO₂ value was 557+94/-82 ppmv.The p CO₂ of ETM2in the study area was the largest(2475+1705/-917 ppmv and 2277+1456/-764 ppmv),followed by ETM3,EECO and MECO.The source of p CO₂ during the extreme heat event was analyzed,and it was considered that the main carbon source might come from the oxidation of terrestrial organic matter,and there was a supply of some volcanic sources.The petrology,sedimentology,paleontology and geochemistry of Eocene in Fushun basin were systematically analyzed,and the paleoclimate evolution characteristics of Fushun basin were revealed.The enrichment conditions and models of organic matter in different climatic zones of Eocene in Fushun basin were reconstructed combined with paleostructure and paleosedimentary environment.The climate zone I(Guchengzi Formation)period is warm and humid climate,the basin is a stable rift stage,the study area is mainly shallow lake and swamp freshwater oxidation environment;the shallow lake environment,carbon burial rate(C _burial,62.97 g/(m²·a))plant net primary productivity(NPP,314.87 g/(m²·a))is higher,but the input of terrigenous clastic is high,sedimentary carbonaceous mudstone;in the swamp environment,C _burial and NPP are extremely high,with the average values of 116.94 g/(m²·a)and 584.68g/(m²·a).The input of terrigenous debris is low,and thick coal is deposited.Moreover,multi-period extreme heat events(ETM2,ETM3 and EECO)occurred in this climate stage,which promoted the increase of biological productivity and further increased the abundance of organic matter.The climate zone ?(Jijuntun Formation)period is generally warm and humid climate,basin rift reached the peak,formed shallow lake oxygen-poor environment and semi-deep lake-deep lake anoxic environment;shallow lake environment,lake biological productivity medium(981.67 g/(m²·a)),there is a certain supply of terrestrial organic matter,water bottom poor oxygen state led to some organic matter is oxidized and decomposed,forming poor quality oil shale;the semi-deep lake-deep lake anoxic environment,organic matter preservation conditions are good,and the lake biological productivity is extremely high(2344.88 g/(m²·a)),deposited with high quality oil shale.Climate subzone ?(Xiloutian Formation)period as a whole warm semi-arid climate,basin structure basically stopped.During this period,the oxygen-poor environment was mainly the transition from shallow lake fresh water to brackish water.The lake had low biological productivity(311.91 g/(m²·a)),poor organic matter preservation conditions and low organic matter abundance mudstone.The extremely hot event(MECO)also occurred during this period,resulting in an increase in lake productivity and an increase in organic matter abundance in mudstone.Climate zone ?(Xiloutian Formation)period as a whole warm semi-humid-semi-arid climate.Controlled by paleoclimate,the period was mainly characterized by shallow lake brackish water poor oxygen and semi-deep lake-deep lake brackish water anoxic environment high frequency alternation;the semi-deep lake-deep lake brackish water anoxic environment,the ancient lake biological productivity is high(1441.53 g/(m²·a)),high deposition rate,resulting in a small amount of organic matter is diluted,and the formation of anoxic environment,making the lake preservation conditions are good,the deposition of moderate organic matter abundance of oil shale;shallow lake brackish water and oxygen-poor environment,ancient lake productivity is low(417.93 g/(m²·a)),high deposition rate,the organic matter is diluted and oxidized decomposition,deposition of low organic matter abundance mudstone.This series model reveals the organic matter enrichment mechanism of Eocene in Fushun Basin,enriches the metallogenic theory of coal and oil shale and plays a guiding role in the exploration and development of oil and gas resources.