Study On The Mechanism Of Organic Matter Enrichment In The Oil Shale-bearing Series Of The Bayingebi Formation In Chulu Uplift,Yin'e Basin

The Chulu Uplift in the study area is a component of the Early Cretaceous rift-depression group in the Yin'e Basin,in which an oil shale-bearing rock series developed in the Bayingebi Formation,combined with the predecessor's zircon radioisotope chronology data and this analysis of the fossil and sporopollen species of the Jehol Biota in the study area,it can be seen that the sedimentary age of the second member of the Bayingebi Formation belongs to the late Barremian-early Aptian of the Early Cretaceous.Based on the study of sedimentary facies and sequence stratigraphy in the study area,this paper analyzes the sedimentary sequence evolution,paleoclimate evolution,hydrothermal sedimentary characteristics,lake sedimentary environment and organic matter source characteristics of the second member of the Bayingebi Formation by means of palynology,petrology,mineralogy,organic geochemistry,element geochemistry and isotope geochemistry,the enrichment mechanism of organic matter was discussed.According to petrology,mineralogy and geochemical analysis,there are abundant oil shale,mudstone and carbonate deposits(such as dolomite,dolomitic mudstone,marl,etc.)in the second member of the Bayingebi Formation in the study area.Oil shale and mudstone are rich in fossils of the Jehol Biota,and their abundance of organic matter is high.Among them,oil shale has a relatively high abundance of organic matter,and the oil yield is between 3.68%and 9.86%,with an average of 6.02%,the total organic carbon(TOC)content can reach up to 13.6%,and the hydrocarbon generation potential S₁+S₂ can reach up to 136.54 mg/g.The total sulfur(TS)content is between 0.04%and 2.49%,and the total nitrogen(TN)content is between 0.07%and 0.47%.Oil shale is mostly horizontal bedding,and the organic microscopic components are mainly algae(lamalginite and telalginite)and sporophyte,and contain a small amount of cutinite and vitrinite.Ro and T_max values indicate that the organic matter in the second stage of Bayingebi Formation is in the immature and low mature stage.The industrial quality analysis of oil shale shows that the Bayingebi Formation oil shale is medium-quality,high-ash,low-calorie,low-sulfur oil shale.On this basis,based on the principles of sequence stratigraphy,a sequence stratigraphic framework of the Bayingebi Formation in the study area was established,and a three-level sequence was identified.In most areas,a transgressive systems tract(TST)and a highstand systems tract(HST)were developed,a lowstand systems tract(LST)and a regressive systems tract(RST)are locally developed.Semi-deep lake-deep lake facies are mainly developed in HST,which is a favorable facies belt and system tract for the formation of oil shale.Shallow lake facies are mainly developed in RST.Nearshore underwater fans develop in the semi-deep to deep lake facies of TST,and are mainly deposited in the northeast of the study area.The southeast of the study area is close to the main control fault of the basin-Altun fault zone,and the syngenetic fault activity may become the trigger mechanism of the nearshore subaqueous fan.At the same time,affected by it,the sedimentary thickness of the second member of the Bayingebi Formation has the characteristics of being thinner in the northwest and thick in the southeast in the study area.Based on the statistics of sporopollen species and quantity,the paleoclimatic evolution process of the second member of the Bayingebi Formation was restored.The paleovegetation types in the study area are dominated by coniferous forests,followed by evergreen broad-leaved forests and herbaceous plants,with a small amount of deciduous broad-leaved forests and shrubs.At the same time,the phenomenon of alpine pollen(spruce,fir and cedar pollen)mixed with tropical-subtropical plant pollen suggests that there is a palaeogeographic pattern of high mountains around the study area.In addition,combined element ratios(CIA,Sr/Cu,Al₂O₃/Mg O,etc.)and organic carbon isotopes(?¹³C_org)and other geochemical data further confirmed that TST,HST and RST of the second member of the Bayingebi Formation mainly experienced the paleoclimate evolution processes of warm-cool semihumid,warm semihumid and warm-cool semiarid semihumid,respectively.The carbon isotope(?¹³C_wood)of plant debris fossils was used to restore the atmospheric CO₂ concentration of the second member of the Bayingebi Formation.The p CO₂ value is between 340.77-1384.74 ppm,and the high p CO₂ value corresponds to the semi-humid warm temperate palaeoclimate characteristics.Combined element geochemistry,biomarkers and organic nitrogen isotopes(?¹⁵N),which reveals the paleowater conditions(redox and paleosalinity),organic matter source,terrigenous debris supply and microbial activity characteristics of organic matter in the second member of the Bayingebi Formation.Element proxy indexes of paleoredox:U_EF,MO_EF,?U and DOP_T indicate that the second member of the Bayingebi Formation mainly experienced the evolution process of suboxic,suboxic-anoxic,anoxic and suboxic.The positive deviation of?¹⁵N value indicates the advantage of incomplete denitrification,and the activity of denitrifying bacteria is strong in anoxic water environment.The element proxy indexes B/Ga,Sr/Ba and S/TOC of paleosalinity suggest that the second member of the Bayingebi Formation mainly experienced the change trend of brackish-saline water,brackish and brackish-saline water.The kerogen type of organic matter in oil shale is mainly type I,and some are type II₁.The Ti content and Ti/Al ratio in HST are relatively low,suggesting that lake sediments are mainly supplied by lake organisms under higher lake level conditions.In the hydrothermal sediments,the symbiosis of plate-like and radial marcasite,vein-like pyrite and dolomite can be seen by scanning electron microscope.Geochemical data confirmed that the hydrothermal activity of the second member of the Bayingebi Formation mainly developed in HST,and part of it occurred in RST.Through temperature measurement and analysis of fluid inclusions in dolomite and calcite,it can be seen that the hydrothermal fluid of the Bayingebi Formation has the characteristics of low temperature,medium salinity and high density.Through detailed analysis of the evolution characteristics of the paleoclimate and sedimentary environment of the second member of the Bayingebi Formation,it can be seen that the warm-semi-humid paleoclimate has brought sufficient rainwater and promoted the rise of the lake level,the increase of the redox interface provides favorable storage conditions for the enrichment of organic matter.At the same time,the warm and humid paleoclimate is conducive to the weathering of rocks and the growth of terrestrial vascular vegetation.The high degree of paleoweathering and strong surface runoff input a large amount of terrestrial nutrients(Fe,P,K,etc.)to the lake,promote the prosperity of planktonic algae and increase the primary productivity,and provide a rich source of organic matter enrichment.Paleoclimatic conditions are the main controlling factors driving the lake sedimentary evolution and organic matter enrichment of the second member of the Bayingebi Formation.The increase of p CO₂ and paleoclimate warming during the deposition of the second member of the Bayingebi Formation oil shale are related to the increase of global paleotemperature in the Aptian Stage of the Early Cretaceous.While paleoclimate controlled the organic matter accumulation mechanism,hydrothermal activity at the lake bottom also had a certain impact on the organic matter accumulation.The hydrothermal activity in the second member of the Bayingebi Formation is related to the extensional faults in the basin,and hydrothermal deposition mainly occurs in HST.Strong hydrothermal deposition often corresponds to high paleosalinity and anoxic water environment.This coupling relationship may imply that strong hydrothermal activity may release more hydrothermal fluid and promote the increase of lake bottom salinity.And the formation of anoxic environment at the bottom of the lake.Moderate hydrothermal activity is coupled with the productivity of the lakes,implying that the nutrient elements released by moderate hydrothermal activities may promote the increase of primary productivity of lakes,while strong hydrothermal activities may inhibit the increase of primary productivity of lakes.Based on the study of sequence stratigraphic framework,paleoclimate evolution,paleophyte types,paleowater conditions,and hydrothermal deposition characteristics in the study area,this paper establishes organic matter enrichment models in different system tracts in the sequence stratigraphic framework.The study area is affected by the extensional tectonic background,and fault depressions are developed,accompanied by volcanism and magmatism.During the TST deposition period,paleovegetation types were dominated by temperate coniferous forests,mainly showing the characteristics of warm and semi-humid paleoclimate,lakes were brackish-salt water and suboxic water conditions,and lakes had relatively low productivity.During this period,the degree of organic matter enrichment showed a gradually increasing trend.During the HST deposition period,with the rapid increase of atmospheric CO₂ concentration,the rapid increase of paleotemperature caused the increase of tropical and tropical-subtropical vegetation.The warm and semi-humid paleoclimatic conditions promoted the elevation of the lake level.The paleosalinity of the water body decreased accordingly,and the oxygen content of the lake bottom decreased,showing brackish water anoxic-suboxic water conditions.At the same time,a suitable lake ecological environment promoted the prosperity of lake organisms and terrestrial vascular plants around the lake,and the paleoproductivity further increased.The bottom of the lake mainly develops moderate hydrothermal activities,and the nutrient elements brought by it promote the flourishing of lake algae.The degree of organic matter enrichment reaches the highest level and thick oil shale is deposited.During the deposition period of RST,the paleoclimate characteristics of semi-arid and semi-humid temperate zone were present.The paleo temperature dropped,the lake level gradually decreased,and the productivity of paleolakes reached the lowest.The lake presents a paleowater condition of brackish water and saline water with suboxic.The hydrothermal activity at the bottom of the lake gradually weakens,and the degree of organic matter enrichment is significantly reduced.This model reveals the organic matter enrichment mechanism of the second member of the Bayingebi Formation in the study area,enriches the mineralization theory of oil shale and plays a guiding role for the exploration and development of oil and gas resources.