High-resolution Research On Organic Matter Accumulation Mechanisms Of Oil Shales In The Lower Cretaceous Jiufotang Formation,Chaoyang Basin,Western Liaoning Province

In western Liaoning Province,the Lower Cretaceous Jiufotang Formation in the Chaoyang Basin is famous for the Jehol Biota and oil shale resources.However,no detailed research has been carried out on organic-rich source rocks that are interbedded with gravity flow deposits and volcaniclastic rocks.The impact of basin evolution,gravity flows,and volcanism on oil shale formation is still unknown.Furthermore,the lacustrine carbon cycle in the Chaoyang Basin has not yet been investigated.This Ph D uses a combination of petrology,sedimentology,sequence stratigraphy,elemental geochemistry,organic geochemistry,organic petrology,zircon U-Pb geochronology,and carbon isotope chemostratigraphy.Based on petrology,sedimentology,and sequence stratigraphic principles,the sequence stratigraphic framework of the third Member of the Jiufotang Formation was constructed.A complete third order sequence which comprises four systems tracts(the lowstand systems tract,the transgressive systems tract,the highstand systems tract and the regressive systems tract)were identified.Deltaic-dominated intervals were well-developed in the lowstand systems tract and the regressive systems tract,the lacustrine-dominated intervals were mainly deposited in the transgressive systems tract and the highstand systems tract,and gravity flows can be observed in every systems tract.According to this study,we propose that it was the multi-episodic extensional tectonic subsidence in the east of the basin that led to the formation of the semi-graben rift basin,led to a base-level variation(a steady rise,followed by a drop trend),and triggered volcanic eruption,earthquake and gravity flows.According to sedimentology,petrology,and industry qualityanalysis,the oil shales of the Chaoyang Basin were deposited in the semi-deep to deep lake facies,and they are characterized by low-medium grade(oil yield 3.5⁷.7 wt%),low calorific value(2261.86⁵325.75 J/g),and high ash content(78⁸8 wt %).Higher quality oil shales are dark black and well laminated,suggesting a still water body and a seasonally stratified lake,without being affected by bioturbation.By using a range of parameters and techniques,the characteristics and formation conditions of the lacustrine source rocks were determined,including palaeoclimate(chemical index of alteration(CIA)and C-value),terrigenous detrital matter input(Zr and Th),palaeosalinity and redox conditions(Sr/Ba,?U,(Cu + Mo)/Zn,pristane/phytane,gammacerane index(GI),and the relative amount of ?-carotane),and organic matter source(Plot of hydrogen index(HI)versus the Rock-Eval temperature of maximum hydrocarbon generation(Tmax),C₂4 tetracyclic terpane/(C₂4 tetracyclic terpane + C₂3 13?(H),14?(H)-tricyclic terpane),C19 tricyclic terpane/(C19 tricyclic terpane + C₂3 13?(H),C₂7/C₂9 ???-20 R steranes,C₂8/C₂9 ???-20 R steranes,4-methylsterane index,dinosterane index,tetramethylsterane index)and primary productivity(Ba/Al?Babio).During the deposition period of lacustrine sediments,climate turned from moist to semimoist-semiarid,showing a climate cooling.Compared with the shallow lake facies,terrestrial input decreased in the semi-deep to deep lake facies,with a dominant aquatic organism contribution,more reducing water condition,higher water salinity and primary productivity.According to the fact that the deposition of organic-rich oil shales was frequently interrupted by gravity flow sediments,the negative relation between Zr and ?U,the samples affected by gravity flows in the semi-deep to deep lake facies have lower total organic carbon(TOC),higher C₂4 tetracyclic terpane/(C₂4 tetracyclic terpane + C₂3 13?(H),14?(H)-tricyclic terpane),C19 tricyclic terpane/(C19 tricyclic terpane + C₂3 13?(H),and pristane/phytane ratios,and a lower relative abundance of gammacerane,with a kerogen type change from type ? to type II1,it is inferred that gravity flows were accompanied by the input of terrestrial detritus and a degree of oxygenation of the bottom waters,reducing organic matter accumulation.Thin section observation,organic petrology,and organic geochemistry were used to figure out the influence of airfall volcanic ash on organic matter accumulation during the formation of oil shales.Fish bones,ostracod shells,and alginite are present below the thick tuff layer.Alginite richness gradually increased from very low just above the tuff layer,to a higher concentration further above the tuff layer.It is proposed that large quantities of volcanic ash led to water column poisoning,causing the sudden mass mortality of some aquatic organisms.The increased proportion of brackish/saline-water dinoflagellates,which is corroborated by the high concentration of telalginite,abundant dinoflagellate-specific biomarkers(C31 4,22,23,24-tetramethylcholestanes),high values of the C₂7/C₂9 ???-20 R and C₂8/C₂9 ???-20 R sterane ratios,4-methylsterane index,dinosterane index,and tetramethylsterane index,suggest that brackish/saline-water dinoflagellates show a positive response to volcanic ash input.Some specific brackish/saline-water dinoflagellates are present in organic-rich tuff laminae,indicating that a minor volcanic ash input was more likely to result in rapid and efficient volcanic fertilisation effects and high-quality oil shale formation.Based on the constraint of absolute age(121.06 ± 0.63 Ma,119.72 ± 0.40 Ma,119.9 ± 0.65 Ma)by zircon U-Pb dating,the oil shale sequence of the third Member of the Jiufotang Formation in the Chaoyang Basin to permits correlation of the coeval marine black shale in the Early Aptian OAE1 a.It is inferred that the exchange of 13C-depleted carbon in water and atmosphere could led to a potential response of lacustrine carbon cycle to global carbon cycle.Influenced by Ontong Java Plateau and local volcano,as well as the enhanced organic carbon burial due to the deposition of global marine black shale and organic-rich oil shale,?13Ccarb in lacustrine deposits underwent a persistent positive shift.Enhanced global organic carbon burial that acted as a sink for excess volcanic CO₂,and lower atmospheric CO₂ concentrations due to reduced Ontong Java Plateau and local volcano,were keys to climate cooling.The climate cooling,which is usually accompanied by stronger evaporation,could increase the water salinity.There is a series of oil shale-bearing rift basins formed in northeastern China that share a similar geological setting,so this research provides a useful example for future resource exploration of coeval oil shales in northeastern China.The study on the interplay among volcanic activity,palaeoclimate,organic carbon burial,and lacustrine carbon cycle in this study can be useful in the research of the coeval lacustrine source rock in north China.