Research On Paleoclimate,paleoenvironment And Source Rock Development Model Of The Cretaceous Shahezi Formation In Xujiaweizi Fault Depression,Songliao Basin

In recent years,the research on the paleoclimate and paleoenvironment of the Cretaceous continental basin has received great attention.The main reason is that the current research finds that the research on the extreme paleoclimate change of the Cretaceous can better help us understand the modern greenhouse climate and predict the future climate change,and explore the development mode of organic rich source rocks under the influence of the global oceanic anoxic event of the Cretaceous,it can also provide theoretical support for solving the energy crisis.At present,it is found that the Cretaceous strata in the Songliao Basin,located in northeast China,have the longest deposition time and the most complete preservation.Therefore,this paper takes the Songliao Basin as the research object,and conducts a systematic study on the paleoclimate and paleolake sedimentary environment for the Shahezi Formation,the main deep source rock section of the Cretaceous,and analyzes the development mode of the source rock under the influence of different climate and sedimentary environment.The research in this paper is based on petrology,mineralogy,logging analysis and seismic analysis,with palynology,organic geochemistry and elemental geochemistry as the main research methods,using continuous and high-precision stable organic carbon isotopes,whole rock nitrogen isotope geochemical stratigraphy as the framework,a systematic study of paleoclimate,paleo-lake sedimentary environment and biological evolution law under the influence of extreme climate during the sedimentary period of the lower Cretaceous Shahezi Formation,and the development model of source rocks in the same period was further discussed.According to the analysis of the petrological,mineralogical and geochemical test results,sandstone,mudstone,conglomerate,pyroclastic sedimentary rock and tuff are mainly developed in the lower Shahezi Formation,and there are abundant pyroclastic materials in the whole,indicating that the volcanic activity has a significant impact on the formation.The total organic matter content（TOC）of mudstone is relatively high,with an average content of about 9.2%,up to 40%.The total nitrogen content（TN）is 0.11%～2.56%,with an average of 0.44%.The total rock inorganic carbon（TIC）content is 4%～27%,with an average of 12%.Tmax-HI crossplot indicates that the lower Shahezi Formation is mainly type III kerogen,and the organic matter is in the mature to over mature stage.Based on the comprehensive analysis of the lithological changes,structural structures,logging and seismic facies characteristics of the cores in the lower section of Shahezi Formation,as well as the structural location near the fault,it can be seen that there are multiple layers of black mudstone interbeds in the lower section of Shahezi Formation in Xujiaweizi fault depression,which are mainly underwater deposits controlled by early fault depression,mainly developing continental deposits dominated by nearshore underwater fan and lacustrine facies,and identifying the inner fan,middle fan and outer fan subfacies of nearshore underwater fan,And semi deep lake and deep lake subfacies of lacustrine facies.Through the statistical analysis of the sporopollen species and distribution range,the paleoclimate changes of the lower Shahezi member were discussed.It was found that the Piceites-Piceaepollenites-Cyathidites assemblage in the lower section of Shahezi Formation was the representative,which was mainly gymnosperms community,and fern spores played a secondary role.The sedimentation period of the lower Shahezi Formation was dominated by tropical temperate climate,with mesogenic humid dry type.The vegetation type is mainly coniferous forest,followed by evergreen broad-leaved forest and herbs,with a small amount of deciduous broad-leaved forest and shrubs.The percentage of pollen species representing tropical type in the lower section of Shahezi Formation increased from bottom to top,indicating a gradual increase in climate.On the basis of sporopollen analysis,in combination with climate change sensitive indicators such as Sr/Cu,Al2O3/Mg O,Mg/Ca,and CIA weathering index,the paleoclimate of Shahezi Formation from the bottom to the formation stage is divided into five different stages,which are stage I＿Warm and cool semi humid climate,stage II＿Warm and humid climate,stage III＿Warm and cool semi humid climate,stage IV＿Warm and cool semi humid-semi-arid climate,stage V＿Warm and humid climate.Based on qualitative to semi-quantitative analysis,the paleo-atmospheric CO2 value of the Shahezi Formation was quantitatively restored by stable isotope to be between 323-1323 ppm,and the change of its content has a good corresponding relationship with the change of different climatic and environmental stages.In order to explore the evolution characteristics of the ancient environment,this paper discusses the redox and paleosalinity changes of the ancient water body during the sedimentation period of the lower Shahezi Formation by means of elemental geochemistry,biomarkers and other research methods,and identifies the sources of organic matter and inorganic terrigenous clastic materials.The change and scope of redox indexes Fe²⁺/Fe³⁺,Ni/Co,Eu*,Ce*,etc.indicate that the lower Shahezi Formation is a sedimentary environment dominated by partial reduction,and has experienced a change process of dysoxic,anoxic,dysoxic,dysoxic-oxic and anoxic.The ratio of Pr/Ph<1 also verifies the partial reduction sedimentary environment.Using proxy indicators of paleosalinity Rb/K,Sr/Ba,triarylsterane-C26（20S）/C28（20S）,it is indicated that the lower Shahezi Formation is dominated by freshwater sediments,mainly freshwater,freshwater-brackish water,and freshwater,freshwater-brackish water,freshwater periodic changes,and the longitudinal change of the gammacerane index further hints that the water body may be stratified.Productivity indicators（P/Ti,Cu EF,Zn EF,Ni EF）increased significantly in stages II-III and V,indicating that there were two high productivity events in the lower Shahezi Formation.The C/N ratio indicates that the organic matter of Shahezi Formation is mainly terrigenous higher plants,and the stage II and V are terrigenous and autogenous mixed organic matter sources,suggesting that the productivity event is the result of biological prosperity.The continuous carbon and nitrogen isotopes are used to connect the paleoclimate and paleoenvironment analysis of the whole lower Shahezi Formation from stage I to stage V,and through the analysis of the reasons for the obvious migration of stable carbon isotopes,it is found that there are two large-scale volcanic activities（stages II and V）in the lower Shahezi Formation,which may be related to the Ontong Java super mantle plume in Aptian period and the giant Aptian volcanism in the Songliao Basin and northeast Asia（SB-V）.The greenhouse gas emissions,warm and humid climate environment and the input of nutrient elements brought by volcanic activities have jointly promoted the prosperity of the lake and the organisms around the lake,improved the productivity,and provided a large amount of organic matter sources for the lake.At the same time,the expansion of lake basin caused by fault activity provides good preservation conditions for organic matter.Comprehensive analysis shows that the development of organic rich source rocks in Shahezi Formation is attributed to the synergistic effect of paleoclimate,biology and preservation environment.According to the multiple seismic deformation structures identified on the core,referring to the conclusions of this paper on warm and humid climate and sufficient precipitation caused by volcanic activity,and the possibility of activation of contemporaneous faults in the basin,it is speculated that the sudden decrease of organic carbon content in Stage III is mainly caused by the dilution of organic matter due to the input of a large number of coarse clastic particles.By combining the special sedimentary environment of the faulted basin and the analysis results of paleoclimate and paleoenvironment at different stages,a development model of source rocks suitable for the faulted basin under the influence of multiple factors is established.That is,the water depth and basin narrow factors are suitable for the early rifting model of organic matter preservation;the fault-volcanic activity period model in which the organic matter is diluted due to the input of multi-terrestrial clasts;the organic-rich source rock deposition under the synergistic effect of climate-biology-preservation environment for the stable late mode.Through the variation range of the stable carbon isotope(δ¹³Corg)in the lower Shahezi Formation and the change trend of obvious excursion combined with chronological constraints,it is found that the lower Shahezi Formation is highly similar to the C5-C8stages of the marine isotope stratigraphy in other parts of the OAE1a.It indicates that the Shahezi Formation may be a continental sedimentary stratum under the influence of a typical global oceanic anoxic event（OAE1a）,and establishes a theoretical research bridge for the study of lakes and global anoxic events in the Aptian period in the Songliao Basin.