Synergistic Effect Of Source And Climate And Its Implications For The Development Of Organic-rich Shale In Typical Hydrocarbonrich Sags In The Bohai Bay Basin

The study of the continental organic-rich shale development mechanism is of great significance for guiding the exploration and development of continental shale oil.Many studies have been carried out on individual regions or sags,focusing on the characterization and interpretation of sedimentary environments,that is,the“site”where continental fine-grained sedimentary rocks form.However,there is a lack of discussion on its development mechanism from a macroscopic perspective,and less consideration is given to macro factors such as parent rock properties,climatic conditions,and tectonic settings.Although the joint control of climate and tectonic subsidence on the evolution of paleolakes and organic matter enrichment have been extensively investigated,little attention has been paid to the parent rocks,which is also an essential factor.In fact,the difference in the parent rock property and climate change in source areas will lead to the difference in sediment yield and type,which will have an important control effect on the sedimentary rock characteristics,but there are few targeted and systematic comparative studies.The Bohai Bay Basin is bounded by a series of uplifts and fold belts,and the types of rocks exposed in these areas are very different.During the Paleogene period,the provenance of the basin came from multiple directions,and different sags in the basin were separated by uplifts or bulges.At the same time,the paleoclimate changed significantly during the evolution of the basin.Therefore,the Bohai Bay Basin is a natural laboratory for studying the control of source and climate on the development of continental organic-rich shale.In this thesis,shales developed in three typical hydrocarbon-rich sags,namely the Raoyang,Dongying,and Bozhong sags,in the Bohai Bay Basin were selected as the research objects.Qualitative observation methods include core,thin section,scanning electron microscope,and quantitative analysis methods include Rock-Eval pyrolysis,whole rock and clay X-ray diffraction,elemental geochemistry,organic and inorganic carbon and oxygen isotope analysis are comprehensively used.Under the guidance of petrology,sedimentology,sedimentary geochemistry,and isotope geochemistry,qualitative,semi-quantitative,and quantitative recoveries of every single factor were carried out,and the differences in shale types and characteristics in different sags and layers were compared and analyzed.The sources,climatic,and paleoenvironmental factors during shale deposition were clarified,and the coupling and internal relationships between various factors were discussed,to reveal the controlling effects of source and climate on the development of continental organic-rich shales.The major breakthrough of this study are as follows:Based on the regional geological background and previous studies,the provenance of fine-grained sedimentary rocks in different sags of the Bohai Bay Basin were analyzed by using element geochemistry data.The results show that there are obvious differences in the properties of parent rocks in the source areas of the Raoyang,Dongying,and Bozhong Sags during the shale deposition.At the same time,the parameters of Y/Ho,La/La*,and ⁸⁷Sr/⁸⁶Sr indicate that the sedimentary environment of the Shahejie Formation shale in the Dongying Sag was a continental environment,which was less affected by seawater intrusion.Base on the strong positive Eu anomaly characteristics,Fe-Mn-（Cu+Co+Ni）×10 and Ni-Co-Zn triangle diagrams,it is considered that the Bozhong Sag were affected by hydrothermal activity during the Shahejie and Dongying Formation shales deposition.Based on previous studies,the paleoclimate evolution process of the Bohai Bay Basin during the Paleogene was discussed from the macro background of the East Asian monsoon evolution.The results show that the basin was influenced by monsoon and planetary winds during the Paleogene period.At the same time,due to the large north-south extension of the basin,it is located at the boundary of the temperate zone and the central arid zone in latitude.Therefore,it has the characteristics of cross-climate zone,which caused the differentiation of paleoclimatic evolution in the southern,the northern and the main part of the Bohai Bay Basin.As for the main part of the basin,it is believed that the global paleoclimate changes and the astronomical driving force caused the reciprocating migration of the climate boundary,which led to the alternating control of different climates in this area,resulting in two large-scale dry-wet climate cycles during the Paleogene.The element geochemistry characteristics of shales from different layers of three sags were analyzed using principal component analysis,and the comparative analysis was carried out by sag and layer.The results show that the source and climate have obvious control on the elemental geochemistry characteristics of shale,and the control of source is reflected in the difference between shales in different sags,while the control of climate is reflected in the difference between shales from different layers in same sag.A comparative study of the inorganic material composition of shales from different layers in three sags based on thin sections and XRD data suggests that,in addition to climate and tectonics,the property of the parent rock is also a first-order control factor for the inorganic material composition of fine-grained sedimentary rocks.The differences in parent rock properties and climate changes in source area will lead to differences in sediment yield and type,which controls the“material basis”for the formation of fine-grained sedimentary rocks.The role of climate in the sedimentary area is mainly manifested in controlling the balance between precipitation and evaporation and regulating the precipitation of carbonate minerals.At the same time,the periodic changes of paleoclimate will also cause shale to have rhythm and cycle.The paleosalinity,paleoproductivity,and bottom water redox conditions of different geological periods in the Raoyang,Dongying and Bozhong sags were qualitatively,semi-quantitatively,and quantitatively characterized by various parameters,and their controlling factors were discussed based on the synergistic effect of source and climate.It is believed that the difference of source and climate caused the temporal and spatial imbalance of paleosalinity of the Bohai Bay Basin.The differences in parent rock properties and climate conditions in source area will cause differences in nutrient supply,water transparency and other factors,thus controlling the productivity of paleolake.The bottom water redox conditions are directly controlled by the physical and hydrochemical conditions of lake,which are deeply influenced by macro factors such as source,climate,and tectonic conditions.Therefore,macro factors also play an important role in controlling the bottom water redox conditions.In this thesis,the development mechanism of continental organic-rich shale was discussed in terms of tectonic and orbital time scales.The dominant factors leading to the differential enrichment of organic matter at different time scales were clarified,and the continental organic-rich shale development model controlled by source and climate at different time scales was established.On the tectonic time scale,the differences in shale types and organic matter enrichment mechanisms in different sags are essentially attributable to the differences in external macro factors such as source and climate.As the first-order controlling factors,macro factors affect the second-order controlling factors such as water salinity and depth,lake stratification,and hydrological conditions,and then determine the relative importance of direct controlling factors such as redox conditions,productivity,and sedimentation rate.On the orbital time scale,high-frequency climate change will amplify or suppress the general sedimentary conditions on the tectonic time scale to some extent,and usually cause the enhancement of heterogeneity of organic-rich shale.