Characteristics Of Event Deposition And Coupling Relationship In The Chang 7 Oil Member Of Triassic Yanchang Formation,Ordos Basin

Ordos Basin is the second largest petroliferous basin in China,with abundant oil and gas resources.It is not only the energy base in Northwest China,but also the main source of the west east gas transmission project.Zhangjiatan shale is the main source rock of Yanchang formation.In recent years,the study of event sedimentation has been paid more and more attention by scholars.Based on the analysis of large sedimentary environment,the application of advanced dating technology,combined with the coupling relationship of event sedimentation,can analyze the sedimentary evolution process qualitatively,quantitatively and qualitatively.It is not only an innovation in the field of sedimentology research,but also has great guiding significance for other related research.In this paper,taking Chang-7 oil member of Triassic Yanchang Formation in Ordos Basin as the research object,on the basis of previous studies and under the guidance of the relevant theories of sedimentology,stratigraphy,event sedimentology and sedimentary geochemistry,the sedimentary characteristics of each event in the surrounding field profile and drilling core of the basin are studied,and a reliable age frame is established to discuss the sub strata of Chang-7 The time sequence and coupling relationship of sedimentary events.Through comprehensive analysis,the following conclusions and understandings are obtained:1.The sedimentary characteristics of seismic event,turbidity current event,volcanic event and anoxic event of Chang-7 member are systematically described.The sedimentary environment of Chang-7 member is mainly from deep lake to semi deep lake,with low sedimentation rate and relatively stable sedimentation,which is an ideal place for the study of sedimentary events in continental lake basin;there are many event deposits preserved in Chang-7 member,such as seismic events,turbidity current events,volcanic events and anoxic events.The main sedimentary response of seismic events is a large number of sedimentary deformation structures,which can be specifically divided into soft sediment liquefaction deformation;gravity deformation;water plastic deformation;brittle deformation,etc.;and the sequence of seismites is established to analyze the relationship between the characteristics of seismites and seismic strength and its plane distribution characteristics.Turbidite is the material manifestation of the turbidity event of Chang-7.The turbidite sand body is fine,with complete and incomplete Bauma sequence,channel model,channel model,two-stage grain size probability curve and C=M diagram parallel to the baseline of C=M,which are the main characteristics of the typical turbidity deposition of Chang-7.The most developed turbidite is Chang-7₃.The material form of volcanic event is tuff,which is judged to be continental granite environment based on its geochemical characteristics;the ages of tuff in the bottom,middle and top of Chang-7 are 243.0 Ma,240.3 Ma and 239.3 Ma respectively by shrimp zircon U-Pb dating;the plane distribution of Chang-7 tuff is NW trending.The material form of anoxic event in Chang-7 formation is rich organic shale,which can be identified from logging curve,lithology and microscopic characteristics;anoxic event runs through the whole deposition process of Chang-7,with the strongest reducibility,Chang-7₂ times and the weakest Chang-7;from the plane distribution law of rich organic shale in Chang-7 sublayers,we can see the distribution rule of anoxic event The law is consistent with the deep and semi deep lake environment of Chang 7.2.For the first time,the astronomical age scale of Milankovitch cycle of Chang 7member in Ordos Basin has been established.Using the spectral analysis method to analyze the Milankovitch cycle of Chang-7,the astronomical age scale of Chang-7 formation is established,and the zircon U-Pb isotopic ages of tuff intercalations at the bottom,middle and top of Chang-7 measured by shrimp are used as the age control points.The accurate age of each event sedimentary layer of Chang-7 can be obtained:the sedimentary time limit of chang-7₃ is 243.0 241.0Ma,241.0-240.125 Ma for Chang-7₂ and 240.125-239.250 Ma for Chang 7₁.3.For the first time,a time scale coupling model of Chang-7 events and deposition has been established.In the vertical direction,the seismites,turbidites,tuffs and high-quality source rocks are developed and influenced each other.The development of seismites is closely related to the tectonic activity.The Chang-7₃ period of seismites is the strongest and the most developed.The formation of turbidites is closely related to the topographic characteristics.Turbidity current events can transport the continental margin materials to the deep lake area along a certain slope.The formation of tuffs is related to the Huoshan event,while volcanoes are mainly formed at the junction of plates,which is related to the tectonic activity The shale is mainly formed in the anoxic environment of Chang-7₃ deep and semi deep lake.In Chang-7₃,there are mainly anoxic events and volcanic events,and in Chang-7₂ and Chang-7₁,there are mainly turbidity current events and seismic events;the high-quality source rocks formed by anoxic events in Chang-7₃ are closely contacted with the large-scale turbidites formed by turbidity current events in Chang-7₂ and chang-7₁,forming a good relationship of"lower generation and upper storage",which creates conditions for the development of dense oil in Chang-7.Under the precise time scale,combined with the coupling relationship of event deposition,the sedimentary process of Chang-7 is described in a timely,qualitative,quantitative and comprehensive way.Finally,the temporal spatial combination model of Chang-7 event deposition in Ordos Basin is established.