Study On The Characteristics Of Hydrothermal-exhalative Sedimentary Rocks(HESR)and Its Relationship With Source Rocks From Chang 7 Section Of Triassic Yanchang Formation In The Southern Margin Of Ordos Basin

The Ordos Basin is one of the largest petroliferous basins in my country.It has rich oil and gas resources and is characterized by the large-scale development of high-quality source rocks of the Yanchang Formation of the Upper Triassic.The Chang 7 source rock has large thickness,wide distribution,and strong hydrocarbon generation ability.It is the main formation for the exploration and development of shale oil and tight oil.Based on the detailed study of field profiles and drilling cores,ordinary thin slices,scanning electron microscopy,electron probes,geochemistry,and mineral in-situ isotopic methods.We have studied the Late Triassic lacustrine hydrothermal sedimentary rocks,including their petrology,mineralogy,structural tectonics,geochemistry and isotopic characteristics,etc.The organic geochemical characteristics of Lacustrine organic-rich shale related to hydrocarbon generation.On this basis,further explore the influence of hydrothermal exhalative sedimentary rocks on the development of Chang 7 source rocks.Mainly obtain the following knowledge and conclusions.（1）Field profiles and drilling core observations from Chang 7 section of the Triassic Yanchang Formation in the southern margin of the Ordos Basin show that there are a large number of sedimentary formations and rock-mineral assemblages related to lacustrine hydrothermal activities in the study area.On the basis of previous studies and based on petrographic characteristics,it is determined from the perspective of genetic petrology that the hydrothermal exhalative sedimentary rocks in this area include hydrothermal rocks（chemical deposition of fluids）,explosion rocks（Contains crystalline particulate debris and irregular clumps）and hydrothermal minerals.Among them,hydrothermal sedimentary rocks include dolomite rocks,calcite rocks and hydrothermal siliceous rocks;explosion rocks include dolomite rocks,calcite rocks and breccia pyrite;hydrothermal minerals are dolomite and calcite in butterfly shape and leaf shape.（2）Through the in-situ geochemical characteristics of single minerals of hydrothermal exhalative sedimentary rocks in the study area and the whole-rock powder analysis of related surrounding rocks,Using Fe-Mn-（Cu+Co+Ni）×10 diagrams,Si O₂/（K₂O+Na₂O）-Mn O/Ti O₂diagrams and discrimination indicators of hydrothermal deposition,combined with its rock and mineral and structural features,It further proves that the Chang 7 sublacustrine hydrothermal activity in the southern margin of the basin participated in the deposition process of lacustrine organic-rich shale.The geochemical characteristics of the sample show that the sample contains high U,Pb,As,Sr,and Mo;some Eu values are high（positive anomaly）,some Eu values are low（negative anomaly）;Ti and Ta show negative anomalies.The analysis of the S isotope characteristics of pyrite shows that the source of the S component is complex.Some of the components come from the mantle source,and there are also components from the crust,basin basement and sedimentary cap layer.The latter is the process of fluid ejection along the channel brought in.The geochemical characteristics of the surrounding rocks show that the Chang 7 lacustrine organic-rich shale was formed in a deep lake environment with anoxic,low salinity and warm and humid climate.（3）Hydrothermal channels and chimney fragments were found in the Chang 7 source rock,which retained relatively complete hydrothermal channels and their mineral assemblages.The channels were columnar and branched.The inner wall of the channel is filled with breccia-like calcite and dolomite minerals,and a large amount of radial,saddle-like dolomite,pyrite and long strip anhydrite are developed on the outer wall.The geochemical characteristics and in-situ isotope characteristics of the minerals related to the hydrothermal chimney show that:Trace elements such as Cs,U,Th,Pb,Ba show positive abnormalities,the U/Th value is high,the content of major elements such as Sr,Mn is high,and the in-situ sulfur isotope of pyrite reaches 7.89‰-10.88‰.On this basis,the formation model of the hydrothermal chimney in the study area was established,and it was considered that the sulfate formation stage,sulfide and carbonate formation stage were the main stages it experienced.The study of hydrothermal chimneys in this geological historical period further supports the existence of hydrothermal activities in the study area during the Late Triassic.It also provides a new example and basis for the study of the effect of hydrothermal activities on high-quality source rocks and a new understanding of the origin of Chang 7 oil shale.（4）Fossils of Triassodus tongchuanlepis sp.nov.are widely developed in the Chang 7sedimentary area of the Triassic Yanchang Formation in the study area.The petrology,mineralogy,geochemistry,and carbon and oxygen isotope characteristics of the fossil and its surrounding rocks prove that hydrothermal activities have an effect on the burial of the fossil.Their unique composition,form,and distribution characteristics have become the key to discuss the living environment of extreme environmental organisms.This discovery provides a fossil record of the evolution of fish in terrestrial sedimentary basins in the Late Triassic.The research is expected to provide new insights into the life forms of various extreme organisms in hydrothermal environment and their effects on the nutrient cycle and ecosystem evolution of deep lakes;Conducive to the realization of comparative studies of fish in Asia,North America,and Oceania during the Late Triassic,and is of great significance for revealing the spatial changes of the ecological environment in the Late Triassic;It also provides a new basis for the study of hydrocarbon-generating parent materials related to source rocks in Chang 7 Member.（5）The hydrothermal exhalative sedimentary rocks in the study area are all developed in Chang 7 high-quality source rocks,and the two are conjugated and associated,and show a good relationship.The research shows that the Chang 7 source rock in the study area is a mixed origin deposit with more terrigenous clastic components and a small amount of thermal fluid input.The mixed-genetic sedimentary strata with thermal material and fluid input are very good—excellent effective source rocks,with organic matter abundance up to 27%.The hydrocarbon generation index and hydrocarbon generation potential of source rocks of mixed origin are generally higher than those of normal lacustrine deposits.Further studies on the geochemical characteristics of single wells longitudinally and multi-wells transversely show that:The correlation coefficients between the TOC value of the source rock and the ratios of Al/（Al+Fe+Mn）and（Fe+Mn）/Ti representing the input of hydrothermal fluid are as high as-0.88 and 0.87,respectively.there is a close relationship between reactive hydrothermal deposition and lacustrine organic-rich shale.That is,with the continuous input of hydrothermal fluid,the hydrocarbon generation index and potential of lacustrine organic-rich shale have also increased correspondingly.Compared with the sections of Chang 7-2 and Chang 7-1,Chang 7-3 section has the largest hydrothermal input intensity and the highest abundance of organic matter.The same horizon（Length 7-3）,compared with normal lacustrine deposits,is located in the southern margin of the basin,and the well sites or sections where hydrothermal is developed in the lake have higher abundance of organic matter.