Research On Fine-grained Sedimentary Microenvironment Of Chang 7 In Ordos Basin

Chang 7 period of Ordos Basin is the largest period of lake basin area in the whole extension period.The source supply of the lake basin is mainly composed of the braided river delta located in the southwest and the meandering river delta in the northeast.the two major sources converged to the middle of the lake basin to form a semi-deep lacustry-deep lake and gravity flow sedimentary system.Due to the small slope of the lake basin,abundant provenance supply and warm climate,the fine-grained sedimentary rocks of Chang 7 period are distributed in a wide overlying pattern.Thus forming a large area of tight oil and gas reservoirs.However the morphology and properties of fine-grained sediments in different regions of the lake basin are different due to the influence of local microenvironmental which difference in the sedimentary period of fine-grained sedimentary rocks.The research on the microenvironment of Chang 7 fine-grained sediments and the study of the characteristics of microenvironment in different morphology fine-grained sediments and the influence factors of different environment on fine-grained sediments are to lay a new theoretical foundation for the development of fine-grained sedimentology in the study area.It is of guiding significance for the exploration and development of further dense oil?gas?and shale oil?gas?.Based on the anatomic core description of outcrop in the field,X-ray diffraction and SEM analysis,6 types of fine-grained sedimentary rocks are summarized.By comparing the differences of trace elements in different fine-grained sedimentary rocks,the microenvironment and its characteristics of different rock types in the process of sedimentation are identified preliminarily.Combined with the above research results,the longitudinal and lateral distribution of fine-grained sedimentary rocks was studied and the profiles were drawn to determine the temporal and spatial distribution of different types of fine-grained sedimentary rocks and their microenvironments in the study area.At the same time,the control of fine-grained sedimentary rocks is studied by studying the changes of various external environments.The 4 factors which have direct influence on the spatial assemblages of fine-grained sedimentary rocks have been preliminarily identified.In the course of the project research,the following understandings were obtained:?1?It is recognized that the distribution of fine-grained deposits in the study area is mainly block mudstone,horizontally bedded mudstone,granular bedding mudstone,microwave shale,straight stripe shale and massive shale.?2?Geochemistry of various fine-grained sedimentary rocks and TOC ratio of clay mineral assemblage can be seen that the water depth,redox,clay content and TOC content of different types of fine-grained sedimentary rocks are different during the deposition period.The average TOC content of the poor organic block montmorillonite mudstone is 3%,and the average relative content of Sr/Ba is 0.34.The average relative content of V/?V+Ni?is 0.51,and the estimated water depth is 50-60m which mainly formed in slope break zone.The average TOC content of the horizontal bedding of the middle organic matter of imimon mudstone is 6%,and the average relative content of Sr/Ba is 0.38.The average relative content of V/?V+Ni?is 0.73,the estimated water depth is 60-80m which mainly deposited near the center of the lake.The average TOC content of the middle organic matter grained sequence stratification Illite mudstone is4%,and the average relative content of Sr/Ba is 0.42.The average relative content of V/?V+Ni?is 0.72,the estimated water depth is 70-105m which mainly formed in half deep lake zone.The average TOC content of microwave Illite shale rich in organic is8%,and the average relative content of Sr/Ba is 0.44.The average relative content of V/?V+Ni?is 0.79,and the estimated water depth is 105-120m which mainly formed in deep lake quieter environment.The average TOC content of Organic matter-rich flat-grain chlorite shaler is 12%,and the average relative content of Sr/Ba is 0.47.The average relative content of V/?V+Ni?is 0.83,and estimated water depth greater than120m which mainly formed in deep lake quieter environment.The average TOC content of organic matter like block chlorite shale is 16%,and the average relative content of Sr/Ba is 0.5.The average relative content of V/?V+N?is 0.88,and estimated water depth greater than 120m which mainly developed in deep lake sag.?3?Through core sampling observation and element geochemical comparison of key wells,it is found that the majority of massive mudstones are developed in the downhill break zone environment of semi-deep lake water,the flat grain shale developed in the deep lake,which is the environment of the lake bottom block sag,the massive shale developed in deep lake,often interbedded with tuff,which is the environment of lake bottom block sag affected by volcanic movement.Chang 7₃-Chang7₁.With the decrease of lake basin area,the sand sediment of larger granularity increases,the fine-grained sedimentary range of smaller grain size is reduced.Many kinds of fine-grained sedimentary rocks located in the depth of lake basin are distributed from continuous to sporadic distribution.?4?Based on the study of the types of fine-grained sedimentary rocks,the characteristics of sedimentary microenvironment and the distribution law,the controlling factors of microenvironment change are summarized,the ancient water depth controls TOC content and distribution range in microenvironment,paleoclimate controls hydrodynamic conditions of microenvironment and variation of water stratification.Sedimentaryfaciesbeltscontroltheformationofshallow microenvironment.Volcanic movement only affects the change of environmental types in the deepest obliterated sag of water.