Karst Planation And Diagenetic Field Evolution Of Ordovician In Ordos Basin

As the largest transgression period in the geological history of the Chinese mainland,carbonate rocks developed in the early Paleozoic.The majiagou formation in ordos basin,represented by it,underwent long-term weathering denudation and fresh water leaching,forming a special karst landform and karst reservoir.However,recent research is more and more contrary to existing cognition,the deviation is derived from the misunderstanding of the ancient karst formation mechanism,to review peneplain process in ordos basin,karst development process and the pore development and filling mechanism caused by the accompanying rock evolution to break through the original ancient karst reservoir.It is of great significance to the formation process of weathering crust,karst development process,pore evolution and preservation mechanism,and the understanding of the north China platform caledonian period missing record On the basis of core observation,logging data statistics,combined with casting thin section observation under the microscope,cathode luminescence,C-O isotope and other analysis methods,this paper makes further discussion on diagenesis,karst leveling characteristics,diagenetic process and pore filling characteristics and rules in the ordos basin:?1?The diagenetic environment in the study area is changeable,and it has mainly experienced near-surface diagenetic environment,shallow burial environment,superficial diagenetic environment,and deep burial diagenetic environment.With the diagenetic fluids and diagenesis of different properties,it can be generally divided into quasi-symbiotic Stage,near surface shallow burial diagenesis stage,superficial atmospheric freshwater diagenesis stage,shallow burial diagenesis stage,deep burial diagenesis stage.The petrological characteristics and geochemical data of karst reservoirs indicate that the gypsum mold holes formed during the quasi-cogenetic period,and intermittent exposure caused early filling of the vadose-silt and I establish the diagenetic evolution sequence of the Ordos Basin.?2?Combined with diagenesis,the karst is divided into quasi-intergrowth karst in phase I,superficial karst in phase II,shallow burial in phase III,and deep karst in phase IV.According to the reflection of logging curves in different karst zones,the karst zones are divided into eluvial zone,surface erosion zones,the three stages of vertical permeating belts combined with horizontal underflow belts,and deep slow flowing belts.The geological background and karst characteristics of the Ordos Basin indicate that the formation of the Ordovician karst landforms in the Ordos Basin was a peneplain leveling process,and the formation of weathered crusts was controlled by the planation surface.The karst cycle in the traditional sense is actually caused by the bottom erosion process of the double karst planation surface.Using the water table as the bottom plate of the double karst planation surface,the Late Ordovician-Early Carboniferous planing plane of the basin can be divided into three phases.Its thickness and depth are stable,and the distribution in the horizontal and vertical directions is regular,which reflects the overall control of karst and paleogeomorphy.The process of peneplain and karst planation of the Caledonian period in the Ordos Basin is summarized with model diagrams.?3?Ordos basin karst reservoir pore space is given priority to anhydrite moldic pores.There are mainly vadose-silt,calcite,quartz,iron dolomite fillings,and the combination of the main mineral assemblages are vadose-silt half filling,vadose-silt+calcite,vadose-silt+quartz sand filling,vadose-silt+quartz+calcite,vadose-silt+ankerite.Karst platform calcite filling degree of Ma5₁₊₂ period is weak and the whole basin calcite filling degree of Ma5₃₊₄ period is stronger.Weathering crust of different landforms and the combination of pore fillers on the horizon are regularly different.The rule of the distribution in time and space is actually affected by the transformation of the diagenetic field.So based on the diagenetic environment and evolution of the Ordovician in the Ordos Basin,the macroscopic karst leveling characteristics,and the microscopic Ma5_1-4 stage anhydrite moldic pores fillings'plane distribution features,the diagenetic field and pore evolution process are clarified.?4?Palaeomorphology determines the planar distribution of the overall reservoir,and karst planation changes the diagenetic environment to make the karst layers have different pore effects,and cause uneven filling of pores on the planar distribution.Based on the understanding of karst planation,diagenetic field evolution characteristics,pore formation and filling process,the favorable distribution areas of Ordovician karst reservoirs in the Ordos Basin are predicted.