Sedimentary Microfacies Of The Middle Jurassic Zhiluo Formation In Yanwu Area,Ordos Basin

In recent years,breakthroughs have been made in the exploration of in-situ leachable sandstone uranium deposits in the Zhiluo Formation on the western margin of the Ordos Basin.Recently,industrial oil flow was discovered in the township's No.369 well area in the west of Yanwu area,indicating that the Zhiluo Formation was relatively Good oil and gas resource potential.However,studies on the sedimentary facies of the Middle Jurassic Zhiluo Formation in the Ordos Basin have focused on the outcrops in the eastern and northern parts of the basin.As far as the Yanwu area is concerned,there is no unified plan for stratigraphic division at the stratum level of the Zhiluo Formation,and there are disputes between the braided river,meandering river and delta in the sedimentary phase.These problems have restricted the understanding of the Middle Jurassic sedimentary pattern in the Ordos Basin and the efficient exploration of resources and energy in the Yanwu area.Based on this,this paper focuses on the description of the sedimentary microfacies of the Zhiluo Formation in the Yanwu area in the southwest of the basin and describes the spatial distribution.The four outcrop sections of the Zhiluo Formation on the periphery of the basin were carefully measured,and four drilling wells in the study area were developed.Detailed observations and descriptions were carried out to analyze the stratum subdivision and log sedimentary microfacies in more than 500wells in the basin,from the paleo-geozoic,paleogeozoic,paleontology,and mineralogy of the rocks before the deposition of the Zhiluo Formation.The type and microfacies of sedimentary facies in the Zhiluo Formation were clarified with respect to structural features,section structure,etc.The Zhi 11-straight-42palaeogeographic map system developed on a small layer revealed the sedimentary evolution of the Middle Jurassic in Yanwu area.The typical outcrop profile at the periphery of the study area is the obvious visible stratification of the two sections of the thick giant sandstone section of the lower section of the Zhiluo Formation and the interbedded section of the sandstone and mudstone of the upper part.The well logging response marker has more than500 natural logging curves in the middle.The conversion surface of gamma,natural potential,sonic time difference and resistivity,this sign is consistent with lithological conversion,and is divided into upper and lower sections according to this,and then according to more regular depositional cycles,combined with electrical and lithologic changes The features are gradually stratified upwards and downwards,and can be further divided into 4 depositional cycles,each of which can identify two sub-cycles.Finally,the principle of subdivision was defined,and the subdivision scheme for the 4th and 8th subsections was determined for the first time.That is,the Zhiluo formation is divided into straight section 1,straight section 2,straight section3,and straight section 4.It is further subdivided into straight 1₁,straight 1₂,straight2₁,straight 2₂,straight 3₁,straight 3₂,straight 4₁,straight 4₂.A total of 8 sub-sections.The thickness of each segment is about 80¹20m,and each sub-section is generally40⁶0m.The collection of near-thousand groups of pristine streamflow data from the four outcrop profile sections of Huluhe,Xujiu Cuijiagou,Binxian Beizigou and Binchuanshayaozi of Zhiluo Town in Yanwu District showed that the ancient direction was mainly northward.The northeast direction indicates that the source of river-delta sediments in the study area is from the southwest region.In combination with paleo-ecology,rock lithology,sedimentary structure,grain size analysis,and structural characteristics of phase profiles,the lower section of the Zhiluo Formation is a braided river–braided river delta sedimentary facies,which can be divided into braided rivers,rivers,and braids.The river delta plain subfacies,the microfacies of the developed riverbank and the braided river distributary channel microfacies;the upper sedimentary facies are meandering river delta facies,which can be divided into meandering river delta plains and meandering river delta front subfacies,developing subaqueous shunts.The river and the diversion bay are in microfacies.After studying the palaeogeographic pattern in Yanwu area,we found that braided and braided river deltas developed from the southwest to the northeast in the4th period of the study area.The braided rivers are mostly coarse sandstone,with large grain sizes and low mud content,at the end of the river channel.The development flood plain,silty mudstone and argillaceous siltstone sandwich each other,and the riverbed-dominated sedimentary environments such as braided river channels of the delta plain are developed.The braided river delta plain was developed in the northeastern period of the 3rd period,the river appeared multiple forked convergence,and the braided channel in the southwest narrowed,and the heart bank was often developed at the bifurcation as the deposition amount increased.With the rise of the datum plane,the increase of accommodation space is still characterized by the development of the braided river and the braided distributary channels.Under the influence of the Yanshan movement curtain,the sedimentary environment of the basin evolved into a meandering river delta deposition with a gentle topography.The meandering river delta plains are developed in the southwestern part of the country.The sub-facies of the river channel are the sub-facies of the floodplain and are mainly fuchsia mudstone.The middle and fine sandstones of the distributary channel pass through the mudstone,and the northeastern part of the Meso-Triangle river develops the front edge of the meandering river.The sub-distributary rivers dominated by fine-grained siltstones and silty mudstones are intertwined.The first straight period is still the meandering river delta environment,the distributary channel is becoming smaller,and a large number of rivers are formed.The mud content in the sandstone is significantly higher.The sand bodies that began to form during this period are not conducive to the development of the reservoir.The sedimentary pattern is similar to that of the previous period.In the plane distribution,the relatively small meandering river delta distributary channel sand body shuttles through the mudstone,indicating that the Zhiluo basin has gradually disappeared.Due to the erosion and erosion of the overlying Anding Formation river-delta system,the sedimentary system during this period remains incomplete.