| With the comprehensive analysis of the sedimentary structures,lithologic composition,geochemistry,micro-characteristics of sandstone as well as the characteristics of the log response,based on many previous research results,using sedimentology,petroleum geology theory,this paper makes full use of information of the drilling,well logging and testing analysis to study the sedimentary microfacies and reservoir characteristics of the chang4+5 oil-bearing Intervals of the Triassic Yanchang formation in the Baibao—Huachi region in Ordos Basin.According to sedimentary cycles,contrasted stratum's isochronism with K5 and K6 marker bed,the chang4+5 oil bearing intervals is divided into two sub-paragraphs and four layers which are chang 4+51,chang 4+52,chang 4+511,chang 4+512,chang 4+521,chang 4+522.Use of the facies marker such as lithology, sedimentary structures, fossils, particle size distribution, log-phase analysis and profile structure of analysis of,etc,it can be recognized delta front, the former delta and shallow lake subfacies in chang4+5 oil-bearing intervals in the research area,and can be more identified diversified sedimentary microfacies such as underwater distributary channels, mouth bars, distal bars, subsea natural barriers and crevasse splays, sand sheets, separate inter-bays, the former delta muds and shallow lake muds and so on.For the first time seismites,deposition is discovered in the chang4+5 oil-bearing intervals in research district,while discussing the characteristics seismites, establishing seismites sedimentary sequence and analysising its causes. Through the observation for the cores, many syndepositional deformational structures such as sand balls,sand pillows,ball-and-pillows, graded micro-faults, micro-cracks, fluidified convolute deformational beddings have been greatly discovered in the lake delta front deposition in the chang4+5 oil-bearing intervals,which are recognized as seismites.On base of the detailed sduty of the seismites characteristics, it can be established different sedimentary sequences,and in-depth analysed its causes. Study on the sandbody distribution and lithofacies palaeogeography mapping from the chang4+522 to chang4+511 sedimentary period suggest that depositional system of the Baibao- Huachi area in chang4+5 sedimentary period is belong to the extension of the northeast Zhi Jing Delta and Wuqi delta to south-west direction.Chang4+5 sedimentary depositional model.is established on combining with study of sedimentary facies marks,sediment type characteristics and dividing as well as characteristics of seismites.For the first time the paper have a quantitative or semi-quantitative study on inversion of the chang4+5 sedimentary depositional environment and paleo-climatic conditions taking full advantage of the sedimentary geochemistry signs.Using the walker method (equivalent boron method),the Adams method and the elemental ratio method(Sr/Ba) conclude that it was the freshwater depositional environment in the period of chang4+5. Using iron mineral assemblage method and Fe2+/Fe3+ method determine that sedimentary environment was the weak reduction condition at that time.Using the method of the combination of elements (Cu,Ba,Ni,etc.) suggest that the sedimentary environment was from lakeshore to shallow lake in depositional period of chang4+5;Using the method of the combination of elements(Fe,Mn,Cr,V,Ni,etc.) and the elements ratio (Sr/Ba, Fe3+/Fe2+) inferred that it was humid weather conditions in the chang4+5 depositional period.Observation of a large number of core and various test analysis conclude that underwater distributary channels and mouth bars, as well as sand bodys which have the complex genetic types in the delta front are favorable facies for development of the reservoir. The rock types of the reservoir sandstone of the chang4+5 oil-bearing intervals in the research area are mainly feldspar,following feldspar sandstone debris.which have lower the mineral maturity and the maturity structure because of the near source deposition. The main porosity types are the remainder inter-particle-hole,following feldspar dissolution porosity.With an average porosity of 14.36% and average permeability of 1.66×10-3μm~2.the reservoir is belong to low porosity and low permeability or ultra-low permeability reservoir Diagenesis of the study area have mechanical compaction, pressure solution, cementation, dissolution,etc,in which early cementation and dissolution have a constructive role in development of the reservoir.Contrasting Jiyuan-Hu Jianshan and Baibao-Huachi with rock types,heavy mineral assemblages,quartz cathodoluminescences,sandbody distribution laws,changes in vertical facies sequence,as well as the causes of the reservoir sandbody suggest that the provenance of the Jiyuan-Hu Jianshan is controlled mainly by Yinshan oldland and Alashan oldland,belonging to the mixed deposition zone of the northwest and northeast river system, in which the sandbody of the chang4+52 deposition period develope relatively.The underwater distributary channels and the overlays of the multi-phase underwater distributary channels are favorable development facies zones of the reservoir sandstone.While the provenance of the Baibao-Huachi is mainly controlled by the Yinshan oldland,belonging to the depositional system of the northeast delta,in which the sandbody of the chang4+51 deposition period is relative development.Favorable reservoir facies zones are the mouth bars and the overlay.of the mouth bars.
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