The Study On Sediment-Diagenesis Of Chang-3 Oil Formation In Zhenbei Aera, Ordos Basin

As the second largest sedimentary basin in China, the Ordos Basin possesses abundant oil and gas resources with a bright prospect for the exploitation and development of oil and gas. Chang-3 oil formation is one of the most important oil-bearing formation in Zhenbei aera which lies in the southwest of the Ordos basin. The trait of low porosity , low permeability and low output of Chang-3 oil formation is very predominant.Based on the synthetic research of core description,drilling section,well log and office compilation, combined with analyses of depositional setting of Yanchang formation , the facies marks of lithological association, sedimentary structure, palaeontologic species,image granularity and trace elements character,the fluvial-dominated shallow delta plain is identified as the sedimentation type of Chang-3, furthermore the two sedimentary microfacies including distributary channel and flood plain are divided. The distributary channel is the skeleton sand bodies.With the method of spot-line-surface combination,the distribution of the sand bodies and sedimentary microfacies in three sub-stratums are detailedly analysed by drawing the sedimentary facies map of key wells, well-tie sedimentary facies profiles,the thickness of sand bodies isopleth map and sand percentage isopleth map. The branching phenomenon of distributary channels in Chang-3 stage is apparent and the ambit of distributary channels is continuously shrinked;the aera is gradually swamped and the thickness of sand bodies is gradually decreased.Based on the depositional feature and the distribution of the sedimentary facies, sedimentary model of Chang-3 is summarized.On the base of the theory of petrology, the paper discusses the characteristic of reservoir petrology and diagenesis in detail with the methods of analysis of popular section, cast section, scanning electron microscope, X-ray diffraction and etc. The reservoir rocks are mainly composed of feldspar lithic fine sandstone and part of lithic fine sandstone with a large variation range of composition of the sandstone fragmens in planar and longitude. The main diagenetic types of the reservoir rock are moderate compaction, multiphase cementation, ubiquitous denudation and the local fracturing, the compaction and carbonate cementation destroy the primary-texture of pore intensively, dissolution changes the pore structure effectively. The diagenesis of the reservoir rock of Chang-3 is during the period of the middle diagenesis A and the pattern of diagenetic and porosity evolution are established. Based on the research of sedimentary facies and diagenesis of Chang-3 in Zhenbei aera,the four sediment and diagenesis facies belt are sketched, delta plain distributary channel secondary solution opening facies and delta plain chlorite clay film cementation primary facies are advantaged sediment and diagenesis facies.It is considered that Reservoir properties are mainly controlled by sedimentary microfacies and diagenesis.The influence of sediment-diagenesis on the sand bodies, physical property, heterogeneity and oil accumulation of sandstone reservoir is analysed in detail.The distribution of sand bodies are fully controlled by the sedimental microfacies which effects on the macroscopic characteristic of sandstone reservoir; The diagenesis affects on the physical property and heterogeneity of sandstone reservoir by reforming the pore texture on the microscopic scale and consequently affects on the oil accumulation.