The Main Lithologic Assemblages Of The Cenozoic In The Western Qaidam Basin And Their Post-sedimentary Structural Deformation

Lithology combination and the sedimentary and tectonic deformation in the formation of unconventional reservoirs have important influence,previous research has focused on a single lithology of fracture development situation,for the different lithology combination of sedimentary rocks and sedimentary tectonic deformation after research in-depth,in this paper,on the basis of core observation,thin section identification,logging data analysis,modeling experiment and research,the western region of Qaidam basin(abbreviation)in western Qaidam Cenozoic sedimentary rock lithology combination of sedimentary tectonic deformation characteristics after preliminary analysis,and obtained the following understanding:Main lithological association types:According to the classification of grain size and composition of sedimentary rocks,sedimentary rocks in qaixi area are divided into three main lithologic assemblages: fine-grained rock assemblages(clay rock,complex rock,carbonate rock,etc.),sandstone-fine-grained rock assemblages,and salt-salt-fine-grained rock assemblages.The structural deformation characteristics reflected by the acoustic time difference log Ninety-seven porosity values were calculated from 19 Wells in the qaixi area,and the scatter plot of the acoustic time difference was drawn.The acoustic time difference of the sandstone--fine-grained rock assemblage and fine-grained rock assemblage decreased rapidly in the range of 0-1000 m with the increase of burial depth,reflecting that they were mainly plastic deformation in this depth section.The change is relatively slow in the range of 1000-2000 m,which reflects that they are mainly elastoplastic deformation in this depth section.Differences in rock stress and strain curves According to 7 group of 28 core samples in western qaidam basin of triaxial rock mechanics test data,combined with previous studies of cream salt rocks stress-strain curve analysis: under the condition of the same stress,sandstone should variables to greater than that of mudstone,mudstone of the dependent variable was slightly greater than carbonate,and paste strain values than sandstone formation strain value,as a whole should be variable is from big to small order: cream salt rocks,sandstone type,fine grained rocks.Fracture development and distribution characteristics in the core According to the observation of cores and thin sections of more than 20 Wells in qaixi area,fractures in cores are divided into parallel plane fractures,low-angle oblique fractures and near-vertical fractures.The burial depth of fractures is different in different lithologies.Fractures are developed in fine-grained rocks at a depth of more than 2000 m.Fractures develop in sandstone approximately at depths greater than 2500 m.Post-depositional structural deformation characteristics of major lithologic association With the increase of burial depth,the deformation of major lithologic association structure in qaixi area can be roughly divided into three stages: plastic deformation stage,elastoplastic deformation stage,and elastoplastic and fracture deformation stage.For sandstone-fine-grained rock assemblages,plastic deformation mainly occurs when the burial depth is less than 1000 m,elasto-plastic deformation mainly occurs between 1000 m and 2000 m,and elasto-plastic and fracture deformation occurs when the burial depth is more than 2000 m.For the salt-fine rock combination,because the salt-fine rock has the characteristics of strong plasticity at high temperature,the structural deformation of the combination is mainly elastoplastic deformation when the buried depth is less than 1000 m,and elastoplastic and fracture deformation when the buried depth is greater than 1000 m.