Characteristics And Genesis Analysis Of Carbonate Lamina In The Upper Fourth Member And The Lower Third Member Of Shahejie Formation In Dongying Depression,Bohai Bay Basin

As a typical sedimentary structure in fine-grained rocks,laminae are widely distributed in shales and mudstones in sedimentary basins.Carbonate lamina,as a very important part of the lamina,was of great significance both for the record of paleoenvironment and the response to TOC in shale.Several research mainly focus on description of the characteristics of laminae have done,however research on the genesis of laminae were relatively less.Some sedimentation models were established in several case stydies based on seasonal sedimention,but there was no clear evidence for the duration of annual laminae.Depend on the systematic cored well of shale,base on the core and thin section observation,together with mineral compositon,major and trace element,whole rock and micro area stable isotope tests,with the help of milankovitch astronomical cycle analysis,the type,characteristic and genesis of carbonate lamina of lacustrine shale in Dongying Derpessin was discussed in this stydy.The main understanding and conclusions were as follow:1.The types of laminae in Dongying Depression were divided and the background environment of these laminae was analyzed.Based on macroscopic and microscopic observation of core and thin section,respectively,four different couplets were divided by the composition of layers including carbonate-clay couplet,carbonate-organic couplet,clay-organic couplet and carbonateclay-organic triplet.Five different laminae were divided by the characteristic of morphology including parallel laminae,interbedded laminae,wavy laminae,lenticular laminae and weak laminae.Combined with various geochemical tests,mineral composition and other factors,a closed brackish lake with high salinity and high productivity under drought background could be concluded.2.The annual laminae affected by seasons in Dongying Depression was clarified.Based on milankovitch theory,astronomical time frame and sedimentation rate curve of the lower third member and upper fourth member of Shahejie Formation in Dongying Depression was established through time series analysis.Combined with the laminar thickness measured by microscopic observation,it can be calculated that the duration range of couplets in study area was between 0.49 yr and 3.74 yr,with an average value of 1.18 yr.The duration range in all samples was relatively small,which showed that the calculation result was relatively stable and that the annual lamina could be identified.3.The genesis and sedimentary process of different types of carbonate laminae in Dongying Depression of Bohai Bay Basin were analyzed.According to microscopic observation,two main types of carbonate laminae were identified including micrity carbonate laminae and fibrous sparry carbonate laminae.Based on the analysis of the variation of carbon and oxygen isotopes in the microzone,it was concluded that the formation of micrite carbonate lamina was mainly affected by the seasonal carbonate deposition caused by the periodic thrive of algae.The deposition of fibrous calcite was mainly affected by overpressure and crystallization of granular calcite caused by undercompaction and dehydration of clay minerals in the diagenetic process.