The Study On Sedimentary Environment Of Fine Grained Rocks Of The Upper Fourth Member Of Paleogene Shahejie Formation，Dongying Sag

The study of sedimentary environment of fine grained rock for the prediction of favorable zones as well as the exploration and development of shale oil has important reference.In this paper,the fine grained rock rich in organic matter of the upper fourth member of Shahejie formation in Dongying sag is regarded as the study target.The sedimentary model and sedimentary system of fine grained rocks were concluded through the analysis of the petrology characteristics,the characteristics of lamination,the overall sedimentary environment and the control factors of lithofacies combination.In the process of study,this article innovatively applied milankovitch cycles to the comprehensive analysis of fine grained rock.As a result,astronomical cycle does have obvious influence on fine grained rock,especially,the precession cycle of 23 ka(millennium)is most evident.The mineral composition and source is the basic problem in the study of fine grained rock.Quartz,carbonate,clay minerals,organic carbon and pyrite are the main composition in study area.Quartz and clay minerals are mainly terrestrial,and a small amount of quartz is authigenic,formed in the process of diagenesis.A majority of carbonate relate to biochemical cause,and a few also derives from terrigenous.The type of organic matter isⅠand Ⅱ1,from algae and plankton.Pyrite is aggregate in strawberry shape and highly idiomorphic.The prime sedimentary structures of fine grained rock are horizontal bedding,cross bedding,massive bedding and lenticular bedding,etc.Fine grained rocks are of laminae in the thin section,the basic laminae is clay laminae,carbonate laminae,organic matter laminae and mixed laminae.According to the characteristics of laminae of fine grained rocks,this paper puts forward the model of muddy water – half muddy water – clear water for the formation mechanism of laminae.In the stage of muddy water,the lake dominately produces clay laminae and mixed laminae.In the stage of half muddy water,the lake mainly produces mixed laminae.In the stage of clear water,the lake primarily produces carbonate laminae.Because of the difference of the duration of the three stages,different laminae composition appear in the study area.The analysis of the natural gamma logging data found that significant milankovitch cycles exist in the fine grained rock.The precession cycle of 23 ka corresponds to the curve of mineral composition of fine grained rock.It is to say that the astronomical cycle control the sedimentary process of fine grain rock,leading to different lithofacies combination.The fine grained rock of the upper fourth member of Shahejie formation in Dongying sag can be divided into four sedimentary system: 1)depositional system of muddy water,mainly for the mudstone of high organic matter and mudstone of medium organic matter;2)depositional system of half muddy water,mainly for the mixed fine grained rock of medium organic matter and mixed fine grained rock of low organic matter;3)depositional system of clear water,mainly for the carbonate rocks of high organic matter and carbonate rocks of low organic matter;4)deposit system of turbidite,mainly for the ribbon siltstone.