Study On Sedimentary Environment And Diagenetic Evolution Of The Zhangxia Formation,Cambrian,Western Hill Of Beijing

Xiaweidian outcrop in Western Hill of Beijing is one of the typical outcrops of the Cambrian system in north China.It has always been the focus of the study of geological scholars.However,the research on sedimentary facies is not precise enough,and a unified understanding has not yet been formed.Moreover,the research on the diagenesis of the oolitic beach in the study area is relatively weak.In this paper,based on previous research,through field observation and measurement,combined with indoor thin section identification,XRD test and other technologies,we did a detailed analysis on the sedimentary of the Cambrian Zhangxia Formation.Characteristics of ooid size in the Zhangxia Formation of the Cambrian at Xiaweidian Outcrop have been discussed and its influence on sedimentary of oolitic beach were revealed.The spatial coupling relationship between characteristics of ooid size and sedimentary environment of oolitic beach was established.The main diagenesis types and diagenetic sequences of the Zhangxia Formation oolitic beach of the Lower Cambrian were clarified,and the prospect of the exploration of the oolitic beach of the Zhangxia Formation was prospected.The mechanism of oolitic dolomitization and silicification was discussed.Through research,the main faces of the Zhangxia Formation is detrital bathyal shallow marine facies and platform facies.The subfacies and shale are limited in shallow sea;The main development of the platform is restricted platform,beach and tidal flat subfacies.The beach is divided into high energy beach and intermittent highenergy beach.The Zhangxia Formation is an offshore and gentle slope Carbonate platform.It consists of many M level cycle which gradually becomes shallow upward.In general,the water from the bottom is growing deeper,Carbonate platform gradually becomes stable.The results show that ooid size,sorting coefficient,kurtosis and the intersection of the component on cumulative probability curve can reflect the hydrodynamic environment of the ooid sedimentation.In general,the ooids in stable hydrodynamic environment of intermittent high-energy beach have the smaller size,better sorting,more sharp kurtosis.The granularity distribution frequency curves are more unimodality and the ooids are mainly suspended.The ooids in turbulent hydrodynamic environment of high-energy beach have the larger size,relatively poor sorting,relatively flat kurtosis,the granularity distribution frequency curves are more multimodal and the cumulative probability curves are mainly “one bouncing population-one suspension population” bi-segment pattern.The size of ooid is closely related to the type of layer structure,the layer of the larger ooids are mainly concentric,concentric-radial and the smaller ooids are mainly radial.The diagenesis experienced by oolitic beaches mainly includes mud crystallization,compaction-pressure dissolution,cementation-filling,dolomitization,dissolution,silicification,and rupture.The dissolution and refilling or neomorphism of the ooid results in the enrichment of organic matter at the edges of the ooids,and the organic acids,released with the evolution of organic matter,are the main factors for siliceous precipitation,the silica mainly derives from the input of the terrestrial clastics and the transformation of the clay minerals.Due to the concentration of organic matters and formation of the organic membranes at the edges of the ooids,silica precipitation mainly occurs along the edges of the ooids,forming a specific silicification sheath along oolitic edges,which also suggests that microbes play an important role in the formation of ooids.The seepage relux of the high-magnesium brine played a vital role in the early dolomite precipitation in the cortical laminae of the ooids before they were illed with interstitial materials.The sudden increased in the dolomite content of the oolitic limestone in carbonate containing gypsum provided key evidence for seepage-relux dolomitization of the ooids.The higher porosity and permeability of the high-magnesium luid in radial ooids demonstrates that the degree of dolomitization was higher than in the concentric ooids.The distribution of the dolomite was strongly inluenced by the original cortical structure of the ooids.Migration and storage complex system,composed of the stylolite and dolomite intercrystal pores,is a possible storage space in the local oolitic bank.In the provision that the hydrocarbon supplying ability of the Proterozoic source rocks is confirmed,the Zhangxia Formation represents a new potential sequence for exploration,and warrants further investigation.