Paleoenvironment Reconstruction Of Lower Cretaceous Chengzihe Formation In Yanxing Basin,Heilongjiang Province

Yanxing Basin,a Late Mesozoic fault Basin is located in Bulieya-Jiamusi-Xingkai massif,Taipinggou uplift area in Jiamusi uplift belt in the east of Mudanjiang fault.In recent years,oil and gas which is found in the Early Cretaceous Basin Group in the east of Heilongjiang shows tha t industrial oil flow were also found in the adjacent Basins in Russia.Therefore,the analysis of sedimentary facies and paleo climate and paleo environment in the region as well as studying the evolution of sedimentary facies and distribution characteristics and the corresponding relationship between the ancient climate and environment will have practical significance for further exploration in this area and provide the reference for the research and for the study of surrounding Basin.According to the basis of previous studies,this paper analyzes the sedimentary face types of Lower Cretaceous Chengzihe Formation in Yanxing Basin through the core observation and logging analysis,than use the well profile to study the vertical and horizontal distribution law of the sedimentary phase,in order to discuss the distribution characteristics and evolution laws.Meanwhile,by using trace elements and REE geochemistry combing with the analysis of the palaeontological sporo pollen fossils,this paper carries on the analysis to the ancient climate and environment in the sedimentary period of Chengzihe Formation in the study area,and discusses the corresponding relationship between sedimentary evolution and paleo climate.Alluvial fan facies,fan delta facies and lacustrine facies was identified in the lower Cretaceous Chengzihe Formation.As we know,alluvial fan facies includes three sedimentary subfacies: fan root,mid-fan and fan margin;Fan delta can be divided into two sedimentary subfacies: fan delta plain and fan delta front;And The lake is divided into two sedimentary subfacies of shore-shallow lake and deep-half deep lake,totally including seven kinds of sedimentary microfacies.Analysis of sedimentary facies of two wells in study area meticulously and researching characteristics of transverse and longitudinal distribution of sedimentary facies according sedimentary well section can promote the analysis of the distribution and evolution characteristics of the sedimentary plane.The sandstone isopach map,contour map and sand ratio of single well sedimentary facies sedimentary facies determine the boundary.Draw the Chengzihe Formation in four layers of sedimentary facies plane to determine the sedimentary facies evolution.Based on the element geochemical analysis method combing with the ancient biological fossil identification,this paper recovers the paleoclimate and paleoenvironment restoration of Chengzihe Formation in study area.As the result,the first section of Chengzihe Formation was in dry climate and the second section started transformation to humid climate and became more humid with the third section sedimentary period was the most humid climate as well as a slightly dry trend shown in the fourth section again.According to the analysis,this paper gets the following conclusions: there was a dry climate in the first sedimentary period of Chengzihe Formation and Alluvial fan phase and fan delta facies are mainly developed in the study area;the climate was more humid during the second depositional period of Chengzihe Formation.In the study area,the deposition range of alluvial fan narrowed and the fan delta front deposition range expanded.The most humid climate in the study area happened in the third sedimentary period and large-scale development of lake facies had begun and fan delta facies range was relatively narrow;The fourth section of Chengzihe Formation sedimentary period have a dry climate trend with the range of lake beginning to narrow and the sedimentary range of fan delta facies expanding relatively.