Sedimentary Characteristics And Evolution Of The Middle Permian Fluvial System In Eastern North China

In this paper,the Middle Permian in eastern North China is taken as the research object,and the sedimentary and evolution characteristics of xiashihezi Formation and shangshihezi Formation in the study area are systematically studied by using outcrop,logging,analysis and laboratory data.The Carboniferous-Permian profiles of Ziboboshan,Shabagou in Xingtai and Shimenzhai in Qinhuangdao are carefully measured and described.It is found that xiashihezi Formation and shangshihezi Formation have obvious vertical cycles and clear binary structure,which are typical fluvial facies sedimentary characteristics.The xiashihezi Formation is mainly composed of interbedded medium-fine lithic feldspar sandstone and mudstone.The single sand body is small in thickness and grain size,and gravel is rare.Mudstone is well developed and sand-land ratio is low.It is a typical meandering river deposit.The shangshihezi Formation is mainly medium-coarse quartz sandstone,with a large thickness of sand body up to 15 meters.Gravel is very common.It can be seen that flint,granite,mud gravel and other types.The sandy land is relatively high.It has obvious braided river characteristics.Further divided into Wanshan section,Kuishan section and Xiaofuhe section,the braided degree is different.Wanshan section often has the characteristics of braided river and meandering river at the same time,which is the transitional stage of braided-meandering transformation.The braided river in Kuishan section is the most developed,while Xiaofuhe section shows the characteristics of meandering river.Based on the comparison of three nearly EW-trending sedimentary facies profiles,the sedimentary model of Middle Permian fluvial facies is established according to the sedimentary characteristics of different strata in several key wells and a large number of grain size characteristics.It is found that during the Middle Permian period,three ancient rivers flowing from north to South developed in the study area.The Western rivers are roughly located in the front line of Zhangjiakou-Shijiazhuang-Xingtai and the central rivers are large.To be located in Chengde-Tianjin-Cangzhou-Dezhou-Jining first line,the eastern river is roughly located in Qinhuangdao-Dongying-Zibo-Linyi first line.From the Heishan section of the xiashihezi Formation to the Kuishan section of the shangshihezi Formation,the estuaries of the Western rivers move southward obviously,and the areas where the braided rivers are transformed into meandering rivers are gradually developing southward,and the scale of the rivers is increasing.Taking Dagang exploration area as a typical block,according to the data of stratum thickness,glutenite thickness and percentage content of sand body in the block,and comparing the distribution characteristics of East-West sand body,it is considered that meandering river sediments are mainly developed in Heishan section of xia Shihezi Formation,with thin sand body thickness and poor connectivity;braided river sediments are developed in Wanshan and Kuishan sections of shang Shihezi Formation,with large sand body thickness and good connectivity.As a better oil and gas reservoir,Xiaofuhe reach is mainly floodplain sediment,mainly fine-grained sediment,which can be used as a good caprock.