下扬子地区上震旦统-下古生界白云岩地球化学特征及成因:以南京地区为例  

Geochemical Characteristics and Genesis of Upper Sinian-Lower Paleozoic Dolomite in Lower Yangtze Region:A Case Study from Nanjing Area

作　　者：郭颖 杜晓峰 杨波 黄振 王军 李志强 Guo Ying;Du Xiaofeng;Yang Bo;Huang Zhen;Wang Jun;Li Zhiqiang(Tianjin Branch,CNOOC China Limited,Tianjin 300459,China)

机构地区：[1]中海石油(中国)有限公司天津分公司,天津300459

出　　处：《地球科学》2023年第12期4558-4574,共17页Earth Science

基　　金：国家科技重大专项(No.2016ZX05024-003)。

摘　　要：扬子地台上震旦统-下古生界白云岩广泛发育,是最重要的碳酸盐岩储集层.目前对下扬子地区白云岩的研究程度较低,严重制约了对该区白云岩储层性能的深化认识和油气勘探工作.研究以南京地区为例,通过对该区上震旦统-下古生界白云岩进行系统的岩石学、主量和微量元素、碳-氧同位素及稀土元素分析,确定了白云岩的类型及成因.结果表明:研究区发育泥晶白云岩、粉-细晶白云岩、细晶白云岩、中晶白云岩4种白云岩类型.上震旦统泥晶白云岩具有较高的Mg含量和较低的Fe、Mn、Sr元素含量,δ^(18)O平均值高达0.2‰,形成温度近于地表温度,盐度指数明显高于海水,∑REE值约为28×10^(-6),δCe和δEu呈负异常,平均分别为0.867和0.917,稀土元素配分模式与海相灰岩类似,为近地表强蒸发浓缩海水流体环境下准同生白云石化成因;中寒武统粉-细晶白云岩晶体具有雾心亮边和环边状结构,Mg、Fe、Mn元素含量和稀土元素特征与泥晶白云岩类似,盐度指数较高,说明有相似的海水流体性质,δ^(18)O值介于-2.5‰~-6.5‰,为浅埋藏环境下渗透回流白云石化成因;上寒武统细晶白云岩Fe元素含量和∑REE值高于前两类白云岩,δ^(18)O值介于-6.5‰~-10‰,形成温度较高,但未越过热蚀变界限,为埋藏白云石化成因;下奥陶统中晶白云岩晶体粗大,孔隙中发育鞍状白云石,具有较高的Fe、Mn、Al、Si、Sr元素含量,δ^(18)O值小于-10‰,形成温度高且呈热蚀变特征,盐度指数低于海水,∑REE值较高,δEu呈强正异常,为热液白云石化成因.The Upper Sinian-Lower Paleozoic dolostones is widely developed in the Yangtze platform,which is one of the most important carbonate reservoirs.There has been few studies on the dolostone in the Lower Yangtze region,restraining our recognition to the properties of dolostone reservoir.In this paper it systematically investigates the geochemical characteristics and formation mechanisms of dolostone in the Lower Yangtze area based on an integrated petrological,major and trace element,rare earth element,and carbon oxygen isotope analyses.The results show that the studied dolostone can be divided into four types,namely,micrite dolostone,powder-fine dolostone,fine crystal dolostone,and medium crystal dolostone.The Upper Sinian micritic dolostone has high Mg content and low Fe,Mn,Sr contents,highδ^(18)O value,forming temperature close to surface temperature,higher salinity index compared with sea water,lowΣREE value,negative Ce and Eu anomalies,and REE distribution pattern which are similar to marine limestone,indicating the dolostone belongs to penecontemporaneous dolomitization in an environment of concentrated seawater fluid with strong evaporation near the surface.The Middle Cambrian powder-fine dolostone is characterized by fog-core bright edge structure,similar contents of Mg,Fe,Mn and REE characteristics compared with micritic dolomite,high salinity index,and relatively mediumδ^(18)O value ranging from-2.5‰to-6.5‰.The genesis of this dolomite is percolation reflux dolomitization in shallow burial environment.Marked by higher Fe content and∑REE values,lowδ^(18)O value(-6.5‰to-10‰),higher formation temperature,the Upper Cambrian fine-grained dolostone is attributed to buried dolomitization genetic.The Lower Ordovician medium crystal dolomite is characterized by high contents of Fe,Mn,Al,Si,Sr and∑REE value,lowδ^(18)O value less than-10‰,lower salinity index compared with sea water,positive Eu anomaly,and high diagenetic temperature with thermal alteration characteristics,suggesting the dolomite i

关 键 词：下扬子 南京地区 上震旦统-下古生界 白云岩 地球化学 成因 

分 类 号：P58[天文地球—岩石学]