渤海湾盆地塘沽地区古近系沙河街组湖相白云岩特征及成因*

渤海湾盆地塘沽地区古近系沙河街组三段沉积早期为半封闭的湖湾沉积环境,底部发育一套白云岩储集层,其产出受区域内北东向古潜山基底断裂控制,沿古潜山两侧发育,分布面积较大,累计厚度可达100多米。系统的岩石学、矿物学和地球化学特征研究表明,该套白云岩常与泥岩互层产出,白云石主要为铁白云石和含铁白云石,普通显微镜下呈微晶状,扫描电镜下可见菱面体状晶型;X射线衍射分析表明其形成于准同生期,是快速结晶的产物;碳氧同位素分析及稀土元素分析显示,这类微晶白云岩形成于较为封闭的高盐度蒸发环境中,具统一的流体来源和相同成因机理。基于上述研究成果,认为塘沽地区沙河街组三段微晶白云岩的形成主要经历了准同生期回流渗透作用和成岩期埋藏环境下类质同象交代作用两个阶段。

Characteristics and origin of lacustrine dolostone of the PaleogeneShahejie Formation in Tanggu area,Bohai Bay Basin

The Member 3 of Paleogene Shahejie Formation,in the Tanggu area of Huanghua Depression,Bohai Bay Basin,was formed in semi-enclosed lake bay environment,with one set of dolostone reservoir developed at its bottom.The dolostone,with a cumulative thickness up to 100 meters,was mainly distributed along the sides of a buried hill as controlled by a NE trending basement fault.Generally,it was interbedded with mudstone in this area.Petrological,mineralogical and geochemical characteristics were studied,which shows:(1)The dolomite minerals are mainly ankerite and ferruginous dolomite,which are micrites under an optical microscope while rhombohedral crystals under a scanning electron microscope(SEM).(2)X-ray diffraction analysis indicates that this kind of micritic dolostone was formed during the penecontemporaneous period,and is the product of rapid crystallization.(3)The analytical result of the carbon and oxygen isotopes and the rare earth elements shows that it was formed in a relatively closed and high salinity evaporitic environment,and owns an unified fluid source and the same genetic mechanism.Based on the above analyses,the author holds the view that the micritic dolostone experienced two stages,namely the seepage-reflux dolomitization during penecontemporaneous period and the isomorphous replacement during burial diagenesis period.