渤海湾盆地北塘凹陷古近系湖相白云岩地质特征及古环境意义

Geological Characteristics and Paleoenvironmental Significance of the Paleogene Lacustrine Dolomite in the Beitang Sag, Bohai Bay Basin

湖相白云岩是渤海湾盆地北塘凹陷古近系沙河街组三段发现的一种特殊油气储层类型。根据钻井岩心、古生物、薄片、扫描电镜及电子探针等分析,确定了该套白云岩的主要岩石类型及矿物特征,并通过地球化学特征分析,对白云岩的形成环境进行了探讨。北塘凹陷白云岩常与泥岩和方沸石岩呈互层状产出,其次为均匀块状;矿物成分主要有白云石、方沸石、粘土矿物等。通过X射线衍射、主微量元素地球化学分析,对白云岩形成时的古盐度、古温度、水深、氧化还原条件等进行了研究,认为研究区白云岩沉积于高盐度、较高水体温度和较高PH值的半咸水湖湾还原环境。海水的侵入使湖水的含盐度增大,提供了有利的物理化学条件,促进了白云岩的形成。

A special type of lacustrine dolomite reservoir was discovered in the Member 3 of Paleogene Shahejie Formation in the Beitang sag, Bohai Bay Basin. The main rock types and mineral characteristics of the dolomites have been studied in detail by a vast amount of data from core drilling, paleontology, rock thin section, scanning electron microscope, and electron microprobe. The generation environments of the dolomites were debated by the X-ray diffraction data, major and trace element geochemistry. The dolomites in the Beitang sag are usually bedded or laminated with mud or analcime, and the mineral components are mainly dolomite, analcime, and clay minerals. The paleosalinity, paleotemperature, paleo-water depth, and redox conditions of the depositional environments of the lacustrine dolomites were discussed carefully. We inferred the dolomites were deposited in the lacustrine bay with a relatively high salinity, high temperature and high PH of the water column. The salinity of the lake increased during the seawater intrusion, providing favorable physical and chemical conditions, subsequently promoted the dolomite formation.