渤海湾盆地北塘凹陷古近系沙河街组三段白云岩中方沸石的特征及成因

渤海湾盆地北塘凹陷古近系沙河街组三段白云岩中有多种矿物,X射线衍射(XRD)分析识别出的主要矿物有白云石、方沸石、石英、长石、黏土矿物、黄铁矿及方解石等,其中方沸石平均含量达38.8%。根据钻井岩心、岩石薄片、扫描电镜、电子探针和地球化学分析等资料,对沙三段白云岩中方沸石的矿物特征进行详细描述并探究其成因。结果表明,沙三段白云岩中的方沸石有2种分布样式,分别是纹层型和充填型,其中前者多与白云石互层、呈纹层状,偶见似结核状;后者形成较晚,方沸石以填隙物形式充填在裂缝、孔隙之中。这2种方沸石的存在形式、晶体形态、伴生矿物各不相同,其中纹层型方沸石多与白云石共生,晶体粒径小于0.04,mm,为泥晶结构;充填型方沸石多与黄铁矿、有机质共生,晶体粒径大于0.04,mm,为粉晶结构。根据主、微量元素及碳、氧同位素分析结果,计算出古盐度平均为28.8‰,Sr/Ba值平均为2.93,表明渤海湾盆地北塘凹陷沙河街组三段沉积时期为碱—咸溶液沉积环境,具备形成方沸石的条件。电子探针测试的Si/Al值表明,这2类方沸石母质来源不同,纹层型方沸石母质为黏土矿物,而充填型方沸石母质与热液有关。与国内含方沸石白云岩的盆地进行对比,并从岩石学特征、形成温度等方面进行分析,认为研究区方沸石有2种成因,其中纹层型方沸石是同生成岩阶段的产物,而充填型方沸石则受到热液流体的影响。

Characteristics and causes of analcime distributed in dolostone of the Member 3 of Paleogene Shahejie Formation in Beitang sag,Bohai Bay Basin

There are many minerals in the dolostone of the Member 3 of Paleogene Shahejie Formation from Beitang sag of Bohai Bay Basin. X-ray diffraction results show that the dolostone contains dolomite,analcime,quartz,feldspar,clay mineral,pyrite,calcite, etc. and the average content of analcime is about 38.8%. In this paper,based on a vast amount of data from core drilling, thin sections,scanning electron microscope,electron probe and geochemistry characteristics,we study the characteristics and causes of analcime in the Member 3 of Shahejie Formation in detail. Analcime occurs as two types,namely,laminae type and filling type. The former one,mainly laminated and sometimes nodular,is interbedded with dolomite. The other one is formed later and distributed in the crack as a kind of interstitial fillings. These two kinds of analcime are different in many aspects,such as existence form,crystal morphology,and associated minerals, etc. The laminae type is associated with dolostone,and the size of crystal is under 0.04,mm. However,the filling type is associated with pyrite and organic matter,and the size of crystal is above 0.04,mm. Based on the major-trace element and carbon-oxygen isotope analysis,the average salinity is 28.8‰ and the average Sr/Ba value is 2.93. It indicates that the sedimentary environment was alkaline-salty during the depositional period of Member 3 of Paleogene Shahejie Formation in Beitang sag of Bohai Bay Basin,which is beneficial to form analcime. The electron probe chemistry analyses show that the analcime of laminae type and that of filling type have different sources. The source of the analcime of laminae type is clay mineral,and the source of the analcime of filling type is related to hydrothermal fluid. Compared with other similar analcime-bearing dolostone basins in China,such as Jiuxi Basin,Santanghu Basin,we gave analyses on petrology characteristics,REE and formation temperature. The results show that the analcime in the study area has two causes: Analcime of laminae type is formed through deposition process,and analcime of filling type is formed through hydrothermal fluid process.