东营凹陷沙四段湖相白云岩沉积特征及成因

Sedimentary characteristics and genesis of lacustrine dolomite in the fourth member of Shahejie Formation in Dongying Sag

以岩石学特征为基础,结合沉积特征和沉积地球化学资料,对东营凹陷沙四段湖相白云岩成因进行了研究。结果表明:不同成因的白云岩,其岩石结构、发育环境和地球化学特征有所不同;该地区白云岩类型多样,普遍具有泥-微晶结构;白云岩主要位于滨浅湖、深湖—半深湖及盐湖环境中,岩性组合上与泥页岩、粉砂岩、膏岩及玄武岩伴生;受层序发育的控制,该时期白云岩多发育在晚期高位体系域之中,白云石化作用不彻底,白云岩含灰质成分较高。地球化学数据表明:白云石有序度普遍较低,Sr质量分数普遍较高,平均值接近海水中的Sr质量分数,说明沙四时期受到了海侵作用和构造热液-深部流体的影响;高Mn、低Fe、高Na的特征反映了该时期主要的沉积环境为干旱—半干旱的蒸发环境。综合构造条件、沉积环境及地球化学分析等因素,建立了沙四段准同生白云岩化和构造热液白云岩化模式。

Based on the petrology characteristics and the data of sedimentary characteristics and sedimentary geochemistry, this article analyzes the genesis of lacustrine dolomite in the fourth member of Shahejie Formation, Dongying Sag. The study result indicates that the dolomites of different genesis have the different rock structures, development environments and geochemistry characteristics. There are many types of dolomites in this area and most of them have the mud-microcrystalline structure. The dolomites were mainly developed in shore-shallow lake, deep lake to semi-deep lake and salt lake environments, associated with mud shale, siltstone, gypsolyte and basalt in the lithologic association. Controlled by the sequence development, the dolomites were mainly developed in the late highstand system tract at this period, with the dolomitization being incomplete and the limy component content of dolomite being high. The geochemistry data indicate that the dolomite ordering degree is generally low, that the Sr content is high and that the average value is close to the Sr content of sea water, which shows that the transgression and struetural hydrothermal fluid-deep fluid occun＇ed in the fourth member of Shahejie Formation. The features of high Mn, low Fe and low Na reflect that the sedimentary environment of this period is mainly the arid-subarid evaporation environment. In the end, based on the data of tectonic setting, sedimentary environment and geochemistry, the penecontemporaneous dolomitization genesis model and structural hydrothermal fluid dolomitization genesis model are established for the fourth member of Shahejie Formation.