柴达木盆地西部第三系蒸发岩微量元素组成及其地球化学特征

GEOCHEMISTRY CHARACTERISTICS OF TRACE ELEMENTS IN EVAPORATES OF THE TERTIARY IN WESTERN QAIDAM BASIN

对柴达木盆地西部第三系盐湖相的研究提出了“大咸盆套小盐湖”的沉积模式 :“小盐湖”是指由深大断裂控制的主要堆积岩盐、石膏、泥灰岩和富含有机质生油岩的面积较小的深水沉积 ;“大咸盆”是指围绕“小盐湖”面积很大的区域 ,其水体较浅 ,以钙质泥岩、泥灰岩和砂泥岩沉积为主。对在沉积中广泛含有的泥灰岩微量元素及其常量元素的分析表明 ,与生油岩共生的泥灰岩和膏盐沉积物是深水强还原环境的产物 ,泥灰岩和钙质泥岩是浅水弱还原条件下的沉积物。同时发现“小盐湖”沉积物微量元素含量特别高 ,这可能与深大断裂的上升卤水或与来自地表水的强烈蒸发作用有关。对盐湖沉积物微量元素组成进行了分析 ,其中包括稀有元素 (含稀土元素 )、铁族元素、亲铜元素、分散元素、钨钼族元素及放射性等 4 0多种元素。阐述了蒸发岩与生油岩的成因联系。

A genetic model for the deposition of evaporites and oil source rocks in the saline lacustrine environment in the Tertiary of the western Qaidam basin was proposted. This model was known as “a small brine lake located in a big saline depression'. The trace elements and the main elements in the evaporites of the Tertiary of Qaidam basin were presented. It was suggested that the evaporites were originated from the mixing of the river water and the deep brine, which came from the crust cut faults in the basin. By strong evaporating, the mixed water was concentrated to deposit carbonates, including marls and calcareous shale, sulfates (e.g. gypsum, anhydrate) and chlorites such as halite in the saline lakes. There was a clear difference in the trace element compositions between calcareous shale deposited in the “small brine lake' and in the “big saline depression', which was sampled from wells drilled in the Tertiary of the western Qaidam basin. The difference approved that the “small brine lake' was in a deep water and strong reductive environment, and the “big saline depression' was in a shallow water and less reductive lake. The deep brine came from the crust cut fault was rich in the heavy metal trace elements and rare elements, which resulted in higher content of the elements in the sulfates and chlorites deposited in the small brine lake than those in the carbonates deposited in the big saline depression. More than forty kinds of trace and rare elements in the evaporites were described, and the relation of the evaporites with oil source rocks was discussed in this paper.