含油气盆地蒸发盐矿物成因类型及其地质意义*

中国是一个多盐湖国家,然而盐湖研究主要集中于分析湖水化学性质、盐类物质来源和盐矿资源开发等,对盐类矿物沉积特征和埋藏成岩改造研究较少,造成从蒸发岩角度去理解古代盐湖盆地的油气富集规律较为困难。在广泛阅读国内外大型盐湖文献的基础上,笔者介绍了盐湖分类方案和蒸发岩中盐类矿物的主要成因类型,并总结了中国陆相含油气盆地中常见的硫酸盐、氯化物、含钠碳酸盐和硼酸盐的沉积—成岩过程及其古环境和古气候意义。同时,尝试利用盐湖沉积最新研究成果去探讨中国含油气盆地蒸发岩研究中存在争议或值得关注的问题,得出: (1)深部热液可为湖泊输送大量元素离子,但要在湖泊环境下富集大量蒸发岩,则(半)干旱气候和蒸发浓缩作用是前提条件;(2)易溶蒸发岩(如石盐)在沉积中心单层厚度大,而在斜坡—边缘区缺失,这是季节性气温变化和温跃层浮动引发“中心聚集效应”的结果;(3)温度可影响蒸发岩中盐类矿物溶解度、晶体结构形态和发育深度,而部分无水盐类矿物在常温常压下却无法结晶,这一现象可用来指示古地温和地层埋藏史;(4)碳酸盐型盐湖中的Na-碳酸盐种类可指示大气CO₂浓度和古温度。

Genesis of evaporites in petroliferous basins and the sedimentary and climatic significances

China is rich in saline lakes. Many previous studies were focused on analyzing the water chemistry of saline lakes,salt origins,the development of salt mine resources,but rarely paid attention on sedimentary characteristics and diagenetic processes of evaporites,making it difficult to understand the accumulation processes of hydrocarbon in ancient saline lacustrine basins. In this paper,the classification of saline lakes and the main genetic types of saline minerals in the evaporites are reviewed based on the extensive literature review on a large number of saline lakes. The sedimentary and diagenetic processes of common minerals including sulphates,chlorides,sodium carbonates and borates in lacustrine petroliferous basins and their significance for paleo-climate and paleo-environment were concluded. Meanwhile,the controversial and noteworthy issues in the studies of evaporites in petroliferous basins were discussed based on the latest research findings in saline lakes sedimentology: (1)The hydrothermal fluids can transport large amounts of elements into lakes,but the semi-arid and arid climate and the evaporation and concentration are the pre-conditions for accumulating the massive evaporates in the lakes. (2)The soluble evaporates(e.g.,halite)more easily concentrate in the sedimentary center characterized as a thick single layer,while they are missing in the slope and margins of the lakes,which is controlled by “halite aggregation effect at the center”caused by seasonal temperature and thermocline fluctuation. (3)Temperature affects the solubility,crystal structural morphology and formation depth of salt minerals in evaporites. Some anhydrous salt minerals cannot crystalize under normal temperature and pressure conditions,which are good indicators of paleo-temperature and burial history. (4)The sodium-bearing carbonates are closely related to pCO₂ and paleo-temperature of saline lakes of carbonate subtype.