四川盆地川西坳陷雷口坡组四段稀土元素地球化学特征及意义

摘要

The composition and characteristic parameters of rare earth elements in sediments provided sedimentary en⁃vironment as well as fluid and source information, which can be used to judge the sedimentary environment of source rocks and reservoirs and the sources of oil and gas. The tests of rare earth elements in carbonate rocks from the fourth member of the Middle Triassic Leikoupo Formation in the western Sichuan Basin indicated that the total REE content was low. The LREE and HREE differentiation were obvious. Obvious negative δCe and positive δEu anomalies were observed. The low total REE content and negativeδCe anomaly reflected a relatively open and weakly oxidizing low⁃energy environment when the fourth member of the Middle Triassic Leikoupo Formation was deposited. This environ⁃ment was favorable for the breeding and growth of hydrocarbon generation organisms, hence providing a material basis for the formation of hydrocarbon source rocks in the Middle Triassic Leikoupo Formation. The positive δEu anomaly suggested a strongly reducing environment for fluid activity during the late period, which was favorable for organic matter preservation. Enriched LREE, poor HREE, positive δEu anomaly and a REE distribution pattern similar to hydrothermal fluid indicated that the fourth member of Leikoupo Formation experienced late hydrothermal alteration. Hydrothermal migration channels also provided good permeability for deep oil and gas migration.%沉积物中稀土元素配分模式及特征参数能够很好地反映沉积环境及流体性质与来源，应用于油气地质研究中可辅助判断烃源岩、储层沉积环境及油气来源等。川西坳陷雷口坡组四段碳酸盐岩稀土元素测试结果表明，稀土元素总量较低，轻、重稀土分异明显，具有明显的δCe负异常和δEu正异常。分析认为，低稀土元素总量、δCe负异常反映了雷四段沉积时为相对开放的弱氧化低能环境，有利于成烃生物的繁育和生长，可为雷四段烃源岩的形成提供物质基础；显著的δEu正异常表明了后期强还原环境下的流体活动，对有机质的保存有着重要作用。轻稀土富集、重稀土亏损、显著的δEu正异常以及与热液流体相似的稀土元素配分模式，表明川西雷四段经历过后期的热液蚀变与改造，而热液运移的通道也可为深层油气向上运移提供良好的输导条件。