鄂尔多斯盆地中部湖相有机质沉积环境特征

Sedimentary environment of lacustrine organic matter in the central Ordos Basin

在采用能量色散X射线荧光技术的测试过程中,受能量配置探索不足的影响,所测元素的数量往往较少,导致无法充分利用元素参数来刻画古沉积环境。此外,陆相有机质的来源包括河流输入有机质和水体生产有机质,如何对其进行定量区分尚存困难。针对能量色散X射线荧光技术的能量配置进行了改进,由此将精确测试的元素数量扩充到28个。选择鄂尔多斯盆地中部延长组7段(长7段)页岩为研究对象,开展了有机碳、主要元素、微量元素的地球化学测试。在分析主要元素、微量元素等地球化学特征的基础上,对长7段有机质的沉积水体、氧化还原状态和生产力等古沉积环境进行了恢复,建立了有机质来源的数学模型,并分析了有机质的供给特征。研究结果表明,长7段有机质形成于偏温暖潮湿的气候背景,水体为偏贫氧和缺氧的淡水环境,古生产力较高但初级生产力不高。对于鄂尔多斯盆地中部地区的页岩有机质,水体来源有机质的贡献略大于陆源输入有机质的贡献。

The numbers of measurable elements in lacutrine shale sampels by energy dispersive X-ray fluorescence(ED-XRF)technology are often limited due to insufficient energy configuration.This makes it difficult to fully restore the paleosedimentary environment for samples concerned.Furthermore,some lacutrine organic matter can be of river input origin or indigenously produced in lake water,quantitatively distinguishing one from the another is challenging.In this study,energy configuration of ED-XRF is improved to increase the number of accurately measurable elements to 28.Shale samples from the seventh member of Yanchang Formation(Chang 7)in central Ordos Basin are tested for geochemical characterization of organic carbon,major elements,trace elements and carbon isotopes.The results are used to restore the paleosedimentary environment of organic matter from Chang 7,including the water column,redox and productivity.A mathematical model is established to analyze the organic matter.The results show that the organic matter of Chang 7 shale is formed in fresh water with oxygen deficiency or anoxia,high paleoproductivity but a moderate primary productivity against a warm and humid climate background.For shale in the central Ordos Basin,organic matter is rather lake water produced than terrestrial input.