火山活动对鄂尔多斯盆地三叠系长7段淡水湖盆富营养化与沉积水体介质环境的影响

The impact of volcanism on eutrophication and water column in a freshwater lacustrine basin： A case study of Triassic Chang 7 Member in Ordos Basin

高嘉洪 金之钧 梁新平 李士祥 杨伟伟 朱如凯 杜晓宇 刘全有 李彤 董琳 李鹏 张旺

Jiahong GAO Zhijun JIN Xinping LIANG Shixiang LI Weiwei YANG Rukai ZHU Xiaoyu DU Quanyou LIU Tong LI Lin DONG Peng LI Wang ZHANG

大规模富有机质页岩多见于盐盆，但鄂尔多斯盆地作为典型的淡水盆地，其三叠系延长组7段（长7段）的总有机碳含量（）最高却达30 %，远超咸化湖盆的平均水平，亦使其有机质富集主控因素成为热点问题。大量研究表明，长7段页岩较高层段中普遍伴生多套凝灰岩层，指示长7段页岩沉积时期伴随着强烈的火山活动，也暗示淡水湖盆富有机质页岩的异常高有机质富集程度与火山活动存在着一定的内在联系。通过对长7段页岩主、微量元素数据分析，发现页岩受火山活动影响，Al和K等为黏土矿物富集元素，Ni，Cr和V等为指示高古生产力和还原环境的元素，而Zr，Th和Hf等不易流失的高场强元素含量相对较高。同时，凝灰岩层段上部有机质明显富集，生烃强度逐渐升高，指示凝灰层上段古生产力的提升。长7段富有机质页岩整体上Fe/Fe ≥ 0.38，Fe/Fe ≤ 0.8，指示缺氧、含铁的还原环境。随着有机质富集程度的提高，Fe/Fe逐渐升高，指示有硫化的趋势。当长7段有机质富集程度较高时（>6 %），较高的Mo元素含量及/比等均说明长7段沉积时期部分富有机质页岩层段形成于Mo富集的硫化环境的形成。综上，火山活动向淡水湖盆输入的火山物质和无机元素提高了古生产力，并促进形成了短时性有利于有机质保存的还原环境，从而发育了有机质异常富集的页岩层段。因此在淡水湖盆富有机质页岩的勘探和开发，应当将含凝灰质层段之上的富有机质页岩层段纳入重点关注对象之一。

Large-scale organic-rich shales are usually formed in saline basins rather than freshwater basins. However， the Ordos Basin， as a typical freshwater lacustrine basin， has a maximum total organic carbon content （） of 30 % in its Triassic Chang 7 Member shale， way above the average content of shales in saline basins， leaving the main controlling factors a hot topic for discussion. The multiple tuff layers occurred frequently in high sections of the member indicate intense volcanic events and a subtle connection between the events and the high value. Analysis of main and trace elements of the shale confirms the impact of volcanic events as indicated by the relatively higher content of elements enriched in clay minerals like Al and K， of elements as proxy of paleo-productivity and reducing environment including Ni， Cr and V， as well as of high field strength elements （Zr， Th， and Hf）. The upper parts of these tuff are even richer in organic matter with increasing hydrocarbon generation intensity that indicates the elevated paleoproductivity. There are trends of Fe/Fe ≥ 0.38 and Fe/Fe ≤ 0.8 in organic-rich shale but with Fe/Fe up to 0.8 with the increase of TOC. The （/） （auth） ratios is 1-3 when the is greater than 6 %. Both the iron speciation and （/） （auth） ratios indicate that there was an euxinic environment for Mo and Fepy enrichment， but the sulfate reduction strength was low （SRI ≤ 1.375）. In summary， the input of volcanic materials and inorganic elements into the freshwater increased paleoproductivity and promoted the formation of a reducing environment. This is favorable for the organic-rich matter accumulation and preservation. The upper shales of the tuff-bearing section are suggested to be one of the key targets for future exploration and development in the basin.

10.11743/ogg20230407