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扬子克拉通秭归地区震旦系陡山沱组第四段黑色泥页岩中广泛发育具明显δ~(13) C负异常的碳酸盐岩结核,其是否与古甲烷天然气水合物渗漏有关值得深入研究.对该碳酸盐岩结核开展的沉积结构构造、岩相学和地球化学研究表明,碳酸盐岩结核具典型韵律环带结构,普遍发育有亮晶球体结构、草莓状黄铁矿,以及与渗漏系统有关的凝块组构,δ~(13) C具明显负异常(-5.65‰~-6.76‰),U、Mo元素强烈富集(U_(EF)=8~26,Mo_(EF)=99~320),Y/Ho比值为31.05~37.31,稀土配分型式为平缓左倾,主微量元素K、Sc、V、Cr、Co、Ni、Rb、Sr、Ba、Th、U和Mo等总体显示为缺氧-硫化环境,与冷泉碳酸盐岩的形成环境和特征一致.碳酸盐岩结核环带SiO_2、MgO、CaO、CO_2等地球化学元素含量呈阶段性连续增减变化,显示碳酸盐岩结核形成经历了初始形成、成岩-交代、成岩后改造3个连续演化阶段.据此,提出碳酸盐岩结核是新元古代末噶斯奇厄斯冰期(582~551Ma)结束温度回暖,黑色泥页岩中低温封存固态天然气水合物发生分解释放和成岩-交代形成的冷泉碳酸盐岩结核,也是古天然气水合物存在的重要地质记录和标志,这一新认识为华南扬子克拉通在震旦系和下古生界沉积盖层中寻找页岩气(甲烷天然气)储集层位提供了重要地质依据.
Carbonate nodules with obvious δ 13 C negative anomalies are extensively developed in the black mud shale in the fourth section of the Doushantuo Formation in the Sinian area in the Yangtze craton, and whether they are related to leakage of paleo-methane gas hydrate It is worth further study.Studies on the sedimentary structure, lithofacies and geochemistry of the carbonate nodules show that carbonate nodules have the typical rhythmic ring structure with spherulitic spherulites and strawberry-like yellow iron (13.6%), and negatively anomalous δ ~ (13) C (-5.65 ‰ ~ -6.76 ‰), and U, Mo = 8 ~ 26 , Mo_ (EF) = 99 ~ 320). The Y / Ho ratio was 31.05 ~ 37.31. The REE patterns of the REEs were gentle left leaning, main trace elements K, Sc, V, Cr, Co, Ni, U and Mo generally show hypoxia-sulfide environment, which is consistent with the formation environment and characteristics of cold spring carbonate rocks.The carbonate elements such as SiO_2, MgO, CaO and CO_2 are continuously increased The results show that the formation of carbonate nodules has undergone three stages of continuous evolution: initial formation, diagenesis-metasomatism, and post-diagenesis transformation.Accordingly, carbonate rocks The nuclei are the cold springs carbonate nodules formed by the decomposition and release of low-temperature sequestrated solid-state gas hydrates from the black mud shale at the end of the Neoproterozoic Kargošky Ice Age (582-551 Ma) This new understanding of important geological records and signs of the existence of gas hydrates provides an important geological basis for the search for shale gas (methane natural gas) reservoirs in the caprock of the Sinian and Lower Paleozoic in the Yangtze Craton of South China .