南黄海地区二叠系孤峰组硅质烃源岩的地球化学特征及上升流成因

Geochemical characteristics and upwelling origin of siliceous source rocks in the Permian Gufeng Formation of the South Yellow Sea area

前人对南黄海地区中二叠统孤峰组层状硅质岩的生烃潜力和成因研究较少,本文利用下扬子-南黄海地区的5口钻井资料,对二叠系孤峰组硅质烃源岩进行了详细的矿物学和地球化学研究,并通过与现代秘鲁上升流区的沉积物进行元素含量对比,分析了南黄海地区孤峰组硅质烃源岩的生烃潜力和成因机制。结果显示,下扬子-南黄海地区孤峰组具有硅质岩和硅质泥岩不等厚互层的特征,是一套呈SWW-NEE向展布的过成熟偏腐殖型优质烃源岩。在地球化学特征方面,南黄海地区孤峰组硅质烃源岩和秘鲁上升洋流沉积物均表现出还原性敏感元素和生产力敏感元素相对富集,K、 Ti、 Mn相对亏损,具有Co×Mn<0.4、Cd/Mo>0.1的特征,显示为大陆边缘的上升流成因。研究显示,南黄海地区孤峰组硅质烃源岩的有机质富集主要受高生产力控制,形成于中等滞留的缺氧-硫化环境。与硅质泥岩相比,孤峰组硅质岩的陆源碎屑输入和Co×Mn值较低,Zr/Rb值较高,这意味着硅质岩沉积时期的上升流活动强度大于硅质泥岩。此外,部分探井中的孤峰组硅质泥岩相对于硅质岩具有较高的化学蚀变指数,说明古气候变暖是造成上升流活动减弱和硅质沉积含量减少的主要原因。

Few studies regarding the hydrocarbon generation potential and genesis of the layered siliceous source rocks in the Mid-Permian Gufeng Formation(GFF) of the South Yellow Sea(SYS) has been conducted. The mineralogy and geochemistry of the siliceous source rocks in the GFF were studied in detail based on the borehole data of five wells located in the Lower Yangtze to South Yellow Sea area, and the element content were compared with those of the sediments in the modern upwelling area of Peru, from which the hydrocarbon generation potential of the GFF siliceous source rocks in the SYS and its origin were revealed. Geological data shows that the GFF in the SYS area is characterized by interbeds of siliceous rocks and siliceous mudstones in unequal thickness, and is a set of over-mature and slightly humic high-quality source rocks extending in the SWW-NEE direction. In terms of geochemistry, both the siliceous rocks and Peruvian upwelling deposits show enrichment in the elements that are sensitive to reduction and productivity, but relative depletion in K, Ti and Mn, and have Co×Mn value lower than 0.4, Cd/Mo value higher than 0.1, indicating their origin of upwelling on continental margin. This study shows that the enrichment of organic matter in the GFF siliceous source rocks in the SYS area is mainly controlled by high productivity and is formed in anoxic to euxinic environment with moderate retention. Compared with siliceous mudstone in the GFF, the terrigenous clastic input and Co×Mn value are lower and Zr/Rb value is higher in the siliceous rocks, which means that the upwelling intensity of the siliceous rocks during sedimentation is greater than that of siliceous mudstones. In addition, the siliceous mudstone of the GFF have higher chemical alteration index than the GFF siliceous rocks in some wells, which suggests that the paleoclimate warming is the main cause for the weakening of upwelling activity and the reduction of siliceous sediment content.