摘要： 准噶尔盆地西北缘二叠系风城组内的云质岩、硅质岩是致密油藏重要的储集岩,目前对云质岩和与之伴生的硅质岩成因存在较大争议。通过岩心观察和对硅质岩的主量、微量、稀土及硅同位素的综合研究,以期揭示硅质岩的成因。研究表明,风城组中的硅质岩SiO₂含量变化范围为55.15%~85.61%,为不纯的云质硅质岩,其内含有较多热水沉积矿物和碱湖沉积成因的盐类矿物。云质硅质岩主要围绕热液喷口分布,它们常与白云岩呈纹层状、条带状互层。云质硅质岩中硼异常富集,硼含量介于90×10^-6~10 000×10^-6之间,平均680×10^-6,与盐类矿物硅硼钠石有关,反映了碱湖沉积成因;Sr/Ba值均大于1,Th/U值多在0.62~2.06之间,Sm/Nd值多在0.20~0.24之间。云质硅质岩的稀土配分显示出Eu负异常、Ce负异常或弱正异常及轻稀土相对重稀土富集的特征。δ³⁰Si值介于0.39‰~1.88‰之间,其δ³⁰Si和微量元素特征不反映典型热水成因。云质硅质岩的地质产状和地球化学特征共同表明,风二段和风三段云质硅质岩主要为热水和碱湖的混合沉积成因。硅质岩形成受喷口的位置、热水喷流期和各喷口喷流热水的温度等多方面因素的控制,硅质岩形成时存在多个喷口,在相对封闭的高盐度—碱性还原环境下的热水喷流期,热水与碱湖湖水混合,混合水沉积形成硅质、盐类矿物及白云石,最终形成含盐的云质硅质岩;热水喷流间歇期形成碱湖成因的含盐云质岩。

关键词: 硅质岩, 热水喷流, 碱湖沉积, 二叠系风城组, 准噶尔盆地

Abstract: The siliceous and dolomitic rocks of the Permian Fengcheng are the most significant oil source rocks in the Junggar Basin. However,there is a debate on the origin of dolomitic and siliceous rocks. Base on the core observation and geochemical analyses including major elements,trace elements and silica isotopes,a more detailed study was made on the origin of siliceous rock. The results indicate the SiO₂ content of siliceous rocks ranges from 55.15% to 85.61%,which indicates the rocks are not purely siliceous and contain a relatively high content of salt minerals formed in an alkaline lake and minerals formed in hydrothermal fluids. The siliceous rock is mainly distributed around the hydrothermal vents and usually interbedded with dolomitic rocks. Boron is enriched in siliceous rock,ranging from 90×10^-6 to 10 000×10^-6 with an average of 680×10^-6,which might be related to the searlesite and reflect an alkaline lake depositional environment the Sr/Ba of siliceous is larger than 1,and the Th/U and Sm/Nd values range from 0.62 to 2.06 and 0.20 to 0.24,respectively. REE distribution pattern of siliceous rock shows Eu negative anomaly and Ce negative anomaly or weak positive anomaly and significant enrichment in LREE. The δ³⁰Si values range from 0.39‰ to 1.88‰. Trace elements and δ³⁰Si values are different from typical hydrothermal siliceous origin. Both geological occurrence and geochemical characteristics of siliceous rocks indicate that the siliceous rocks of Members 2 and 3 of Fengcheng Formation are formed in an alkaline lacustrine involves hydrothermal activities. The formation of siliceous rock is controlled by many factors,such as the location of hydrothermal vents,the hydrothermal exhalative period and the temperature of each hydrothermal vents. During the hydrothermal exhalative period,the hydrothermal fluids mix with the alkali lake water to form the siliceous rocks that contain salt and dolomite. The sedimentary environment of siliceous rocks is a relatively closed environment with high salinity and alkaline reduction. During the intermittent stage between hydrothermal exhalative activity,the dolomite rocks containing salt minerals are the major products.

Key words: siliceous rock, hydrothermal exhalative activity, alkaline lacustrine sediment, Permian Fengcheng Formation, Junggar Basin

中图分类号: 

• P59