“化学-沉积相”分析:一种研究细粒沉积岩的有效方法

“Chemo-sedimentary facies” analysis:An effective method to study fine-grained sedimentary rocks

细粒沉积岩作为致密油气和页岩油气重要的勘探对象,具有粒度细、颜色均匀、非均质性强等特点,传统的岩心观察方法难以适用于细粒沉积岩的精细研究。通过获取细粒沉积岩手标本(岩心尺度)高分辨率(30μm)的二维原位无机地球化学元素分布信息,结合图像分析,建立岩心尺度精细的"化学-沉积相"模型。在模型中,可以根据无机地球化学元素的平面分布清晰的识别细粒沉积岩的矿物成分和沉积构造。在此基础之上,能够提取不同部位的原位地球化学信息,针对细粒沉积岩沉积周期长厚度小的特点,实现在毫米-厘米级尺度上表征古环境、沉积物来源和成因,同时避免常规测试中粉碎均一化样品后所造成的信息干扰,为致密油气与页岩油气的勘探开发提供更加精细的沉积相模型和可靠的数据来源。

Fine-grained sedimentary rocks,a key rock type bearing shale oil and gas,are characterized by fine grain size,colour uniformity and strong heterogeneity.Traditional sedimentological study based on visual observation and optical images alone becomes inadequate to precisely characterize such fine-grained rocks.Therefore,a “chemo-sedimentary facies” model for analyzing fine core-scale samples is established based on the high-resolution(down to 30 μm measurement intervals) 2D inorganic geochemical element mapping of rock slabs obtained by in-situ XRF scanning and optical images.Through the model,it is possible to unambiguously recognize the rock compositions and sedimentary fabrics.And then certain environment-sensitive elemental parameters(ratios) can be calculated in situ from individual chemo-sedimentary facies at millimeter-centimeter scales for the effective interpretation of depositional environment and provenance of fine-grained sediments,as well as their origin.Meanwhile,the method also avoids information interference in the conventional bulk compositional analyses whereby homogenized sample powders are often used.In conclusion,the method can help to establish more fine-scale sedimentary facies models and to provide reliable data for the exploration and development of tight and shale oil & gas.