Inter-layered clay stacks in Jurassic shales

Abstract

As the average shale contains ∼60% clay minerals¹, an understanding of the physical and chemical characteristics of these minerals is central to the question of burial diagenetic reactions and hydrocarbon generation. Much existing evidence concerning burial diagenesis relies on X-ray diffraction (XRD) data, particularly of the clay-size (<2 µm) fraction of shales². Here we use scanning electron microscopy (SEM) in the backscattered electron (BSE) mode, together with energy-dispersive X-ray microanalysis (EDXA), to show that Lower Jurassic shales from the North Sea Basin contain large numbers of clay mineral stacks up to 150 µm in size. By studying polished shale sections with BSE and EDXA, the size, shape orientation, textural relationships and internal compositional variations of the clays can be observed in situ. We present here preliminary evidence which suggests that the clay stacks studied are authigenic, and may have formed at shallow burial depths during early diagenesis.