Drilling- and coring-induced artefacts in poorly indurated clays

Abstract

Macroscopic and microscopic drilling-induced deformation structures were documented in cores of Ypresian Ieper Group and Rupelian Boom Formation clays using macro-polished slabs and polarizing microscopy of thin sections. The cores were recovered in north Belgium from a depth of a few hundred metres by pushing a steel barrel with a cutting shoe into the non-indurated clays that are in a ductile to brittle transition state. The outer border of the clay cores was bent downwards due to the pushing action. The geometry of the drilling-induced fractures can be described as vertically stacked axisymmetrical conical fractures complexed by a general concave and undulating shape, and by fractures splaying from other fractures. At the microscopic scale (2D), the fractures are micro-faults that produce a breccia texture and which consist of thin planes, lines in 2D, of orientated clay minerals. The preferred orientation of these clay minerals in the fractures point to shear movement and explain the shiny appearance of these slickenside-like fractures when viewed in 3D. The micro-faults produce clay domains tilted with respect to each other. Recognizing exclusively drilling-induced structures helps in the selection of cores and core fragments for further laboratory testing, and may be of help in distinguishing them from regional faults or fracture systems in homogeneous clays. Thematic collection: This article is part of the Sustainable geological disposal and containment of radioactive waste collection available at: https://www.lyellcollection.org/topic/collections/radioactive