The Origin and Properties of Flint in the Upper Cretaceous Chalk

One of the most critical geological/technical challenges affecting engineering projects in chalk with flints either for hydrocarbon/water exploration or infrastructural development is encountering flints, which mostly result in excessive wear or damage to cutting tools or drilling/tunnelling equipment. Overcoming this requires a regional understanding of the engineering properties of flints which are linked to various factors including the genesis/diagenesis of flints. To date, the genesis of flints remains debatable and the controls on engineering properties of flints on a regional basis have not been systematically investigated. This study sets to investigate: the genesis of flints, the engineering properties of flints across various geographical locations, the variations in these properties with colour, structures, origin, and morphology of flints, and the tectonic history of the host chalk. To achieve these objectives a series of physicomechanical and petrophysical tests on flints and associated structures sampled from the Danish, English and French chalks have been carried out. A drillability test focusing on the grey and dark brownish grey flints using a standard tricone bit to reflect the rock mass characteristics of flint has been conducted. Petrographic studies comprising scanning electron microscopy, scanning electron microcopy-cathodoluminescence and X-ray diffraction have then been applied to analyse the genesis, diagenesis, microstructures, microtexture, and mineral phase/composition of flints. Results support the biogenic origin of flints through a calcite-silica replacement process and illustrate that engineering properties of flints correlate with colour, origin, and the diagenesis of flints, plus the properties and tectonic history of the host chalk. This study demonstrates that physicomechanical properties of flints are controlled by their micro-structures/texture/fractures, structures, calcite inclusions and mineral compositions. The dark brownish grey flints with finer microtexture are the strongest, stiffest, densest, hardest, most abrasive and least porous materials with extremely high potentials to cause drill wear. By comparison, the grey flints located in a highly tectonized zone behave differently and are the weakest, less dense, less abrasive and the most porous flints with relatively lower potentials to cause drill wear. These findings offer new contributions to the understanding of the material properties of flints applicable to drillability prediction, tool wear evaluation and drilling/tunnelling system specifications in chalk with flints.