Bit-metamorphism, illustrated by lithological data from German North Sea Wells

Abstract

Permo-Carboniferous redbeds encountered in some deep German North Sea wells are commonly altered when drilled with diamond bits. The process has been referred to as 'bit-metamorphism' by well-site geologists and others. Severely altered cuttings constitute over 80% of samples; they are hard, and when oil-based muds have been used are almost black and generally magnetic. Study of cuttings using petrographic, X-ray and chemical methods, supplemented by scanning electron microscopy, demonstrates that the process involves the reduction of particle size by shearing, followed by cementing or welding of the particles into a matrix superficially resembling a glass. The bulk chemical analysis of strongly altered cuttings is similar to that of accompanying unaltered material, though slightly richer in silica, but X-ray reflections of all minerals except quartz and feldspar are reduced or lost. All rock types are believed to be affected, but abundance of quartz and accompanying above-average hardness probably favour the process. Oil-based muds are not thought to be a necessary factor, but when used their breakdown may give rise to spurious gas readings. It is not certain whether the altered cuttings are bound together by precipitation of colloidal silica or by fusion to a glass. Further research is needed and might be expected to lead to better cuttings, reduced drilling torque, and less bit wear.