Petrography, mineralogy and diagenesis of overpressured Tertiary and Late Cretaceous mudrocks from the East Shetland Basin

Abstract

AbstractThe petrography, mineralogy and diagenesis of an overpressured Tertiary and Late Cretaceous mudrock sequence from a single well in the East Shetland Basin (North Sea) are described. The clay fraction is believed to be dominated by volcanic glass above 1400 m depth and by smectite below 1400 m. The smectite content increases with depth due to an increase in detrital smectite and to in situ alteration of volcanic glass and grains; this commenced almost immediately after deposition and is still not complete at burial depths >2300 m. There is little evidence of alteration of the smectite to illite despite a local available supply of K+ in the form of K-feldspar and the presence of K in the more deeply buried smectite. Very little leaching or precipitation of quartz has occurred. Microfossil tests filled with authigenic clays such as have been reported in other Tertiary mudrocks in the North Sea were not observed, though a few fossil cavities are filled with carbonate cements, and very rarely with microquartz. The suprisingly limited reactivity of this mudrock sequence may be due to the early development of overpressure, resulting in a “closed” system, and to the absence of interbedded higher permeability layers or open faults through which fluids could be exchanged.