The spatial distributions of faults and deep sea carbonate mounds in the Porcupine Basin, offshore Ireland

Abstract

Localised populations of deep water carbonate mounds occur throughout the NE Atlantic margin of Ireland, the UK and Norway, but the mechanisms responsible for their nucleation and growth are not well known. Based in part on the interpretation of seismic data from the Porcupine Basin, offshore Ireland, it has been proposed previously that deeply rooted faults are present immediately beneath mounds and act as conduits for the vertical migration of mound-feeding hydrocarbons. Several discrete carbonate mound populations or provinces are present in the Porcupine Basin above a number of distinct fault systems at different levels in the stratigraphy. However, detailed mapping of the distributions of both mounds and faults for two of these provinces in the northern part of the basin, using 3D and 2D seismic data, demonstrates that there is a poor spatial relationship between the two. Furthermore, virtually all the reflector offsets directly beneath the mounds, which have previously been interpreted as faults can be attributed to seismic artefacts such as velocity pull-ups and diffraction cones. Therefore, our findings strongly suggest that seismically mappable faults do not play a pivotal role, as conductive fractures, in the evolution of the mounds. However, mounds located towards the NW margin of the Porcupine Basin are underlain by a shallow, intensely faulted slide package, which provides one potential association between faults and mounds.