Large-scale conical sandstone intrusions and polygonal fault systems in Tranche 6, Faroe-Shetland Basin

Abstract

Kilometre-scale sandstone intrusions are important components of many deep-water clastic systems. They result from the remobilisation and injection of sand during early stages of burial, and are commonly found in fine-grained mudrocks containing a pervasive polygonal fault network. This paper uses three-dimensional seismic data, from the Faroe-Shetland Basin, on the UK Atlantic margin, calibrated to nearby wells, to examine the relationship between an extensive system of large downward-tapering conical sandstone intrusions to the pervasive polygonal fault network present within the surrounding encasing host mudstones. The intrusions were injected upwards during the Late Miocene through Eocene/Oligocene claystones with a pre-existing polygonal fault network to form conical bodies up to 300 m high within the ∼700-m-thick claystone sequence. This study shows that polygonal faults and the discordant limbs of conical sandstone intrusions form two distinct dip populations with little overlap that have average dips of 58° and 26°, respectively. Polygonal faults display decreasing dip with increasing depth. However, conical sandstone intrusion dip is uniform throughout the Eocene–Oligocene succession. These observations lead us to conclude that the geometry of conical sandstone intrusions is not greatly affected by the presence of a pre-existing polygonal fault network, nor is it affected by increased overburden pressure with depth. Therefore, the 10–50° dip range displayed by the limbs of conical sandstone intrusions observed within the study area is a fundamental property of conical sandstone intrusions. The presence of a widespread region of conical sandstone intrusions in the Faroe-Shetland Basin has important implications for vertical fluid migration in the region. The Eocene–Oligocene claystones in the region have very low permeability, but the presence of large-scale conical sandstone intrusions and any possible feeder systems is likely to provide high-permeability fluid conduits through this sequence. In effect, the otherwise good regional caprock provided by the Eocene–Oligocene claystone sequence is breached by a network of sandstone intrusions possibly resulting in reduced hydrocarbon-trapping potential beneath the seal.