Background and Context
In 2016, the Loanan well (214/23-1) located in the Faroe-Shetland Basin was prematurely TD’d as a result of inadequate kick tolerance given the probability of encountering an overpressured igneous intrusion within the secondary target (see Mark et al. 2017). Within this contribution we will explore the evidence base that lead to that decision.
Mafic sill complexes are common features of sedimentary basins at rifted continental margins. Such complexes often comprise networks of interconnected sills, dykes and inclined sheets that facilitated the transfer of magma over considerable vertical distances to shallow depths within sedimentary basins. One such basin is the Faroe-Shetland Basin on the NW European Atlantic Margin, where the extensive Faroe Shetland Sill Complex (FSSC) was emplaced during the Paleogene across an area >22,500 km2. The FSSC was predominantly emplaced within Cretaceous shales, and is characterised by a pervasive network of interconnected, vertically stacked intrusions that extends from ~2 km beneath the present-day seabed, to depths >6 km.
The FSSC has been penetrated by 27 wells, with many wells experiencing loss of circulation events when drilling through intrusions. Combined with core data that provide evidence for fractures, these observations suggest that certain intrusions within the FSSC may host permeable, open fracture systems, even at present-day depths >5 km. Most notably, well 214/28-1 encountered 9 mafic intrusions at depths between ~3.8 and 5 km, including overpressured, thin (<8 m), fractured gas-charged intrusions at ~4.6 and 5 km, respectively. These overpressured intrusions form part of a series of interconnected sills and inclined sheet intrusions that can be mapped on seismic reflection data, and appear to connect to the deepest parts of the basin.
The overpressures could possibly reflect local gas generation related to thermal maturation of the surrounding Cretaceous shales during emplacement of the intrusions, though this would require the overpressures to have been sustained for >50 Myr. Our preferred interpretation for the origin of the overpressure is that the interconnected intrusions have acted as fractured conduits that link to pressure regimes from deeper in the basin. This hypothesis is consistent with the observations of open fractures within deeply buried intrusions in the FSSC (and in deep intrusions in other basins), and with evidence that fractured intrusions have provided migration pathways for petroleum fields located at the edge of the FSSC, such as the Tormore gas field. Our results suggest that transgressive, interconnected sill complexes may represent a previously unrecognized means of vertically transferring pressures (and indeed hydrocarbons) from deep to shallow levels in sedimentary basins. Given that the majority of intrusions in sedimentary basins are too thin to be resolved on seismic reflection data, our results highlight the difficulties associated in planning exploration drilling near interconnected intrusive complexes.
Mark, NJ., Schofield, N., Pugliese, S., Watson, D., Holford, S., Muirhead, D., Brown, R. & Healy, D. (2017) 'Igneous intrusions in the Faroe Shetland basin and their implications for hydrocarbon exploration: new insights from well and seismic data'. Marine and Petroleum Geology. DOI: 10.1016/J.MARPETGEO.2017.12.005