Seismic expression of large scale fluid escape pipes using time lapses seismic surveys: examples from the Loyal Field (Scotland, UK)

Fluid escape pipes are key features of primary interest for the analysis of vertical fluid flow and secondary hydrocarbon migration in sedimentary basin. Identified worldwide (Løset et al., 2009), they acquired more and more importance as they represent critical pathways for supply of methane and potential structure for leakage into the storage reservoir (Cartwright & Santamarina, 2015). Therefore, understanding their genesis, internal characteristics and seismic expression, is of great significance for the exploration industry. Here we propose a detailed interpretation of a 4D seismic survey (released by the BP) recently acquired in the Loyal Field aiming at characterizing the large scale leakage system constituted by big-scale fluid escape pipes affecting the Upper Paleogene/Neogene stratigraphic succession in the Loyal Field, Scotland (UK). The Loyal field, is located on the edge of the Faroe-Shetland Channel slope, about 130 km west of Shetland (Quadrants 204/205 of the UKCS) and has been recently re-appraised and re developed by a consortium led by BP. The 3D detailed mapping analysis of the full and partial stack survey (processed using amplitude preservation workflows) indicate that the overall complex system of fluid pipe structure is rooted in the pre Lista formation and developed across the Paleogene and Neogene Units. Geometrical analysis show that pipes got diameter varying between 100-300 m and a length of 500 m to 2 km. Most pipes seem to terminate abruptly at discrete subsurface horizons or in diffuse termination suggesting multiple overpressured events and lateral fluid migration (through Darcy flows) across the overburden units. The internal imaging of the large pipes shows a tendency of upbending of reflection (rather than pulls up artefacts) affected by large scale fracture (semblance image) and consistent with a suspended mud/sand mixture non-fluidized fluid flow. Two main classes of pipes are encountered: a) pipes reactivating existing clustered faults (enucleating above the Lista formation) and characterized by a rather elliptical geometry and b) large circular pipes apparently unrelated to any seismically visible pre-existing faults and concentrated in the crest of the slope structure of the loyal field. Most of the mapped fluid escape pipes suggest a post late Paleogene- Neogene genesis although some few examples indicate a termination near the seabed suggesting a very recent genesis. Near-Middle-Far offsets amplitude analysis confirms that most of the amplitude anomalies within the pipes conduit and terminus are only partly related to gas. Possible hypothetical formation mechanisms for those Pipes are discussed