Understanding the Full Potential of the East Shetland Platform

UK frontier areas are hot again with the award of blocks for the 31st offshore licensing round. The East Shetland Platform abuts the mature graben axes of the North Sea rift system. Analysis of modern imaging applied to 3D GeoStreamer data has revealed a local working petroleum system in its large Palaeozoic structures with the potential to charge multiple targets throughout the entire sedimentary column.

Historically, the East Shetland Platform (ESP) was dismissed as a broad and flat high, with shallow basement and few visible structures. Results derived from PGS GeoStreamer data over the area revealed a raft of new exploration potential. From 2011-2016, PGS acquired and imaged 17 200 sq. km of multisensor broadband data in UKCS quadrants 3, 8–9, 14–16. The GeoStreamer data clearly illuminated sub-BCU Paleozoic structures and potential closures throughout the entire Devonian to Neogene column in areas where none had been discernible before.

ESP Revelations 

  • New GeoStreamer 3D seismic helped to make sense of old well results
  • Explains reported Devonian oil shows and source rock penetrations
  • Greatly improves understanding of the migration pathways from the prolific Viking Graben
  • Reveals a working petroleum system localized within the ESP

Petroleum System Summary

PGS has studied the ESP for several years, and findings have been previously published in First Break 2016, 2017 (Patruno and Reid), GeoExpro 2017 (Patruno and Roche) and Geological Society, London 2018 (Patruno et al.). The results of these studies show that the ESP petroleum system comprises multiple proven and potential reservoir and source intervals, with viable play fairways and Palaeozoic mature source rocks.

The ESP comprises an alternation of persistent basement highs and intra-platform Permo-Triassic depocenters that contain a nearly continuous Palaeozoic-Mesozoic succession, such as the ‘Crawford-Skipper Basin’. Over most of the ESP, Mesozoic units are thin or absent.

Positive features on the petroleum system include the formation of Meso-Cenozoic closures, the Middle Devonian source rocks in the present-day oil-window, and fault- and tilting-related migration pathways. Mudstone-rich, Carboniferous to Triassic successions provide effective sealing of deep targets.

Noteworthy discoveries on the ESP, such as Nevis, Crawford, and Dumbarton, are found less than 7 km from intra-platform Permo-Triassic basin margins. These are assumed to have been charged either via lateral migration from the Viking Graben, or as a result of vertical migration from the Paleozoic mini-basins.

Fluid Escape Features

Numerous vertical seismic amplitude anomalies have been observed within the Cenozoic succession of the ESP in recent studies, which are interpreted to be a result of hydrofracturing associated with vertical fluid migration through the sedimentary rocks.

Fluid conduits seen over the ESP are for the most part clustered, in association with the edges of the Crawford-Skipper Basin and other Permo-Triassic fault-bounded depocenters.

As highlighted by a paper in press (Karstens et. al, 2019), most of the 450 vertical seismic anomalies classified as gas chimneys originate from the base tertiary unconformity. Evidence that further strengthens the hypothesis that the preserved intra-platform Permo-Triassic basin fill plays a pivotal role in the maturation of the Devonian source rock and the upward and outward migration of hydrocarbons. Spatial correlation of these pipe structures, with leads identified on GeoStreamer data, supports the probability that these targets are charged by deeper-sitting source rocks and through the pipes. Those parts of the ESP where identifiable leads are out of reach for petroleum charge by lateral migration from the Viking Graben are now interesting frontier play areas, given the deeper source and vertical migration.

Quantitative Interpretation

A successful QI study, based on the GeoStreamer seismic data, targeted rocks as deep and as old as the Devonian. This confirmed that towed-streamer seismic data, gathered using multisensor broadband, and subjected to modern processing routines, enable geoscientists to derive reliable prestack information (acoustic impedance and Vp/Vs), even at depth.

A very high correlation was observed between the wells and seismic in the Palaeozoic section and there were also clear indications that Upper Devonian rocks are characterized by surprisingly high effective porosities, of up to 22%.

Well analysis suggested that relative acoustic impedance for the upper Devonian is a proxy for effective porosity, and this was successfully tested for on the broadband seismic data.

Unraveling Prospectivity

Modern imaging and analysis of GeoStreamer 3D seismic data over the East Shetland Platform have radically altered how geoscientists understand the area and revolutionized its status as a frontier play area. The revelation of Palaeozoic reflectors with significantly more clarity than had previously been achieved made it possible to unravel the true geological story of ESP.

PGS geoscientists then identified a number of targets within the Devonian to Neogene and highlighted the presence of multiple reservoir intervals. A mature and working source was postulated, as highlighted by direct well penetrations and indirect evidence, such as vertical amplitude anomalies and the numerous oil seeps reported by Richardson et al. (2005). Quantitative interpretation was used to further de-risk Palaeozoic plays at the reservoir scale. The presence of intra-platform Permo-Triassic basins with a more complete stratigraphic succession represent inherently less risky exploration areas.

This ESP study demonstrates how reliable multisensor data and advanced imaging can transform explorationists’ understanding of frontier areas. The UK Oil and Gas Authority (OGA) commissioned a 2D GeoStreamer and a reservoir interpretation study based on that data in the resource pack for the 29th round, and included the East Shetland Platform again in the 31stUKCS frontier licensing round. The PGS team congratulates this year’s the award winners and hopes to support further exciting exploration developments on the UKCS.

 

Schematic play concept, idealized for the Crawford-Skipper Basin area of the East Shetland Platform (after UK Discovery Digest, 1999)
Schematic play concept, idealized for the Crawford-Skipper Basin area of the East Shetland Platform (after UK Discovery Digest, 1999)

 

The Devonian Buchan Field in the neighboring Moray Firth shows similarities to undrilled structures in the Permo-Triassic intra-platform basins.
The Devonian Buchan Field in the neighboring Moray Firth shows similarities to undrilled structures in the Permo-Triassic intra-platform basins.

 

Relative impedance can be used as a proxy for effective porosity. Hard impedances correspond to low porosities and soft impedances to high porosities (PGS BLYM2013)
Relative impedance can be used as a proxy for effective porosity. Hard impedances correspond to low porosities and soft impedances to high porosities (PGS BLYM2013)

 

Two vertical seismic anomalies associated with vertical fluid migration (PGS 3D GeoStreamer PGS15010).
Two vertical seismic anomalies associated with vertical fluid-migration (PGS 3D GeoStreamer PGS15010).