Historically, the East Shetland Platform (ESP) was just seen as a broad and flat high, with shallow basement and few visible structures. Mesozoic units are thin or absent. On older seismic data, Palaeozoic reflectors were often interpreted as basement, this despite significant discoveries within Palaeogene-age reservoirs.
Between 2011 and 2016, PGS used GeoStreamer technology to acquire several 3D surveys covering 17 200 sq. km in UKCS quadrants 3, 8–9, 14–16. The imaging results revealed a raft of new exploration potential, clearly illuminating sub-BCU (Base Cretaceous Unconformity) Palaeozoic structures and potential closures in areas where none had been discernible before.
The new seismic helped to make sense of old well results, including reported Devonian oil shows and source rock penetrations.
Petroleum System Summary
PGS has studied the ESP for several years, and findings have been previously published in First Break (Patruno and Reid, 2016, 2017) and GeoExpro November 2017 (Patruno and Roche). The results show that the ESP petroleum system comprises multiple proven and potential reservoir and source intervals, with viable play fairways and possibly mature source rocks.
The ESP comprises persistent basement highs and predominantly subsiding Permo-Triassic depocenters that contain a nearly continuous Palaeozoic-Mesozoic succession, such as the newly-defined ‘Crawford-Skipper Basin’.
Existing hydrocarbon discoveries and shows on the ESP are found less than 7 km from intra-platform Permo-Triassic basin margins. That makes exploration inherently less risky due to possible positive influences of the basin on deep-seated structures in the petroleum system. These influences include the formation of Meso-Cenozoic closures. The maturity of the Devonian source and the presence of simple fault-related migration pathways are also a factor. As is the viability of sub-Cretaceous reservoir trap-seal configurations.
Fluid Escape Features
Numerous vertical seismic amplitude anomalies have been observed within the Cenozoic succession of the ESP recently, which we interpreted to be a result of hydrofracturing associated with vertical fluid migration through the sedimentary rocks.
Anomalies seen over the Greater ESP are for the most part clustered, in association with the south-eastern edge of the Crawford-Skipper Basin and other PermoTriassic fault-bounded depocenters. Evidence that further strengthens the hypotheses 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.
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 enables geoscientists to derive reliable prestack information (acoustic impedance and Vp/Vs), even at depth.
A very high correlation between the wells and seismic in the Palaeozoic section was observed and there were also clear indications that Upper Devonian rocks are characterized by surprisingly high effective porosities, 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
Awareness of the untapped potential of the ESP began to spread, thanks to PGS marketing efforts at conferences, numerous webinars, publications, and client presentations. Interest in the new insights provided by the initial GeoStreamer data led to the acquisition of additional 3D GeoStreamer data (2015-2016). It also encouraged the UK Oil and Gas Authority (OGA) to include the ESP in the 29th and 31st UKCS Frontier Licensing Rounds.
Modern seismic data over the East Shetland Platform has radically altered how geoscientists understand the area and revolutionized its status as a frontier play. Imaging of 3D GeoStreamer data revealed the Palaeozoic reflectors with significantly more clarity than had previously been possible, making it possible to unravel its true geological story.
PGS geoscientists identified a number of sub-BCU targets 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 seep surveys. Intra-platform Permo-Triassic basins with a more complete stratigraphic succession are present and represent inherently less risky exploration areas. Quantitative interpretation was used to further de-risk Palaeozoic plays at the reservoir scale.
The results encouraged the UK Oil and Gas Authority (OGA) to include the ESP in both the 29th and 31st UKCS frontier licensing rounds. END ON SOMETHING ABOUT RESULTS and award winners.
IMAGE: The Devonian Buchan Field in the neighboring Moray Firth shows similarities to undrilled structures in the Permo-Triassic intra-platform basins.
IMAGE: 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)
IMAGE: Two vertical seismic anomalies associated with vertical fluid migration (PGS 3D GeoStreamer PGS15010).