Central North Sea: Regional Dataset Supports Continued Exploration

Author M. Jameson, S. Ragbir, C. Loader, T. Bird, C. Smith & C. Reiser (PGS)

The Central North Sea (CNS), although a mature hydrocarbon province, continues to provide new exploration prospects and reserves.

The United Kingdom’s 26th Licensing Round revealed increased demand for CNS acreage combined with a number of recent discoveries including the Culzean, Shaw and Catcher fields. A change in the focus of exploration from structural traps to stratigraphic traps, such as the submarine fans of the Forties Sandstone has highlighted the demand for accurate seismic data.

The PGS MegaSurveyPlus is a regional pre-stack merged and state-of-the-art reprocessed seismic dataset covering nearly 17,000 km² of the core CNS area, combined with regionally consistent horizons tied to well control and AVO attributes), all of which contribute to the search for new reserves.

This seismic cross section trends north-west to south-east, crossing 150 kms of the core of the Central North Sea. The section reveals the horst and graben structure of the Central North Sea and crosses the Western Platform, Western Central Graben, Montrose-Arbroath Ridge, East Central Graben and Cod Terrace. It highlights the Base Cretaceous Unconfomity and its truncation of tilted Triassic-Jurassic Fault blocks. Subsequent Cretaceous horizons onlap and are truncated by the Sele formation. The Top Hordaland Group reveals a regional unconformity that is overlain by sediments of the Nordland Group. The section also reveals the Zechstein Salts of the Cod Terrace that appear to have mobilized along an existing Permo-Triassic Fault.

Recent Discoveries in The Central North Sea

The Central North Sea (CNS) has been a focus of hydrocarbon exploration and production for decades. Even now the area remains a hive of activity, with an estimated 5 billion barrels of oil and gas undiscovered (UK DECC Estimates of Undiscovered Resources, 2010).  

Recent discoveries in the CNS include the Culzean Gas and Condensate field (block 22/25a) by Maersk Oil, and the Shaw Discovery (block 22/22a) by Marubeni and Talisman in 2009. In 2010 EnCore announced the Catcher and Catcher East discoveries (block 28/9) with estimated reserves of ~300 MMBoe, representing one of the largest discoveries in the Central North Sea for over a decade.  

In 2010, 144 Licenses were awarded in the United Kingdom’s 26th Licensing Round. With increasing exploration and investment in the CNS there is greater demand for high quality, increasingly detailed and innovative seismic data to assist in the exploration, understanding and de-risking of this prolific hydrocarbon province.  

Petroleum Geo-Services (PGS) has been working in the Central North Sea for nearly 20 years. Since 2002, the company has been delivering merged 3D seismic data known as the MegaSurvey, which has mapped, in both regional and prospect level detail, 60,000km² of the Central North Sea. This has now been upgraded to the MegaSurveyPlus; a pre-stack merge of nearly 17,000 km² of Central North Sea seismic which, with horizon interpretation and highlighted AVO anomalies, allows rapid regional screening for prospect and lead identification.

RMS Seismic Attribute of the Forties Member, showing deep marine channels. Image: PGS

Hydrocarbon Plays

The Central North Sea is a focus of exploration for structural, stratigraphic and combination traps. A range of structural traps are present in the CNS, ranging from tilted fault blocks to crestal slumps. However as exploration and development has progressed, significant new structural closures are increasingly rare. The PGS MegaSurveyPlus covers a number of large scale Cretaceous slumps similar to the productive Centurion field, representing a new play type in the CNS.   

Stratigraphic traps are thought to hold up to 75% of the undiscovered oil in the UK continental shelf. Stratigraphic traps range from onlap to pinchouts, spanning single seal and polyseal traps. The Forties sandstone stratigraphic trap in the Callanish Field is an example of deep water mounded turbidites that have suffered differential compaction and pinch out towards the southwest. Combination trapping mechanisms combining structural and stratigraphic features are also present, for example in the Britannia field in the Lower Cretaceous.  

Although a range of hydrocarbon play types exist in the CNS, this article focuses upon deep water Paleocene and shallow to deep marine Jurassic plays.   

Paleocene and Jurassic plays

The Catcher discovery (~300 MMboe) has highlighted the potential of Paleocene plays in the Central North Sea. The Forties and Maureen Sandstone are key reservoirs and represent a series of deep marine sand rich submarine fans. Fields producing from these reservoirs include the Forties (~340 MMboe), Arbroath (~170 MMboe) and Montrose (~236 MMBoe). Paleocene reservoirs are charged by the Heather and Kimmeridge Clay formations. The Lista and Sele formation shales represent a key upper seal to Paleocene reservoirs which are also underlain by hemipelagic shales forming a bottom seal. The MegaSurveyPlus has highlighted the distribution of the Forties Member in a regional context, with increased accuracy attributed to the reprocessing of field tapes on a regional scale.  

Paleocene plays are dominantly stratigraphic traps, although some combination traps associated with salt doming are also present. Combination traps are favoured targets due to a higher chance of success.   

Jurassic plays represent key targets in the Central North Sea. Jurassic traps may be structural or stratigraphic with key reservoir intervals spanning the shallow marine facies of the Fulmar formation and deep marine, turbidite facies of the Kimmeridge Clay. The Fulmar differs from the Kimmeridge Clay in that it represents a high pressure, high temperature reservoir (Erratt et al., 2005), but despite this added risk the Fulmar remains a popular play in the Central North Sea.

Fulmar formation (green) and Base Cretaceous (blue) Horizons displaying a combination trap of tilted fault blocks truncated by the Base Cretaceous Unconformity. Image: PGS

Fields producing from Middle to Late Jurassic reservoirs include the Fulmar (~567 MMBoe), Clyde (~130 MMBoe) and Kittiwake (~74 MMBoe) fields. Jurassic reservoirs are charged by the Heather and Kimmeridge Clay formations. Seals are abundant, from intraformational shales to the source-seal Kimmeridge Clay, Cretaceous Shales, Chalk and overlying Cenozoic clastics.   

The MegaSurveyPlus reveals the distribution of the Middle - Upper Jurassic reflectors and has also been reviewed for AVO anomalies. Several additional horizons have also been interpreted for the MegaSurveyPlus, aiding in the exploration of subtle stratigraphic traps.      Reservoir Characteristics The previous sections have outlined the regional coverage of the MegaSurveyPlus, however the dataset can also be utilised at reservoir level.   

To attain details at reservoir level the PGS Prospect Scanner workflow was used in the identification and mapping of AVO related anomalies over the dataset. Prospect Scanner combines seismic AVO analysis and inversion technology to derive relative elastic properties (i.e. P and S-wave impedances, Vp/Vs ratio).   

Further analysis of the MegaSurveyPlus at various reservoir intervals reveals lithology trends of play fairways, local fluid effect anomalies corresponding to discoveries, and in some cases undrilled stratigraphic features.

3D view of part of UK Quad 22 looking North, showing Envelope Attribute of extra far offset stack at the Top Forties TWT structure horizon. The image shows high amplitudes (red) corresponding to field outlines of Montrose field (centre rear, showing two prominent channel trends) and Arbroath field (centre). There are also several bright anomalies below the Arbroath field, located on undrilled structural culminations. Source: PGS

The future of the CNS

This article has briefly covered the recent discoveries and plays of the Central North Sea and demonstrates how the upgraded data coverage of the MegaSurveyPlus can help to identify more leads and prospects with a level of confidence never possible before. With an estimated five billion barrels of oil and gas undiscovered and significant in-situ infrastructure, the Central North Sea is still a very attractive area to explore and operate.

As exploration continues in this prolific hydrocarbon province, comprehensive, state-of-the-art data is essential for delineating prospects and reducing risk. The application of pre-stack seismic data combined with refined well-tied horizons and AVO anomalies provides a wealth of information. Furthermore, the future application of PGS’ GeoStreamer® (dual-sensor towed streamer technology) will further enhance data coverage of the Central North Sea and continue to add value at the reservoir level.