Abstract: Exploration for Submarine Fans and Channels: Regional View and Analogs From the North Sea Fields

Castano, Gustavo and Donal O'Driscoll - Texaco, UK

Approximately 20% of the total North Sea hydrocarbon reserve base occur in sands derived from the erosion of the quartz-rich Shetland Platform during the Lower Tertiary and from uplifted graben flanks during the Lower Cretaceous and Upper Jurassic. The sands were deposited as submarine channels and fans and commonly preserve excellent reservoir characteristics. Combined structural/ stratigraphic traps associated with this type of reservoir can be viewed as important field analogs for exploration in other parts of the world.

The Late Cimmerian rifting event affected the North Sea between Late Oxfordian and early Ryazanian times. Sands and conglomerates were deposited as restricted fan aprons close to the eroded graben margins and intra-basinal highs of the Moray Firth and South Viking Graben. Sands were also transported some 10?s kms basinward, forming channelised fan complexes, as seen in the Miller and Galley Fields, Figure 1. Laterally, the sands are intercalated with the organic-rich shales of the Kimmeridge Clay Formation, the major source rock of the North Sea. Poroperm characteristics can be excellent where these reserviors are buried to moderate depths of about 9,000 ft, e.g., Claymore and Magnus Fields with average porosity of 20% and 100 mD permeability. Moderate overpressure at the Galley Field maintains porosity of 15% and permeability of > 100mD at depths of 13,000ft. Because of their relatively deep burial and the overlying high-velocity Cretaceous Chalk, these Jurassic turbiditic reservoirs are not usually imaged clearly on seismic. Depositional models often rely on sedimentological interpretations. These reservoirs are associated with about 3.5 BBOE. The discovered reserves are largely confined to the South Viking Graben - Outer Moray Firth area of the UKCS, with a significant outlier to the north, in the Magnus Field (850 MMBO), and minor commercial reserves in the Central Graben.

Subsequent tectonic pulses during Early Cretaceous times generated the submarine fan sands of the Scapa (Valanginian - Hauterivian) and the Captain/ Britannia (Aptian- Albian) plays. These sands are transitional in style between the aereally restricted Jurassic fans and the larger scale fans of the Tertiary and are confined to the Moray Firth Basin. They account for 1 BBOE reserves in the UK sector.

The main Tertiary uplift of the British Isles is dated at about 60 Ma (late Danian), and is related to the opening of the Atlantic. The exposed Precambrian and Paleozoic highs of Scotland and the Shetland Platform, along with polycyclic Mesozoic sediments provided the sands that were transported eastward to the North Sea Basins, in some cases over distances of 100 km or more. The sands were deposited in relatively large scale, well organised submarine depositional systems, as discrete channels, isolated fan lobes and massive amalgamated fans, re 2. Mounding due to differential compaction may provide 4-way dip closure. Updip pinchout or lateral facies change often provides a stratigraphic element to trapping. These features are frequently visible on seismic, especially 3D data. Depth of burial is usually between 5,000 and 10,000 ft and porosities are commonly >25%, with permeabilities > 1 Darcy. Tertiary fields with turbiditic reservoirs account for > 9 BBOE in the North Sea, the majority in the UK sector.

A related, but separate Tertiary submarine fan provides an emerging play to the west of Shetland, with 1 BBOE discovered to date.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil