Remobilization and Injection of Clastic Sediments above an Eocene Depositional Fan Body in the Northern North Sea
Remobilization and injection of sands have been extensively documented in Paleogene sediments of the Outer Moray Firth and South Viking Graben of the North Sea. Examples are associated with hydrocarbon producing fields such as the Alba, Balder, Chestnut, Grane, Gryphon, Harding, Sleipner, Volund, etc. These products have been genetically linked in the sub-surface to either normal depositional bodies directly below them or completely transported from deeper sources. In contrast,the North Viking Graben(NVG) and adjacent parts of the Northern North Sea has only been sparsely documented so far, perhaps due to a paucity of hydrocarbon discoveries in the Paleogene succession in this northernmost part of the North Sea. A 50x50m bin spacing 3D seismic volume covering an area of 29,000km2 and an extensive well database has provided a wealth of information that, aided the description and understanding of the relationship between a newly studied 25km long/300m thick Eocene age depositional body and overlying distinctive high amplitude seismic reflection interpreted as the Opal A/CT diagenetic boundary. These occur below series of high amplitude seismic anomalies within the Oligocene polygonally faulted mudstone dominated unit. Seismically, the depositional body is characterized by high amplitude reflections corresponding to sand as shown by well calibrations. Its paleogeographical location and geomorphology suggests a slope fan body. Amplitude analysis and isochron thickness reveal a lobate geometry with well correlation showing change from massive to mudstone-intercalated sandstones towards the NVG and upwards chronologically. Sediment source is likely from the western south Norwegian mainland, although major channels/canyons are not observed. Our analysis suggests that the fan body underwent post-depositional remobilization/injection, transporting sands into overlying mudstones where they are manifested as high amplitude seismic anomalies. We interpret these as remobilised/injected sandstones and infer that the fan body could have been within a paleodiagenetic zone which has now advanced into the Oligocene unit. The interaction of the advancing Opal A/CT diagentic front, perhaps in association with deeper fluid flow is proposed as possible overpressure generation mechanisms that triggered this process. These Oligocene injectites can serve as standalone reservoirs; provide vertical connectivity and evidence of seal by pass system if the fan was hydrocarbon charged.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California