WATSON, ROSELEEN S., Department of Geology and Petroleum Geology, Kings College, Aberdeen University, Aberdeen, Scotland

ABSTRACT: The Forth Field, British Sector, North Sea: Evidence of Paleo-Oil Leakage in a Near Surface Reservoir

The Forth field consists of Palaeogene, sand rich, submarine fan lobes present between 1.6 and 2.2 km burial depth, in the Beryl Embayment of the Northern North Sea. They contain biodegraded gas and oil sourced from the Kimmeridgian hydrocarbon kitchen 20 km to the north of the field.

Early concretions are present within all of the reservoir units. High pre-cement porosities and the presence of bitumen inclusions in early diagenetic calcites suggest oil emplacement and carbonate precipitation occurred simultaneously, fairly close to the depositional interface. Bitumen filled coprolites within the surrounding shales indicate that oil migrated through poorly consolidated shales, eventually collecting in the more porous sand units, when a sufficient shale seal was deposited.

Biodegradation of the migrated oil at the paleo-oil water contact by different bacterial species produced a series of cements with isotope{13}C<PDB> values ranging from -31 to +12 per mil.

The diagenetic sequence observed suggests oil migration was initiated in the late Palaeocene but hydrocarbon leakage may have occurred a number of times, producing a complex sequence of carbonate cemented horizons, which have the potential to act as major barriers to vertical fluid movement.

Breach of the reservoir seal may have been initiated by soft sediment deformation, tectonic activity or fluid/gas overpressure. Active seismicity was concurrent with sea-floor spreading in the Norwegian-Greenland Sea at this time. However oil and gas accumulation and subsequent pressure release cannot be ruled out as a possible mechanism for fluid discharge or for the creation of sand injection structures observed above the main reservoir sand.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.