ILIFFE, JAMES, Conoco Inc., E&PT, Houston,Texas, USA

Abstract: Basin Dynamics, Plumbing, Pressures and Hydrocarbon Charge of Tertiary Reservoirs of the UKCS

The link between basin dynamics, plumbing, pressure and hydrocarbon migration is illustrated by a charge model for the Foinavon oilfield,West of Shetlands, where well connected, laterally continuous, sands fan out from the Westray Ridge over the basin. These sands are important lateral migration pathways for hydrocarbons moving from the centre to the sides of the basin, and also effective vehicles for the distribution of pressure. As pressures built up in the basin during the Paleocene aged rapid deposition, exported excess pressures caused hydrofracturing or fault dilatancy at the shallow proximal part of the sands, facilitating vertical migration and charge into Paleocene reservoirs. The "Whoopee Cushion" sand geometry facilitates the hydrofracturing by making the average overpressure in the sand at the shallow valve end of the sand closer to the greater overpressures in the basin, increasing the potential for fracture. Anomalous S1 pyrolysis values within normally barren overlying shales and exceptional overpressures in the sands from well 204/19-1 support the vertical migration pathway concept. Hydrofracturing and vertical migration probably reactivated during a regional Oligo-Miocene uplift and erosion event, providing a later charge of oil to Foinavon.

Regional uplifts can result in a series of unconnected pressure cells forming effective stress domains. These domains are interpreted to be the vestiges of past vertical migration epsiodes, for example in the North Viking Graben. Identification of these events and their positions has important implication for charging Tertiary sands.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England