Abstract

Overpressure distributions within the Paleocene Forties, Cromarty and Eocene Tay sandstones in the Central North Sea have been compiled to gain better insight into hydrocarbon migration pathways. Drainage within the Forties Sandstone Member is westwards at the toe of the fan, where overpressures are >1000 psi (7 MPa), swinging round to northwestwards up-dip, towards the Moray Firth. Overpressure distributions in the Cromarty and Tay sandstone members indicate that these formations are draining westwards, where fluids probably escape through shelfal sandstones of the Mousa and Dornoch formations. Locations of vertical drainage pathways can be interpreted from local anomalies in overpressure distribution, and good hydraulic connectivity may be indicated where overpressures in different sandstone members are the same. Fluids within the Forties Sandstone Member appear to be draining vertically upwards into the Cromarty Sandstone Member within UK blocks 22/29 and 22/30, 29/05 and 29/10, and 29/01, with drainage pathways in UK block 29/01 permitting migration of good quality oils into the recently discovered accumulations of Catcher, Vardero, Burgman and Carnaby. The lateral overpressure gradients within the Cromarty and Tay sandstone members, the pressure evidence for vertical drainage, and the presence of oil accumulations showing limited biodegradation imply connection to the deeper graben through still-active pathways for the migration of oil and gas.

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