Pressure regression, fluid drainage and a hydrodynamically controlled fluid contact in the North Sea, Lower Cretaceous, Britannia Sandstone Formation

Abstract

The Lower Cretaceous Britannia Sandstone Formation is located below overpressured Sola Formation shales ( c . 2800 psi overpressure) and above Jurassic reservoirs (up to 5700 psi overpressure) and represents a pressure regression, recording overpressure values ranging from approximately 100 psi in Block 15/30 (in the extensional Outer Moray Firth/Central North Sea basin) up to a maximum of approximately 1600 psi in Block 22/4. The Britannia Sandstone has an anomalously low overpressure when compared with deeper Cretaceous (Valhall) and Jurassic reservoir overpressures locally, and further afield in the Central North Sea at similar depths. The pore pressures in the Sola Formation shales have been estimated using conventional porosity/sonic-based prediction methods, indicating strong overpressure disequilibrium between the Sola Formation shales and the Britannia Sandstones, both in the Britannia Field area and in the laterally age-equivalent sands to the west referred to as the Kopervik Fairway (the Aptian sands of the South Halibut Basin, forming the reservoirs of the Blake, Captain and Goldeneye fields). Burial curve modelling indicates that it is likely that the Sola Formation shales have overpressured for the last 4 Ma, and likely much longer, suggesting an early history of pressure build up (now preserved in the Sola Formation shales and deeper, Jurassic reservoirs and associated sediments). The adjacent Britannia Sandstone Formation therefore lost fluid and pressure due to local availability of a fluid conduit. Resulting active fluid flow, driven by overpressure differences in the Britannia Sandstone Formation, has created a hydrodynamically tilted hydrocarbon–water contact with lateral flow from east to west. Well evidence indicates vertical flow from shallower reservoirs into the stratigraphically deeper, main Britannia reservoir. The fluid escape could be either laterally via the Kopervik Fairway and/or vertically through the Upper Cretaceous Chalk facilitated by fractures/faulting.