Modelling the effects of transient overpressure on the petroleum systems of the UK North East Atlantic Margin and northern North Sea areas: implications for the deep water South Atlantic and Gulf of Mexico
Authors: SCOTCHMAN I. C. , CARR A. D. 
Published: April 2010

Transient fluid overpressure development appears to be a major control on the contrasting petroleum systems of Faroe–Shetland Basin (FSB) on the UK North East Atlantic margin and the northern North Sea North Viking Graben (NVG). Middle and Upper Jurassic shales are the main source rocks in both the NVG and the FSB, but the hydrocarbons within these basins differ significantly. Oil is predominant with only minor associated gas in the FSB while the NVG has a significantly higher proportion of dry gas. Both are rifted basins but the amount of rifting, heat flow and depth of burial were all considerably greater in the FSB, and consequently higher maturity hydrocarbons should be present in the FSB than in the NVG.

The difference in hydrocarbon type between the basins arises from the different Cenozoic burial and, importantly, pressure histories in the two basins. Oil generation commenced during the early Cretaceous in both basins and some Mesozoic reservoirs may have been charged at this time. Rapid burial of thick argillaceous sediments in both basins from the late Cretaceous into the Tertiary led to overpressure development. The thermodynamic effect of this fluid overpressure in the source rocks was the retardation of hydrocarbon generation. In the NVG, regional uplift during the Tertiary resulted in transient overpressure reductions in the source rocks such that hydrocarbon generation, which had ceased prior to the uplift events, restarted due to the pressure reduction outstripping the effects of reduced temperatures. Although the source rocks were cooler than those in the FSB, the reduction in overpressure was sufficient to result in gas generation from both the Middle Jurassic and lower part of the Upper Jurassic source rocks. Conversely, although the FSB was also uplifted during short-lived Paleogene and Neogene events, the greater thickness of argillaceous late Cretaceous sediments reduced the transient overpressure loss in the source kitchens compared with the NVG. The maturity of the source rocks in the FSB was therefore held in the late oil window and, as a consequence, oil is the predominant hydrocarbon phase in this basin.

The importance of overpressure development (or lack of it) and its subsequent release has major implications for petroleum prospectivity of basins. This is illustrated using the NE Kwanza Basin (Angola) and the Mississippi Fold Belt in the Gulf of Mexico (GoM). Whereas there are no commercial hydrocarbon discoveries in the deepwater Kwanza Basin due to lack of regional seal between the oil mature Pre-Salt source and the overlying migration conduit into the shelf, significant oil reserves occur in the deepwater GOM where the source rock depth (∼10 km) might be expected to preclude the presence of oil. The key to the release of oil from the source kitchens is the salt movement that occurs during the Neogene with the formation of salt canopies. As the salt moves vertically, it reduces the overpressure, thereby enabling oil generation to restart and migrate vertically along faults.