Arctic Hyperextended Margins and Basins - Implications for Exploration

Erik Lundin¹ and Anthony G. Doré^2
1New Ventures, Statoil ASA, Trondheim, Norway.
²New Ventures, Statoil Gulf Services LCC, Houston, TX.

Slow and extreme extension of continents may lead to the development of magma-poor rifted margins, e.g. the well-known Iberian - Newfoundland margins. The Arctic is dominated by magma-poor rifted margins: Labrador Sea, Baffin Bay and probably Amerasia Basin. However, the process can become interrupted prior to break-up. Oblique opening of the NE Atlantic split a c 3000 km long hyperextended basin chain, resulting in fragmentation thereof.

Implications of the hyperextension process include a significant reduction of the strength of the lithosphere by: a) crustal thinning, and b) associated upper mantle hydration. The resulting weakness is long-lived and acts as a stress-guide, making such margins and basins prone to compressional deformation. A number of such compressional folds have been drilled along the NE Atlantic margins with varying results. In the case of the NE Atlantic, hyperextension was of Early Cretaceous age, hence post-dating the Upper Jurassic source rock. This resulted in fragmentation of the source rock, deep burial, and early maturation, often well before reservoirs and traps were in place. The mentioned compressional folds are of Cenozoic age, primarily Neogene. However, petroleum systems may also work well in hyperextended settings, e.g. the South Atlantic (Angola), probably as a function of the presence of a post-rift source rock.
More conceptual implications of hyperextension are reduced syn-rift subsidence due to the buoyancy of the hydrated mantle (serpentinized); on a geologic time scale the serpentinization process is rapid. If subsequently deeply buried, the temperature-governed serpentinization reaction may become reversed, leading to a density increase and water release. The increased density may induce subsidence well into the post-rift phase without associated crustal extension. Such released water is generally super-critical and may hence rise through the overlying crust and sediments as a vapour phase, until cooled below the stability threshold (374 C at 22 KPa). The volume of released water is estimated to be quite large. Possible effects on petroleum systems of such water are poorly understood, but a suggestion of this having taken place is a reported correspondence between trace elements of oils and serpentinite in Brazil. This is not surprising considering that super-critical water is highly reactive.

Appreciating the existence of hyperextension is clearly relevant when assessing the petroleum potential of rifted margins, including those of the Arctic.

AAPG Search and Discovery Article #90130©2011 3P Arctic, The Polar Petroleum Potential Conference & Exhibition, Halifax, Nova Scotia, Canada, 30 August-2 September, 2011.