THE PALEOCENE SANDY SIRI FAIRWAY: AN EFFICIENT “PIPELINE” DRAINING THE PROLIFIC CENTRAL GRABEN?

Abstract

A new petroleum charge model is presented for the sand-dominated Paleocene channel system known as the Siri Fairway in the Central Graben of the North Sea. The Siri Fairway is located in the platform area along the Danish - Norwegian border and extends from the Norwegian palaeo shelf into the Tail-End Graben and Sogne Basin. The nearest known expelling source rocks are located in the Central Graben. The discovery of the Siri oilfield and later the Cecilie and the Nini fields proves that petroleum has migrated through these Paleocene sandstones for up to 70 km, which is a considerable distance in the North Sea. If the Siri Fairway has acted as a “pipeline” for petroleum migrating from the Graben to the platform area, the chemical composition of the hydrocarbons discovered in the Graben and within the Fairway itself should be similar in terms of maturity and organic facies signature. This study shows this not to be the case. The Graben oils have a mature signature, whereas the oils from the Siri field have an early mature signature and are mixed with biogenic gas generated in situ. The biogenic gas “signature”, which was inherited from gas which accumulated in the trap before the arrival of the oil charge, should have disappeared if petroleum had continuously been introduced to the Fairway. It therefore appears that hydrocarbon charging to the Fairway ceased for some reason before the source rocks in the Graben entered the main oil window; the Siri Fairway therefore represents an aborted migration route, and limited charging of the Paleocene sandstone deposits in the platform has occurred. The chemical composition of the oils from the Siri field indicates that the Fairway was charged from two different basins with different subsidence histories. The Siri-2 trap is thus interpreted to have been filled with the same oil as that found in Siri-1 and Siri-3, but this oil was later partly displaced by oil generated in a shallower sub-basin. The sandstones in the Siri Fairway were deposited as turbidites and/or gravity slides in the Late Paleocene, and consist of stacked interfingering sandstone lobes which are encased to varying degrees in fine-grained sediments. Although long distance migration through the sandstones has been proved to occur, connectivity between individual sandlobes may be problematic. The number of dry wells drilled in the Fairway and the early-mature character of the analysed oils, together with the general absence of more mature later petroleum, indicate that migration routes in this region are limited and difficult to predict.