PETROLEUM SEEPAGE AT ASUK, DISKO, WEST GREENLAND – IMPLICATIONS FOR REGIONAL PETROLEUM EXPLORATION
J. A. Bojesen-Koefoed1*, T. Bidstrup1, F. G. Christiansen1, F. Dalhoff1, U. Gregersen1, H. P. Nytoft1, H. Nøhr-Hansen1, A. K. Pedersen2 and M. Sønderholm1
1Geological Survey of Denmark and Greenland (GEUS), 10 Øster Voldgade, DK-1350K Copenhagen, Denmark.
2Geological Museum, Øster Voldgade 5-7, DK-1350K Copenhagen, Denmark.
*Author for correspondence, email: jbk@geus.dk
ABSTRACT
Organic geochemical screening and biological marker analysis was carried out on a total of 45 Albian–Cenomanian sandstone and mudstone samples collected from a landslide block on the north coast of Disko island, central west Greenland. The landslide block covers an area of several square kilometres, and originated approximately 400 m up-section from where it moved to its present position after the last glaciation. The mudstones are generally rich in organic carbon, but show no potential for petroleum generation. However, biodegraded oil stains were found in the poorly lithified sandstones. Staining by undegraded or only slightly degraded oil in volcanic rocks is common in this region, but the occurrence described here is the only known outcrop where staining occurs in siliciclastic sediments, as well as the only one in which the oil is severely biodegraded.
The biodegradation shows preferential elimination of cheilantanes. This unusual feature is probably an effect of the permafrost prevailing in the region. The oil stains appear to represent a biodegraded variety of the Cretaceous marine shale-derived Itilli oil type which is known from many locations in the Disko–Nussusaq–Svartenhuk Halvø region. The oils entered the sandstones before the landslide event, probably during or before the extrusion of the volcanic succession in the Paleogene.
This is the first time that a petroleum seepage has been found east of the Kuugannguaq-Qunnillik fault zone, which is located approximately 30 km to the west of Asuk. The presence of staining by marine oil at Asuk demonstrates that marine petroleum source rocks were deposited far further eastwards than previously thought, thus expanding the area of potential exploration significantly. The presence of marine source rocks to the east of the Kuugannguaq-Qunnillik fault zone may explain the frequent observation of “Direct Hydrocarbon Indicators” in seismic data collected in the Vaigat Sound.