The Petroleum System of the Oslo Graben, Southern Norway

Jon Halvard Pedersen
Lundin Norway AS, Lysaker, Norway

The Oslo Graben is a Palaeozoic rift basin located in southern Norway. Rock strata in the Oslo Graben region consist of pre-rift marine sedimentary rocks of Cambro-Silurian age, and syn-rift Carboniferous-Permian continental sedimentary and volcanic rocks. The Cambro- Silurian rocks were deeply buried during the Caledonian orogeny (Late Silurian-Middle Devonian), and experienced elevated heat flows during the Carboniferous-Permian volcanism.

Hydrocarbons have been sampled at three outcrop locations in the onshore Oslo Graben rift basin; from a Cambrian sandstone, an Upper Ordovician carbonate rock, and a Permian volcanic intrusive. Hydrocarbon samples appear as solid bitumen, light oil, and gas. Extracted hydrocarbons have been analyzed using gas chromatography, and δ¹³C isotope values have been established for gas and extracts. Scanning electron microscopy has been applied on insoluble bitumen, and microthermometry has been used to investigate fluid inclusions.

Geochemical analyses point to a marine shale as source rock for the extracted hydrocarbons. Hydrocarbon maturity corresponds to peak oil to late oil stages of generation. Source rock candidates include the marine, organic rich Cambrian Alum shale and Ordovician marine mudstones. Traces of migrated hydrocarbons suggest that a petroleum system has been active in the Oslo Graben. This has interesting implications for the southern continuation of the Oslo Graben; the offshore Skagerrak Graben, where the rift system is overlain by Mesozoic sedimentary rocks.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013