OIL-PRONE LOWER CARBONIFEROUS COALS IN THE NORWEGIAN BARENTS SEA: IMPLICATIONS FOR A PALAEOZOIC PETROLEUM SYSTEM
J.H. van Koeverden1*, D.A. Karlsen1, L. Schwark2,
A. Chpitsglouz3 and K. Backer-Owe1
1 Department of Geosciences, University of Oslo, PO Box 1047 Blindern, N-0316 Oslo, Norway.
2 Institute for Geosciences, University of Kiel, Ludewig-Meyn-Str. 10, D-24118 Kiel, Germany.
3 University of Cologne, Institute of Geology and Mineralogy, Zülpicher Str. 49a/b, D-50674 Cologne, Germany.
* corresponding author, email: vankoeverden@geo.uio.no
In this study, we assess the oil generation potential of Lower Carboniferous, liptinite-rich coals in the Tettegras Formation on the Finnmark Platform, southern Norwegian Barents Sea. Oil from these coals has been expelled into intercalated sandstones. The coals may have contributed to petroleum recorded in well 7128/4-1 on the Finnmark Platform and may constitute a new Palaeozoic source rock in the Barents Sea.
The Tettegras Formation coals contain up to 80 vol. % liptinite (mineral matter free base) and have low oxygen indices. Hydrogen indices up to 367 mg HC/g TOC indicate liquid hydrocarbon potential. In wells 7128/4-1 and 7128/6-1, the coals have vitrinite reflectance Ro = 0.75–0.85 %. Compared to shale and carbonate source rocks, expulsion from coal in general begins at higher maturities (Ro = 0.8–0.9% and Tmax = 444–453°C). Thus, the coals in the wells are mostly immature with regard to oil expulsion. The oil in well 7128/4-1 most likely originates from a more mature part of the Tettegras Formation in the deeper northern part of the Finnmark Platform. Wide variations in biomarker facies parameters and d 13C isotope values indicate a heterogeneous paralic depositional setting. The preferential retention by coal strata of naphthenes (e.g. terpanes and steranes) and aromatic compounds, compared to n-alkanes and acyclic isoprenoids, results in a terrigenous and waxy oil. This oil however contains marine biomarkers derived from the intercalated shales and siltstones. It is therefore important to consider the entire coal-bearing sequence, including the intercalated shales, in terms of source rock potential.
Coals of similar age occur on Svalbard and Bjørnøya. The results of this study therefore suggest that a Lower Carboniferous coaly source rock may extend over large areas of the Norwegian Barents Sea. This source rock is mature in areas where the otherwise prolific Upper Jurassic marine shales are either immature or missing and may constitute a new Palaeozoic coal-sourced petroleum system in the Barents Sea.
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