Evaluation of Cretaceous Source Rocks at the North Atlantic Passive Margin and the Arctic Ocean Connection

Monika Majewska-Bill, Maarten Felix, Ute Mann, and Joachim Rinna
Basin Modelling Group, SINTEF Petroleum Research, Trondheim, Norway.

Mid-Cretaceous source rocks have charged more than one-third of petroleum reservoirs worldwide (e.g. Larson 1991). Cretaceous source rocks are known to occur in areas stretching from the central Arctic Ocean and Greenland to the Norwegian North Sea, and elsewhere in the Atlantic during this period (e.g. Stein 1991 and references therein), but no occurrence has been proven in the Norwegian Sea. The black mudstones of the Blodøks Formation of the Norwegian North Sea are of latest Cenomanian to early Turonian age (Surlyk et al., 2003), while the black mud deposits of the Alpha Ridge in the central Arctic Ocean are somewhat younger (Dell’Agnese and Clark 1994; Firth and Clark 1998; Stein and Macdonald 2004). The shallow marine mudstones and siltstones on East Greenland are also rich in organic carbon, but have poor hydrocarbon potential due to high maturity (Tmax over 500°C) (Larsen et al. 1999).

Øygard and Olsen (2002) discussed geochemical indicators supporting the idea of a Cretaceous source rock at the mid-Norwegian shelf and source rock modelling studies were carried out along 2D sections to understand the variability of this time interval and to identify the depositional controlling factors of these rocks (Mann et al. 2002, Inthorn et al. 2007). In the present study we have for the first time a full 3D data set available to quantify the initial potential of the mid Cretaceous deposits. The modelled area extends from 62° to approximately 68° 20’ North, and from the Vøring Marginal High to the coast of Norway (approximately 0° to approximately 12.5° east). Input data for the model consisted of interpreted seismic horizons and palaeo-water depth reconstructions of the top and base of the Lange Formation together with geochemical data from 135 wells off mid-Norway. Three different stratigraphic models were created to account for the uncertainty in age assignments and diachronous bounding surfaces. These were combined with three organic sedimentation settings with increasing preservation factors, to investigate possible source-rock potential and quality variations.

The modelling results supported earlier conclusions that the most important factors controlling organic matter accumulation in the area were the variability in sedimentation rate combined with the intensity of anoxia. Only the setting with the most intense anoxia resulted in deposits with a good source-rock potential during intervals and in areas of condensed deposition. Therefore, only locally some thicker successions of sediments with good source rock potential occurred. However, accumulated thickness of good source rock potential can reach more than 500 m in distinct local areas.

 

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

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