ALBERT

Description: We investigated active methane seeps in a water depth of 200 m in the Hola area off the coast of Vesteralen, northern Norway, to assess (1) hydrocarbon sources, (2) migration pathways and (3) the influence of hydrocarbon seepage on sediment pore water and water column chemistry. The seepage area is characterised by the presence of gas flares in the water column as revealed by hydro acoustic surveys and elevated methane concentrations of up to 42 nM ca. 5 m above the seafloor. Pore water analyses of three gravity cores from the seepage area show varying depths of the sulphate-methane-transition zone (SMTZ) between 80 cm and 〉 250 cm indicating spatially heterogeneous methane ascent. The isotopic composition of methane (d13C from - 40per mil to - 63per mil and d2H from - 191per mil to - 225per mil) and d13C depth profiles of methane and dissolved inorganic carbon show that the hydrocarbons are predominantly of thermogenic origin, consistent with d13C values of C2 to C4 hydrocarbons. Isotope data also indicate considerable biodegradation of propane. Seismic profiles from the study area reveal major faults and steeply dipping unconformities between the basement and overlying Mesozoic sedimentary rocks. We propose that these act as migration pathways for the hydrocarbons from late Jurassic to early Cretaceous source rocks.

Description: With the recently recovered organic-rich sediments of early Tertiary age from the Lomonosov Ridge by the Integrated Ocean Drilling Program (IODP) Leg 302, first direct source rock data are now attainable for the central basins of the Arctic Ocean. Results on seismic interpretations and sedimentological, micropalaeontological and organic geochemical data from Leg 302 have been published (Backman et al. 2006, Brinkhuis et al. 2006, Jokat et al. 1992, Jokat 2005, Knies et al. 2007, Moran et al. 2006, Sluijs et al. 2006, Stein et al. 2006, Stein 2007) and provided the framework for the first quantitative assessment of source rock quality and distribution of the Palaeogene sediments in the central Arctic Ocean.Three major objectives were addressed in the modelling study: (1) Application of new proxy data for input in the source rock modelling; (2) Improvement of the understanding of the major processes and the Palaeogene depositional environment, testing of different conceptual scenarios; (3) Quantitative assessment of the source rock potential in an area stretching over the Lomonosov Ridge into the adjacent Amundsen Basin.The modelling results can be summarized as follows: An approximately 80 m thick sedimentary sequence of continuous good to very good source rock quality is identified along a 75 km long transect across the Lomonosov Ridge. Even though in-situ generation of hydrocarbons is rather unlikely as overburden (~200-250 m) and thermal maturity are too low, Rock Eval data indicate an early generation or impregnation of the sediments with hydrocarbons (Stein 2007). This is supported by our modelling approach showing that the initial richness of the sediments might have been higher then measured HI values indicate. However, burial history and thermal modelling revealed that an overburden of at least 1000 m is necessary to start generation at place. Based on that, we tested if lateral equivalents of the rich Lomonosov Ridge sequence may continue into the adjacent Amundsen Basin where an overburden of ca. 1000 1200 is indicated on the seismic and a higher temperature regime can be expected due to the vicinity to the Gakkel Ridge spreading centre.References:Backman, J., K. Moran, D. B. McInroy, L. A. Mayer, and the Expedition 302 Scientists (2006), Proceedings of the Ocean Drilling Program, Vol. 302, Ocean Drill. Program, College Station Tex. (Available at http://www.ecord.org/exp/acex/vol302/104/104_.htm).Brinkhuis, H., et al. (2006), Episodic fresh surface waters in the Eocene Arctic Ocean, Nature, 441, 606609.Jokat, W., Uenzelmann-Neben, G., Kristoffersen, Y., Rasmussen, T. (1992), ARCTIC91: Lomonosov Ridgea double sided continental margin. Geology 20, 887890.Jokat, W. (2005), The sedimentary structure of the Lomonosov Ridge between 88oN and 80oN. Geophys. J. Int. 163, 698-726.Knies, J., Mann, U., Popp, B., Stein, R., Brumsack, H. J. (2007), Nitrogen cycling in the Arctic Ocean over the past 60 Ma; Palaeoceanography, in review.Moran, K., et al. (2006), The Cenozoic palaeoenvironment of the Arctic Ocean, Nature, 441, 601 605.Sluijs, A., et al. (2006), Subtropical Arctic Ocean temperatures during the Palaeocene Eocene thermal maximum, Nature, 441, 610613.Stein, R., B. Boucsein, and H. Meyer (2006), Anoxia and high primary production in the Paleogene central Arctic Ocean: First detailed records from Lomonosov Ridge, Geophys. Res. Lett., 33, L18606, doi:10.1029/2006GL026776.Stein, R. (2007), Upper Cretaceous/lower Tertiary black shales near the North Pole: Organic-carbon origin and source-rock potential, Marine and Petroleum Geology 24, 6773.