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Middle Triassic Source Potential and Lateral Variation in Organic Facies, Southwestern Barents Sea and Svalbard

Abstract

Shale and mudstone beds of the Steinkobbe and Kobbe formations from the Norwegian Barents Sea, and the Botneheia Formation from Svalbard have been geochemically studied to determine their hydrocarbon generation potential and lateral variations in organic facies. The sample set covers cores from various locations in the Barents Sea such as the Svalis Dome, Bjarmeland Platform, Nordkapp and Hammerfest basins, and outcrops from Svalbard. The results show that the hydrocarbon generation potential and organic facies of the Middle Triassic-dominated beds are highly variable, laterally, from one location to another. The Total organic carbon (TOC) contents for the Botneheia and Steinkobbe formations from Svalbard and Svalis Dome, respectively, are generally high and associated with high Rock-Eval S2 values and hydrogen index (HI) of 300 to 600 mg HC/g TOC for most samples, indicating Type II kerogen and are generally considered to be potential sources of oil. By contrast, further to the east in the Bjarmeland Platform and Nordkapp Basin, where the formation name changes to Kobbe, the TOC, S2 and HI values decrease significantly, indicating lean source and is generally gas prone. In the Hammerfest Basin, the formation has good potential with a mixture of Type III and Type II kerogen, and can generate gas and some oil. Tmax and molecular geochemical data indicate that all samples analysed are immature to early oil window mature. This suggests that the observed lateral variation in organic content and kerogen type, and hence in the generation potential cannot be attributed to variations in maturation. Molecular data from gas chromatography show a wide lateral variation in organo-facies signatures, suggesting differences in paleo-depositional environments and/or input material. During the Middle Triassic, a more reducing environment prevailed in Svalbard (Botneheia Formation) and Svalis Dome (Steinkobbe Formation) compared to the Nordkapp Basin and Bjarmeland Platform where the sediments are dominated by Type III organic matter, suggesting more coastal setting. The terrigenous effect on the organic matter in the Kobbe Formation in these eastern areas is furthermore clear from the gas chromatograms which are dominated by long-chain n-alkanes with high odd predominance. In the Hammerfest basin, the condition varies from somewhat dysoxic to anoxic marine. This variation in organic facies is consistent with the palaeogeography of the Barents Sea and Svalbard.