The occurrence and potential origin of asphaltite in bedrock fractures, Forsmark, central Sweden

Abstract

Occurrences of asphaltite in fractures in the Svecofennian basement have been documented in drill cores from the site investigation for a deep repository for nuclear waste in Forsmark, central Sweden. The carbonaceous material has been found at depths down to 124 m and appears as hardened black fracture fillings. The δ13C values of the asphaltite are relatively uniform with values between -29.5 and -30.1‰ (PDB) indicating an organic origin. These results are consistent with previous analyses of asphaltite occurrences in Sweden. One sample collected from a fracture in the Ordovician limestone in Kinnekulle, south-western Sweden was analysed for comparison and shows a similar δ13C signature (-29.5‰). Biomarker analyses of the asphaltite samples confirm an organic origin and the organic-rich early Paleozoic Scandinavian Alum Shale is suggested as the source rock. This is supported by comparison of the asphaltite with published biomarker and carbon isotopic data from the Alum Shale. Downward migration of hydrocarbons into the crystalline basement due to the presence of an overpressured basin is a possible mechanism for the emplacement of the asphaltite in the bedrock fractures. It is suggested that the heat source responsible for the migration of the asphaltite was the overburden of the Caledonian foreland basin.