Regional studies of fluid inclusions in Paleozoic sediments from southern Scandinavia

Abstract

Fluid inclusions in diagenetic quartz and calcite have been investigated in order to determine minimum temperatures attained during diagenesis and obtain information on composition of porewaters in the Paleozoic sediments. Surface samples from Bornholm, Skane and Gotland and drill core samples from Frederikshavn 1, R0nde 1, N0vling 1, Abenra 1, Slagelse 1 and 0rslev 1 have been studied. Two major types of fluids are described: aqueous and hydrocarbon mixtures. The aqueous inclusions can roughly be divided into 3 different groups based on homogenisation temperatures with arithmetic means of 82°C (with a salinity around 25 eq. wt. % salts) and 136° and 175°C (with salinities around 5 eq. wt. % salts). The correlation of temperatures and salinities of porewater is tentatively explained by dehydration of clay minerals diluting the porewaters. The hydrocarbon inclusions, some of which fluoresce green-yellow, homogenize into the liquid phase at temperatures between -44° to + 138°C, and are only found in calcite veins from the Silurian of N0vling 1 and the Lower Ordovician Komstad limestone, Bornholm. With regard to the thermal history the data indicate that 1: major thermal maturation occurred in pre­Carboniferious times 2: all Cambro-Ordovician samples are post-mature with respect to hydrocarbon generation and 3: only small, dry gas potentials are present in the upper Silurian to Carbonifereous rocks.