Shallow hydrocarbon gas in the northern Jutland-Kattegat region, Denmark

Carbon and hydrogen isotopic compositions and chemical analyses are reported for gas samples collected from 11 gas wells, two gas seeps and 8 water wells in Vendsyssel, northern Jutland, and the island of Læsø in Kattegat, Denmark. Tue chemical composition shows methane-rich gas, poor in heavy gaseous hydrocarbons with a ½+ concentration less than 0.01 % . Tue methane is relative ly depleted with respect to the heavy isotopes, i.e. b13C: -63.6%o to -89.2%o b2H: -177%o to -2288%.. Tue data fall within the range which is generally considered to characterize microbial gas formed via CO2 reduction. Toere is no evidence of contribution of gas from thermogenic sources. Tue gas is known from a large number of gas wells and water wells in Upper Pleistocene marine deposits and from submarine gas seeps in the Kattegat. Tue gas field has a NW to SE areal extent subparallel to the northem limit of the Danish sub-basin and the major fault-systems of the Fennoscandian border zone. This distribution coincides with the occurences of a tectonic depression in the pre-Quaternary surface which primarily is filled in with Eemian and Weichselian marine sediments. Tue gas most likely derives from degradation of organic material in the Upper Pleistocene marine sediments themselves and is subsequently trapped in restricted reservoirs of sand and grave!.