Hydrocarbon gases in Palaeogene volcanic rocks from the Lopra-1/1A well, Faroe Islands

• Main Author: Troels Laier
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Faroes; Lopra-1/1A; volcanics hydrocarbon gas; isotopes; headspace methane; cuttings; Faroe Islands; Lopra
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.540.699; http://www.geus.dk/publications/bull/nr9/nr9_p79-90.pdf

Hydrocarbon gases were monitored in the drilling fluid during deepening of the Lopra-1 well from 2178–3565 m, in which thermogenic, methane-rich gases had been found previously. The mud gas con-centration, up to 105 ppm of methane, was generally higher in the hyaloclastite sequence, 2470 m – terminal depth (TD), than in the overlying lavas of the lower basalt formation. The highest concen-trations of mud gas in the lower basalt formation were associated with the more porous tuffaceous zones, whereas no simple relationship could be established between measured mud gas concentrations and porosity of the hyaloclastic rocks, which showed less marked porosity variations than the lavas. Chemical (C2+ < 1%) and isotopic (δ13C1: –34 to –39‰) compositions of seven samples of mud gas collected at peak gas concentrations between 2657 m and 3442 m compare well with those of the hydrocarbon gases which had been seeping more or less continuously into the existing well since 1983, suggesting a common origin of the gases. Headspace methane concentrations measured in 135 canned samples of cuttings were scattered between 10 ppm and 6 × 103 ppm, with the exception of six samples from a short interval, 2685– 2745 m, which showed consistently high values> 104 ppm. No particularly gas-rich zones were indi-cated, however, by the mud gas, nor was any significant change in lithology noted for this interval. It