Accumulation and seepages of biogenic gas in Northern Denmark

Gas from two shallow submarine seeps in northern Kattegat were found to consist of methane (94.8-98.6%), carbon dioxide (0.3-2.1%) and nitrogen (up to 3%). The stable isotopic ratios of methane, delta-C-13: -65.3 to -68.4 parts per thousand and delta-H-2: -168 to -191 parts per thousand, clearly show it to be biogenic. The gas most likely originates from the Late Pleistocene marine deposits in which gas with similar chemical and isotopic composition has been found by the authors in several on-land shallow (100-120 m) wells. These marine deposits were found to contain 0.6-0.9% total organic matter of mainly terrestrial origin (based on Rock-Eval data), deposited at high sedimentation rates. The processes of generation and accumulation of biogenic gas during the Late Pleistocene is found to parallel those known to be acting today in a major depocentre in northern Kattegat. Recent tectonic activity in the area has created fractures through which the gas escapes. With regard to sediment type, seeps in the seabed from the area are more likely to occur in coarser sediments rather than fine grained sediments, since consumption of methane by sulphate reduction is probably more important in the latter.