Indication of methane movement from petroleum reservoir to surface, Löningen oilfield, NW-Germany

An existing method of detecting microgas seeps was applied in a free soil gas investigation above the Löningen oilfield, NW-Germany. Simple gas surveying was combined with sampling of soil gases for gas-chromatographic treatment and examination of the carbon isotopic composition. A zone of biological gas formation and an area of thermocatalytic methane were discovered, the latter probably caused by migration from the subsurface petroleum accumulation.The methane in the reservoir differs in isotopic composition depending on its origin: gas from gascap or petroleum. Methane dissolved from petroleum is depleted in ¹²C by about 5 ppt compared to methane from the gascap. In relation to the reservoir a general ¹³C-enrichment between 2.5 and 4.6 ppt is observed in the soil gas methanes which is supposedly due to isotopic fractionation during migration. The two species of reservoir methane are still distinguishable on the surface by their different composition. Gaseous hydrocarbons from C₂ to C₄ seem to be restricted by migration. Bacterial oxidation appears not to affect the isotopic composition of the thermocatalytic methane seriously.