Hydrocarbon sources of cold seeps off the Vesterålen coast, northern Norway

•Active hydrocarbon seeps in the Hola area on the northern Norwegian shelf•Ascending methane produces strong heterogeneity in sediment pore water profiles.•Isotopic data suggest a predominantly thermogenic origin of the Hola gases.•The contribution of microbial methane varies in the studied gravity cores.•A possible late Jurassic to early Cretaceous source rock is suggested.

We investigated active methane seeps in a water depth of 200 m in the Hola area off the coast of Vesterålen, northern Norway, to assess (1) hydrocarbon sources, (2) migration pathways and (3) the influence of hydrocarbon seepage on sediment pore water and water column chemistry. The seepage area is characterised by the presence of gas flares in the water column as revealed by hydro acoustic surveys and elevated methane concentrations of up to 42 nM ca. 5 m above the seafloor. Pore water analyses of three gravity cores from the seepage area show varying depths of the sulphate-methane-transition zone (SMTZ) between 80 cm and > 250 cm indicating spatially heterogeneous methane ascent. The isotopic composition of methane (δ13C from − 40‰ to − 63‰ and δ2H from − 191‰ to − 225‰) and δ13C depth profiles of methane and dissolved inorganic carbon show that the hydrocarbons are predominantly of thermogenic origin, consistent with δ13C values of C2 to C4 hydrocarbons. Isotope data also indicate considerable biodegradation of propane. Seismic profiles from the study area reveal major faults and steeply dipping unconformities between the basement and overlying Mesozoic sedimentary rocks. We propose that these act as migration pathways for the hydrocarbons from late Jurassic to early Cretaceous source rocks.