The geochemical characteristics of the hydrate-bound gases from the Nyegga pockmark field, Norwegian Sea

The Nyegga pockmark field located south east of the Vøring Plateau, Norwegian Sea, was investigated on the UoB Cruises GS07-148 (2007) and GS08-155 (2008). Remotely operated vehicle (ROV) monitoring and successful recovery of gas hydrate give insights to the origin of the hydrocarbon gases in the complex pockmark feature named G11. Isotope values of methane (δ13C ranging from −72.4 to −66.2‰ and δD from −202.0 to −198.0‰) in the hydrate bound gas suggest that the methane is microbially produced and originates from CO2 reduction. It is proposed that the ethane from the gas hydrate has mixed thermogenic and microbial contributions, which is supported by the fact that the gas hydrate also contains propane and isobutane. The composition of the hydrate bound gas thus leads to small inclusions of structure II (sII) in the hydrate while the major part is structure I (sI). The isotope data and gas composition together with the geological setting suggest that the hydrate bound gas originated predominantly from a free gas system located beneath the gas hydrate stability zone and accumulated under glaciated debris formations. The free gas system is mainly microbial methane from in situ production. However, a contribution of thermogenic gas from deeper polygonal faults to the free gas system seems evident from the composition of the gas. The ROV monitoring strengthens previous suggestions that the fauna in the complex pockmark G11 are supported by ongoing micro-seepage.