Ocean bottom seismometer investigations in the Ormen Lange area offshore mid-Norway provide evidence for shallow gas layers in subsurface sediments

A multi-component Ocean Bottom Seismometer (OBS) survey in the Ormen Lange area of the Storegga Slide constrained the existence of free gas and possibly gas hydrates in the shallow subsurface. The three investigated areas (A, B, C) lie in close vicinity to the slide scar above one of the largest deep-water gas reservoirs on the mid-Norwegian margin. Generally, P-wave and S-wave velocities are high compared to average velocities at a given burial depth due to an exposure of deeper sediments as a consequence of sediment mass movement by the Storegga slide. Indications for the presence of gas within the sediments exist for two of the three investigated areas. The gas accumulates beneath less permeable layers of glacigenic debris flow deposits. Average gas concentration of pore space in both areas is 0.9% (area A) and 0.15% (area B). The geophysical data do not allow a conclusive answer about the occurrence of gas hydrates. Their presence might be masked by high-velocity debris flow deposits, which occur in the subsurface. Nevertheless, gas hydrate concentrations of pore space have been estimated to about 9% in area A and 7% in area B.