Abstract
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.
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