However, these conditions are offset by the following:
• Mechanical characteristics below 1500 m depth: according to Grollimund et al. (2000), sediments are not sufficiently consolidated to support stress, which is in keeping with the porosity values over 27% and permeability over 1D in the Utsira Formation
• The question of whether faults actually exist in and above the Utsira Formation. A priori, the apparent structures are due to mud volcanoes and intraformational faults are more likely to affect the underlying Oligocene sediments (as in the Troll field
northeast of Sleipner)
As a rule, the injection of the CO2 in sands of the Utsira Formation should not trigger any measurable microseismicity except in impermeable or semi-permeable shale lenses that block the rise of the CO2 toward the top of the formation. This could be an indication of the presence of CO2 insofar as it would allow the detection of the conduits used for CO2 migration. The start of this passage still has to be established in order to define the advance of the CO2 front. Similarly, microseismicity may appear at the top of the formation. This could be an evidence of the initiation of open fractures that could subsequently give rise to leakage.