Sediment porosity at the Scanner Pockmark in the North Sea from controlled source electromagnetic data
Romina A. S. Gehrmann, Giuseppe Provenzano, Christoph Böttner, Naima K. Yilo, Gaye Bayrakci, Hector Marin-Moreno, Jonathan M. Bull, Timothy A. Minshull and Christian Berndt
As part of the EU Horizon2020 ‘STEMM-CCS’ project, controlled source electromagnetic (CSEM) and seismic data were acquired in 2017 at the Scanner Pockmark in the UK sector 15/25 of the North Sea, which is actively venting methane gas, to contribute to the evaluation of risk from potential fluid pathways to the sequestration of carbon dioxide in geological formations. We will present some preliminary results of the deep-towed CSEM study and relate electrical resistivities to sediment properties such as porosity and gas saturation.
Sub-vertical fluid migration pathways may occur where pressurized fluids penetrate upwards. These structures are often identified as ‘chimneys’ on reflection seismic profiles with distinctive ‘blanking’ (lack of reflection) within the chimney. If free gas is present, ‘bright spots’ (high reflectivity with polarity reversals) occur at impermeable layers or ‘pockmarks’ (sub-rounded depressions) on the seafloor where gas is released into the water column.
The CSEM data presented were acquired with a deep-towed source and two towed receivers measuring the potential difference of the electric field. The data were processed in the frequency domain and the electrical resistivity structure was inferred with a 2D regularized inversion algorithm (MARE2DEM). The electrical resistivity in marine sediments depends strongly on the salinity and connectivity of pore water (low resistivity) and on the concentration of hydrocarbons such as free gas (high resistivity). Therefore, the resistivity of active fluid migration pathways will contrast with that of shallow gas accumulations and the background resistivity of the glacio-marine sediment sequence, which is mainly composed of clays with some silts and sands.
To estimate porosities to about 200 m below the seafloor, we use the empirical Archie’s law and calibrate Archie’s coefficient using physical properties measured with the multi-sensor core logger on gravity cores and sediment cores from the British Geological Survey Rock Drill 2 rig. Geological horizons identified on reflection seismic data are used as constraints in the resistivity model. The porosity decreases with depth due to compaction.