Indicators for fluid pathways at the Scanner Pockmark in the North Sea from electromagnetic data

As part of the EU FP7 'STEMM-CCS' project, controlled source electromagnetic (CSEM) and seismic data were acquired in 2017 at the Scanner Pockmark in the UK sector 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. We will present the preliminary results of the CSEM study and relate electrical resistivities to physical properties of the sediment such as porosity and permeability, as well as to gas saturation.

Sub-vertical fluid 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) occur at impermeable layers or 'pockmarks' (depressions) on the seafloor where gas was released.

The CSEM data were acquired with a deep-towed source, and ocean bottom and 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 pathways will contrast with that of shallow gas accumulations and the background resistivity of the glacio-marine sediment sequence, composed mainly of clays with some silts and sands.

Preliminary results show a steady resistivity increase with depth in the background sediments, most likely related to compaction. The top of the chimney is more resistive than sediments at comparable depths elsewhere, which can be related to the presence of free gas. The results agree well with preliminary seismic and core data analysis.