Controlled Source Electromagnetic (CSEM) and Seismic Interpretation of Fluid Escape Features at the Vestnesa Ridge, West Svalbard Continental Margin

Abstract:

Numerous seafloor pockmarks with episodic methane venting have been reported at the Vestnesa Ridge (1200-1350 mbsl), a sediment drift in the West Svalbard continental margin gas hydrate province. What is driving the free gas through the hydrate stability zone (HSZ) to the seafloor pockmarks in the study area is not well understood, although over-pressured gas, water limitation and high salinity caused by the formation of hydrate are likely to be contributory factors. Knowledge of the fluid constituents within the acoustic chimney imaged by seismic reflection data could help in understanding how methane gas migrates through the HSZ. Electrical resistivity obtained from controlled source electromagnetic (CSEM) data offers improved information on fluid saturation, which helps predict the saturations of hydrate, gas, water and its salinity, within the fluid migration pathways. With the objective of a joint interpretation, we acquired coincident seismic reflection and CSEM data on the Vestnesa Ridge during 2011 and 2012. The CSEM survey was conducted over an active pockmark using a 100-m horizontal electric-dipole-source, 9 ocean-bottom electric-field (OBE) receivers and a towed tri-axis electric-field receiver. Preliminary interpretation of CSEM and seismic reflection data suggests high concentrations of free gas (40-65% pore saturation) within the main chimney, with a possible hydrate plug underneath the seafloor pockmark. High resistivity anomaly and high percentage of resistive fluids predicted within the chimney indicate water limitation to be an important factor enabling free gas migration to the seafloor. A deeper gas reservoir (>30% pore saturation) can also be observed, which appear to feed free gas into the HSZ. The rising gas from the deeper reservoir forms a heterogeneous distribution of hydrates (~10% pore saturation) and also feeds the fluid-escape features.