Processing and interpretation of the Svyatogor 2016 high-resolution P-Cable 3D seismic dataset. Investigating the dynamics of a sub-seabed gas hydrate system with a potential abiotic methane source

Abstract

A new high-resolution P-Cable 3D seismic dataset was acquired in July 2016 targeting a seafloor pockmark cluster at the northern end of Svyatogor Ridge, offshore west Svalbard. The processing and interpretation of this dataset formed the primary focus of this thesis. The seismic processing sequence was designed to enhance the signal-to-noise ratio of the data while preserving the useful signal bandwidth and was implemented using the RadexPro 2016.3 software package. For example, burst-noise filtering allowed useful signal to be extracted from channels that would otherwise have been discarded, improving the overall trace density of the dataset. The suppression of bubble effects, ghost waves and random noise lead to a significant improvement in the useable bandwidth. In addition, “high-resolution static” correction proved an important means of improving reflector continuity and suppressing acquisition footprint noise caused by tides and streamer depth variations. Considerable effort was also spent on improving the receiver geometry based on a least squares type inversion of direct wave arrivals and produced a noticeable, if subtle, improvement in reflector continuity. This method of assigning geometry also has a potential application in 4D (time-lapse) seismic processing where the small-magnitude but extensive quantitative differences in amplitude, compared to the conventional method of assigning geometry, may become more critical.

Interpretation of the Svyatogor 2016 3D dataset indicates that the gas-hydrate, free-gas system has been relatively stable with respect to leakage and the seafloor pockmarks have been inactive and infilling for some time. However, significant evidence of paleo fluid migration was observed and the continued re-opening of fractures at fault tips or fault-segment junctions may be an important mechanism facilitating focussed, vertical fluid migration. The episodic fluid flow regime is postulated to be driven by 1) gas migration into the system along faults, probably as a dissolved phase 2) gas-hydrate formation at the base of the gas-hydrate stability zone (BGHSZ) produces a hydrate-cemented seal that results in a structurally enhanced trap 3) continued gas migration from depth and recycling of hydrate at the BGHSZ leads to accumulation of a free-gas phase contained beneath the BGHSZ 4) overpressure builds beneath the BGHSZ as gas charge continues eventually resulting in episodic gas release triggered in combination with dynamic stresses from earthquakes. The degree to which free-gas zone overpressure or external tectonic stresses control fault-slip is difficult to differentiate, but free-gas zone overpressures may significantly increase the slip tendency of faults at Svyatogor ridge.

It appears unlikely that the Svyatogor gas-hydrate/free-gas system could have been supplied by in-situ methane production alone. It remains difficult to conclusively rule out the contribution of a thermogenic source, but this would likely be dependent on lateral migration pathways that were not studied in detail in this thesis. However, it does appear plausible that the observed free-gas zone could have been charged by abiotic methane migrating along axial detachment faults during the period of active sedimentation on Svyatogor ridge assuming a similar flux rate to that reported by Cannat et. al. (2010) for the Rainbow hydrothermal field. A small elongated pockmark located above the lateral tip of an underlying fault may be associated with the most recent episode of seabed gas leakage. It is therefore considered the most promising target for future sediment coring aiming to recover gas or hydrate bearing samples for geochemical analysis to potentially resolve the methane source question.