GEOHAZARDS IN QUEEN CHARLOTTE BASIN, OFFSHORE BRITISH COLUMBIA: GEOPHYSICAL EVIDENCE FOR SHALLOW GAS ORIGINATING FROM A DEEP SOURCE

Seafloor geohazards pose a potential threat to emplacement and operation of infrastructure associated with petroleum exploration, telecommunications and other offshore developments. This paper describes seafloor geohazards resulting from gas in shallow sediments at sites within Queen Charlotte Basin (QCB); a sedimentary basin situated offshore British Columbia, western Canada. The basin underlies the continental margin which was covered in glacial ice during the Pleistocene. Significant petroleum reserves are estimated to be present within QCB and identification of potential geohazards is necessary to understand the feasibility of any future seafloor development or installations.

Numerous surficial and shallow sediment features related to both ice cover and shallow-gas have been identified in high-resolution multi-channel seismic, Huntec deep-towed seismic and multi-beam bathymetry data that were acquired over a 10km by 10 km grid in an eastern part of QCB. Based on limited data this area was previously inferred to contain gas in the shallow sediments; however, its source was not determined. Pockmarks and seafloor mounds are visible in the multi-beam data; small-scale folds and vertical blank zones are observed in the Huntec data. The multi-channel seismic data show that one seafloor mound lies directly above a shallow fault that offsets an asymmetric fold. The remaining mounds lie in close proximity to asymmetric folds that are faulted. Bright spots are observed along some fault traces and within a cut and fill sequence at two-way travel times between 0.42 and 0.750 seconds. These observations indicate that gas has been migrating upward through the rock column and through the seafloor. Based on the position of the bright spots gas must have originated from a source in excess of 700 meters depth, making it most likely thermogenic. Three of the six mounds in the study area show surficial pockmarks in the multibeam data. The remaining mounds show no surficial pockmarks but two of these display high-amplitude base reflectors in the seismic data. This suggests that they were likely gas-charged at the time of surveying.