Surface and subsurface signatures of gas seepage in the St. Lawrence Estuary (Canada): Significance to hydrocarbon exploration

The seabed of the St. Lawrence Estuary is characterized by many fluid releasing features. On multibeam bathymetric images these features correspond to crater-like depression (pockmarks) predominantly found on the northwestern shoulder of the ∼300 m deep Laurentian Channel, as well as on the channel floor. Aligned pockmarks, which define segments up to 12 km long, are frequent in the Laurentian Channel, whereas they are preferentially associated with submarine landslides on the northwestern shoulder of the channel. On high-resolution seismic profiles, pockmarks found in the Laurentian Channel are characterized by seismic chimneys that may be traced down to the autochthonous Paleozoic rocks (St. Lawrence platform), suggesting a thermogenic origin for the gas. On the northwest shoulder of the Laurentian Channel, the seismic signature of pockmarks does not extend downward to the reflector that corresponds to bedrock (St. Lawrence platform and/or Grenvillian province) suggesting, together with high sedimentation rate, a biogenic origin for the gas. These results are discussed in the light of the data collected in the onshore parts of the St. Lawrence platform and suggest the presence of a mature hydrocarbon source. The conclusions are supportive arguments for the hydrocarbon prospectivity of both the onshore and offshore segments of the Paleozoic autochthonous domain (St. Lawrence platform).