Analysis of P-wave and S-wave Data on Near-surface Potential Hydrocarbon Indicators

The combined use of compressional (P-waves) and shear-waves (S-waves) seismic reflection data is appealing for providing insights into the prospectivity of petroleum systems because P-waves are sensitive to fluids and S-waves are not. This analytical method is tested on two potential hydrocarbon indicators located in a prospective area of the St. Lawrence Platform in Eastern Canada: a zone where P-bedrock reflections are blanked and a zone where high-amplitude P-reflections within the unconsolidated sediments are pulled down. For the blanked zone, S-reflections remain bring suggesting that gas is migrating vertically through fractured gas-prone shales. This is supported by P-velocity analysis showing a marked decrease of velocity over the blanked zone. The high-amplitude pulled down P-reflections appear mostly flat on S-wave data but S-amplitudes show a vertically disturbed zone sharply bordered by bright reflections. Velocity analyses indicate a drastic and slight decrease for P- and S-wave data respectively. P-anomalies are believed to result from gas venting through fractured shales into the unconsolidated sedimentary column. However, S-amplitude anomalies are probably due to local changes of the physical properties (e.g. density) of the matrix induced by fluid migration.