Detection of near-surface hydrocarbon seeps using P- and S-wave reflections

Abstract

The combined use of P- and S-wave seismic reflection data is appealing for providing insights into active petroleum systems because P-waves are sensitive to fluids and S-waves are not. The method presented herein relies on the simultaneous acquisition of P- and S-wave data using a vibratory source operated in the inline horizontal mode. The combined analysis of P- and S-wave reflections is tested on two potential hydrocarbon seeps located in a prospective area of the St. Lawrence Lowlands in Eastern Canada. For both sites, P-wave data indicate local changes in the reflection amplitude and slow velocities, whereas S-wave data present an anomalous amplitude at one site. Differences between P- and S-wave reflection morphology and amplitude and the abrupt decrease in P-velocity are indirect lines of evidence for hydrocarbon migration toward the surface through unconsolidated sediments. Surface-gas analysis made on samples taken at one potential seeping site reveals the occurrence of thermogenic gas that presumably vents from the underlying fractured Utica Shale forming the top of the bedrock. The 3C shear data suggest that fluid migration locally disturbs the elastic properties of the matrix. The comparative analysis of P- and S-wave data along with 3C recordings makes this method not only attractive for the remote detection of shallow hydrocarbons but also for the exploration of how fluid migration impacts unconsolidated geologic media.