Anisotropy at the Lacq hydrocarbon field (France) from shear‐wave splitting

Abstract

Three‐component seismograms from local earthquakes located beneath the reservoir of the Lacq hydrocarbon field have been analysed for shear‐wave splitting. Shear‐waves recorded at the station USI located on top of the hydrocarbon reservoir show a clear splitting in an average polarization direction N30°–40°E which is different from the present‐day local and regional stress direction. This direction seems approximately parallel to the Tertiary period stress field in this area which corresponds to the measured main crack directions in the gas reservoir. At station LDB, located outside of the anticlinal structure, no splitting was detected but due to a poor signal‐to‐noise ratio, only nine events were available and anisotropy could be absent or obscured. For a 250 m thick oil or gas field the velocity anisotropy is estimated at about 6–10%, which is in the same range as values obtained from VSPs in other hydrocarbon fields. Monitoring induced‐seismicity by three component stations could be a useful tool to characterize hydrocarbon fields in term of the rock's average properties in a large volume.