The present-day stress regime of the southwestern part of the Aquitaine basin, France, as indicated by oil well data

Abstract

In the Aquitaine Basin, there is little information available for estimating subsurface in situ stress magnitudes, but a small number of oil industry drilling leak-off tests suggest that σ h approaches 80% of σ v in magnitude. Horizontal principal stress orientations have been obtained by analysing drill ‘breakout ovalization’ in 55 wells. However, there is no clearcut directional homogeneity to the indicated stress trajectories and ovalization commonly parallels well deviation. Around Bordeaux, σ H exhibits a consistent NW-SE orientation. As one approaches the Pyrenees, the situation becomes less definitive. NE-SW σ H orientations are widely present and NW-SE σ H orientations also occur. ‘High-grading’ the data does not improve the picture significantly. This lack of directional homogeneity in areas adjacent to the Pyrenees is not completely understood. The data points could straddle a sinuous boundary between two stress provinces, but this explanation is not favoured. Alternatively, they may indicate that there is some local structural control of stress orientations. Whatever the cause, there are grounds for suspecting that horizontal stress anisotropy is weakly developed in this region today.