The logs and drilling records of exploration wells often can provide enough information to characterize the stress regime of a sedimentary basin, provided its geomechanical framework is understood. In the Aquitaine Basin, there are not many measurements for estimating subsurface in-situ stress magnitudes, but a significant number of wells have been logged suitably to provide stress directional data. Enough stratigraphic and structural information is available to assess the geomechanical response of the basin fill compared to that of the underlying basement. This study utilizes breakout ovalization analysis of 55 wells. The resulting horizontal principal stress orientations show no clearcut directional homogeneity. Around Bordeaux, σH exhibits a consistent NW-SE orientation. Towards the Pyrenees, the situation becomes less definitive. NE-SW σH orientations are widely present, but NW-SE σH orientations also occur. High-grading the data does not improve the resolution.This widespread lack of directional homogeneity, and the frequency with which ovalization was found to be aligned with well deviation, suggest that horizontal stress anisotropy is weakly developed in much of the Aquitaine Basin today. It is thought that this condition has arisen because most of the rocks in which stress orientations were measured are geomechanically detached from “basement” and cannot transmit a crustal stress signature. The detachment agent is believed to be early Mesozoic salt. As a result, the propagation directions of hydraulic fractures may vary considerably from place to place. It is also likely that preferred orientations for open fractures will not be consistent regionally.
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- A Reinterpretation of the Stress Regime of the Aquitaine Basin, Southwestern France, and Implications for Hydrocarbon Recovery
J. S. Bell
- Springer Berlin Heidelberg