Stress magnitude and orientation in the Potiguar Basin, Brazil: Implications on faulting style and reactivation

We analyzed borehole breakout data and drilling-induced tensile fractures derived from resistivity image logs run at 10 oil wells to derive the orientation of the maximum horizontal stress S-Hmax from the Potiguar Basin in the continental margin of Brazil. Stress magnitudes are derived from density logs for the vertical stress, mini-frac tests for the minimum horizontal stress S-hmin, and rock strength laboratory analysis to estimate the S-Hmax magnitudes. We compared these results with the stress regime and S-Hmax orientation derived from nine earthquake series located in the crystalline basement, where seismicity is concentrated, and previous breakout data from the basin. In the basin, the S-Hmax gradient is 20.0MPa/km, and the S-Hmax/S-hmin ratio is 1.154, indicating a normal tectonic stress regime from 0.5 to 2.0km, whereas the S-Hmax gradient of 24.5MPa/km and S-Hmax/S-hmin ratio of 1.396 indicate a transition from a normal to strike-slip stress regime at 2.5 to 4.0km. The deeper stress regime in the basin is similar to that in the basement at 1-12km depth. This transition of the tectonic stress regime is consistent with an incipient tectonic inversion in the basin. We note that the S-Hmax orientation rotates from NW-SE in the western part of the Potiguar Basin to E-W in its central and eastern parts, roughly following the shoreline geometry, indicating that local features such as flexural stresses influence the local (scale<100km) stress pattern. We also conclude that the basement is critically stressed, but not the basin.