Orientação e magnitude de tensões na bacia potiguar: implicações para evolução de bacias em margens passivas

Abstract

This study presents new stress orientations and magnitudes from the Potiguar basin in the continental margin of Brazil. We analyzed breakout and drilled induced fractures derived from resistivity image logs run in ten oil wells. We also used direct Shmin measurements determined from hydraulic fractures and rock strength laboratory analysis. In addition, we compared these results with 19 earthquake focal mechanisms located in the crystalline basement. We observed that stress directions and magnitudes change across the basin and its basement. In the basin, the SHmax gradient of 20.0 MPa/km and the SHmax/Shmin ratio of 1.154 indicate a normal stress regime from 0.5 to 2.0 km, whereas the SHmax gradient of 24.5MPa/km and the SHmax/Shmin ratio of 1.396 indicate a strike slip stress regime from 2.5 to 4.0 km. The deeper strike-slip stress regime in the basin is similar to the regime in the basement at 1-12 km deep. This stress regime transition is consistent with an incipient tectonic inversion process in the basin. We also noted that the SHmax direction rotates from NW SE in the western part of the Potiguar basin to E W in its central and eastern part, following roughly the shoreline geometry. It indicates that local factors, as density contrast between continental and oceanic crust and sediment loading at the continental shelf influence the stress field. The concentration of fluid pressure in faults of the lowpermeability crystalline basement and its implications to establish a critically stressed fault regime in the basement is also discussed