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Petrobras Research Center — Cenpes, Ilha do Fundão Q-7, 21949-900-Rio de Janeiro, Brazil claudioc{at}cenpes.petrobras.com.br
The South American plate is now in horizontal compression and shortening. This is shown by stress data compilations, intraplate stress field numerical models and space-based geodetic results. Consistent with the compressive scenario, analyses of leak-off and hydraulic fracturing data indicate that the maximum principal stress is horizontal for most Brazilian basins. The observed compression/shortening is probably mostly due to the convergence of the South American and the Nazca plates and the divergence of the South American and the African plates.
Plate-wide deformation related to the Andean tectonics has been evidenced by analyses of integrated visualizations of available plate-scale information on tectonics, continental geology, topography/bathymetry, seismicity, stresses, active deformation, residual isostatic anomalies, fission track analyses, and seismically derived Moho depths and P and S wave velocity anomalies. Here, some results of these analyses are presented and some implications of the ongoing compression for petroleum exploitation and exploration are discussed.
A conceptual model for the plate-wide deformation is presented and numerically tested using elasto-plastic rheologies. The model states that in response to the compression, the lithosphere as a whole (or only the crust if thermal gradients are high enough) tends to fold and fracture. This tendency is stronger during peaks of the Andean orogeneses. The forming antiforms are responsible for uplift along the erosional basin borders, whereas the forming synforms are sites of continental sedimentation, at basin centres. The denudation of sedimentary covers promotes the exhumation of deeper and deeper rocks, cropping out at the foot of retreating scarps. Consequently, the erosional borders of the basins form local topography highs with respect to the adjacent basement.
In exploration, neotectonics is usually simply disregarded, at least with regard to the exploratory activities developed in ‘passive’ margin basins. Nevertheless, we infer that neotectonics has an important role in the distribution and preservation of petroleum accumulations, since: (i) accumulations are ephemeral in a geological time scale, being strongly dependent on the seals’s fine geometry and biodegredation; (ii) a strong positive correlation exists between permeability anisotropy and maximum horizontal stress; (iii) the source rocks of the most important Subandean and intraplate petroleum systems are still in the oil generation window; and (iv) the disruption of ‘kitchens’ of generation is a positive factor to primary migration. This inference seems to be confirmed, since the most important South American petroleum accumulations are found along the actively deforming border between the South American and Caribbean plates. Indeed, the most important accumulations in marginal basins are found in the southeastern Brazilian margin (the Campos basin), which has been deformed the most with respect to other margins during the Cenozoic, and continues to be the most seismically active.
