Tectonic subsidence history and source-rock maturation in the Campos Basin, Brazil

Abstract

The Campos Basin is a petroleum-productive, marginal sag basin along the Brazilian margin. It contains a stratigraphic sequence recording lithospheric extension and rift tectonics developing to a fully evolved post-break-up setting. We present a combined approach using subsidence analysis and basin history inversion models to make predictions on source-rock maturation. The classical uniform stretching model does not account for the observed tectonic subsidence , and so we consider the thermal and subsidence implications of incorporating different events of magmatic underplating. The post-rift is characterized by one major phase of post-rift thermal subsidence: changes in sediment supply and load, as well as halokinetic movements, cause deviations from normal thermal subsidence and relaxation. The dimensions of these events have been examined and quantified. Our best-fit forward model is tested against measured borehole temperature (BHT) data. Results show a systematic overestimation of present-day temperatures in the shallow-water wells. Including the effects of hydrothermal convection by raising the conductivity of the top part of the stratigraphic sequence results in an almost perfect fit between predicted and measured BHTs in the shallow-water wells. Based on these forward models, we predict that the lacustrine shales of the syn-rift Lagoa Feia Fm. could be mature to generate oil and/or gas over a larger area than previously expected. Furthermore, our results suggest that the early post-rift Macaé Fm. is locally mature to generate oil, and that the middle/late post-rift Carapebus/Ubatuba Fm. shales are likely to generate oil as well, if organic-rich facies are indeed present. In light of the most recent pre- and post-salt discoveries made near established oil fields in the Campos Basin, our results support the idea that many more such accumulations may be found.