Abstract: Regional Rules of Overpressure Rift Compartments in Campos Basin

Toledo, Marco A.S.; Verney J. Stumpf and Francisco F. Lima Neto - Petrobras/E&P

The Campos Basin presents a exploratory success in shallow waters (<250m) of the Jequia (123 -126 my) rift levels mainly confirmed in the 1970s. Some wells were characterized by hazards for drilling because of overpressure occurrences. In the deep water, a huge hydrocarbon province has been proved in the post-rift plays since the late 1980s, nevertheless the deep horizons of rift sections still remains on a reasonable prospecting immaturity. The aims here regard to understand the geological controls of pore pressure and geotemperatures distribution in order to help the exploration in rift section.

The data set of this work contains more than 200 exploratory wells widely spread in the basin, corresponding to 1,200 temperature records derived from wireline logs and 200 pressure records from wireline flow tests or drill steam tests. The rift reservoirs are bioclastic calcirudite and fractured basalt of basin substratum. The pressure gradient estimates refer to the datum at the sea level. Mudlog information was also used from the wells lacking direct pressure measurement.

Temperature records were corrected for cooling effect of well mud circulation, grouped into 40 different regional areas and plotted versus sediment thickness. These procedures avoided any misapprehending related to sea bottom low temperature, especially in deep water. The plots indicate that geotemperatures follow linear equations across the major lithostratigraphic units and the angular and linear coefficients vary specifically from area to area in the basin. The association of those equations to structural maps and water depth composed the methodology for modeling geotemperatures at any stratigraphic level.

The integration of pressure and temperature data treated independently revealed a pressure cell system in which overpressure can reach gradients of 18.4 MPa/km in the northern part of the basin. Generically, the overpressured cells are associated to low permeability reservoir rocks and are overlaid by the thickest post-rift packages in the basin, which is confirmed by a regional heat anomaly (260° - 300° F) modeled at top of the evaporitic unit. The evaporitic unit marks the transition from the rift to drift phase.

On a regional scale of observation, the Campos Basin is divided into a post-rift succession under hydropressure regime equalized by an expressive "drain" efficiency of the sand prone Eocene-Miocene sections, and a rift section in which the structural architecture delineates the lateral pressure compartments in SW/NE direction. The presence of mature source rock in rift points out the hydrocarbon generation as the main overpressure mechanism, that could be complemented by disequilibrium compaction in consequence of sedimentation rates higher than 50 m/my during the thermal subsidence of late Tertiary. The "hard" overpressure occurs below of 4.5 km. depth in the deepest rift troughs while rift margins and intra-highs present hydrostatic regime or low levels of overpressure and reveal that the conditions for pressure dissipation in this tectonic setting were primarily depth dependent.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil