Bentham, Peter¹, Mark Longacre², Kevin Boyd¹
(1) BP, Houston, TX
(2) MBL Inc, Denver, CO

ABSTRACT: Impact of Gravity Modeling in the Foz do Amazonas Basin, Offshore NE Brazil

The Foz do Amazonas basin is located along the continental margin of NE Brazil, offshore from the modern-day Amazon River mouth. It is a Tertiary basin developed in response to deposition of the Amazon Fan during Miocene-Recent time. Listric extensional faults within the shelf and upper slope of the fan are linked directly to contemporaneous compressional deformation beneath the lower slope. Linkage occurs through a mobile shale substrate of Late Cretaceous-Paleogene age, although the absolute level of this detachment varies laterally. 2D & 3D Gravity modeling was undertaken as part of a regional structural and petroleum systems analysis to help to define the large-scale crustal distribution basin. Higher-frequency variations within the Bouguer gravity data also provided additional information about the primary structural trends. Detailed 2D gravity modeling was undertaken to investigate the density distribution consistent with the observed residual anomalies. Somewhat surprisingly, the main structural highs corresponded closely with linear lows in the residual gravity data. These are interpreted to reflect the presence of low density shales within the Paleogene interval, immediately beneath the base of the Amazon Fan. This information, combined with seismic observations across the study area suggest that development of the main compressional trends began at or just prior to Base Amazon Fan time and continued throughout Amazon Fan deposition. As a result, Paleogene shales within the main culminations remained under-compacted relative to the equivalent strata within the adjacent synclines. This structural history has important implications for the analysis of the active petroleum system within the basin.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.