RADARSAT-1 images in support of petroleum exploration: the offshore Amazon River mouth example

Abstract

RADARSAT-1 images were used as an attempt to identify seepage slicks in the Foz do Amazonas Basin, one of the most promising oil and gas exploration frontiers in the Brazilian equatorial margin, a region that is cloud-covered for most of the year. An unsupervised semivariogram textural classifier algorithm was used to enhance areas of smooth texture and low radar backscatter, indicative of these seepage slick targets. Complementary information related to sea surface temperature, cloud top temperature, wind velocity, and modelling for the tidal regime (all obtained as close as possible to the RADARSAT-1 acquisition) was used to support image interpretation and to exclude false targets also characterized by low radar backscatter (e.g., local upwelling, heavy rain cells). Twenty-one target areas were interpreted as seepage slicks. Considering the influence of the oceanographic and environmental conditions on drifting of the interpreted seepage slicks, they were spatially compared with available geological information, including seismically derived structural and isopach maps, and bathymetric data. Most of the interpreted seepage slicks occur in an extensional structural domain of growth faults. These structures are related to Cenozoic gravity tectonics that result from tensional stress in the continental slope due to the huge load of fluvial sediments transported by the Amazon River. Together with previously discovered subcommercial shallow water oil accumulations, the remote detection of seepage slicks is additional evidence of present-day generation and migration phenomena. The results indicate that the use of a remote, swift method to identify offshore natural oil seepage related to active petroleum systems may constitute a well-accepted approach to support exploration in frontier areas like the Foz do Amazonas Basin.