|MERIS satellite image showing slick in Campos Basin off Brazil, November 11, 2011. Image courtesy European Space Agency.|
It is possible, but call us skeptical. From my previous years working as an exploration geologist I know there are natural seeps off Brazil (that was a study I did for BP back in the day...). But I've never seen a natural seep create a slick this large on a satellite image.
One way to verify if it's a natural seep: go back and look at historical satellite images for this area, and if it doesn't show up then either it's a brand-new natural seep or it's a human-caused slick. We're doing that right now. We'll let you know what we find.