Even if a tiny amount was sequestered each year over the course of 55 million years there would be a huge pile of carbon there. But it isn’t there, is it?
my initial reply was a post about oil mining in the amazon...
Additionally tectonic plates move, 55 million years ago the amazon probably wasn't even there. Additionally oil found on land could've been produced by biomass under the ocean and vice versa it's all moving around all the time. Likely the oil found underneath the amazon could have been produced at a time where that tectonic plate was under water.