The Giant Oil Fields of the Santos basin and the role of Mg-silicates
Paul Wright (PW Carbonate Geoscience; Natural Sciences, National Museum of Wales)
The huge oil and gas discoveries in the Santos Basin, offshore Brazil are mainly hosted in thick Cretaceous lacustrine carbonates with apparently no modern or ancient analogues. Many of the textures in the carbonates have never been encountered before and the pore systems are also unique. In such a situation it is necessary to go back to basics and try to explain the reservoir from first principles. What has emerged is a model of an extreme geochemical system in which abiotic carboantes accumulated to hundreds of metres but that the carbonate textures were controlled by interactions with Mg-silicate gels, which themselves converted to clays and were then dissolved to produce the porosity. There are still huge uncertainties about the general depositional model and the whole exercise serves to illustrate that there are important, non-actualisitc successions still to be discovered that challenge our current knowledge of how reservoirs form.