Petroleum System Overview of São Francisco Basin: ANew Gas Province Onshore Brazil
São Francisco, a proterozoic basin in the central portion of Brazil, has its petroliferous potential supported by the presence of surface gas seeps and gas flows observed in the drilled wells, outcrops, rivers and lakes around the whole basin. Geochemical analyses of black shales of the Bambuí and Paranoá groups indicate potential source rocks presenting carbon content ranging from 2-4%, attaining locally up to 15%. Isotopic analyses of the gas seeps (δ13C ranging from -38‰ to -56‰) corroborates the rock data and points to an thermochemical origin with a very high maturation level, attesting the predominant gas-prone character of the basin.
Aiming to establish an initial geological overview that comprehend source rocks thermal evolution, charge and migration through time, a 2D petroleum system modeling along a regional basin section was performed using well data, previous studies and analogous models. Thermal calibration model considered surface heat flow and geothermal gradient analyses and, that most of the source rocks have already achieved the oil window stage (in such a way that the observed measured TOC values can be considered residual). Expulsion time is about 365 My for the Bambuí source rocks. A conceptual source rock within the Canastra/Paranoá passive margin sequence was simulated, presenting its expulsion time at 820My.Two main reservoir lithofacies was considered: the carbonates of Bambuí Group and the fractured sandstone of Paranoá/Canastra Group. Porosities values taken from well reports are too low (averaging 1%) but the occurrence of secondary porosity could improve the permo-porosities characteristics of the carbonates through dissolution (in some cases wireline logs indicate an average of 8%), and of the sandstones through intense fracturing. Therefore, the low porosity values should not be a problem as gas is the main fluid target.
The results of the petroleum system modeling together with new geological, geophysical and geochemical data, open up a new frontier of exploration, inside the São Francisco Basin, to hold significant gas accumulations.