| Summary: | Exploratory work for hydrocarbons along the southeastern Brazilian Margin discovered high concentrations of CO<sub>2</sub> in several fields, setting scientific challenges to understand these accumulations. Despite significant progress in understanding the consequences of high CO<sub>2</sub> in these reservoirs, the role of several variables that may control such accumulations of CO<sub>2</sub> is still unclear. For example, significant differences in the percentages of CO<sub>2</sub> have been found in reservoirs of otherwise similar prospects lying close to each other. In this paper, we present a hypothesis on how the rifting geodynamics are related to these CO<sub>2</sub>-rich accumulations. CO<sub>2</sub>-rich mantle material may be intruded into the upper crustal levels through hyper-stretched continental crust during rifting. Gravimetric and magnetic potential methods were used to identify major intrusive bodies, crustal thinning and other geotectonic elements of the southeastern Brazilian Margin. Modeling based on magnetic, gravity, and seismic data suggests a major intrusive magmatic body just below the reservoir where a high CO<sub>2</sub> accumulation was found. Small faults connecting this magmatic body with the sedimentary section could be the fairway for the magmatic sourced gas rise to reservoirs. Mapping and understanding the crustal structure of sedimentary basins are shown to be important steps for “de-risking” the exploration process.
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