CO2 occurrences are common in hydrocarbon reservoirs in different geologic and geotectonic environments. In most of the Brazilian sedimentary basins, carbon dioxide occurs in minimal amounts. However, more recent deepwater exploration in the Santos Basin discovered significant concentrations of CO2 in some petroleum fields geographically dispersed. Adjacent fields within a very similar geologic context can hold a few percent to 80% of CO2 creating great scientific challenges for understanding the origin of CO2 and the identification of the main controls that govern its erratic distribution. That is the case of the Tupi (Lula) field, which has low CO2 content, and the neighbor Jupiter field, with an abnormal 80% of CO2 in the gas cap. The origin of this CO2 from earth’s mantle was already proved by isotopic analyses of noble gases. We have developed some hypotheses to explain this mantle sourced carbon dioxide in hydrocarbon reservoirs of the Santos Basin, namely, regional crustal thinning, deep-seated faults, high fault density, igneous intrusions, among others. The CO2 is abundant in the mantle, and the continental crust can act as a seal that inhibits these fluids from flowing toward the earth’s surface. The highly stretched continental crust in the Santos Basin allows CO2-rich mantle material to intrude the upper levels of the continental crust in some locations. Fault systems associated with these magmatic intrusions can direct the carbon dioxide into the upper sedimentary section in a trajectory toward the surface. Understanding the crustal structure of a sedimentary basin is an important step in the exploratory process for the assessment of CO2 risk. The integration of geophysical (grav-mag) and geochemical data prove great efficacy in identifying the origin of CO2 and the most important controls on its distribution in the Santos Basin. Such results strongly suggest this approach as a valuable tool for derisking new exploration projects.

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