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Origin of CO2 in Brazilian Basins

Santos Neto, Eugenio V.*1; Cerqueira, José Roberto 1; Prinzhofer, Alain 2
(1) Geochemistry, Petrobras R&D Center, Rio de Janeiro, Brazil.
(2) Geochemistry, IFPEN, Rueil Malmaison, France.

Carbon dioxide is one of the most common non hydrocarbon gas found in petroleum reservoirs. However, petroleum accumulations with CO2 > 20% can be considered relatively rare. The most important source of the large volumes of CO2 found in petroleum accumulations is the mantle. Commonly, areas with major CO2 risk are associated with “hot basement” (GG > 30 degrees C/km), deep seated faults, igneous intrusions and basin rifting.

Representative gas samples were collected in petroleum accumulations from 11 Brazilian basins aiming to identify the origin of the produced CO2. Emphasis was given to the pre-salt basins occurring along the southeastern Brazilian margin. Results used in this investigation include delta 13C of C1-C4 and CO2; percentages of C1-C4, CO2 and other non-hydrocarbons, as well as, concentrations and isotopic ratios of associated noble gases. Data were complemented with delta 13C and delta 18O measured in CO2 from micropyrolysis experiments using pre-salt carbonates and kerogens.

Regarding the origin of CO2 results have shown a clear separation in two groups. One represented by almost exclusively CO2 originated in the mantle (delta 13CCO2 mostly within -7‰ and -5‰, and, relatively high values for 3He/4He represented by R/Ra up to 5.60). This type of CO2 can be found in the pre-salt petroleum accumulations with abundances of up to 80%. The second group is represented by CO2 derived from organic matter originated from microbiological or diagenetic processes (highly enriched or highly depleted in 13C). Many examples of this group have shown a detectable mantle contamination (0.02 < R/Ra < 1.81).

The results of this investigation corroborate the worldwide trend that correlates directly CO2 abundance with intensity of mantle contribution. However, not all of our studied cases have shown a recognizable relationship between CO2 abundance and deep faults, igneous rocks, or anomalies in the geothermal gradient. These observations suggest that more investigations are necessary to improve our knowledge about the mechanisms and processes of CO2 generation and migration upwards from mantle to petroleum accumulations in sedimentary basins.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California