Study of the origin of anomalous CO2 concentrations in Cerro Bayo de la Sierra Negra area (Neuquén Basin) In the Cerro Bayo de la Sierra Negra area (Neuquén Basin, Argentina), anomalous high levels of CO2 (65-90%) were recognized in operating oil wells. The hydrocarbon gases are found in proportions lower than 25%, with methane d13C values ranging between -43 and -38 ‰ and the distribution of d13C values for C1 - C3 alkanes that denote an organic thermogenic origin for light hydrocarbons. In contrast, CO2 shows d13C values between -4 and -8 &‰ and suggest an inorganic origin with contributions of magmatic and carbonatic origin. The values of 3He/4He ratio in these gases are between 3.7 and 5.1 R/Ra, indicating high proportion of mantle derived He. The high relative contents of He relative to N2-Ar allowed to identify inorganic sources of these gases involves both deep magmatic mantle derived and cortical material. These values are consistent with the "plume" of low resistivity model associated with a mantle flow would affect the study area and the thermal characteristics of the region. It is possible the interaction processes between magmatic fluids ascending and Mulichinco and Vaca Muerta formations, favor the additional release of CO2 from carbonate and organic matter of these units. Thus, the deep magmatic mantle derived source is recognized as the responsible source (directly and indirectly) for the CO2 accumulations in the study area. Variations and areal distribution of CO2 would be related to a local structural control, zones with structural damage would favor circulation and accumulation of deep origin fluids, enriched in CO2 and strongly related to the development of the thermal anomaly.