In the framework of a global investigation of the Spanish natural analogues of CO2 storage and leakage, four selected sites from the Mazarron-Ganuelas Tertiary Basin (Murcia, Spain) were studied for computing the diffuse soil CO2 flux, by using the accumulation chamber method. The Basin is characterized by the presence of a deep, saline, thermal (similar to 47 degrees C) CO2-rich aquifer intersected by two deep geothermal exploration wells named "El Saladillo'' (535 m) and "El Reventon'' (710 m). The CO2 flux data were processed by means of a graphical-statistical method, kriging estimation and sequential Gaussian simulation algorithms. The results have allowed concluding that the Tertiary marly cap-rock of this CO2-rich aquifer acts as a very effective sealing, preventing any CO2 leak from this natural CO2 storage site, being therefore an excellent scenario to guarantee, by analogy, the safety of a CO2 storage. (C) 2013 Elsevier Ltd. All rights reserved.