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In the framework of a Spanish project focused on carbon capture and storage technologies, the Gañuelas-Mazarrón Tertiary Basin (SE Spain) was studied as a natural analogue of a CO2 reservoir affected by anthropogenic leakages. It has been accepted that the main objective of natural analogue studies is to predict the long-term performance of a natural CO2 reservoir to be extrapolated to the operation of a CO2 deep geological storage, but these studies can also provide valuable information to select a site for CO2 storage. Thus, the comprehensive study performed in this Spanish basin has allowed the establishment of a guide to be applied to other similar natural systems with deep saline aquifers that are able to store CO2. This guide comprises 5 phases: (1) the compilation of existing information from the site; (2) the geological and structural study of the natural CO2 reservoir; (3) the characterisation of the main components (waters–rocks–gases) of the natural CO2 reservoir; (4) the identification of analogies between the natural CO2 reservoir and a potential site for CO2 geological storage; and (5) the implications for the long-term behaviour and safety of a CO2 storage system. Whilst the three first stages are well known since they are exclusively focused on the site characterisation, the latter two are poorly developed. For this reason, it has been considered convenient to go deeper into both the identification of analogies and the assessment of their performance in storing CO2. The main results of this study are aimed at elaborating a practical guide for regulatory agencies that are responsible for making decisions about sites that have been selected for anthropogenic CO2 geological storage. [ABSTRACT FROM AUTHOR]

• Current travertines precipitation from CO 2 -rich groundwater was studied. • Artificial CO 2 leakages are analogous to those from CO 2 storage sites. • δ13C signatures could be good tracers for the CO 2 escapes from a CO 2 storage site. • Physicochemical variables controlling aragonite/calcite precipitation were discussed. In the framework of a natural CO 2 reservoir with CO 2 leakages as an analogue of a failed CO 2 deep geological storage, the current precipitation of travertines and the associated upwelling of CO 2 -rich saline groundwater were analysed. This natural analogue is located in the Gañuelas-Mazarrón Tertiary Basin (SE Spain). The study comprises of the chemistry of both groundwater and travertines, including stable isotopes, mineralogy and petrography of the travertines, all this performed after a review of the geology of the basin. In this sense, the basin gathers the main features of a safe natural CO 2 reservoir in a deep saline aquifer sealed by a thick marl formation. The aquifer was artificially perturbed by the drilling of wells, inducing the travertines precipitation at these water discharge points. Groundwater is saline, slightly acid, oversaturated in aragonite and calcite and with significant concentrations of heavy elements, some of them toxic. From an isotopic viewpoint, the relative constant δ13C-DIC values suggest that carbon is mainly inorganic in origin with minor organic and mantle contributions. Travertines are basically composed of aragonite or calcite, their precipitation being controlled by a sudden CO 2 degassing and minor biological activity. Their δ13C signatures indicate that carbon mainly has an inorganic origin, although some contribution of organic carbon must be considered as well. Furthermore, these carbonate deposits did not precipitate in isotopic equilibrium, as determined by δ18O values. Finally, it is suggested that the appearance of travertines along with their carbon isotopic signatures represent efficient tools for detecting CO 2 leakages from any CO 2 storage site. [ABSTRACT FROM AUTHOR]