CO2 degassing from a Spanish natural analogue for CO2 storage and leakage: implications on 222Rn mobility

Abstract

The Gañuelas-Mazarrón Tertiary basin, located in the Murcia region (SE Spain) and filled by a thick marly Neogeneformation, hosts a deep saline CO2-rich aquiferfrom which CO2escapes occurs. Consequently, this basin has been considered as a natural analogue for CO2storage and leakage. Recent studies performed in wellheads showed a close association between CO2 and222Rn, suggesting a further investigation of this association, particularly in diffuse escapes through the basin soils. For that, concentrations of CO2 and 222Rn in soils,at 1m depth approximately, were measured by usingthe TRT-SEG04 and RM-2 Radon devices, respectively. These measurements were performedon selected areas according to their high density of lineament intersections, since they could be structural weakness zones and consequently favourable pathways for CO2 and 222Rn. The orientedmeasurement profileswere correlated with their corresponding geological cross-sections.The results allowed concluding that: i) both gases show a significant correlation only in faulted zones and/or wellheads; ii) the CO2 leakages could be doubly dangerous due to its toxicity and the radioactivity ofassociated 222Rn; and iii) the Neogene marly cap-rock of the basin acts as a sealing formation against CO2 and 222Rn leakages. Finally,it can be additionally stated that 222Rn can also be used as an alert of CO2 escapes from a CO2natural reservoir or from a CO2deep geological storage.