A comparison of leaking and non-leaking CO2 reservoirs as natural analogues for geological CO2 sequestration (Sesta and Latera geothermal fields, central Italy),

The Latera and Sesta areas are two geothermal fields located in central Italy which both exhibit significant quantities of associated CO2 gas. The primary difference between these two sites, however, is that there is large scale leakage of CO2 to the atmosphere in the Latera area while the CO2 at Sesta was only discovered at depth during exploratory geothermal drilling. These sites were thus selected as natural analogues for the geological sequestration of CO2 and were studied with both geophysical and shallow geochemical techniques in order to better understand the differences between a leaking and a sealed CO2 reservoir. The Latera geothermal field is located within the extinct Latera caldera, a large, elliptical, NNE-SSW trending structure which has long been know for carbonate rich springs and CO2-rich gas vents at surface. Sesta, on the other hand, is located within a NNW-SSE trending graben having associated boundary faults and a thick Pliocene clay cover. The CO2 at both sites is believed to be the result of decarbonisation of carbonate minerals due to the local high heat flow. Work conducted on these sites included soil gas surveys to delineate zones of elevated CO2 leakage and gas flux measurements to quantify the amount of CO2 leaking to the atmosphere. In addition electrical tomography surveys were performed at the Latera site to delineate migration pathways (faults and fracture networks). Soil gas results from Latera show CO2 concentrations which range from normal values of 1% up to 95% in gas vents cores. Associated with the elevated CO2 values are anomalous concentrations of trace gases which are transported from depth within the carrier stream, including H2S, He, H2 and CH4. Gas flux measurements range from 3 x 10-7 to 7.7 x 10-5 m3 m-2 sec- 1, with measurements on one gas vent (70 m2) indicating a total mass flux of about 108 kg day-1. In sharp contrast Sesta data showed low CO2 soil gas concentrations, on the order of shallow biological production, and gas flux rates which were 3 orders of magnitude lower than those observed at Latera. Gas migration to surface at Latera thus appears to occur along sub-vertical collapse structures associated with caldera formation, whereas the thick clay sequences at Sesta appear to have prevented large-scale escape of deep gases along the local graben boundary faults. The authors gratefully acknowledge the support of the EC in partly funding this work (EC Nascent project).