Latera

The Latera geothermal field is located in the west-central part of the Italian peninsula within the Latera caldera. Although studied since the early 1970s for geothermal energy, the area was well known long before that for carbonate rich springs and CO2-rich gas vents which inferred the occurrence of CO2 production / reservoirs at depth. The great age of these gas reservoirs (more than 0.1 Ma old) and the fact that local inhabitants have lived for thousands of years above these reservoirs and associated gas vents means that the Latera site is an excellent natural analogue of what might happen at surface in the unlikely event of leakage from an engineered geological storage site for anthropogenic CO2.

Locally the geology consists of a metamorphic basement, "Tuscan" limestones, "Ligurian" flysch, volcanic units and post-orogenic sediments. Formation of the local geothermal reservoir was likely due to the combination of compressional tectonics during the Eocene to Miocene

(which formed the subsurface structural high within the host Tuscan limestones - Bertrami et al., 1984; Barberi et al., 1984) and subsequent volcanic activity and tensional tectonics (which supplied the necessary fluids and high heat flow for the thermo-metamorphic formation of CO2 - Duchi et al., 1992). Finally the collapse structures associated with the various stages of local volcanism have either remained open for fluid flow (resulting in springs or gas vents on surface) or have become sealed by fault gouge or secondary mineral precipitation (thereby isolating entire blocks and creating heat/water convection cells).

Extensive work has been performed on the Latera test site by the authors during the NASCENT project, including regional, detailed and highly detailed soil gas surveys, detailed gas flux measurements, electrical tomography surveys, gas vent characterization and aqueous geochemistry. This research has been conducted to outline major gas migration pathways and to quantify the amount of gas reaching the atmosphere. Only some of the gas geochemistry data will be discussed here; the interested reader is referred to the final reports of the project itself (Pearce, 2004) for a more extensive treatment of the data.

Figure 1. Soil gas CO2 concentration distribution obtained during a regional geochemical survey in the Latera area. The two small squares in the central portion of the map indicate the location of the two detailed soil gas surveys.

A preliminary, regional soil-gas survey was conducted in the western sector of the Latera caldera with the goal of locating the fault systems which dissect the collapsed area and form the main route of gas migration towards the surface. Sampling was performed in an area of about 65 km2 using a uniform distribution of about 2 samples/km2 for a total of 110 samples. The CO2 contour map shows the spatial distribution of this gas species (Fig. 1). The major anomalies occur in the central and south-western parts of the Latera caldera, in good correspondence with mapped structural elements shown in the figure. Previous geophysical investigations in these areas highlighted a series of NW-SE and NE-SW oriented discontinuities which offset the carbonate substratum and formed the structural high hosting the geothermal reservoir. Elevated values are likely due to gas migration along these geochemically active faults, which result in the clearly anisotropic CO2 anomalies. This migration of deep (magmatic) gases is also indicated by the presence of He and Rn anomalies in the same area (not shown).

Two subsequent detailed surveys (locations shown in Figure 1) were conducted in areas which exhibited elevated soil gas CO2 concentrations during the regional survey. As samples were concentrated around gas vents the observed concentrations are significantly higher than those measured in the regional survey. For example during the first detailed survey, CO2 concentrations of up to 97% and CH4 values of over 1000 ppm were measured. Radon values (up to 480 Bq/l) also exceeded the amount expected if this radioactive gas were only produced in situ via the decay of 226Ra in the local shallow volcanic rocks. Contour maps of these gas species (not shown) highlight two clear circular anomalous zones, one next to a straight creek segment with gas bubbling at its base (fault?) and the other near a geothermal exploration well. Both of these circular anomalies correspond to vent structures that leak significant quantities of CO2 to the atmosphere. In contrast the second detailed survey, conducted about 1km north of the first in an area defined by electrical tomography anomalies, had slightly lower mean CO2, CH4 and Rn values but higher He values (Table 1). Contour maps of CO2 and He (not shown) give a similar distribution of spotty anomalies aligned parallel to the known NE-SW tectonic elements, with the major anomalies occurring in the area of significant gas vents.

Table 1. Minimum, maximum and mean values for four gas species analysed during the two detailed soil gas surveys (DS1 and DS2)._

CO2

CH4

Rn

He

Min

Max

Mean

Min

Max

Mean

Min

Max

Mean

Min

Max

Mean

DS1

0.05

97.3

9.1

0.26

1076

57.6

12.7

480

127

-540

310

-88

DS2

0.04

97.2

3.1

n.d.

1120

12.0

14.6

313

109

-913

2943

-105

Finally detailed soil gas profiles were conducted across a known gas vent occurring in the area of the second detailed profile to outline relative gas distributions and the size and form of the vent. In this work samples were collected about every 1 m and analysed for a series of different gas species, both in the field and in the laboratory.

100

)

80

0s

60

o1

40

C

20

0

800

)m

600

Q.

n.

400

CO^

nf

200

Distance (m)

5000 4000 3000 2000 1000 0

-

r 1600

-

h2s

- 1200

e

- -

— CH4

- 800

a.

r\

1

400 0

Distance (m)

Figure 2. Soil gas concentrations of various species, measured along a profile crossing numerous gas vents in the Latera area.

Figure 2 shows the results for four of these gas types, CO2, He, H2S and CH4. As can clearly be seen in this figure the survey crossed a number of gas vents, however the distribution of the various species is not consistent from one vent to another. For example CO2 and He are the two gases with the most significant number of anomalies, with both showing a similar spatial and relative concentration distribution. In contrast H2S and CH4 are much more spatially restricted, with H2S showing elevated values in the vent located around 25m while CH4 is more concentrated in the one at 200m. The different relative distribution of CO2 / He versus H2S / CH4 can be explained in terms of gas abundance and reactivity. For example He is a highly mobile and non-reactive molecule, and thus it is logical that it shows a large number of wide peaks along the entire profile. CO2, on the other hand, which shows essentially the same distribution, is far more reactive due to its high solubility in water and its involvement in acid-base reactions. The similarity of the reactive CO2 and the conservative He implies that the high CO2 flux / concentration (>90%) supplies an excess mass of CO2 at a rate which is greater than that of dissolution and eventual reaction. In contrast the relatively low concentrations and reactive nature (be it chemical or

microbiological) of both H2S and CH4 has resulted in the consumption of these gases in the unsaturated zone. Only in the centre of vents, which have sufficiently high flux rates, is it possible for these species to migrate closer to the soil-atmosphere interface, as is observed in the two vents at 25 and 200m.

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