Gases and water isotopes in a geochemical section across the Larderello, Italy, geothermal field

Abstract

Steam samples from six wells (Colombaia, Pineta, Larderello 57, Larderello 155, Gabbro 6, and Gabbro 1) in a south to north section across the Larderello geothermal field have been analyzed for inorganic and hydrocarbon gases and for oxygen-18 and deuterium of steam. The wells generally decrease in depth and increase in age toward the south. The steam samples are generally characterized by

• (1) Total gas contents increasing south to north from 0.003 to 0.05 mole fraction;

• (2) Constant CO₂ (95±2 percent); near constant H₂S (1.6±0.8), N₂ (1.2±0.8), H₂ (2±1), CH₄ (1.2±1), and no O₂ in the dry gas;

• (3) Presence of numerous, straight chain and branched C₂ to C₆ hydrocarbons plus benzene in amounts independent of CH₄ contents with highest concentrations in the deeper wells;

• (4) Oxygen-18 contents of steam increasing south to north from −5.0‰ to −0.4‰ with little change in deuterium (−42±2‰).

These observations are interpreted as showing:

• (1) Decreasing gas contents with amount of production because the proportion of steam boiled from liquid water increases with production;

• (2) Synthesis of CH₄ from H₂ and CO₂ with CO₂ and H₂ produced by thermal metamorphism and rock-water reactions;

• (3) Extraction of C₂ to C₆ hydrocarbons from rock organic matter;

• (4) Either oxygen isotope exchange followed by distillation of steam from the north toward the south (2 plates at ∼220°C) or mixture of deeper more-exchange waters from the north with shallow, less-exchanged recharging waters from the south.