Drilling into an active mofette: pilot-hole study of the impact of CO₂-rich mantle-derived fluids on the geo–bio interaction in the western Eger Rift (Czech Republic)

Abstract. Microbial life in the continental deep biosphere is closely linked to geodynamic processes, yet this interaction is poorly studied. The Cheb Basin in the western Eger Rift (Czech Republic) is an ideal place for such a study because it displays almost permanent seismic activity along active faults with earthquake swarms up to M_L 4.5 and intense degassing of mantle-derived CO₂ in conduits that show up at the surface in form of mofettes. We hypothesize that microbial life is significantly accelerated in active fault zones and in CO₂ conduits, due to increased fluid and substrate flow. To test this hypothesis, pilot hole HJB-1 was drilled in spring 2016 at the major mofette of the Hartoušov mofette field, after extensive pre-drill surveys to optimize the well location. After drilling through a thin caprock-like structure at 78.5 m, a CO₂ blowout occurred indicating a CO₂ reservoir in the underlying sandy clay. A pumping test revealed the presence of mineral water dominated by Na⁺, Ca²⁺, HCO₃⁻, SO₄²⁻ (Na-Ca-HCO₃-SO₄ type) having a temperature of 18.6 °C and a conductivity of 6760 µS cm⁻¹. The high content of sulfate (1470 mg L⁻¹) is typical of Carlsbad Spa mineral waters. The hole penetrated about 90 m of Cenozoic sediments and reached a final depth of 108.50 m in Palaeozoic schists. Core recovery was about 85 %. The cored sediments are mudstones with minor carbonates, sandstones and lignite coals that were deposited in a lacustrine environment. Deformation structures and alteration features are abundant in the core. Ongoing studies will show if they result from the flow of CO₂-rich fluids or not.