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)

• Main Authors: R. Bussert, H. Kämpf, C. Flechsig, K. Hesse, T. Nickschick, Q. Liu, J. Umlauft, T. Vylita, D. Wagner, T. Wonik, H. E. Flores, M. Alawi
• Format: Article
• Language: English
• Published: Copernicus Publications 2017-11-01
• Series: Scientific Drilling
• Online Access: https://www.sci-dril.net/23/13/2017/sd-23-13-2017.pdf

Microbial life in the continental <q>deep biosphere</q> 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_<i>L</i> 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&thinsp;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.