Methane anomaly in soil gas above an abandoned oil & gas exploration borehole in NE Czechia

Abstract

In the 1950s–1960s, 71 coal and gas exploration wells were drilled in the area of interest in the Western Carpathian Flysch Belt (WCFB) in the NE Czech Republic. Some wells were abandoned while others were used for gas production and storage in the Třanovice underground gas storage (UGS). A monitoring campaign was carried out in place of an earlier abandoned exploration well Žu-108 aiming at proving or ruling out a geochemical relationship of the soil gas and the stored gas in the UGS. The measurements were carried out each month from June 2018 to October 2019 and included amount of methane (CH4), total petroleum gases (CH4 to C4H10), carbon dioxide (CO2) and oxygen (O2) in soil gas in a grid of up to 41 probes. The results show a methane gas peak occurrence with 10 m diameter around the well-top with maximum methane content of 57%vol, while the natural background value of methane content outside the plume is < 500 ppm. The position of the methane peak is associated with significant O2 drop down to 0.25%vol and is surrounded by up to 8.34%vol CO2 halo produced most probably by methanotrophic archea-bacteria. The natural background CO2 values are ca. 0.80%vol. CH4 flux to the atmosphere from the topsoil directly above the well-top of Žu-108 well was measured using the accumulation chamber and range from 2126 to 26 197 g d−1 m−2. The flux increased during drops in atmospheric pressure and ground water level in situ. In contrast, low methane flux was registered after strong rainfalls and when the soil was covered by ice and snow which acted as seal. Based on chemical composition and isotopic signature of methane, the gas is of oil-associated type and methane is thermogenic, similar to that of the original (i.e. pre-UGS) Žukov gas field as well as other small accumulations in the WCFB on the territory of the Czech Republic. It statistically differs from imported mixture of Siberian and Norwegian gas stored in the Třanovice UGS. Based on it, the possible natural gas leakage from the UGS to the Žu-108 is in this case ruled out.