Mofettes are gas emission sites where high concentrations of CO₂ ascend through conduits from as deep as the mantle to the Earth’s surface and as such provide direct windows to processes at depth. The Hartoušov mofette, located at the western margin of the Eger Graben, is a key site to study interactions between fluids and swarm earthquakes. The mofette field (10 mofettes within an area of 100 m × 500 m and three wells of 28, 108, and 239 m depth) is characterized by high CO₂ emission rates (up to 100 t/d) and helium signatures with (³He/⁴He)_c up to 5.8 Ra, indicating mantle origin. We compiled geological, geophysical, geochemical, and isotopic data to describe the mofette system. Fluids in the Cheb basin are mixtures between shallow groundwater and brine (>40 g/L at a depth of 235 m) located at the deepest parts of the basin fillings. Overpressured CO₂-rich mineral waters are trapped below the mudstones and clays of the sealing Cypris formation. Drilling through this sealing layer led to blow-outs in different compartments of the basin. Pressure transients were observed related to natural disturbances as well as human activities. External (rain) and internal (earthquakes) events can cause pressure transients in the fluid system within hours or several days, lasting from days to years and leading to changes in gas flux rates. The 2014 earthquake swarm triggered an estimated excess release of 175,000 tons of CO₂ during the following four years. Pressure oscillations were observed at a wellhead lasting 24 h with increasing amplitudes (from 10 to 40 kPa) and increasing frequencies reaching five cycles per hour. These oscillations are described for the first time as a potential natural analog to a two-phase pipe–relief valve system known from industrial applications. Full article

Volatiles transported from the Earth’s interior to the surface through permeable faults provide insights on the gas composition of deep reservoirs, mixing and migration processes, and can also be applied as gas-geothermometer. Here, we present carbon (δ¹³C), hydrogen (δ²H) and nitrogen (δ¹⁵N) isotopic data of CO₂, CH₄, and N₂ from gas samples collected from the Kızıldere and Tekke Hamam geothermal fields, located along the eastern segment of the Büyük Menderes Graben, Turkey. The stable isotopic composition of carbon (δ¹³C) ranges from +0.30 to +0.99‰ (PDB) for CO₂ from Kızıldere and is slightly more variable (−0.95 to +1.3‰) in samples from Tekke Hamam. Carbon isotope data in combination with CO₂/³He data reveal that ~97% (Tekke Hamam) to ~99% (Kızıldere) of CO₂ derives from limestone sources, with the residual CO₂ being magmatic in origin with no evidence for CO₂ from organic sources. The slightly higher contribution of limestone-derived CO₂ in Kızıldere, compared to Tekke Hamam can be attributed to the higher temperatures of the Kızıldere reservoir and resulting amplified fluid–limestone interaction, as well as helium depletion during phase separation for Kızıldere samples. In contrast to the carbon isotopic composition of CO₂, the δ¹³C values of methane from Kızıldere and Tekke Hamam are clearly distinct and vary between −23.6 and −20.8‰ for Kızıldere and −34.4 and −31.7‰ for Tekke Hamam, respectively. The δ²H-CH₄ composition is also distinct, measured as −126.7‰ for Kızıldere and −143.3‰ for Tekke Hamam. CO₂-CH₄ carbon isotope geothermometry calculations based on the isotopic fractionation of δ¹³C between the dominant component CO₂ and the minor component CH₄ reveals temperatures 20–40 °C and 100–160 °C higher than the bottom–hole temperatures measured for Tekke Hamam and Kızıldere, respectively. Based on the CO₂-CH₄ carbon isotope disequilibrium, unusual high methane concentrations of ~0.3 to 0.4 vol.-% and CH₄/³He-δ¹³C-CH₄ relationships we suggest thermal decomposition of late (Tekke Hamam) to over-mature (Kızıldere) organic matter and, to some extent, also abiogenic processes as principal source of methane. The N₂/³⁶Ar ratios of most samples reveal the existence of a non–atmospheric nitrogen component within the gas mixture issuing from both fields, in addition to a constant contribution of atmospheric derived nitrogen accompanied into the system via the meteoric recharge of the geothermal system. Based on the δ¹⁵N isotopic ratios (varying between −4.44‰ and 4.54‰), the non–atmospheric component seems to be a mixture of both sedimentary (crustal organic) and mantle nitrogen. The thick Pliocene sedimentary sequence covering the metamorphic basement is the likely major source for the thermogenic content of CH₄ and crustal N₂ gas content in the samples. Full article

We investigated fault gases (helium, radon, CO₂) in two seismically active Cenozoic sedimentary basins: (a) Meinweg (in 2015), at a tectonically quiescent horst structure in the Lower Rhine Embayment; and (b) Bodanrück (in 2012; Lake of Constance), in the Molasse Basin and part of the seismically active Freiburg–Bonndorf–Bodensee Fault Zone (FBBFZ). Both study areas were selected because recent “GeoBio-Interactions” findings showed that red wood ants (RWA) are biological indicators of otherwise undetected degassing systems. We combined presence/absence data of RWA nests, their spatial pattern analysis (prototype lines), seismicity and known tectonic settings with soil gas analyses (a total of 817 samples) to unveil geochemical anomalies related to tectonic developments unknown so far. Currently, Meinweg can be considered “no ants land” due to the very low background-level of geogas concentrations. Thus, anomalies (Rn-CO₂) weakly trending in NE-SW extension direction emerged. This could probably indicate the onset of (re)activation of the NE-SW-trending (Variscan) structures or the development of new fractures as an aftershock process of the 1992 Roermond earthquake that occurred about 15 km to the west. Results at Bodanrück (three RWA clusters and two RWA-free corridors) revealed degassing patterns in NW-SE and NNE-SSW directions in the clusters corresponding to re-activated and recent strike-slip fault systems. No gas anomalies were found in RWA-free corridors. The RWA nest distribution was shown to be a valuable tool for identifying areas of even actively degassing spotty anomalies caused by macro- and microscale brittle deformation masked by sediment cover. Full article

This study assesses potential geological connections between the unconventional petroleum plays in the Beetaloo Sub-basin, regional aquifers in overlying basins, and the near surface water assets in the Beetaloo Sub-basin Northern Territory, Australia. To do so, we built an innovative multi-disciplinary toolbox including multi-physics and multi-depth imaging of the geological formations, as well as the study of potentially active tectonic surface features, which we combined with measurement of the helium content in water sampled in the aquifer systems and a comparative analysis of the surface drainage network and fault lineaments orientation. Structures, as well as potential natural active and paleo-fluid or gas leakage pathways, were imaged with a reprocessing and interpretation of existing and newly acquired Beetaloo seismic reflection 2D profiles and magnetic datasets to determine potential connections and paleo-leakages. North to north-northwest trending strike slip faults, which have been reactivated in recent geological history, are controlling the deposition at the edges of the Beetaloo Sub-basin. There are two spring complexes associated with this system, the Hot Spring Valley at the northern edge of the eastern Beetaloo Sub-basin and the Mataranka Springs 10 km north of the western sub-basin. Significant rectangular stream diversions in the Hot Spring Valley also indicates current or recently active tectonics. This suggests that those deep-rooted fault systems are likely to locally connect the shallow unconfined aquifer with a deeper gas or fluid source component, possibly without connection with the Beetaloo unconventional prospective plays. However, the origin and flux of this deeper source is unknown and needs to be further investigated to assess if deep circulation is happening through the identified stratigraphic connections. Few north-west trending post-Cambrian fault segments have been interpreted in prospective zones for dry gas plays of the Velkerri Formation. The segments located in the northern part of the eastern Beetaloo Sub-basin do not show any evidence of modern leakages. The segments located around Elliot, in the south of the eastern Beetaloo Sub-basin, as well as low-quality seismic imaging of potential faults in the central part of the western sub-basin, could have been recently reactivated. They could act as open pathways of fluid and gas leakage, sourced from the unconventional plays, deeper formations of the Beetaloo Sub-basin or even much deeper origin, excluding the mantle on the basis of low ³He/⁴He ratios. In those areas, the data are sparse and of poor quality; further field work is necessary to assess whether such pathways are currently active. Full article