Coalbed gases in the Velenje basin are highly variable in both their concentrations and stable isotope composition. Major gas components are CO₂ and methane. The CDMI carbon dioxide-methane index [CDMI = (CO₂/(CO₂ + CH₄))·100(%)] varies from 0 to 98.8%, δ¹³C_CO2 from -34.1 to 2.9‰ and δ¹³C_CH4 from -74.0 to -34.7‰. According to the geochemical results (recalculated on an air-free basis) for coalbed gases, several types of origin of CO₂ and methane could be recognized: thermogenic methane, endogenic and thermogenic CO₂, microbial methane and CO₂. The phenomenon of thermogenic methane in the lignite seam could be explained by microbial CO₂ reduction and/or methane oxidation processes, causing enrichment in the heavy ¹³C isotope of methane. Considering geological events at the time of formation of the lignite seam in the Velenje basin, it was found that most of the gas in the lignite seam is of bacterial origin, indicating CO₂ reduction processes mixed with external CO₂. A considerable fraction of the CO₂ in the lignite seam is of external origin. A crucial factor determining the formation of coalbed gases in the Velenje basin was bacterial activity. The distribution of coalbed gases (CO₂ and methane) is a result of the different physicochemical properties of CO₂ and methane. Methane accumulates at the subsurface of the Velenje basin, while carbon dioxide remains adsorbed on the coalbed surface and in micropores of the coal structure. CO₂ is the major gas component in the lignite seam and represents a permanent danger for gas outbursts. Isotopic fractionation due to migration of methane from the lignite seam to the surface is reflected in isotopically light methane in the water saturated layers. CO₂ accumulating at the subsurface of the Velenje basin is mainly derived from hinterland water and oxidation of methane.

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The Quaternary biogenic gas province in the east of the Qaidam basin is the largest biogenic gas province in China. We collected 21 gas samples from the different gas pay beds of the Sebei 1 gas field and the Sebei 2 gas field, measured their chemical compositions and carbon isotopes, and emphatically discuss their methanogenic pathway and significance. The CH₄ and CO₂ of the biogenic gases were enriched in ¹³C with increasing depth, which indicates that the gases are CO₂ reduction-derived gases. The δ¹³C₁, δ¹³C_CO2 and δD values of the biogenic gases approximate the range of the corresponding values of CO₂ reduction gases. The values fall in the CO₂ reduction field on genetic diagrams based on δ¹³C₁, δ¹³C_CO2 and δD values of natural gases. The fractionation coefficients (α_c) of the carbon isotope of the CH₄ and CO₂ of the biogenic gases are more than 1.005, characteristic of a CO₂ reduction pathway. During the Quaternary due to the dry climate, low temperature, high sedimentation rate and a high content of sulfate in the east of the Qaidam basin, methane bacteria could not propagate until sediments were buried at deeper depth. These conditions benefit a CO₂ reduction pathway of the gases. These results are of importance in discussing the biogenic gas origin and formation conditions, determining the methods and conditions for biogenic gas simulation, accessing biogenic gas reserves, establishing a pool-forming model and selecting targets for natural gas exploration.

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Experiments on CO₂-water-rock interaction at hydrothermal temperatures have been performed to investigate dissolution and precipitation phenomena, including Ca extraction from rocks that might occur during CO₂ sequestration into geothermal fields. Distilled water samples were exposed to a CO₂ atmosphere at a temperature of 25°C and pressures up to 6 MPa. The resulting solutions were then reacted with granodiorite samples from the Ogachi hot/dry rock field and labradorite, at 200°C and 120°C respectively. The calcium concentrations in the solutions that had reacted with CO₂ were twice those with N₂ instead of CO₂. Combined with the results of thermodynamic calculations, these observations indicate that calcium can be released from rocks (silicates) easily and might be removed as CaCO₃ and/or CaSO₄ during CO₂ sequestration into geothermal fields.

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The soil-gas method is based on the principle that faults and/or fractures are highly permeable pathways in rock formation where gases can migrate upward from the deep crust and/or mantle and retain their deep-source signatures in the soil cover. This method is adopted because it can give results in short time and at low costs. In this work, soil-gas compositions are measured and synthesized in conjunction with the geological, geophysical and geomorphological information along the Chaochou Fault, which is considered as an active fault in southern Taiwan. More than 500 soil-gas samples were collected along 18 traverses crossing the observed structures and analyzed for He, CO₂, CH₄, O₂ + Ar and N₂. The results show that both helium and carbon dioxide concentrations in the soil gas have anomalous values at the specific positions in each of the traverses. The trace of these positions coincides with the N-S trending faults and/or fractures, that is, the postulated trend and pattern of the faults in southern Taiwan. Hence, helium and carbon dioxide are useful index gases in this area. Based on the helium and carbon dioxide concentrations of the soil gases, at least three components are required to explain the observed variations. In addition to the atmospheric air component, two gas sources can be recognized. One is the deep crust component, exhibiting high He and CO₂ concentrations, and considered as best indicator for the surface location of fault/fracture zones in the region. The other component could be a shallower gas source with high CO₂ concentration, and low He concentration. Moreover, helium isotopic compositions of representative samples vary from 0.52 to 1.05 Ra (the ³He/⁴He ratio of air), illustrating that most samples have soil air component and may be mixed with some crustal component but no significant input of mantle component. Carbon isotopic composition (δ¹³C) of carbon dioxide in the soil samples vary from -11.8 to -23.4‰, which could be the result of mixing of organic and limestone components. Both helium and carbon isotopic results support the multiple gas sources in studied area. Meanwhile, continuous monitoring indicates that soil gas variations at fault zone may be closely related to the local crustal stress and hence, is suitable for further monitoring on fault activities.

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Long-term radiometric and hydrological investigations at the Wettinquelle mineral spring in Bad Brambach demonstrated the existence of a fluidal connection to the currently most frequent earthquake-swarm hypocentre at Novy Kostel, 10 km east of Bad Brambach. The gas composition and δ¹³C_CH4 values of this mineral spring were monitored from May 2000 until October 2003, i.e., before, during and after the protracted swarm earthquake period from late August until late December 2000. About eight weeks after the beginning of the seismically active period, we observed an increase in the methane concentration (from ≈40 up to ≈250 ppmv) accompanied by a decrease in the methane δ¹³C values from ≈-50 to ≈-70‰. For more than two years, such periods of variations were repeatedly observed before returning to the "baseline" signature. It is assumed that this additional methane was microbially produced in the granite-enclosed aquifer using H₂, which was released (seismically triggered) from the fissured granite in which the Wettinquelle spring capture is located. The additional methane production might have started as a co-seismic event, with only the migration from the deep granite to the surface being responsible for the eight-week delay.

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The total amount of CO₂ released at Ischia Island has been estimated from soil gas flux measurements and from chemical composition of the gases released by fumaroles or dissolved in groundwaters. The preliminary results indicate an overall CO₂ output of about 15 kg s^-1 from the entire island (46 km²). The main contribution to the total output from diffuse soil degassing is about 14.8 kg s^-1, followed by dissolved CO₂ of about 0.3 kg s^-1. The contribution of fumaroles to the total output was found to be negligible (about 0.03 kg s^-1). Ischia's output, although being considerably less than that of open conduit volcanoes, is higher than many other volcanic systems, especially those related to volcanic arcs. The recent tensile tectonic regime of the area allows probably an easier upflow of CO₂ from the mantle sustaining the diffuse degassing of the island.

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The possibility of understanding natural processes leading to explosive events in volcanic systems provides advantages for a better management of possible volcanic crises. On account of the possibility of the occurrence of other phenomena, such as tsunamis, the explosions driven by submarine volcanic systems are of particular interest, although little investigated. The recent sudden increase in the degassing activity of the submarine geothermal system of Panarea Island (Aeolian arc), has allowed us to better understand the way in which the quiet degassing activity of a submarine hydrothermal system may develop if new magma or magmatic gases feed it. We focused our investigations on the crater-shaped area where the volcanic crisis started, with the aim of evaluating whether the crater was formed by an explosive event or by sediment erosion due to the intense gas flow rate. The calculated energetic conditions, coupled with the computed physic-chemical state of the fluids at the level of the deep reservoir, provided the theoretical boundary conditions of the occurred event, while suggesting that a low-energy explosion was responsible for producing the crater at the sea bottom.

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Kueishantao is a Holocene volcanic islet (<7,000 yrs) located at NE offshore Taiwan, and tectonically is part of western extension of the Okinawa Trough. Magmatic activities are considered very active around this area on the basis of the fact that on-land fumaroles and submarine hydrothermal systems are prevailing currently. Representative bubble gas samples from submarine hydrothermal vents were collected for gas composition and helium isotope analysis. The gases show similar compositions of low temperatre fumaroles in the world, i.e., with high CO₂ and H₂S but low SO₂ and HCl contents. They exhibit consistent high ³He/⁴He ratios (7.3-8.4 R_A, where R_A is the ³He/⁴He ratio of air), probably the highest ³He/⁴He values of gases ever reported in active hydrothermal areas of the western Pacific region. Meanwhile, seawater samples around Kueishantao and other fluid samples from I-Lan Plain, the land area closest to the Kueishantao and also the southernmost part of the Okinawa Trough, show a significant excess of ³He compositions as well. This indicates that the mantle component plays an important role for their gas sources, and implies that the mantle fluids may have invaded into I-Lan Plain. The westward opening of the Okinawa Trough may have caused thinning of the continental crust and produced deep normal faults and hence, the primordial ³He is able to degas from mantle source region without significant crust contamination.

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