Exploring the 2013-2018 degassing mechanism from the Pesje and Preloge excavation fields in the Velenje Coal basin, Slovenia: insights from molecular composition and stable isotopes.

Gas samples were collected from 25 m long horizontal boreholes drilled into the excavation field at 10° inclination to the longwall face in two mining areas, Pesje and Preloge, in the Velenje Coal Mine, Slovenia, from 2013 to 2018. The degassing mechanism of coalbed gas and its stable isotopic composition (δ13CCO2, δ13CCH4, and δ2HCH4) were investigated in boreholes in advance of eight working faces. The major coalbed gas constituents were CO2 and methane. Gas concentrations and isotope values revealed that the methane is biogenic in origin with δ13CCH4 values of -69.4 to -29.5 ‰, δ2HCH4 values of -301 to -222 ‰, and a fractionation factor (αCO2-CH4) of 0.998-1.073, suggesting that methane derives from microbial acetate fermentation and CO2 reduction. The carbon dioxide methane index values ranged from 50.0-98.3 vol.% and δ13CCO2 values from -11.8 to -0.5 ‰, indicating that CO2 is biogenic and endogenic in origin. The degassing mechanism results in isotope fractionation of methane and CO2 for carbon isotopes up to 39.9 ‰ and up to 8.5 ‰, respectively, depending on the position of the excavation fields in space, e.g. under pre-mined coal area, fresh overburden.

Interlaboratory test for stable carbon isotope analysis of dissolved inorganic carbon in geothermal fluids.

RATIONALE: Stable carbon isotope ratios have many applications in natural sciences. In the first worldwide interlaboratory proficiency test, the discrepancies in measured δ13 CDIC values of natural waters were up to σ = ±3‰. Therefore, we continued the investigation on the analytical data quality assurance of individual laboratories and internal consistency among laboratories worldwide. METHODS: We designed and performed an interlaboratory comparison exercise for δ13 C analyses of ten water and two solid samples (Na2 CO3 , CaCO3 ), including two synthetic samples prepared by dissolving the carbonates individually. Three laboratories analyzed an additional sample set to assess solution stability, at least one month after the first set analysis period. The δ13 C values were measured using dual inlet isotope ratio mass spectrometry (DI-IRMS) or continuous flow (CF)-IRMS. RESULTS: The δ13 C values of solid Na2 CO3 and its aqueous solution were -5.06 ± 0.21‰ and 5.32 ± 0.24‰, respectively, while the δ13 C value of solid CaCO3 was -4.49 ± 0.93‰. Similarly, the lake water has a consistent value (2.45 ± 0.19‰). The δ13 C values of geothermal water have a wide dispersion among individual laboratory measurements and among those of different laboratories; however, a trend exists in the δ13 C values measured at the three sampling points of each well. CONCLUSIONS: The δ13 C values of solid Na2 CO3 and its solution, and lake water (i.e. DIC concentration samples >100 mg/L carbon) are consistent among all the participating laboratories. The dispersion in the δ13 C values of solid CaCO3 is associated with its lower chemical affinity than that of Na2 CO3 . The poor reproducibility in the δ13 C values of geothermal fluids, collected at three points of a geothermal well, despite overall consistent trends regarding their collection points suggests inadequate sample handling (atmospheric CO2 exchange) and/or inappropriate analytical approaches (incomplete H3 PO4 acid reaction).

Inter-laboratory test for oxygen and hydrogen stable isotope analyses of geothermal fluids: Assessment of reservoir fluid compositions.

RATIONALE: Knowledge of the accuracy and precision for oxygen (δ18 O values) and hydrogen (δ2 H values) stable isotope analyses of geothermal fluid samples is important to understand geothermal reservoir processes, such as partial boiling-condensation and encroachment of cold and reinjected waters. The challenging aspects of the analytical techniques for this specific matrix include memory effects and higher scatter of delta values with increasing total dissolved solids (TDS) concentrations, deterioration of Pt-catalysts by dissolved/gaseous H2 S for hydrogen isotope equilibration measurements and isotope salt effects that offset isotope ratios determined by gas equilibration techniques. METHODS: An inter-laboratory comparison exercise for the determination of the δ18 O and δ2 H values of nine geothermal fluid samples was conducted among eleven laboratories from eight countries (CeMIEGeo2017). The delta values were measured by dual inlet isotope ratio mass spectrometry (DI-IRMS), continuous flow IRMS (CF-IRMS) and/or laser absorption spectroscopy (LAS). Moreover, five of these laboratories analyzed an additional sample set at least one month after the analysis period of the first set. Statistical evaluation of all the results was performed to obtain the expected isotope ratios of each sample, which were then subsequently used in deep reservoir fluid composition calculations. RESULTS: The overall analytical precisions of the measurements were ± 0.2‰ for δ18 O values and ± 2.0‰ for δ2 H values within the 95% confidence interval. CONCLUSIONS: The measured and calculated δ18 O and δ2 H values of water sampled at the weir box, separator and wellhead of geothermal wells suggest the existence of hydrogen and oxygen isotope-exchange equilibrium between the liquid and vapor phases at all sampling points in the well. Thus, both procedures for calculating the isotopic compositions of the deep geothermal reservoir fluid - using either the analytical data of the liquid phase at the weir box together with those of vapor at the separator or the analytical data of liquid and vapor phases at the separator -are equally valid.

Stable carbon isotope analysis of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in natural waters--results from a worldwide proficiency test.

RATIONALE: Stable carbon isotope ratios of dissolved inorganic (DIC) and organic carbon (DOC) are of particular interest in aquatic geochemistry. The precision for this type of analysis is typically reported in the range of 0.1‰ to 0.5‰. However, there is no published attempt that compares δ(13)C measurements of DIC and DOC among different laboratories for natural water samples. METHODS: Five natural water samples (lake water, seawater, two geothermal waters, and petroleum well water) were analyzed for δ(13)CDIC and δ(13)CDOC values by five laboratories with isotope ratio mass spectrometry (IRMS) in an international proficiency test. RESULTS: The reported δ(13)CDIC values for lake water and seawater showed fairly good agreement within a range of about 1‰, whereas geothermal and petroleum waters were characterized by much larger differences (up to 6.6‰ between laboratories). δ(13)CDOC values were only comparable for seawater and showed differences of 10 to 21‰ for other samples. CONCLUSIONS: This study indicates that scatter in δ(13)CDIC isotope data can be in the range of several per mil for samples from extreme environments (geothermal waters) and may not yield reliable information with respect to dissolved carbon (petroleum wells). The analyses of lake water and seawater also revealed a larger than expected difference and researchers from various disciplines should be aware of this. Evaluation of analytical procedures of the participating laboratories indicated that the differences cannot be explained by analytical errors or different data normalization procedures and must be related to specific sample characteristics or secondary effects during sample storage and handling. Our results reveal the need for further research on sources of error and on method standardization.

Nitrogen isotopes determination in natural gas: analytical method and first results on magmatic, hydrothermal and soil gas samples.

A continuous-flow GC/IRMS technique has been developed to analyse delta(15)N values for molecular nitrogen in gas samples. This method provides reliable results with accuracy better than 0.15 per thousand and reproducibility (1sigma) within+/-0.1 per thousand for volumes of N(2) between 1.35 (about 56 nmol) and 48.9 muL (about 2 mumol). The method was tested on magmatic and hydrothermal gases as well as on natural gas samples collected from various sites. Since the analysis of nitrogen isotope composition may be prone to atmospheric contamination mainly in samples with low N(2) concentration, we set the instrument to determine also N(2) and (36)Ar contents in a single run. In fact, based on the simultaneously determined N(2)/(36)Ar ratios and assuming that (36)Ar content in crustal and mantle-derived fluids is negligible with respect to (36)Ar concentration in the atmosphere, for each sample, the degree of atmospheric contamination can be accurately evaluated. Therefore, the measured delta(15)N values can be properly corrected for air contamination.