Alexandre A. Ferreira

Formation temperatures of thermogenic and biogenic methane

Making of methane deep underground Technologies such as hydraulic fracturing, or “fracking,” can now extract natural gas from underground reservoirs. Within the gas, the ratio of certain isotopes holds clues to its origins. Stolper et al. analyzed a wide range of natural gas, including samples from some of the most active fracking sites in the United States. Using a “clumped isotope” technique, the authors could estimate the high temperatures at which methane formed deep underground, as well as the lower temperatures at which ancient microbes produced methane. The approach can help to distinguish the degree of mixing of gas from both sources. Science, this issue p. 1500 Isotopic analysis of methane indicates the timing and location of hydrocarbon gas formation in natural settings. Methane is an important greenhouse gas and energy resource generated dominantly by methanogens at low temperatures and through the breakdown of organic molecules at high temperatures. However, methane-formation temperatures in nature are often poorly constrained. We measured formation temperatures of thermogenic and biogenic methane using a “clumped isotope” technique. Thermogenic gases yield formation temperatures between 157° and 221°C, within the nominal gas window, and biogenic gases yield formation temperatures consistent with their comparatively lower-temperature formational environments (<50°C). In systems where gases have migrated and other proxies for gas-generation temperature yield ambiguous results, methane clumped-isotope temperatures distinguish among and allow for independent tests of possible gas-formation models.

Comparison of calibration curve fitting methods in the determination of H2S mass fractions in natural gas by GC-SCD

ABSTRACT This paper discusses metrologically the best practice regarding the calibration curves applied to H2S mass fractions determination in natural gas by gas chromatography with sulfur chemiluminescence detection (GC-SCD). Three calibration curves were constructed by performing GC-SCD analysis of different H2S gas standard concentrations (from 3 mg kg−1 up to 500 mg kg−1). These experimental curves are better fitted by an unweighted quadratic calibration curve considering ANOVA approach compared to ASTM D5504-12. Despite this, the obtained results show that these two different calibration curve approaches (ASTM and ANOVA) lead to comparable results. Hence, there are no significant statistical differences between these two approaches based on the hypothesis test applied. However, the quadratic calibration curve presents measurement uncertainties of H2S mass fractions much lower than the ASTM approach.