Abstract
Abstract The large-scale extraction of unconventional hydrocarbons in the United States has led to fears of methane contamination of shallow groundwaters. Differentiating between the deep gas released during extraction (shale gas, coal bed methane and underground coal gasification) and natural shallow-sourced methane is imperative for the monitoring and managing of environmental risks related to the extraction process. Here, for the first time, we present measurements of the major gas, and stable and noble gas isotope composition of coal bed methane (CBM) from central Scotland and coal mine methane (CMM) from central England, UK. The molecular (C1 / (C2 + C3) = 21 to 121) and stable isotope compositions (δ13CCH4 = − 39.5 to − 51.1‰; δDCH4 = − 163 to − 238‰) indicate a thermogenic origin for the methane. They are distinct from the majority of shallow-sourced gases in UK. Both sample suites exhibit high He concentrations (338 to 2980 ppmv) that are considerably above atmospheric and groundwater levels. Simple modelling shows that these high 4He concentrations cannot be solely derived from in situ production since coal deposition, and hence the majority is derived from the surrounding crust. The Scottish CBM contains a resolvable mantle He, Ne and Ar contribution that may originate from melts in the deep crust, demonstrating the UK coals have acted as a store for deep volatiles for 10s of millions of years. The high 4He in the coal-derived gases has the potential to be used as a novel diagnostic fingerprint to track fugitive release of deep methane from future unconventional gas extraction operations in the UK.
Highlights
The development of horizontal drilling and hydraulic fracturing techniques has permitted the extraction of oil and gas from an array of unconventional reservoirs (e.g. Tour et al, 2010)
Many existing (North America, Australia) and emerging (e.g. China, Argentina, Russia, Brazil) unconventional gas reservoirs are close to potable water resources (Conti et al, 2013; Day, 2009; Mauter et al, 2014; Measham and Fleming, 2014; Vörösmarty et al, 2010), and concern has been raised over the impact of unconventional hydrocarbon exploitation, often by hydraulic fracturing, on groundwater resources (e.g. Vengosh et al, 2014)
The C and H isotopic compositions of the CH4 exhibit narrow ranges (δ13CCH4 = −39.5 to − 41.8‰; δDCH4 = −163 to − 220‰) that are typical of thermogenic methane (Table 1, Figs. 3 & 4)
Summary
The development of horizontal drilling and hydraulic fracturing techniques has permitted the extraction of oil and gas from an array of unconventional reservoirs (e.g. Tour et al, 2010). Many existing (North America, Australia) and emerging (e.g. China, Argentina, Russia, Brazil) unconventional gas reservoirs are close to potable water resources (Conti et al, 2013; Day, 2009; Mauter et al, 2014; Measham and Fleming, 2014; Vörösmarty et al, 2010), and concern has been raised over the impact of unconventional hydrocarbon exploitation, often by hydraulic fracturing, on groundwater resources The molecular (e.g. C1 / (C2 + C3)) and stable isotopic (e.g. δ13CCH4, δDCH4 or Δ13C = δ13CCH4 − δ13CC2H6) composition of hydrocarbon gases can be used to differentiate between thermogenic and biogenic sources (Jackson et al, 2013a; Kornacki and McCafrey, 2011; Osborn et al, 2011; Whiticar, 1999). Simple mixing between biogenic and thermogenic methane can mask the initially diagnostic isotopic composition (e.g. Whiticar, 1999)
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