Assessing Connectivity Between an Overlying Aquifer and a Coal Seam Gas Resource Using Methane Isotopes, Dissolved Organic Carbon and Tritium

Abstract

Coal seam gas (CSG) production can have an impact on groundwater quality and quantity in adjacent or overlying aquifers. To assess this impact we need to determine the background groundwater chemistry and to map geological pathways of hydraulic connectivity between aquifers. In south-east Queensland (Qld), Australia, a globally important CSG exploration and production province, we mapped hydraulic connectivity between the Walloon Coal Measures (WCM, the target formation for gas production) and the overlying Condamine River Alluvial Aquifer (CRAA), using groundwater methane (CH4) concentration and isotopic composition (δ13C-CH4), groundwater tritium (3H) and dissolved organic carbon (DOC) concentration. A continuous mobile CH4 survey adjacent to CSG developments was used to determine the source signature of CH4 derived from the WCM. Trends in groundwater δ13C-CH4 versus CH4 concentration, in association with DOC concentration and 3H analysis, identify locations where CH4 in the groundwater of the CRAA most likely originates from the WCM. The methodology is widely applicable in unconventional gas development regions worldwide for providing an early indicator of geological pathways of hydraulic connectivity.

Biogeochemical constraints on the origin of methane in an alluvial aquifer: evidence for the upward migration of methane from underlying coal measures

“…A low-permeability layer (ranging from 8 × 10 −6 to 1.5 × 10 −1 m day −1 ) has been reported between the CRAA and the underlying WCM (KCB, 2011;QWC, 2012). This has been referred to as the "transition layer" (QWC, 2012) or a "hydraulic basement" to the alluvium (KCB, 2011).…”

“…The WCM are thicker (150 to 350 m) along the western margin of the CRAA and thin to around 50 m in the east, where the unit outcrops (KCB, 2011); however, only around 10 % of this is coal. The unit consists of very fineto medium-grained sandstone, siltstone, mudstone and coal, with minor calcareous sandstone, impure limestone and ironstone (KCB, 2011). The coal consists of numerous discontinuous thin lenses separated by sediments of low permeability (Hillier, 2010).…”

“…The WCM overlie the Eurombah Formation (consisting of conglomerate sandstone with minor siltstones and mudstone beds) and underlie the Kumbarilla Beds (mainly sandstone, with lesser mudstone, siltstones and conglomerates) (KCB, 2011).…”

“…We have previously shown that there may be local natural connectivity between the WCM and the CRAA (Iverach et al, 2015 ], δ 13 C-CH 4 , δ 13 C-DIC, δ 13 C-DOC and δ 2 H-H 2 O), as well as characterisation of microbiological communities present, can inform the discussion surrounding the occurrence of CH 4 and its potential for upward migration in the groundwater of the CRAA.…”

“…There is concern regarding the potential impacts of gas and fluid movement, as well as depressurisation, on groundwater quality and quantity in adjacent aquifers used to support other industries (Atkins et al, 2015;Heilweil et al, 2015;Iverach et al, 2015;Moritz et al, 2015;Owen et al, 2016;Zhang and Soeder, 2016).…”