Hydrocarbon‐Rich Groundwater above Shale‐Gas Formations: A Karoo Basin Case Study

Abstract

Horizontal drilling and hydraulic fracturing have enhanced unconventional hydrocarbon recovery but raised environmental concerns related to water quality. Because most basins targeted for shale-gas development in the USA have histories of both active and legacy petroleum extraction, confusion about the hydrogeological context of naturally occurring methane in shallow aquifers overlying shales remains. The Karoo Basin, located in South Africa, provides a near-pristine setting to evaluate these processes, without a history of conventional or unconventional energy extraction. We conducted a comprehensive pre-industrial evaluation of water quality and gas geochemistry in 22 groundwater samples across the Karoo Basin, including dissolved ions, water isotopes, hydrocarbon molecular and isotopic composition, and noble gases. Methane-rich samples were associated with high-salinity, NaCl-type groundwater and elevated levels of ethane, He, and other noble gases produced by radioactive decay. This endmember displayed less negative δ C-CH and evidence of mixing between thermogenic natural gases and hydrogenotrophic methane. Atmospheric noble gases in the methane-rich samples record a history of fractionation during gas-phase migration from source rocks to shallow aquifers. Conversely, methane-poor samples have a paucity of ethane and He, near saturation levels of atmospheric noble gases, and more negative δ C-CH ; methane in these samples is biogenic and produced by a mixture of hydrogenotrophic and acetoclastic sources. These geochemical observations are consistent with other basins targeted for unconventional energy extraction in the USA and contribute to a growing data base of naturally occurring methane in shallow aquifers globally, which provide a framework for evaluating environmental concerns related to unconventional energy development (e.g., stray gas).

Structural and Hydrogeological Controls on Hydrocarbon and Brine Migration into Drinking Water Aquifers in Southern New York

“…The methods for the analysis of inorganic constituents were identical to those reported previously in this volume (Harkness et al ). The methods for the analysis of major gases and noble gases were also identical to those reported previously (Darrah et al ; Kang et al ; Moore et al ; Eymold et al ).…”

“…Elevated methane concentrations corresponded to samples with elevated levels of salts with a Na‐Cl composition (Figure ; Tables and ), higher relative ethane levels, markedly higher levels of 4 He, and elevated 20 Ne/ 36 Ar (Figures , and ). Groundwater samples that exhibited correlations between these components have been attributed previously to the migration of a hydrocarbon‐rich brine from source rocks to shallow aquifers over unknown periods of geological time (Warner et al , ; Jackson et al, ; Darrah et al ; Eymold et al ). These observations are consistent with other areas in the NAB and around the world (Darrah et al , , ; Harkness et al ; Moore et al ; Eymold et al ).…”

“…The process of hydrocarbon gas and brine migration along faults and zones of intense fracturing has also been widely documented in the literature (Evans ; Caine et al ; Jacobi ; Moritz et al ; Harkness et al , ; Eymold et al ). Although south‐central New York is often considered structurally featureless, consisting only of a gently southward‐dipping sequence of broadly folded anticlines, the area contains many well‐documented zones of regional‐scale faults.…”

“…The emergence of unconventional energy development from tight shales has catalyzed concerns about the potential for compromised drinking‐water quality near areas of shale‐gas development (e.g., Osborn et al ; Warner et al ; Jackson et al ; Molofsky et al ; Brantley et al ; Darrah et al , ; Vengosh et al ; Warner et al ; Siegel et al ; Humez et al ; Harkness et al ; Nicot et al , , ; Darrah ). Fugitive methane contamination has been documented in a subset of wells near drill sites (Osborn et al ; Jackson et al ; Darrah et al ; Heilweil et al ; Sherwood et al ), while others have documented natural methane contamination unrelated to shale gas development (Warner et al ; Molofsky et al ; Baldassare et al ; Darrah et al ; Siegel et al ; Harkness et al ; Eymold et al ).…”

“…Specifically, we interrogate the geospatial patterns and mechanisms of hydrocarbon gas and brine contamination in shallow aquifers with respect to known and mapped geological structures. To accomplish this, we examine a suite of geochemical tracers (i.e., dissolved inorganic ions, the molecular and isotopic composition of hydrocarbons, and noble gas elemental abundances and their isotopes) that have proven to be effective tracers of crustal fluid migration in other locations within the NAB and elsewhere (Warner et al ; Jackson et al ; Darrah et al , , ; Wen et al ; Harkness et al , ; Eymold et al ).…”