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

William K. Eymold, Kelley Swana, Myles T. Moore, Colin J. Whyte, Jennifer S. Harkness, Siep Talma, Ricky Murray, Joachim B. Moortgat, Jodie Miller, Avner Vengosh, Thomas H. Darrah

Research output: Contribution to journalArticlepeer-review

20 Scopus citations

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, 4He, and other noble gases produced by radioactive decay. This endmember displayed less negative δ13C-CH4 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 4He, near saturation levels of atmospheric noble gases, and more negative δ13C-CH4; 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).

Original languageEnglish
Pages (from-to)204-224
Number of pages21
JournalGround Water
Volume56
Issue number2
DOIs
StatePublished - 1 Mar 2018
Externally publishedYes

ASJC Scopus subject areas

  • Water Science and Technology
  • Computers in Earth Sciences

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