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

Abstract

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, ⁴ He, and other noble gases produced by radioactive decay. This endmember displayed less negative δ ¹³ C‐CH ₄ 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 ⁴ He, near saturation levels of atmospheric noble gases, and more negative δ ¹³ C‐CH ₄ ;more » 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).« less

Authors:

Eymold, William K. ^[1]; Swana, Kelley ^[2]; Moore, Myles T. ^[1]; Whyte, Colin J. ^[1]; Harkness, Jennifer S. ^[1]; Talma, Siep ^[3]; Murray, Ricky ^[4]; Moortgat, Joachim B. ^[1]; Miller, Jodie ^[2]; Vengosh, Avner ^[5]; Darrah, Thomas H.

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+ Show Author Affiliations

1. School of Earth Sciences The Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall Columbus OH 43210
2. Department of Earth Sciences, Stellenbosch University, Private Bag XI Matieland 7602 South Africa
3. Natural Resources and the Environment, CSIR Pretoria, P.O. Box 395 Pretoria 0001 South Africa
4. Groundwater Africa, 38 Disa Ave., Kommetjie 7975 South Africa
5. Division of Earth and Ocean Sciences, Nicholas School of the Environment Duke University Durham NC 27708

Publication Date:

Tue Feb 06 00:00:00 EST 2018

Sponsoring Org.:

USDOE

OSTI Identifier:

1419705

Grant/Contract Number:  

DE‐FE0024357

Resource Type:

Publisher's Accepted Manuscript

Journal Name:

Ground Water

Additional Journal Information:

Journal Name: Ground Water Journal Volume: 56 Journal Issue: 2; Journal ID: ISSN 0017-467X

Publisher:

Wiley-Blackwell

Country of Publication:

United States

Language:

English