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Nelson Mandela University (2019)

Identifying methane emissions with isotopic and hydrochemical clues to their origin across selected areas of the Karoo Basin, South Africa

Campbell, Richard Duncombe

Titre : Identifying methane emissions with isotopic and hydrochemical clues to their origin across selected areas of the Karoo Basin, South Africa

Auteur : Campbell, Richard Duncombe

Université de soutenance : Nelson Mandela University

Grade : Magister Scientiae 2019

Résumé partiel
Currently, only sparse data exists on the methane emissions from the Main Karoo Basin, South Africa, where the potential discovery of vast quantities of unconventional natural gas from the black shales of the Whitehill Formation has sparked great interest in the prospect of hydraulic fracturing. In this study, a new infield methane quantification instrument (Picarro G2201-i) is used to identify freely emitted methane and more importantly collect stable carbon isotope ratios (δ13C-(CH4)g) that is used in obtaining information regarding the genetic origin and thermal maturity of the methane. Hydrochemical information (TOC, δ13C-TOC, 3H, δ18O, δ2H, and anions) were also scrutinised to assist in the determination of the origin of methane across the Karoo Basin. Twenty-one sites were investigated across four provinces in the Main Karoo Basin. Six of these sites are thermal springs, three are shallow boreholes (<100 m in depth), and twelve were Soekor (The Southern Oil Exploration Corporation)/deep boreholes (>1500 m). Of these 21 sites, 17 were suitable for either methane analysis or groundwater sampling, with four of the Soekor boreholes unsuitable. The presence of methane in groundwater and being freely emitted seems to be a common occurrence above the Main Karoo basin and of the 17 sites investigated, 14 had freely emitted methane emission. All but one of these sites had δ13C-(CH4)g signatures greater than -50 ‰, indicating a thermogenic origin. Combining the results obtained from the Picarro instrument with those compiled by Talma & Esterhuyse (2015), a higher resolution distribution map was created. The δ13C-CH4 signatures show patterning with a decreasing trend from the southern Karoo Basin to the north, which corresponds to the general decrease in thermal maturity of the Ecca shales (Whitehill Formation) northward across the Karoo Basin. The δ13C-(CH4)d results from a case study conducted by Eymold et al. (2018) differ significantly with the data collected in this study that included several of the same sampling locations.

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