- Title
- Identifying methane emissions with isotopic and hydrochemical clues to their origin across selected areas of the Karoo Basin, South Africa
- Creator
- Campbell, Richard Duncombe
- Subject
- Methane -- Environmental aspects -- South Africa
- Subject
- Atmospheric methane -- Environmental aspects -- South Africa
- Subject
- Groundwater -- Research
- Date Issued
- 2019
- Date
- 2019
- Type
- Thesis
- Type
- Masters
- Type
- MSc
- Identifier
- http://hdl.handle.net/10948/42218
- Identifier
- vital:36636
- Description
- 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. This is explained by a two phase partitioning (gas + water) that leads to the thermogenic endmember of methane being released in its free state (analysed by Picarro G2201-i) and microbial methane that is formed in situ remains dissolved in the water (analysed by Eymold et al. 2018). Soekor and deep sites; SA 1/66, KA 1/66, and KWV-1 that have direct pathways for methane migration from the Whitehill are deemed the best proxies to resolve the thermogenic endmember of methane, with δ13C-(CH4)g signatures of -26.32‰, 31.66‰, and -34.57‰, respectively. The hydrochemistry results suggests that that free methane emissions do not necessarily have to be associated with saline Cl- waters, as multiple sites have CH4 emissions with low salinities (Cl < 50 mg/L) and that methane in its free state can migrate to the surface due to buoyancy. The results also indicate that dolerite intrusions act as conduits for upward migration of groundwater from depth, but that the deep groundwater signatures proposed by Murray et al., (2015) are related to their different migration pathways and water-rock interactions rather than being representative of the deep formation waters. Using an initial assessment δ13C-CH4, TOC concentration [TOC] and the tritium (3H) values, where water samples that have 3H ≤ 1 TU, detectable TOC and δ13C-CH4 signatures > -50‰ could indicate hydraulic connectivity between the shallow aquifer and an organic/CH4 rich sedimentary layer, which may or may not be from the Whitehill Formation. However, this method for determining aquifer connectivity requires further investigations in the Karoo Basin context. The results obtained in this study add to the limited isotopic data of methane across the Karoo Basin and demonstrates the effectiveness of an infield identification of methane emissions using the Picarro G2201-i.
- Format
- xviii, 126 leaves
- Format
- Publisher
- Nelson Mandela University
- Publisher
- Faculty of Science
- Language
- English
- Rights
- Nelson Mandela University
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