Abstract
Shale gas has generated economic interest as a hydrocarbon resource for energy production globally. The shale gas exploration initiatives in the Karoo Basin are encouraged by the successes achieved by the United States of America (USA) with regards to unconventional shale gas exploration. In this study, the Cisuralian shales of the Karoo Basin and the Devonian/Carboniferous Berea Sandstone project samples from the Appalachian Basin, USA, were characterized and compared using geochemistry, organic petrology and Raman spectroscopy characterization techniques. The Devonian/Carboniferous Berea Sandstone project samples were included in order to assess the characterization techniques used in South Africa as compared to those used in USA and also for quality assurance purposes. This assessment aided in strengthening the local analytical competency and bolstered confidence in the researcher’s capability to conduct analysis such as total organic carbon (TOC) and thermal maturity assessments. Horizons from two boreholes (KZF-01 and KWV-01) drilled in the southern and south-eastern Karoo Basin were evaluated. Analytical techniques used to characterize the shale samples included TOC, total sulphur (TS), organic petrology (solid bitumen and vitrinite reflectance, and maceral analysis), X-ray Diffraction (XRD), carbon isotopes, N2 and CO2 adsorption, programmed pyrolysis, and Raman spectroscopy. The TOC data indicates that the Whitehill Formation of the Karoo Basin has a good to excellent shale gas resource potential, with TOC’s averaging at 4.35 wt%. However, thermal maturity data of the Whitehill Formation shows the organic matter to be overmature, with vitrinite reflectance equivalent values greater than 3% RoV. The Devonian/Carboniferous Berea Sandstone project samples are thermally immature, with the abundance of solid bitumen macerals. Comparisons were drawn between the Whitehill Formation data (KZF and KWV results) generated from this study and the literature data from the overmature shales from gas producing areas such as Middle Devonian Marcellus and Silurian Longmaxi marine shales. The analyses results generated from KZF and KWV samples are comparable to the literature data from the Marcellus and Longmaxi Formation shales, in terms of the thermal maturity (all are overmature), TOC (all rich in organic carbon) and also contain major minerals such as quartz and clay. The presence of dolerites sills and pyrrhotite in KWV-01 of the Whitehill shales clearly differentiate the shales from the Marcellus and Longmaxi Formation shales. Total sulphur data for the Whitehill
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Formation and the Devonian/Carboniferous shales suggests that both formations were influenced by marine environments. The Berea Sandstone project samples contain high amounts of quartz, and clay mineral contents. Carbon isotope analysis reflected the absence of carbonates in both Berea Sandstone project samples and KWV samples. A positive correlation between the volume of micropores as determined by CO2 adsorption and the TOC is observed for the Berea Sandstone project samples, implying that micropores in these shales were mainly developed in the organic matter. However, the poor correlation between micropores volume as determined by CO2 adsorption and the TOC in the KWV samples implies that there are factors other than the organic fraction which influence their micropore structure, and such factors may include inorganic materials such as clay minerals. Programmed pyrolysis data indicated that the KWV samples are overmature, and they fall in the category of poor generation resource potential. Raman spectroscopy demonstrated good potential to predict thermal maturity in shales as the technique provided insights about the molecular structure of the organic matter. In combination with petrographic approaches, Raman spectroscopy can be used as a complementary analytical technique with potential to replace conventional methods that are usually time consuming in terms of sample preparation and performing the analysis. In conclusion, Raman spectroscopy was successfully applied as a complementary analytical technique in organic matter characterization of potential source rocks. Collectively, this research provides valuable data for the enhanced understanding of analytical techniques for shale gas exploration in the Karoo Basin, South Africa.