Mineralogical, petrographic and geochemical characteristics of shales of the Karoo supergroup, South Africa
The main objective of this research study is to gain a mineralogical and geochemical understanding of the potential of the source rocks of the Ecca Group to predict the success of hydraulic fracturing in the study area. A total of 12 samples were collected from the Prince Albert, Whitehill and Collingham Formations along the R67 provincial route between Grahamstown and Fort Beaufort in the Eastern Cape Province of South Africa. These samples were subjected to petrographic, mineralogical and geochemical analyses using petrographic microscopy, x-ray diffraction, x-ray fluorescence and inductively coupled plasma mass spectrometry. The main mineral constituents of the Lower Ecca Group in the study area are quartz (65.6 wt.%), illite (12.19 wt.%), smectite (0.99 wt.%), chlorite (1.19 wt.%), calcite (12.19 wt.%) and dolomite (1.19 wt.%). The Prince Albert and Whitehill Formations contain a high amount of quartz, some carbonates and the least amount of clay, while the Collingham Formation contains a high amount of quartz, some clay and the least amount of carbonate. The mineralogical composition of the Lower Ecca Group compares favourably with gas-producing shales such as the Barnett Shale of the Fort Worth Basin in the United States. The Lower Ecca Group has high quartz (68.6 wt.%) content relative to the Barnett Shale (43 wt.%). The concentration of clay minerals in the Lower Ecca Group (4.8 wt.%) is generally lower than in the Barnett Shale (31 wt.%). Dolomite is the dominant carbonate mineral in the Barnett Shale, while calcite predominates in the Lower Ecca Group. In essence, the Lower Ecca Group of the Main Karoo Basin contains a high brittle mineral and low ductile mineral content relative to the Barnett Shale. The petrographic analysis reveals that the mudstones and shales from the study area have been altered mainly due to carbonate and quartz cementations. Post-depositional diagenesis, such as the precipitation of calcite cements, occurs mainly in the Prince Albert Formation. The petrographic analysis shows that quartz and calcite cements are the most abundant, occurring mainly as fracture filling, pore-space filling and grain and matrix replacement. The deformation in the study area is largely linked to the Cape Fold Belt and assisted by dissolution. Large-ion lithophile elements (Rb, Ba, Sr, Th and U), high field strength elements (Zr, Hf, Y and Nb) and transition trace elements (Sc, V, Cr, Co, Pb, Ni and Zn) in the mudstones and shales of the study area are generally depleted compared to the upper continental crust. The concentrations of Cu, Ni, Zn and V are depleted in the Lower Ecca Group relative to the Barnett and Marcellus Shales in the United States. The concentration of major elements in the current study is generally higher than in the Barnett Shale, while the SiO2 content averages 63 wt.% in the study area-higher than in the Barnett Shale (15.16 wt.%). The Whitehill Formation of the Main Karoo Basin and the Barnett Shale of the Fort Worth Basin share the following similarities: they are composed of black carbonaceous shales, high brittle mineral constituents and high total organic content accommodated by an anoxic depositional environment. Based on this research study, the Prince Albert, Whitehill and Collingham Formations possess more brittle mineral content (quartz and carbonate) than ductile mineral content (clay). An increase in brittle minerals generally suggests that the formations are conducive to hydraulic fracturing. The overall mineralogy promotes natural fractures and the generation of induced network fractures. The Collingham Formation may enable a good seal and trap shale gas from migrating out of the shale below due to the high clay content (9.1 wt.%) compared to the Prince Albert (3.69 wt.%) and Whitehill Formations (8.75 wt.%). The performance of hydraulic fracturing would be successful if these formations were to be developed. Diagenetic transformations (cementation) may cause loss of porosity, permeability reductions and reservoir quality degradation, increasing the requirement for hydraulic fracturing and additional stimulation methods to enhance shale gas recovery and flow capacity.
A thesis submitted to the Faculty of Science, University of the Witwatersrand in fulfilment of the requirements for the degree of Master of Science, 2021