- Title
- Petrophysical analysis of thermo-tectonic effects linked to Lower Jurassic dolerite intrusions in sedimentary rocks of the Main Karoo Basin, with implications for shale gas development in South Africa
- Creator
- Nengovhela, Vhuhwavhohau
- Subject
- Sedimentary rocks -- South Africa -- Research
- Subject
- Sedimentation and deposition Geology, Structural -- South Africa Geology -- South Africa
- Date Issued
- 2018
- Date
- 2018
- Type
- Thesis
- Type
- Masters
- Type
- MSc
- Identifier
- http://hdl.handle.net/10948/33802
- Identifier
- vital:33034
- Description
- Prevalent dolerite sills and dikes emplaced in the sedimentary rocks of the main Karoo Basin are associated with contact metamorphism and devolatilization of carbon-rich shales of the Lower Ecca Formation. These thermal effects may have significant bearings on the potential of shale gas development in the Karoo. This research aims to study and quantify the effects of dolerite intrusions on the Karoo sedimentary rocks along contact aureoles in shales and sandstones of the Ecca Group based on drill core samples collected from deep SOEKOR boreholes, and field samples from the Lower Beaufort Group of the Eastern Cape Province. Variations in host rock minerals, chemistry, and porosity are determined using analytical techniques including: petrography, scanning electron microscopy (SEM), X-ray diffraction (XRD) on samples collected along the thermal aureoles at various distances away from the intrusions. Using the determined mineral phase equilibria, geochemistry and thermal modelling, the pressure-temperature (P/T) conditions that prevailed during contact metamorphism are evaluated to constrain the thickness of the contact aureoles in deep boreholes, and to further evaluate potential loss of shale gas. The mineral composition of the samples generally consists of quartz, feldspar, illite/smectite, calcite, muscovite, and pyrite. Metamorphic minerals close to contacts with the sills include andalusite-chiastolite, biotite, muscovite, illite, forsterite, and diopside. SEM imaging confirms that the hornfels samples are compact and that the metamorphic minerals reduce and limit porosity to grain boundaries, and in few cases intra-mineral porosity occurs within individual crystals. The porosities are in the range of mesopores (2-50 nm) and micropores (< 2 nm). The data also suggests that the thermal alteration of the Ecca shales were short lived (e.g. 10-100 years). Disequilibrium metamorphic textures such as irregular grain boundaries, and inclusions in andalusite and cordierite confirm that the elevated temperatures did not last long enough to accomplish (re)crystallization and create equilibrium textures. To test the effect of geothermal gradients on the initial host rock temperatures prior to intrusive activity, and how these temperature differences affect the contact aureole thickness, thermal models were simulated for the deep boreholes using three geothermal gradients: 30 ◦C/km, 35 ◦C/km, and 40 ◦C/km. In addition, the depths were re-calculated based on post-emplacement erosion of the overlying host rocks (2000-3000 m). Heat flow away from dolerite sills through the host shales were modelled using convection and conduction heat flow mechanism separately, to assess differences and/or similarities in cooling trends. The models show that a 10 ◦C increase in geothermal gradient (from 30 ◦C /km to 40 ◦C/km) significantly affects the initial temperature of the host rocks and in turn decreases the rate at which the dolerite sills cool. Thermal modeling data is coherent with the mineralogy of the contact zones. Andalusitechiastolite and cordierite porphyroblasts occur within 10-20 m of the contact aureole, together with biotite and authigenic muscovite. Literature data shows that crystallization of this mineral assemblage in pelitic hornfels occurs at temperatures ranging between 450 ◦C and 600 ◦C. Thermal modelling results indicate maximum temperatures of shales at the contact with dolerite sills range between 650 ◦C to 700 ◦C. Temperatures decrease rapidly to approximately 400 ◦C within 10-20 m away from the contacts. The data also shows that thermal perturbation flanking the sills were short-lived, with maximum temperatures reached within the first 100 years of cooling depending on the thickness of dolerite sills and the combined effect of multiple sill intrusions. Data acquired from the thermal models and literature was used to evaluate gas loss estimations. The calculations conducted around a 45 m thick sill, suggests that at least 4- 6 TCF of gas was liberated from the Whitehill Formation during Karoo sill emplacement. These values are significantly less than previously calculated losses (22 TCF), and thus have a significant bearing on further predictions of shale gas potential of the Karoo Basin.
- Format
- xii, 166 leaves
- Format
- Publisher
- Nelson Mandela University
- Publisher
- Faculty of Science
- Language
- English
- Rights
- Nelson Mandela University
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