3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Application of 3D seismics to enhance mapping of potholes in the western Bushveld Complex, South Africa(2019) Sehoole, Lebogang Tshepiso CharmaineThe ~2.06 Ga Bushveld Complex in South Africa is widely known as the world’s largest layered igneous intrusion. The Bushveld Complex hosts the world's largest platinum and chromium deposits. The two of the major economic platinum-bearing ore bodies (known as reefs) are the Merensky and the Upper Group-2 (UG-2) reefs, which are located at depths between 500 m and 1.5 km below surface. Mining operations on these horizons are often complicated by the presence of geological features such as faults, dykes, potholes and iron-rich ultramafic pegmatite (IRUP) bodies. This emphasises the importance of characterising these features and predicting their occurrence. In this study, the 3D reflection seismic method is employed to delineate these structures (mainly potholes) within the Western Bushveld Complex, South Africa. Potholes are slump structures that disturb mining processes and ultimately decrease the mining productivity. Various state-of-the-art techniques were used to identify and analyse potholes, including (i) horizon-based seismic attributes, (ii) complex-trace attributes, (iii) 3D volumetric attributes, (iv) difference-of-two-surfaces, and (v) geostatistical methods. The seismic techniques complimented each other in detecting and identifying approximately 43 potholes, which were then used for geostatistical analysis. The results suggest that pothole structures are asymmetric and are often associated with faults. This study also reveals that a majority of the potholes imaged in the seismic data have propagated between the Merensky Reef and the UG-2 levels, i.e., MR and UG-2 are affected by the same potholes. These potholes are randomly distributed and exhibit some clustering. A k-mean cluster analysis was implemented and revealed two clusters that are present in the study area. The various hypotheses on the formation of potholes are investigated. This study rules out some of the hypotheses for the formation of potholes and proposed that the potholes might have formed from multiple mechanisms as opposed to just one. Statistical analyses suggest a positive linear relationship between the pothole depth and the pothole diameter, indicating that the pothole may have grown in width and depth at the same time.Item Tectonic evolution of the deepwater Orange Basin (offshore South Africa) using 3D reflection seismic data: implications for hydrocarbon systems(2018) Mahlalela, VuyolwethuThis study applies seismic attributes to the high-resolution 3D reflection seismic data acquired for the deepwater exploration block in the Orange Basin, offshore South Africa, to provide insight into the mechanisms controlling deformation in hydrocarbon systems. The seismic interpretation was carried out on high-resolution, prestack time-migrated 3D seismic data that image post-rift sequences from the Upper Cretaceous to the Quaternary. Three post-rift sequences were identified in the seismic sections; post-rift sequences II, IV and V. Two slumping events caused by gravity failure of the margin were also identified in the Coniacian to Santonian (post-rift sequence II) and Late Cenozoic (post-rift sequence V) sequences. The seismic data also image a thrust fault system in the lower slope, which is interpreted to have been formed by episodic gravity failure from the Coniacian to Santonian. The geometry of this thrust fault system formed independently from the two detachment levels which coincide with the Cenomanian-Turonian boundary source rocks and top of the Turonian marine shale unit. The shale detachments are inferred to be moderately overpressured. The Late Cenozoic slumping event creates S to WSW and N to ENE dipping normal faults that cross-cut one another. The thrust faults and S to WSW dipping normal faults were interpreted as possible conduits for hydrocarbon migration to the seafloor to form pockmarks. Large submarine canyons were interpreted to have formed by a combination of the two slumping events. This study is the first attempt to constrain the tectonic history of the deepwater Orange Basin through the interpretation of high-resolution 3D seismic data acquired for hydrocarbon exploration. In this study, it is proposed that high-resolution 3D seismics should be recognised as a fundamental geophysical method in studying the tectonic history of the region and to detect hydrocarbons and structures that affect these economic resources.Item Integrated interpretation of 3D seismic data using seismic attributes to understand the structural control of methane occurrences at deep gold mining levels: West Wits Line Goldfield, South Africa(2017) Mkhabela, MbaliAt a number of gold mines in South Africa, the presence of methane gases has been encountered when drilling into faults and/or dyke structures extending to depths beyond 4.5 km. Methane gas has been reported to have migrated through structures from within the basin to the mine working environments (~3.0 km depths) and caused explosions. The Booysens Shale is considered one of the possible source rocks for hydrocarbons and it forms the footwall to the gold-bearing Ventersdorp Contact Reef (VCR, ~ 1.5 m thick). The Booysens Shale lies at depths between 3.5 km and 4.5 km below land surface and can be best described as the base of the divergent clastic wedge which thickens westward, hosting the quartzite and conglomerate units that sub-crop against the VCR towards the east of the gold mining areas. Geometric attributes (dip and dip azimuth) and instantaneous attributes (phase, frequency and envelope) computed for the Booysens Shale and Ventersdorp Contact Reef horizons (interpreted from 3D prestack time migrated data acquired in the Witwatersrand goldfields) provide insight into structures that extend from the Booysens Shale into the overlying mining level, the Ventersdorp Contact Reef. These attributes provide high-resolution mapping of the structures (faults, dykes, and joints) that have intersected both the Ventersdorp Contact Reef and Booysens Shale horizons. Volumetric fault analysis using the ant-tracking attribute incorporated with methane gas data also show the continuity and connections of the faults and fracture zones possibly linked to methane gas and fluid migration. Correlation between the known occurrence of fissure water and methane with geologically- and seismically-mapped faults show that steeply dipping structures (dip>60°) are most likely to channel fracture water and methane. δ13C and δ2H isotope results suggest that the methane gas (and associated H2 and alkanes) from the goldfields, particularly along seismically delineated faults and dykes, have an abiogenic origin produced by water-rock reactions. Isotopic data derived from adjacent goldfields also suggests the possibility of mixing between microbial hydrocarbons (characterized by highly depleted 2HCH4 values) and abiogenic gases. It is, therefore, possible that the propagation of these structures, as mapped by 3D seismics and enhanced volumetric attributes, between Booysens Shale and Ventersdorp Supergroup provide conduits for mixing of fluids and gases encountered at mining levels. The study may provide new evidence for the notion of hydrocarbons, particularly CH4, having migrated via faults and dykes from depth, within the Witwatersrand Basin, to where they are intersected at mining levels. The research gives new insight into mixing between microbial and abiogenic end-members within hydrogeologically isolated water pockets.