3. Electronic Theses and Dissertations (ETDs) - All submissions
Permanent URI for this communityhttps://wiredspace.wits.ac.za/handle/10539/45
Browse
5 results
Search Results
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 Application of reflection seismics to the assessment of cool-mining-related subsidence in the Karoo Basin and neotectonic activity in the offshore orange basin(2018) Isiaka, Ibrahim AhmedThe general aim of this thesis was to use the high-resolution seismic reflection method to investigate coal-mining related subsidence in the Karoo Basin and to assess hydrocarbon leakage and neotectonic activity in the offshore Orange Basin. This research work is divided into three independent parts. The first part is a shallow sub-surface investigation conducted at old coal mines located in the Benoni area and the Springlake Colliery mine, where room and pillar method was used for coal-mining. At the Benoni study area, the high-resolution seismic reflection investigation was used to detect near-surface voids related to coal mining, as well as the dissolution cavities within the underlying dolomite that are responsible for the subsidence- and sinkholes-related features commonly observed in the area. Similarly, the technique was used to delineate subsidence zone in the Springlake mine, where subsidence is caused by the collapse of the overlying strata into subsurface voids due to the removal of coal. In the second part of the thesis, a major half-graben bounded fault within the Ibhubesi gas field of the Orange Basin was investigated. The objective was to assess the vertical migration of hydrocarbon and the recent tectonic activities along the fault. It was found that the vertical migration of hydrocarbon along the fault occurred through fault-parallel extensional fractures within the fault zones, as well as in the relay zones within the fault segment boundaries. The results also provide evidence that the fault is tectonically active, and therefore it is recommended that a seismic hazard assessment is conducted around the vicinity of the fault. The third part of the thesis is focused on the investigation of an enigmatic circular geological structure in the Orange Basin. Volume rendering of high-resolution 3D reflection seismic data revealed the detailed morphology of the circular structure. The morphological characteristics exhibited by the circular structure resemble a complex impact crater. However, the confirmation that this enigmatic feature is an impact structure is subject to the analysis of core samples from boreholes within the vicinity of the structure.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.Item Seismological and mineralogical studies of the world’s deepest gold-bearing horizon, the Carbon Leader Reef, West Wits Line goldfields (South Africa): implications for its poor seismic reflective character(2016) Nkosi, Nomqhele ZamaswaziThe measurements of physical rock properties, seismic velocities in particular, associated with ore deposits and their host rocks are crucial in interpreting seismic data collected at the surface for mineral exploration purposes. The understanding of the seismic velocities and densities of rock units can help to improve the understanding of seismic reflections and thus lead to accurate interpretations of the subsurface geology and structures. This study aims to determine the basic acoustic properties and to better understand the nature of the seismic reflectivity of the world’s deepest gold-bearing reef, the Carbon Leader Reef (CLR). This was done by measuring the physical properties (ultrasonic velocities and bulk densities) as well as conducting mineralogical analyses on drill-core samples. Ultrasonic measurements of P- and S-wave velocities were determined at ambient and elevated stresses, up to 65 MPa. The results show that the quartzite samples overlying and underlying the CLR exhibit similar velocities (~ 5028 m/s-5480 m/s and ~ 4777 m/s-5211 m/s, respectively) and bulk densities (~ 2.68 g/cm3 and 2.66 g/cm3). This is due to similar mineralogy and chemical compositions observed within the units. However, the CLR has slightly higher velocity (~ 5070 m/s-5468 m/s) and bulk density (~ 2.78 g/cm3) than the surrounding quartzite units probably due to higher pyrite content in the reef, which increases the velocity. The hangingwall Green Bar shale exhibits higher velocity (5124 m/s-5914 m/s) and density values (~ 2.89 g/cm3-3.15 g/cm3) compared to all the quartzite units (including the CLR), as a result of its finer grain size and higher iron and magnesium content. In the data set it is found that seismic velocities are influence by silica, iron and pyrite content as well as the grain size of the samples, i.e., seismic velocities increase with (1) decreasing silica content, (2) increasing iron and pyrite content and (3) decreasing grain size. Reflection coefficients calculated using the seismic velocities and densities at the boundaries between the CLR and its hangingwall and footwall units range between ~0.02 and 0.05, which is below the suggested minimum of 0.06 required to produce a strong reflection between two lithological units. This suggests that reflection seismic methods might not be able to directly image the CLR as a prominent reflector, as observed from the seismic data. The influence of micro-cracks is observed in the unconfined uniaxial compressive stress tests where two regimes can be identified: (1) From 0 - 25 MPa the P-wave velocities increase with progressive loading, but at different rates in shale and quartzite rocks owing to the presence of micro-cracks and (2) above stresses of ~20 - 25 MPa, the velocity stress relationship becomes constant, possibly indicating total closure of micro-cracks. The second part of the study integrates 3D reflection seismic data, seismic attributes and information from borehole logs and underground mapping to better image and model important fault systems that might have a direct effect on mining in the West Wits Line goldfields. 3D seismic data have delineated first-, second- and third-order scale faults that crosscut key gold-bearing horizons by tens to hundreds of metres. Applying the modified seismic attribute has improved the imaging of the CLR by sharpening the seismic traces. Conventional interpretation of the seismic data shows that faults with throws greater than 25 m can be clearly seen. Faults with throws less than 25 m were identified through volumetric (edge enhancement and ant-tracking seismic attributes) and horizon-based (dip, dip-azimuth and edge detection seismic attributes) seismic attribute analysis. These attributes provided more accurate mapping of the depths, dip and strikes of the key seismic horizon (Roodepoort shale), yielding a better understanding of the relationship between fault activity, methane migration and relative chronology of tectonic events in the goldfield. The strato-structural model derived for the West Wits Line gold mines can be used to guide future mine planning and designs to (1) reduce the risks posed by mining activities and (2) improve the resource evaluation of the goldbearing reefs in the West Wits Line goldfields.