ETD Collection

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    Tectonic evolution of the deepwater Orange Basin (offshore South Africa) using 3D reflection seismic data: implications for hydrocarbon systems
    (2018) Mahlalela, Vuyolwethu
    This 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.
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    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, Mbali
    At 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.