Application of reflection seismics to the assessment of cool-mining-related subsidence in the Karoo Basin and neotectonic activity in the offshore orange basin

Abstract

The 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.