The effect of physical properties on the reflection seismic method: implications for shale gas exploration in the main Karoo Basin, South Africa

Abstract

The Main Karoo Basin of South Africa is being investigated for its shale gas potential. The presence of a possibly large hydrocarbon deposit (i.e., shale layer of the Whitehill Formation) is still underexplored. The reflection seismic method, amongst other geophysical methods, could yield more certainty and information about the geology at the subsurface of the basin. For the application of the reflection seismic technique to be successful, it is crucial to investigate and understand the physical properties (e.g., seismic velocities, bulk density and porosity) associated with hydrocarbons and their host rocks. To provide guidelines for future seismic surveys in the Main Karoo Basin, this study aims to determine the physical properties of Karoo Basin rocks and to better understand the nature of the seismic reflectivity of the hydrocarbon deposit (mainly shale unit), its underlying and overlying rock units. This was achieved by conducting laboratory experiments to measure P-wave (Vp) and S-wave (Vs) velocities at ultrasonic frequencies on drill-core samples as a function of stress and bulk density. The core samples were taken from the deep (~2.5 km) Karoo borehole (KWV borehole) that penetrated the entire Beaufort Group, the Fort Brown, Whitehill and Prince Albert Formations of the Ecca Group, and the Dwyka Group in the Main Karoo Basin.This study further investigates the influence of mineralogy, stress, fractures and mineral alignment on the seismic velocities of the rocks. The shale samples collected from borehole KWV (15 in total) exhibit average P-wave and Swave velocities of 3859 m/s and 2116 m/s, respectively. The shale samples have an average bulk density of 2.74 g/cm3 and porosity of 1.66 %. On average, sandstone samples (10 in total) exhibit an average Vp of 4904 m/s, Vs of 2623 m/s, bulk density of 2.68 g/cm3 and porosity of 1.12%. The shale and sandstone samples exhibit higher seismic velocities than typical soft rocks; which may be attributed to the presence of pyrite content in these samples. However the bulk densities of these samples fall within the published range of typical shale and sandstone samples. Twelve dolerite samples show average Vp and Vs values of 5105 m/s and 2718 m/s, respectively. The average bulk density and porosity for dolerite is 2.95 g/cm3 and 1.65%, respectively. No correlation exists between the bulk density and P-wave velocities of the samples from borehole KWV.

The seismic velocities of all samples tend to increase as a function of uniaxial stress, indicating a possible closure of pore spaces, macro- and micro-fractures. The velocity-stress dependence behaviour varies considerably, depending on the rock type. No correlations were observed between the bulk density, depths and porosities of the samples. A sandstone exhibits a seismic anisotropy of 5.9%, while two shale samples exhibit seismic anisotropy of 5.8% and 11.4%, respectively. The fast seismic velocity is along the direction of the mineral SPO and micro-fracture line orientations. The synthetic seismogram indicates that significant reflection coefficients (Rc) occur at most stratigraphic contacts (e.g., Rc~ 15% for shale-sandstone contact at the top of the Whitehill Formation of the lower Ecca Group), implying that these horizons, at least in terms of acoustic impedance contrasts, are favourable targets for surface reflection seismic surveys. The observed relationship between seismic velocities, mineralogy, fractures, mineral alignment and stress has particularly important implications (e.g., velocity analysis for seismic processing and time-to-depth conversion of the seismic data) for the design of future deep seismic surveys that may be used for future shale gas exploration.