| dc.description.abstract | This thesis focusses on the hydrocarbon potential of the Prince Albert Formation in terms of
its shale gas potential. Unconventional gas production from hydrocarbon-rich shale
formations, known as “shale gas”, is one of the most rapidly expanding trends in onshore oil
and gas exploration and exploitation today. In South Africa, the southern portion of the main
Karoo Basin is potentially favourable for shale gas accumulation and may become a game
changer in the energy production regime of the country. The Prince Albert Formation was
selected for research, since previous studies in South Africa have focused on shale from the
Whitehill Formation, which together with the underlying Prince Albert Formation, occur
within the lower Ecca Group in the main Karoo Basin.
The petrophysical properties and shale gas potential of the Prince Albert Formation was
determined using the parameters of mercury porosimetry, total organic carbon (TOC),
vitrinite reflectance, Rock-Eval and residual gas measurements. The lithostratigraphy, rock
classification, and depositional environment of the Prince Albert Formation, together with
the adjacent parts of the overlying Whitehill Formation and underlying Dwyka Group within
the southern part of the main Karoo Basin were addressed. Rock types were characterised
using X-ray fluorescence (XRF), X-Ray diffraction (XRD) and statistical analysis. Geochemical
proxies and stable isotopes (δ18O, δ13C and δ15N) were used to identify the depositional
environment. Additionally, the Dwyka Group, Prince Albert and Whitehill formations were
correlated with equivalent Southern Gondwanaland units to understand basin development.
The results of the shale gas study of the Prince Albert Formation showed porosities ranging
between 0.08 and 5.6%, permeabilities between 0 and 2.79 micro-Darcy, TOC between 0.2
and 4.9 weight % and vitrinite reflectance values between 3.8 to 4.9%. Rock-Eval analysis
indicated that the kerogen in the shale was Type III and IV and hydrogen indices were less
than 65 mg/g. Free or absorbed gas was not detected in the recently drilled boreholes KZF-01
and KWV-01 used in this study. It is probable that the absence of gas is a result of overmaturity
due to tectonic duplication in KZF-01 and thermal degassing associated with dolerite
intrusions in KWV-01. Although the porosity and TOC values of the Prince Albert Formation
shales across the southern part of the main Karoo Basin are comparable with, but at the lower
limits of, those of the gas-producing Marcellus shale in the United States (porosities between
1 and 6% and TOC between 1 and 10 weight %), the high vitrinite reflectance values indicate
that the shales are overmature with questionable potential for generating dry gas.
A comprehensive rock classification were compiled for the Prince Albert Formation, which
consists of shale and minor ferruginous shale ranging between thicknesses of 30 and 168 m
based on field work and core descriptions. Mineralogical, geochemical and statistical data,
classified collected samples as Fe-shale, phosphatic shale, manganiferous shales, shale,
wacke, Fe-sand and litharenite.
In unconventional resources, understanding the depositional environment is important in
delineating the depositional process and bottom water conditions. The Prince Albert
Formation was interpreted as marine forming under dysoxic to euxinic conditions. δ 13C values
range between -17.5 and -23.1 ‰ and δ 15N between 8.5 and 11.1 ‰ reflecting marine
conditions. Sediments of the Prince Albert Formation were interpreted as middle to outer
continental shelf deposits based on various lithologies identified and XRF data (geochemical
proxies).
Compiled literature of the karoo- type basins in Southern Gondwanaland provides a
correlation of the Dwyka Group, Prince Albert and Whitehill formations in South Africa. Based
on stratigraphy and radiometric dating, the Dwyka Group, Prince Albert and Whitehill
formations have been correlated with lithostratigraphic units in the Falkland Islands, Namibia
(Huab, Karasburg and Aranos basins), the Ellsworth Basin of Antarctica and the Sauce Grande
and Parana basins of South America. This correlation suggest that the main Karoo foreland
system was subjected to very similar tectonic influences present in the other karoo-type
basins of Southern Gondwanaland.
In conclusion, results from this research indicate that viable conditions for shale gas might
exist within the “sweet spot” areas constrained by formation thickness being > 30 m, relative
dolerite intrusion of < 20%, relative total organic carbon content > 4 weight %, and maturity
of < 3.5%. It is essential that new exploration boreholes be drilled within the “sweet spot”
areas to test whether the lower Ecca Group (Prince Albert and Whitehill formations) has the
potential to generate viable shale gas. | en_US |
