Seismic imaging of dolerite sills and volcanic vents in the Central Karoo, South Africa: implications for shale gas potential

2021 ◽  
Author(s):  
S.E. Scheiber-Enslin ◽  
M. Manzi ◽  
S.J. Webb
Keyword(s):  
South Africa ◽  
Shale Gas ◽  
Global Climate ◽  
Magnetic Data ◽  
Seismic Reflection Profile ◽  
Magnetic Studies ◽  
Seismic Line ◽  
Seismic Section ◽  
Karoo Basin ◽  
Gas Potential

Abstract The Karoo Basin of South Africa covers an area of 700 000 km2 and has been identified as a possible shale gas reserve. Any evaluation of the shale gas potential of the basin must consider the widespread dolerite dykes and sills. These intrusions were emplaced into the Karoo Supergroup and are well dated at around 183 Ma. Their intrusion triggered the explosive releases of gas in the basin, marked on surface by breccia pipes and hydrothermal vents. This outpouring of gas has been proposed as a significant contributor to global climate change. Research into the three-dimensional interconnected structure of these dolerite sills and dykes and their interaction with the hydrocarbon rich layers in the lower part of the Karoo Supergroup has been limited to localized observations of outcrop, magnetic data, legacy seismic data (from the 1970s) and well core. Here we present an interpreted 65 km long higher-resolution 2D seismic reflection profile across the Karoo Basin, approximately 100 km southeast of Trompsburg. These data were collected in the 1990s and at the time deeper structures along the line interpreted. In this study we focus on the top 0.6 to 2 seconds TWT of the data. The seismic line images the interconnected and cross cutting nature of the dolerite dykes and sills along the profile. We also report possible evidence of a gas escape structure (approximately 2.5 km in diameter at surface) emerging near the edge of a dolerite sill in close proximity to the Whitehill Formation, which is the main target for shale gas exploration. This suggests that gas vents in the eastern Karoo Basin close to Lesotho are due to the release of gas from the carbonaceous shales of the Ecca Group. This is similar to breccia pipes mapped on surface in the western part of the Karoo Basin. This seismic section highlights why dolerite sills and dykes must be considered when evaluating the shale gas potential of the Karoo Basin. We propose that better characterization of the Karoo Basin subsurface by seismic and magnetic studies is necessary prior to any efforts to calculate shale gas reserves.

Shale gas leakage in lower Ecca shales during contact metamorphism by dolerite sill intrusions in the Karoo Basin, South Africa

2021 ◽  
Author(s):  
V. Nengovhela ◽  
B. Linol ◽  
L. Bezuidenhout ◽  
T. Dhansay ◽  
T Muedi ◽  
...  
Keyword(s):  
South Africa ◽  
Grain Boundaries ◽  
Shale Gas ◽  
Elevated Temperatures ◽  
Contact Metamorphism ◽  
Microscopy Imaging ◽  
Gas Leakage ◽  
Karoo Basin ◽  
Maximum Temperatures ◽  
Host Rocks

Abstract Contact metamorphism along widespread dolerite sills and dykes, emplaced at 182 to 183 Ma through the sedimentary host rocks of the Karoo Basin, triggered devolatilization of carbon-rich shales of the Lower Ecca Group. Hornfel samples collected from drill cores that intersect dolerite sills were analyzed for mineral phase equilibria, chemistry and porosity to characterize thermal aureoles at various distances from sill intrusions. Andalusite-chiastolite and cordierite porphyroblasts with biotite and muscovite occur within 10 to 20 m of many intrusive contacts. These metamorphic minerals crystallized when host shales attained maximum temperatures ranging between 450 and 600°C. Scanning electron microscopy imaging confirms that the hornfels are compact and that their metamorphic minerals limit porosity along grain boundaries. In few cases intra-mineral porosity occurs within individual crystals such as calcite, andalusite and cordierite. Disequilibrium metamorphic textures such as irregular grain boundaries, and inclusions in andalusite and cordierite reveal that the elevated temperatures were too short-lived to accomplish complete (re)crystallization. Thermal modeling results are consistent with the observed metamorphic mineral assemblages. Gas leakage calculations along a 7 m and a 47 m thick dolerite sill that intrude toward the top of the Whitehill Formation suggest that methane volumes ranging between 8 to 15 Tcf were generated during the sill emplacement. Methane was likely released into the atmosphere through hydrothermal vent complexes that are well preserved in the western Karoo Basin. If such loss was widespread across the entire basin, the implications for paleo-climate change and preserved shale gas reserves in the Karoo Basin of South Africa would be significant.

scholarly journals Prospective policy safeguards to mitigate hydrogeological risk pathways in advance of shale gas development in the Karoo basin, South Africa

2020 ◽  
pp. 100499
Author(s):  
Kevin Pietersen ◽  
Luc Chevallier ◽  
Audrey Levine ◽  
Thandokazi Maceba ◽  
Zaheed Gaffoor ◽  
...  
Keyword(s):  
South Africa ◽  
Shale Gas ◽  
Karoo Basin

Geochemical Evaluation of the Permian Ecca Shale in Eastern Cape Province, South Africa: Implications for Shale Gas Potential

2018 ◽  
Vol 92 (3) ◽  
pp. 1193-1217 ◽  
Author(s):  
Christopher BAIYEGUNHI ◽  
Kuiwu LIU ◽  
Nicola WAGNER ◽  
Oswald GWAVAVA ◽  
Temitope L. OLONINIYI
Keyword(s):  
South Africa ◽  
Shale Gas ◽  
Eastern Cape Province ◽  
Eastern Cape ◽  
Geochemical Evaluation ◽  
Gas Potential

scholarly journals O, H, CDIC, Sr, B and 14C Isotope Fingerprinting of Deep Groundwaters in the Karoo Basin, South Africa as a Precursor to Shale Gas Exploration

2015 ◽  
Vol 13 ◽  
pp. 211-214 ◽  
Author(s):  
J. Miller ◽  
K. Swana ◽  
S. Talma ◽  
A. Vengosh ◽  
G. Tredoux ◽  
...  
Keyword(s):  
South Africa ◽  
Shale Gas ◽  
Karoo Basin

Shale gas potential of the Prince Albert Formation: A preliminary study

2019 ◽  
Vol 122 (4) ◽  
pp. 541-554 ◽  
Author(s):  
H. Mosavel ◽  
D.I. Cole ◽  
A.M. Siad
Keyword(s):  
Shale Gas ◽  
Vitrinite Reflectance ◽  
Marcellus Shale ◽  
Mercury Porosimetry ◽  
The United States ◽  
Karoo Basin ◽  
Type Iv ◽  
Volume Porosity ◽  
Rock Eval ◽  
Gas Potential

Abstract Recent investigations of the shale gas potential in the main Karoo Basin have concentrated on the Whitehill Formation within the Ecca Group. This study focuses on the shale gas potential of the underlying Prince Albert Formation using the parameters of volume porosity, permeability, total organic carbon (TOC), vitrinite reflectance and Rock-Eval data. Shale samples were retrieved from three surface localities in the southern part of the main Karoo Basin and from core of three boreholes drilled through the Prince Albert Formation near Ceres, Mervewille and Willowvale. The sampling localities occur near the borders of the prospective shale gas areas (“sweet spots”) identified for the Whitehill Formation. Kerogen was found to be Type IV with hydrogen indices less than 65 mg/g. Shale porosities are between 0.08 and 5.6% and permeabilities between 0 and 2.79 micro-Darcy, as determined by mercury porosimetry. TOC varies between 0.2 and 4.9 weight % and vitrinite reflectance values range from 3.8 to 4.9%. Although the porosity and TOC values of the Prince Albert Formation shales 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. This overmaturity is probably a result of an excess depth of burial, tectonic effects of the Cape Orogeny and dolerite intrusions. However, viable conditions for shale gas might exist within the “sweet spot” areas, which were defined for the Whitehill Formation.

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