CO2 storage potential of basaltic rocks, Mpumalanga: Implications for the Just Transition

Abstract

South Africa is the largest CO₂ emitter on the African continent. These emissions stem from a heavy reliance on coal as the primary energy fuel and contributor toward socio-economic development. The South African government has targeted reducing CO₂ emissions by more than half in the next 10 years. To meet climate change mitigation scenarios, while alleviating continued emissions, South Africa will look to technologies such as carbon capture, utilisation and storage. Initial assessments of South Africa’s potential for CO₂ storage have focused on deep saline aquifers within volcano-sedimentary sequences along the near and offshore regions. Sustaining the Just Transition will, however, require additional storage capacity. In this study, we make an initial assessment of possible CO₂ storage in basaltic sequences of the Ventersdorp Supergroup. Geological and mineralogical information was ascertained from borehole data. The geological information suggests that the subsurface extent of the Ventersdorp Supergroup is at least 80 000 km² larger than previously mapped, extending beneath major point-source CO₂ emitters and active coalfields. Furthermore, petrographic analyses suggest pore space of up to ca 15% with minimal alteration, and preservation of mafic silicate minerals that would enable reactive carbonation of injected CO₂. Notable metasomatic and hydrothermal alteration is confined to significant contact horizons, such as the lowermost Ventersdorp Contact Reef. These results suggest that basaltic sequences may exponentially increase South Africa’s CO₂ sequestration storage capacity and may have a significant impact on the country’s Just Transition.

Significance:
This study shows that basaltic sequences may support the permanent storage of anthropogenic CO₂ in South Africa, in particular, proximal to significant point-source CO₂ emitters. South Africa has voluminous and widespread basaltic sequences, which, in combination, increase South Africa’s geological CO₂ storage potential by several orders of magnitude. These storage reservoirs can have a direct impact in South Africa by enabling a sustainable Just Transition toward a low-carbon economy while meeting intended climate change mitigation scenarios.