Assessing CO2 storage potential in a structurally complex depleted gas reservoir, offshore South Africa

• A complex, fault-bounded reservoir structure characterises the F-O Gas Field. • The CO₂ Storage Capacity ranges between 37.1 and 74.1 Mt after accounting for sweep efficiency. • Supports South Africa’s climate goals through carbon capture and storage (CCS) and a Net-Zero decarbonisation agenda. As global efforts to mitigate greenhouse gas emissions intensify, carbon capture and storage (CCS) has emerged as a key strategy for reducing the environmental impact of fossil fuel use. However, geological storage of CO₂ in structurally complex and heterogeneous reservoirs presents a range of issues due to the geological intricacies, with implications for storage capacity estimation, CO₂ injection, migration, and even long-term containment, which pose environmental risks. Therefore, this study assesses the CO₂ storage potential of the depleted F-O Gas Field in the Bredasdorp Basin, offshore South Africa, using a robust modelling approach based on the analysis of a suite of exploration and production datasets from the field. A high degree of structural compartmentalisation with a fault-bounded anticlinal trap characterises the field. The Valanginian-age marine sandstone reservoirs exhibit low to moderate porosity and permeability. In total, a CO₂ storage capacity of 185.3 Mt was determined for the F-O gas field, which reduces to 37.1–74.1 Mt after accounting for reservoir heterogeneity and sweep efficiency. This reduction reflects the impact of the field's complex structural architecture, variable facies distribution, and petrophysical variability, which collectively limit the effective pore volume accessible for CO 2 storage. By rigorously integrating the structural architecture of the field, sedimentary processes, facies distribution, and petrophysical variability of the candidate reservoir, this study provides critical insights and strategies into the feasibility of CCS in structurally complex depleted gas fields. Significantly, these findings contribute to ongoing national CCS assessments and support South Africa’s long-term decarbonisation agenda.