Evaluation of the lithofacies, petrography, mineralogy, and geochemistry of the onshore Cretaceous Zululand Basin in South Africa for geological CO2 storage

•Characterisations of the Zululand Basin rocks for geological CO2 sequestration.•Description of the mineralogy and the geochemistry of the Zululand Basin.•Descriptions of the petrophysical properties of the sedimentary lithofacies.•Impacts of CO2-H2O-rock reactions on the geochemical and geomechanical properties.•Implications of the changes of the geochemical and geomechanical properties on the Zululand Basin rock integrity for geological CO2 storage. The onshore Cretaceous Zululand Basin of South Africa was investigated for CO2 storage potential using NZA, ZA, ZB, and ZC boreholes drilled by Petroleum Oil and Gas Corporation of South Africa in the 1960s for hydrocarbon exploration. Basin fill comprises clastic sedimentary rocks, pyroclastic deposits and carbonates of the Makatini, Mzinene and St. Lucia formations. Digital image analyses showed 20% average porosity and ca. 1.0 mD permeability for Aptian and Cenomanian sandstones, identified for CO2 storage. Siltstones overlying the sandstones were identified as caprocks. XRD analysis shows that the average composition of the sandstones is 34.45 wt% quartz, 29.53 wt% feldspars, 2.40 wt% micas, 32.91 wt% clays, 3.10 wt% Fe-oxides, 4.44 wt% carbonates and 2.00 wt% organic materials, with sulphides and sulphates traces, which is supported by petrography and SEM. XRF results for sandstone geochemistry range from 29.72 – 62.51 wt% SiO2, 6.95 – 13.44 wt% Al2O3, 3.06 – 48.81 wt% CaO, 1.90 – 4.51 wt% MgO, 1.04 – 2.19 wt% K2O, 1.00 – 3.67 wt% Na2O, and low TiO2, Cr2O3, P2O5 contents. Siltstones show similar mineral composition and geochemistry to sandstones, but are fine-grained, impermeable with porosity below 5%, and have a high clay content. CO2-H2O-rock interaction experiments were performed under 100 °C and 100 bars using autoclaves and showed reaction of sandstones and siltstones with scCO2. These rocks are geomechanically soft and recorded a maximum of 15 MPa using Enerpac P141 apparatus. Further geochemical and geomechanical modelling is necessary to fully understand the in-situ behaviour of these rocks.