The Cretaceous Zululand Basin is one of proposed areas for geological CO2 storage in South Africa. The knowledge of the basin is however very limited. This thesis aims to describe the sedimentary facies of the ZA core, drilled onshore Zululand Basin, in KwaZulu-Natal, South Africa, and to assess their depositional environment as well as possible lateral correlation. Further aim is to investigate the suitability of these rocksfor a permanent sequestration of CO2, based on their geochemical and petrographic characteristics.
The ZA drilling penetrated the Zululand Basin strata to a depth of 1779.9 m. The lowermost part of the ZA core from the recorded bottom at 1779.9 to 1602 m containing the Makatini Formation is however missing. Most of the core nevertheless is well preserved. The preserved ZA core from 1602 to 1041 m depth is characterised by dark grey coloured, bioturbated calcareous siltstone beds, rich in foraminifera, echinoderm and algae fossils and a dark grey coloured laminated glauconitic siltstone, overlain by a succession of siltstone strata with some interbeds of bioclastic packstones, calcareous and glauconitic siltstones (Lower Mzinene Formation). The Upper part of the Mzinene Formation starts from 1041 to 675 m depth, displaying a bioclastic packstone layer followed by bioturbated and wavy bedded arkosic wacke beds interbedded with calcareous sandstone layers rich in fossils and parallel bedded subarkose strata. The boundary between the Mzinene and St Lucia Formation is defined by the presence of a laminated siltstone bed at 675 m. The St Lucia Formation closing-up the Zululand Group, is comparable to the Mzinene Formation, except that the St Lucia Formation contains more fossil rich beds and glauconite. Above 675 m depth, the core displays siltstone layers intercalated by a bioturbated arkosic wacke bed, followed by 245 m thick calcareous siltstone beds. These 245 m
thick calcareous siltstone strata are overlain by cross-bedded arkose and glauconitic arkosic wacke layers, followed by calcareous and bioturbated siltstone units and bioclastic grainstone bed.
Only the sandstone strata between 1035 to 678 m, meet the criteria of a suitable reservoir for a permanent sequestration of CO2. The sandstone strata are bound on top by a 42 m thick siltstone layers as potential cap rocks, covered by 242 m thick calcareous siltstone and sandstone. These depths however are by far too shallow for CO2 sequestration. Nevertheless, petrographically, these sandstones are characterised as arkosic wackes, calcareous sandstone and subarkose and they contain mainly quartz, calcite and plagioclase minerals with a respective average of 47, 21 and 15% vol. Smectite, glauconite, mica, zeolite, hematite, and lithic fragments are present. The geochemical results of these sandstones supporting their mineralogical contents shows the predominance of SiO2 (55 wt%), CaO (11 wt%), Al2O3 (10 wt%) and Fe2O3 (7 wt%) followed by other oxides such as MgO, Na2O, K2O, H2O, MnO, TiO2, P2O5 and Cr2O3 summing up to c. 17 wt%. The permeability and porosity of the middle sandstone rocks are respectively 8% and 10mD. The mineralogy of the possible cap rock layer is similar to the afore-mentioned sandstone units, dominated by quartz, calcite and plagioclase minerals. However, the clay mineral contents (mostly smectites) in the caprock bed are higher than in the possible sandstone reservoir. This higher clay content of the possible caprock is in accordance with the mineral spectral results showing the abundance of smectites and clay minerals in different depths.
The rock samples were treated with CO2 under supercritical conditions of 100 bars and 100⁰C for the duration of four weeks. The treatment results show a dissolution on the surface of quartz grains. The calcite cement was dissolved, created
secondary porosity and increasing the porosity and permeability of the rock. Therefore, the possible sandstone reservoir and the overlying siltstone rocks are probably unsuitable for injecting carbon dioxide because of the significant amount of calcite and authigenic minerals which may be dissolved during the scCO2 - mineral reaction, creating secondary porosity and can lead to the disintegration of entire layers.