RESUMO
The world has set the goal of reducing CO2 emissions from burning fossil fuels by using carbon capture and storage (CCS) as one of the major solutions. A sudden and complete switch from fossil fuels to renewable resources cannot be achieved immediately. Therefore, CCS remains an essential techniques to reduce CO2. In this work, the 180 - 65 Ma old onshore part of the Zululand Basin in KwaZulu-Natal in South Africa was investigated for geological CO2 sequestration. A total of 160 core samples of sandstone, conglomerate, tuff, rhyolite, breccia, and siltstone were taken from NZA, ZA, ZB, and ZC drill cores. The wells were drilled in the 1960s by the South African Petroleum and Gas Corporation Company for hydrocarbon exploration. In order to examine the basin suitability for CO2 storage, porosity and permeability, mineralogy, geochemistry, geomechanical properties, and H2O-CO2-rock interactions were investigated using geological core logging, spectral scanning, petrography, X-ray diffraction (XRD), X-ray fluorescence (XRF), inductively coupled plasma mass spectrometry, uniaxial compressive stress, and scanning electron microscopy. The basin comprises clastic sedimentary rocks, pyroclastic deposits and carbonates from the Makatini, Mzinene and St. Lucia formations. Aptian and Cenomanian sandstones are identified as CO2 reservoirs, and the siltstone above is considered capstone. The sandstone comprises on average 34.45 wt% quartz, 32.91 wt% clays, 29.53 wt% feldspars, 4.44 wt% carbonates, 3.10 wt% Fe-oxides, 2.40 wt% micas, and 2.00 wt% organic materials as per XRD data, also contains trace amounts of sulphides and sulphates. Geochemical XRF data for sandstone are 29.72 - 62.51 wt% SiO2, 6.95 - 13.44 wt% Al2O3, 3.06 - 48.81 wt%, 1.90 - 4.51 wt% MgO, 1.04 - 2.19 wt% K2O, 1.00 - 3.67 Na2O wt%. The content of TiO2, Cr2O3 and P2O5 is below 0.01 wt% each. Siltstone has similar mineralogy and geochemistry as sandstone, but high clay content, fine-grained, impervious, with porosity <5%. The sandstone and siltstone are geomechanically soft and recorded 15 MPa on the Enerpac P141 device. CO2-H2O-rock interaction experiments performed at 100 °C and 100 bar using autoclaves showed that sandstone and siltstone react with scCO2.
