Physico-chemical properties of South African shales and siltstones in the context of geological CO2 storage

Abstract

Shale is normally impermeable and has a low porosity; it was therefore not considered by the project on geological storage of CO2 in South Africa (the “Atlas” project) as a potential storage reservoir. However, research in other parts of the world such as the United States of America (USA) on carbonaceous shales similar to those of the Central Karoo Basin of South Africa has proved that shales store significant amounts of CO2 in an adsorbed state within their organic matter, also known as kerogen. The paucity of conventional storage reservoirs for the geo-sequestration of CO2 in the interior of South Africa, where most sources of the anthropogenic CO2 are situated, has prompted this study, which focuses on the carbonaceous shales and siltstones of the Karoo Basin of South Africa and their potential for CO2 adsorption. Six samples from five old Soekor boreholes were selected for the study and their physico-chemical properties were studied through parameters such as petrography, mineralogy, elemental composition, bulk density, maturity (vitrinite reflectance), kerogen, total organic content (TOC), low pressure adsorption and high pressure adsorption. In terms of TOC, which is one of the important parameters for gas adsorption onto shales, the studied carbonaceous shale samples compare quite favourably to those from other parts of the world that have been found to successfully store significant amounts of CO2. These include the Barnett Shale of the USA which measures 4.00 wt.% of TOC and the famous Marcellus shale (USA) with 1–10 wt.%. The Irati and the Rio Bonito formations of Brazil have the highest TOC values found in the literature, measuring 2.3–26.30 wt.% and 1.7–43.90 wt.%, respectively. The Whitehill Formation of the Karoo Basin of South Africa has values of 0.5–14.70 wt.% TOC as described in the literature. In this study a sample from the Whitehill Formation from borehole G39974 gave a measured value of 4.52 wt.%, while a sample from the underlying Prince Albert Formation, from borehole KL 1-65, measured a value of 1.17 wt.% TOC. In the north-eastern part of the Karoo Basin, two samples from the Pietermaritzburg Formation, a north-eastern stratigraphic equivalent of the Whitehill Formation which pinches out in this part of the basin intersected by boreholes SW 1-67 and LA 1-68, measured 2.50 wt.% and 0.77 wt.% TOC, respectively. Sample 1124.5 from the Volksrust Formation intersected by borehole BE 1-67 measured 0.21 wt.% TOC. For maturity studies, all the six samples measured reflectance values above 1.4%Ro and are therefore classified as overmature. A plot on the HI vs Ol index plot revealed that these samples plot predominantly below the gas prone Type III kerogen curve. The samples exhibit a specific surface area of 17.40–21.88 m2/g, with an average of 19.66 m2/g. This compares quite favourably with the gas-bearing Silurian shales in the Sichuan Basin (China) which exhibit a specific surface area of 17.83–29.49 m2/g, with an average of 22.18 m2/g. There is also a trend of increasing surface area with TOC content, affirming the influence of organic matter on the surface area. The high pressure adsorption experiments on the samples exhibited anomalous behaviour which affected the credibility of the results. Most experiments showed an unprecedented increase in pressure instead of the expected decrease due to the CO2 being adsorbed onto the shale samples. The samples are from the old Soekor drill cores which have been exposed to the atmosphere for over 40 years and have undergone severe weathering during that period. The weathering and the degree of oxidation and break-down of pyrite are strongly suspected to be the cause of the anomalous behaviour. Based on the content of organic matter and kerogen, these samples have potential to store CO2. However, maturity studies have revealed that these shales and siltstones are in fact overmature and at this stage are likely to have less gas generating capacity and thus less affinity to adsorb CO2. This could be argued also in favour of storage capacity as the gas does not disturb contact of CO2 to rock, however, the high tightness as a consequence of over-maturity will have a higher negative effect on adsorption potential. Further studies on freshly drilled core samples are recommended to ascertain these findings given the limitations posed by the condition of the current samples, especially in studying the adsorption behaviour. Only then can we conclusively deduce whether or not the shales and siltstones of the Karoo Supergroup have a role to play in South Africa’s anthropogenic CO2 geo-sequestration endeavours.