Evaluation of Rocks in Boreholes ZA, ZB, ZC and NZA of the

Onshore Zululand Basin for Carbon Dioxide Storage

Landman, B.E.1,2; Ndongani, F.L.1,2; Hugo, D. 1; Tibane, L.V. 1; Altermann, W.1

1University of Pretoria; 2South African Centre for Carbon Capture and Storage

Sedimentary basins and their deep saline aquifers in particular, are being investigated as possible repositories for large volumes of

anthropogenic CO2 to mitigate global warming and related climate changes (Cloete et al., 2010).

The Cretaceous Zululand Basin of South Africa is one of the possible sites proposed in the “Atlas on Geological Storage of Carbon Dioxide” for

geological CO2 storage, however little is known on its sedimentary facies, geochemical and mineralogical properties.

Thus, this contribution aims to investigate the suitability of the sedimentary rocks of the onshore Zululand Basin in KwaZulu Natal, South Africa,

for a permanent sequestration of CO2. Different methods were performed in order to develop a sedimentological and petrographic model for the

basin, such as the determination of the lithostratigraphy, petrography, mineralogy and geochemistry, based on the Zululand cores.

The middle sandstone of the Mzinene Fm and the overlying clastic and calcareous siltstone rocks of the St Lucia Fm are unsuitable for injecting carbon

dioxide because of the important amount of calcite content and authigenic minerals which are dissolved during the scCO2 - mineral reaction, creating

secondary porosity. The progressive increase of the pore pressure may lead to the disintegration of the entire beds.

Laboratory tests and modelling, and geomechanical tests must be performed meticulously to understand the physical changes of the rocks caused by

interaction with CO2 and improve the storing condition of the reservoir and the cap rocks. During injection, the interaction between minerals and scCO2

must be monitored in order to avoid leakage or even collapse of the reservoir and cap rock.

The studied drill cores are stored in the National Core Library in Donkerhoek, Pretoria and were provided by the Council for Geosciences of

South Africa.

Techniques carried out on the forty samples from each borehole, after core logging, include petrography (visual observation and thin section), X-

Ray diffraction (XRD), X-Ray fluorescence (XRF) and spectral logging of the entire core, in order to characterise the onshore Zululand

lithofacies. Furthermore, laboratory experiments were completed in Germany, at the Martin-Luther University in Halle under reservoir conditions

(scCO2) to investigate the positive or negative environmental impacts resulting from CO2 injection and to understand the petrography, in order to

avoid increase of pore pressure in the target reservoir and to minimise the risk of leakage during the CO2 injection process.

Andre´, L., Audigane, P., Azaroual, M., Menjoz, A., 2007. Numerical modelling of fluid–rock chemical interactions at the supercritical CO2 - liquid interface during supercritical CO2

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Cloete M., Viljoen J.H.A., and Stapelberg F.D.J. 2010. Technical Report of the Geological Storage of Carbon Dioxide in South Africa. Council for Geosciences. 236pp.

Gerreard, I. 1972a. Report on progress on the evaluation of the Zululand Basin. Report. SOEKOR, PSV 1325 (unpubl.) In: Cloete M., Viljoen J.H.A., and Stapelberg F.D.J. 2010.

Technical Report of the Geological Storage of Carbon Dioxide in South Africa. Council for Geosciences. 236pp.

Gaus, I., Audigane, P., Andre´, L., Lions, J., Jacquemet, N., Durst, P., Czernichowski-Lauriol, L., and Azaroual, M., 2008. Geochemical and solute transport modelling for CO2 storage,

what to expect from it? International journal of greenhouse gas control , pp. 60 5 – 62 5.

Kennedy, W.J., Klinger, H.C. (1972). Hiatus concretions and hard ground horizons in the Cretaceous of Zululand. Palaeontology. 15: 539-549.

Pettijohn, F. J., P.E. Potter and Siever 1973. Sand and Sandstone. Berlin: Springer-Verlag in Greensmith, J.T. 1978. Petrology of the sedimentary rocks, London: Allen & Unwin, 6th

ed. 17-67pp.

Tucker, M. E. 1991. Sedimentary petrology: an introduction to the origin of sedimentary rocks, second edition, Blackwell Scientific, Oxford, 1991, 1992. 260pp.

Van Vuuren, CJ., Broad, D.S. Jungslager, E.H.A., Roux, J and McLachlan, I.R. (1998). Oil and Gas. In: Cloete M., Viljoen J.H.A., and Stapelberg F.D.J. 2010. Technical Report of the

Geological Storage of Carbon Dioxide in South Africa. Council for Geosciences. 236pp.

Fig.2. Submature arkosic wacke consisting of angular grains of quartz and

plagioclase, supported by clayey matrix. Fig. 3. Rock sample A31 from borehole ZA shows the texture of quartz,

plagioclase and clay grains before and after treatment with CO2 at 100°C and

100 bars for two weeks, and the creation of a secondary porosity (B).

GEOLOGICAL BACKGROUND

METHODOLOGY

RESULTS

DISCUSSION AND CONCLUSIONS

The onshore Zululand Basin is located on the north coast of South Africa, in the KwaZulu-Natal Province (Fig. 1). The basin covers an area of

7500 km2, extending from St Lucia northwards into Mozambique. The lower most rock units are of the volcanic Fenda Formation, related to the

late Karoo volcanic events and are overlain by sedimentary rocks of the Zululand Group. The Zululand Group encompasses the pre-upper

Aptian to Lower Maastrichtian (Kennedy and Klinger, 1972). The oldest, Makatini Formation comprises of basal conglomerates, sandstones,

siltstones and limestones (Kennedy and Klinger, 1972). The following Mzinene Formation comprises of fine grained sandstones, glauconitic

siltstones, interspersed limestone containing invertebrate fossils throughout the formation (Kennedy and Klinger, 1972). The St Lucia Formation

closing-up the Zululand Group is similar to the Mzinene Formation except, that it is thicker, more fossil rich and contains more glauconite

(Kennedy and Klinger, 1972).

Fig.4. (A) Enlarged XPL microphoto of plagioclase grain prior to reaction with scCO2. (B) Enlarged XPL microphoto after treatment with scCO2. Calcite appears smooth and

with no visible pockets on mineral surface. The blue dye appears more prominent around quartz and feldspar clasts where calcite cement has dissolved due to reaction with

scCO2

Fig.1. (A) Location of the onshore Zululand Basin after (Van Vuuren et al., 1998, in Cloete et al., 2010). (B) Location of drill cores of the onshore Zululand Basin (modified after

Gerrard, 1972,a in Cloete et al., 2010).

Fig.5 Correlation of the studied Zululand Basin drill cores.

REFERENCES

CONTACT DETAILSNames: Prof Wladyslaw Altermann, Kumba-Exxaro Chair in Geodynamics

University of Pretoria, Department of Geology

Email. wlady.altermann@up.ac.za Website: up.ac.za

A B

A B

The NZA drill core displays strikingly different

lithofacies to the other Zululand boreholes and thus

cannot be correlated based only on lithological

grounds. Biostratigraphy is needed.

The bottom of the NZA sequence is characterised

by the pyroclastic rocks of the Fenda Formation. It is

overlain by the calcareous sandstones of the Makatini

and St Lucia Formations. Dividing the Makatini and St

Lucia Formations is a pink terrestrial limestone at ca.

480 m depth.

The Mzinene Formation is not identified in the

stratigraphy of the NZA drill core.

ZA , ZB and ZC drill cores display similar lithologies

from the bottom to the top of the sequences. The

reddish paleosol sediment observed at the bottom of

the ZC sequence indicates the presence of the

weathered Lebombo Basement, it is overlain by

alternating sandstone and siltstone beds of the

Makatini Formation. A shallow dip angular

unconformity is encountered marking the limit of the

Makatini and Mzinene Formations.It is charaterised by

the occurrence of the sandstone beds across the drill

cores.

The following Mzinene Formation comprises

sandstone and calcareous sandstone beds. The

upper angular unconformity indicating the boundary

between the Mzinene and St Lucia Formaions is

located at ca. 680-650m depth. This upper angular

unconformity is characterised by the presence of a thin

sandstone bed ca. 20-90m thick. It is known from

outcrops but invisible in the drill cores.

The uppermost St Lucia Formation comprises mostly

calcareous sandstone beds with glauconitic siltstones,

sandstones beds and bioclastic limestone bed on top.

St Lucia

Fm

.M

zin

ene F

m.

Makatin

i F

m.

St Lucia

Fm

.M

akatin

i F

m.

Fe

nda F

m.

Unconformity

SW NE