Abstract:
The drill core of borehole NZA, drilled in the southwest margin of the onshore
Zululand Basin to a depth of ca. 571 m was investigated in this study. In this thesis
descriptions of the geochemistry, mineralogy and physical properties of major lithofacies of
the NZA drill core are followed by an interpretation of the suitability of NZA rock units for
injection of carbon dioxide (CO2). A representative NZA sandstone was subjected to reaction
with super critical (sc)CO2 and water under reservoir conditions for two weeks to gain insight
into the effects of rock-CO2 interactions on rock porosity. Rock porosity was estimated by
petrographic study for clastic sedimentary rocks of NZA.
Petrographic study served to identify major lithofacies associations of the NZA drill
core, as well as the depositional environments; interpreted after studying primary
sedimentary structures, chemistry and biogenic features. Trends within the lithofacies were
used to discriminate various formations of the Zululand Group observed in the NZA drill core.
The Zululand Group rocks of the NZA drill core were generally deposited in a shallow shelf
environment, while the pyroclastic rocks of the Fenda Formation identified in the drill core
were deposited in a subaerial volcanic environment. Calcareous sandstones, greywackes,
coquina limestone, beachrock, lapilli-stone, lapilli-tuff, tuff-breccia and tuff were identified as
the major lithofacies of the NZA drill core and are consistent with literature data.
Major element XRF results and mineralogical results formed an integral role in
investigating the chemistry and its effects on the mineralogy of the NZA drill core’s rocks.
Major element analysis was further utilised for discriminating tectonic setting and provenance
of siliciclastic rocks. From geochemical data the source area of the Zululand Group
sedimentary rocks of the NZA core is consistent with the inferred continental rift setting for
the Zululand Basin. The provenance of the NZA drill core’s siliciclastic rocks is consistent
with chemically immature terrigenous detritus, sourced from the proximal Lebombo Group
volcanic rocks.
Borehole NZA was drilled to a relatively shallow depth which is insufficient, when
considering its use for geological storage of CO2. Furthermore, the low porosity of ca. 1-2 %,
identified in the rock samples suggests low storage potential. Dissolution after CO2 injection
may significantly increase porosity after reaction with scCO2 while rock stability could be
severely compromised due to dissolution of carbonates. Lastly, lithologies of the NZA drill
core do not correlate well with the adjacent ZA, ZB and ZC drill core lithologies; thus the NZA
borehole is a seemingly unsuitable site for geological storage of CO2 as CCS technology
requires lateral continuity of suitable rock packages. Further study of the chemistry and
physical properties of the ZA, ZB and ZC drill core could reveal a suitable site for injection of
CO2 in the onshore Zululand Basin.
