Petrography mineralogy and geochemistry of the rock formations of the Zululand Basin based on borehole NZA

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.