- Title
- Sedimentology and shale gas potential of the Ecca Group, Karoo Supergroup in the Eastern Cape, South Africa
- Creator
- Nemanashi , Tshisikhaiwe
- Subject
- Sedimentology
- Date
- 2019
- Type
- Thesis
- Type
- Masters
- Type
- MSc
- Identifier
- http://hdl.handle.net/10353/14616
- Identifier
- vital:40023
- Description
- The aim of the study was to establish a deeper understanding on the stratigraphy, sedimentology, petrology, organic geochemistry and diagenesis of the Ecca Group, and to provide new insight on the shale gas occurrence and potential of the Ecca Group, Karoo Supergroup in the southern Karoo Basin. The Ecca Group stratigraphy is divided into five formations from the bottom upwards, namely the Prince Albert, Whitehill, Collingham, Ripon and the Fort Brown Formations. The stratigraphy of the five studied stratigraphy’s formations is now sub-divided into two new different members each. These subdivision was based on field investigation of lithological features, sedimentary structures, facies characteristics and stratigraphic correlation points. Each member has been assigned a lithological name. Sixteen sedimentary facies have been identified in the study area and were subdivided into six distinctive facies associations (FA 1, FA 2, FA 3, FA 4, FA 5 and FA 6). Sedimentological characteristics of facies associations identified indicate that the Ecca Group sediments initially accumulated in a deep marine environment, progressed through turbidite, shallow marine and ended in lacustrine and deltaic environments. The stratigraphic succession of the Ecca Group constitutes a perfect regression sequence, indicating that the marine water gradually retreated and the sea-level gradually dropped. Grain size analysis was performed on twenty four Ecca Group sandstone samples. Statistical parameters of grain-size statistical parameters, linear discriminate functions, passega diagrams and bivariate analysis were used to reveal the hydrodynamic conditions and depositional environments. The results indicated that the Ecca Group sandstones are mostly fine to very fine grained, near-symmetrical, mesokurtic and indicative of dominance of low energy environments. The linear discriminate function analysis for the Prince Albert, Whitehill, Collingham and Ripon Formations samples indicates that the majority of the deposits were by turbidity currents all in a deep marine environment; whilst of the Fort Brown Formation samples are lacustrine/deltaic deposits. The depositional mechanism C-M plot indicates that majority of the Ecca Group sediments clustered in the PQ and QR field suggesting deposition mainly by suspension, rolling or saltation as well as graded suspension. Modal mineral composition analysis indicates that the main framework grains of the Ecca Group sandstones are quartz, feldspar as well as lithic fragments derived from metamorphic, igneous and sedimentary rocks with a few from volcanic origin. The Ecca Group sandstones iii are immature compositionally and can be classified as feldspathic wackes and lithic wackes. The QFL ternary diagram revealed a dissected and recycled orogen arc provenance; whereas QmFLt ternary diagram point to dissected arc and transitional arc sources to an active continental margin as well as recycled provenance. These provenance characteristics suggest a metamorphic and plutonic terrains influence as the main source rock with minor debris derived from recycled sedimentary rocks. The weathering diagram and semi-quantitative weathering index suggests that the sandstones from the Ecca are mostly from a plutonic source area under arid to humid climatic conditions. The detrital modal compositions of these Ecca Group sandstones are related to a strike-slip setting, back arc to continental margin setting. Diagenetic features of the Ecca sandstones and shales are subdivided into early, late (burial) and uplift-related diagenetic stages. Mechanical compaction, recrystallization, cementation, replacement and the dissolution of framework grains or cements are some of the main diagenetic processes that largely affected the Ecca sediments. Early diagenetic processes include cementation, point/planar contact, formation of pyrite, hematite cements and mineral inversions. Recrystallization, replacement, compaction, overgrowth, albitization, seritisation, illitization, concave-convex and suture contacts as well as dissolution took place mostly in the later diagenetic stage due to increase of temperature and pressure as burial depth increased. The uplift-related diagenetic stage was mostly affected by deformation and fracturing, calcitization, dissolution, erosion and weathering. Diagenetic processes largely affected the porosity and permeability of the reservoir rock properties of the Ecca Group. Organic geochemistry results indicate that the sediments were highly weathered and the TOC values ranging from 0.10 to 7.35 wt% with mostly less than 0.5%, which indicate the source rocks have poor oil potential. The majority of the Ecca Shales have HI values less than 50 mg HC/g, TOC indicating Type-IV kerogen mostly derived from reworked organic matter with very little hydrocarbon generation potential. The relatively high Tmax (oC) and vitrinite reflective values indicate that most shales are thermally over-matured thus they have low hydrocarbon pontential.
- Format
- 259 leaves
- Format
- Publisher
- University of Fort Hare
- Publisher
- Faculty of Science and Agriculture
- Language
- English
- Rights
- University of Fort Hare
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UFH Department of Geography & Environmental Science
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