Characterization of the heat source of thermal aquifers within the Soutpansberg Basin in the Limpopo Province, South Africa: Evidence from geophysical and geological investigations
- Authors: Nyabeze, Peter Kushara
- Date: 2019
- Subjects: Basins (Geology) -- Analysis Geology, Structural -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD (Geology)
- Identifier: http://hdl.handle.net/10353/15167 , vital:40192
- Description: The research was conducted to contribute towards the knowledge base on the potential for geothermal energy in the Soutpansberg Basin, located in the Limpopo Province of South Africa. The focus area was Siloam, an area that hosts a hottest spring with the highest recorded temperature of 67.5°C. The research involved visits to the Soutpansberg Basin, water sampling, carrying out ground geophysical surveys, and high-level processing of airborne magnetic data to determine depths and temperatures of magnetic sources. The water samples from the hot springs were found to be enriched in sodium, bicarbonate and chlorine with very low concentrations of other element species. The chemical composition of the spring water indicated a source chemistry comprising of the Na-ClHCO3 water assemblage that is a typical signature for deep circulating groundwater of meteoric origin. The circulation depth was inferred to be 2.0 km. The increased resolution of the ground magnetic, electrical resistivity tomography, and electromagnetic conductivity methods data made it possible to delineate subsurface structures at the spring such as dykes, sills, faults and fractures from generated depth models. Modelling of ground magnetic data showed that the Siloam hot spring occurred between two interpreted north dipping dykes approximately 150 m apart. The minimum depth extent of the dykes was interpreted to be 650 m. The magnetic susceptibility values determined from rock measurements and modelling of magnetic data indicated the presence of volcanic and metamorphic rocks. Electromagnetic profiling data showed that there were three main high conductivity zones in the study area with values above 100 mS/m; A central zone associated with the spring; A zone to the south of the spring and a north zone associated with the Siloam Fault. Ground geophysics survey results confirmed the existence of the Siloam Fault. Two artesian boreholes with water warmer than 40 °C were identified to the south of the Siloam hot spring. Both electromagnetic conductivity and electrical resistivity tomography surveys delineated lateral and vertical variation in the bedrock to depths of 40 m to 60 m. Water bearing structures that could be faults, or fractures were identified. Layering due to weathering and water content was found to be in the depth range of 20 m to 40 m. The depths of the potential heat sources were computed from the radially averaged power spectrum of airborne magnetic data for square blocks with side dimensions L of 51 km, 103 km, and 129 km. Spectral analysis based approaches namely Centroid method, Spectral peak method, and the Fractal based approach were used for computing depth and temperatures to heat sources. Airborne magnetic data sets with larger window sizes were preferred for depth computations, as they preserved spectral signatures of deeper sources and reduced the contribution of shallower sources. The size of the data windows did not have a marked effect of depth and temperature values. Shallower magnetic sources depths of approximately 2.0 km were delineated using the Euler deconvolution method. An anticlinal feature at depths of 2.0 to 4.5 km was 4 Final Submission of Thesis, Dissertation or Research Report/Project, Conference or Exam Paper delineated in the central part of the basin. Spectral analysis results indicated that the depth to the top of magnetic sources was at 3.5 km to 6.2 km; the centroid of the basement at 7.92 km to 13.41 km, and the basal below 11.09 km and 14.08 km. The lower end depth spectrum was determined from application of the Centroid method with the deeper being results from the Fractal based approach. The Spectral peak method was useful for determining the depth to the top of magnetic sources. The temperature of the top of magnetic sources and basement centroid were computed to be in the range 234.00 °C to 281.34 °C. Magnetic source depths and basal temperatures that were in the Curie point range within which rocks lose magnetism due to heat were determined, using a computation approach that utilised fractal parameters, to be 21.39 km and 577.42 °C, respectively. Increasing the value of the fractal parameter β from 0 to 4, had an effect of retaining deeper depths and higher temperatures. The fractal parameter β range of 3 to 4 that gave the Curie point parameters indicated basal rock types with an igneous predisposition. The research highlighted evidence for the existence of the Soutpansberg Basin Geothermal Field (SBGF). The area around Siloam is a potential target for drilling exploration geothermal energy boreholes based on the occurrence of hot springs, shallow heat source depths, anticlinal structure, high formation temperatures, deep circulating water and the achieved Curie point temperature.
- Full Text:
- Authors: Nyabeze, Peter Kushara
- Date: 2019
- Subjects: Basins (Geology) -- Analysis Geology, Structural -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD (Geology)
- Identifier: http://hdl.handle.net/10353/15167 , vital:40192
- Description: The research was conducted to contribute towards the knowledge base on the potential for geothermal energy in the Soutpansberg Basin, located in the Limpopo Province of South Africa. The focus area was Siloam, an area that hosts a hottest spring with the highest recorded temperature of 67.5°C. The research involved visits to the Soutpansberg Basin, water sampling, carrying out ground geophysical surveys, and high-level processing of airborne magnetic data to determine depths and temperatures of magnetic sources. The water samples from the hot springs were found to be enriched in sodium, bicarbonate and chlorine with very low concentrations of other element species. The chemical composition of the spring water indicated a source chemistry comprising of the Na-ClHCO3 water assemblage that is a typical signature for deep circulating groundwater of meteoric origin. The circulation depth was inferred to be 2.0 km. The increased resolution of the ground magnetic, electrical resistivity tomography, and electromagnetic conductivity methods data made it possible to delineate subsurface structures at the spring such as dykes, sills, faults and fractures from generated depth models. Modelling of ground magnetic data showed that the Siloam hot spring occurred between two interpreted north dipping dykes approximately 150 m apart. The minimum depth extent of the dykes was interpreted to be 650 m. The magnetic susceptibility values determined from rock measurements and modelling of magnetic data indicated the presence of volcanic and metamorphic rocks. Electromagnetic profiling data showed that there were three main high conductivity zones in the study area with values above 100 mS/m; A central zone associated with the spring; A zone to the south of the spring and a north zone associated with the Siloam Fault. Ground geophysics survey results confirmed the existence of the Siloam Fault. Two artesian boreholes with water warmer than 40 °C were identified to the south of the Siloam hot spring. Both electromagnetic conductivity and electrical resistivity tomography surveys delineated lateral and vertical variation in the bedrock to depths of 40 m to 60 m. Water bearing structures that could be faults, or fractures were identified. Layering due to weathering and water content was found to be in the depth range of 20 m to 40 m. The depths of the potential heat sources were computed from the radially averaged power spectrum of airborne magnetic data for square blocks with side dimensions L of 51 km, 103 km, and 129 km. Spectral analysis based approaches namely Centroid method, Spectral peak method, and the Fractal based approach were used for computing depth and temperatures to heat sources. Airborne magnetic data sets with larger window sizes were preferred for depth computations, as they preserved spectral signatures of deeper sources and reduced the contribution of shallower sources. The size of the data windows did not have a marked effect of depth and temperature values. Shallower magnetic sources depths of approximately 2.0 km were delineated using the Euler deconvolution method. An anticlinal feature at depths of 2.0 to 4.5 km was 4 Final Submission of Thesis, Dissertation or Research Report/Project, Conference or Exam Paper delineated in the central part of the basin. Spectral analysis results indicated that the depth to the top of magnetic sources was at 3.5 km to 6.2 km; the centroid of the basement at 7.92 km to 13.41 km, and the basal below 11.09 km and 14.08 km. The lower end depth spectrum was determined from application of the Centroid method with the deeper being results from the Fractal based approach. The Spectral peak method was useful for determining the depth to the top of magnetic sources. The temperature of the top of magnetic sources and basement centroid were computed to be in the range 234.00 °C to 281.34 °C. Magnetic source depths and basal temperatures that were in the Curie point range within which rocks lose magnetism due to heat were determined, using a computation approach that utilised fractal parameters, to be 21.39 km and 577.42 °C, respectively. Increasing the value of the fractal parameter β from 0 to 4, had an effect of retaining deeper depths and higher temperatures. The fractal parameter β range of 3 to 4 that gave the Curie point parameters indicated basal rock types with an igneous predisposition. The research highlighted evidence for the existence of the Soutpansberg Basin Geothermal Field (SBGF). The area around Siloam is a potential target for drilling exploration geothermal energy boreholes based on the occurrence of hot springs, shallow heat source depths, anticlinal structure, high formation temperatures, deep circulating water and the achieved Curie point temperature.
- Full Text:
Sedimentology and shale gas potential of the Ecca Group, Karoo Supergroup in the Eastern Cape, South Africa
- Authors: Nemanashi , Tshisikhaiwe
- Date: 2019
- Subjects: Sedimentology
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10353/14616 , 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.
- Full Text:
- Authors: Nemanashi , Tshisikhaiwe
- Date: 2019
- Subjects: Sedimentology
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10353/14616 , 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.
- Full Text:
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