Evidence of Late Holocene extreme marine wave event (tsunami) deposits along the South African coast and implications for coastal zone management
- Authors: Mfikili, Athi Nkosibonile
- Date: 2023-12
- Subjects: Tsunamis -- South Africa , Coasts -- Risk assessment , Sediments (Geology)
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10948/62517 , vital:72811
- Description: Although extreme marine wave events such as tsunamis and storm surges have always been part of human existence, their impact on ever-growing coastal population of the world’s coast was highlighted by the recent catastrophic events over the past three decades. Although in situ observations and eyewitness accounts have improved our ability to record and interpret wave inundation by modern extreme marine events, records documenting inundation by pre-historic and historical events are relatively sparse to provide reasonable information on their distribution, magnitude and frequency intervals in most parts of the world. Owing to consequences these natural coastal hazards have on coastal population and infrastructure of the world’s coast, onshore sedimentary deposits preserved in the geological records have been extensively utilized by scientists worldwide to reconstruct the record of pre-historic and historical events and establish the recurrence interval of future events. Although studies investigating inundation of past tsunami has increased worldwide over the past two decades, research on extreme marine inundation has received little attention along the South African coast, despite known inundation by recent events and potential source mechanisms. Therefore, the aim of this study is to investigate sedimentary evidence of past tsunamis and related extreme marine wave events (i.e., storm surges) preserved in the recent geological archive of several coastal environments along the South African coast. The Swartkops, Kabeljous, Kromme and Keurbooms estuaries located in log-spiral bays along the south-east coast and sheltered coastal systems of the Berg Estuary and Verlorenvlei as well as coastal lowlands of the Dwarskersbos on the west coast were selected as the study sites. The sediment cores were collected from these coastal environments and identification of extreme marine wave inundation was based on the recognition of distinct anomalous sand layers. To characterise sediment transport and reconstruct provenance source origin of the deposits, a combination of sedimentological (i.e., grain size distribution), exoscopy (i.e., quartz grain surface microtextures) and micropaleontological (i.e., foraminiferal assemblages) proxies were utilized. Furthermore, age estimation of the deposits was determined using a combination of radiocarbon (14C) and radiometric (137Cs and 210Pb) dating methods.Evidence of coastal flooding by high-energy marine wave events is found preserved in the stratigraphic records of Swartkops and Kabeljous estuaries along the south-east coast and lowlands of Dwarskersbos on the west coast in the form of anomalous sand deposits (layers). The anomalous deposits of Swartkops and Kabeljous estuaries generally consist of gravel-sized shell fragments often with a sharp erosive contact suggesting deposition by a highly energetic marine wave event. Although the preservation varies between cores, a maximum of three anomalous sand depositional layers are preserved in the Swartkops whereas only a single layer in the Kabeljous Estuary with a basal layer of calcrete deposits. 210Pb and 14C dating, constrain the ages of the younger deposits (i.e., the uppermost layer) at Swartkops between 1770-1804 AD and 339-3 cal. BP (1611-1947 cal. AD), respectively. Although these ages span over a wide period, they are interpreted to represent evidence of one of the historical tsunamis generated in the subduction zones of the Indian Ocean e.g., 1762 AD and 1797 AD events generated by 8.8 and 8.0 Mw earthquakes in the Arakan and Sunda Subduction Zones, respectively. Furthermore, the deposits could also represent geological evidence of the 1833 Mentawai events whereas evidence of 1883 Krakatau tsunami is not disregarded, since this event reportedly inundated Algoa Bay. On the other hand, the 14C dating constrain the ages of the second anomalous deposits between 1526-1253 cal. BP (424-697 cal. AD), whereas the older anomalous deposits preserved in the deeper sections of the Swartkops cores as well as the backwater channel of the Kabeljous Estuary are constrained to have been deposited around 4000 cal. BP. Although there is no known locally documented extreme marine wave event during this period, these deposits are interpreted to represent late and mid-Holocene tsunamis associated with mega-thrust subduction zones of the Indian Ocean whereas local submarine landslide generated event are also not disregarded entirely. On the west coast, three anomalous deposits representing extreme marine wave inundation are preserved in the coastal lowlands of the Dwarskersbos. Although the general sedimentary features and thickness vary between the deposits, the first anomalous deposits in the uppermost section of the cores consist of medium sand with scattered shell fragments whereas, the third anomalous deposits in the deeper section of the cores are generally coarser with gravel-sized shell fragments and sharp erosive basal contacts. By contrast, the second anomalous deposits consist of three successive sub-layers of yellow sand with variable sedimentary features and these layers represent successive wave inundation events. Although the sedimentary characteristics of these sub-layers varies, it was noted that the lowermost layers consist of coarser and less well sorted sediments often with basal erosive contact, suggesting that the first wave was more energetic than the two-succeeding waves. Furthermore, calibrated 14C dated shell fragments from the first and second anomalous deposits yielded ‘post-bomb’ ages suggesting deposition after 1950 AD. Given their position within the stratigraphic records, the two deposits are interpreted to represent first geological evidence of the 2008 west coast and 1969 Dwarskersbos events, respectively. On the other hand, the 14C dated shell fragments constrain the deposition of the third anomalous deposits between 315-0 cal. BP (1635-11950 cal. AD) and 358-0 cal. BP (1592-1950 cal. AD), respectively. The third anomalous deposits are interpreted to the 07 April 1620 event, which was described as ‘Two startling thunderclaps’ near Robben Island. Although initially associated to earthquakes, there are no records of an earthquake during this period, therefore, we postulate that the 1620 event could have been a similar event to that of 2008 that inundated the west coast region and/or an unknown event. While it is acknowledged that the absence of reliable source mechanisms poses a great challenge in the reconstruction of extreme marine wave events inundation along the south-east coast, the findings of exoscopy and micropaleontological analyses suggest that the deposits were transported to their depositional environments, although resultant microtextural features and occurrence of individual foraminiferal species and their taphonomical conditions is dependent on coastal setting. The south-east coast deposits consist of highly diverse foraminiferal assemblages, which are dominated by marine and marginal-marine taxa with relatively low abundances of estuarine-brackish species and variable taphonomical conditions. Furthermore, the mechanical-induced features such as percussion marks, fresh surfaces with sharp edges were still preserved on quartz grain surface of the Swartkops deposits, despite dominance by chemical-induced features such as dissolution. By contrast, grain surface microtextural signatures of the Dwarskersbos deposits are dominated by mechanical features such as fresh surfaces and v-percussion marks. Furthermore, the foraminiferal assemblages of the Dwarskersbos deposits consist of exclusively marine and marginal-marine species, suggesting that the sediments were sourced from shallow water environments as well as the adjacent beach. It was further noted that the foraminiferal tests of these deposits consist of higher abundance of corroded and fragmented tests indicating deposition by high-energy conditions. , Thesis (D.Phil) -- Faculty of Science, School of Environmental Sciences, 2023
- Full Text:
- Date Issued: 2023-12
- Authors: Mfikili, Athi Nkosibonile
- Date: 2023-12
- Subjects: Tsunamis -- South Africa , Coasts -- Risk assessment , Sediments (Geology)
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10948/62517 , vital:72811
- Description: Although extreme marine wave events such as tsunamis and storm surges have always been part of human existence, their impact on ever-growing coastal population of the world’s coast was highlighted by the recent catastrophic events over the past three decades. Although in situ observations and eyewitness accounts have improved our ability to record and interpret wave inundation by modern extreme marine events, records documenting inundation by pre-historic and historical events are relatively sparse to provide reasonable information on their distribution, magnitude and frequency intervals in most parts of the world. Owing to consequences these natural coastal hazards have on coastal population and infrastructure of the world’s coast, onshore sedimentary deposits preserved in the geological records have been extensively utilized by scientists worldwide to reconstruct the record of pre-historic and historical events and establish the recurrence interval of future events. Although studies investigating inundation of past tsunami has increased worldwide over the past two decades, research on extreme marine inundation has received little attention along the South African coast, despite known inundation by recent events and potential source mechanisms. Therefore, the aim of this study is to investigate sedimentary evidence of past tsunamis and related extreme marine wave events (i.e., storm surges) preserved in the recent geological archive of several coastal environments along the South African coast. The Swartkops, Kabeljous, Kromme and Keurbooms estuaries located in log-spiral bays along the south-east coast and sheltered coastal systems of the Berg Estuary and Verlorenvlei as well as coastal lowlands of the Dwarskersbos on the west coast were selected as the study sites. The sediment cores were collected from these coastal environments and identification of extreme marine wave inundation was based on the recognition of distinct anomalous sand layers. To characterise sediment transport and reconstruct provenance source origin of the deposits, a combination of sedimentological (i.e., grain size distribution), exoscopy (i.e., quartz grain surface microtextures) and micropaleontological (i.e., foraminiferal assemblages) proxies were utilized. Furthermore, age estimation of the deposits was determined using a combination of radiocarbon (14C) and radiometric (137Cs and 210Pb) dating methods.Evidence of coastal flooding by high-energy marine wave events is found preserved in the stratigraphic records of Swartkops and Kabeljous estuaries along the south-east coast and lowlands of Dwarskersbos on the west coast in the form of anomalous sand deposits (layers). The anomalous deposits of Swartkops and Kabeljous estuaries generally consist of gravel-sized shell fragments often with a sharp erosive contact suggesting deposition by a highly energetic marine wave event. Although the preservation varies between cores, a maximum of three anomalous sand depositional layers are preserved in the Swartkops whereas only a single layer in the Kabeljous Estuary with a basal layer of calcrete deposits. 210Pb and 14C dating, constrain the ages of the younger deposits (i.e., the uppermost layer) at Swartkops between 1770-1804 AD and 339-3 cal. BP (1611-1947 cal. AD), respectively. Although these ages span over a wide period, they are interpreted to represent evidence of one of the historical tsunamis generated in the subduction zones of the Indian Ocean e.g., 1762 AD and 1797 AD events generated by 8.8 and 8.0 Mw earthquakes in the Arakan and Sunda Subduction Zones, respectively. Furthermore, the deposits could also represent geological evidence of the 1833 Mentawai events whereas evidence of 1883 Krakatau tsunami is not disregarded, since this event reportedly inundated Algoa Bay. On the other hand, the 14C dating constrain the ages of the second anomalous deposits between 1526-1253 cal. BP (424-697 cal. AD), whereas the older anomalous deposits preserved in the deeper sections of the Swartkops cores as well as the backwater channel of the Kabeljous Estuary are constrained to have been deposited around 4000 cal. BP. Although there is no known locally documented extreme marine wave event during this period, these deposits are interpreted to represent late and mid-Holocene tsunamis associated with mega-thrust subduction zones of the Indian Ocean whereas local submarine landslide generated event are also not disregarded entirely. On the west coast, three anomalous deposits representing extreme marine wave inundation are preserved in the coastal lowlands of the Dwarskersbos. Although the general sedimentary features and thickness vary between the deposits, the first anomalous deposits in the uppermost section of the cores consist of medium sand with scattered shell fragments whereas, the third anomalous deposits in the deeper section of the cores are generally coarser with gravel-sized shell fragments and sharp erosive basal contacts. By contrast, the second anomalous deposits consist of three successive sub-layers of yellow sand with variable sedimentary features and these layers represent successive wave inundation events. Although the sedimentary characteristics of these sub-layers varies, it was noted that the lowermost layers consist of coarser and less well sorted sediments often with basal erosive contact, suggesting that the first wave was more energetic than the two-succeeding waves. Furthermore, calibrated 14C dated shell fragments from the first and second anomalous deposits yielded ‘post-bomb’ ages suggesting deposition after 1950 AD. Given their position within the stratigraphic records, the two deposits are interpreted to represent first geological evidence of the 2008 west coast and 1969 Dwarskersbos events, respectively. On the other hand, the 14C dated shell fragments constrain the deposition of the third anomalous deposits between 315-0 cal. BP (1635-11950 cal. AD) and 358-0 cal. BP (1592-1950 cal. AD), respectively. The third anomalous deposits are interpreted to the 07 April 1620 event, which was described as ‘Two startling thunderclaps’ near Robben Island. Although initially associated to earthquakes, there are no records of an earthquake during this period, therefore, we postulate that the 1620 event could have been a similar event to that of 2008 that inundated the west coast region and/or an unknown event. While it is acknowledged that the absence of reliable source mechanisms poses a great challenge in the reconstruction of extreme marine wave events inundation along the south-east coast, the findings of exoscopy and micropaleontological analyses suggest that the deposits were transported to their depositional environments, although resultant microtextural features and occurrence of individual foraminiferal species and their taphonomical conditions is dependent on coastal setting. The south-east coast deposits consist of highly diverse foraminiferal assemblages, which are dominated by marine and marginal-marine taxa with relatively low abundances of estuarine-brackish species and variable taphonomical conditions. Furthermore, the mechanical-induced features such as percussion marks, fresh surfaces with sharp edges were still preserved on quartz grain surface of the Swartkops deposits, despite dominance by chemical-induced features such as dissolution. By contrast, grain surface microtextural signatures of the Dwarskersbos deposits are dominated by mechanical features such as fresh surfaces and v-percussion marks. Furthermore, the foraminiferal assemblages of the Dwarskersbos deposits consist of exclusively marine and marginal-marine species, suggesting that the sediments were sourced from shallow water environments as well as the adjacent beach. It was further noted that the foraminiferal tests of these deposits consist of higher abundance of corroded and fragmented tests indicating deposition by high-energy conditions. , Thesis (D.Phil) -- Faculty of Science, School of Environmental Sciences, 2023
- Full Text:
- Date Issued: 2023-12
Estimating estuarine suspended sediment concentration through spectral indices and band ratios derived from Sentinel-2 data: a case of Umzimvubu Estuary, South Africa
- Authors: Tshazi, Zamavuso
- Date: 2022-11
- Subjects: Sediments (Geology) , Suspended sediments , Remote sensing
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10353/27743 , vital:69406
- Description: The current study was aimed at evaluating the reliability and efficacy of selected remote sensing band ratios and indices in accurately estimating the spatial patterns of suspended sediment concentration level in Umzimvubu Estuary, Eastern Cape, South Africa. Sentinel-2 imagery was acquired on the 29th of March 2022. Band reflectance values were extracted from Sentinel -2 imagery, and laboratory measurements of suspended sediment concentration were obtained from samples collected from fifty (50) sampling points in the estuary on the 29th of March 2022. Sentinel-2 imagery was then validated with the field data in estimating and mapping the suspended sediment concentration. Several remote sensing band ratios Red/(Green plus Near-Infrared), Near-Infrared/Green, Red plus Near-Infrared/Green, Blue(Green plus Red)/Blue and Green plus Near-Infrared)/Blue and indices, that is the Normalised Difference Turbidity Index (NDTI), Normalized Difference Suspended Sediment Index (NDSSI) and Normalized Suspended Material Index (NSMI)) were then used to predict the suspended sediment concentration from Sentinel-2 imagery. The accuracy of band ratios and indices was evaluated by correlating the prediction against the observed suspended sediment concentration from Sentinel-2 imagery. A total of 50 points were randomly surveyed in the Umzimvubu estuary for analyzing suspended sediment concentration. Results indicate that the Blue (Green plus Red)/Blue, the Green plus Near-Infrared)/Blue and NMSI performed well based on their R-squared. The Blue (Green plus Red)/Blue and Green + Near-Infrared)/Blue band ratios had 0.86 and 0, 94, respectively. While NSMI yielded an R-squared of 0,76 and RMSE of 19,2 mg/L. The results in the current study indicate that Sentinel-2 imagery can reliably estimate the concentration of suspended sediment level in the Umzimvubu Estuary using band ratios and indices. , Thesis (MSc) -- Faculty of Science and Agriculture, 2022
- Full Text:
- Date Issued: 2022-11
- Authors: Tshazi, Zamavuso
- Date: 2022-11
- Subjects: Sediments (Geology) , Suspended sediments , Remote sensing
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10353/27743 , vital:69406
- Description: The current study was aimed at evaluating the reliability and efficacy of selected remote sensing band ratios and indices in accurately estimating the spatial patterns of suspended sediment concentration level in Umzimvubu Estuary, Eastern Cape, South Africa. Sentinel-2 imagery was acquired on the 29th of March 2022. Band reflectance values were extracted from Sentinel -2 imagery, and laboratory measurements of suspended sediment concentration were obtained from samples collected from fifty (50) sampling points in the estuary on the 29th of March 2022. Sentinel-2 imagery was then validated with the field data in estimating and mapping the suspended sediment concentration. Several remote sensing band ratios Red/(Green plus Near-Infrared), Near-Infrared/Green, Red plus Near-Infrared/Green, Blue(Green plus Red)/Blue and Green plus Near-Infrared)/Blue and indices, that is the Normalised Difference Turbidity Index (NDTI), Normalized Difference Suspended Sediment Index (NDSSI) and Normalized Suspended Material Index (NSMI)) were then used to predict the suspended sediment concentration from Sentinel-2 imagery. The accuracy of band ratios and indices was evaluated by correlating the prediction against the observed suspended sediment concentration from Sentinel-2 imagery. A total of 50 points were randomly surveyed in the Umzimvubu estuary for analyzing suspended sediment concentration. Results indicate that the Blue (Green plus Red)/Blue, the Green plus Near-Infrared)/Blue and NMSI performed well based on their R-squared. The Blue (Green plus Red)/Blue and Green + Near-Infrared)/Blue band ratios had 0.86 and 0, 94, respectively. While NSMI yielded an R-squared of 0,76 and RMSE of 19,2 mg/L. The results in the current study indicate that Sentinel-2 imagery can reliably estimate the concentration of suspended sediment level in the Umzimvubu Estuary using band ratios and indices. , Thesis (MSc) -- Faculty of Science and Agriculture, 2022
- Full Text:
- Date Issued: 2022-11
Sedimentology, reservoir properties and hydrocarbon potential of the southern Bredasdorp Basin, offshore of the Western Cape Province, South Africa
- Authors: Baiyegunhi, Temitope Love
- Date: 2021-05
- Subjects: Sediments (Geology) , Hydrocarbon reservoirs
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10353/20753 , vital:46546
- Description: The Bredasdorp Basin has become the focus for exploration activity (i.e., seismic exploration and drilling) since the discovery of gas-condensate and oil reservoirs in the early 1980s. The basin has proven hydrocarbon reserves and potential for future discoveries. However, uncertainty about the sedimentological and petrographic characteristics, reservoir qualities, thermal maturity and hydrocarbon potential of the source/reservoir rocks has hindered further exploration, particularly in the southern part of the basin. To date, this part of the basin remains unexplored and partially understood with respect to petroleum systems evolution when compared to the central and northern parts of the basin. To fill the research gaps, exploration wells E-AH1, E-AJ1, E-BA1, E-BB1 and E-D3 were investigated so as to unravel the petrographic characteristics, depositional processes and paleoenvironment, tectonic provenance, paleoweathering, hydrocarbon potential, thermal maturity, diagenetic characterisitcs and reservoir qualities of the southern Bredasdorp Basin. The grain size textural parameters revealed that the southern Bredasdorp Basin sandstones are unimodal, predominantly fine-grained, moderately well sorted, mesokurtic and near-symmetrical. The bivariate plots of grain size textural parameters indicate that the depositional environments had been influenced mainly by river/beach/coastal dune conditions. The linear discriminate functions (LDF) diagrams show that the sediments are turbidity current deposits in a shallow marine environment. The Passega diagram revealed that the studied sandstones were mainly deposited by traction currents and beach process. In addition, the grain size log-probability curves and Passega diagram show the predominance of saltation and suspension modes of sediment transportation. Based on the inter-relationship of the various statistical parameters, it is deduced that the southern Bredasdorp Basin are mainly shallow marine deposits with signature of beach and coastal river processes. Based on the lithofacies analysis of the southern Bredasdorp Basin borehole cores, thirteen lithofacies were identified and grouped into six facies associations (FAs). The facies associations are: matrix supported conglomerate and massive sandstone (FA 1), ripple cross laminated, trough cross bedded and bioturbated sandstone (FA 2), massive sandstone with mudstone and shale interbeds (FA 3), alternating laminated to interbedded sandstone/siltstone and mudstone (FA 4), massive mudstone with minor interlamination of clay-rich sandstones and siltstone (FA 5), and carbonaceous laminated shale and mudstone with occasional siltstone laminae (FA 6). Facies associations FA1, FA2, FA3, FA4, FA5 and FA6 are interpreted as submarine channel-fills, submarine channel-levee, submarine sheet lobe, submarine lobe fring/overflow, basin plain deposits and deep sea floor/basin plain deposits, respectively. Sedimentological evidences from lithofacies interpretation revealed shallow marine environment as the main depositional environment, with minor contribution from the deep marine environment. Petrographic studies show that the southern Bredasdorp Basin sandstones chiefly consist of quartz (52.2–68.0percent), feldspar (10.0–18.0percent), and lithic fragments (5.0–10.2percent). The modal composition analysis revealed that the sandstones could be classified as subarkosic arenite and lithic arkose. The provenance ternary diagrams revealed that the rocks are mainly of continental block provenances (stable shields and basement uplifted areas) and complemented by recycled sands from an associated platform. The tectonic provenance studies show that the sandstones are typically rift-derived arenites and have undergone long-distance transport from the source area along the rift. In the regional context of the evolution of the Bredasdorp Basin, the results suggested that the basin developed on a passive rift setting (trailing edge) of the stable continental margins. The provenance discrimination diagrams based on major oxide geochemistry revealed that the sandstones are mainly of quartzose sedimentary provenance, while the mudrocks are of quartzose sedimentary and intermediate igneous provenances. The discrimination diagrams indicate that the southern Bredasdorp Basin sediments were mostly derived from a cratonic interior or recycled orogen. The bivariate plots of TiO2 versus Ni, TiO2 against Zr and La/Th versus Hf as well as the ternary diagrams of V–Ni–Th×10 suggest that the mudrocks and sandstones were derived from felsic igneous rocks. The tectonic setting discrimination diagrams support passive-active continental margin setting of the provenance. Chemical index of alteration (CIA) indices observed in the sandstones suggest that their source area underwent low to moderate degree of chemical weathering. However, the mudrocks have high CIA indices suggesting that the source area underwent more intense chemical weathering, possibly due to climatic and/or tectonic variations. The organic geochemistry results show that these rocks have total organic carbon, TOC contents ranging from 0.14 to 7.03 wt.percent. The hydrogen index (HI), oxygen index (OI), and hydrocarbon index (S2/S3) values vary between 24–263 mg HC/g TOC, 4–78 mg CO2/g TOC, and 0.01–18 mgHC/mgCO2 TOC, respectively, indicating predominantly Type III and IV kerogen with a minor amount of mixed Type II/III kerogen. The mean vitrinite reflectance values vary from 0.60–1.20percent, indicating that the samples are in the oil-generation window. The Tmax and PI values are consistent with the mean vitrinite reflectance values, indicating that the southern Bredasdorp Basin source rocks have entered the oil window and are considered as effective source rocks in the southern Bredasdorp Basin. The hydrocarbon genetic potential (SP), normalized oil content (NOC) and production index (PI) values all indicate poor to fair hydrocarbon generative potential. The main diagenetic processes that have affected the reservoir quality of the southern Bredasdorp Basin rocks are cementation by authigenic clay, carbonate and silica, growth of authigenic glauconite, dissolution of minerals and load compaction. These aforementioned diagenetic processes act differently in each borehole and at different depths. The influence of cementation and compaction is complex with no particular pattern with increasing depth, suggesting that diagenesis is the main challenge to reservoir characterization in the southern Bredasdorp Basin. The clays in the sandstones act as pore choking cement, which reduces porosity and permeability of the reservoir rocks. Reservoir quality of the sandstones has been improved to various extents due to the development of secondary porosity as a result of partial to complete dissolution of early calcite cement and some detrital grains (feldspars) and also affected by grain fracturing. The scattered plots of porosity and permeability versus cement+clays show good inverse correlations, suggesting that the reservoir quality is mainly controlled by cementation and authigenic clays. Based on the diagenetic study, it can be inferred that the potential reservoir quality of the southern Bredasdorp Basin sandstones is poor-moderate. , Thesis (PhD) -- Faculty of Science and Agriculture, 2021
- Full Text:
- Date Issued: 2021-05
- Authors: Baiyegunhi, Temitope Love
- Date: 2021-05
- Subjects: Sediments (Geology) , Hydrocarbon reservoirs
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10353/20753 , vital:46546
- Description: The Bredasdorp Basin has become the focus for exploration activity (i.e., seismic exploration and drilling) since the discovery of gas-condensate and oil reservoirs in the early 1980s. The basin has proven hydrocarbon reserves and potential for future discoveries. However, uncertainty about the sedimentological and petrographic characteristics, reservoir qualities, thermal maturity and hydrocarbon potential of the source/reservoir rocks has hindered further exploration, particularly in the southern part of the basin. To date, this part of the basin remains unexplored and partially understood with respect to petroleum systems evolution when compared to the central and northern parts of the basin. To fill the research gaps, exploration wells E-AH1, E-AJ1, E-BA1, E-BB1 and E-D3 were investigated so as to unravel the petrographic characteristics, depositional processes and paleoenvironment, tectonic provenance, paleoweathering, hydrocarbon potential, thermal maturity, diagenetic characterisitcs and reservoir qualities of the southern Bredasdorp Basin. The grain size textural parameters revealed that the southern Bredasdorp Basin sandstones are unimodal, predominantly fine-grained, moderately well sorted, mesokurtic and near-symmetrical. The bivariate plots of grain size textural parameters indicate that the depositional environments had been influenced mainly by river/beach/coastal dune conditions. The linear discriminate functions (LDF) diagrams show that the sediments are turbidity current deposits in a shallow marine environment. The Passega diagram revealed that the studied sandstones were mainly deposited by traction currents and beach process. In addition, the grain size log-probability curves and Passega diagram show the predominance of saltation and suspension modes of sediment transportation. Based on the inter-relationship of the various statistical parameters, it is deduced that the southern Bredasdorp Basin are mainly shallow marine deposits with signature of beach and coastal river processes. Based on the lithofacies analysis of the southern Bredasdorp Basin borehole cores, thirteen lithofacies were identified and grouped into six facies associations (FAs). The facies associations are: matrix supported conglomerate and massive sandstone (FA 1), ripple cross laminated, trough cross bedded and bioturbated sandstone (FA 2), massive sandstone with mudstone and shale interbeds (FA 3), alternating laminated to interbedded sandstone/siltstone and mudstone (FA 4), massive mudstone with minor interlamination of clay-rich sandstones and siltstone (FA 5), and carbonaceous laminated shale and mudstone with occasional siltstone laminae (FA 6). Facies associations FA1, FA2, FA3, FA4, FA5 and FA6 are interpreted as submarine channel-fills, submarine channel-levee, submarine sheet lobe, submarine lobe fring/overflow, basin plain deposits and deep sea floor/basin plain deposits, respectively. Sedimentological evidences from lithofacies interpretation revealed shallow marine environment as the main depositional environment, with minor contribution from the deep marine environment. Petrographic studies show that the southern Bredasdorp Basin sandstones chiefly consist of quartz (52.2–68.0percent), feldspar (10.0–18.0percent), and lithic fragments (5.0–10.2percent). The modal composition analysis revealed that the sandstones could be classified as subarkosic arenite and lithic arkose. The provenance ternary diagrams revealed that the rocks are mainly of continental block provenances (stable shields and basement uplifted areas) and complemented by recycled sands from an associated platform. The tectonic provenance studies show that the sandstones are typically rift-derived arenites and have undergone long-distance transport from the source area along the rift. In the regional context of the evolution of the Bredasdorp Basin, the results suggested that the basin developed on a passive rift setting (trailing edge) of the stable continental margins. The provenance discrimination diagrams based on major oxide geochemistry revealed that the sandstones are mainly of quartzose sedimentary provenance, while the mudrocks are of quartzose sedimentary and intermediate igneous provenances. The discrimination diagrams indicate that the southern Bredasdorp Basin sediments were mostly derived from a cratonic interior or recycled orogen. The bivariate plots of TiO2 versus Ni, TiO2 against Zr and La/Th versus Hf as well as the ternary diagrams of V–Ni–Th×10 suggest that the mudrocks and sandstones were derived from felsic igneous rocks. The tectonic setting discrimination diagrams support passive-active continental margin setting of the provenance. Chemical index of alteration (CIA) indices observed in the sandstones suggest that their source area underwent low to moderate degree of chemical weathering. However, the mudrocks have high CIA indices suggesting that the source area underwent more intense chemical weathering, possibly due to climatic and/or tectonic variations. The organic geochemistry results show that these rocks have total organic carbon, TOC contents ranging from 0.14 to 7.03 wt.percent. The hydrogen index (HI), oxygen index (OI), and hydrocarbon index (S2/S3) values vary between 24–263 mg HC/g TOC, 4–78 mg CO2/g TOC, and 0.01–18 mgHC/mgCO2 TOC, respectively, indicating predominantly Type III and IV kerogen with a minor amount of mixed Type II/III kerogen. The mean vitrinite reflectance values vary from 0.60–1.20percent, indicating that the samples are in the oil-generation window. The Tmax and PI values are consistent with the mean vitrinite reflectance values, indicating that the southern Bredasdorp Basin source rocks have entered the oil window and are considered as effective source rocks in the southern Bredasdorp Basin. The hydrocarbon genetic potential (SP), normalized oil content (NOC) and production index (PI) values all indicate poor to fair hydrocarbon generative potential. The main diagenetic processes that have affected the reservoir quality of the southern Bredasdorp Basin rocks are cementation by authigenic clay, carbonate and silica, growth of authigenic glauconite, dissolution of minerals and load compaction. These aforementioned diagenetic processes act differently in each borehole and at different depths. The influence of cementation and compaction is complex with no particular pattern with increasing depth, suggesting that diagenesis is the main challenge to reservoir characterization in the southern Bredasdorp Basin. The clays in the sandstones act as pore choking cement, which reduces porosity and permeability of the reservoir rocks. Reservoir quality of the sandstones has been improved to various extents due to the development of secondary porosity as a result of partial to complete dissolution of early calcite cement and some detrital grains (feldspars) and also affected by grain fracturing. The scattered plots of porosity and permeability versus cement+clays show good inverse correlations, suggesting that the reservoir quality is mainly controlled by cementation and authigenic clays. Based on the diagenetic study, it can be inferred that the potential reservoir quality of the southern Bredasdorp Basin sandstones is poor-moderate. , Thesis (PhD) -- Faculty of Science and Agriculture, 2021
- Full Text:
- Date Issued: 2021-05
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