Browsing by Author "Kahle, Beth"

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    A 3D fault seal analysis study conducted in the Ibhubesi Gas Field Offshore the West Coast of South Africa
    (2018) Ntombela, Nondumiso; Kahle, Beth
    A three dimensional fault seal analysis study of the AK fault situated offshore of the West Coast of South Africa is presented. This study is aimed at informing the development plan of the Ibhubesi gas field with regards to the compartmentalization of the reservoirs, by understanding whether a key fault, the AK fault facilitates hydrocarbon migration to the Ibhubesi field reservoirs or whether it provides a seal. In order to address this research aim, a seismic interpretation of the area was carried out and combined with an interpretation of well data to construct a 3D structural model from which the fault seal analysis was carried out. Juxtaposition analysis was used to determine the lithologies that had been juxtaposed across the fault. Fault clay was determined using the Shale Gouge Ratio (SGR) algorithm and permeability and threshold pressure were also used to quantify the fault seal capacity. The results from the SGR were correlated to the across fault pressure difference and plotted on a reference diagram which is a comparison of sealing faults vs. leaking faults. This diagram indicates that faults with an SGR greater than 25% and threshold pressures greater than 8bars, have a high potential to seal. The AK fault SGR results range between 25 and 55 % with a threshold pressure of up to 20bars. These results indicate that the AK fault is likely to be a sealing fault.
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    Open Access
    4D seismic analysis: discriminating between saturation and pressure changes form a data acquired Offshore Equatorial Guinea
    (2017) Msolo, Andile Lucky; Kahle, Beth; Gidlow, Maurice; Kahle, Richard
    4D seismic analysis is used to understand the performance of a producing oil field in order to increase production and mitigate risk. This is done by interpreting changes in water saturation and changes in effective pressure in the reservoir, which allows production to be maximized safely. In this project, I determined saturation and pressure changes in an oil field offshore Equatorial Guinea. I used Landrø's technique as a basis for the analysis, however Landrø's technique requires the presence of reservoir samples. For this project, rock samples of the reservoir were not available; this led me to modify Landrø's methodology using approximations based on well-accepted rock physics relationships. I tested this new methodology on a synthetic model, which gave encouraging results, after which it was applied to real seismic data. Pressure and saturation changes estimated in the reservoir indicate that the northern part of the reservoir experienced a decrease in pore pressure. The saturation changes over the northern part of the reservoir are not prominent. The central and southern parts of the reservoir show an increase in water saturation accompanied by an increase in pore pressure. The results are consistent with the production and injection history of this area. A comparison between saturation before production, saturation changes after production and pressure changes after production indicates an area (on the northern part of the reservoir) which shows fluid anomalies before production and does not show any production (or 4D) effects. New production wells could potentially be drilled in this area to increase production.
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    Evolution of tectonic and stratigraphic geometries in the Southern Outeniqua Basin
    (2019) Smithard, Travis; Kahle, Beth; Sloan, Alastair
    The evolution of the Southern Outeniqua Basin has been analysed within a tectono-stratigraphic framework by the interpretation of 3,700km of 2D seismic data constrained by five exploration wells, drilled on the adjacent shelf. The mapping of different fault families into distinct structural orientations, NE-SW, NNE-SSW and NW-SE, was done and the basin sub-divided into four sectors (far-west, west, central and east) based on the geographical position of these faults. Sequence stratigraphic analysis of the basin lead to the identification of seven secondorder unconformities which have been further defined into five main seismo-stratigraphic units. The variation in major structural orientations suggest the evolution of the basin developed during three phases of extension, which have been related to the broader plate tectonic context during the Mesozoic break-up of Gondwana. Early Jurassic (170-150Ma) rifting between West and East Gondwana is proposed to have created weaknesses in the crust along the Southern African margin in a NNE-SSW orientation leading to the development of small rift basins and the deposition of syn-rift sediments. A series of intra-continental strike slip faults, including the proto-Agulhas Falklands Fracture Zone, are proposed to have developed along the southern margin during the incipient formation of the Weddell Sea (147Ma) between Antarctica and West Gondwana. Later mid-Jurassic (150-130Ma) extension along the North-Falkland rift zone lead to the oblique extension along the southern margin and the development of normal faults orientated NE-SW. Continued rifting (134Ma) in the proto-South Atlantic lead to translation along the southern African margin and the coalescence of the individual segments of the intra-continental strike slip faults and ultimately the formation of the Agulhas Falklands Fracture Zone. This phase of wrench faulting (134- 130Ma) is characterised by the strike-slip deformation of syn-tectonic sediments defined by second-order unconformities BR and B - late Valanginian - Hauterivian (134-130Ma). The Diaz Marginal Ridge (DMR), which bounds the Southern Outeniqua Basin and AFFZ to the south and north respectively, shows a marked variation is seismic character along strike. In the eastern and western sectors of the basin the DMR is made of syn-rift sediments. The discontinuous and non-parallel seismic character of the DMR in the central sector of the basin is interpreted to be continental basement. Significant bevelling of the top of the DMR in this sector, along with on-lapping reflector geometries of horizon B, Hauterivian (130Ma), is proposed to signify that the continental block existed prior to the formation of the Southern Outeniqua Basin and remained a basement promontory until the development of horizon G - Tertiary (66Ma). The Southern Outeniqua Basin is therefore proposed to have formed through four periods of tectonism: 1) early Jurassic (170-150Ma) orthogonal rifting of West from East Gondwana; 2) late Jurassic to early Cretaceous (150-130Ma) oblique extension and translation along intra-continental strike-slip faults; 3) early-late Cretaceous deformation associated with transverse motion along the AFFZ and development of an active transform margin; 4) mid-late Cretaceous thermal uplift of the Southern Outeniqua Basin associated with migration of the Natal Valley spreading centre along the AFFZ. Thermal subsidence and true passive transform margin conditions developed in the Southern Outeniqua Basin from early Tertiary to present day.
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    Identification of the physical controls on the deposition of Aptian and Albian deep water sands in the Bredasdorp Basin, South Africa
    (2016) Van Rensburg, Tamsyn; Kahle, Beth; Dekker, Peter
    In the petroleum industry, the location of a new well is selected based on several factors, one of which is the presence of reservoir-quality sands. To determine the lateral extent of these sands away from well control, the depositional environment and character of the deposit must be adequately identified. This study aims to explain the physical controls on the deposition of the 13A (Aptian) and 14A (Albian) sequence sands within the deep water region of the Central Bredasdorp Basin through identifying the mass transport facies and processes and relating these to tectono-eustatic factors. Since a primarily seismic-based approach was used to achieve the project objective, the results reflect findings based on 3D seismic data interpretations as well as seismic surface and volume attribute extraction supported by wireline well logs and well completion reports. This dataset contains information that enabled the identification of the structural and stratigraphic architecture of the 13A and 14A sequences as a whole, the location of the sediment provenances and possible triggers of the mass flows as well as the consequential sand distribution trends from the basin slopes to across the basin floor during the Aptian-Albian time. The onshore Tankwa Basin was studied as an analogue to the Bredasdorp Basin because it hosts world class outcrops of deep water lowstand fan deposits and therefore shows the finer-scale details of the associated depositional stratigraphy. The 13A and 14A sequence sands would have entered the Bredasdorp Basin in progradational pulses alternating with mud-rich successions associated with local sea level fluctuations that were on trend with the gradual global sea level rise from the Aptian to the Albian. These alternating successions are identified as lowstand, transgressive and highstand systems tracts in the seismic and wireline well log data used in this study. The presented depositional model of the 13A sequence sands is a system of northwest to southeast sediment transport across the Central Bredasdorp Basin with indications of a final swing in orientation towards the east. The sands were mainly sourced from the paleo shelf edge on the northwest margin, although additional sediment input may have come from the west too. The faults that were active before and during the deposition of the Aptian-aged (13A) sands appear to have been the main control on sand distribution across the basin, guiding the sands from slope channels into basin floor fans and from shelf edge slumps into base of slope fans in a basinwide northwestsoutheast trend. The model of deposition of the 14A sequence sands is based on a channelised flow of sediment from the Central Bredasdorp Basin paleo shelf edge, down the slope and onto the basin floor primarily from the onshore source on the western margin. Supplementary sediment input may have originated from the Agulhas basement high on the southern margin of the basin in the form of less confined channels and mass wasting deposits. Inherited topography of the sea floor at the Albian time appears to have been the primary control on 14A sand distribution, causing bypass zones and giving rise to narrow, confined channel complexes despite some of the active faults possibly redirecting some of the sands from their initial trend. Overall the pattern of deposition of the Aptian and Albian deep water sands in the Bredasdorp Basin appears to have been physically controlled by the regional paleo seabed topography and fault activity until the late Aptian.
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    Micro-seismic observations in Leeu Gamka, Karoo, South Africa
    (2018) Fynn, Melody; Kahle, Richard; Kahle, Beth
    This thesis documents a microseismicity study in the interior of South Africa. The study area is centred on Leeu Gamka in the Western Cape province, a tectonically stable intraplate setting and is therefore expected to be seismically quiet. The International Seismological Centre (ISC) catalogue reported localised anomalous seismicity in the region between 2007 and 2013 with local magnitudes up to 4.5. The short apparent duration and time history of this anomalous reported seismicity is likely a reporting artefact. An array of 23 geophones was deployed for three months (March - June) in 2015, covering an area of 60 km - 65 km centred on the zone of anomalous seismicity. Using this array, I identified a total of 106 earthquakes over this period, with almost all events clustering in a surprisingly small area (75% of the epicentres fall within a one square kilometre block). Double-difference relocation resolved the hypocentres onto a structure with an apparent NW - SE orientation, consistent with large-scale fabric that can be recognised in satellite imagery. The focal mechanisms display strike-slip faulting with the fault plane likely in a NW - SE orientation, consistent with the distribution of the earthquakes. The velocity model was tested by varying the thickness of the Karoo supergroup to investigate the sensitivity of the depths of the earthquakes. An average hypocentral depth of approximately 6 km was calculated for the earthquakes, assuming a depth to the base of the Karoo of 5km. This places the earthquakes just below the base of the Karoo in the Cape Supergroup. The magnitudes of the earthquakes recorded range from -1.5