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Площадь: 3722.93 км²
Gamtoos Basin
Gamtoos Basin is an echelon sub-basin under the Outeniqua offshore Basin of South Africa. It is a complex rift-type basin with both onshore and offshore components and consists of relatively simple half-grabens bounded by a major fault to the northeast. This study is mainly focused on the evaluation of the reservoir heterogeneity of the Valanginian depositional sequence. The prime objective of this work is to generate a 3D static reservoir model for a better understanding of the spatial distribution of discrete and continuous reservoir properties (porosity, permeability, and water saturation). The methodology adopted in this work includes the integration of 2D seismic and well-log data. These data were used to construct 3D models of lithofacies, porosity, permeability, and water saturation through petrophysical analysis, upscaling, Sequential Indicator Simulation, and Sequential Gaussian Simulation algorithms, respectively. Results indicated that static reservoir modeling adequately captured reservoir geometry and spatial properties distribution. In this study, the static geocellular model delineates lithology into three facies: sandstone, silt, and shale. Petrophysical models were integrated with facies within the reservoir to identify the best location that has the potential to produce hydrocarbon. The statistical analysis model revealed sandstone is the best facies and that the porosity, permeability, and water saturation ranges between 8 and 22%, 0.1 mD (< 1.0 mD) to 1.0 mD, and 30–55%. Geocellular model results showed that the northwestern part of the Gamtoos Basin has the best petrophysical properties, followed by the central part of the Basin.
Geological setting
Gamtoos is an echelon twin is along with the Algoa subbasin of the Outeniqua Basin (Fig. 1) is a simple half-graben feature controlled by the Gamtoos' fault extending deep into the crust (McMillan et al. 1997). The onshore parts of the Gamtoos fault have a throw of about 3 km, while offshore, the throw increases to about 12 km (Thomson 1999). Major tectonic movements in the offshore Gamtoos Basin occurred on the eastern flank of the St. Francis arch. In the offshore 1 Port Elizabeth and Uitenhage troughs top of the basement (Horizon D reached depths of 6.5 km and 8 km, respectively.
Fig. 1 Location map of the study area in the red circle, Gamtoos Basin. (modified after McMillian et al. (1997)
Along the southern coastline of the Western Cape and Eastern Cape Provinces on the Eastern edge of the continental shelf is the Agulhas bank with water depths of less than 200 m increasing rapidly beyond 500 m in the south at the present-day shelf edge due to the erosion caused by the Agulhas current flowing along the south-westward in the shelf edge (Malan 1993; Broad et al. 2006). The reservoir sandstones in the Gamtoos Basin mainly occur as shallow marine sandstones in the Kimmeridgian (late Jurassic) stage in the western part of the Basin and the Late Valanginian sequence of the Basin (Fig. 2). The Valanginian sandstone sequence shows the potential for hydrocarbon exploration (Malan 1993) and deposited within the continental shelf to a deep marine depositional environment (Ayodele et al. 2020).
Also, numerous stacked gas-charged submarine sandstone fans defining a Kimmeridgian to Berriasian age reservoir were interconnected in the southern parts of the Gamtoos Basin. The offshore part of the Gamtoos Basin is located along with the other echelons of Outeniqua Basin, South Africa, with reservoir sand indicating a normal pressure with no overpressure zones (Ayodele et al. 2016; Ayodele et al. 2020).
Fig. 2 Chronostratigraphic chart of Gamtoos Basin showing major unconformities, depositional sequences distribution of sandstone, source rocks, and hydrocarbon shows, the time of the events that influence the developments of the basins, Soekor (1994) and adapted & modified after McMillan et al. (1997)
Data source: Static reservoir modeling using stochastic method: a case study of the cretaceous sequence of Gamtoos Basin, Offshore, South Africa. O. L. Ayodele, T. K. Chatterjee, M. Opuwari. 2021
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