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Площадь: 19898.38 км²
Bredasdorp basin
The Bredasdorp basin forms one of the sub-basins of the Outeniqua basin. Turner, Rosendahl, and Wilson (2003) described the basin as one created by the extension encom passing the early stages of rifting during the Jurassic. The maximum deposition of shallow-marine and continental sediments in the basin was from the Early Jurassic to the Late Cretaceous. The Bredasdorp basin has had several eco nomic hydrocarbon discoveries, mainly gas, oil and conden sate. The Oribi, Sable and Oryx fields in the basin have recovered 23 million, 25 million and 7 million barrels of oil (MMbbl) respectively (Davidson and Winkler 2003; McMillan et al. 1997; Scholvin 2020).
Overview of geological setting In South Africa, the offshore basins exist as three distinct tectonostratigraphic zones along the coastline. These zones are defined as the Eastern, Western and Southern zones. Their development preceded and occurred during the Permo-Triassic and the Jurassic period (Jungslager 1999; McMillan et al. 1997). The Eastern zone comprises the Durban and Zululand basins, which formed due to the breakup of Africa, Madagascar and Antarctica during the Jurassic period. This breakup occurred as part of the African Rift system, and a narrow passive margin characterizes the zone. The Western zone consists of the Orange Basin, located on South Africa’s west coast. This basin is a diver gent plate margin associated with half-graben structures that have a trend sub-parallel to the coastline (Brown 1995; Jikelo 2000; Opuwari 2010). Finally, the Southern tectono stratigraphic zone is given by the Outeniqua basin, an exten sive complex intra-cratonic rift basin formed by the amalgamation of rift sub-basins. From west to east, these sub-basins are; Bredasdorp, Pletmos, Gamtoos and Algoa basins. The sub-basins consist of half-graben structures due to rifting, which underlie drift sediments of variable thick nesses. The half-graben characteristic is created when the normal faults in a sedimentary basin have a unidirectional dip, leading to the slipping and tilting of adjacent fault blocks relative to the neighbouring faults (Broad et al. 2006).
Figure 1. Map of the study area showing the position of wells and seismic data used in the study modified after (PASA 2003 and Opuwari and Dominick 2021)
The Outeniqua Basin is characterized by four sub-basins (Bredasdorp, Pletmos, Gamtoos and Algoa basins) with deep-water extensions that join at its southern region. It is a sizeable intracratonic basin that formed its structure via the dextral shearing mechanisms that South Africa’s margin underwent from the Early to Mid-Cretaceous period during the breakup and drifting of Gondwana (PASA 2003). Complex sequences of micro-plates (e.g. Falkland Plateau) travelled in a west-south-west direction past Southern Africa’s coastline and led to sub-basins with an oblique rift half-graben structure. These sub-basins are young in the western regions and mature towards the East. The Bredasdorp basin could be considered a failed rift of the resulting half-grabens (Masindi, Trivedi, and Opuwari 2022; PASA 2003).
The Bredasdorp basin is located between the Cape Infanta and Cape Agulhas anticlinal basement highs. The basin is described by Dingle, Siesser, and Newton (1983) as a broad depression with an asymmetrical cross-section. The southern limit of the Bredasdorp basin was developed by the slightly northward-tilting faulted boundary of the Agulhas Arch. Post-Palaeozoic rocks are visible, exposed as Arch’s upper division, and are characterized by Bokkeveld shales and Table Mountain Group quartzite rocks, which overlie a granite core (de Wet and Compton 2021). The basin floor is relatively level but inclined toward the northwestern direc tion. Therefore, the deepest region of the basin is situated adjacent to the northern margin. The basement of the northern Bredasdorp basin descends rapidly through numer ous minute Infanta Arch boundary faults (Dingle, Siesser, and Newton 1983). The structural development of the Bredasdorp occurred in several prominent events, catego rized into phases (Figure 2).
Figure 2. Schematic cross-section of the Bredasdorp basin, (after PASA 2023)
Sequence stratigraphy concepts were applied to various post-rift sediments deposited during the Lower Cretaceous. These concepts allowed for the correlation of regions with related depositional systems and facies within the basin. A distinct cyclic depositional sequence resulted from the inter active behaviour of rifting, thermal cooling, and global eustatic fluctuation of sea levels. Nearly ten regular and mega-cyclic sequences can be identified as deposits between the mid-Valanginian and lower Santonian (Broad et al. 2006).
The concept of cyclic sequences was employed to recog nize hydrocarbon plays in the basin and was used as a local region-based tool instead of a global system. A chronostrati graphic framework created by age dating was developed after identifying and mapping the recognized sequences, which were amassed with Exxon’s global system (Figure 3). However, the global-cyclicity model from Exxon is indeed present within the basin. Local overprinting of this ‘global effect’ happens from tectonic processes that allowed the Bredasdorp basin to have its entire sedimentation history (Broad et al. 2006). There is favourable hydrocarbon reser voir potential in the low-stand system tracts recognized in the sequences. Low-stand system tracts formed upon an ero sional unconformity (due to the decline of the sea level below the sea shelf edge) are also defined as ‘Type 1’ uncon formities. Transgressive shale rocks deposited during the sea-level rise are the source and seals of the resulting floor fans, channel fills and wedges. The terrigenous clastic mater ial characterized by the Bredasdorp allows for the interpret ation of the low-stand system tracts as a composition of basin floor turbidite fans, channels and/or sheets (Brown 1995; McMillan et al. 1997). The transgressive tracts are poorly defined because the sea-level rise acceleration coin cided with the shelf’s flooding. The relative sea level increase led to the development of deltaic systems with basin-ward progradation and the formation of distinct clinoforms. The principal hydrocarbon prospects in the low-stand systems are located up-dip in the pinch-out of deltaic sandstones, and these exist as draped sheets, mounded turbidite fans, and channel fills (Broad et al. 2006).
Due to low sedimentation rates, shale rocks with high dry- to-wet gas potential are deposited along with oil-prone organic shale rocks in the Barremian age. These identified shale rocks serve as the thickest and highest quality of basinal source rocks (Burden 1992). The deposition of organic-rich shales in the central basin occurred during sediment starva tion in a vast region following the mid-Aptian unconformity (Magoba and Opuwari 2020). The Bredasdorp basin’s main marine sandstones are interbedded with claystones of lagoonal and fluvial origin. The uppermost layer of the sequence is composed of a thick marine sandstone complex that was deposited during the Early Cretaceous, which is now a part of the vital gas reservoirs in the North flank gas field (Burden 1992, Saffou et al. 2020). Channel lobes also combine to form sandstone reservoir rocks. These sandstones resulted from the sediments of the pre-existing high-stand shelf, which are transported into the central area of the basin by turbidity currents. Channel reservoir rocks are predominant in the western region of the basin, where there are multiple fan lobes, and these reservoirs remain unaffected by faults. Sandstone units from the synrift and drift phases can be observed in the basin under standard conditions (PASA 2003). Both stratigraphic and structural traps occurring in the Bredasdorp basin (PASA 2003, Opuwari et al. 2022) were formed from the Late Cretaceous to the Early Tertiary period. The reservoirs in the basin (shallow-marine to fluvial) have structural and truncational traps. In contrast, the drift reser voirs comprise trap anticlines of a compressive nature, strati graphic pinch-out traps, and closures associated with inversion (Jungslager 1999; PASA 2003).
Data source: Evaluation of carbon dioxide storage potential in wells of the Bredasdorp Basin offshore South Africa. Luyanda Ngcobo, Blessing Afolayan and Mimonitu Opuwari. 2024
Следующий НГО: Pletmos Offshore