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Orange Basin
STRUCTURAL SETTING
The Orange Basin forms part of the Southwest African Coastal Basin which lies offshore Namibia and western South Africa, extending from the Walvis Ridge in the north to the Agulhas Falkland Fracture Zone in the south. Three depocentres are recognized from north to south: the Walvis, Lüderitz and Orange sub-basins (Figure 1). The Orange Basin which is the largest of the three, covers an area of approximately 160 000km to the 2000m isobath, defined by the extent of the infill post-rift sediments.
Figure 1: Structural framework and main depocentres of the Southwest African Coastal basin (modified after IHS Energy Group, 2008)
BASIN EVOLUTION
The Orange Basin formed in response to the fragmentation of Western Gondwanaland, and the associated opening of the South Atlantic Ocean through sea-floor spreading. As with classic divergent plate tectonic settings, the geological succession displays a typical two-fold subdivision. This comprises an older syn-rift unit, overlain by a younger post-rift unit. The South West African continental margin is a rifted plate margin that is underlain by pre-rift and syn-rift grabens, covered by post-rift sediments. Four main tectono-stratigraphic sequences represent the major phases that resulted in the opening of the South Atlantic Ocean namely: Syn-rift, Post-rift (Transitional), Drift and Late-Drift (Cretaceous and Tertiary).
During the Middle to Late Jurassic, the extensional stress associated with the breakup, caused the formation of a series of coast-parallel graben systems (Figure 2). These graben systems are separated laterally from an outbound synrift wedge by a flexural high which is known as the Medial Hinge (Gerrard & Smith 1982). The graben fill comprises Lower Cretaceous siliclastics and volcanics, the former including fluvio-lacustrine sediments and coarse continental clastics.
Figure 2: Schematic geologic cross section across the central Orange Basin (Jungslager, 1999)
The drift succession is characterized by a thick seaward and landward thinning sedimentary wedge that overlies the rift basins representing a longstanding sedimentary depocentre, mainly supplied by the Orange and Olifants Rivers and their ancestral equivalents. The margins of the Orange Basin depocentre, in a strike sense, are defined by flanking basement highs. These comprise the Kudu Basement Arch in Southern Namibia, and the Agulhas Columbine Arch in the South (Figure 1).
The termination of rifting and onset of drifting on the Southern African West Coast occurred in the Hauterivian. This marks the time at which the proto-South Atlantic Ocean was established, with the permanent imposition of a marine influence on the west coast. The Cretaceous succession is subdivided into five major super-sequences each defined by major, tectonically enhanced, eustatic sequence boundaries (Brown et al., 1996) (Figure 3). During the initial phase of this period of the margin's history, the South Atlantic remained relatively shallow and restricted. The depositional setting thus was transitional between rift and drift. The continental shelf/slope profile in existence at this time was poorly developed and ramp-like. Associated depositional environments included restricted marine, deltaic and coastal plain settings.
Figure 3: Generalized chronostratigraphy of the Orange Basin
A prolonged northerly sediment input during the Mid to Late Cretaceous explains both the presence of the principal depocenter in this region, as well as the relatively more basin-ward offlap position of younger super-sequences, when, compared to in the south. Subsidence rates on the South African West Coast had diminished by the onset of the Tertiary, with the underlying Cretaceous platform becoming increasingly stable (Dingle & Scrutton, 1974). As a result, Tertiary depocenters shifted significantly basinward of their Cretaceous counterparts, with sedimentation now being focused in a former slope setting. Prolonged vertical stacking of continental slopes during the aggradational phases resulted in spectacular gravity faulting at the shelf edge, occurring contemporaneously with sedimentation.
ALBIAN
The Albian Stratigraphic-Structural Play has been confirmed in several gas discoveries off South Africa, the best of which is the A-K1 (Ibhubesi) (Figures 3,4,5). It is the most significant play in the South African portion of Orange Basin to date. The more important features of this play type exemplified by the A-K1 discovery are its stratigraphic trapping; the excellent porosity-permeability qualities and the extremely good ow rates for relatively thin sandstones; seismic amplitude anomalies associated with the gas-bearing sandstones; and the absence of gas-water contacts indicating possibly large stratigraphically trapped systems. Although this play can be confidently mapped seismically over the entire main depocentre area between the A-K1 location and the Namibian border, individual traps are difficult to identify mainly because of the almost total absence of structure.
Figure 4. The active marine source rock kitchen at the lower Aptian level (after Jungslager, 1999)
BARREMIAN
The Barremian Stratigraphic Play is confirmed by the Kudu gas field offshore southern Namibia, in the northern part of the Orange Basin depocentre (Figures 3,4,5). The most striking features of this gas accumulation are that it is stratigraphically trapped in continental aeolian sandstones. No water contact was encountered; the reservoir is some 1,600 psi overpressured; the porosities average 12% and permeabilities 43 mD at about 4,400 m. The deliverability of the reservoir is substantial despite the depth of burial and the reservoir sands are closely associated with basalts. The gas is about 96% methane and the reservoir is overlain by a gas-charged, but tight, shallow marine sandstone, itself overlain by some 200 m of spent source rocks (Jungslager, 1999).
Figure 5. The main petroleum discoveries and shows in the Orange Basin (after Jungslager, 1999).
Expulsion and migration postdated the sealing of the structural-stratigraphic traps, which was penecontemporaneous with reservoir deposition in the Early Cretaceous.The source for A-F1 and A-K1 (Albian) is accepted to be the widespread Lower Aptian shales. The sealing of the structural- stratigraphic traps, was penecontemporaneous with reservoir deposition in the Early Cretaceous.
HAUTERIVIAN
To date, this play is known in only one discovery in the AJ Graben at well A-J1. (Figures 3,4,5. The significance of the play is that it establishes a working petroleum system in the non-marine part of the syn-rift succession and in the eastern marginal graben province. It is also the only play in which oil has been found in the Orange Basin.
The Hauterivian Stratigraphic Play is located in the eastern marginal graben province, beneath the Aptian transgression.
Trapping results from the pinching out of the syn-rift reservoir in a halfgraben structure. The reservoir comprises lacustrine, fluvial or deltaic sandstones. Porosity (10-15%) and permeability (0.01-468 mD) are locally reduced by calcareous cementation and silicification. Although the petroleum system is represented by a single small oil accumulation, it is important as it demonstrates that the basin is not exclusively a gas basin. Oil and gas prone Hauterivian lacustrine syn-rift shales are the main source rocks. These shales are presently within the oil window but probably reached maturity during the Late Cretaceous as there is virtually no Tertiary overburden in the inner graben area where A-J1 is located. Oil migrated short distances laterally to syn-rift lacustrine sandstone reservoirs in structural-stratigraphic traps. Sealing took place soon after deposition of the reservoir rock.
Data source: Petroleum exploration potential of the Orange Basin. Petroleum Agency SA
Следующий Бассейн: Gabon