Risks and prospects of South Africa's South Outeniqua Basin

Contents
Basin prospectivity
Geological evolution
Exploration history
Zones of hydrocarbon prospectivity
The promise of South Outeniqua

South Africa's aggressive search for oil, especially since it's re-entry into the comity of nations in 1994, has yielded two marginal oil fields, a large gas discovery, and a thick pile of geologic data for operators to investigate.

The E-AR/Oryx Field, an extension to Oribi Field in the Bredasdorp Basin, started flowing at a rate of 10,500 b/d on 25 May 2000. South Africa's total production now stands at about 25,000 b/d, or 6% of the country's daily crude requirements. Meanwhile, The Sable oil and gas field lies 17 km to the west of Oribi's platform, Orca. Soekor has already encountered oil and gas in five wells drilled here, and peak oil production could be 40,000 b/d if results from the appraisal well currently being drilled are as expected.

In June, a 2.5 tcf gas discovery was made in Block 2A, southwest of the Orange River mouth. The operator, Forest Oil, followed-up by commenting that a preliminary evaluation has only been made of Soekor's 1988 gas discovery AK-1, roughly in the middle of the block, over which 3D seismic was run last year. AK-1 has estimated potential recoverable reserves of about 200 bcf, with possible reserves of 2.5 tcf.

Meanwhile, the sum of activity in the Bredasdorp and Pletmos Basins, as well as the Falklands Graben in South America, is proving that the next oil field in the area might yet come from the yet undrilled South Outeniqua Basin, the contiguous deepwater extension of the Gamtoos/Algoa and Platmos Basins. It spreads out in 24,000 sq km in an elongate, southwest-northeasterly fashion, paralleling the adjacent South African coastline.

Basin prospectivity(Back to Top)
The stratigraphic succession is correlatable in age with much of the coastal basins of East Africa. But there the comparison ends, for there are, in South Outeniqua, more geologic reasons to drill for oil than in Tanzania, Uganda, Mozambique, or Kenya.

The Southern Outeniqua Basin is part of a large, intracratonic rift basin, the Outeniqua Basin. A recent report by Ranger Oil, contends that it is a "Late Jurassic trans-tensional rift basin formed during the break up of Gondwana". This is at odds with a 1997 Soekor paper, which put the basin's age as middle to late Jurassic, "before the separation of east and west Gondwanaland". Both papers agree that the Falklands Graben, offshore South America, is part of the South Outeniqua, but Ranger's paper goes further to propose a syn-Gondwana and post-Gondwana evolution. Delivered at the Offshore West Africa Conference, it identifies two principal play systems: a late Jurassic syn-rift gas play and an early Cretaceous post-rift oil play. "Both have been proven in the adjacent Block 9, resulting in production from both the syn- and post-rift accumulations," according to Ranger's David Parker. The 30 million bbl Oribi Field, currently producing 15-20,000 b/d, is part of the post-rift play accumulation. Ranger has traced the two play systems to two of the three leases in the South Outeniqua Basin and is sure there is a southward growth in sand reservoirs from the Breadasdorp/Pletmos Basins to South Outeniqua. "Although the field size is only moderate in Block 9, the structures identified in the Southern Outeniqua Basin are much larger," said Parker, "due to a marked change in structural evolution occurring southwards from Block 9."

Geological evolution(Back to Top)
The early rift sequence in the entire Outeniqua Basin is filed with thick late Jurassic sediments that occupied a number of southeasterly trending grabens. Some of these early depocenters, such as the Plettenberg graben and the Southern Outeniqua Basin, are predicted to contain late Jurassic oil-prone shales, similar to those intersected in the adjacent Gamtoos and Algoa Basins and in the Deep Sea Drilling Project boreholes on the Falkland plateau.

Early fill is overlain by thick aggradational fluvial sediments in the northern Pletmos Basin and marginal sandstones in the southern Pletmos Basin. These sedi-ments were sourced directly off the flanks of the basin and down the axis of the grabens in a southeasterly direction.

The late synrift interval comprises fluvial, shallow marine, and shelf deposits of early Cretaceous age. The sandstone content of the entire synrift succession increases towards the Southern Outeniqua Basin in a southwesterly di-rection away from the sandstarved Plettenberg graben.

The folding and strike-slip movement noticeable along existing normal faults, were created by right-lateral shear stresses which developed along the Agulhas-Falkland fracture zone during the early Cretaceous. The resultant right lateral movement along the fracture zone separated the Falkland plateau from the African plate and bisected the Outeniqua rift basin.

The angular unconformity marked by the top of the synrift succession is regarded as the drift-onset unconformity. It is highly erosional on the basin flanks and on other basements highs. The Outeniqua sub-basins subsided, resulting in deep marine, poorly oxygenated conditions over most of their extent.

During a transitional phase characterized by transform tectonics, an aggradational onlap infill succession of deep marine claystones and thin bedded turbidites of early Cretaceous age was deposited.

A later sequence is characterized by high energy shelfal progradation from the northern margin of the Pletmos Basin shelf to shallow marine sandstones around the northern rim of the basin and across the entire Jnfanta embayment. Slope and basin floor fan sandstones have been proven by drilling toward the southward downdip of the paleontological shelf edge.

The development of the early Cretaceous unconformity is also associated with the deposition of an extensive lowstand wedge prognosed to be present throughout most of the Southern Quteniqua Basin. This was followed by a period of relative sand starvation over a large area of the basin during which time organic rich shales were deposited in the southern Pletmos and the Southern Outeniqua Basins.

A high subsidence rate during this period diminished the effect of eustatic sea level changes, which resulted in an aggradational shelf buildup throughout the mid-Cretaceous. Younger, mid-Cretaceous sediments prograded from the northwestern rim of the basin. The deep marine intervals in the over the Southern Outeniqua and the southern Pletmos Basins remain untested, but the regional geological model suggests that deep marine fan systems fed by canyons from the proximal parts of the basin can be expected.

Exploration history(Back to Top)
Soekor's records show that Ga-Al, the first borehole in the Pletmos Basin, was drilled in 1968. It produced gas at potentially commercial rates (22 million cf/d) from synrift shallow marine sandstones and fractured quartzite basement of Ordovician to Devonian age.

More than 40 wildcats, mostly on synrift structures, have been drilled throughout the Pletmos Basin and Infanta embayment north of the Southern Outeniqua Basin. Gas flow rates between 24 and 5 million cf/d with minor amounts of NGL were measured.

Sandstones from 1,565-2,500 meters subsea have porosities of up to 25%, averaging between 11% to 18%, and permeabilities which range between 10 and 100 md, with a maximum of 450 md. One test performed within the fractured Table Mountain Group quartzite basement flowed 11 million cf/d of gas.
Boreholes drilled in the southern Pletmos Basin intersected gas-saturated sandstones within the synrift succession. It is considered that a large percentage of the dry gas in synrift reservoirs has been derived by updip migration from early-rift shales (Kimmeridgian) lying within the Southern Outeniqua Basin. This area is regarded as a major hydrocarbon kitchen with multiple drift source rocks lying within the oil window.

Zones of hydrocarbon prospectivity(Back to Top)
In the early cretaceous, synrift sequence, good reservoirs are expected within deep marine fan and channel sandstones in the Southern Outeniqua Basin. Good quality wet gas to oil-prone shales are postulated to occur at the base of the aggradational infill succession over the Southern Outeniqua Basin. Source shales varying from dry gas to wet gas and oil have been encountered in the southern Pletmos Basin and Plettenberg graben and are expected to follow the established regional trend of improvement in quality towards the Southern Quteniqua. They presently fall in the wet gas-to-oil maturity window.

In the mid, post rift sequence in the Cretaceous, wet gas to oil-prone source shales have been intersected in the southern Pletmos Basin and are expected to follow the established regional trend of improvement in quality and thickness towards the Southern Outeniqua Basin.

The promise of South Outeniqua(Back to Top)
Combined mapping and evaluation by Soekor and Ranger suggest there is enough potential in this basin in water depths in excess of 300 meters. The inferred distribution of reservoirs and rocks in the untested basin, according to the two papers, suggests there is substantial potential in this large 20,000 sq km, 4,942,000-acre area. Prospect mapping reveals untested, mostly domal structures that are so large that if only half contain oil, there should be 300 million to 1.6 billion bbl oil or 500 billion to 2.7 tcf gas (excluding additional subcrop potential), according to Soekor. Deep marine fans developed within the drift succession are expected to contain oil and gas sourced from the same succession. These fans are expected to include good quality reservoir sandstones similar to those intersected in some of the sequences in the Bredasdorp Basin to the West.

Ranger holds Blocks 11a and 12b and is looking for farm-in partners. Block 13 is entirely available for sub-lease on a noncompetitive basis. The nearest deepwater harbor, Port Elizabeth, is 80-250 km to the northeast. The coastline is 55-130 km away.

References: Jacques Roux, Soekor E&P: Potential Outlined in Southern Outeniqua Basin, off S. Africa, Oil and Gas Journal, July 21, 1997.
Ranger U. K. Limited: Leads and Prospects in South Outeniqua Basin, offshore South Africa, Offshore West Africa Conference , Abidjan, Cote d'Ivoire, 2000. (Back to Top)