Continental hinge is the key to deepwater reservoirs
JEBCO Seismic (UK) Ltd.
Global Exploration Services Ltd
The last two years have seen increasing numbers of awards of acreage along the East Africa margin. Now, after initial forays by smaller companies, the majors are active in the area.
The deepwater situation is akin to that in West Africa about 15 years ago just before systematic deepwater exploration began. Since then, much more is known about the mechanical and thermal evolution of continental margins. It is routine today to predict using gravity analysis where the sweet spots are.
Continental margin geology
East Africa, unlike the three other margins of Africa, is considered to host few significant source horizons. The two main reasons for this widely held view are the paucity of exposed, quality source rocks and the often erroneous attribution of dry holes to a lack of source. Yet there are as many oil and gas seeps as in West Africa, and western Madagascar hosts one of the world's largest heavy oil accumulations with 25,000 MMbbl of oil in place. This accumulation is larger than many of the West African basin-margin asphalts.
Reconciliation of the perceived wisdom with the actual reality of the region is readily possible if the exploration consequences of the geological history of East Africa are examined.
Firstly, much of the onshore succession is more disrupted than that along the western margin of Africa. This is because of the presence of a compound rift system evolving over 180 million years.
This chronostratigraphic schematic shows potential source rocks and major tectonic events.
Tectonism related to the formation of the East Africa rift in the last 31 million years breached previously existing onshore closures and removed through uplift much of the associated source succession from the active oil window. Although West Africa also experienced uplift during the later Tertiary, the overall levels of tectonism and disruption were much lower.
Finally, the impact of the Karoo Plume (183 Ma) severely damaged much of the older source rock section in the Mozam-bique region through for-ced maturation. Madagas- car also suffered younger volcanism.
A major positive factor is the huge volumes of sediment delivered offshore, notably in the last 31 million years following the onshore uplift associated with the East Africa rift and shouldering of the eastern coastal flanks. These sediments include quality sands derived from the basement or from the reworking of Karoo aged clastics. The optimal location for deepwater sand accumulations is, as is the case in West Africa, immediately oceanward of the continental hinge, which is of the main fault defining the continental margin.
Although the rivers draining into the Indian Ocean are generally shorter than those draining into the Atlantic, the higher topography in the eastern part of the continent, together with high seasonal rainfall, ensures that they carry equal, or greater, sediment loads. This fact has been overlooked.
As well as extensive deepwater areas (200 to 2,500 m water depths), the region includes some of the world's most lightly explored continental shelves. There are only three deepwater wells in the entire area and all three were dry. The wells were drilled off Kenya, Tanzania, and southern Mozambique. All three wells were dry, but that is only one well for each country!
This almost complete lack of deepwater well control makes it difficult to demonstrate pros-pectivity and therefore, to prove the potential of the region.
Volumes of sediment were delivered following the onshore uplift associated with the East Africa rift. Deepwater sand is deposited oceanward of the continental hinge.
Along the under-developed East Africa margin, significant accumulations of sediment occur outboard of the continental hinge in several vast areas. In the south, the Tugela Cone is now the focus of exploration activity. Further north, the full extent of the Limpopo Cone is being examined by Jebco outboard of the early BP World Bank Xai Xai graben test. DNO has joined forces with Wilrusco to explore the Inhaminga permit. Sasol and ENH are developing the Pande and Termane gas fields to supply gas to South Africa, and Petronas is revisiting the poorly understood Zambezi Delta.
In Tanzania, Pan-Africa, Aminex, Maurel et Prom, Shell, Petrobras, Jebco, and Antrim International are evaluating source reservoir complexes in axial to distal continental hinge settings.
To the north, in Kenya, Dana, Woodside, PanContinental, and Afrex are examining the same Tertiary turbidites in the Lamu basin region where 12 km plus of sediments have been recorded.
A conventional time-temperature modeling run (left) compared to the PreRo model (right) for a pseudo well northwest of the Simba-1 deepwater well off Kenya. Part of the predicted Jurassic source section is in the gas window and no longer prospective.
The richest well and outcrop-attested source rocks in the East Africa region are associated with the mid-Jurassic rift-drift transition section of the Somali basin and the syn-rift, lacustrine units within the Middle Sakamena of Western Mada-gascar. Kimmeridgian aged source rocks are present in the Maurice Ewing Bank (now west of the Falklands) and marine Early Cretaceous units characterize the south coast basins of South Africa.
Palaeoclimate modeling predicts the presence of upwelling settings along the southern margin of the Tethys Sea for the Triassic and Jurassic. Optimal environments for source rock accumulation include transgressive shallow seas and paralic lakes. Since the East African source units of these ages also occur in the Middle East and western India, the source section likely extends offshore into the present-day deepwater outward to the continent-ocean boundary, provided that there was no sea floor topography, such as that produced by contemporaneous volcanism. The excellent quality of the source rocks in Tanzania bodes well for similar quality sources offshore, as does the description of high quality, age-equivalent sources in western Madagascar. Possible Kimmeridgian sourced oil has been found in South Africa's offshore Durban basin.
In Mozambique, oil shows have been reported above the continental hinge in the Inhambane province, and gas was found in the Beira-1 well. Shows are being investigated throughout the length of Tanzania, where a complex multiple source scenario is emerging. This scenario is expected to extend into Kenya where significant gas shows have already been examined.
A key question for the deepwater is whether there are any significant developments of oil-prone source rocks within the post-middle Jurassic drift section. Middle Eocene and Campanian sources are present in Tanzania, but their offshore extent is unknown. This is also true of the Cretaceous source sections in Somalia, Kenya, Mozambique, and Madagascar (Moron-dava basin).
DSDP corehole results for the Somali and Mozambique basins located, at best, only poor to moderate quality sources anywhere within the penetrated sections. There are only eight holes, two of which were at the same location, and only two of which (Sites 241 in the Somali basin and 249 on the Mozambique Ridge) entered the pre-Tertiary. The basal sections of both these coreholes provided the best total organic content levels.
In Mozambique, anoxia continued in shelf settings into the earliest Coniacian (c. 88 Ma), and source horizons are present in the 15A (Turonian) seismic sequence in the Bredasdorp basin of South Africa. These two observations, when combined with the constraints imposed by the DSDP coreholes, suggest that widespread anoxia continued in protected settings until the Turonian (c. 90 Ma).
The models used by most geochemists assume that pressure plays no role in source rock maturation, despite the fact that the boiling point of fluids vary as pressure changes. Thus, the boiling point of water, which is 100° C at the surface, rises to >300° C at 3,000 psi (the hydrostatic pressure at 13,000 ft). The result is that volume expansion reactions like maturation and hydrocarbon generation are increasingly retarded by the high overpressures that develop within rapidly subsiding basins, and reactions that would readily run to completion at surface conditions cannot progress.
In deepwater petroleum basins, the oil window extends to much greater depths than previously recognized, and hydrocarbon generation continues to occur at depths and temperatures that are not possible under the conventional models.
Overpressure is expected to be present in the rock succession offshore of the mouths of all the major rivers. It is also predicted to exist in regions such as the Pemba Channel and the Lamu basin where multiple smaller rivers have delivered large volumes of sediment in short geological time periods. Where there is no over- pressure, conventional time-temperature modeling will continue to provide correct answers.
Examination of the satellite gravity shows that the East African and Madagascar margins are segmented by abrupt boundaries, many of which relate to features inherited from the Pre-Cambrian framework.
Jointly, bathymetry and gravity can also allow qualitative estimates to be made of the location of the continental hinge. This zone, usually expressed as a single fault line, defines the shoreward limit of major extension. It can be a permanent bathymetric feature, but it can be masked by later progradation.
In the last 15 years or so, the importance of continental margin hinge lines in creating and preserving deepwater plays has become apparent. In West Africa, the details of the relationship between the distribution of fields and the track of what is known there as the Atlantic hinge have yet to be published. However, visual examination of the positions of the fields reveals the following hinge-related play types:
- Tilted fault blocks in the rift section with either dip-slope reworked clastics and/or crestal lacustrine carbonate reservoirs
- Drapes above the tilted fault block crests within the rift-drift transition sequence
- Grainstone and sands shoaling above and along the flanks of the crests
- Marine sand pondings in lows located landward of the hinge line
- Salt-related closures controlled by the underlying hinge
- Deepwater channels and fans downdip of the hinge.
The similarities and the presence of an analogous continental margin hinge line together with multiple deepwater fan systems indicate an encouraging future for East Africa.
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