‘Hidden Rifts’ in Central and Southern Africa – Implications for Regional Basin Framework and Petroleum Potential*
Steve Lawrence1 and Ian Hutchinson1
Search and Discovery Article #10187 (2009)
Posted April 16, 2009
*Adapted from oral presentation, along with the short and extended abstracts, at AAPG International Conference and Exhibition, Cape Town, Africa, October 26-29, 2008.
1 RPS Energy, Henley-on-Thames, United Kingdom
Several phases of rifting have affected the interior of Sub-Saharan Africa during the Phanerozoic. These took place in the Triassic (Late Karoo), the Cretaceous (Early-Mid and Late) and Tertiary-Quaternary (Mio-Pliocene-Recent of the East African Rift System or EARS). These rift episodes have different directions of extension meaning that rift trends often intersect. Conversely, rifting is strongly controlled by basement structure so that, if local conditions are right, there will be a propensity for the coincidence of rifts even if rifting is oblique to the prevailing direction of extension. These possibilities lead to the probability of ‘hidden’ rifts. In addition, in areas where there is a paucity of subsurface data there is the possibility of ‘hidden’ rifts through ignorance.
We review regional rift models. Karoo rifting has been related to ‘continental escape’ where cratonic blocks have moved differentially along transcontinental shear-zones. The multi-phase Cretaceous rifting is related to regional NE-SW extension. Models for the EARS invoke mantle-driven ‘active’ rifting. We use these models to predict ‘hidden’ rifts in Central and Southern Africa.
The petroleum prospectivity of these interfering rift systems is evaluated on the basis of potential source rock development in the Karoo and Cretaceous rift sections. Lower Karoo (Permian) deltaic/paludal shales have a dominantly gas-prone character and where appropriately buried represent a huge, untapped gas resource. Upper Jurassic / Lower Cretaceous, Stanleyville lacustrine shales, sampled in Northeast DRC, are organically very rich and oil-prone. These would represent a source of oil resources where buried by superimposed Cretaceous or EARS rifts.
It is well known that several phases of rifting (Figure 1) have affected the interior of Sub-Saharan Africa during the Phanerozoic. We highlight an ‘Eastern Break-up Domain’ which has had a propensity for attempted continental break-up associated with progressive Gondwana dispersal. Rifting took place in the Permo-Triassic (Late Karoo) (Figures 2 and 3), the Cretaceous (Early-Mid and Late) and the Tertiary-Quaternary (Mio-Pliocene-Recent of the East African Rift System or EARS).
These rift episodes have different and varying directions of extension which provides the opportunity for ‘hidden rifts’ by:
Figure 1 shows the position of the neWe review regional rift models (including direction and mechanisms of extension and the control provided by basement fabric) for the recognized phases of rifting in the ‘Eastern Break-up Domain’. We use these models and the pattern of currently known occurrences of ‘rift’ section to predict ‘hidden’ rifts in Central and Southern Africa (Figures 4 and 5) .
Karoo rifting has been explained by a ‘continental escape’ model where cratonic blocks have moved differentially along transcontinental shear-zones in response to Gondwanide collision in the south (Cape Fold Belt). Tectonic development has resulted in partition of a western compressional terrane with extensive Lower Karoo ‘sag’ basins and an eastern extensional terrane characterized by pull-apart rifts. In the east regional extension directions are E-W or NW-SE, but the main control for rift development is provided by more localized shear movement between cratonic elements and usually within intervening Proterozoic mobile (orogenic) belts. Using this model and recognizing basement structural trends, ‘hidden rifts’ for the Karoo are predicted in the Albert-Edward Rift System where they are superimposed by EARS rifts (and possibly Cretaceous rifts – see below) and the newly-named ‘Tuli-Sabi Pull-apart Basin System’ (Figure 5) which is obscured by ‘post-rift’ Cretaceous-Tertiary cover in Central Mozambique. The ‘Tuli-Sabi Pull-apart Basin System’ is sub-parallel to the better known ‘Tan-Zam Pull-apart Basin System,’ which constitutes a link to Tethys and includes the oil-bearing Majunga and Morandava basins of Madagascar. Lower Karoo (Late Carboniferous-Permian) deltaic/paludal shales and carbargillites in the Luangwa and Zambezi basins are known to be organic-rich with a dominantly gas-prone character (Figure 6). Where appropriately buried by Upper Karoo or younger rift sediments, the Lower Karoo represents a huge, untapped gas resource in conventional reservoirs, or in the form of ‘shale gas’ or coalbed methane. Speculation can also be made on the possible extension of Tethyan marine influences into these pull-apart rifts, imbuing an oil potential.
The multi-phase Late Jurassic-Cretaceous rifting, epitomized by the petroliferous Sudanese-Anza rift, was related to regional NE-SW extension widely affecting the ‘Eastern Break-up Domain.’ By the Late Jurassic Tethyan oceanic development had extended along the East African margin but was decoupled from African continental development which was related to South Atlantic formation. Late Jurassic-Cretaceous NW-SE trending rifts with NE-SW transfer trends are recognized extending into Central and Southern Africa, linking southwards with the site of eventual break-up and spreading in the Natal Valley off southeastern South Africa (Figure 7). This Late Jurassic - Cretaceous rift network interferes with the Karoo rifts in the ‘Eastern Break-up Domain’ and is further overprinted by EARS rifting. ‘Hidden rifts’ (Figure 8) are predicted in the Albert-Edward Rift System as a transfer link between the Ulema-Rukwa rift and the Sudanese-Anza rift and in the Lake Malawi Basin. In these areas they are superimposed by EARS rifting. In Southern Mozambique a series of Cretaceous rifts have been identified on magnetic data and link the Natal Valley rifting with the Zambezi rifting. Lower Cretaceous lacustrine shales have proven to be prolific source rocks in the Sudanese rift system and Late Jurassic/Early Cretaceous Stanleyville Formation lacustrine shales (Figure 9), sampled from outcrop and shallow boreholes around and south of Kisangani in Northeast DRC, are organically very rich and oil-prone. These could represent a source of large oil resources in Cretaceous rifts where buried by superimposed EARS rifts. There has been a suggestion that Lake Albert oil seeps and the Lake Tanganyika offshore oil seep may have a marine Mesozoic source. In this regard, minor marine influences in the Stanleyville Formation and the recent discovery of marine Lower Cretaceous in the Turkana Depression are a very interesting development. The presence of marine sediments in Cretaceous rifts may have enhanced their oil potential.
We submit that the ‘hidden rifts’ of Central and Southern Africa constitute a vast unexplored and untapped hydrocarbon resource. Exploration will be high risk and operationally difficult. It will require state-of-the-art geophysical technology and well thought-out geological models involving interfering rift systems. We also emphasize the aspects of source rock maturity and hydrocarbon expulsion since the burial and thermal history of these interfering rifts will have been complex. As well as multi-phase subsidence, the effects of uplift/inversion recognized in the Cretaceous and Tertiary and, in some cases, volcanicity have to be taken into account in any assessment of the maturity history of Karoo and Lower Cretaceous source rocks.