THE PETROLEUM POTENTIAL OF THE MAFIA DEEP OFFSHORE BASIN. SOUTHERN TANZANIA

The Mafia Deep Offshore Basin (MDOB) of southern Tanzania is one of the several East African basins which resulted from the break-up of the Gondwana continent from the Middle Jurassic onwards (Figs. 1, 2).  The basin occupies an area of some 75,000 km2 developed between the Tanzanian continental shelf edge and the Davie Fracture Zone 200 km east of the coast. The water depth ranges form approximately 500m to a maximum of 3300m in the southeast of the basin.  The availability of a modern Western Geophysical high resolution seismic survey has allowed the geological development of the basin to be interpreted in far greater detail than previously. Prior to this study the geology of the basin had been interpreted only from very widely spaced reconnaissance seismic data of the Sea Gap Group proprietary survey (1976) and the Lamont-Doherty East Africa Margin study (Coffin & Rabinowitz, 1988).  The MDOB offers some of the few remaining frontier exploration opportunities in Africa. It is situated adjacent to the petroliferous Tanzanian Coastal Basin, where there are known onshore oil seeps and subsurface gas discoveries. the extrapolation of well data from this coastal area in combination with the stratigraphic and structural interpretation of the MDOB has facilitated the definition of its petroleum potential. Multi-play potential is recognized and the possibility exists for several world-class hydrocarbon discoveries, comparable with those recently made in West Africa.

FIGURE 1.
COASTAL TANZANIA & MDOB: OIL SEEPS & GAS DISCOVERIES

FIGURE 2.
GONDWANA LAND BREAK-UP AND EAST AFRICAN SEDIMENTARY BASINS

Regional Setting and Structure

The MDOB has developed along a transform plate margin formed after the break-up of Gondwanaland during Bajocian times.  The transform element, now represented by the Davie Fracture zone, acted as a zone of dextral displacement of the Madagascar block with respect to Africa, probably until the Neocomian.  This is consistent with a relict spreading centre located some 200 km east of Mafia Island which is identified from magnetic anomalies dated at 110-120 MaBP.  For the greater part, however, the basin is floored by Karroo-aged rocks with the total post-Karroo sedimentary thickness estimated from seismic profiles to be up to 10 km thick.  Oceanic crust is only present beneath the post-break-up sedimentary cover in the extreme eastern parts of the basin, beyond the area of the new seismic survey.
The present-day structure of the basin is dominated by prominent north-south lineations corresponding to the Sea Gap and Davie Fracture Zones, both dextral strike-slip fault complexes. 

Locally there is an associated series of NNW-striking faults related to both extensional and compressional regimes caused by Neogene inversion movements along the fracture zones.  Both inversion-related anticlines and faulted closures are associated with the major fault lineations and provide a number of potential traps for hydrocarbons.