Seismic Data Profiles Reveal Traps

Return to East Africa

In addition to the ship tracks acquired as part of the geopolitical maneuverings during the Cold War, the petroleum industry has acquired over the years seismic data offshore East Africa. Deepwater seismic acquisition in the Mozambique Channel project area commenced in 1969.

Approximately 31,000 kilometers of seismic profiles have been interpreted and integrated with well, gravity and magnetic data for the study by Rusk, Bertagne & Associates.

The seismic data -- not available until recently -- are 1972 to 1989 vintage, multi-channel recordings with Flexotir source. These data can be integrated with the existing 50,000 kilometers of seismic control acquired by oil companies on the Mozambique shelf and 40,000 kilometers on the Madagascar shelf.

Seismic refraction data, recorded in tandem with reflection data along several profiles, provides additional control, he said. Well control from about 80 onshore and 22 offshore wells in Mozambique, Madagascar and southern Tanzania was used in the interpretation and evaluation process.

According to the study, recurrent rifting from the late Carboniferous to the late Triassic-early Jurassic, which resulted in Karoo continental deposition, deformed East Africa. During a post-Karoo early Jurassic transgression, clastic, carbonates and evaporates were deposited along the pre-breakup plate margins. A subsequent marine transgression took place in mid-Jurassic with widespread deposition of shelf carbonates and basinal sediments.

At this stage -- probably Bajocian -- the onset of drift occurred.

The Madagascar-India-Antarctica plate separated from Africa, moving incrementally southward along the Davie Fracture Zone until early Aptian. During the drift phase, predominately marine deposition took place throughout the region. The post-drift marginal sag phase, which followed an Aptian hiatus and persisted until the present, resulted in the deposition of thick marine siliclastics.

Nine major basins or discrete geological areas can be identified in the coastal and offshore sector of east Africa, the report said. In the general sense, each basin area experienced a similar poly-history development.

Basically, five main tectonic phases or cycles formed the basins and intra-basin framework. These tectonic phases are:

Karoo rift phase, late Carboniferous to early Jurassic.

Post-rift transition phase -- onset of thermal subsidence, early Jurassic.

Initial drift phase -- separation of the Madagascar-Antarctica-India plate from Africa, Bajocian to Aptian.

Passive margin subsidence and second drift phase -- separation of the India-Seychelles plate from Madagascar, Albian to Maestrichtian.

Passive margin subsidence, Tertiary.

The Evidence Suggests

Bertagne and Rusk said numerous sandstones and, to a lesser degree, carbonate quality reservoir objectives have been penetrated in wells along the shelf and coastal areas. In consideration of the thick sedimentary section further offshore, equally prospective reservoir sequences are expected.

Also, Permo-Triassic lacustrine shale -- the established source for the Bemolanga tar sand deposit -- can be expected in any of the offshore sectors underlain by continental crust.

"Lower Jurassic rich, oil-prone shale has been identified in two basins, and most likely is present in others," the authors said. "Also, geochemical results from well data indicate extensive occurrence of oil and gas-prone source beds of Middle Jurassic to Upper Cretaceous age."

Throughout the study area is evidence for a consistent increase of argillaceous facies and organic matter basinward (they wrote). This evidence, coupled with the likelihood that faulting associated with the Davie Fracture Zone formed barriers to circulation, suggests that deposits with oil-prone kerogen may be prevalent within the Jurassic-Cretaceous section.

Trapping mechanisms also are present offshore East Africa.

"A wealth of trap types of potentially major dimensions has been identified, including salt structures, tilted fault blocks, horsts, drape anticlines and flower structures," the report said. "The combination of multiple reservoirs and source rocks and the seismic evidence for numerous structural leads suggests the presence of a wide variety of plays with major resource potential."

-- KATHY SHIRLEY