Undiscovered Oil in the East African Rift: the 2016 USGS Assessment

Using a method based on geology, the U.S. Geological Survey estimates that the East African Rift Province holds, on average, 13.4 billion barrels of undiscovered conventional oil and 4.6 trillion cubic feet of gas that could be recovered with current technology.

The rift

The province is a geologically complex area of about 2,916,160 square kilometres. Its rifts are usually described as an Eastern Branch and a Western Branch.

• The East African Rift System began in the late Oligocene in the Afar region of Ethiopia, shortly after an Eocene–Oligocene outpouring of flood basalt around Lake Turkana and the South Lokichar Basin, in the Eastern Branch, where extensional faults developed within the Precambrian craton.
• In the middle to late Miocene, rifts opened within the craton between Ethiopia and Mozambique, in the Western Branch.

The rift basins hold up to 5,000 metres of sedimentary rock laid down from the Oligocene to the Holocene. At the time of the assessment, the province had 30 oil fields — 18 in Uganda and 12 in Kenya — and was considered underexplored.

The East African Rift Province and its four assessment units, along the Western and Eastern Branches. USGS map, on a National Park Service base.

One petroleum system, four units

The USGS defined an Oligocene–Pleistocene Total Petroleum System with four assessment units — Albertine Graben, Tanganyika-Rukwa, Lake Malawi and Eastern Arm Rift Basins — taking in parts of Burundi, the Democratic Republic of the Congo, Ethiopia, Kenya, Malawi, Mozambique, Rwanda, South Sudan, Tanzania, Uganda and Zambia. The system is defined by:

1. lake-bed source rocks of Oligocene to Pleistocene age, with 1 to 17 percent organic carbon by weight;
2. clastic reservoirs of the same age, with porosity of 15 to 30 percent;
3. permeability from 50 millidarcies to several darcies;
4. shale seals;
5. traps that are mostly structural, from extensional faulting and minor inversion, with some stratigraphic traps.

Oil began forming and migrating in the Miocene and most likely still is.

The smallest field counted was 5 million barrels. In the Ugandan part of the Albertine Graben unit, 15 fields at least that size had been found, and in the Kenyan part of the Eastern Arm unit, 8; none had been found in the other units. The number and size of undiscovered fields were guided by similar basins elsewhere.

The results

Natural gas liquids add a mean of 24 million barrels. The Lake Malawi unit was given a geological probability of 0.9 of holding oil at all, which is why its low end is zero. The largest undiscovered field expected in each unit ranges from 449 million barrels (Albertine Graben) to 1,098 million (Eastern Arm).

Sources

• East African Rift Province Assessment Team (Michael E. Brownfield, Christopher J. Schenk, Timothy R. Klett, Tracey J. Mercier, Stephanie B. Gaswirth, Kristen R. Marra, Thomas M. Finn, Phuong A. Le and Heidi M. Leathers-Miller), Assessment of Conventional Oil Resources of the East African Rift Province, East Africa, 2016, USGS Fact Sheet 2017–3006, March 2017. https://doi.org/10.3133/fs20173006
• Results: USGS Energy Resources Program, http://energy.usgs.gov