Usgs assessment: Undiscovered Oil and Gas Resources of Four East Africa Geologic Provinces

Four geologic provinces along the east coast of Africa recently were assessed for undiscovered, technically recoverable oil, natural gas, and natural gas liquids resources as part of the U.S. Geological Survey’s (USGS) World Oil and Gas Assessment. Using a geology-based assessment methodology, the USGS estimated mean volumes of 27.6 billion barrels of oil, 441.1 trillion cubic feet of natural gas, and 13.77 billion barrels of natural gas liquids.

The main objective of the U.S. Geological Survey’s (USGS) World Petroleum Resources Project is to assess the potential for undiscovered, technically recoverable oil and natural gas resources of the world, exclusive of the United States. As part of this program, the USGS recently completed an assessment of four geologic provinces: three along the eastern part of the African coast and one more than 900 miles east of the African coast and extending to water depths ranging from 2,000−3,000 meters (m) (fig. 1). From north to south,

the provinces are as follows:
(1) the Tanzania Coastal, containing rift, marginal sag, and passive margin rocks of Middle Jurassic to Holocene age;
(2) Seychelles, characterized by rift, marginal sag, and drift rocks;
(3) the Morondava, containing failed rift, marginal sag, and passive margin rocks; and
(4) the Mozambique Coastal, described by rift, marginal sag, and passive margin rocks.
These assessments were based on data from oil and gas exploration wells and published geologic reports. The four provinces were related to the breakup of Gondwana (fig. 2) in the late Paleozoic and Mesozoic (Reeves and others, 2002), and developed similarly through two tectonic phases (fig. 3): (1) a syn-rift phase that was started during the Permo–Triassic and continued

into the Jurassic, resulting in the formation of grabens and half-grabens and (2) a drift phase that began in the mid-Jurassic and continued into the Paleogene. A later passive margin phase began in the late Paleogene and continues to the present in the Morondava, Mozambique, and Tanzania Coastal Provinces, whereas in the Seychelles Province the drift phase continues to the present because there is no significant sediment source after the Seychelles-India breakup. The total thickness of the Mesozoic to Cenozoic stratigraphic section is more than 5,000 m on the outer parts of the continental shelf along the east Africa coast in the Morondava and Mozambique Coastal Provinces and more than 4,000 m in the Seychelles Province.

The four provinces and associated assessment units (AU) were assessed for the first time because of increased exploratory activity, recent discoveries, and increased interest in their future potential. The assessment was geology based and used the total petroleum system (TPS) concept. The geologic elements of a TPS include hydrocarbon source rocks (source rock maturation and hydrocarbon generation and migration), reservoir rocks (quality and distribution), and traps for hydrocarbon accumulation.

Using these geologic criteria, the USGS defined four TPSs and one AU for each TPS (table 1). The TPSs were defined to include Mesozoic to Paleocene source rocks and conventional reservoirs (fig. 3). The Permian to Triassic contains fluvial and lacustrine source rocks, and the Jurassic contains restricted marine Type II kerogen source rocks and marginal marine and deltaic Types II and III kerogen source rocks. Types II and III kerogen source rocks of Cretaceous age have been identified in the Morondava, Mozambique, Seychelles, and Tanzania Provinces, and Types II and III kerogen source rocks of Paleogene age have been identified in Mozambique, Seychelles, and Tanzania Provinces. Permian to Triassic source rocks contain 1.0 to 6.7 weight percent total organic carbon (TOC), with some samples having as much as 17.4 percent. The Early to Middle Jurassic restricted marine Type II source rocks contain as much as 12 weight percent TOC. Upper Jurassic and Cretaceous marine strata include (1) Aptian source containing Type II kerogen, ranging from 2.0 to 4.28 weight percent TOC; and (2) Cenomanian–Turonian source rocks containing Type II kerogen, ranging from 1.0 to 3.0 weight percent TOC. All four AUs contain Mesozoic and Cenozoic clastic reservoirs. Traps are mostly structural within the syn-rift rock units and both structural and stratigraphic in the postrift-rock units. The east African provinces (Mozambique, Morondava, and Tanzania, fig. 1) contain reservoirs that mostly are associated with growth-fault-related structures, rotated fault blocks within the continental shelf, deep water fans, turbidite channels and sandstones, slope truncations along the present-day shelf and paleoshelf edge. Permian to Triassic sandstone and Late Jurassic reefs and platform limestone also are possible reservoirs. The primary seals are Mesozoic and Cenozoic mudstones and shales. The Seychelles Province contains possible reservoirs in Permian to Middle Jurassic rift-related sandstones, Middle Jurassic carbonates, Lower and Upper Cretaceous turbidite sandstones, and Tertiary carbonates. The primary seals are intraformational shales.

At the time of the assessment, the four east African provinces contained 1 oil and 11 gas accumulations (HIS Energy, 2009), thus exceeding the minimum size of 5 million barrels of oil equivalent and 30 billion cubic feet of gas; these provinces are considered to be underexplored for their size. The Seychelles Province contained no discoveries and was also underexplored.

Exploration wells and discovered accumulations on the continental shelf and upper slope (IHS Energy, 2009) provide evidence for (1) the existence of an active petroleum system containing Mesozoic source rocks, (2) the migration of the hydrocarbons most likely since the Late Cretaceous, and (3) the migration of the hydrocarbons into Cretaceous and Cenozoic reservoirs.

Resource Summary
The results of the USGS assessment of undiscovered, technically recoverable conventional oil and gas resources in the east Africa provinces are listed in table 1.
The mean volumes are estimated at (1) 10,750 million barrels of oil (MMBO), 167,219 billion cubic feet of gas (BCFG), and 5,176 million barrels of natural gas liquids (MMBNGL) for the Mesozoic-Cenozoic Reservoirs AU in the Morondava Province; (2) 11,682 MMBO, 182,349 BCFG, and 5,645 MMBNGL for the Mesozoic-Cenozoic Reservoirs AU in the Mozambique Coastal Province; (3) 2,394 MMBO, 20,376 BCFG, and 739 MMBNGL for the Seychelles Rifts AU in the Seychelles Province; and (4) 2,806 MMBO, 71,107 BCFG, and 2,212 MMBNGL for the Mesozoic-Cenozoic Reservoirs AU in the Tanzania Coastal Province.
For this assessment, a minimum undiscovered field size of 5 million barrels of oil equivalent (MMBOE) was used. No attempt was made to estimate economically recoverable reserves.

— — — — — — —
27.6 billion barrels of oil = 3.76 млрд. т. (геол. запасы); *0.3 (КИН) = 1.25 млрд. т. (извлекаемые запасы);
441.1 trillion cubic feet of natural gas = 12.348 трлн. куб. м. (геол. запасы);


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