Regional Tectonics

Tanzania is located in an area affected by major tectonic events commencing in the Permo-Triassic with the breakup of Gondwana and continuing to present-day. The development of the post-Gondwana East Africa Continental Margin is separated into two major phases, ?rift and drift?.

Rift Phase (Permian, Triassic and Early Jurassic)

During the rift stage, the Gondwana continent was split into many parts, which remained in contact with each other. These splits gave rise to basins in which predominantly continental sediments were deposited at relatively high sedimentation rates. During later episodes of extension, sea-floor spreading (drifting) and the development of a passive margin generally followed rifting of continental crust. A few rifts, however, ?failed? and did not progress to the drifting stage.

Coastal Tanzania

A combination of four major tectonic events has contributed to the evolution of Coastal Tanzania; a failed NNE rift, a successful NNW rift, an EW graben and the right-lateral drift of Madagascar. The petroleum province is delineated to the east by the Davie Fracture Zone (DFZ) which represents the right-lateral wrench fault along which Madagascar moved. West of this zone, synrift sediments of Triassic to Early Jurassic age overlain by post Middle Jurassic to Recent post rift sequences are preserved. The Selous Basin occupies the failed rift in the southern area of the Coastal Basin. To the north, the rift is occupied by the Ruvu Basin, to the north- west of the Dar es Salaam Platform. The successful rift margin is occupied by the present-day Ruvuma and Mandawa basins, which are situated east of Selous and separated by the Masasi Spur basement high. North of these basins the east-west trending Rufiji Trough cuts across both failed and successful rifts. Offshore Tertiary Basins parallel the coast and are associated with large, down-to-the-basin faults, which demarcate the present coastline. A series of broad troughs and anticlines parallel the coast offshore. The islands of Pemba, Zanzibar, Latham, and Mafia appear to be basement-cored uplifts.

Inferior Basins

These are characterized by half grabens controlled by major boundary faults. Some of these faults exhibit an oblique strike-slip behavior causing flower structure fault deformation. This has resulted in narrow, but very deep, sediment-filled basins exhibiting arrange of structural trapping possibilities. The geological map illustrates the present day distribution of outcropping basement rocks and sedimentary basins.

Regional Statigraphy

The break up of Gondwana initiated the formation of basement-controlled coastal and early interior basins. The Precambrian basement, penetrated by three wells, and exposed at the edges of the basins, consists of high grade metamorphic rocks, mostly gneisses. The early depositional history of the coastal basins was initiated in the Late Paleozoic, (Carboniferous?) or Permo-Triassic and continued through the rift phase until the end of the Early Jurassic (Karoo of some authors). The rift phase, or synrift stratigraphic units, from the top of basement to the end of Early Jurassic, are best represented in the Selous, Ruvu, Mandawa, and Ruvuma basins of eastern Tanzania, and in the Rukwa Basin in the southwest. In the Selous Basin, the Carboniferous to Permo-Triassic interval consists of about 10 km of fluvial, deltaic, continental and lacustrine deposits of the Dwyka and Stigo Series, overlain by the Triassic Tanga Beds (Figure 6).

This sequence is, in part, equivalent to the Lower Sakamena source rocks of Madagascar. In the Mandawa Basin, the Tanga Beds are overlain by clastics and evaporites of the Nondwa Formation (Early Jurassic). Significant thicknesses of salt are associated with these sediments and have been identified throughout the basin. Pre Middle Jurassic sections are also preserved in the Rufiji Trough and Dar es Salaam Platform. From Middle Jurassic onwards, the coastal basins were established as the continental shelf of a passive margin and subjected to various deltaic and marine depositional environments. Several transgressive and regressive phases resulted in 4,000 m of Mesozoic and another 4,500 m of Tertiary sediments being deposited. These sediments consist of marine marls, shales, sandstones and limestones.

In the interior basins deformation commenced in the carboniferous with an estimated 4000 m of Karoo deposits. Further thicknesses of up to 2000 m of continental sandstones and shales were deposited during the late Jurassic and early Cretaceous eras. The post-rift phase started in the Middle Jurassic with a regional unconformity, and initiated a marine transgression, accumulating thick shales of the Makarawe Formation and limestones of the Kidugalo and Mtumbei Formations. The regional unconformity separated the Karoo continental deposits from the marine Jurassic sediments. North of the Kisarawe-1 well, the Jurassic has not been reached by drilling. The seismic control suggests that the Jurassic is buried very deep offshore, beneath the thick Cretaceous-Tertiary cover.

The Middle to Late Jurassic transgression was followed by regression in the Early Cretaceous, resulting in predominantly clastic sediments in the south, including the Neocomian Songo Songo reservoir sandstones of the Kipatimu Formation, and limestones and conglomerates in the north. A regional Middle Cretaceous unconformity and the deposition of thick platform carbonates culminated the regression and initiated another major marine transgression in the Late Cretaceous. This resulted in the deposition of the shelf sands and slope shales of the Ruaruke Formation. At this time there was also some tectonic activity resulting in the development of several fault systems separating the shallow marine environment in the west from the deep marine environment in the east.

Paleocene deposition was associated with the last phase of transgression continuing from Late Cretaceous times and with contemporaneous subsidence in the coastal and offshore areas. Several uplifts occurred in the Late Tertiary, represented by unconformities at the base of the Eocene and Miocene, followed by Middle Eocene to Recent regressive sediments of continental, deltaic and marine facies. In the interior basins a late phase of deposition occurred during the late A number of oil seeps and slicks have been reported from the Interior Rift Basins. Oil seeps were reported from Lake Tanganyika, as early as 1896, and in more recent times, project PROBE found an oil film on the lake. Oil shows have been reported from the Pemba-5, Mandawa-7, Mafia-1, Mita Gama-1 wells. Cut fluorescence and staining from seven other wells and three boreholes have been observed. Songo Songo wells yielded small amounts of oil, which are low in sulfur with 33E – 47E API.

Songosongo Gas Fields

The field is located on and offshore Songo Songo Island, about 15 km from the mainland and 200 km south of Dar es Salaam.The discovery well, Songo Songo-1, was drilled in l974 by AGIP. Songo Songo is a large N-S trending structure containing one to two TCF of gas reserves. The gas is contained in Lower Cretaceous inner shelf sand reservoirs with porosities averaging greater than 20%, and net porous intervals up to 155 m thick. The structure was uplifted during the Early Cretaceous and the reservoir modified by the Mid Cretaceous regional unconformity. The trap consists of two faulted highs separated by a saddle. A series of N-S and NW-SE trending faults step down to the east. This faulting, regional eastward tilting, and rapid Tertiary sedimentation created more growth faults which mostly sole out in the sealing shales of the Upper Cretaceous Ruaruke Formation. The Lower Cretaceous reservoirs are not affected by this later episode of faulting. Continued activity along these faults resulted in the development of broad, low relief anticlines. The gas appears to have been sourced from post mature organic material of Jurassic or Early Cretaceous age. The associated liquids are probably sourced from mature organic matter of Early Cretaceous and Middle Cretaceous age and seem to have migrated from a nearby basin.

Mnazi Bay

AGIP drilled the discovery well, Mnazi Bay-1, in l982. The well encountered commercial quantities of gas in two Oligocene sands within the Mnazi Bay Clay Formation. These sands have porosities of 15 to 25% and permeabilities up to 560 md. Drill stem tests produced flows of 3.5 x 105 m 3 / day (12.5 MMCF/d). Based on one well and existing seismic coverage, the reserves at Mnazi Bay are estimated to be in the 1 TCF range. The gas is considered thermogenic but no source identification has been made.

Deep Sea

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Hydrocarbon Discoveries

Several oils shows have been earmarked. The known oil seeps of Tanzania are located at Tundaua on the west coast of Pemba Island, Wingayongo and at Msimbati near Mnazi Bay and in the Interior Rift Basins. The Pemba Island seep indicates a marine source, which has been correlated with Campanian-Maastrichtian shales. However, the typing of the seep does not correlate with oil shows from the nearby Pemba-5 well. It appears that two separate sources are involved.

The Wingayongo oil seep at the northern flank of the Rufiji Trough shows unusual biomarker characteristics, which have been interpreted to be derived from a source rock deposited in a restricted carbonate, lacustrine environment. The Makarawe Shale (Bajocian) is a possible source of this seep. Seeps at Msimbati near the Mnazi Bay gas discovery indicate two families of oil; one is interpreted to be generated from a carbonate source rock deposited in a strongly reducing environment; the other contains significant terrestrial organic matter and biomarkers of Late Cretaceous or younger age.

A number of oil seeps and slicks have been reported from the Interior Rift Basins. Oil seeps were reported from Lake Tanganyika, as early as 1896, and in more recent times, project PROBE found an oil film on the lake. Oil shows have been reported from the Pemba-5, Mandawa-7, Mafia-1, Mita Gama-1 wells. Cut fluorescence and staining from seven other wells and three boreholes have been observed. Songo Songo wells yielded small amounts of oil, which are low in sulfur with 33E – 47E API.

Source Rock Characteristics

Oil prone source rocks have been identified in the Lower to Middle Jurassic Pindiro or Nondwa carbonate/evaporites, as shown in the Mandawa-7, Mbuo-1, and Mita Gamma-1 wells. They contain a rich mixture of Type I and III kerogen, with TOCs greater than 9%. The Permo-Triassic and possibly even some early Jurassic sediments are dominated by Type III kerogen, although rocks with TOC of 7% and a HI of 386 mgHC/g TOC occur in the Lukuledi-1 well, in the Ruvuma Basin.

The section above the Middle Jurassic is typically dominated by Type III kerogen and is essentially gas prone. The Upper Cretaceous in Kimbiji East-1 contains TOCs up to 12%. Cretaceous and Tertiary source potential has also been speculated. Eocene lignites and organic rich shales are present in wells around the Songo Songo Gas Field. Occurrences of oil in a Songo Songo well and in the Pemba-5 well have been described in the section on Hydrocarbon Occurrences. Basin studies, chemical analysis and burial history and maturation modeling indicate that extensive hydrocarbon generating ?kitchens? are likely to occur within the basins.

Hydrocarbons generated in the Karoo basins may have accumulated in reservoirs of the same age, or migrated into Lower Cretaceous sands. The regional Upper Cretaceous shales of the Ruaruke hydrocarbons generated in the Lower Cretaceous shales in the distal (offshore) areas could be expected to migrate laterally and updip into the younger Cretaceous and Tertiary sequences. The Interior Basins contain Lacustrine Karoo sediments (4 to 5% TOC), with 30% Type I and II kerogen. The Tertiary Lake Bed Formation contains up to 4.9% TOC, consisting of Type I and II kerogen.

Deep Offshore Basins

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 km 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 high resolution seismic data 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. Following licencing of the areas and subsequent exploration drilling by Petrobas, BG and Statoil, sizeable gas discoveries have been made mainly in the cretaceous sections. Further exploration is underway.