Introduction
Tanzania can be considered to be under-explored. There have been positive indications of hydrocarbon potential in the form of oil seeps, gas and oil shows but only relatively few exploration and development wells have been drilled. The biggest discoveries have been the Songo Songo and the Mnazi Bay gas fields.
History
The history of oil and gas exploration in Tanzania falls into three phases, beginning in 1952 when BP and Shell were awarded a concession covering the onshore coastal basins and the islands of Zanzibar, Pemba and Mafia. Following extensive geological and geophysical surveys, which included 40 stratigraphic boreholes, four wildcat wells were drilled, one each on the three islands and one in the Mandawa Salt Basin. None of these wells encountered hydrocarbons in sufficient quantities to justify further drilling at a time when better opportunities were available in the Middle East. Accordingly, the entire concession was relinquished in 1964.
The second exploration phase coincided with the formation of TPDC in 1969 when Agip was awarded a concession covering a similar onshore areas to that held by BP but which also included the continental shelf to a water depth of 200m. From 1973 to 1982 Agip was joined by Amoco and drilled three onshore and 3 offshore wells resulting in the discovery of the Songo Songo and Mnazi Bay gas fields.
The third exploration phase was stimulated by high oil prices during the early eighties at which time licences were awarded to Shell, IEDC, Elf and Amoco. In 1981 Shell, later to be joined by Esso , was assigned five licences covering the Ruvu and Selous Basins. Between 1981 and 1985 Shell carried out an extensive exploration campaign which included the acquisition of over 6,400 km of seismic. Despite this extensive seismic campaign much of the thick sedimentary section in the Rufiji Trough and Selous Basin remains covered by only a broad regional grid. Subsequent drilling was limited to three wells; Ruaruke North-1, Liwale-1 (the only well in the Selous proper) and Lukuliro-1, a dry hole on the flank of the Rufiji Trough. No wells were drilled in the Ruvu Basin.
At the same time, the remaining part of the Ruvu Basin was awarded to IEDC, later joined by BHP and Elf, who after a limited seismic campaign drilled two wells on the western side of the rift-Makarawe-1 and Kiwangwa-1.
From 1978 onwards TPDC made a significant contribution to exploration, conducting onshore seismic programs in the Ruvu, Kimbiji / Bigwa, Pemba, Mafia and Ruvuma areas and offshore around Songo Songo, Pemba, and Zanzibar. In 1982/83 they drilled the Kimbiji East-1 and Kimbiji Main-1 wells. Petro-Canada drilled an aid funded well, Tan Can-1, close to the Kimbiji area in 1983. TPDC also carried out an appraisal program on the Songo Songo gasfield comprising seven wells (Songo Songo-3 to 9).
During 1983-84 the attention of the international oil industry was drawn to the Modern Rift System in Tanzania by geophysical surveys funded by PCIAC and the World Bank, and by seismic surveys conducted by Duke University as part of Project PROBE on Lake Tanganyika and Nyasa. Subsequently, exploration licences were assigned to Amoco, who drilled two shallow wells in the Rukwa Basin – Galula-1 and Ivuna-1.
At the end of the 1980s, licences were held by Shell over the Mandawa and Mafia Basins and by Texaco in the Ruvuma Basin, one exploratory well was drilled in each area.
In 1995, Irish company Dublin International Petroleum Limited signed a PSA over Mandawa and Rufiji Basins and two exploration wells were drilled in 1996 at Mandawa.. In 1997, two Canadian companies, Antrim Resources (now Antrim Energy Limited) and Canop World-wide signed PSAs over the Pemba/Zanzibar region and the Dar-es-Salamm/Mafia region respectively. In 1998, Gulf Western Mining signed an agreement covering parts of the Tanga Region, Ruvu and Kimbiji areas.
Canop’s acreage consists of three contiguous blocks, the Kisangire Block, Dar-es-Salaam Platform Block and the Mafia Island Basin Block.
In 1998, Tanzania offered acreage up for foreign investment. In March 1999, Ndovu Resources of Australia acquired the rights to explore 31 blocks offshore for oil for a period of 11 years. The area covers eastern Songo Songo Island, and stretches to the Rufiji Delta in the Coast Region.
Early in 1999, Western Geophysical signed an agreement to carry out deep-sea speculative surveys and in August 2000, Western Geophysical announced that it would be carrying out phase II of this 2-D seismic acquisition programme covering the Pemba Basin, offshore Tanzania in 1999. The data is to be available in time for the licensing round planned for the fourth quarter of 2000.
Tanzania plans to publicise the launch of its first offshore exploration licensing round by holding road-shows in the United Kingdom and the United States in September 2000.
Exploration Database
The petroleum exploration data base in Tanzania consists of wide ranging geological and geochemical studies, geological and drilling records from nearly one hundred boreholes and twenty-eight deep wells; and many thousand kilometers of gravity, airborne magnetometer and seismic surveys. All this data is available for inspection at TPDC’s Exploration Office in Dar-es-Salaam. A Technical report has been prepared which reviews this data and provides an assessment of the hydrocarbon potential of Tanzania.
The country was initially mapped by the Tanganyika Geological Survey which is now based in Dodoma and maps are available at a scale of 1:50,000. Only twenty wildcat exploration wells and eight development wells have been drilled in Tanzania, giving a drilling density of one well per 12,000 sq.km for the onshore sedimentary basins and one well per 8,000 sq.km for the offshore.
A number of these wells, both early and more recent, were located on the basis of poor quality or inadequate seismic control and in retrospect turned out to be invalid structural tests. Others were drilled on the less prospective basin margins which happened to be regional structural culminations.
There is excellent coverage of aeromagnetic data over the whole of Tanzania, both onshore and offshore, from a 1971 survey by Agip and a 1976 survey by Geosurvey International.The coverage of gravity data can best be described as ‘patchy’, consisting mainly of regional surveys. The best coverage is the coastal area over the Mandawa and Ruvuma Basins, the larger offshore islands and in the Lake Rukwa Basin. In the Ruvu, Selous and Rufiji areas the coverage is mainly limited to data along the more recent seismic lines.
A total of over 24,000 km of onshore seismic (including lake surveys in the Modern Rift Basins) and 21,000 km of offshore seismic (including onshore surveys on the islands) have been recorded in Tanzania.
Participating Organisations
Energy Africa Ltd
Energy Africa has acquired a 25% interest in the Mandawa licence in Tanzania.
Tanzanian Petroleum Development Corporation
TPDC operates the Tiper refinery in Tanzania as well as developing the Songo Songo gas field.
