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Informations géologiques sur la Tanzanie 10/2008

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Geochemical Evaluation of Source Rocks and Oil Seeps from Tanzania's Sedimentary Basins

Albert Maende1, Hossein Halimi1, Dick Drozd1, Dan Jarvie1, and Meshack Kagya2
1Humble Geochemical Services, Humble, TX
2Tanzania Petroleum Development Corporation, Dar es Salaam, Tanzania

Recent discoveries in SongoSongo, Mnazi Bay and Mkuranga wells have substantiated the presence of an effective petroleum system in Tanzania. The discoveries of Agip and Maurel et Prom have confirmed commercial gas flow rates. Agip drilled the Songo Songo-1 well in Mafia Basin/Songo Songo area and made a gas discovery in Lower Cretaceous shelf sands. Agip then drilled Mnazi Bay-1 well in Ruvuma Basin and made a gas discovery in Oligocene deltaic sands.The drilling of Mkuranga-1 well by Maurel et Prom resulted in a gas discovery within the Cretaceous Ruaruke Formation.

Cuttings from both drilled wells and outcrops in Tanzania’s Ruvuma, Dar es Salaam Platform, Mandawa, Mafia/SongoSongo and Pemba/Zanzibar Basins, have been sampled and analyzed for their source potential and those with adequate TOC content and suitable Rock-Eval values for hydrocarbons generation can be broadly classified into Type I oil prone, Type II oil prone and Type III gas prone Kerogen Types.

Four oil seep samples and three probable bituminous rock samples obtained from three basins in Tanzania were analyzed to assess their geochemical characteristics; three of the oil seep samples are moderately biodegraded light oils but still showing presence of normal alkanes. Whereas two of these seep samples exhibited little to no natural oil content, the third sample is indicative of thermogenic oil. Extracts from the three probable bituminous rock samples have low saturates and aromatics contents in the range of 3.2 - 7.7% and 2.6 - 11.6% respectively and fairly high resins and asphaltenes contents ranging from 29.8 - 50.8% and 32.7 - 59.9% respectively; these oils are characteristic of altered(mainly evaporated) relatively immature to early mature oils. It appears that at least one of these oils is mixed with highly mature biodegraded oil previously migrated to this seepage location.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 ©️ AAPG Search and Discovery

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