ABSTRACT: Hydrocarbon potential of the deep sea (Indian Ocean) - off Tanzania Coastal Basin

Kagya, Meshack , Tanzania Petroleum Development Corporation, Dar Es Salaam, Tanzania

Oils with gravity ranging from 33 to 47 API have been observed in the Lower to Mid- Cretaceous sandstone of Songo Songo gas field. Their geochemical characteristic based on isoprenoid to n-alkane ratios and carbon isotopic signature predict multi-source rock origin. Mixed organic matter source deposited in transitional environment grading to peat-coal organic matter deposited in sub-oxic to oxic environment is indicated. Lignitic sediments were encountered in several Tertiary formations of the Songo Songo wells; on pyrolysis they gave good hydrocarbon yield (13.8 to 71.5 kg/tonne). This kind of organic matter are the likely source of oil plotting in the peat-coal zone of the isoprenoid/n-alkane cross plots. In Songo Songo field the Tertiary section is quite immature to generate hydrocarbon. It is therefore anticipated that the source of such oil is apparently from the deep sea basin, east of Songo Songo. Marine source characteristics shown by the carbon isotopic signature (d ¹³C₁₂) of saturated and aromatic hydrocarbon fractions, and n-alkane to isoprenoid ratios correlate to restricted marine source observed in Mandawa Basin. Recent geophysical data in the deep sea, apparently show structural setting similar to that of Jurassic formations in Mandawa Basin where kerogen TypeI/II source rocks have been reported. Teartiary to Cretaceous-turbidites and deltaic sands suitable for potential reservoirs have been interpreted along the shelf, thus, together with source rock formations form a potential petroleum system.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia