Hydrocarbon potential of the Deep Sea off Tanzania coastal basins as indicated by geochemistry of source rocks and oils from Songo Songo gas field

摘要

Oils with gravity ranging from 33 to 47°API have been observed in the Lower to Middle Cretaceous sandstone of Songo Songo gas field. Their geochemical characteristics based on isoprenoid to n-alkane ratios and carbon isotopic signature, predict a multi-source rock origin. Mixed organic matter deposited in transitional environment grading to peat-coal deposited in sub-oxic to oxic environment is also 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-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 stable carbon isotopic signature (δ~(13)C) of saturated and aromatic hydrocarbon fractions, and n-alkane to isoprenoid ratios correlate to restricted marine source observed in Mandawa Basin, west of Songo Songo. Recent geophysical data in the Deep Sea apparently show structural setting similar to that of Jurassic formations in Mandawa Basin where kerogen Type Ⅰ/Ⅱ source rocks have been reported. Turbidites, sea floor fans and deltaic sands suitable for potential reservoirs have been interpreted. Thus good reservoir implications together with source rock formations form a potential petroleum system in Deep Sea, off Tanzania (East African) coast.