25th International Meeting on Organic Geochemistry IMOG 2011

P-105
Challenges on the origin of oil/condensate and presence of
bitumen and oil seeps in the Tanzania Coastal Basins
Meshack Kagya
Tanzania Petroleum Development Corporation, Dar es Salaam, United Republic of Tanzania
(corresponding author:mkagya@tpdc-tz.com)
Oil or condensates have been encountered in Songo
Songo and Mnazi Bay gas fields. In Songo Songo, the
oil/condensate are produced from the Albian and
Neocomian sandstone. Physically, they are colourless
to brown in colour with gravity ranging from 33 to 47˚
API. Isoprenoid to n-alkane ratios envisage a
transitional to peat-coal depositional environment of
parent organic matter. However, the carbon isotopic
signature (� 13 C12) of saturated and aromatic
hydrocarbon fractions, and other parameters correlate
to a restricted marine source. Diasterane/regular
sterane ratios (0.92) infer a highly mature source
origin (>1%Ro), whereas regular sterane ratios have
values around 0.52 and 0.6 suggesting a thermal
maturity of >0.7%Ro. A maturity value of ~0.8%Ro
was deduced from naphthalenes and phenanthrenes
ratios. Source rock of predominant terrestrial organic
matter is indicated in Songo Songo by lignitic
sediments encountered in Tertiary formations. On
pyrolysis they gave good hydrocarbon yield (13.8 to
71.5 kg/tonne), likely to source oil plotting in the peatcoal
zone of the isoprenoid/n-alkane cross plots.
However the Tertiary section in this region is quite
immature to generate neither oil nor gas. Therefore
the source of such oil is possibly being generated
from deeper shelf off Songo Songo Island.
Nevertheless another source could be restricted
marine source observed in Mandawa Basin west of
Songo Songo. In Mnazi Bay the Tertiary formations
produce oils/condensate with API gravity ranging from
18˚ to 32˚. Such gravities are apparently indicative of
heavy crude, contrary .to their physical appearance
and volatility They are maturity range between 0.67
and 0.77% Rc. Oleanane compounds are apparently
present indicating the input of Tertiary organic matter.
The source rock data from the Tertiary interval is
shown to be immature to generate the observed oil.
Several oil seeps and bitumen occur along the coast.
The bitumen occurs as bituminous sands and tar balls
along the shore line. The Gas Chromatograms of
extracts from some of bitumen and oil seeps have
light end hydrocarbon signature, thus an indication of
being active seeps. The origin for these seeps
remains debatable.
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