THE HYDROCARBON POTENTIAL OF THE CENTRAL, SOUTHERN
AND NYANZA SEGMENTS AND CORRELATION WITH LAKE
TURKANA SEGMENT OF KENYA (GREGORY) RIFT
The Kenya Rift region is a part of the Eastern branch of the East Africa Rift
System that consists of a linear structure that extends over 900 km from the
Turkana basin at 4030‘N to the North Tanzanian Divergence (NTD) at 20 to the
south, while the Nyanza half graben which trends east-west (N80°E), branches off
from the main Kenya Rift at Menengai where there is a triple junction and
disappears under Lake Victoria. This Rift can be divided into four major segments
which are the Northern, Central, Southern and the Nyanza segments. These
segments contain sedimentary basins of different thicknesses.
Petroleum has been discovered in the Northern part of the Kenya Rift Valley which
has confirmed productive reservoirs for commercial development of hydrocarbons
in the Lokichar basin. Source rocks in Lorkichar basin includes:i)
ii)
Lokhone Shales- (organic rich), of the Early Miocene, containing TOC
values of 4- 17.5% mean 8.8% and it is of the Type I kerogen
Loperot Shales of the Early Oligocene- Eocene Age with TOC values of
approximately 2.2% and it is of the type III kerogen. The hydrocarbon
reservoirs in Lorkichar basin are the Upper and Lower Lokhone Sandstone
comprising of quartz and feldspars with subangular to angular grains, have
good intergranular porosity and finally the seals in this basin are
a. Argilaceous basin fill sealing the Upper Lokhone Sandstone and
b. Lokhone shales sealing the Lower Lokhone Sandstones. Central,
Southern and Nyanza Rift basins, being on the same Rift valley; hence
have the almost similar geology and useful factors in evaluating
potential occurrence for hydrocarbons.
The basin sediment fills in the Central, Southern and Nyanza Rift basins are 7 to 8
km, 2 to 4 km and 2 to 5 km respectively. This places them in a position of having
favourable potential source rock and reservoirs for hydrocarbons. Fault structures
present in the basins can act as traps and also reservoirs. TOC values for Central
Rift Segment basin are approximately 20 % in the Poi Shales (similar to OligoMiocene shales in Lokichar) while TOC values in Nyanza basin ranges from 4 to
20%. This indicates favourable source rock conditions. Arkosic sandstone with
porosity up to 25% in the Central Rift basins, Holocene to Recent Lacustrine and
fluvial sandstones in Southern Rift basins and Paleogene-Miocene to Recent
fluvial-deltaic and lacustrine porous and permeable sandstones in the Nyanza Rift
basin demonstrates favourable potential hydrocarbon reservoirs. Geothermal
signatures along the Rift Valley, offers favourable conditions for the maturity of
the
hydrocarbons.
Key words: Rift Valley Segments, Hydrocarbon, Source rock, Resevoir and Traps.