Etude stratigraphique, pétrographique et diagénétique des grès d ...

tel-00534181, version 1 - 9 Nov 2010
Chapter I
6. Conclusions
The age of the uppermost parts of the LSF has been conclusively fixed, from stratigraphic field work and
numerous radiometric age determinations using K/Ar and Ar/Ar methods, as ranging from 37 to 33 Ma (upper
Eocene-early Oligocene). We do not have new ideas on the age of the base of the LSF, since no new fossil evidence
was obtained from the field, apart from the strongly fragmented dinosaur bones encountered near the base of the typesection
north of the Lokitaung Gorge. This basal part of the formation will therefore continue to be considered as
Cenomanian in age, in agreement with previous authors. The environments of deposition of the basal and the topmost
parts of the LSF have been interpreted as distal alluvial fans while the middle part of the formation corresponds to a
braided stream system. There must be many stratigraphic gaps which were not evaluated in the field due to the fact
that the one lithologic section measured in the field was not adequate to identify such gaps and further field studies
are recommended in order to fully understand the gaps within the sedimentary unit. Diagenetic evolution of the formation
has shown that the dominant cement material changes from dominant calcite at the base to hematite within
the middle zones of the formation to kaolin towards the top. This is consistent with climatic conditions prevailing in
the central-eastern Africa region during the deposition of various zones which evolved from mainly semi-arid during
the Cenomanian (99.6-93.5 Ma) to a more humid environment starting from the Maastrichtian (70.6-65.5 Ma).
Reservoir potential of the LSF, based on results of porosity measurements which have yielded values ranging
from a minimum of 3 % to a maximum of 25 %, is generally fairly good. The top zone of the formation has the
best preserved porosity values (12-22 %) and therefore represents the best potential reservoir section. These values
compare well with other observed porosity preservation in sandstones of similar depositional environments such as
the ones in the southern Sudanese basins where porosities have been reported to be fairly good with values ranging
from 8 to 38 % in the Unity Field of the Muglad Basin (Schull, 1988), where Giedt (1990) reported permeabilities in
the range of 40 to more than 10,000 millidarcies in the main reservoir horizons which are mainly in the kaolin-cemented
horizons. For the reservoirs of the Lokichar basin, the Lomerimong sandstones have a residual porosity of ranging
from 1-15 % (Tiercelin et al., 2004), the low values are caused by the cementation by calcite as well as due to
the presence of micritic and clay matrix. Subsurface samples from sidewall cores obtained from the Loperot-1 well
show drastic reduction of porosity due to laumontite and quartz cementation. The porosity values at the same time
show a discrepancy between point count values and wireline log-calculated values, the later being much higher than
the former due to the presence of microporosity attributed to the abundant detrital and authigenic clays (Visser, 1993).
Considering the fact that the reservoirs of southern Sudan and other basins of the Central African Rift System (CARS)
are confirmed oil-producing reservoirs and that the LSF reservoir appears to compare favorably with these known
producers, the overall oil exploration potential of northern Kenya should be comparable to the other basins of the
CARS, at least from a reservoir point of view. Even with the expected reduction of porosity with depth of burial due
to diagenetic alteration of the sandstones, residual porosity can be expected to remain of reasonable reservoir quality
between 12-14 % (Morley, 1999). Source rocks have been confirmed in the Turkana area in the Loperot-1 well
(Lokichar Basin) where total organic carbon (TOC) values are reported to be as high as 17 %, with an average of 4.5
% (Morley et al., 1999b; Talbot et al., 2004). On the outcrop level, the claystones identified within the LSF (lower
and upper terms) were not subjected to geochemical analysis to determine their organic matter richness and hence
their source rock potential. Palynofacies studies conducted on a restricted number of samples did not demonstrate the
presence of organic debris. This could be an interesting target for future work, perhaps engaging proper methods of
sampling in the field. However, due to the lack of adequate well data with only two wells drilled in the entire northwestern
Turkana area, these ideas can only be confirmed after further exploration drilling. With the area having been
licensed to oil exploration companies, this may be expected to happen in the not too distant future.
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