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

tel-00534181, version 1 - 9 Nov 2010
Chapter III
poorly studied. In terms of reservoir rocks, arkosic sandstones near the base of the Tambach Formation are
quartz-rich and well sorted, and reflect dominant basement provenance. Cements are calcite and iron oxides,
as it is in the Lapur Sandstone Formation in Northern Kenya Rift. Quartz and albite overgrowths are locally
present. Near the top of the formation, the sandstone composition is characterized by an increase in volcanic
grains as a consequence of the occurrence of regional volcanic activity (Ego, 1994).
3.1.2. Source Rocks
The main component of the Ngorora Formation is formed by lacustrine shales that were described by
Pickford (1978) as being petroliferous and thus, the Tambach and Ngorora Formations were considered as
attractive for hydrocarbon exploration. The lacustrine facies in the Tambach Formation are dominated by pale
green, grey and white shales. Some are waxy in feel and appearance or show a “paper shale” facies (Renaut
et al., 1999). They are interpreted as accumulated in fresh to slightly brackish waters. Of the two lacustrine
members of the Ngorora Formation, one (Member C) shows interest in terms of source rock/hydrocarbon
potential. Both members represent a succession of lake environments from shallow, freshwater lakes to small
saline, alkaline lakes resembling the present-day Lake Baringo, or Lake Bogoria, respectively (Tiercelin and
Vincens, 1987; Renaut and Tiercelin, 1994). Petroliferous shales with up to 4.3 % TOC have been described
at Poi locality on the western flank of the Tugen Hills. The shales are dark green or dark grey in colour, and
contain very well preserved fossil fish and leaf fragments. Presence of abundant zeolites such as analcite and
clinoptilolite associated with other authigenic minerals indicate a saline, alkaline lacustrine domain, certainly
hydrologically closed and submitted to frequent fluctuations in lake level as well as in salinity. An analogue
can be found in the Central Kenya Rift with the modern lakes Nakuru and Elmenteita, or with Lake Bogoria,
a meromictic lake with an anoxic monimolimnion, preserving the accumulation of organic-rich (up to 12 %
TOC) zeolitic muds in the deeper parts of the lake (Tiercelin and Vincens, 1987). The Poi shales can also be
compared to the Oligo-Miocene shales found in the Lokichar Basin.
122