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

tel-00534181, version 1 - 9 Nov 2010
Chapter III
3.1.1. Reservoir Rocks
The lowest part of the Kerio Basin infill is of sedimentary origin and possibly of Paleogene age. At
the southern end of the Kerio Basin, arkosic sandstones known as the “Kimwarer Formation” outcrop along
the footwall of the western faulted margin of the basin - the Elgeyo Border Fault, where they overlie the
Precambrian basement. Renaut et al. (1999) describe green, waxy, laminated tuffaceous shales associated to
these sandstones. To the east, in the Baringo Basin, the oldest sediments exposed along the western faulted
side of the basin – the Saimo Border Fault - are known as the “Kamego Formation” (Fig. 7). They are mainly
arkosic fluviatile sandstones sourced from Precambrian rocks (Chapman et al., 1978; Renaut et al., 1999).
These Kimwarer and Kamego sediments represent the earliest sections of the Kerio and Baringo Basins infill,
possibly indicating fluvial and lacustrine environments similar to the ones found to the north in the Lokichar
and North Kerio Basins. Fluvial networks of Paleogene age draining the basement floor of the Kerio and
Baringo Basins are interpreted as flowing northward, feeding the Lokichar-North Kerio Basins (Morley and
Wescott, 1999; Tiercelin et al., 2004). The Paleogene history of the Kerio and Baringo Basins is unfortunately
poorly known because outcrops are scarce and poorly exposed and due to the absence of sufficient seismic
data and exploration wells. Nevertheless, possible similarity between the Kimwarer-Kamego Formations and
the Lokichar-North Kerio sediments may encourage oil exploration in Blocks 13T and 12A (presently licensed
to Platform Resources).
The Miocene history of the Kerio-Baringo half-grabens is much better documented by sediments and
lavas outcropping along the Elgeyo Border Fault and the western flank of the Tugen Hills fault block (Saimo
Border Fault) (Fig. 41). Thick sedimentary deposits are found interbedded between the volcanic units and
relate to the existence of at least two wide lacustrine domains known as the Tambach Lake and the Ngorora
Lake, both dated middle Miocene. In the Kerio Basin, more than 2 km of volcanic rocks fill have been deposited
between 23 and 15 Ma, then between 14 and 7 Ma (Chapman et al., 1978) (Fig. 41a). The oldest sequence
of Neogene age is the Tambach Formation. Up to 400 m of colluvial, fluvial and lacustrine sediments are
exposed discontinuously along the Elgeyo Border Fault, and lie upon an irregular topography of basement
rocks (Ego, 1994; Renaut et al., 1999). The upper limit of the Tambach Formation is provided by the overlying
Uasin Gishu Phonolite Formation (described by Lippard, 1973), that is dated from 14.5-12 Ma. The overall
Tambach depositional sequence is formed by alluvial fan and gravelly braided stream deposits near the base
of the sequence, followed by sandy braided deposits, to shallow deltaic and lacustrine sediments at the top part
of the sequence (Renaut et al., 1999).
The Ngorora Formation is of Miocene age but younger than the Tambach Formation (Bishop and
Chapman, 1970) (Fig. 41). The 400-m thick Ngorora sediments lie upon lavas of the Tiim Phonolite Formation
dated 13.1 Ma and are unconformably overlain by the Ewalel Phonolite Formation dated 8.5 Ma (Hill, 1999).
Clastic sediments in the Ngorora Formation are mainly fluvial sediments and lahar deposits, and have been
Figure 41. (a) Interpreted stratigraphic series of the Central Kenya Rift, from the Kerio Basin to the west, to the Baringo
Basin to the east. The logs for the Kerio Basin (SEE; South Elgeyo Escarpment; NEE, North Elgeyo Escarpment; KB,
Kerio Basin; TH, Tugen Hills, eastern flank of the Kerio Basin) show the stratigraphic position of the Paleogene(?) old
Kimwarer and Kamego Formations, of unknown thickness but presenting an interesting reservoir potential (see KB and
TH logs), and the Neogene Tambach and Ngorora Formations. To the east, the Baringo Basin (BB) log shows the deep
structure of the basin below the 2 km-thick Samburu Basalts (21-11 Ma). This deep part of the basin can offer potential
source rocks and reservoirs of Paleogene age, but this can only be confirmed by deep drilling. (b) Simplified diagram of
the Central Kenya Rift (Kerio-Baringo Basins) evolution from Paleogene times to present Central Kenya Rift. The early
stages of rifting are contemporaneous of the Lokichar-North Kerio rifting events in the Northern Kenya Rift. But, contrary
to the case of the Lokichar Basin, basin evolution in the Kerio Basin has continued until upper Miocene, and up to present
in the Baringo Basin (modified from Hautot et al., 2000; Tiercelin and Lezzar, 2002).
121