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

tel-00534181, version 1 - 9 Nov 2010
Chapter I
setting of the LSF, its diagenetic evolution and the reservoir potential as well as the implications of the overall
hydrocarbon prospectivity of the northern Turkana region. These results also contribute to a better understanding
of the evolution of the Cretaceous-early Cenozoic continental basins in central and eastern Africa, in
terms of associated palaeogeography and regional palaeoclimate.
2. Regional geological setting
The Turkana depression is a vast lowland lying at a mean elevation of ~400 m above sea level, where the
main geographical feature is Lake Turkana, characterised at its northern end by a wide delta built by the Omo River
(Fig. 7). This region has been described as the most important area in eastern Africa for studying a long-lived segment
of the East African Rift System and how it interacted with the Cretaceous-Paleogene southern Sudan and
Anza rifts (Fairhead, 1986; Morley et al., 1999b). The present-day Lake Turkana Basin is part of a string of major
N-S oriented half-grabens that started to develop from Cretaceous(?) through Eocene-Oligocene to Plio-
Pleistocene (Mugisha et al., 1997; Morley et al., 1999b; Hautot et al., 2000; Tiercelin and Lezzar, 2002; Tiercelin
et al., 2004) (Fig. 6c). The northern end of this basin is a N-S trending half-graben that formed during middle to
late Miocene times, following a major phase of volcanic activity dated late Eocene to middle Miocene, that resulted
in the deposition of the 2.5 km-thick Turkana Volcanics (Bellieni et al., 1981, 1987; Zanettin et al., 1983;
Clément, Bellon et al., work in progress). The structure of the northern half of the Turkana Basin has been partly
imaged using offshore reflection seismic data of the PROBE Project (Rosendahl et al., 1986; Dunkleman et al.,
1988, 1989) and few seismic lines acquired by Amoco Kenya Petroleum Company (Wescott et al., 1999). The
major N-S oriented border faults that delineate to the west the Turkana Rift Basin (morphologically represented
by the Murua Rith-Lapur (MRL) fault escarpment) are imaged on the onshore seismic line TVK-10 (Figs. 7, 8 and
9a). Exposed in the footwall of the MRL fault are the Lapur sandstones that overlie Precambrian basement and in
turn are overlain by the Turkana Volcanics (Figs. 8 and 9b, sections 1, 2 and 3). The deep basin organization below
the Turkana Volcanics is almost unknown because of the considerably degraded quality of seismic data below the
subsurface volcanics (Morley et al., 1992; Wescott et al., 1999). Nevertheless, an alternative interpretation of the
TVK-10 seismic line is proposed further in this work (see paragraph 4.2.).
To the west of the MRL escarpment, a gravity and seismic survey conducted in 1985 by Amoco Kenya
Petroleum Company indicated the presence of two deep-buried, elongate (50 km long; 20 km wide) sedimentary
basins, the Lotikipi Basin to the west, and the Gatome Basin to the east, separated by the Lokwanamoru structural
high (Figs. 6c and 7). Depth to basement was interpreted to be 4 km and 6 km for the Lotikipi and Gatome
Basins, respectively (Wescott et al., 1999; Desprès, 2008). Seismic data over the Gatome Basin revealed one major
half-graben basin consisting of a late Eocene-Miocene volcanic sequence (Turkana Volcanics) and pre-volcanics
sedimentary rocks overlying Precambrian basement. The structure of the Lotikipi Basin was interpreted either as
a rift basin or a thermal sag basin, infilled by the late Eocene-Miocene Turkana Volcanics. Below the volcanics are
possible Cretaceous-Tertiary sediments resting on Precambrian basement, resembling to the Lapur Sandstone
Formation (Wescott el al., 1999). In terms of regional structure, the two Lotikipi and Gatome Basins appear to be
on trend with the basins of the Abu Gabra, Blue Nile and Muglad Rifts in southern Sudan (Bosworth, 1992;
Hendrie et al., 1994; Ebinger and Ibrahim, 1995; Wescott et al., 1999) (Fig. 6c). To the east of Lake Turkana, the
presence of Cretaceous strata in the Sirius-1 exploration well (Fig. 6b) has suggested similarity between the Anza
Rift and the southern Sudan rifts (Bosworth, 1992; Schull, 1988; Morley et al., 1999b). Nevertheless, no direct evidence
relates the Lotikipi-Gatome rifting to the Cretaceous-Paleogene south Sudan-Anza rift systems or to the
Paleogene northern Kenya-southern Ethiopia segment of the Eastern Branch of the East African Rift System.
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