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

tel-00534181, version 1 - 9 Nov 2010
Chapter III
between 11 and 25 %. Kaolin cements are also present within the section where it occurs as a later cement
after hematite. Though the measured porosity values are fair to good, the presence of iron crusts, if these are
laterally continuous in reservoir scale, could create possible reservoir heterogeneities resulting in compartmentalization
and hence production difficulties. In the top zone of the LSF (Fig. 10; Zone III), the most prominent
cement is kaolin with subordinate amounts of hematite (except for the topmost calcite-cemented beds).
Porosity in this zone has been relatively well-preserved, with values ranging from 12 to 22 %. Secondary dissolution
porosity from the dissolution of feldspars and secondary intragranular porosity from partial dissolution
of feldspars are important contributors to the total porosity. It has however been noted (Schmid et al.,
2006) that secondary porosity is not in most cases of real benefit to reservoirs since it is redistributional in
nature, where the material from dissolved minerals is re-precipitated as new cement elsewhere.
Microporosity is significant within the vermicular variety of the pore-filling kaolin. Due to its generally
small size, however, the microporosity is unlikely to contribute significantly to permeability of the reservoir.
Kaolin associated with kaolinization of detrital clays does not appear to produce significant amounts of
microporosity. From a production point of view, the high amounts of microporosity associated with kaolin
cements causes high residual water saturation and the small kaolin crystals can easily be moved from pore centers
to pore throats by the moving fluids resulting in the clogging of pore throats and thus reducing the permeability
of the reservoir (McBride et al., 1996). Overall, the top half of the LSF constitutes a more prospective
reservoir zone than the lower half.
The Muruanachok Sandstone Formation (MSF) (Figs. 7, 27 and 28) is composed of conglomerates
and sandstones deposited within a fluvial setting within alluvial plains and fluvial channels, with limited
amounts of argillaceous matter. At the type locality, the thickness of the measured section is slightly over 110
m and this therefore represents a significant potential reservoir unit. The MSF is not a well-cemented formation.
Total cement ranges in abundance between trace amounts to 30 %. The three main cements are, as it is
in the LSF, kaolin, calcite and hematite. Iron oolites are concentrated near the middle part of the MSF containing
abundant hematite cement. Porosity values based on visual estimation in thin sections studied throughout
the vertical section of the MSF range from 5 to 20 %, mainly in the form of primary intergranular porosity as
well as secondary intragranular and dissolution porosity resulting from the partial and occasionally total dissolution
of feldspar grains. The formation contains fractures which are partially healed by the deposition of
calcite cement and these could significantly enhance permeability thereby increasing the reservoir potential of
the MSF.
2.1.2. Source Rocks
Potential source rock occurrence is also a major question within the Gatome and Lotikipi Basins.
Good quality source rocks in continental rift basins are typically shales of lacustrine facies (Talbot, 1988), as
it is for the Lokichar Basin (Morley et al. 1999b; Talbot et al., 2004). In the Gatome and Lotikipi Basins, there
is no seismic evidence for thick accumulations of lacustrine shales. Only lenses of dark grey or black silty
mudstones a few metres to few tens of m in thickness can be seen at the outcrop scale in the Lapur Sandstone
Formation, where they may represent floodplain or shallow lake deposits (Thuo et al., in press) (Fig. 32c).
Nevertheless, these mudstones do not demonstrate abundant organic content. Only few wood stems are found
in the sandstone facies of LSF. Erosive discontinuities seen on top of mudstone beds and the presence of mud
clasts in sandstone facies indicate strong erosive processes affecting such deposits (Fig. 32d). This also suggests
that thicker piles of mudstones may be present in different environmental settings within the LSF. There
is no source rock evidence from the Muruanachok Sandstone Formation. Only one metre-thick layer formed
by grey and purple silty mudstone has been found containing small dark organic debris and prints of plant lea-
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