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

tel-00534181, version 1 - 9 Nov 2010
Chapter I
Hematite cements. In Zone III, the hematite cements are particularly common in the section measured
within the Lokitaung Gorge, and they are clearly early cements; as they occur as pore-lining cements
followed, in many instances, by later generations of siliceous and kaolin cements (sample LOK 11/04)
(Fig. 23g, h). This type of cementation is very common within this zone and the different cements and their
relative timing of generation are clearly distinguishable.
Siliceous cements. These occur only in the top zone of the LSF and they are amorphous non-crystalline
in texture (sample LOK 11/04) (Fig. 23g, h), identifiable only through SEM-EDS analysis. This
type of cement is predated by the reddish-brown hematite cement and post dated by pore-filling vermicular
kaolin cements.
4.4.3. Porosity
Porosity measurements were performed on a total of 24 samples taken from the LSF type-section
(Fig. 24). The present total porosity in the samples is the net porosity after the original porosity at deposition
has been reduced by diagenetic processes. These processes include mechanical compaction and the precipitation
of cements. From the samples studied, the LSF has total porosity values ranging from a minimum of 3 %
to a maximum of 25 %. While this porosity appears to be evenly distributed, it is markedly less abundant in
the lower-most parts and the top-most parts of the LSF (which are cemented by poikilotopic calcite). This is
as would be expected that the presence of pervasive cementation by poikilotopic calcite would drastically
reduce the amount of primary intergranular porosity and this is what is observed in the LSF samples at the base
of the formation. The percentage of calcite cement in the samples is close to the assumed original intergranular
porosity of about 40 %. A significant part of the remaining porosity can be attributed to microporosity
inside the kaolin cements and to K-feldspar dissolution.
4.4.4. Source of cements ad diagenetic reactions
This discussion attempts to postulate the possible sources of the major cement types in the LSF in view
of the important role these cements play in the determination of the reservoir quality of any sandstone unit.
4.4.4.1. Calcite cementation. In the basal section of the LSF, calcite cements were probably precipitated
prior to any significant sediment compaction. The main diagenetic events in this section were the deposition
of grain-coating hematite cement, the formation of kaolin cements mainly from the alteration of feldspars
and the precipitation of poikilotopic calcite cement. Assuming that the sandstones had 45 % initial intergranular
porosity and the present residual porosity is less than 5 %, and the amount of cement is approximately
40 %, this suggests that there has been no appreciable loss of porosity through compaction. There is no
evidence of a significant mechano-chemical (pressure-solution) compaction; most of the compaction is therefore
mechanical and it involves ductile deformation, re-arrangement and fracturing of grains. In other parts of
the formation that do not have calcite cement, there is evidence of a higher amount of compaction with the
presence of broken muscovite grains. Early calcite cement may have greatly inhibited sediment compaction
and the mechanical compaction is assumed to have been early.
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