Mechanisms of Silica Transport

The calculation of the hydrological parameters necessary for establishing the above-mentioned silica balance appears to be a rather complicated problem as intergranular pressure solution leads to a dynamic reduction in rock mass and volume and frequently also to a lowering of the porosity. These parameters have been simulated for different petrophysical and geochemical conditions. The first results show that for the large amount of secondary silica observed to become exported, 0.5 x io -o.5 x lo cm of water is required for every cm of the Cambrian sandstones of the Oued el-Mya Basin. If we assume, on the other hand, that the quartz cement in the Ordovician sandstones from Ahnet Mouy-dir resulted within the rocks themselves from pressure solution then the water flux necessary was small or virtually nil and this formation approached an isochemical system. 

In general, sandstones with little detrital clay actually possess an elevated permeability and exhibit a notable secondary silicification as if they were part of a subarkosic 

Sources of Fluids Transporting Silica. Boles and Frank (1979) expressed the reaction of illitization by the following formula  

At a temperature of 75 °C this reaction is not yet complete. In order to dissolve the 26 moles of silica liberated during this reaction we need 38 X 10 moles of water (Walther and Helgeson 1977). It is thus obvious that even in the case of the silica supply being accompanied by dehydration of the enclosing argillaceous strata there has to be a supplementary source of water. 

In the case of the Devonian sandstones of the Tiguentourine field in the Illizi Basin the mean porosity is 13% and the mean content of secondary silica is also 13%. Under the temperature conditions prevailing in this region (70-100 °C) the solubility of quartz drops from 0.056 to 0.0316 g Tb When the diagenetic fluid cools from 100 to 80 °C SiOj will be deposited at a rate of 0.0010 g /1 °C (Holland and Malinin 1979). Then per cubic centimeter of sandstone a volume of 130 X 10 cm of pore fluids will deposit 0.290 g of silica for each temperature drop of 1 °C and we need actually 1.4 x 10 cm of fluid at a drop of 1 °C to deposit the 13% of authigenic quartz observed in these sandstones. 

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Aplin, A. C. and E. A. Warren, 1994, Oxygen isotopic indications of the mechanisms of silica transport and quartz cementation in deeply buried sandstones. 

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