Silicification from Underground Waters

The silicification of sediments little affected by diagenesis under surface conditions is a pronounced frequent phenomenon entailing the dissolution of silica by surface waters, as well as hydrolysis of silicates with the subsequent redeposition of the silica in the discharge zone of the surface waters (Taylor 1950 Thompson 1959 Weyl 1959 Trumit 1968). These cases of silicification are widespread under geochemical conditions favourable for the dissolution of silicates. 

Our observations have shown that secondary quartz is abundant in those beds in which the quantity of argillaceous cement is negligible. This situation is also well developed on the scale of thick beds in thin section studies. It is obvious that layers of more argillaceous rocks are characterized by a lower permeability and vice versa. Consequently, in less argillaceous (more permeable) strata, deep-seated mineralized waters and formation waters should be able to migrate provided the other conditions are favourable. To some extent these conditions probably existed in a number of provinces cut by a dense network of fractures. 

We may assume that a certain amount of secondary quartz could be deposited from thermal and formation waters. These processes could take place in permeable layers in which formation waters and deep waters can circulate as well as a result of the lowering of the partial pressure of CO under these conditions. In any case the presence of gypsum, anhydrite and baryte in numerous sections of the rocks studied is evidence of the penetration of mineralized waters derived especially from the salt-bearing sections of the Triassic. We have also tried to find a connection between the permeability of the sandstones and the distance of these horizons from tectonic disturbances. It has been noted that the mean permeability of reservoir rocks in holes situated less than 500 m from fractures or faults differs very little from that in rocks from other holes (Fig. 3.9). 

We may suppose that certain formation and thermal waters could be displaced along fracture zones but that the circulation of these waters could take place essentially within layers possessing a certain permeability (slightly argillaceous beds). Under these conditions some of the sihca could be deposited in the pores from these solutions as shown above. 

The presence of faults and fractures taken as a sign of silicification of reservoir rocks is one precondition but not the only one. This criterion becomes real in those cases where the beds are permeable and little argillaceous. In permeable beds we consequently have to analyse the relation between the content of secondary quartz and the fractured zones. In these cases old but reactivated fractures like, e.g., those of the Triassic and Cretaceous are of particular interest as they possess a higher flow potential and thus do not represent screens isolating displaced reservoir blocks. 

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