Early Invasion of the Reservoirs by Hydrocarbons

The arrival of the hydrocarbons in the reservoir rocks slowed down these reactions and the hydrocarbons themselves also modified the petrophysical characteristics of the environment, in particular in the contact zone with water. This phenomenon contributed to the decompaction of the reservoir rocks by dissolution in the transition zones, of cementing materials and of the matrix components by the products of incomplete oxidation and transformation of hydrocarbons like, e.g., the transformation of an oil deposit into a gas-condensate system. 

Silica Solid Phase Transformation A New Concept for Sandstone Diagenesis as Revealed in North African Basins 

This section is written together with Mrs LI. Plyusnina. 

Many mechanisms have been suggested to explain quartz cementation processes in sandstones. Amongst these, pressure solution has been the focus of considerable attention as a principal process for supplying silica for quartz cementation (e.g. Taylor 1950 Heald 1956 Thompson 1959 Weyl 1959 Houseknecht 1984,1988). These observations led to the elaboration of theoretical geochemical models of intergranular pressure solution and a limited number of experimental simulations (Ernst and Blatt 1964 Heald and Renton 1966 Renton et al. 1969). There is still no consensus as to the importance of pressure solution. Some researchers insist that pressure solution considerably reduces the porosity of sandstone (e.g. Houseknecht 1984,1988) whereas others conclude it is of limited importance (e.g. Sibley and Blatt 1976 Blatt 1979). 

The idea that clay mineral diagenesis and particularly the iUitization of smectite might be an important source of authigenic silica have been previously discussed (Siever 1962 Towe 1962 Keller 1963 Fuchtbauer 1967 Dunoyer De Segonzac et al. 1968 Burst 1969 Perry and Hower 1970 Hower and Eslinger 1973 Schmidt 1973 Yeh and Savin 1973 McBride 1989). Hower et al. (1976) and Blatt (1979) also confirmed that dis- 

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