Model of Diagenesis in Space and Time

A general model of the diagenesis of the sandy reservoirs of the Sahara is presented in Fig. 4.17. The sequence of the diagenetic stages is controlled by temperature and time. Vitrinite reflectance in shales adjacent to or intercalated with the quartz sandstones indicates their exposition in time and space to temperature and may be used also as an indicator of the thermal maturity of the quartz sandstones. In this way the vitrinite reflectances correspond to the stage of mesodiagenesis. Mechanical compaction dur- 

Organic acids generation Hydrocarbon generation (in intercalated shales) 

Gain of porosity (permeability) Loss of porosity (permeability) 

Stage I of maturity is characterized by maximum decarbonatization and decarboxylation of organic matter. The generation of hydrocarbon fluids is overall moderate, increasing to its maximum towards the end of this stage. Carbonatization, quartz diagenesis and mechanical compaction proceed rather moderately. The establishment of secondary porosity is at its maximum and results essentially from decarbonatization and dissolution of feldspars. This increase notably compensates for the loss of porosity. 

The initial phase of stage II maturity corresponds to moderate to low levels of carbonatization, decarbonatization, decarboxylation, mechanical compaction and quartz diagenesis, while the generation of hydrocarbon liquids takes place over a short period only. The loss of secondary porosity is minimal and its increase is equally negligible. 

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