Silurian Graptolitic Source Clays

On the whole, although the Silurian source rocks are found at present in the gas window , they have never been strongly warmed up. This apparent contradiction may plausibly be explained by the uplift and erosion of the Paleozoic sediments during the Hercynian orogeny. In many instances, the present-day subsidence depth of source rocks is smaller than it was before the Hercynian uplift. In the Illizi Basin, in particular in its southern and western regions, the deepest subsidence took place prior to the Hercynian uplift. A slight uplift could not cause a substantial temperature decrease  

The effective-to-general shale thickness ratio is a factor which determines the ability (or inability) of hydrocarbons to migrate from source rocks. In this context, of definite interest for evaluating the amount of generated hydrocarbons were also the less thick shale beds in the Ghadames, Illizi, Triassic and Reggane Basins. 

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