Lithostratigraphic Traps

The nature of the lithostratigraphic traps varies with the level of the reservoirs located above the volcanic rocks (T-i and T-2) as well as below them (Lower Series). In the upper reservoirs (T-i and T-2) penetration of saline waters from the Saline Series (S1-S2-S3...) higher up has led to blockage of these sandstones which in petrophysical aspects are better sorted than those of the Lower Series. The same saline solutions almost completely destroyed the permeability of the sandstone T-2 which could have represented, among the Triassic reservoirs, a storage area of some importance for the accumulation of hydrocarbons. 

In the Lower Series low permeabilities are developed throughout because of  

the enormous weight exerted by the overlying volcanic rocks  

the concentrated solutions derived from the overlying volcanic rocks and flowing towards base within the Lower Series  

the absence of a pronounced relief which could have acted as a source area for the sedimentation of the Lower Series by shedding coarse clastic elements leading to the development of thicker and more extensive sandy formations  

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The Lower Series was deposited on a flooded erosion surface over a Paleozoic substrate, a marine surface on which were developed vast lagoons, volcanic flows, tidal flats and barriers. Truely marine conditions prevailed only over the northern part of the region. The sandstones of the Lower Series are not as uniformly continuous as those of the Tj and T. They are generally lenticular or sometimes form wedges interrupted by the volcanic rocks. A great potential for the accumulation of hydrocarbons in lithostratigraphic traps may develop under these conditions like in the sandstones in the upper part of the Lower Series in TKT-i, BKZ-i and BKZ-2. 

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