Lithological Traps

Lithologic and stratigraphic traps in the lUizi Basin, a Location map b paleostructural profile at the south of the Ghadames Basin 

We thus observe in the northern part of the Oued el-Mya Depression a little south of the line Hassi R Mel-Hassi Messaoud the regional boundary of the wedging-out of the TAG reservoir rocks. With these wedging zones and those in which permeable facies become replaced laterally by impermeable ones traps of the lithological type may be associated. 

Other examples of lithological traps in the lUizi Basin are shown in Fig. 2.15. 

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In the upper structural level the presence of lithological traps at the base above the unconformity is more likely. This is observed in the northern part of the Oued el-Mya... 

Depression where there is a south-facing wedge of productive sandy horizons in certain parts of the Triassic and Permo-Triassic (Fig. 2.4). Along this wedge zone of reservoir rocks we may observe lithological traps in the basal rocks of the Mesozoic complex. It is even possible to locate stratigraphic traps resulting from the closure of reservoir rocks by internal unconformities. 

Fig. 2.15c-e. Lithologic and stratigraphic traps in the Illizi Basin, c Paleostructural profile at the east of the Illizi and Ghadames Basins d stratigraphic and lithologic traps associated with Complexe Terminal Zarzaitine field, Illizi Basin e Paleostructural profile at the south-east of the Saharan Platform... 

The reservoirs in the Flamra Quartzites and the El-Atchane sandstones which had been eroded along the Hercynian unconformity are wedging out around this uplift. In the Triassic sediments around the northern edge of the uplift a lithological trap might be developed. The reservoirs lie at a depth of 3 000-4 200 m and the quality of the Triassic reservoirs improves towards the north. 

The attempt to locate on the Saharan Platform and interparticular in the Triassic Province traps of the non-structural type is of particular interest for prospecting in view of the exhaustion of the inventory of unexplored structural traps. The stratigraphic traps are more characteristic of the Paleozoic sedimentary complex because of the presence of wedging zones and angular unconformities in the respective basins. Lithological traps are developed essentially in the sandy-argillaceous formations of the Triassic and result from the facies variations characteristic of these sediments. 

Lithology may exercise a primary control of hydrate deposition, resulting from permeability, faults, and traps. 

According to Hubbert (1967), considering hydrodynamic conditions in the carrier-reservoir rock the number of permutations of lithological and hydrodynamic conditions, and of oil and gas densities that can combine to produce hydrocarbon traps is unlimited. Hubbert distinguished two groups of traps ... 

Those that occur in conventionally closed lithological structures (i.e. in hydrostatic traps). In these traps the hydrocarbon-water contact may have any degree of tilt from the horizontal to the maximum dip of the barrier boundary at the downstream side of the closure. Although hydrocarbons may become trapped in the conventional hydrostatic traps of sufficient sealing capacity, the hydrocarbon accumulation is not necessarily present in the same position within the trap, as its actual position depends on the hydrodynamic condition in the carrier-reservoir rock (Figure 5.10). 

The pattern of excess " °Ar trapped in minerals can be used to estimate the bulk diffusivity through the rock. In theory, the concentration of " °Ar in a mineral above its closure temperature should be close to zero. However, if the Ar is produced in the rock faster than it can diffuse out, then the mineral is likely to retain some excess " °Ar. The build-up of " °Ar in a mineral above its closure temperature is a function of the relative rates of Ar production and diffusion out of the rock, not the mineral. Utilizing outcrop-scale gradients in " °Ar concentrations in an amphibolite from the Simplon pass Switzerland, Baxter et al. (2001) calculate a bulk Ar diffusivity of the order 10 cm s at a temperature of 500°C (the conditions of their study). These results probably define the upper limit on the bulk Ar diffusivity in the upper crust (at least, for the immediate lithology of amphibolite) as diffusion rates are likely to decrease at lower temperatures. Studies by Poland (1979) and Scailliet (1996) also correlate excess " °Ar buildup with limited bulk diffusivity. 

Battani A, Sarda P, Prinzhofer A (2000) Basin scale natural gas soiuce, migration and trapping traced by noble gases and major elements the Pakistan Indus Basin. Earth Planet Sci Lett 181 229-249 Baxter EF, DePaolo DJ, Reime PR (2002) Spatiahy correlated anomalous " °Ar/ Ar "age" variations in biotites about a lithologic contact near Simplon Pass, Switzerland A mechanistic explanation for excess Ar. Geochim Cosmochim Acta (submitted)... 

Traps in the western Haltenbanken region like Lavrans, Kristin, Trestakk and Smorbukk contain mainly petroleum of high maturity, yet in the compartmentalized Smorbukk field some medium maturity bitumen remains in particular reservoir sections. This is most likely due to strong compartmentahzation caused primarily by the lithological heterogeneities and secondarily by diagenetic poroperm modifications. 

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