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Controls on hydrocarbon occurrence and productivity in the F6 reservoir, Tin Fouyé-Tabankort area, NW Illizi Basin

Abstract Several oil accumulations of various sizes have been found within the F6 reservoir of Upper Silurian-Lower Devonian age in the Tin Fouyé region of the Illizi Basin, about 1500 km from Algiers. Lithologically, the F6 consists of interbedded sandstones and shales and is subdivided into units: M1, M2, A, B1, B2, C1, C2 and C3, the upper units often being missing through erosion. These reservoirs, which were deposited in offshore coastal bars and tidal channels, show a high degree of lateral and vertical facies variation. Reservoir quality is maximized in medium- to coarse-grained clay-poor bar sands within units C1 and B2, which contain significant secondary porosity. Hydrodynamic activity is very important in controlling the distribution of hydrocarbons. Study of the hydrology of the F6 reservoir shows that connate water and oil have been frequently flushed by fresh water in the most permeable units, particularly C1 and B2. This finding is supported by hydrochemical data, by the inclination of oil-water contacts and by a high observed hydraulic gradient. This mechanism has led to the flushing of some oil from the Tin Fouyé Est (TFE) structural trap and a concentration of oil in an accumulation with a tilted contact on the Tin Fouyé Nord structural nose. Many kinds of traps have been recognized in this region. These include a simple structural trap at Tin Fouyé, a combination hydrodynamic-stratigraphical-structural trap at Tin Fouyé Nord, a hydrodynamic-structural trap at TFT-2 and a pure hydrodynamic trap at TFT 8. Hydrodynamic activity is not favourable for the accumulation of hydrocarbons in low-amplitude structures but can lead to significant accumulations in favourably orientated structural noses, even when these lack closure.