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Abstract: Oil and Reservoir Core Extracts Compositional Variations in the Kerkennah Ouest Fields, Tunisia

Rhamzi Ghenima, Eric Lafargue

A suite of oils and reservoir core extracts from the Kerkennah oil fields in Tunisia has been analyzed by various geochemical techniques to elucidate the geological processes which cause variations in oil and extracts composition and their resulting fingerprinting in the different reservoirs of the field. More generally, the results obtained greatly helped in the understanding of filling directions which is valuable for future exploration of satellite fields.

The oil pools studied are parts of a large geologic province ("the pelagienne plateforme") where the main oil fields are limited by NW-SE major faults. The two main reservoirs are encountered in the carbonate series of Turonian and Eocene ages and the best reservoir qualities are found in the packstone and grainstone Nummulites facies. Numerous fractures are observed through the fields and we could demonstrate their influence on the filling history of the different fields as well as on the present oil production.

The detailed analysis of the light hydrocarbons (C20-) as well as the complete study of the C15+ hydrocrabons indicate compositional variations between the hydrocarbons stored in the Eocene and Turonian reservoirs. The core extracts from the two reservoirs also shows some variations with in particular maturity differences. Apparently the only possible source rock in the area is represented by the Bahloul formation of Turonian age. Due to unsufficient burial, the possible contribution of an Eocene second source rock from the Bou Dabbous formation was discarded.

Knowledge of oils and core extracts compositions from one reservoir to another, source rock maturity, geochemical modelling and tectonic history of the fields allow the setting of a scenario involving multiple-charge process for the oils to fill these reservoirs over an extended period of time. Within this scenario, we proposed the hypothesis of different behaviours of the main faults over geological time: a first period where the faults acted as conduits for hydrocarbon migration towards both Turonian and Eocene reservoirs and a second period where the faults became impervious to the hydrocarbons moving towards the Eocene reservoirs thus resulting in the storage of more mature hydrocarbons in the Turonian reservoirs only. Also of interest is the observation of different levels of homogenization in the Turonian reservoirs from one field to another.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France