Spatial heterogeneities in bulk and molecular composition of petroleum within individual fields due to gravity segregation, or secondary alteration processes such as biodegradation or water washing have been known for decades (Baily et al., 1973; Milner et al., 1977; Hunt, 1979; Tissot & Welte, 1984; Connan, 1984). However, recently, attention has been geared towards a different way of interpreting the reservoir compositional heterogeneities. The organic constituents of petroleum reservoirs frequently show measurable compositional variations both laterally (between wells) and vertically (within a single well). These phenomena are mainly controlled by the geological setting of the reservoir, which influences the way in which migrating petroleum fills the reservoir from one or more source kitchens. England et al., 1987; England and Mackenzie, 1989; Karlsen and Larter, 1989; Larter et al., 1989, have shown that once the reservoir filling is achieved, the reservoir fluids may then redistribute themselves at a rate which is often comparable to geological time periods. Because of the low rates of in-reservoir mixing of successive petroleum charges from the source drainage area, in many cases, the compositional variations "inherited" from the filling process persist to the current day.

My project will concern the Hassi Messaoud giant oilfield, known by its vertical and lateral facies heterogeneities and presence of faults which can play the role of barriers preventing oil mixing and hence disturbing reservoir continuity. We seek to understand how the field was filled, from which source rock (Silurian in Oued Mya, in Berkine basin or both or from the North).

This study will help to:

1. Better appraise, develop and improve the production of the field by identifying compartments,
2. Increase potential oil recovery of the field, by understanding and determining the reservoir continuity,
3. Orient the focus of explorationists working in the surrounding area by identifying the main source rock(s), and reconstructing migration pathways from the source to the reservoir.

A set of 100 oil samples have been collected from all over the Hassi Messaoud field, el Baguel, el Gassi - el Agreb fields and some of the satellite fields (Rhourde chegga, Hassi Guettar and OL). These samples will undergo the following geochemical analyses :

• Thin Layer Chromatography/FID (TLC/FID) of all the crude oils,
• Gas Chromatography (GC) of the crude oils, gasoline, aliphatic and aromatic fractions,
• Gas Chromatography/Mass Spectrometry (GC/MS) of aliphatic and aromatic fractions and polar compounds (phenols and carbazoles),
• Isotope Ratio/ Mass Spectrometry (IR/MS) of crude oils, aliphatic, aromatic, resins and asphaltene fractions.