The hydrocarbon reservoir of the Hasdrubal field (offshore Tunisia) lies within the Eocene El Garia Formation. This formation was deposited on a shallow north- to NE-facing vamp in the Early Eocene and is composed of a belt of nummulitic wackestones-grainstones. The nummulitic facies occupies a range of depositional environments from outer to mid ramp. In addition to Hasdrubal, several other producing oil- and gasfields have been discovered in the variably dolomitised El Garia Formation offshore Tunisia. Cores from three Hasdrubal wells were examined. Reservoir quality shows a limited relationship to primary depositional fabric and has been influenced significantly by compaction and later diagenesis. The highest permeabilities are typically developed within a dolomitised zone which occurs near the middle of the reservoir interval across the entire field, and which may follow a primary, wackestone lithofabric (typically 20-30% bulk volume dolomite, with porosities of 15-22% and permeabilities of 1-30mD). Fractures, particularly in zones surrounding faults, have resulted in enhanced permeabilities. Combined results of isotope (delta O-18 -5.0 to -7.3 parts per thousand PDB) and fluid inclusion (Th 80-90 degreesC) analyses of dolomites from this dolomitised zone indicate that matrix dolomites are burial diagenesis cements. Dolomitisation of the reservoir was a "closed system" event and was not the result of major fluid flow or mixing. Magnesium ions for dolomitisation were derived from the transformation of high-Mg to low-Mg calcite in nummulite tests within the reservoir facies.