Constraints on diagenesis and reservoir quality in the fractured Hasdrubalfield, offshore Tunisia

The hydrocarbon reservoir of the Hasdrubal field (offshore Tunisia) lies wi thin the Eocene El Garia Formation. This formation was deposited on a shall ow north- to NE-facing vamp in the Early Eocene and is composed of a belt o f nummulitic wackestones-grainstones. The nummulitic facies occupies a rang e of depositional environments from outer to mid ramp. In addition to Hasdr ubal, several other producing oil- and gasfields have been discovered in th e variably dolomitised El Garia Formation offshore Tunisia. Cores from three Hasdrubal wells were examined. Reservoir quality shows a l imited relationship to primary depositional fabric and has been influenced significantly by compaction and later diagenesis. The highest permeabilitie s are typically developed within a dolomitised zone which occurs near the m iddle of the reservoir interval across the entire field, and which may foll ow a primary, wackestone lithofabric (typically 20-30% bulk volume dolomite , with porosities of 15-22% and permeabilities of 1-30mD). Fractures, parti cularly in zones surrounding faults, have resulted in enhanced permeabiliti es. 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 d olomitised zone indicate that matrix dolomites are burial diagenesis cement s. 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 nummu lite tests within the reservoir facies.