C. I. Macaulay1,5, D. Beckett2, K. Braithwaite3, D. Bliefnick3 and B. Philps4
1Isotope
Geosciences Unit, Scottish Universities Research and Reactor Centre, East Kilbride
G75 0QF.
2BG
International, 100 Thames Valley Park Drive, Reading, Berkshire RG6 1PT.
3BG
Technology, Gas Research and Technology Centre, Ashby Road, Loughborough, Leicestershire
LE11 3GR.
4Royal
School of Mines, Imperial College, London SW7 2BP.
5
Corresponding author: email Calum.Macaulay@glg.ed.ac.uk
Present
address: Department of Geology and Geophysics, University of Edinburgh, Grant
Institute, West Mains Road, Edinburgh EH9 3JW.
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 ramp 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 (?18O -5.0 to -7.3‰PDB) and fluid inclusion (Th 80-90°C)
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.
Our
analyses indicate that calcite cements were precipitated at temperatures of
up to almost 150°C in primary and secondary pores and in variably-sealed fractures.
Fracture lining and filling cements show a range of ?18O values, which suggests
that the fractures acted as fluid conduits over a range of temperatures during
burial diagenesis. Fracture densities measured in core increase rapidly close
to seismically-resolvable faults in the reservoir facies. Fracturing probably
resulted in the leakage of hydrocarbons through the Compact Micrite Formation
seal which overlies the accumulation, as well as facilitating the ingress of
hot fluids from stratigraphically deeper levels in the basin.