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Abstract: Carbonate Reservoir Modelling: The Early Eocene El Garia Formation, Offshore Tunisia.

JACKSON, HELEN and KAREN BRAITHWAITE, BG plc (Research and Technology); GREIG COWAN BG plc (Exploration and Production)

Hasdrubal is a pre-development offshore field in the Gulf of Gabes, Tunisia. The main productive zone is in the nummulitic El Garia Formation which is bounded by a pinchout to the North and by a combination of faults and the gas/water contact to the West, South and East.

Data from three wells were used in the study, with porosity and permeability data only available from two of them. The formation was deposited in a series of nummulite banks with associated nummuloclastics and phosphatic mudstones. A thin shale bed was also observed in one well. Although there appears to be little correlation between facies types and poroperm there appears to be a good correlation between wells allowing the reservoir to be layered into different zones. The horizons having the highest porosity and permeability occur in the middle of the reservoir, roughly corresponding with the areas with the highest percentage dolomite and highest carbonate dissolution.

The reservoir was modelled using STORM, a stochastic reservoir modelling package supplied by Smedvig Technologies inc. The nummulite banks, phosphatic mudstone and shale were modelled as rectangular sheets using a general marked point process with nummuloclastics forming the background facies. Statistical analysis of the porosity and permeability showed a high coefficient of variation for each zone and for the purposes of the petrophysical model data was taken from the average of all zones with the exception of the high permeability zones which were modelled seperately.

The objectives of this study were:

1) To produce a “worst case” layer cake model to in the planning of a horizontal well

2) To produce a model which is robust and readily updated as further wells are drilled.

3) To produce models which investigate the effects of:

a) Sensitivity to size and shape of of nummulite banks. Different sizes are quoted in the literature and there are no present day analogues.

b) The importance of the shale bed. Although shale is only found in one well, it will have an important effect on reservoir performance if present in continuous layers.

c) Different diagenetic scenarios.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah