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Group, 100 Thames Valley Park Drive, Reading, Berkshire, RG6 1PT.
email
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Eocene
nummulite accumulations, also referred to as nummulite "banks", form
important hydrocarbon reservoirs in Tunisia and Libya and may constitute exploration
targets in other parts of North Africa, the Mediterranean and the Middle East.
Porosities commonly average 10-20% and permeabilities 10-50md. Foraminifera
of the genus Nummulites may comprise up to 98% of the bioclasts in these carbonate
reservoirs, although only one or two species may be present. The absence of
associated fauna is generally taken to indicate an oligotrophic depositional
environment.
In
this paper, the palaeoecology of the genus Nummulites is discussed together
with depositional models for two nummulitic carbonate reservoirs -- the Middle
Eocene Seeb Limestone of Oman and the Lower Eocene El Garia/Jdeir Formation
of Tunisia and Libya. The El Garia and Seeb Limestone Formations were deposited
in ramp settings, and comprise a series of amalgamated sheets or low-relief
banks. In the Hasdrubal field offshore Tunisia, where the El Garia Formation
constitutes the reservoir rock, most of the nummulites have been redeposited
from shallow into deeper waters whilst in the Bourri field (offshore Libya)
they occur as an in situ "bank".
Nummulite
accumulations often show evidence that both physical reworking (scouring, winnowing
and imbrication) and biological processes (reproduction strategies and bioturbation)
have influenced their formation. A general model is outlined for discriminating
between physically and ecologically produced biofabrics, and the implications
for reservoir quality are discussed.