A review of eocene nummulite accumulations: Structure, formation and reservoir potential

Eocene nummulite accumulations, also referred to as nummulite "banks", form important hydrocarbon reservoirs in Tunisia and Libya and may constitute e xploration targets in other parts of North Africa, the Mediterranean and th e Middle East. Porosities commonly average 10-20% and permeabilities 10-50m d. Foraminifera of the genus Nummulites may comprise up to 98% of the biocl asts 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 toget her with depositional Models for two nummulitic carbonate reservoirs - the Middle Eocene Seeb Limestone of Oman and the Early Eocene El Garia/Jdeir Fo rmation of Tunisia and Libya. The EI Caria and Seeb Limestone Formations we re 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 fiel d (offshore Libya) they occur as an in situ "bank" Nummulite accumulations often show evidence that both physical reworking (s couring, winnowing and imbrication) and biological processes (reproduction strategies and bioturbation) have influenced their formation. A general mod el is outlined for discriminating between physically and ecologically produ ced biofabrics, and the implications for reservoir quality are discussed.