Exploring for Lower Eocene-Nummulitic Banks in the Gabes-Tripolitania Basin of Offshore Tunisia

N. Y. Abraham

Nummulitic limestones of the lower Eocene Metlaoui Formation form an important productive trend in the north African Offshore Gabes-Tripolitania basin. Coquinalike banks of Nummulites constitute the prospective reservoir bodies. These may be stratigraphically trapped and sourced by adjacent off-bank facies. Nummulitic banks differ from other bioherms because their development is controlled more by hydrodynamic processes than by a strict biologic "growth." Bathymetric highs in the lower Eocene outer platform are considered a prerequisite for bank development. Field evidence suggests the prospective bank reservoir actually comprises a number of stacked packstone-grainstone lobes (up to 30 m thick). Rimming the bank and platform edge is the forebank lithofacies of fine (silt size) nummulitic skeletal debris. This lateral transition of bank to forebank is fairly abrupt, and the transition from forebank to the basinal pelagic-foraminiferal (Globigerina) marls is more gradational. The highly oil-prone and organic-rich character of these marls can be attributed to semirestricted depositional conditions prevalent in the lower Eocene basin.

In 1987, Marathon drilled a well on the Metlaoui trend to evaluate a specific seismic anomaly. Detailed microfacies and biostratigraphic analyses of the well and subsequent seismic and stratigraphic evaluations suggest it was drilled in a basinal reentrant of the outer platform. An exploration model has been developed to pursue the nummulitic bank play based upon (1) a study of available trend wells, (2) a review of seismic data on the Ashtart field and other discoveries, and (3) geologic field studies. Seismic evaluations have focused on delineating bankstratigraphic anomalies along the more prospective Metlaoui outer platform edge through isochronal relationships of the Metlaoui and adjacent sequences as well as seismic and stratigraphic analysis of amplitude variation and onlap/offlap relationships.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.