The Eocene Ramp Complex of Al Jabal al Akhdar, Cyrenaica, NE Libya: A Surface Analogue for Nummulite Reservoirs

By

Ahmed S. El Hawat¹, Bruno Caline², Stephan Jorry³, Eric Davaud³

(1) Garyounis University, Benghazi, Libya (2) Totalfinaelf Exploration & Production, Pau, France (3) Dept. of Geology, University of Geneva, Geneva, Switzerland

 The well-exposed and continuous outcrops of Al Jabal al Akhdar, Cyrenaica, are appropriate to study the nummulite body’s geometry, which form major hydrocarbon reservoirs in the offshore of Libya and Tunisia. The Eocene succession forms a shoaling up megasequence up to 600 m thick. It consists of interbedded mudstone and porous chalk-dominated, Apollonia Formation, which grades upward and laterally up the regional structural axis into porous nummulite-rich facies of Dernah Formation. These represent outer, and middle-inner ramp facies belts respectively. Sedimentation of the ramp complex was controlled by the pre-depositional inversion configuration of Al Jabal al Akhdar anticlinorium. It influenced the regional facies belts width, their thickness, and the change of the mid-ramp facies from nummulite dominance west of the area to coralgal-reefal dominance to the east. Syndepositional reactivation of the structures also, triggered mass transport of the nummulite deposits into deeper waters, and controlled the accumulation, geometry and internal heterogeneities of the nummulite bodies. Since the nummulite bodies were accumulated in an essentially mud-rich, low-energy environment, reworking by waves and currents led to improvement of the grain-mud ratio in upward shallowing sequences. Meanwhile, the outer-ramp transport of lime-mud, enhanced by mass-movement of mid-ramp deposits produced seaward prograding clinoforms on which the nummulite facies and associated inner-ramp shoals advanced. Below sequence boundaries, eustatic and tectonic controls were not only necessary in improvement of the grain-mud ratio of the nummulite bodies, but also led to enhancement of the rock pore network by subaerial meteoric water diagenesis over structural highs. Deepening events led to transgression of operculina-rich mudstone facies up to the nummulite platform. Four depositional ramp models are characteristic of the Cyrenaican inversion setting. These depend on facies-type dominance, ramp slope angle and depositional energy distribution in relation to structural orientation.