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

Forty eight high resolution stratigraphic sections and more than 850 thin sections were used to study the Eocene of Cyrenaica. Sections are located on the northern limb of Al Jabal al Akhdar inversion anticlinorium that formed a series of submerged Cretaceous structural highs and islands. These played an important role in the development of the Eocene nummulitic ramp complex. Evolved regionally parallel facies belts consisted of outer ramp mudstone clinoforms and mass transported facies, mid-ramp nummulite facies and inner ramp miliolid-orbitolites, coral-coralgal and skeletal facies respectively. Progradation of these belts led to an upward shallowing mega-sequence constituting Apollonia and Dernah Formations. It consisted of three correlatable nummulite dominated sequences in Ras al Hilal area. The top two of which pass eastward into structurally controlled reefal-coralgal-discocyclinid dominated facies. Reworking of nummulite bodies by waves and currents over structural highs led to improvement of primary porosity. Syndepositional reactivation of the structures not only triggered mass transport of the mid-ramp deposits, but also controlled nummulite body geometry, internal heterogeneities and diagenesis by spatially shifting high-energy and subaerial exposure zones through time. There no single nummulite ramp model that would characterize this active inversion setting, as depositional parameters changes in space and time.