CONTROLS ON THE DISTRIBUTION OF NUMMULITE FACIES: A CASE STUDY FROM THE LATE YPRESIAN EL GARIA FORMATION (KESRA PLATEAU, CENTRAL TUNISIA)

Abstract

Eocene nummulite deposits along the southern Tethys margin locally constitute important hydrocarbon reservoir rocks. In order to understand the heterogeneity of these complex reservoirs, we have carried out a detailed field study of the nummulite limestones which crop out at the Kesra Plateau in Central Tunisia. The main contributions of this paper can be summarised as follows: Various species of Nummulites and planktonic foraminifera were identified in late Ypresian carbonates in the Kesra area and provided accurate biostratigraphic ages. Nummulitic limestones occur in the Chouabine and El Garia Formations representing deposition over a period of about 2Ma. Six depositional facies have been identified in the El Garia Formation, ranging from high‐energy nummulitic grainstones to laterally‐equivalent lower‐energy nummulithoclastic accumulations. Variations in nummulite morphology (size and shape) were used to distinguish a number of subfacies which correspond to different depositional environments. We provide evidence that nummulite accumulations were locally sub‐aerially exposed. A 3D reconstruction of the Kesra Plateau outcrop identified two stacked nummulite bodies, deposited under high‐energy conditions, which pass laterally into two isolated bodies in more distal parts of the platform. Regional‐scale observations indicate that the geometry of the late Ypresian nummulite platform was strongly influenced by the presence of the Oued Bahloul anticline structure, which formed a major environmental barrier between open‐marine and more restricted settings. Facies distribution in the Kesra area was strongly controlled by the inherited topography, which reflects the reactivation of NW‐SE trending faults during the Late Cretaceous and Paleocene. A tentative 3D facies model illustrates the relationship between facies and fault distributions in the Kesra area. The facies map shows that the best reservoir potential is associated with palaeo‐highs, and consists of nummulite limestones deposited in very shallow waters (a few metres) under high‐energy conditions.