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.

Microfacies Analysis of Upper Eocene Shallow-water Carbonates from the Rodnei Mountains (N Romania)

“…Assuming that the same ecological conditions are valid for fossil nummulitids, the presence of large numbers of tests concentrated in distinct levels throughout the sandstone could be the result of episodic remobilization of the highly buoyant bioclasts by waves or marine currents (Jorry et al, 2003). This is also supported by the monospecific character of the deposits, the presence of imbricate structures within densely packed nummulitic shoals, and the lack of encrustations, the latter suggesting continuous movement (Racey, 2001).…”

Hydrodynamic behaviour of Nummulites: implications for depositional models

“…Aigner (1982) pointed out the presence of "reminiscent cross-bedding stratification," small-scale scours and fill structures and Nummulites imbrications in the Gizehensis bed of Egypt. Jorry et al (2003b) mentioned the presence of large-scale cross bedding in the Eocene El Garia Formation in central Tunisia. However, most of the time, grain-supported facies show a rather chaotic pattern and no obvious sedimentary structures.…”

“…According to Loucks et al (1998), these deposits are not high-profile or relief banks (they have no steep slopes or landward dipping strata), and they formed in moderately low-energy environments between fair-weather and storm wave base. More recently, it has been recognised that Nummulites occur reworked in high-energy environments such as shoals, forereef-channels and storm deposits (Racey 2001;Hasler and Davaud 2001;Jorry et al 2003b;Jorry 2004). From a sedimentological point of view, nummulite-rich facies may result from three distinct processes: (1) the representation of the undisturbed record of a prolific biocoenosis, (2) the accumulation of tests transported by wave-or tide-induced currents and (3) the residual concentration of tests after repeated winnowing events (Aigner 1985).…”

“…• Shoals formed in proximal up-ramp settings ). This facies, showing large-scale cross-bedding structures, can be observed in central Tunisia (Juggurta and Kesra Plateau; Jorry et al 2003b;Jorry 2004); • Nummulites "bars" developed in very shallow environments in front of coralgal reefs bordering a carbonate ramp system (Eichenseer and Luterbacher 1992). This Depending on the model, Nummulites-rich sediments are considered to be autochthonous deposits (biocoenoses) or para-autochthonous to allochthonous deposits (taphocoenoses), resulting from landward or seaward transportation.…”

“…In the fossil record, several authors consider the shape and wall thickness of foraminiferal tests as a depth indicator (Kulka 1985;Eichenseer and Luterbacher 1992;Loucks et al 1998;Racey 2001;Jorry et al 2003b;Vennin et al 2003;Hasler 2004;Jorry 2004). This assumption is based on observations of LBF from present-day environments.…”