A Review of Eocene Nummulite Accumulations: Structure, Formation and Reservoir Potential

Abstract

Eocene nummulite accumulations, also referred to as nummulite "banks", form important hydrocarbon reservoirs in Tunisia and Libya and may constitute exploration targets in other parts of North Africa, the Mediterranean and the Middle East. Porosities commonly average 10-20% and permeabilities 10-50md. Foraminifera of the genus Nummulites may comprise up to 98% of the bioclasts in these carbonate reservoirs, although only one or two species may be present. The absence of associated fauna is generally taken to indicate an oligotrophic depositional environment.In this paper, the palaeoecology of the genus Nummulites is discussed together with depositional models for two nummulitic carbonate reservoirsthe Middle Eocene SeebLimestone of Oman and the Early Eocene El GaridJdeir Formation of Tunisia and Libya. The El Garia and Seeb Limestone Formations were deposited in ramp settings, and comprise a series of amalgamated sheets or low-relief banks. In the Hasdrubal field offshore Tunisia, where the El Garia Formation constitutes the reservoir rock, most of the nummulites have been redeposited from shallow into deeper waters whilst in the Bourri field (offshore Libya) they occur as an in situ "bank". Nummulite accumulations often show evidence that both physical reworking (scouring, winnowing and imbrication) and biological processes (reproduction strategies and bioturbation) have influenced their formation. A general model is outlined for discriminating between physically and ecologically produced biofabrics, and the implications for reservoir quality are discussed.

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). …”

“…In addition to biological processes, observed associations of fossil LBF were often affected by physical processes, leading to the selective removal or concentration of different components as a function of their primary skeletal porosity which can allow for easy remobilization by waves and marine currents (Racey, 2001;Jorry et al, 2006), resulting in para-autochthonous or allochthonous deposits (Aigner, 1985).…”

Late Eocene transgressive sedimentation in the western Swiss Alps: Records of autochthonous and quasi-autochthonous biofacies on a karstic rocky shore

“…Therefore, the absence of red-algal encrustations within the Diablerets Mb seems to sustain the existence of a protected depositional setting during the Early Priabonian. Similarly, the absence of large benthic foraminifera such as nummulitids and discocyclinids, which inhabited a broad range of open marine settings, is consistent with a restricted environment (e.g., Racey, 2001). On the other hand, nummulitids and discocyclinids exhibit prolific occurrences throughout the open-marine and deeper limestones of the Pierredar Mb (Fig.…”

Paleontological, mineralogical and chemical studies of syngenetic and epigenetic Pb–Zn–Ba–P mineralizations at the stratotype of the K/P boundary (El Kef area, Tunisia)

“…The interstitial fluids percolating through the carbonates in section I were of marine phreatic origin, whereas in section II the sparry blocky cement reflects not only replacement of marine phreatic water by meteoric water but also the involvement of hydrothermal fluids which are discussed in the succeeding paragraphs. The paleoenvironmental models of Eocene assemblages described inter alia by Eichenseer and Luterbacher (1992), Loucks et al (1998), andRacey (2001) refer to a ''bank'' of larger foraminifera made up of Nummulites and Alveolina. A subtidal (to intertidal) depositional environment down to a depth of 40 m on the middle ramp with sea grass meadows was described for the calcareous lower Eocene Seeb Formation in Oman and may be a facies description also to be adopted for the lithofacies of section I (Dill et al 2007).…”