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Fabio Lottaroli:
The stratigraphic framework of the Plio/Pleistocene sequence of the offshore eastern Nile Delta (eastern Mediterranean-Egypt)
(Talk)

In these last few years, several wells were aimed towards gas-bearing sands within the Plio/Pleistocene sequence of the Mediterranean offshore of the Nile Delta region (Egypt). This exploration stage started at the beginning of the '90s and took advantage of newly-acquired 3-D seismic surveys and a more-effective seismic interpretation approach (Barsoum et al., 1998).

Following this renewed explorative interest, the need for a stratigraphic framework aimed to support well correlations and geological modelling became of greatest importance.

In the studied area (Figure 1), the Plio/Pleistocene succession is made up of shelfal and slope to basinal deposits arranged as a thick siliciclastic wedge (locally up to 4000m). Extensive growth-faulting, acting during the Plio/Pleistocene, is a key point controlling the sedimentary evolution of this sequence. Listric faults, mainly NW-SE-oriented, are generally based on the Messinian evaporites. Great tectonic complexity, coupled with such a dynamic depositional setting, make it often impossible to correlate different tectonically-bounded sectors without reliable reference time-lines.

The palaeontological record is often scanty within the Plio/Pleistocene. Planktonic components are rare within foraminiferal assemblages. Though the assemblages are often poorly diversified, calcareous nannofossils prove an effective tool, at least in identifying some biostratigraphic horizons of regional value. The main emphasis was put on last occurrences (LO), since the biostratigraphic work was carried out on ditch-cuttings only. Total abundance variations, as well as abundance peaks of certain genera (Helicosphaera spp., Discoaster spp.) were checked to assess their reliability for local correlation. The Rio et al. (1990) biozonation was chosen as a reference, and successfully applied.

A sector displaying a relatively complete section was identified southward. In this area, a mainly silty/shaley Early Pliocene overlies the Messinian evaporites. The Early Pliocene generally lacks the lowermost part (MNN12 and MNN13 p.p. Biozones), and is fairly rich in nannofossils. The top of the Early Pliocene has been approximated to the top of MNN14-15, since G. puncticulata is generally missing. On the contrary, the LO of Sphenolithus spp., together with the LO of Reticulofenestra pseudoumbilicus, generally coinciding with a relative increase of nannofossil abundance, is one of the most reliable time-lines. The first occurrence (FO) of Pseudoemiliania lacunosa within MNN14-15 proved a useful marker, also working with cuttings, while we found Helicosphaera sellii to be extremely rare in the lowermost part of its stratigraphic range.

Within the Middle Pliocene succession, locally highly sandy, the top of MNN16b/17 (LO of Discoaster surculus, D. pentaradiatus) is always well marked. Generally it is underlined by a relative increase of the total abundance, and by a peak, of Helicosphaera carteri. Discoasters are generally extremely rare within our record, with the exception of D. surculus and D. asymmetricus, even if the latter was found only within MNN14 -15.

The Late Pliocene is always sand-dominated, and some wells were located on the shelf through this time-interval. Nannofossil occurrences are extremely discontinuous between top MNN16-17 and the base of the Pleistocene (FO of Gephyrocapsa n.s.).

The Pleistocene is characterised by great variations in facies and thickness. Nannofossil abundance is hence controlled by several factors. Anyway, the different Pleistocene successions are well-characterised through nannofossil biostratigraphy. Generally, the top of MNN19d (LO of H. sellii) was the youngest reliable biohorizon in the studied wells. Nannofossil indications were of basic importance for the identification of a sector where syntectonic subbasins were rapidly filled by immature Pleistocene turbiditic flows at a sedimentation rate exceeding 3000m/my.

The biostratigraphic information was finally applied to 3-D seismic lines in order to extrapolate them to undrilled sectors and to improve the regional correlation framework. This integration work formed the basic geologic framework for any further modelling activity. Moreover biostratigraphy gave precise time-constraints to the occurrence of gas-bearing sands and to the growth-faulting activity.

References

Barsoum, K., Aiolfi, C., Dalla, S. & Kamal, M. 1998. Evolution and hydrocarbon occurrence in the Plio-Pleistocene succession of the Egyptian Mediterranean margin: examples from the Nile Delta Basin. Proc. 14th EGCP Conference, Cairo, Egypt, October, 1998.

Rio, D., Raffi, I. & Villa, G. 1990. Pliocene-Pleistocene calcareous nannofossil distribution patterns in the western Mediterranean. Proc. ODP, Sci. Res., 107: 513-533.

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