The North Ras Kanayis Offshore block in the underexplored Herodotus Basin (East Mediterranean)

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The great Eastern Mediterranean exploration game continues! The big development in early 2021 is the award of the North Ras Kanayis Offshore (NRKO) block on the southern margin of the Herodotus Basin to a consortium operated by Total and including Shell, KUFPEC and Tharwa. The commitment work programme is reported as consisting of a 3D seismic acquisition programme (www.total.com/media/news/press-releases/total-enters-new-operated-exploration-permit-Egypt).

This area is included in Strata GeoResearch’s recently completed review of carbonate-hosted petroleum systems in and around the Mediterranean and so we thought a brief summary of the area in which this block lies might be of interest to Strata’s friends. Clearly carbonates are not the only exploration objective in this area, given its location just to the west of the Nile Delta with its prolific Tertiary clastic plays, but carbonates are what get us out of bed in the morning so that’s what we’ll be focusing on here.

One of the things that is exciting about this under-explored frontier area is the potential for the emerging petroleum systems of the deepwater Eastern Mediterranean to overlap with the much maturer petroleum systems of the onshore Western Desert of Egypt (Figure 1).

Figure 1: Late Jurassic gross depositional environment map showing: (A) proven and possible polygons for thermogenic Triassic-Jurassic petroleum systems of the Egypt and the Levant Margin; and (B) proven and possible polygons for Tertiary biogenic petroleum systems (Strata GeoResearch, 2020). Wells highlighted in yellow encountered good to very good quality oil and gas prone source rocks of early Jurassic and early Cretaceous age and/ or oils interpreted to have been generated from potential Jurassic or Cretaceous source rocks (Wever,2000; Shaaban et al, 2006).

The former have, to date, been mainly explored in the Levant Basin and on the flanks of the Eratosthenes seamount and have produced major biogenic gas discoveries in both Tertiary deepwater clastics (eg Tamar and Leviathan) and in Cretaceous platform carbonates (eg the Zohr field).

The latter is characterized by polyphase intracratonic rift basins that have been explored since the late 1960s, mainly targeting Jurassic and Cretaceous paralic and shallow marine clastic reservoirs, but with secondary shallow marine carbonate reservoirs (Dolson et al, 2014). This area continues to be the focus of significant, mainly infrastructure led, exploration activity, see for example: www.energy-pedia.com/news/egypt/eni-announces-new-oil-discovery-and-new-production-in-the-western-desert-of-egypt-181517.

The NW Egyptian shelf formed as a transform margin during NW-SE oriented extension associated with the opening of this sector of Neotethys (Jagger et al, 2020). Initial continental rifting during the Upper Triassic to Mid Jurassic resulted in formation of the Levant and Herodotus Basins, whilst subsequent continental breakup resulted in the emplacement of oceanic crust in the Herodotus Abyssal Plain from the Mid Jurassic onwards. As is typical for transform margins, the NW Egyptian margin is characterized by a narrow continental shelf and a steep, narrow slope across which there is a sharp transition to the oceanic crust under the Herodotus Abyssal Plain. A large post-rift, land-attached, mixed carbonate-clastic shelf or platform developed with the platform margin interpreted to be located over the crustal-scale faults that define the rift shoulders, some 50 km offshore, running approximately parallel to the present day coastline (Tassy et al, 2015). To the south of the margin, facies belts become increasingly clastic-dominated as the influence of the continental African craton increases, as is well documented in the Western Desert, whilst to the north a steep slope system appears to connect the carbonate platform system to the deep water deposits of the Herodotus Basin (Figure 1 and Dolson, 2014). During the Jurassic, the platform system appears to be largely aggradational, with a significant backstepping event occurring prior to the Cretaceous margin becoming establishing some 25 km landward of the Jurassic margin (Figure 2 and Tassy et al, 2015)

The NRKO block is located on the platform margin-to-slope transition on the outer edge of the NW Egyptian shelf facing the Herodotus Basin and it appears probable that the block contains both the Jurassic and Cretaceous margins (Figure 1). Based on the limited public domain data available form this area, combined with much better documented systems elsewhere in the Mediterranean, from the Levant Margin in the east to Italy and Tunisia in the west, we can speculate about the nature of the carbonate petroleum systems that might be exploration targets in the area.

Prolific biogenic petroleum systems are well documented in the deepwater Levant basin sourced from thick, Tertiary, distal clastic sequences containing relatively low quantities of land-derived Type III kerogen (Perez Drago et al, 2020). Similar source rocks are likely to be developed in the Herodotus Basin and have the potential to charge carbonate reservoirs where the Tertiary clastics onlap the underlying carbonate sequence in the platform margin and slope region, as at Zohr on the flanks of the Eratosthenes Seamount to the north (Figure 1B and Figure 2).

A number of potential thermogenic petroleum systems also could charge carbonate reservoirs developed along the NW Egyptian margin. Here we’ll focus on the Triassic and Jurassic, but a number of younger possibilities also exist. In the Western Desert, the Mid Jurassic mixed clastic carbonate paralic source rocks of the Khatatba Formation are well documented: these source rocks were deposited in the interior of the land-attached platform to the south of the carbonate-dominated outer shelf and margin belt within which the NRKO block is located. The richest source intervals occur in coals and estuarine lagoons and contain a mix of Type III and Type II kerogens (Dolson et al, 2014), resulting in expulsion of both gas and oil. In general, this interval becomes leaner to the north towards the more open, carbonate-dominated shelf environments.

Maturation is controlled by tectonic subsidence and is only achieved in graben centres, with local migration charging reservoirs developed on the highs flanking the graben, as for example in the case of the Mid Cretaceous lagoonal dolomite reservoirs of the Alamein Dolomite (Abdine et al, 1993) in a number of fields in the Alamein Basin just to the south-east of the NRKO Block. This combination of depositional controls on source quality and structural controls on maturation results in a complex and localized charge distribution but charge from the south from source rocks developed within the platform/ shelf interior is certainly a model worth considering for prospects developed along the margin in the NRKO block and analogous positions.

A number of more speculative but potentially prolific thermogenic source rocks developed in more basinal areas could also charge Mesozoic carbonate prospects along the margin of the NW Egyptian shelf. Mid to Upper Triassic sabkha and evaporitic lagoonal carbonate source rocks are well documented further east on the Levant margin (Figure 1A) and could potentially be developed in the early syn-rift fill of half grabens developed in the Herodotus Basin. Lower and Mid Jurassic shelfal and deepwater carbonate or mixed carbonate clastic source rocks deposited as a consequence of the drowning of extensive Triassic and Lower Jurassic carbonate platforms and shelves are widely documented in the Mediterranean from the Levant margin to Italy and such source intervals could also be developed in the half graben of the Herodotus Basin. Most of these source rocks are likely to be associated with local anoxia arising either from local paleo-oceanographic circulation patterns or in tectonically restricted basins, but more regional source deposition could be associated with the Toarcian Ocean Anoxic Event. Organic rich deposits associated with this event are widely documented across the region (Farrimond et al, 1989). The combination of complex paleobathymetry associated with rift development and ocean anoxia can produce spectacularly rich source intervals, as is the case with the Toarcian Posidonia shales of the Ionian Basin (Rigakis & Karakitsios, 1998) and in northern and central Tunisia (Soua, 2014).

As the brief summary above shows, a wide range of different petroleum systems could be developed along the NW Egyptian margin. Similarly, regional analogues suggest that a wide range of different reservoir types could be developed associated with the margins of both the Jurassic and the Cretaceous platform systems, including Cretaceous rudist banks, Jurassic coral reefs, oolitic shoals, lagoonal dolomites and slope carbonates. However, that’s a story for another day…

Figure 2: SW-NE offshore seismic profile showing the seismo-stratigraphic architecture of the Jurassic mixed shelf edge and slope along the NW Egyptian continental margin. The seismic interpretation is calibrated by 3 industrial onshore located between 15km and 30km south of the platform margin (modified after Tassy at al, 2015).

If you’re looking for training in evaluating carbonate petroleum systems or need to put your opportunity into its regional context or are looking for analogues, then get in touch with us – we’d be pleased to discuss how we can help you!!

References

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• Dolson, J.C., Atta, M., Blanchard, D., Sehim, A., Villinski, J., Loutit, T., Romine, K., 2014. Egypt’s future petroleum resources: A revised look into the 21st century, in: Marlow, L., Kendall, C., Yose, L. (Eds.), Memoir 106: Petroleum Systems of the Tethyan Region. AAPG Memoir 106, pp. 143–178.

• Jagger, L. J., Bevan, T. G., & McClay, K. R. (2020). Tectono-stratigraphic evolution of the SE Mediterranean passive margin, offshore Egypt and Libya. Geological Society, London, Special Publications, 476(1), 365-401.

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• Shaaban, F., Lutz, R., Littke, R., Bueker, C., & Odisho, K. (2006). Source‐rock evaluation and basin modelling in NE Egypt (NE Nile delta and Northern Sinai). Journal of Petroleum Geology, 29(2), 103-124.

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• Strata GeoResearch (2020). Carbonate Petroleum Systems and Isolated Carbonate Platforms of the Mediterranean. Geological features, seismic expression, hydrocarbon occurrence and potential. Multiclient Report

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