M. A. El-Ghamri1, I.C. Warburton2 and S.D. Burley2&3

1Geology Dept., Faculty of Science, Alexandria University, Egypt. email: m_elghamri@yahoo.com

*2Exploration and Development Geoscience, BG Group, UK. email: ian.warburton@bg-group.com.

3Basin Dynamics Research Group, University of Keele, UK. email: stuart.burley@bg-group.com

Source rocks on-structure in the October field are not sufficiently mature to have expelled hydrocarbons at the present day. 1-D modelling of a pseudo-well in the deep October South Trough demonstrates that the Eocene Thebes and Senonian Sudr Formations are presently mature at depths greater than 3km. However, only the carbonate-rich Sudr Formation has generated sufficient hydrocarbons for expulsion to take place at depths shallower than 3.5km, suggesting that reservoired oil in the October field originates predominantly from source rocks of this unit. Model results indicate that hydrocarbon expulsion from this source may have commenced as early as 20Ma in kitchen depocentres adjacent to the October field, and continued through the Pliocene.

Migration routes predicted from 2-D modelling are complex and require downward expulsion from the Sudr Formation east and west of the October field to charge the Palaeozoic Nubian Sandstone. Mature Sudr Formation in the October South Trough first charges the Nubian and then te lower Senonian Matulla Formation reservoirs in the NW compartments of the October field via flow across the NW bounding fault. The mature Sudr Formation in the Baba Trough to the SE charged the Matulla Sandstones on the eastern flanks of the October field which then back-filled to spill into the Nubian Sandstones. Hydrocarbons generated from the Thebes Formation contribute only minor volumes to the predicted accumulations.

The presence of trap-bounding faults and their sealing properties is shown to be a key sensitivity for charging the October field and related structures. Fault seal is required to charge known accumulations and match known hydrocarbon column heights, but extending shale smear into the Nubian Sandstone prevents the crestal October field structure from charging.

The Matulla Formation sandstones are a regional migration fairway providing charge for the Ras Budran field and Nubian Sandstone prospects to the west of the October field. Younger Mesozoic sandstones are charged by vertical migration through seals and along faults. This regional migration sequence as described for the October field is likely to apply to other tilted fault-block structures in the Gulf of Suez and provides a regional predictive exploration tool. Back filling and spilling beneath sealing faults is likely to be the characteristic charging mechanism for the Nubian Sandstones in this setting.

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