Lower miocene Nukhul formation, Gebel el Zeit, Egypt: Model for structuralcontrol on early synrift strata and reservoirs, Gulf of Suez

The Aquitanian-early Burdigalian Gower Miocene) Nukhul Formation at Gebel e l Zeit, Egypt, was deposited during early stages of Gulf of Suez rifting. T he unit dips 8-15 degrees less than underlying prerift strata, indicating t hat significant rotation and extension preceded subsidence of the Gebel el Zeit fault block. The Nukhul Formation at Gebel el Zeit is up to 75 m thick in outcrop and consists of a lower sandstone and an upper carbonate unit. The formation varies considerably along strike because of syndepositional d ifferential movement of small fault-bounded blocks. The lower clastic unit at South Gebel el Zeit contains poorly sorted, conglomeratic, marly sandsto ne that commonly displays grading and Bouma sequences. Beds were deposited below storm base by sediment gravity flows. Thicker intervals are inferred to fill small, structurally controlled, submarine gullies that funneled san d and gravel southwestward to a half-graben basin. In contrast, an inferred correlative, thin, basal conglomeratic unit in North Gebel el Zeit was dep osited in a shallow-marine setting. The presence of basement clasts in Nukh ul strata indicates early syndepositional uplift due to structural tilting. The upper carbonate unit consists of bioclast, peloid, and intraclast packs tone, wackestone, and grainstone with minor floatstone, rudstone, and coral -algal boundstone. Carbonate strata were deposited variously in deep-marine , low-energy peritidal and subtidal, and reefal environments. Deeper submer ged blocks were the site of carbonate resedimentation or deeper shelf depos ition. Reefs and shallow-marine bioclast shoals formed on higher submerged blocks. Nukhul strata show that synrift reservoir prediction in the Gulf of Suez, the Red Sea, and presumably in other rifts requires mapping of synri ft cross faults and fault block by fault block facies analysis.