Lower Miocene Nukhul Formation, Gebel el Zeit, Egypt: Model for structural control on early synrift strata and reservoirs, Gulf of Suez

Abstract

The Aquitanian-early Burdigalian (lower Miocene) Nukhul Formation at Gebel el Zeit, Egypt, was deposited during early stages of Gulf of Suez rifting. The unit dips 8-15° less than underlying prerift strata, indicating that 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 differential movement of small fault-bounded blocks. The lower clastic unit at South Gebel el Zeit contains poorly sorted, conglomeratic, marly sandstone 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 sand and gravel southwestward to a half-graben basin. In contrast, an inferred correlative, thin, basal conglomeratic unit in North Gebel el Zeit was deposited in a shallow-marine setting. The presence of basement clasts in Nukhul strata indicates early syndepositional uplift due to structural tilting. The upper carbonate unit consists of bioclast, peloid, and intraclast packstone, 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 submerged blocks were the site of carbonate resedimentation or deeper shelf deposition. 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 synrift cross faults and fault block by fault block facies analysis. Robert Winn is a consulting geologist. He received a Ph.D. from the University of Wisconsin-Madison in 1975 and joined Marathon Oil Company as a research geologist in 1977. He left Marathon in 1994 for the University of Papua New Guinea, where he was senior lecturer and then associate professor. He was head of the Geology Department from 1996 to 2000. His primary interests are in clastic sedimentology and sequence stratigraphy.Paul Crevello is a consulting geologist and technical director of PetrexAsia, which he formed in 1997. He received an M.S. degree from the University of Miami (1978) and a Ph.D. from Colorado School of Mines (1989). He was employed by Marathon Oil as a research geologist from 1978 to 1994 and by the University of Brunei as senior lecturer from 1994 to 1997. His specialties are in sequence stratigraphy and sedimentology of carbonate and turbidite systems and the integrated calibration of 3-D reservoir models. William Bosworth is employed by Marathon Oil Company. He joined Marathon in 1984 and has worked principally on international exploration and development projects. From 1980 to 1984 he taught structural geology and tectonics at Colgate University in Hamilton, New York. His research interests are principally in extensional tectonics, continental stress field evolution, and the paleogeodynamics of the African plate.