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ABSTRACT

The establishment of a robust, paleontologically defined chronostratigraphic framework coupled with detailed facies analysis of outcrops resulted in the development of a sequence stratigraphic model for the Miocene synrift of the Gulf of Suez. Application of this model, along with 3-D seismic, has had a major impact on the ability to recognize stratigraphic and subtle combination traps within this mature basin.

Graphic correlation of paleontological data from wells and outcrops reveals that the Neogene section consists of at least eight biostratigraphic sequences (S10-S80) separated by graphic terraces (T00-T70) or geologic lacunae. Outcrop analysis of terraces T00 to T30 and their associated fossil assemblages indicates that they represent either regional regressive or transgressive events. The number of depositional sequences (those bounded by regressive erosional surfaces) is therefore less than the number of paleontologically defined sequences. Terraces T00 and T20 are sequence boundaries, and subsurface evidence suggests that T40 is a condensed interval and that T50 is an erosional unconformity (sequence boundary). Field observations of T10 at Wadi Thal in the Sinai indicates that it consists of at least two ravinements and a condensed section within a narrow stratigraphic interval. Similarly, the T30 lacuna associated with the Markha Anhydrite at Wadi Feiran is composed of several stacked flooding and regressive surfaces. These surfaces at both Wadi Thal and Wadi Feiran represent minor time breaks. These small lacunae cannot be individually resolved by graphic correlation, but their sum total within a thin rock (hiatal) interval is detectable. Despite limitations in resolution, graphic correlation of paleontological data was crucial for recognizing key surfaces and intervals that allowed us to decipher the sequence stratigraphy of the Miocene synrift section of the Gulf of Suez.

A regional analysis of the Miocene outcrops exposed along the Sinai margin of the Gulf of Suez within this constrained chronostratigraphic framework resulted in a depositional model for these strata that integrates tectonic history and sedimentary response during rift initiation and clysmic phases of rift basin evolution. The rift initiation phase is recorded by deposition of the Nukhul Formation. Nukhul depositional facies include: continental alluvial valley fill, estuarine, tidal flat, tidal channel complexes, and shallow off-shore marine. The clysmic stage of rifting is recorded by deposition of relatively deep marine mudstones, basin-floor fan sandstones, and footwall-margin conglomeratic-talus cone and fan delta deposits of the Mheiherrat Formation. The later stages of the clysmic rift were documented in the channelized submarine fan, offshore marine, deltaic, lacustrine, and hypersaline lagoon/sabkha deposits of the Hawara, Asl, and Ayn Musa formations. The cessation of active rifting is recorded by open marine mudstones and delta front deposits of the Lagia and Ras Budran members of the Ayun Musa Formation.

The depositional history of the Suez Rift from the Aquitanian through Langhian stages of the Miocene began with northerly flowing fluvial systems occupying the down-thrown portions of asymmetrical half grabens. Increased subsidence resulted in a relative sea level rise that flooded the half grabens forming elongate estuaries and ultimately shallow open-marine environments. Uplift of the rift shoulders and basin subsidence during the clysmic stage resulted in relatively deep marine conditions within the rift and initially sediment starvation. As sediment supply readapted to the new topography submarine fans were deposited on the basin floor. Continued extension resulted in crustal thinning and an isostatic uplift of the rift, shallowing of the detachment depth and increased rotation of the tilted fault blocks. Shallower water open and marginal marine deposits prograded into and filled in the more subtle rift.

Within the subsurface of the Gulf of Suez, seismic data is generally poor because of energy attenuation by shallow evaporites, multiple reflections, and complex structure. These conditions make traditional seismic sequence stratigraphy techniques difficult to apply. However, the tectono-sequence stratigraphic model developed from outcrops and the paleontologically-defined chronostratigraphic framework provides tools that allow for better subsurface correlations by systematically mapping stratal geometries using sequence boundaries and flooding surfaces defined by high-resolution biostratigraphic data. The stratigraphic picture that emerges from application of these concepts creates a profound change in quantification of fault throws and recognition of stratigraphic and combination traps. In addition to revealing new plays, application of the tectono-stratigraphic model also results in a better understanding of reservoir geometry and distribution.

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