London July Evening Lecture

Recent work by a multi-disciplinary team has led to a better understanding of the prospectivity of the North Red Sea. A sequence of thick halite with interbedded evaporite and clastics poses seismic imaging challenges. Recent reprocessing and newly acquired seismic data has imaged the prospective prerift section. New petroleum systems modelling of rift timing, crustal thinning and source rock temperature through time indicate oil generation in the inboard section of North Red Sea - Block1. This agrees with hydrocarbon shows in the drilled offshore wells which can be typed to pre-rift source rocks. Extensive onshore fieldwork and mapping of sediment input points, fault orientations and fault linkages has helped to develop an understanding of the expected controls on syn-rift sandstone and carbonate deposition offshore. Although the Red Sea has experienced additional extension to the Gulf of Suez since the activation of the Aqaba-Jordan transform, all the key elements of the Gulf of Suez petroleum system exist in the North Red Sea. An integrated exploration approach in this paper has enabled lead areas in the North Red Sea – Block 1 to be high-graded. Technical work is ongoing and Hess plans to make a drilling decision on two wells in the fourth quarter of this year.

Introduction
The North Red Sea – Block 1 concession is located in the North-West of the Red Sea, near the mouth of the Gulf of Suez and covers ~9,450km2. Esso held the block from the mid 1970s through to 1984. They acquired 2D seismic and drilled five unsuccessful wells between 1976 and 1981. Phillips also drilled two dry wells to the south of the block in 1977. Failure of these wells was largely attributed to the difficulties in imaging and drilling valid structures on poor quality seismic, which suffered from serious multiple contamination and poor subsalt illumination. BG acquired the block in 1995, acquired 1,450km2 of 3D seismic in 1999 and then relinquished in 2000. BP took the block in 2004, then farmed out to Hess who shot 2,000km2 of 3D seismic in 2006 and a further 750km2 in 2008.

Stratigraphy and Tectonic Framework
Prior to the development of the Red Sea rift system this part of North Africa was located on the southern margin of the Tethys Ocean. An extensive passive margin succession developed showing an overall transgressive character, culminating with extensive carbonate deposition in the Late Cretaceous and Eocene. This passive margin megasequence forms the pre-rift stratigraphy of the Red Sea and Gulf of Suez. This pre-rift sequence contains the primary reservoir target of the Nubian sandstone as well as the prolific Brown Limestone/Dakhla source rock. Onshore exposure of the pre-rift shows depositional thickness variations along old structural trends, as well as some erosion from footwall highs exposed during the Miocene. South of Quseir the pre-rift section is not exposed onshore and is postulated to be absent offshore due to non-deposition and/or erosion. These onshore observations significantly upgrade the exploration potential of the pre-rift in the Northern Red Sea relative to the central area south of Quseir.

A wide variety of fault orientations are mapped onshore and can be interpreted on seismic. These can be grouped as:
The Pre-Cambrian – ESE-WNW Najd shear zone group of sinistral transform faults [Polis et al., 2005]
The SE-NW Clysmic trend faults from Gulf of Suez rift extension - 24-15Ma [Patton et al., 1994; Bosworth and Burke, 2004]
Gulf of Aqaba parallel NNE-SSW structures due to the oblique spreading - 15- 0Ma [Patton et al., 1994, Ripple 2001]

Source
The Cretaceous age Dakhla formation, a limestone facies with an organic-rich base, overlies the Duwi regionally. Dakhla total organic content (TOC) data from the onshore as well as regional studies indicates a world class source rock facies present across much of Egypt. Hydrogen index (HI) values for the Dakhla range from 250 to 500. Estimated average source rock thickness is ~30m, similar to that of the Brown Limestone in the Gulf of Suez. Petroleum systems modelling suggests peak oil generation occurred at 17Ma.

Reservoir and Seal
The Cretaceous and older Nubian sands are the principal reservoir interval within the pre-rift. The Nubia represents the initial clastic dominated succession of the Tethyan passive margin sequence deposited immediately upon basement and was deposited in a continental to shallow marine setting. The Nubian section thins southwards through the Gulf of Suez and is locally absent across the Ras Mohammed Arch at the southern tip of the Sinai Peninsula. South of this feature, the Nubia thickens dramatically as seen in outcrop at Gebel Duwi, located onshore of the North Red Sea Block 1, where in excess of 500metres of Nubia is present. At outcrop the Nubian clastics thin due to erosion on footwall highs. Pressure seals are present in the lower Miocene, as well as an ultimate seal at the evaporite layers of the South Gharib and Zeit.

Seismic Acquisition and Processing
The new seismic data was shot in 2008 with deeper streamers and sources to help maximise low frequency signal penetration of the thick evaporite section. A 3D surface related multiple elimination process was used to reduce the impact of multiples from the rugose seabed, Top Zeit and numerous interbed multiples from within the Zeit evaporite sequence. PDSM model building with tomography resulted in a laterally varying interval velocity field within the evaporite sequence. Overall, this has resulted in a better subsalt image.

Seismic Mapping
Although the imaging of the pre- and syn-rift sequences has markedly improved from the original time-migrated data, the current 3D data quality still does not lend itself to an intricate sequence-stratigraphic or AVO/amplitude mapping approach. Rather, the top and base of gross tectonic sequences have been mapped, such as the top evaporites (Top Zeit), the base evaporites (base South Gharib), the rift-onset unconformity and the top of igneous basement. Mapping at these levels combined with detailed structural interpretation and geological models from the onshore has identified prospectivity at both syn-rift and pre-rift levels.

Gravity / Magnetics
Aeromagnetic coverage provides a link between the outcrops of the Red Sea Hills, offshore well penetrations and the seismic data offshore. Ship based magnetic and gravity data were collected during the 2008 3D acquisition. The integration of both gravity and magnetic data with onshore mapping and offshore seismic helps constrain seismic interpretation. 2D gravity and magnetic profile modelling has been particularly beneficial over lead areas.

Summary
The western margin of the North Red Sea is thought to be a continuation of the Gulf of Suez. The primary play is the pre-rift Dakhla source rock charging pre-rift Nubian sands in structural traps set up by extension, such as tilted fault blocks. Onshore the basis for a working petroleum system can be demonstrated and oil shows in the wells offshore North Red Sea have been typed to pre-rift Dakhla source rock. This confirms that the pre-rift play extends into the offshore region. There is also a secondary play that exists with a syn-rift source rock and both clastic and carbonate potential reservoirs. Improved seismic imaging has de-risked these plays. An integrated exploration approach has enabled lead areas to be high-graded. Hess plan to make a drilling decision in Q4 2009 for 2 wells to be drilled Q1 2011.

Acknowledgements
The authors would like to thank EGPC for their permission to present this paper. Surface geological map courtesy of Dr. Samir Khalil, Suez Canal University, Egypt.

Special thanks for the contributions made by Dave Peel, Dr Paul Whitehouse and Dr Ahmed Barkooky.