OVERBURDEN ROCK

Thickness

All post-Cretaceous rocks comprise Sudr-Nubia(!) overburden in the Gulf of Suez (Figure 6a and Figure 7a). Pre-rift Paleocene and Eocene marine carbonates and shales, also potential source rocks, are several hundred to 500 meters thick and are overlain by a syn- and post-rift section of many lithologies and depositional facies ranging to 4000 meters in thickness. Evaporite lithologies can constitute several thousand meters of overburden. Some deeper grabens in the Gulf of Suez contain as much as 7000 meters thickness of rock above the pre-Miocene section (Patton and others, 1994). A burial history chart (Figure 6a) from the central Gulf of Suez near Ramadan field shows the Sudr Formation, currently at 5.5 km (18,000 ft) depth, generating oil 6 Ma when it was at a 4.6-km (15,000 ft) depth.

The Maqna(!) total petroleum system is represented by a burial history curve from the Midyan basin in northwestern Saudi Arabia (Figure 6b). Overburden is restricted to middle Miocene and younger siliciclastics, evaporites and carbonates (Figure 7b). Oil expulsion began here at a depth of 3 km, 8 Ma for the basal gas-prone Burqan shale and 3 Ma for the oil-prone Maqna carbonate source rocks. Less overburden is required for oil generation outside the Gulf of Suez where thermal gradients are higher. Oil maturity windows as shallow as 1000 meters are documented in the western Red Sea (Barnard and others, 1992).

Thermal Gradient

Thermal gradients in the Gulf of Suez basin range from 27° to 46° C/km (1.5° to > 2.5° F/100 ft), higher to the south and at rift margins and lower in deeper grabens (Alsharhan and Salah, 1997a; Schutz, 1994). Such trends and irregularities in thermal gradient are explained by heterogeneities in the crust causing hot spots and by laterally exposed basement increasing the horizontal component of heat flow.

Throughout the remaining Red Sea, thermal gradients and heat flow increase non-uniformly southward and from the basin margins inward toward the axial rift. An anomalously hot area is in the Tokar Delta of southern Sudan (Figure 3a), trending onshore southward toward the Danakil rift near Asmara, Eritrea. Another high thermal anomaly is in a Red Sea coastal well of south central Saudi Arabia. Deep Sea Drilling Project boreholes have registered axial Red Sea thermal gradients at least as high as 219° C/km (12° F/100ft), and the highest thermal gradient recorded in a southern Red Sea oil and gas test is 76° C/km (4.2° F/100 ft). Coastal Red Sea basin locations likely had higher geothermal gradients in the past when closer to the spreading center, whereas Gulf of Suez thermal gradients are probably highest now (Feinstein and others, 1996). Regional and temporal thermal variability and the block-faulted nature of the province results in a pod-like distribution of mature source rock with different stratigraphic horizons potentially mature in different areas.

TRAP STYLE OF OIL AND GAS FIELDS

Known Traps and Their Timing

In Gulf of Suez producing fields, most traps are described as "tilted fault blocks," "faulted," or "structural" (Petroconsultants, 1996). Reservoirs within the fields have additional stratigraphic complexity, especially Tertiary syn-rift and post-rift strata, with varying lithologies and depositional facies ranging from carbonate build-ups and deep submarine clastics to terrigenous clastics (Figure 4).

Recoverable reserves in the Gulf of Suez are widely distributed stratigraphically (Table 2). Upper Cretaceous to Eocene carbonate source rocks (Figure 1 and Figure 2) were deposited uniformly over a large area during tectonic stability, as was the Paleozoic to Cretaceous Nubia Formation, the dominant pre-rift sandstone reservoir. Trap and local seal formation occurred just prior to generation of substantial hydrocarbons (Figures 6a and 6b; Figures 7a and 7b). Deposition of the regional evaporitic seal was in progress or complete at the time of oil expulsion.

Gulf of Suez rifting peaked at "Mid-Rudeis" time, 18 Ma, and most pre-rift fault-block traps were thus formed, although reduced subsidence and lesser block rotation continue locally to the present. Younger Miocene reservoirs were deposited during the next 6 or 7 m.y. with depositional processes and facies distribution impacted by local topography in fault-bounded depocenters. As regional evaporites were being deposited, basinal source rocks were maturing, and expulsion of oil was occurring by 8 Ma. Deeper or hotter sub-basins could have experienced Sudr expulsion as early as 12 or 14 Ma and would be mature for gas now.

Hydrocarbon generation from potential Jurassic source rocks also likely occurred after fault-block traps formed, particularly because rifting began first in the southern Red Sea. Shahin (1997) suggested that Jurassic sources in the northernmost Gulf of Suez expelled petroleum from 20-10 Ma. Although hotter at the southern end of the Red Sea, Jurassic source rock might be similarly mature because of lesser burial depth.

Younger Miocene source rock matured more recently in the southern Gulf of Suez, except in hotter areas where Miocene expulsion is nearly coeval with Cretaceous expulsion farther north. Most Red Sea thermal modeling from south of the Gulf of Suez shows Miocene expulsion to have begun at various times, but largely coeval with Cretaceous expulsion in the Gulf of Suez. Expulsion from youngest Pliocene potential sources in the southern Red Sea is current (Cole and others, 1995; Alsharhan and Salah, 1997a; Savoyat and others, 1989). Red Sea halokinesis occurred simultaneously during late stages of salt deposition because of the instability created by sea-floor spreading and continued subsidence. Salt-related structures are mostly less than 10 m.y. old.

Undiscovered Traps

Undiscovered Gulf of Suez traps for either the Sudr-Nubia(!) or Maqna(!) total petroleum systems are likely to be subtle block-faulted structures with stratigraphic complexity (e.g., Tertiary drapes, pinchouts and facies changes) – more difficult to delineate and less prolific than the more predictable and extensive Nubia reservoirs. Reserves might also be discovered in smaller, fault-sealed compartments within and adjacent to existing fields and associated with the limited salt diapirism at the southern end of the Gulf of Suez. Sudr-charged traps similar to those in the Gulf of Suez are expected in the sparsely drilled northwestern Red Sea along the Egyptian coast south to where organic-rich Brown/Duwi phosphates were found onshore near Quseir.

For the major part of the Maqna(!) total petroleum system outside the Gulf of Suez, similar block-faulted trap styles likely are present around both margins of the Red Sea basin. Seaward of the coastal margins, syn- and post-rift salt-related structures and stratigraphic traps are expected.

Traps in the Red Sea are also likely to preserve hydrocarbons from younger Miocene and Pliocene source rocks where thermal gradients are high. South of the Farasan and Dahlak Islands, potential source rocks in Jurassic marine shales might also charge coastal block-faulted structural traps. Details concerning the quality and distribution of reservoir and source rock for the Red Sea are summarized by Mitchell and others (1992).