The Red Sea occupies a long (2000 km) linear rift of late Oligocene age 180–50 km wide (Hughes et al., 1991). The conjugate margins are bounded by a series of large fault terraces with ≤2500 m of relief. Within the sea itself, there are three physiographic elements: (1) shallow shelfal areas, narrow north of 21°N but wider to the south; (2) a main trough 600–1000 m deep occupying most of the sea area to the north of 21°N; and (3) a narrow axial trough, ~2000 m deep and 5–30 km wide (Coleman, 1993).
The crust beneath the axial trough has been deter- mined as oceanic on the basis of magnetic anomalies (Girdler and Styles, 1978), with the oldest crust thought to have been formed ~5 Ma. There remains much doubt as to the nature of the crust beneath the shelfal areas. It may be oceanic crust formed during the Oligo– Miocene (Hall, 1989), thinned continental crust (Egloff et al., 1991), or a bit of both (Cochran, 1983). Sea-floor spreading is still active within the Red Sea area.
The thin continental crust and active sea-floor spreading has resulted in heat flows, typically >200
mWm2(megawatts per square meter) in the axial
trough (Coleman, 1993), and almost everywhere
greater than the world average of 55 (mWm2). As a
consequence, thermal gradients are also high, from
~73°C km–1at the basin center to ~45°C km–1at the
basin margin. Volcanism associated with the rifting continued throughout the Miocene (Davison and Rex, 1980).
Stratigraphy
Four megasequences have been identified (Figure 2; Hughes and Beydoun, 1992; Mitchell et al., 1992). These, with their approximate ages, are described below.
Prerift Pre-Late Oligocene, 26 Ma or Older The oldest megasequence comprises minor marine deposits generated by occasional flooding by the Indian Ocean into the incipient rift in the Meso- zoic. The Oligocene is dominated by regional flood volcanism. BP Antufash License Antufash-1 Al Meethag 1 (W1) Al Meethag 2 (W2) Wadi Mawr YEMEN YEMEN RED SEA 42 o E 15 o N 16 o N 43 o E Figure 1. Location map for the Antufash license in the Yemeni Red Sea.
Synrift Early to Middle Miocene, 26–16 Ma (“Infra-Evaporite”)
In quick succession, the rift phase was typified by uplift, high rate of extension, and subsidence as Arabia and Africa separated. Transgression occurred from the north, and widespread marine conditions were estab- lished. By the middle Miocene, the environment remained shallow marine, but water circulation was restricted.
Postrift Middle to Late Miocene, 16–5 Ma (“Evaporite”)
Following development of a silled basin, thick evaporite deposits were developed during lowstand drawdown. Uplift of the rift shoulders resulted in deposition of thick clastic wedges along the basin mar- gin. Intermittent connection with the Indian Ocean and periods of anoxia led to the development of poten- tial petroleum source rocks (El-Anbaawy et al., 1992; Cole et al., 1995). The massive salt at the base of this megasequence also began to move at this time due to the gravity loading in the coastal areas (Heaton et al., 1993; Davidson et al., 1994, 1995). Cyclic deposition of
source reservoir and seal parasequences occurred in the salt withdrawal basins.
Axial Rift Pliocene–Recent, 5 Ma–Present (“Supra-Evaporite”)
Eustatic sea level fall accentuated erosion on the basin margins. Spreading continued with rapid subsi- dence of the axial trough. New oceanic crust was formed in the south, and open marine conditions were established with the Indian Ocean; major carbonate deposition occurred. The continued clastic deposition at the basin margins and the resultant salt movement accentuated earlier developed structural traps.
Reservoir Development
A combination of seismic reflection data mapping and information obtained from existing wells revealed that reservoir potential was likely to be best developed within upper Miocene highstand progradational sys- tems and associated lowstand systems tracts (Crossley et al., 1992). In both tracts, basin-fringing alluvial/flu- vial systems were predicted to be the most likely reser- voirs. Some marine influence was likely to have occurred during maximum flood.
Table 1. Core Porosity and Permeability Data for Regional Wells in the Antufash License.
Average Mean Mean
Depth Number Porosity 1σ Permeability 1σ
Well (m subsea) of Plugs (%) (%) (md) (md)
Al Meethag 1 1417 51 28.5 5.3 34.7 77.0
(W1)
Al Meethag 2 1540 29 21.6 3.7 11.5 19.0
(W2)
CHRONOSTRATIGRAPHY LITHOSTRATIGRAPHY GLOBAL SEQUENCE STRATIGRAPHY MIOCENE PLIO- CENE PLEISTOCENE OLIGO- CENE U L U M L HOLOCENE CALABRIAN - MILAZZIAN PIACENZIAN ZANCLEAN MESSINIAN TORTONIAN SERRAVALLIAN LANGHIAN BURDIGALIAN AQUITANIAN
SERIES STAGES SOUTHERN
RED SEA SUPRA- EVAPORITE EVAPORITE INFRA- EVAPORITE
MARINE SEDIMENTS & FLOOD VOLCANICS RELATIVE CHANGE OF COASTAL ONLAP SEQUENCE B'NDARY AGE LANDWARD BASINWARD 0.8 1.6 2.4 3.0 4.2 8.0 5.6 6.3 10.6 12.5 13.6 15.6 16.5 17.5 21 22 24
Figure 2. Stratigraphy of the Red Sea area (R. Jones, 1994, personal communication).
Sand Provenance/Composition
The last major uplift in the Red Sea area began dur- ing the middle Miocene and continues today (David- son et al., 1994). As a consequence, the present-day geological maps of the circum Red Sea area are taken to represent the potential provenance area for Upper Miocene sediments (Sudan, 1963; U.S. Geological Sur- vey, 1963; Kazmin, 1973; Merla, 1979). In broad terms there were two very different provenance terrains dur- ing the mid-Miocene:
• Pre-Cambrian acid and acid-intermediate meta- morphic and igneous granites and gneisses with minor Jurassic and Cretaceous sandstones (Taw- ila Formation).
• Pre-Cambrian basic metamorphic and igneous rocks, Oligo–Miocene volcanics, and minor Juras- sic limestones.
The following depositional sand compositions were estimated using descriptions of SEM preparations from core and cuttings obtained from the two Al Meethag wells (Figure 1).
These wells showed basic/volcanic-derived and acid-derived deposits as follows: quartz 30 ± 20% and 50 ± 20%, respectively; feldspar 30 ± 20% and 30 ± 20%, respectively; various rock fragments 40 ± 20% and 20%, respectively.
The rock fragments include volcaniclastic grains, mafic mineral grains, and a little glauconite.
In the Antufash acreage, as evidenced by the Al Meethag wells, much of the provenance appears to have been from the basic metamorphics and volcanics. Only in the area west of Wadi Mawr (Figure 1) is this basic/volcanic input likely to have been diluted. This wadi drains along a Jurassic transfer fault and taps into an area that may have shed large quantities of arkosic Tawila Sandstone during the middle Miocene.