A regional structure contour map at Near Top Cretaceous is based on hundreds of well tops from extensive bibliographic references from throughout the Middle East. This structure map shows strong basin asymmetry. Major faults are shown in outcrop and/or suspected at basement or intermediate levels, based in part on gravity and magnetics modeling, published and in ‘open file’ studies. Major uplifts associated with several super-giant oil and gas fields are clearly indicated even at the shallow Cretaceous level (Ghawar Anticline, Qatar Arch, Burgan-Khurais Trend, etc.), even at a very regional scale with a contour interval of 1,000 feet. Isopach maps of Upper, Middle, and Lower Cretaceous are contoured at intervals of 500 feet. Each of these three isopach maps is overprinted in color to show generalized lithofacies trends. Lower and Middle Cretaceous deltaic sandstone fairways on the western shelf provide excellent reservoir rocks for a trend containing many of the world’s very largest oil fields. Somewhat more basinward, predominantly carbonate facies include oil reservoirs in the Upper, Middle, and Lower Cretaceous. Deepest facies lie beneath the Zagros Foothills Belt in coastal Iran and eastern Iraq. This is particularly true for the Upper Cretaceous, where Coniacian to Maestrichtian thick, deep basinal shales, cherts, clastic turbidites, and slumped exotic blocks of ophiolites mark the northeastern border of the Late Cretaceous basin as it approaches the Main Zagros Fault and an assumed subduction zone underthrusting the Iranian Plate or Eurasia.
The reservoirs of many super-giant oil fields of the Middle East are Lower to Middle Cretaceous deltaic sandstones. Some examples are Kuwait’s Burgan field with 86 billion barrels of oil (BBO), Saudi Arabia’s Safaniya (32 BBO and the largest offshore field in the world) and Iraq’s Majnoon (30 BBO). Table 1 and Figure 1 show the largest 33 Middle East oil fields.
Lower and Middle Cretaceous carbonate reservoirs also account for huge oil reserves in such fields as Iraq’s Rumaila (22 BBO), Abu Dhabi’s Zakum (17 BBO) and Saudi Arabia’s Zuluf (14 BBO). Upper Cretaceous carbonate reservoirs also contain giant oil reserves in such fields as Dubai’s Fateh (6 BBO) and Abu Dhabi’s Shah (3 BBO).
Out of a total of 33 Middle East oil fields with ultimate recoverable reserves greater than about 5.0 BBO, 17 have Cretaceous reservoirs (Figure 1 and Table 1). Most of the super-giant oil fields in the Middle East are trapped in large anticlines or faulted anticlines (Aramco Staff, 1959; Bou-Rabee, 1986; Beydoun, 1988; Edgell, 1991, 1992; Carman, 1996). This is specifically true for the giant and super-giant Cretaceous oil fields which are identified in the oil productive Cretaceous fairway shown on Figure 1. Approximate reserves are rounded to the nearest billion barrels (Adasani, 1965; Schlumberger, 1975; Ibrahim, 1983, 1984; Beydoun, 1988; Brennan, 1990, 1991; Ivanhoe and Leckie, 1993).
In spite of the importance of the Cretaceous Middle East petroleum system there have been only limited published regional maps of Cretaceous structure, isopachs, and lithofacies distribution. Most of the published regional maps are areally limited, or include only sparse or out-of-date well control (e.g. Kaymen-Kaye, 1970; Murris, 1980; Koop and Stoneley, 1982).
Regional maps are important tools in exploration. Isopach and lithofacies maps not only indicate important reservoir fairways along which large structural closures are productive but also areas with stratigraphic trap potential. They may show key sub-basins where good source rocks are developed. They may also be used to gain a better understanding of tectonic activities, growth history, petroleum migration pathways and timing of trap formation.
In this paper regional maps of the Near Top Cretaceous structure are shown for a contour interval equal to 1,000 feet (ft). Also shown are isopachs (contour interval = 500 ft), with generalized lithofacies patterns for the Lower, Middle and Upper Cretaceous. The maps are based on several hundred wells published in the international geological literature. Also stratigraphic and structural sections and detailed isopach, facies and structure maps of small local areas helped provide control for overall regional patterns.
Standard international Cretaceous subdivisions normally group Berriasian to Albian stages in the Lower Cretaceous, while Cenomanian to Maestrichtian stages are grouped traditionally in the Upper Cretaceous. However, for purposes of this study, the Nahr Umr shale is recognized as a regionally significant upper seal for the Aptian Shu’aiba/Upper Thamama carbonate reservoir rocks (Alsharhan and Nairn, 1990; Alsharhan, 1994). The Coniacian Laffan/Aruma/Tanuma formations and equivalents are recognized as being similarly important regional upper seals for the Turonian Mishrif and equivalent oil reservoirs (Carman, 1996).
For this reason, the European (French) subdivisions of Upper, Middle, and Lower Cretaceous are used in this study, as shown in Figure 2.
Gross regional structure of the Middle East Basin, the richest oil and gas province in the world, is reasonably well known (Dubertret and André, 1969; Brown, 1972; Iranian Geological Survey, 1976; Murris, 1980; Demidov, 1981; Lovelock, 1984; Beydoun, 1988). Near Top Cretaceous structure contour map (Figure 3) shows very clearly:
The Precambrian Shield and Central Arabian Arch region west of Riyadh.
The deepest part of this very asymmetrical basin beneath the Zagros Foothills Foldbelt of coastal Iran and eastern Iraq on the east side of the Arabian Gulf (Dubertret and André, 1969; Brown, 1972; Morris, 1977; Setudehnia, 1978; Demidov, 1981).
A northern “Mesopotamian Sub-basin” and a southern “Rub’ Al-Khali Sub-basin” and a broad positive centrally located structural region composed of several individual highs (Qatar-South Fars Arch, Ghawar Anticline, and Burgan-Khurais Trend, in particular) on which are found several of the largest oil and gas fields on our planet (Edgell, 1989).
In northern Iraq fold axes change direction from northwest to westerly, becoming southwesterly in northern Syria and in adjacent parts of southeast Turkey (Ponikarov et al., 1967).
Several types of fault patterns and mini-rift basins of differing ages complicate the northern part of the basin, especially in southeast Turkey and Syria (Cline, 1989; Çemen and Ediger, 1990; Çançar, 1990-93; Tür-Kan Petroleum, 1991; Christian, 1992; de Ruiter et al., 1994).
In a very regional sense the left-lateral Dead Sea Fault, trending southwesterly through Lebanon, Palestine and Jordan (Dubertret, 1967; Freud et al., 1970; Gvirtzman and Weissbrod, 1984) and the Main Zagros Fault, with right-lateral as well as thrust component (Gidon et al., 1974), trending northwesterly through Iran and Iraq, form a conjugate pair related to Late Cretaceous-Tertiary northward subduction of the Arabian Plate beneath Eurasia along the Taurus Mountains Thrust Zone (Fairbridge and Badoux, 1960; Brinkman, 1976; Hatcher et al., 1981).
Inferred from Bouguer gravity, magnetotelluric sections, limited well data and subcrop patterns, basement faults are shown trending northwesterly through Central Saudi Arabia and western Iraq (Sallomy and Al-Khatib, 1986a, 1986b; Johnson and Stewart, 1993; Stewart et al., 1996). Northerly trending ‘basement faults’ also appear to form the western boundary of the Ghawar, Khurais, and Qatar highs (Edgell, 1989, 1991, 1992; McGillivray and Husseini, 1992; Johnson and Stewart, 1993; Christian, 1994b). Some of these faults are confirmed by seismic data, although Saudi Aramco and former Aramco personnel claim major offsets are in Triassic or older and tend to die out upward and may even be undetectable at Cretaceous/Jurassic levels (McGillivray and Husseini, 1992; Johansson, 1997).
In western Iraq faults of similar northwest strike are inferred from gravity and magnetic data (Sayyab and Valek, 1967; Abbas, 1983; Ministry of Geology, Moscow, 1984; Alomari and Alnaib, 1986). Seismic and well data are at present too sparse to confirm these faults.
The above faults, as well as graben faults in Jordan, are all, more or less, parallel to the Najd Fault System exposed in the Arabian Shield, and also nearly parallel to the Main Zagros Fault of Iran. Although Zagros faulting is supposed to be primarily young, it seems likely that all of these faults of similar strike may be inherited from pre-Hercynian or perhaps even Precambrian structural trends, and have been reactivated at various times with, in some cases, demonstrated reversals in the direction of strike-slip motion (Agar, 1987; Husseini, 1988; Jackson and McKenzie, 1988; Christian, 1994a).
The Zagros Fold and Thrust Belt shown in Iran is too complex to be contoured at the Cretaceous level at this map scale (Académie des Sciences de L’U.R.S.S., UNESCO, 1981; Berberian and King, 1981); but major (super-giant) oil and gas fields are indicated, with outcrop structure based on Ion et al. (1959), Slinger and Crichton (1959), U.N. Economic Commission for Asia and Far East (1963) and Geological Survey of Iran (1976-77).
Abundant Top Cretaceous well points have been published in the Rub’ Al-Khali region of southern Saudi Arabia, Oman and Yemen, but the Top Cretaceous horizon is too shallow to show the deeper structures which produce oil in this region (Soliman and Shamlan, 1982).
LOWER CRETACEOUS ISOPACH-FACIES
Very generalized Lower Cretaceous isopachs are drawn with a contour interval of 500 ft. The superimposed color pattern indicates major lithofacies trends (Figure 4).
Shallow marine deltaic sandstone facies on the western shelf side of the basin include major oil producing reservoirs in Iraq, Kuwait and Saudi Arabia (pre-Aptian/pre-Shu’aiba Zubair and Biyadh formations). Eastward these sands shale-out basinward and become largely replaced by carbonates in Abu Dhabi and adjacent areas, where the carbonates include major oil reservoirs in the Thamama Formation and equivalents. Zubair sands productive at Kifl, in Central Iraq, suggest the Lower Cretaceous deltaic sands are prospective at least 400 kilometers (km) north of the Kuwait border along trend with established production (Figures 1 and 4) in an area of very inadequate exploration. There is as yet no facies evidence of Zagros folding nor subduction at this early stage.
MIDDLE CRETACEOUS ISOPACH-FACIES
Figure 5 maps Middle Cretaceous isopachs, also with a 500 ft contour interval and the same color scheme, to show generalized facies distribution patterns. Enormous oil reserves are contained in the extensive deltaic sands mapped in Saudi Arabia, Kuwait and Iraq in the western and central parts of the greater Middle East Basin (Burgan, Khafji, and Safaniya reservoir sands).
Major oil is also produced from overlying carbonates (Mishrif-Rumaila-Mauddud reservoirs), especially in Iraq, United Arab Emirates, and adjacent areas. The existence of Middle to Lower Cretaceous reservoir sands at East Baghdad field extends these subsurface porosity objectives to at least 450 km north of the Kuwait border. Structural leads within this stratigraphic trend should be well worth seismic investigation and drilling.
Restricted rift deposits with thick evaporites are found in the Levant, extending northeastward from Syria toward Mosul, in northern Iraq (a late stage of the old Triassic-Jurassic rift system where thick evaporite and carbonate sequences have been well described by Dubertret (1967), Druckman (1974), Bach Imam et al. (1980), Beydoun (1981), Bach Imam (1985) and May (1991). Post Cretaceous inversion of this rift basin created the Palmyra Foldbelt in Syria and smaller mini-rift inversions extending east-northeast into northern Iraq (Tel Hajar, Abd El Aziz, Sinjar Anticline).
These Middle Cretaceous rocks still seem to be ‘pre-Zagros folding’ except that a thick deep basinal turbidite facies with exotic blocks (Boote et al., 1990) exists in the foredeep immediately west of the Oman Mountain Front (Figure 5).
Facing page:Lithofacies legend systems with sandstone and shale end members tend to dominate the western shelf of the Cretaceous isopach-facies maps (see Figures 4, 5 and 6), while carbonate and shale end members are more usual in the central and eastern parts of the Arabian Gulf Basin. Carbonate-evaporite facies and deep marine (bathyal to abyssal) turbidite-chert-shale-ophiolite facies are color coded as local extremes of the predominantly carbonate or predominantly shaley facies.
UPPER CRETACEOUS ISOPACH-FACIES
Figures 2 and 6 indicate Late Cretaceous transgressive facies systems, reversing the extensive regressive deltaic sandstone developments of the Middle Cretaceous. Not only is there very little basin margin sandstone development in the west, but also open marine shelf to deeper marine shales and carbonates predominate over most of the greater Middle East Basin. Locally deeper silled basins (Coniacian Karabogaz Formation, for instance) preserved organic-rich source rocks, although much of the Cretaceous reservoirs are actually charged with Jurassic-sourced oil (Dunnington, 1967; Avedisian and Hammosh, 1970; Ala et al., 1980; Harput et al., 1982; Bordenave and Burwood, 1990; Harput and Ertürk, 1991).
A deepening marine pulse in Coniacian time probably reflects downwarping and northeastward tilting of the Arabian Plate accompanying subduction beneath the Iranian/Eurasian Plate (Erdogan and Akgül, 1981; Soylu, 1991). Shale seals critical to the trapping of numerous Mishrif and Mishrif-equivalent oil fields represent this marine deepening pulse in the Gulf region (Laffan Shale), Iraq (Tanuma Shale), and southeastern Turkey (Karabogaz Formation), as has been suggested by Karig and Kozlu (1990) and Christian (1992). Detailed stratigraphic and subcrop patterns controlling reservoir distribution immediately below this level need further investigation in the United Arab Emirates and Oman (Harris and Frost, 1984; Jorden et al., 1985; Pascoe et al., 1994).
Although old relict north-south isopach trends are still evident on Figure 6, northwesterly trending shallow reef trends parallel to the Zagros Fault in western Iran, eastern Iraq, and northeastern Syria are evident, such as the proposed Suwaidah-Ain Zalah-Alan-Kirkuk trend (Weber, 1963; Nikolayevskiy, 1972; Christian, 1992). Closer to the subduction zone along the Zagros and Taurus Mountain Fronts, thicker and deeper marine facies include shales, red radiolarian cherts, turbidite clastics, and detached slide blocks of serpentines. A similar association of deep bathyal or abyssal lithofacies has been observed being overthrust along the Oman Mountain Front (Alsharhan, 1995; Boote et al., 1990).
These rocks, of Coniacian to Maestrichtian age, undoubtedly were deposited during an early stage of northwest striking Zagros folding, accompanying the Arabian Plate’s subduction beneath Iran. Between Coniacian and latest Cretaceous time, then, is bracketed the moment when the Zagros Orogeny first began in a major way. Subduction continued well into the Paleocene and Eocene along the Zagros and Oman Mountain Fronts, and to some extent even into the Oligocene and Early Miocene, particularly at the northern margins of the basins beneath the Taurus Mountains in southeastern Turkey (Académie des Sciences de l’U.R.S.S., UNESCO, 1981; Snyder and Barazangi, 1986). Zagros folding has continued to the end of the Tertiary and even Quaternary.
Local mini-rift basins in Turkey, Syria and northwestern Iraq were particularly active during Late Cretaceous: the Upper Euphrates Rift in Syria, Anah Graben in Iraq, and the Tel Hajar-Abd El Aziz Rift in Syria-Iraq (Youash and Naoum, 1970; Leonov et al., 1986; Leonov et al., 1989); the Abba-Furat mini-rift in Syria (de Ruiter et al., 1994); and the Akçakali Graben in southeastern Turkey (Tardu et al., 1987).
The regional Cretaceous structural and isopach/lithofacies maps of the Middle East provide a unique overview of the major tectonic and stratigraphic developments in the leading petroleum basin of the World. The structural map of the Near Top Cretaceous and the Upper Cretaceous isopach show that most of the tectonic elements of the region, including the main anticlinal trends with oil fields remained active during and after the Late Cretaceous. The isopach and lithofacies maps of the Lower, Middle and Upper Cretaceous indicate the development of broad intra-shelf basins along the Cretaceous platform which controlled the regional distribution of lithofacies.
Figure 7 shows the reserve size distribution of 400 oil fields in the Arabian Gulf region, ranked and plotted on logarithmic graph paper. The 50 percentile (median) reserve size for the Middle East is 150 million barrels (MMBO). Similar individual plots for Iraq, Iran and Saudi Arabia, separately, would show median reserve sizes in the 250 to 500 MMBO range. Simplified plots for several lesser oil provinces are shown for comparison (Christian, 1994b).
Vast future potential in the Middle East can be forecast for Cretaceous reservoirs, as for rocks of certain other ages. Perhaps the greatest potential for new giant-sized oil fields in the Middle East lies along a 400-kilometer trend of inadequately explored Lower Cretaceous Zubair sandstone and Middle Cretaceous Khafji-Safaniya sandstone deltaic shelf edge “fairways” between Burgan in Kuwait and Baghdad in central Iraq (Jamil, 1978; Jawad Ali and Aziz, 1993; Christian and Johnston, 1995).
The author thanks Professor Abdulrahman Alsharhan for inviting this paper for GeoArabia, the referees and the editor for their useful suggestions. Gulf PetroLink redrafted the final figures.
ABOUT THE AUTHOR
Louis Christian’s career began with Chevron in Bakersfield and Los Angeles. After three years in Manila with Anglo-Philippine Oil and Mobil Oil as Senior Geologist and Field Party Chief, he was posted by Mobil to Libya and Tunisia for several years as Staff Geologist, Planning Analyst and Exploration Supervisor. Following a three-year assignment with Mobil Oil Indonesia, Christian spent several years as Associate Geological Advisor in Mobil’s International New Ventures Group in Dallas and was involved with Australia, China, Iraq, Abu Dhabi and other Middle East areas. In 1987-88, Christian was International Advisor to Nelson Bunker Hunt in Dallas. In 1988-89 he directed Middle East New Ventures studies for Anschutz Overseas Corporation in Denver and Dallas. Since 1990 Christian has continued Middle East exploration consulting, advising Summit Exploration, Enron, Arco, Amoco, Transworld, BHP, Mosbacher Energy, Mobil and Tür-Kan Petroleum Ltd. In 1990-91, he compiled a five-volume study on subsurface geology and oil potential of the Turkey-Syria-Iraq border area. Chrisitan holds BSc and MSc degrees in Geology from Stanford University.