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Why So Much Oil?

Rasoul Sorkhabi, Ph.D.

The term “Middle East” is not without problems. First, it has colonial connotations, as the term first appeared in the mid-nineteenth century as part of the Europe-centered division of the East into the Near, Middle and Far East. Second, there is no consensus on the geographic extent of the Middle East. Some define it as the region between India and Egypt, in which case it has been aptly designated by the United Nations as Western Asia. Other definitions also add North Africa or central Asia.

For the purpose of this article, the term “Middle East” focuses on the oil-rich countries in southwest Asia including Iran, Iraq, Syria, Kuwait, Saudi Arabia, Bahrain, Qatar, United Arab Emirates (UAE), Oman, and Yemen. These ten countries together have an area of 5.1 million square kilometers or about 3.4% of Earth’s land surface, but they possess 60% of world’s known oil reserves and 41% of natural gas reserves.

60% of the world’s known oil reserves

From time to time, regions such as the Caspian or West Africa have been fantasized as “another Middle East” but none has materialized, which begs this important question: Why is there so much oil in the Middle East? This article synthesizes our knowledge about this question and briefly describes the geological factors for the abundance of oil in the Middle East.

Of the Earth’s total land surface (149 million square kilometers), ten countries in the Middle East account for only 3.4% of the area but contain 60% of world’s known oil reserves and 41% of natural gas reserves. Despite decades of exploration worldwide, we have not found “another Middle East.” Illustration: Rasoul Sorkhabi

Gondwana Margin and Tethys

The Proterozoic-Cambrian transition at 542 Ma opened a new chapter in the geologic history of the Middle East with far-reaching implications for petroleum resources of this region. After a long history of ocean subduction and micro-continental collision along the northeastern margin of Africa, from about 700 Ma to 600 Ma, the basement rock of the Middle East (the Nubian-Arabian shield) was consolidated. This event, together with some other collisions assembled the supercontinent of Gondwana in the southern hemisphere, comprised by Africa, India, Australia, Antarctica, and South America. The Middle East was then positioned on the north-facing passive margin of Gondwana. Throughout Paleozoic and Mesozoic times, the Middle East was a locus of sedimentation over a long and wide shelf affected, from time to time, by sea level changes. As Gondwana consolidated at the turn of the Cambrian, microorganisms and marine species also rapidly evolved and diversified, thus enriching marine sediments with organic carbon necessary for oil generation.

Tectonic framework of the Middle East divided into (1) Zagros fold-and-thrust belt, (2) Unstable Arabian shelf, and (3) Stable Arabian shelf. The Arabian continental plate, which collided with the Asian plate along the Bitlis-Zagros suture during the Eocene is still converging with Asia at rates of 1.9 to 2.3 cm per year based on GPS measurements. (Global measurements of relative plate motions of Arabia with respect to Eurasia show higher velocities of 2.4-3.5 cm per year). This continental collision gave rise to the Zagros Orogen and its Cenozoic foreland basin which was superimposed on the Paleozoic-Mesozoic Tethys shelf. The cumulative thickness of sediments in the region thus reaches up to 12 km. The Zagros deformation and salt domes have folded the sedimentary beds into large, gentle anticlines (“whaleback” structural traps). The western and southern boundaries of the Arabian plate are bounded by the Red Sea and Gulf of Aden rifts. These Neogene continental rifts have separated Arabia from Africa, and are further pushing Arabia against Asia. The rift-shoulder uplifts have outcropped the Precambrian rocks (part of the Nubian-Arabian shield) along the Red Sea and are capped at places by rift volcanic. Illustration: Rasoul Sorkhabi

The Tethys Ocean, which washed the northern margin of Gondwana, was not a single ocean, but developed in at least three oceanic basins: Proto-Tethys (Infracambrian-Carboniferous), Paleo-Tethys (Ordovician-Early Jurassic), and Neo-Tethys (Permian-Paleocene). This sequential development of Tethys took place as continental fragments successively broke away from the Gondwana margin and drifted northward to join the paleo-Asian continent. The opening of each Tethys ocean was accompanied by extensional tectonics which, in turn, created continental rifts during the Infracambrian (Ediacaran-Cambrian transition period), the Ordovician, and the Permian in the Middle East. These rift basins added favorable sites for petroleum generation in the region.

At the end of the Paleozoic, Gondwana merged with Laurasia to form the largest supercontinent ever, Pangea. It was also during this time that Proto-Tethys closed and Neo-Tethys began opening. In the Late Cretaceous, Neo-Tethys began to subduct beneath the southern margin of paleo-Asia (along the Anatolia-Iran-Tibet margin) and shrank in size until the ocean completely disappeared in the Eocene and gave way to a foreland basin in front of the rising Bitlis-Zagros-Himalayan mountains.

The Phanerozoic sedimentary succession in the Middle East reaches up to of 12,000 meters and may be divided into three mega-sequences: (1) The Infracambrian-Carboniferous sequence, consisting largely of siliciclastic rocks and showing long gaps in sedimentation (during the middle-Paleozoic lower sea levels); (2) the Permian-Cretaceous sequence, made up of mainly carbonate rocks with only brief hiatuses; and (3) the Cenozoic foreland sequence including both carbonates and siliciclastics (toward the top) and with early Paleocene and Oligocene hiatuses in some areas.

This tectonic-sedimentation framework impacted on petroleum generation and accumulation in the Middle East in ways unique to this region.

Movement of the Kuwait-Persian Gulf area throughout Phanerozoic times. Also shown are major geologic events relevant for sedimentation history of the region. Compiled from various sources including Beydoun (Episodes, June 1998). Illustration by Rasoul Sorkhabi

Back to the Source

Organic-rich source rocks (with total organic carbon >5%) are not unique to the Middle East. What is remarkable about Middle East’s source rocks, though, is that they were deposited on a passive continental-shelf margin spanning Paleozoic and Mesozoic times in relatively stable conditions, which took advantage of sea-level rises, anoxic (reducing) environments, and nutrient-rich upwelling sites off the cost. Moreover, these marine shales and marls contained oil-prone kerogen types I (protein/lipid-rich algal organic matter) and II (lipid-rich but with lower hydrogen-to-carbon ratios).

One of the lucky horizons for petroleum generation in the Middle East is the Silurian “hot” shale called the Qusaibah Shale in Saudi Arabia but also found in some other parts of the Middle East and North Africa. Paleogeographic modeling of Paleozoic continents and atmospheric circulations (Judith Parrish, AAPG Bulletin, June 1982) shows that during the Silurian the Middle East was close to a major upwelling current that enriched the sediments with exceptional amounts of organic matter.

Over 70% of the Middle East oil was generated from Jurassic-Cretaceous sediments (accounting for less than 30% of the Phanerozoic eon). Some of these classic source rocks include the Sargelu Formation (Middle Jurassic, 150-200 m thick in type locality), Garau Formation (Lower Cretaceous, over 800 m thick), Gadvan Formation (Lower Cretaceous, 100 m thick), and Kazhdomi Formation (Middle Cretaceous, 200 m thick) in the Zagros basin of SW Iran and age-equivalent limy-shale formations in Iraq and the Persian Gulf areas.

To explain these rich source rocks we need to consider the position and extent of the Neo-Tethys shelf during Jurassic and Cretaceous times. Neo-Tethys was then located close to the warm, organic-rich Equator; it enjoyed a 2000-3000 km-wide wide shelf and a length of at least twice that (Murris, AAPG Bulletin, May 1980). Moreover, Neo-Tethys was triangular in shape pointed (thinning) toward west; it was thus a partly enclosed basin with its wide shelf oriented almost west-east, and in a favorable position to benefit from organic-rich sedimentation processes and high stand sea-levels.

In Late Jurassic-Early Cretaceous times, Gondwana was split apart, and as rift volcanism and sea-floor spreading developed, ocean temperatures rose and ocean waters became enriched with nutrient elements from volcanic outpourings (along mid-ocean ridges and continental or island arcs). Carbon-isotope analyses of marine limestones have shown that 125-85 Ma was a time of severe global warming due to a rapid increase in atmospheric carbon dioxide concentrations (mainly from increased volcanic activities). This is consistent with sequence stratigraphic evidence for sea-level maxima in mid-late Cretaceous times. Warm climate, high-stand seas, and increases in the nitrogen-phosphorus-carbon contents of oceans, in turn, led to a profuse radiation of plankton populations - a key factor in the organic richness of marine sediments laid down during that period. Neo-Tethys most benefited from these events and the Middle East was in a right position at a right time.

Based on their sedimentary facies distributions, R. J. Murris has divided the Mesozoic carbonates of the Middle East into two categories: (1) Carbonate ramps (mixed with clastic clayey beds) with cycles (“layer cakes”) ranging in thickness from 30 to 100 m and deposited during marine regressions when clastics were brought into the basin, and (2) differentiated carbonate shelf or platform carbonates, which were deposited under transgressive conditions, and differentiation is marked with starved (sediment-deficient) euxinic basins separated by high-energy margins from carbonate-evaporite platforms. Rich source rocks are included in the second category.

World paleogeography in the Early Jurassic (~200 Ma) when the Middle East was part of Gondwana passive margin submerged under the warm equatorial waters of Neo-Tethys. The triangular Neo-Tethys Ocean had a wide and long shelf, restricted on the western end but open to the east. The idea of trade winds and Neo-Tethys ocean currents (concentrating nutrients for planktons) comes from Irving et al. (Canadian Journal of Earth Science, Jan. 1974). These conditions, which favored deposition of organic-rich source rocks and thick carbonates, prevailed in the Middle East until the latest Cretaceous. Illustration by Rasoul Sorkhabi

Into the Storage and Trap

The Middle East is renowned for its rich carbonate oil reservoirs. Such reservoir rocks are also found in North America and Western Siberia (mainly Paleozoic), Central-South America (mainly Mesozoic), and Southeast Asia (mainly Miocene). In the Middle East, however, carbonates were deposited on a long and wide shelf from the Permian to the Paleocene with insignificant hiatus. Even after the Arabia-Asia collision, carbonate sedimentation continued in a very shallow marine environment (an ongoing process in the Persian Gulf). Indeed, large oil fields in the Middle East have thick stacks of multiple carbonate payzones.

Limestone and dolomite reservoirs of the Middle East have fairly good porosity and permeability. Primary porosity has been well preserved in packstones and grainstones such as those of the Late Jurassic Arab Formation widely spread in the Middle East. In Saudi Arabia’s Ghawar field (the world’s largest oil field), two producing members (C and D) of the Arab Formation, respectively, have thicknesses of 30 m and 80 m, and a porosity of 20%. The same formation in the UAE ranges from 130-240 m in thickness and 10-30% in porosity. Moreover, the Zagros deformation has created fracture networks enhancing permeability, especially in cemented limestones such as the Oligocene-Miocene limestones of southwest Iran (Asmari Formation) and Eastern Iraq (Jerribe Formation), where its thickness ranges from 120-480 m, and has a porosity of 8-24%.

Apart from marine shale and marl cap rocks, a vast part of the Middle East basins also contains evaporite beds, which are efficient seals because of their ductility. The main evaporite horizons include (1) the Infracambrian Hormuz salt; (2) Triassic interbedded evaporites; (3) Late Jurassic Gotnia-Hith Formation; and (4) and the Miocene Gachsaran Formation. In this manner, the entire Phanerozoic succession is bounded by evaporite seals both at the bottom and on the top.

These ductile evaporite and shale beds have also acted as detachment horizons for the Zagros thrust structures, thus producing vertical compartments of petroleum systems within the sedimentary succession.

Stratigraphy and petroleum source-reservoir rocks of selected areas in the Middle East. Data compiled mainly from Beydoun (1999) and Alsharhan & Nairn (2003). Illustration by Rasoul Sorkhabi

Zagros Deformation and Salt Domes

Basin modeling from various parts of the Middle East has shown that the Jurassic-Cretaceous source rocks were buried to oil-generation-window temperatures during the Neogene, and this partly overlapped in time with Zagros deformation and development of large anticlines as structural traps.

Three important features of Zagros deformation helped to preserve oil accumulations in the Middle East:

Firstly, the Cenozoic Zagros foreland basin was superimposed on the Paleozoic-Mesozoic Tethyan sediments, providing an overburden necessary for preservation as well as burial heating of the underlying source rocks.

Secondly, the relatively slow motion of the African-Arabian plate during the Cenozoic prevented a wide-spread, severe tectonic deformation unlike what occurred in other parts of the Tethyan belt, most notably in the Himalayas. (The Indian plate has moved at a rate of 5-6 cm per year after the India-Asia collision at 50 Ma, which is about three times faster than that of the African-Arabian plate over the same period.)

Thirdly, the presence of ductile Hormuz salt at the base of the Phanerozoic sediments helped to detach the sedimentary cover from the underlying igneous-metamorphic basement rock. Therefore, Zagros deformation evolved as a thin-skinned tectonic deformation in which the basement rock did not override the sedimentary cover nor did it exhume the entire Tethyan shelf sediments. (This scenario has happened in the thick-skinned tectonic deformation and uplift of the Himalayas.)

Indeed, in the Zagros Mountains, Precambrian metamorphic rocks are not outcropped, and those metamorphic schists and granites in Zagros, which were once thought to be of Precambrian age, have been dated as Paleozoic by high-resolution radiometric methods. Note that a thin-skinned deformation in the Zagros fold-and-thrust does not overrule the possibility of basement deformation at deeper levels; it simply means that deformation of the sedimentary cover has been decoupled from that of the basement (of which we have little knowledge). Perhaps, millions of years in the future, Zagros will also look much like today’s Himalayas, especially if the motion of the Arabian Plate, currently being pushed by the Red Sea rifting, becomes faster as the Red Sea develops into a wide ocean.

The Infracambrian Hormuz salt has also created salt diapris and folds, which provide favorable structural traps for oil; these salt movements date back to the Late Cretaceous. Although the Hormuz salt has been mapped only in some areas of the Middle East, its true extent is not precisely known as it has not been drilled in areas such as southern Iraq or the UAE.

View of the Qatif Central Producing Plants during its construction in the early 2000s. Completed in 2004, these facilities, according to Saudi Aramco, produce 800,000 barrels per day from rejuvenation of the onshore Qatif field (first produced in 1951) and the offshore Ab Safah field (first produced in 1966). Photo courtesy of Saudi Aramco

Summing-up

For petroleum to be successfully generated, migrated, accumulated, and preserved, all elements and processes of the petroleum system, including organically rich and thermally matured source rocks, porous-permeable reservoir rocks, effective extensive cap rocks, and appropriate time relations between oil migration and trap formation should be present. Obviously, the Middle East qualifies all these conditions to a high degree and quality.

Since our discussion is at the regional scale of the Middle East, we have to simplify and generalize the issue and thus ignore variations in structural and sedimentation histories within the region. One such variation, for example, is that as we move away from the Precambrian outcrops in western Saudi Arabia toward the High Zagros in southwest Iran, the basin(s) become deeper and Mesozoic-Cenozoic sediments attain enormous thicknesses. Few wells have, indeed, penetrated the Paleozoic sections in these deeper parts of the basin. Moreover, some parts of the Middle East, notably the central and northern Iran, virtually remain unexplored. While keeping these shortcomings in mind, several key points may be highlighted to explain the abundance of oil in the Middle East:

1. A 500-million history of sedimentation on the passive continental margin of Gondwana;
2. Sedimentation largely within latitudes 30° North-30° South where climate is warm and organic activity is at high levels;
3. Wide, long (east-west oriented) shelf of Neo-Tethys during the Jurassic-Cretaceous;
4. Favorable position of the Middle East during the mid-Cretaceous global warming, sea level rise, and plankton abundance;
5. Marine source rocks containing oil-prone kerogen types;
6. Proximity of interbedded source rocks to reservoirs minimized oil migration pathways;
7. An enormous thickness of sediments (predominantly carbonates but some clastics as well) provided the necessary storage for the huge amounts of generated oil;
8. Presence of effective regional seals, both marine shale and evaporites, at the bottom, on the top, and within the sedimentary package;
9. Superimposition of the Cenozoic Zagros foreland basin atop the Tethyan shelf sediments;
10. Relatively slow motion of the Arabian plate and the presence of ductile evaporite-shale layers at the base of the Phanerozoic sediments resulted in a thin-skinned tectonic regime in which the Precambrian igneous-metamorphic basement did not override or exhume the entire Tethyan shelf sediments;

Large “whaleback” anticlines generated by the “gentle” Zagros compression (folding and thrusting) or salt diapirs provided abundant structural traps for oil accumulations.

Updated: 02.06.2010 14:44 by Alf Kvassheim