Part One of a two part series by author Rasoul Sorkhabi, focussing on oil and gas in the Middle East. Part Two - How Much Oil in the Middle East?
The term ‘Middle East’ is not without problems. First, it has colonial connotations, as the phrase first appeared in the mid-nineteenth century as part of the Europe-centred division of the East into the Near, Middle and Far East. Second, there is no consensus on the geographical 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 south-west 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 kilometres, or about 3.4% of the Earth’s land surface, but they possess according to BP’s 2012 Statistical Review of World Energy, 48% of world’s known oil reserves and 38% of natural gas reserves.
From time to time, regions such as the Caspian or West Africa have been fantasised as ‘another Middle East’ but none has materialised, which begs this important question: why is there so much oil in the Middle East? This article synthesises our knowledge about this question and briefly describes the geological factors for the abundance of oil in the Middle East.
Gondwana Margin and Tethys
The Proterozoic-Cambrian transition at about 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 north-eastern 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, comprising Africa, India, Australia, Antarctica, and South America. At this time the Middle East was 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 hydrocarbon generation.
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-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 favourable sites for petroleum generation in the region.
At the end of the Palaeozoic, 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 is up to 12,000m thick and can be divided into three mega-sequences:
- The Infracambrian-Carboniferous sequence, consisting largely of siliciclastic rocks and showing long gaps in sedimentation (during the middle-Paleozoic lower sea levels);
- the Permian-Cretaceous sequence, made up of mainly carbonate rocks with only brief hiatuses; and
- the Cenozoic foreland sequence including both carbonates and siliciclasitcs (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.
Back to the Sources
Organic-rich source rocks (with total organic carbon >5%) are not unique to the Middle East. What is remarkable about the Middle East’s source rocks, though, is that they were deposited on a passive continental-shelf margin spanning Palaeozoic and Mesozoic times in relatively stable conditions, which took advantage of sea-level rises, anoxic (reducing) environments, and nutrient-rich upwelling sites off the coasts. 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.
However, 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, 150-200m thick in type locality), Garau Formation (Lower Cretaceous, over 800m thick), Gadvan Formation (Lower Cretaceous, 100m thick), and Kazhdomi Formation (Middle Cretaceous, 200m thick) in the Zagros Basin of south-west Iran, and age-equivalent limey-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 possessed a broad 2,000-3,000 km-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 the 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 were enriched with nutrient elements from volcanic outpourings (along mid-ocean ridges and continental margins or island arcs). Oxygen-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 the right position at the right time.
Based on their sedimentary facies distributions, R. J. Murris has divided the Mesozoic carbonates of the Middle East into two categories. The first one is carbonate ramps (mixed with clastic clayey beds), with cycles (‘layer cakes’) ranging in thickness from 30 to 100m and deposited during marine regressions when clastics were brought into the basin. The second category is differentiated carbonate shelf or platform carbonates, which were deposited under transgressive conditions, where 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.
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 Palaeozoic), Central and South America (mainly Mesozoic), and South East 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, have thicknesses of 30m and 80m respectively, and a porosity of 20%. The same formation in the UAE ranges from 130 to 240m 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 south-west Iran (Asmari Formation) and eastern Iraq (Jerribe Formation), where their thicknesses range from 120-480m, and porosities from 8-24%.
Apart from marine shale and marl cap rocks, many Middle East basins also contain evaporite beds, which are efficient seals because of their ductility. The main evaporite horizons include:
- the Infracambrian Hormuz salt;
- Triassic interbedded evaporites;
- Late Jurassic Gotnia-Hith Formation; and
- 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.
Zagros Deformation and Salt Domes
Basin modeling of various fields of the Middle East has shown that the Jurassic-Cretaceous source rocks were buried to oil-generation temperatures during the Neogene, and that the oil window partly overlapped in time with Zagros deformation and the 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. (Such diastrophism has happened in the thick-skinned tectonic deformation and uplift of the Himalayas.)
In fact, in the Zagros Mountains Precambrian metamorphic rocks do not outcrop, 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.
Why So Much Oil?
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 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 south-west 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 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:
- A 500-million history of sedimentation on the passive continental margin of Gondwana;
- Sedimentation occurred largely within latitudes 30° North-30° South, where the climate is warm and organic activity is at high levels;
- The wide, long (east-west oriented) shelf of Neo-Tethys during the Jurassic-Cretaceous;
- The favourable position of the Middle East during the mid-Cretaceous global warming, coupled with sea level rise, and plankton abundance;
- Marine source rocks containing oil-prone kerogen types;
- The proximity of interbedded source rocks to reservoirs minimised oil migration pathways;
- An enormous thickness of sediments (predominantly carbonates but some clastics as well) provided the necessary storage for the huge amounts of generated oil;
- The presence of effective regional seals, both marine shales and evaporites, at the bottom, on the top, and within the sedimentary package;
- Superimposition of the Cenozoic Zagros foreland basin above the Tethyan shelf sediments;
- 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.