In regards to oil reserves, the Middle east is second to none. This “oil miracle” of the human being has to be shaped by a collection of favorable factors, some worldwide and rather local, enrolled in the geologic history of the region.
Source: Halfdan Carstens

Part among a two part series by author Rasoul Sorkhabi, focussing on oil and also gas in the middle East. Part Two - exactly how Much Oil in the center East?

Ten countries in the Middle east account for just 3.4% that the area however contain 48% the world’s recognized oil reserves and 38% of natural gas reserves. Despite years of expedition worldwide, we have not discovered ‘another middle East.’ Source: Rasoul SorkhabiThe ax ‘Middle East’ is no without problems. First, it has early american connotations, together the phrase very first appeared in the mid-nineteenth century as component of the Europe-centred department of the eastern into the Near, Middle and Far East. Second, there is no consensus on the geographical extent of the center East. Some define it together the region between India and also Egypt, in which instance it has actually been suitable designated by the United countries as west Asia. Other meanings also add North Africa or central Asia. For the objective of this article, the hatchet ‘Middle East’ concentrates on the oil-rich nations in south-west Asia consisting of Iran, Iraq, Syria, Kuwait, Saudi Arabia, Bahrain, Qatar, united Arab Emirates (UAE), Oman, and also Yemen. These ten nations together have an area that 5.1 million square kilometres, or around 3.4% of the earth land surface, yet they possess according come BP’s 2012 Statistical evaluation of civilization Energy, 48% the world’s recognized oil reserves and also 38% of natural gas reserves. Indigenous time to time, regions such together the Caspian or West Africa have actually been fantasised together ‘another center East’ however none has materialised, i beg your pardon begs this vital question: why is there so lot oil in the center East? This write-up synthesises our knowledge around this question and also briefly defines the geological determinants for the variety of oil in the center East.

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Gondwana Margin and also Tethys

The tectonic frame of the Middle east is split into (1) Zagros fold-and-thrust belt, (2) turbulent Arabian shelf, and also (3) steady Arabian shelf. The Arabian continental plate, which collided with the eastern plate along the Bitlis-Zagros suture during the Eocene, is quiet converging with Asia at a price of 1.9 to 2.3 centimeter per year based on GPS measurements. (Global dimensions of loved one plate activities of Arabia with respect to Eurasia show greater velocities of 2.4-3.5 centimeter per year). This continental collision offered rise come the Zagros orogen and also its Cenozoic foreland basin, which to be superimposed top top the Paleozoic-Mesozoic Tethys shelf basin. The accumulation thickness of sediments in the region reaches as much as 12 km. The Zagros deformation and also salt domes have folded the sedimentary beds into large, gentle anticlines (‘whaleback’ structural traps). The western and also southern boundaries of the Arabian plate are bounded respectively by the Red Sea and also Gulf the Aden rifts. These Neogene continental rifts have separated Arabia native Africa, and are further pushing Arabia versus Asia. The rift-shoulder uplifts have actually outcropped the Precambrian rocks (part that the Nubian-Arabian shield) follow me the Red Sea and are capped at locations by rift-related volcano rocks. The Proterozoic-Cambrian shift at around 542 Ma opened up a new chapter in the geologic background of the Middle eastern with significant implications for petroleum resources of this region. After a long history of ocean subduction and micro-continental collision follow me the north-eastern margin the Africa from around 700 Ma come 600 Ma, the basement rock of the Middle eastern (the Nubian-Arabian shield) to be consolidated. This event, together with some various other collisions, assembled the supercontinent that Gondwana in the southerly hemisphere, consisting of Africa, India, Australia, Antarctica, and South America. At this time the Middle eastern was positioned top top the north-facing passive margin of Gondwana. Transparent Paleozoic and Mesozoic times, the Middle east was a locus the sedimentation over a long and vast shelf affected, indigenous time to time, by sea level changes. Together Gondwana consolidated at the revolve of the Cambrian, microorganisms and marine species also swiftly evolved and diversified, thus enriching naval sediments through organic carbon vital for hydrocarbon generation. The Tethys Ocean, i m sorry washed the northern margin that Gondwana, was no a solitary ocean, but occurred in at the very least three oceanic basins: Proto-Tethys (Infracambrian-Carboniferous), Paleo-Tethys (Ordovician-Jurassic), and Neo-Tethys (Permian-Paleocene). This sequential breakthrough of Tethys took ar as continental fragments successively damaged away native the Gondwana margin and also drifted northward to sign up with the paleo-Asian continent. The opened of every Tethys s was attach by extensional tectonics which, in turn, produced continental rifts during the Infracambrian (Ediacaran-Cambrian change period), the Ordovician, and the Permian in the center East. These rift basins added favourable sites because that petroleum generation in the region.At the end of the Palaeozoic, Gondwana an unified with Laurasia to kind the largest supercontinent ever, Pangea. It was additionally during this time the Proto-Tethys closed and also Neo-Tethys started opening. In the late Cretaceous, Neo-Tethys began to subduct in ~ the southerly margin the paleo-Asia (along the Anatolia-Iran-Tibet margin) and shrank in size until the ocean fully disappeared in the Eocene and also gave way to a foreland container in prior of the climbing Bitlis-Zagros-Himalayan mountains.The Phanerozoic sedimentary succession in the Middle east is up to 12,000m thick and also can be separated into three mega-sequences:

The Infracambrian-Carboniferous sequence, consisting greatly of siliciclastic rocks and also showing long gaps in sedimentation (during the middle-Paleozoic reduced sea levels);the Permian-Cretaceous sequence, made up of largely carbonate rocks through only short hiatuses; andthe Cenozoic foreland sequence including both carbonates and also siliciclasitcs (toward the top) and also with beforehand Paleocene and Oligocene hiatuses in some areas.

This tectonic-sedimentation framework impacted on petroleum generation and build-up in the Middle east in ways unique to this region.

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Back come the Sources

Movement of the Kuwait-Persian Gulf area throughout Phanerozoic times. Likewise shown are significant geologic events relevant for sedimentation background of the region. Compiled from miscellaneous sources including Beydoun (Episodes, June 1998). Organic-rich source rocks (with complete organic carbon >5%) room not distinctive to the center East. What is remarkable around the center East’s resource rocks, though, is the they to be deposited on a passive continental-shelf margin covering Palaeozoic and Mesozoic times in reasonably stable conditions, which took benefit of sea-level rises, anoxic (reducing) environments, and nutrient-rich upwelling sites turn off the coasts. Moreover, these naval shales and marls contained oil-prone kerogen types I (protein/lipid-rich algal necessary matter) and also II (lipid-rich however with lower hydrogen-to-carbon ratios).One the the happy horizons for petroleum generation in the Middle east is the Silurian ‘hot’ shale, referred to as the Qusaibah Shale in Saudi Arabia but also found in some other parts of the middle East and also North Africa. Paleogeographic modeling of Paleozoic continents and atmospheric circulations (Judith Parrish, AAPG Bulletin, June 1982) shows that throughout the Silurian the Middle eastern was close come a major upwelling existing that enriched the sediments with exceptional amounts of essential matter.However, over 70% the the Middle eastern oil was created from Jurassic-Cretaceous sediments (accounting for less than 30% of the Phanerozoic eon). Few of these classic resource rocks encompass the Sargelu development (Middle Jurassic, 150-200m special in kind locality), Garau development (Lower Cretaceous, end 800m thick), Gadvan formation (Lower Cretaceous, 100m thick), and also Kazhdomi formation (Middle Cretaceous, 200m thick) in the Zagros basin of south-west Iran, and age-equivalent limey-shale formations in Iraq and also the Persian Gulf areas.To explain these rich source rocks we need to think about the position and extent of the Neo-Tethys shelf throughout Jurassic and also Cretaceous times. Neo-Tethys to be then situated close to the warm, organic-rich Equator; it own a wide 2,000-3,000 km-wide shelf and also a length of at the very least twice the (Murris, AAPG Bulletin, may 1980). Moreover, Neo-Tethys to be triangular in shape, pointed (thinning) toward the west; the was thus a partly enclosed basin with its wide shelf oriented nearly west-east, and in a favorable place to advantage from organic-rich sedimentation processes and also high was standing sea-levels. 

World paleogeography in the beforehand Jurassic (~200 Ma) once the Middle east was component of Gondwana passive margin and was submerged under the warm equatorial waters that Neo-Tethys. The triangular Neo-Tethys Ocean had actually a large and lengthy shelf, restricted on the western end however open to the east. The idea of profession winds and Neo-Tethys s currents (concentrating nutrient for planktons) comes from Irving et al. (Canadian journal of planet Science, Jan. 1974). These conditions, i m sorry favoured the eposition that organic-rich resource rocks and thick carbonates, dominated in the Middle east until the recent Cretaceous. In so late Jurassic-Early Cretaceous times, Gondwana was break-up apart, and as rift volcanism and also sea-floor dispersing developed, ocean temperatures rose and ocean waters were enriched with nutrient elements from volcanic outpourings (along mid-ocean ridges and also continental spare part or island arcs). Oxygen-isotope analyses of naval limestones have presented that 125-85 Ma to be a time of severe worldwide warming as result of a rapid rise in atmospheric carbon dioxide concentration (mainly from raised volcanic activities). This is continual with sequence stratigraphic proof for sea-level maxima in mid-late Cretaceous times. Warm climate, high-stand seas and increases in the nitrogen-phosphorus-carbon materials of oceans, in turn, led to a profuse radiation of plankton populations - a an essential factor in the organic richness of naval sediments to adjust down throughout that period. Neo-Tethys most benefited from these events and the Middle eastern was in the right position at the appropriate time.Based on their sedimentary facies distributions, R. J. Murris has split the Mesozoic carbonates of the Middle eastern into 2 categories. The very first one is lead carbonate ramps (mixed through clastic clayey beds), v cycles (‘layer cakes’) ranging in thickness indigenous 30 come 100m and deposited throughout marine regressions when clastics were lugged into the basin. The 2nd category is differentiated carbonate shelf or communication carbonates, which were deposited under transgressive conditions, wherein differentiation is significant with starved (sediment-deficient) euxinic containers separated by high-energy margins from carbonate-evaporite platforms. Rich resource rocks are consisted of in the 2nd category. 

Into the Storage and also Trap

Stratigraphy and petroleum source-reservoir rocks the selected locations in the middle East. Data compiled largely from Beydoun (1999) and also Alsharhan & Nairn (2003). Illustration by Rasoul SorkhabiThe Middle eastern is renowned because that its wealthy carbonate oil reservoirs. Such reservoir rocks are also found in north America and Western Siberia (mainly Palaeozoic), main and south America (mainly Mesozoic), and also South eastern Asia (mainly Miocene). In the center East, however, carbonates to be deposited ~ above a lengthy and wide shelf indigenous the Permian come the Paleocene through insignificant hiatus. Also after the Arabia-Asia collision, carbonate sedimentation continued in a very shallow marine atmosphere (an ongoing procedure in the Persian Gulf). Indeed, large oil areas in the Middle east have thick stacks the multiple carbonate payzones. Limestone and also dolomite reservoirs of the Middle eastern have fairly an excellent porosity and also permeability. Main porosity has actually been well kept in packstones and also grainstones such as those the the so late Jurassic Arab formation widely spread in the center East. In Saudi Arabia’s Ghawar field (the world’s biggest oil field), two creating members (C and also D) that the Arab Formation, have thicknesses of 30m and 80m respectively, and also a porosity that 20%. The same development in the UAE varieties from 130 come 240m in thickness and 10-30% in porosity. Moreover, the Zagros deformation has developed fracture networks improving permeability, specifically in cemented limestones such together the Oligocene-Miocene limestones of south-west Iran (Asmari Formation) and eastern Iraq (Jerribe Formation), wherein their thicknesses variety from 120-480m, and also porosities indigenous 8-24%. Personally from marine shale and also marl lid rocks, countless Middle east basins additionally contain evaporite beds, i m sorry are efficient seals because of your 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 sequence is bounded through evaporite seals both at the bottom and also on the top.

These ductile evaporite and shale beds have additionally acted as detachment horizons because that the Zagros thrust structures, thus creating vertical compartments of petroleum equipment within the sedimentary succession.