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Shaikan Oil Field

Geographic Location

Located 60 kilometers northwest of Erbil, Iraq in the foothills of the Zagros Mountains near the Turkey-Iraq border.

Map of the Middle East with Shaikan starred. Free use from Wikimedia Commons.

Geologic Setting and History

The Shaikan Oil Field is on the Arabian Plate near the convergent boundary with the Eurasian Plate. For most of geologic history, the Arabian plate is connected to the African Plate. The first parts of the Arabian Plate formed in the middle to late Proterozoic from island arcs and small pieces of continental crust subducting and accreting onto the Pan African Craton, adding Arabia to the super-continent Gondwana. ^[2] In the Ordovician, there was an expansion of polar glaciers in Gondwana that then melted in the Silurian, raising sea levels.

In the Devonian to Carboniferous, the Hercynian Orogeny began forming Pangea from a collision between Gondwana and Euramerica at present day South America and Africa. This Orogeny uplifted the Arabian plate and tilted it, exposing the Devonian and Silurian formations to erosion and causing a hiatus in sedimentation.^[2] Pangea was almost fully formed by the end of the Carboniferous.

In the Late Permian, the Cimmerian continent rifted and separated from Gondwana. This continent migrated northeast, beginning to close the Paleo-Tethys Sea and open the Neo-Tethys Sea. The Cimmerian continent is the crustal material on the northeastern side of the present day Zagros Mountains that are in the collision zone. ^[3]

In the Triassic, the Cimmerian continent continued to travel across the Paleo-Tethys Sea while opening the Neo-Tethys Sea. By the end of the Triassic, the Cimmerian continent closed the Paleo-Tethys Sea and collided with present-day China and Siberia. ^[3]

During most of the Jurassic and Cretaceous, the eastern edge of the Arabian Plate was a large, shallow sea shelf near the equator. As the Cretaceous went on, the African and Arabic plate began rotating counterclockwise and moving northeast toward Eurasia. As the plates moved toward Eurasia, they started to close the Neo-Tethys Sea. ^[2]

The Neo-Tethys Sea completely subducted under the Eurasian plate during the Oligocene-Miocene. Once all of the oceanic crust had subducted, the Arabian and Eurasian plates began colliding, starting the Zagros Orogeny and building the Zagros Fold and Trust Belt. ^[4] The Red Sea Rift began separating the Arabian Plate and the African plate in the early Oligocene, beginning to form the Red Sea. ^[5]

The stratigraphy of the region shows the development and history of the area. The Precambrian basement is andesitic volcanic and dioritic plutonic rocks. The end of the Precambrian then developed the Hormuz salt that is the main source for diapirs in the area. The Ordovician and Silurian sea level transgression and regression deposited an upward-coarsening shale member then there was a long hiatus of deposition at the end of the Silurian through the Carboniferous. In the Late Permian, after the breakup of the Gondwana super continent, the Arabian plate became a tectonically stable coastal shelf environment. This shelf deposited the many limestones, shales, and marls and stayed a shelf environment until the Late Cretaceous. In the Late Cretaceous, as the Neo-Tethys Sea was subducting, the accretionary wedge caused ophiolite obduction. This obduction caused another hiatus of deposition until the collision of the Arabian and Eurasian plates in the Zagros Orogen. ^[2]

Zagros Fold Belt

The Zagros Fold and Thrust Belt has been studied since 1855 with papers submitted to the Geological Society of London from Loftus.

The oil field is located in the northwestern part of the Zagros Fold Belt. The Zagros Fold Belt is formed by collision of the Arabian and Eurasian tectonic plates starting in the Oligocene.

The Zagros Fold Belt is tectonically affected by many salt diapirs, particularly in the eastern end of the range. These diapirs are mainly sourced from the Late Precambrian-Early Cambrian Hormuz salt deposits. These deposits are 5-10 km below the surface of the earth. ^[8] They were deposited in an evaporite basin in the Precambrian. The thickness of the salt is debated but most estimates put it at 1000-2000 m with some estimates going as thick as 4000 m. These overlying sediments are penetrated by the Hormuz diapirs. ^[9] The Hormuz salt is composed of salt, anhydrite, dolomite, shale, siltstone, sandstone, and some basement rock that was brought up by the diapirs. There are many different morphologies of these diapirs. They range from high relief, surface glaciers to dead, eroded structures. The movement of salt diapirs in the Zagros is attributed to the folding and faulting of the mountains. The main phase of folding is recorded in the growth strata of the Pliocene Aghajari formation. ^[9] The folding of the Zagros is influenced by the plastic décollement of the Hormuz Salt which allows the thick sedimentary strata to glide over the basement. These folds have a fold axis parallel to the mountain belt. The folding and faulting is also influenced by basement and very deep faults, which create horst and graben structures that have a significant influence on uplift of the crust and the distribution salt diapirs. ^[10][11]

Shaikan Oil Field is located in Kirkuk Embayment which is part of the Simply Folded Zone of the Zagros Fold Belt. This is an area that is tectonically less influenced by the Hormuz Salt and is mainly influenced by basement faults and detachment surfaces. ^[6]

Hand drawn picture of Zagros Fold Belt.

Stratigraphy

Stratigraphy of the Hydrocarbon-Bearing Formations of the Shaikan Oil Field

Period	Stage	Age	Formation	Description
Lower Cretaceous	Hauterivian-Barremian	130 Ma	Sarmord	115 m[12] of thinly bedded limestone and marl. Deposited on the shallow shelf during the Lower Cretaceous[13]
Lower Cretaceous	Valaginian-Hauterivian	140-130 Ma	Garagu	180 m[12] of oolitc limestone (mudstones and wackestones) with shale, sandstone (lithic subarkose to arkose), and marl. Benthic forams, gastropods, and brachiopods. Deposited on the shallow shelf or lagoon. [14]
Upper Jurassic-Lower Cretaceous	Tithonian-Berriasian	150-140 Ma	Chia Gara	230 m of thin-bedded medium-yellow limestone with some shale. Mostly micrite. [15] Deposited in the clear, shallow shelf. The contact with the Barsarin is a basal conglomerate.[16]
Upper Jurassic	Callovian-Tithonian	165-150 Ma	Barsarin	20-60 m of thick black limestone with laminated and nodular massive beds. These limestones are believed to be stromatolites. This formation was deposited in the lagoon of a rimmed shelf on a carbonate platform. [17]
Middle Jurassic	Bajocian-Bathonian	170-165 Ma	Sargelu	50-150 m of thick bituminous and dolomitic limestone with black shale and marl. Algae, dinoflagellates, spores, and pollen all found. [18] Deposited in a euxinic marine basin. [16]
Lower Jurassic	Toarcian	180-175 Ma	Alan	70-90 m of bedded anhydrite, oolitic limestones, and brown limestone (not very carbonaceous). No fossils present, deposited in an inland coastal flat environment.[19][16]
Lower Jurassic	Pliensbachian-Toarcian	190-180 Ma	Mus	25-50 m of carbonaceous limestone, mostly wackstone. Has dolomitized limstone with calcareous shale. Deposited in a transgressive marine environment. [19]
Lower Jurassic	Sinemurian	199-180 Ma	Adaiyah	60-90 m of bedded anhydrites with beds of brown limestone, black shale, and green marl with anhydrite nodules. [20][16]
Lower Jurassic	Hettangian-Sinemurian	200-190 Ma	Butmah	500 m of oolitic limestone, detrital limestone, shale, and anhydrite. The middle of the section contains sandstone and shale beds. This formation was deposited in a shallow water lagoon. [16]
Upper Triassic	Norian-Rhaetian	220-200 Ma	Baluti	60 m of shale with thin dolomitic and oolitic limestone and recrystallized breccias. Rare fossils. Restricted lagoon and estuary depositional environment. The last green shale bed is the contact with the Kurre Chine. [16]
Upper Triassic	Carnian-Norian	235-220 Ma	Kurre Chine	Up to 850 m of black and brown limestones in thick and thin beds with some thick dolomite and shales. Some recrystallized breccias. Some evaporites present. Lagoonal depositional environment. [16]

Hand drawn stratigraphic column of the Mesozoic sections of Shaikan Oil Field.

Cross Section

Digitally illustrated cross section of Shaikan Oil Field.

Petroleum System

Seal

A seal is a rock that acts as a barrier to migrating hydrocarbons. Anhydrites, shales, and salts are some of the most prominent seals, but as long as the displacement of the seal is higher than the buoyancy of the hydrocarbons. There are regional seals that stop migration as well as local seals that capture small accumulations.^[21]

The main type of seal in the Shaikan oil field is from anhydrites. There are seals from the Hormuz salt in the region but not in the Shaikan Field.

The anhydrite and shale beds of the Adaiyah formation are a good seal for the Butmah reservoir. The porosity of the Adaiyah Formation is between 0 and 7%. ^[22] It is mosly kerogen type IV. It has a low TOC of 0.08 to 0.47%. ^[23]There also may be some local anhydrite seal layers in the Butmah and Kurre Chine formations.

Source

Source rocks are rich in organic matter and will produce oil or gas if in the right thermal conditions. These source rocks (generally limestone and shale) have about 1% organic material and at least 0.5% total organic carbon. ^[24]

The main source rocks in the Shaikan field are the Kurre Chine, Sargelu, Butmah, and Chia Gara formations.

The Kurre Chine total organic carbon values indicate a fair to good amount of carbon. Kurre Chine has a mixed type II-III and type III kerogen. Kurra Chine entered the early oil window at 143 Ma and reached peak oil window at 130 Ma. It is currently in the main oil window and is thermally mature. ^[25]

The Sargelu is a 100 m thick limestone that has a total organic carbon composition of about 1-5 % and generates type II and III kerogens. The thermal maturity is not very high in the area. The Sargelu entered the oil generation phase in the Eocene. The generation potential is good at 5 mg HC/g of rock. It is one of the most important source rocks in the area. ^[26]

The Butmah formation is a fair source rock due to total organic carbon levels that aren't exceptionally high. It mainly has type III kerogen with some type II and IV in areas. ^[27] The average porosity is 8%, showing that it is a source rock but also a reservoir rock being sealed by the Adaiyah Formation. ^[28]

The Chia Gara is a 230 m thick rock with a total organic carbon composition of around 1.7 %. The kerogen in the formation belong to type II and III. The pyrolysis analysis shows that the maturity of the formation is mature. This is a fair source rock.

Reservoir

A hydrocarbon reservoir is a rock that has enough permeability for fluid migration, enough porosity to store the hydrocarbons, and a high saturation of hydrocarbons.

The Sarmord and Garagu are two of the younger reservoir rocks in the field. The average porosity of the Sarmord and the Garagu are 10 and 14%, respectively. They also have an average of about 30% water saturation, meaning that water takes up 30% of pore space in the rock. The Sarmord has a "pay zone" (area of reservoir with economically extractable hydrocarbons) of 36m and the Garagau has a "pay zone" of 93m. ^[29] The kerogen types are mostly type III and IV and the organic matter is all still immature.

The Alan and Mus formations are thin reservoir layers. The Alan formation has a total organic carbon of 1.47-1.82% while the Mus has a TOC between 0.85 and 1.75. The kerogen type is mostly type II in this area. ^[23]

The Barsarin formation is a good reservoir and source rock in the region. The average value of total organic carbon is 1.7%. The kerogen in Barsarin sections are type III and IV. Barsarin is in the mature stage and is in the oil and early gas window. It has an average of 0.71 mg HC/g of rock. ^[30]

Trapping Mechanism

Folds and faults from the Zagros Fold Belt convergence cause structural oil traps in Shaikan. The anticlinal traps in the area have a fold axis that is parallel to the Zagros Mountain Belt. The anticlines make a dome that will accumulate hydrocarbons under a seal rock layer. There is also a major fault surface cutting through the section, creating structural traps along the fault plane where the fault causes an impermeable and a permeable layer to be adjacent to each other. The combination of a source rock, a seal rock, and a trapping mechanism lead to the formation of hydrocarbon reservoirs. ^[31]These trapping mechanisms prevent further migration of hydrocarbons, which is the process in which hydrocarbons seep from source to reservoir rocks until reaching a trap, either a stratigraphic seal or a structural seal. The hydrocarbons can go through primary migration, where they seep from the source rock to the reservoir rock. They can also go through secondary migration along a "carrier bed" or tertiary migration where the hydrocarbon moves from one trap to another.^[32]

Thermal History

The thermal history of Shaikan is found from the depth, temperature, and oil/gas window. The different methods of determining thermal maturity are through calculations using peak temperature and vitrinite reflectance. As source rocks are buried, they pass through different windows of thermal maturity. The shallowest and lowest temperature hydrocarbon formation is kerogen, followed by oil, then gas. The oil window is 60-120 degrees Celsius (at 2-4 km depth) and the gas window is 120-180 degrees Celsius (4-6 km depth).^[33] All but one of the hydrocarbon bearing formations are oil bearing, meaning that they have not been above 120 degrees Celsius while at the appropriate depth. The gas bearing section of the Kurra Chine Formation is the only formation in the field that is in the proper window of thermal maturity and peak temperature over 120 degrees Celsius while being at 4-6 km deep. ^[34]

Petroleum Exploration and Drilling

Shaikan was discovered in August 2009 with production starting in July 2013 and exportation starting December of that year. Operated by Gulf Keystone Petroleum.

Produced over 70 million barrels with a capacity of 40,000 to 50,000 barrels a day. Estimates put the oil in place volume at 13 billion barrels. Through June 2020, the average barrels per day was at 37,000.