Abstract There are more than 100 oil and gas fields in Iraq, containing more than 137 billion barrels of recoverable oil and more than 106 TCF of recoverable gas. Of this large resource, about 25 billion barrels of oil and 11 TCF of gas have been produced to date. Nearly all of the oil and gas occurs in fields located within the Mesopotamian foredeep, Gotnia Basin, and Zagros foldbelt. Minor discoveries and shows have been found on the Arabian platform along the western flank of the Mesopotamian foredeep. There is one gas discovery (Akkas field) on the Arabian platform in western Iraq.Ninety-eight percent of the oil and gas occurs in reservoirs of Cenozoic and Cretaceous age. The largest reserves occur in: 1) carbonate rocks of the Kirkuk Group (Lower Miocene–Oligocene), in fields within the Zagros foldbelt of northeastern Iraq, the largest being Kirkuk field; 2) carbonate rocks of the Mishrif Formation (Turonian–Cenomanian), in fields within the Mesopotamian fore-deep and Zagros foldbelt in southern and central Iraq, including Rumaila, West Qurna, Majnoon, Halfayah, Zubair, and Buzurgan fields; and 3) siliciclastic rocks of the Zubair Formation (Albian– Barremian), in fields within the Mesopotamian foredeep and Zagros foldbelt in southern and central Iraq, including East Baghdad, Rumaila, West Qurna, and Zubair fields. Large reserves also occur in carbonate rocks of the Upper Cretaceous above the Mishrif Formation and in the Lower Cretaceous below the Zubair Formation. Smaller reserves occur in other Neogene and Paleogene carbonates and siliciclastics, in Jurassic and Triassic carbonates, and in Ordovician siliciclastics.Most of the oil and gas that have been discovered were generated from organic-rich, oil-prone carbonates of the Jurassic Sargelu and Naokelekan Formations. These source rocks are widely distributed and mature for oil and gas generation across the Mesopotamian foredeep and Zagros foldbelt. Lesser amounts of oil and gas are derived from: 1) Upper and Lower Cretaceous oil-prone source rocks within the Zagros foldbelt; 2) Triassic oil-prone source rocks in northwestern Iraq; and 3) Silurian gas-prone source rocks in western Iraq. The oil generated from the Jurassic source rocks migrated vertically to fill stacked reservoir intervals in many fields. Lateral migration of oil occurred along the western margin of the Mesopotamian foredeep, as proven by small fields and large seeps that are located where source rocks are absent or immature for oil or gas generation. The tectonics and sedimentation during the Phanerozoic created the source-rich setting of Iraq. Sedimentation was widespread across Iraq during most of the Paleozoic, although intervals of strata are absent and interpreted either not to have been deposited or to have been eroded following deposition. A notable example is the absence of Early and Middle Devonian strata from Iraq. The distribution of Pre-Permian strata, including the gas-prone Silurian source rocks, and therefore the Paleozoic hydrocarbon system and plays, was controlled in large part by erosion at the unconformity that separates Late and Early Carboniferous. Following siliciclastic infill of the topography at this unconformity, Iraq was covered by platform-wide deposition of carbonate sediments on an east-dipping ramp in the Permian, Triassic, and Early Jurassic. Continental fragments rifted from the northern and eastern margins of Arabia during these times, after which a large portion of eastern Iraq subsided. This intra-shelf basin, named the Gotnia Basin, was the location of organic-rich “starved” sedimentation in the Middle to Late Jurassic that formed the important Sargelu and Naokelekan source rocks. The Gotnia Basin was rimmed by carbonate shelf margins.The Gotnia Basin filled with anhydrite and salt layers in the latest Jurassic and then continued to fill throughout the Cretaceous, primarily by prograding carbonate and siliciclastic sediments that entered the basin along the southern, western, and northern margins. These progradational deposits contain the major reservoir units of Iraq, whereas the basinal sediments include most of the sealing facies, predominantly marine shales, as well as some locally important source rocks. The basinal area continued to contract by sedimentary infill until the Turonian, when the Arabian plate was disturbed by a compressional event related to the initial closing of the Tethys Ocean. This event is marked by a hiatus in deposition and, at least locally, erosion at an unconformity. Faults were inverted and folding of anticlines in the Mesopotamian foredeep (and probably in the Zagros foldbelt) occurred at this event.Marine waters flooded across this unconformity in the Late Turonian and Coniacian, with renewed carbonate deposition. Ophiolites were obducted onto Arabia in the middle part of the Late Cretaceous and are present in far northeastern Iraq. At the same time, thick calcareous and muddy sediments filled rapidly subsiding extensional grabens present in western Iraq. A subsiding trough formed in the position of the existing Mesopotamian foredeep and Zagros foldbelt of Iraq, and siliciclastic sediments started to fill the deep-water trough from the north and northeast due to the encroaching Tethys closure. Basinal carbonate and shale were deposited in the center of the trough in the Paleocene and Eocene, while the basin was partly filled by prograding carbonate shelf margins located on the southwestern and north-eastern sides of the basin. This continued through the Oligocene and Early Miocene, until the remaining basins were filled with anhydrite of the Dhiban Formation and the overlying carbonates of the Jeribe Formation in the Early Miocene. Deposition throughout the Late Cretaceous and Cenozoic provided the burial for the Mesozoic source rocks to generate oil and gas. Closure of the Tethys Ocean continued, with local uplift and erosion of areas of Iraq in the Early Miocene. Thick evaporitic deposits of the Fatha Formation lap onto and over the older Cenozoic strata. The marine seaway along the Mesopotamian foredeep was closed by the Middle Miocene, with ensuing non-marine siliciclastic deposition and folding associated with west-verging shortening deformation in the Zagros foldbelt. Oil and gas plays are proven and their distribution is well known for the Cenozoic and most of the Cretaceous. The deeper plays of the Lower Cretaceous, Jurassic, and older reservoirs are less well explored in the Mesopotamian foredeep, Gotnia Basin, and Kirkuk embayment of the Zagros foldbelt. Exploration is occurring in the marginally drilled interior of the Kurdistan region of the Zagros foldbelt in northeastern Iraq. Paleozoic plays in western Iraq are also marginally explored.

Structural disharmonies between rock masses lying above and below the salt-bearing Lower Fars formation are characteristic of the Alpine foothill folds of the Zagros mountain belt in parts of northern Iraq and southwestern Iran. Most impressive and what appear to be chaotic structural discrepancies have been described from the Iran sector of the saliferous Fars basin, where depositional thicknesses of salt were great. Salt deposits were much thinner in Iraq, and lacked the spectacular disharmonies of the Iran deposits. The typical disharmonic habit in northeastern Iraq is the overthrusting of the northeastern flanks of the long and strongly developed anticlines. Overthrusting has generally been attributed to tectonic compression in the later phases of folding, or to southwestward gliding of the post-salt cover, as a unit, off the rising “high” of the Zagros. Both interpretations are inadequate in some particulars to account for some features of the overthrusts. The thesis is advanced that the overthrusts resulted from gliding adjustments of each synclinal fill to gravitational changes caused by isostatic recovery of the Zagros after the fold-producing orogeny. Overthrusting essentially postdated the completion of folding, being facilitated by decreases in competency of the anticlinal crests as these became deeply eroded. It was controlled by the excess of load in the northeast of each syncline, which resulted from development of high relief during progressive regional tilting.

Abstract Outcrop studies integrated with subsurface data of core, cuttings and different well logs were used to investigate reservoir characteristics of the Middle–Late Eocene Pila Spi Formation at Taq Taq oil field of Kurdistan Region of northeastern Iraq. Reservoir studies include petrographic investigations, microfacies analyses and petrophysical evaluation in an attempt to provide an insight regarding the reservoir potential. The Pila Spi Formation is subdivided into four distinctive lithofacies (P1–P4), characterized by dolostones, dolomitic limestones and limestones. Several types of dolomite were recognized ranging from early diagenetic fenestral fine crystalline to late diagenetic coarse crystalline dolomite, which had positively influenced the reservoir characteristics by enhancing inter-crystalline, intra-skeletal and micro-vug porosity, especially for lithofacies units P2 and P3. Reservoir porosity is heterogeneous in distribution and ranges from 5 to 20%. Using porosity cut-off values of 8.2%, and water saturation cut-off values of 24%, six porosity units were identified from top to bottom (PU1–PU6). The best unit is PU2 (15 m thick), which is characterized by medium crystalline dolomite mosaic with average effective porosity of 21.5%. Permeability ranges between 0.1 and 1 md. Flow unit differentiation is discussed in term of porosity–permeability cross-plot and reservoir pore-throat classification (R35). Results indicate that most of the Pila Spi reservoir is of micro-port (matrix) flow type. However, reservoir quality enhancement is attributed to fracturing.

Abstract The Taq Taq Field is located within an anticline in the gently folded zone of the Zagros mountains, northeastern Iraq, approximately 50 km ESE of Erbil. The main reservoirs are fractured limestones and dolomites of Late Cretaceous age, with an oil column exceeding 500 m in thickness. Eocene limestones and dolomites at shallow depth form a subsidiary reservoir. The structure is a gentle thrust-related fold which has also been affected by dextral transpression. A pervasive fracture system is present within the reservoirs, giving good connectivity and deliverability. Initial discovery and appraisal was made in 1978 when three wells were drilled. The recent appraisal programme started in 2005 and by the end of 2008 two seismic surveys had been acquired and eight additional wells had been drilled. Mapping has incorporated a seismic principal component analysis for horizon and lithology identification. Modelling of the fractures has utilized a comprehensive data set derived from core and image logs. Special core analysis has been directed towards the understanding of the pore system and its interaction with the fractures. Synthesis of all these elements is performed in a dual-media dynamic model which is currently in use for development planning.

Abstract Two depositional basins were present in the Kirkuk Basin of northeastern Iraq during the Burdigalian and Early Langhian ages (Late Lower–Early Middle Miocene) and have been studied at several oilfields. Both basins display shallowing upward sequences composed of carbonate and evaporite sediments. Evaporites form part of the sedimentary sequences that begin with normal open marine and shelf deposits and pass upward into thickly bedded and massive- nodular anhydrite that alternates with thinly bedded limestone (dolomitized with anhydrite nodules), and that is unconformably overlaid by massive bedded lagoonal limestone. The first basin of (Burdigalian age) is represented by the Serikagni, Euphrates and Dhiban formations, whereas the second of Langhian age includes the Jeribi and Fat'ha formations. The early part of the Burdigalian age is characterized by a marine transgression that covered a large part of the region, resulting in a broad spectrum of marine environments ranging from the open marine in the uppermost part of the gentle slope environments, evidenced in the Serikagni Formation, to the shallower environments of fore- and back-shoals of the Euphrates Formation. Regression took place at the end of this period and resulted in the deposition of the Dhiban Anhydrite Formation. The basin of deposition of the Dhiban Formation is an inherited basin from the older Serikagni and Euphrates basins. This is reflected in an inhomogeneous distribution, both of type and thickness, of evaporites. The Early Langhian age of the Kirkuk Basin is also characterized by a shallowing-upward sequence begun by sediments rich in planktonic foraminifera for the lower part of the Jeribe Formation and then shallow water and lagoonal carbonates for the upper part of the Jeribe Formation. The environment passed upwards into the evaporitic sequence of the Fat'ha Formation.