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.

ABSTRACT A migration geochemical study was conducted over an area of approximately 15,000 km 2 (5792 mi 2 ) in northeast Saudi Arabia to define regional migration patterns and de-risk oil charge away from the Late Jurassic source kitchen of the Gotnia Basin to prospects further to the south and west on the shelf margin and Summan Platform. Discovered accumulations range in depth from more than 10,000 ft (3048 m) subsea on the shelf margin in the north to around 5000 ft (1524 m) on the Summan Platform in the south. Shallowing south/southwestward is associated with a wide API gravity variation (15–35°) and gentler molecular and isotopic maturity trends. Defining migration patterns based on bulk, isotopic, and saturated and aromatic hydrocarbon distributions alone proved ineffective, especially given that all oils examined from the target Arab-A reservoir have a narrow peak-oil window maturity and a largely common source, that is sulfur-rich (up to 6.1 wt.%) anoxic marine carbonate, presumably within the Hanifa/Tuwaiq Mountain formations or their equivalent Najmah/Sargelu formations in the Gotnia Basin. Molecular geotracers, based primarily on compositional correlation coefficients of aromatic nitrogen compounds, identified several southwest-trending migration pathways that have charged traps in the Late Jurassic Arab Formation. Prospects falling along inferred migration pathways or entry/spill routes are therefore high-graded.

Abstract In basin and petroleum system modeling, the spatial resolution of models is too coarse to cover all of the relevant geologic processes that occur in reservoirs. Reservoir models are built on a static (time invariant) grid and cannot cover the charge history of a field or processes related to changing geologic structures through time. A new method to combine regional-scale petroleum system models and local reservoir- or prospect-scale models was developed and applied to an oil field in Kuwait. In the field, heavy oil zones occur at the original oil-water contact and also in stratigraphic and structural positions above it. Heavy oil occurs in the highly permeable Fourth Sand and Middle Third Sand of the Burgan Formation in the field. This study demonstrates that the heavy oil distribution in those layers can be explained by the petroleum charge history. An early charge from the Cretaceous Makhul Formation was replenished by the Cretaceous Kazhdumi Formation, the stratigraphic equivalent of the Burgan Sands. These sediments were deposited in the area of the Dezful Embayment of the Zargos Fold Belt. No Jurassic charge, breaking through the Gotnia evaporites, is needed to fill the structures of the Cretaceous Burgan reservoirs in the field. Well data and the results of the high-resolution petroleum system model covering the area of the oil field and describing the distribution of charge from the regional model to the field scale lead to the conclusion that the heavy oil zones are mainly the result of gravity segregation, although some influence of water washing cannot be excluded.

Abstract Kuwait has proven oil reserves and production from supergiant and giant fields that include the Greater Burgan (Burgan, Ahmadi, and Maqwa), Raudhatain, Sabriya, and Minagish fields. These fields are associated with very gentle oval anticlines interpreted as drape structures over deep-seated fault scarps or as growth structures related to halokinesis. These structures are generally very simple, consisting of a series of roughly parallel, anticlinal uplifts trending NNW-SSE, with a few having a more N-S to NNE-SSW trend. Reservoir rocks are found in the Jurassic Marrat, Sargelu, and Najmah Formations (carbonates), the Lower Cretaceous Ratawi and Minagish Formations (sandstones and carbonates), and the Middle Cretaceous Burgan and Wara Formations (sandstones), as well as the Mauddud and Mishrif Formations (carbonates). Depth of reservoirs range from 3680 m (12,073 ft) in the Middle Jurassic to 2000–3650 m (6561–11,975 ft) in the Lower Cretaceous and 1000–2570 m (3281–8432 ft) in the Middle Cretaceous. The most important reservoirs are the Lower and Middle Cretaceous sandstones, which are sealed by interbedded and overlying shales. Several Jurassic and Cretaceous limestone units form additional, but subordinate, reservoirs that are generally sealed by shales. Only the Upper Jurassic Gotnia salt and the overlaying Hith Anhydrite seem to act as a regional seal for Middle Jurassic limestone reservoirs. Proven and potential source rocks with high TOC values, characterized by a mixture of marine and terrestrial sapropelic organic matter, are present in the upper-Lower and Middle Jurassic and the Lower and Middle Cretaceous. Kerogens from these rocks fall between Type II and II–III. The maturity level and quality of the kerogen in the Makhul (Sulaiy) Formation suggests that they are the most likely source rocks for the Cretaceous reservoirs, and responsible for generating part of the oil which has accumulated in present structures. Source rock characteristics for the Jurassic succession vary and range from moderate to excellent TOC values in the Sargelu and Najmah Formations. Similarly, the Middle Jurassic succession potentially represents mature oil generation. Oil generation from Jurassic source rocks began in the Late Cretaceous at the time when structural traps had already started to form. The Makhul (Sulaiy) source rock entered the oil window during the Early Tertiary, whereas oil expulsion occurred throughout Tertiary time.

Abstract Geologic Problem Solving with Microfossils: A Volume in Honor of Garry D. Jones SEPM Special Publication No. 93, Copyright © 2009 SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-137-7, p. 127–152. Depositional and potential reservoir layers in the Jurassic carbonates of Saudi Arabia cannot necessarily be recognized by core description or wireline log interpretation alone. Recent studies based on thin-section micropaleontology of closely spaced core samples have led to elucidation of such layers in heterogeneous reservoir carbonates, based on the vertical tiering of various paleoenvironmentally sensitive biocomponents. When integrated with core descriptions, such biofacies and their lateral variations provide a powerful tool for optimal reservoir development. The biofacies guide sequence-based frameworks towards three-dimensional lithofacies and reservoir facies distribution models, with the added power of facies prediction away from drilled locations. The 13 carbonate reservoirs in the Jurassic Shaqra Group of Saudi Arabia are developed in seven formations, of which the Arab Formation hosts the world’s largest onshore reservoir. The lithostratigraphic succession is composed of the Lower Jurassic Marrat Formation, the Middle Jurassic Dhruma and Tuwaiq Mountain formations, and the Upper Jurassic Hanifa, Jubaila, and Arab formations, which terminate with a succession of interbedded carbonates and evaporites of which the final thick evaporite is termed the Hith Formation. The Marrat Formation, of Toarcian age, hosts the Marrat Reservoir. The Dhruma Formation, of Bajocian–Bathonian age, hosts the Faridah, Sharar, and Lower Fadhili reservoirs. The Tuwaiq Mountain Formation hosts the Upper Fadhili and Hadriya reservoirs, and is of Callovian age. The Oxfordian Hanifa Formation hosts the Hanifa reservoir, which is overlain by the Jubaila and Arab formations, of Kimmeridgian age, and together host the Arab Reservoir. The Hith Formation is of probable Tithonian age and hosts the Rimthan and Manifa reservoirs. An ascending order of tiered deep- to shallow-marine foraminiferal assemblages has been determined for each formation and applied to distinguish both long-term and high-frequency paleobathymetric variations. Micropaleontology has regained its importance in such industrial applications, and has significantly outgrown its traditional, but equally important, chronostratigraphic use. Because age-significant foraminiferal species are rare in the Shaqra Group, micropaleontological biofacies and their paleoenvironmental aspects are of especial importance. In addition to the intrareservoir applications, recent studies are focusing on the application of biofacies for regional studies and delimitation of potential reservoir targets in frontier areas. Using this technique for the Hanifa Formation, a regional lithofacies trend map has been deduced, which is currently assisting seismic program planning for exploration activities.

Abstract This chapter summarizes the tectonic events that have affected the region of the Arabian Intrashelf Basin and the development of the intrashelf basin. Precambrian–Infracambrian fault systems provided a structural framework, which was later reactivated during the Late Paleozoic. Further development along the structural trends continued in the Triassic and Early Jurassic with the development of an Early Jurassic tectonically controlled intrashelf basin. The accommodation space was filled with Dhruma Formation carbonates by the early Bathonian, resulting in a broad and fairly flat platform. A crucial factor is how suppressed tectonism was during the Mid- and Late Jurassic. The intrashelf basin developed on the broad, tectonically stable Tethyan passive margin continental shelf 200--300 km distant from the Tethyan outer shelf edge. During the Mid- and Late Jurassic, this tectonic stability provided the foundation for a broad, stable Tethyan continental shelf region at least 1000 × 1200 km in area, far removed from siliciclastic sources, within which the huge Arabian Intrashelf Basin formed and the sequences of carbonate rocks and evaporites that created the Jurassic hydrocarbon system were deposited. Tectonism during the Mid- to Late Jurassic evolution of the Arabian Intrashelf Basin initially included only moderate subsidence. There was subtle uplift along some of the structural trends in the Late Jurassic and uplift and westwards structural tilt along the Tethys oceanic margin. Relatively stable tectonism continued after the Jurassic and was a major factor in the accumulation of the vast reserves of Jurassic sourced oil. Tectonic stability prevented major faulting and structural compartmentalization of the basin features, which provided large areas for hydrocarbon migration as the large and broad anticlines developed into the huge structural traps. The lack of major faulting limited the movement of burial fluids, preserving the early formed porosity and the regionally extensive seals. The present day structures primarily developed from the Late Cretaceous to Miocene as the Tethys Ocean closed.

Abstract This is the first of two chapters in which the data presented in the first four chapters of this Memoir are reviewed and interpreted. A summary cross-section across the basin from the Saudi outcrop to Abu Dhabi and the Tethyan margin is used to summarize the regional setting and sequences as interpreted in this Memoir. The key points illustrated by the summary cross-section are stated and discussed in the first part of this chapter. Two different interpretations for the eastern margin of the intrashelf basin are reviewed. In the first of these interpretations, the margin with the deeper waters of the Tethys Ocean shelf is adjacent to the intrashelf basin rim in Abu Dhabi, whereas in the second, preferred in this Memoir, a broad, shallow Tethyan shelf platform (200–300 km wide) extends from the intrashelf basin rim to the Tethys continental shelf edge. The implications of Late Jurassic exposure and erosion on the adjacent Tethyan shelf are discussed. The development of the Arabian Intrashelf Basin during the Callovian–late Oxfordian (the Tuwaiq Mountain Formation, the source rock and the Hanifa intervals and associated sequences) is interpreted and discussed with illustrations, including step-by-step facies maps. The interpretation integrates depositional, eustatic and tectonic factors in the evolution of the intrashelf basin. The interpretation in this Memoir is that the Arabian Intrashelf Basin began with isostatic and extensional subsidence on top of a broad Dhruma Atash Member platform, which had largely filled the accommodation space up to the wave base and to near sea-level. It developed fully into an intrashelf basin during the deposition of the Callovian Tuwaiq sequence, with rising sea-levels coincident with a productive shallow water carbonate factory resulting in a rim of shallow water carbonate. An end-Callovian to early Oxfordian lowstand terminated the Tuwaiq sequence on the basin rim. During the lowstand, restricted conditions in the basin deposited the rich Lower Hanifa source rock as a lowstand systems tract. As more normal conditions returned in a subsequent sequence, the source rock facies graded upwards into Hanifa reservoir facies, which partially filled the basin. Hanifa progradation was terminated by another lowstand during which a subaqueous gypsum/anhydrite marker bed was deposited in at least part of the remnant basin. Earlier interpretations of these sequences are also discussed.

Abstract The Jurassic Arabian Intrashelf Basin provides the setting for the world's greatest conventional oil reserves, including the world's largest oilfield, the supergiant Ghawar field. The stratigraphic interval corresponding to the development and infill of the Arabian Intrashelf Basin is from the uppermost Dhruma Formation to the top of the Hith Anhydrite Formation, spanning the late Bathonian–early Callovian to Tithonian. Many areas of the intrashelf basin have been well described in recent years and the stratigraphic succession has been defined in sequence concepts, but the regional development of the intrashelf basin has not been well synthesized. This Memoir builds on published data to give a regional interpretation of the geological evolution of the Arabian Intrashelf Basin. This introductory chapter reviews some of the earlier work, summarizes the key events and elements in the geological history of the Arabian Intrashelf Basin and gives a brief review of the history of petroleum exploration in this region. It is intended to serve as an extended abstract to introduce the general setting and summarize the contents of this Memoir, including some of the proposed revisions of depositional models, correlations and the sequence nomenclature, providing a context for considering and evaluating each subsequent chapter. The themes summarized in this chapter are documented and discussed in much greater detail in the subsequent chapters of this Memoir.

Abstract This chapter includes 11 cross-sections and one well log profile to show the depositional geometry and setting in specific areas around the basin: the Saudi Arabia outcrop belt; the Rimthan Arch; and the eastern and central areas of the intrashelf basin in Saudi Arabia, Bahrain, Qatar, Abu Dhabi and Oman. These cross-sections are used to demonstrate the similarity and degree of continuity of the upper Dhruma Formation, the Tuwaiq Mountain Formation, the source rock, the Hanifa, Jubaila–Arab and Arab–Hith formations and depositional sequences in these different locations in the basin. They show the manner in which the underlying platform formed, the rim developed, the source rock was deposited and the basin progressively filled. The blanket deposition of the Arab-D anhydrite was followed by the Arab-C to Arab-A and Hith carbonate and evaporite sequences. The cross-sections provide the framework used in subsequent chapters to make a series of facies maps and other interpretative diagrams and cross-sections that summarize and, for some intervals, revise the interpretation of the settings and geological events that formed the Arabian Intrashelf Basin.