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The Palaeozoic basin of Jordan and Iraq occupies the NW part of the Arabian Plate. The Lower Palaeozoic succession is the most under-explored succession in Iraq, particularly in western Iraq where it is considered the most promising target for future exploration. The western Iraqi subsurface and Jordan geology are characterized by similar major chrono-stratigraphic rock units. In western Iraq, the Akkas-1 well is the only gas and condensate discovery in sandstone reservoirs of Early Silurian and Late Ordovician age. Lower Silurian sandstones of the Akkas Formation are an oil reservoir characterized by a mean porosity of 6.5% and permeability of 0.2mD. Underlying Upper Ordovician sandstones of the upper part of the Khabour Formation are also reservoir, but for sweet gas and condensate. The latter are comparable to both the Risha and Wadi Sirhan discoveries in Jordan. Several deeper potential sandstone reservoirs (of older Ordovician and Cambrian ages) are also present across the region. Equivalent Ordovician age Jordanian reservoirs are primarily developed in homogeneous, highly mature quartz arenites of storm-dominated clastic-shelf stratigraphies. Within the Risha Field and the Wadi Sirhan area, are also end-Ordovician (Hirnantian) glaciogenic and periglacial sandstones with more variable reservoir quality characteristics compared to the underlying quartz arenites. These often comprise highly immature arkoses (as diamictites) that provide a mineralogically much less stable assemblage in which there is a possibility of developing secondary porosity. Sedimentological and XRD mineralogical studies suggest that diagenesis is an important factor in developing of reservoir quality. Nevertheless, reservoir quality is a major exploration risk where burial was significant that led to quartz overgrowth and clay cementation. It is notable that some sandstone reservoirs have chlorite-clay coatings, which prevented formation of the quartz overgrowth and preserved good reservoir quality. The widespread occurrence of associated source and seal rocks together with the proven and potential reservoirs, suggests that Lower Palaeozoic prospects below the deserts of western Iraq are promising targets for future hydrocarbon exploration.

The Middle Jurassic–Lower Cretaceous strata of the NE Arabian Plate contain several prolific source rocks providing the charge to some of the largest world-class petroleum systems. They are located within the Zagros Fold Belt and Mesopotamian Foreland Basins covering the northern, central and southeastern parts of Iraq, Kuwait and western and southwestern Iran, particularly the Lurestan and Khuzestan provinces. These source rocks include the Bajocian–Bathonian Sargelu, the Callovian–Lower Kimmeridgian Naokelekan and the Upper Tithonian–Lower Berriasian Chia Gara formations of Iraq and their chronostratigraphic equivalents in Kuwait and Iran. They have charged the main Cretaceous and Cenozoic (Tertiary) reservoirs throughout Iraq, Kuwait and Iran with more than 250 billion barrels of proven recoverable hydrocarbons. These formations represent the transgressive system tracts of sequences deposited within deep basinal settings and anoxic environments. They are dominated by black shales and bituminous marly limestones, with high total organic carbon (TOC) contents (ranging from 1–18 wt%), and by marine Type IIS kerogen. Their Rock-Eval S2 yields may reach up to 60 mg HC/g Rock, particularly along the depocentre of the Mesopotamian Foreland Basin. The immature hydrogen index (HI) values might have been up to 700 mg HC/g TOC, whereas the present-day observed values vary depending on the location within the basin and the present-day maturity. The Source-Potential Index (SPI; i.e. mass of hydrocarbons in tons, which could be generated from an area of 1 sq m in case of 100% transformation ratio) averages around 2–3, but can even reach up to 14–16 along the basins’ centres. The Sargelu and the overlying Naokelekan-basinal Najmah formations (and their equivalents) could represent the best potential shale-gas/shale-oil plays in Iraq, Kuwait and Iran, due to their organic richness, favourable maturity and the presence of regional upper and lower seals. The estimated oil-in-place for the potential Sargelu shale-oil play in Iraq only is around 1,300–2,500 billion barrel oil-equivalent (BBOE) and in Kuwait is about 7–150 BBOE.

The middle Cenomanian – early Turonian Mishrif Formation, a major carbonate reservoir unit in southern Iraq, was studied using cuttings and core samples and wireline logs (gamma-ray, density and sonic) from 66 wells at 15 oilfields. Depositional facies ranging from deep marine to tidal flat were recorded. Microfacies interpretations together with wireline log interpretations show that the formation is composed of transgressive and regressive hemicycles. The regressive hemicycles are interpreted to indicate the progradation of rudist lithosomes (highstand systems tract deposits) towards distal basinal locations such as the Kumait, Luhais and Abu Amood oilfield areas. Transgressive hemicycles (transgressive systems tract deposits) represent flooding of the shallow carbonate platform and are recorded in oilfields such as Amara, Halfaya and Zubair. A sequence stratigraphic framework has been constructed for the Mishrif Formation based on correlation of the transgressive and regressive hemicycles which are separated by maximum flooding surfaces. Three third-order sequences are identified which show lateral and vertical facies variations depending on relative sea-level changes. Sequence boundaries are characterized by karstic, exposure and drowning features. Middle Cenomanian – Turonian eustatic sea-level changes together with regional-scale tectonic deformation of the Arabian Plate controlled the availability of accommodation space and therefore the depositional profile during development of each sequence. Both of these factors controlled the maximum flooding surfaces and sequence boundaries which have been identified. The sequence stratigraphic key surfaces presented in this study represent typical candidate horizons or datum surfaces for future seismic or resevoir modelling studies. Also, lateral facies variations in each transgressive-regressive sequence and associated carbonate bodies (i.e. prograding shelf margin, forced regressive wedge) may form important stratigraphic traps in the Mesopotamian Basin.

The CenomanianÐEarly Turonian reservoirs of the Mishrif Formation of the Mesopotamian Basin hold more than one-third of the proven Iraqi oil reserves. Difficulty in predicting the presence of these mostly rudistic reservoir units is mainly due to the complex paleogeography of the Mishrif depositional basin, which has not been helped by numerous previous studies using differing facies schemes over local areas. Here we present a regional microfacies-based study that incorporates earlier data into a comprehensive facies model. This shows that extensive accumulation of rudist banks usually occurred along an exterior shelf margin of the basin along an axis that runs from Hamrin to Badra and southeast of that, with additional interior rudist margins around an intra-shelf basin to the southwest. Regional tectonism defined the accommodation sites during the platform development. Facies analysis allowed the recognition of 21 microfacies types and their transgressive-regressive cyclic stacking pattern. Sequence-stratigraphic analysis led to the recognition of three complete third-order sequences within the studied Mishrif succession. Eustatic sea-level changes were the primary control on this sequence development but local tectonics was important at the Cenomanian/Turonian boundary. Rudist biostromes are stacked as thicker shallowing-up cycles composed of several smaller-scale cycles. In places, smaller cycles are clearly shingled (stacked laterally). Iraq’s Mishrif sequences are thus analogous to coeval systems across the Arabian Plate in Oman, United Arab Emirates, offshore Saudi Arabia and Kuwait, southwest Iran and the Levant. Analysis of poroperm trends shows porosity increasing beneath sequence boundaries due to karstification and meteoric dissolution. The presence of interconnected vugs in grain-dominated fabric make the rudist biostromes the best reservoir units. Dissolution of aragonitic components of rudist shells was the most important diagenetic process that enhanced reservoir characteristics. The presence of rudist-bearing facies with their diagenetic overprint within regressive cycles is considered the primary factor in effective porosity development and distribution. As a result, because of depositional heterogeneities (facies type distribution and their 3-D geometries) and the influence of sequence boundaries on reservoir quality, each field shows unique geometrical combinations of pay zones, barriers and seals.

The carbonate reservoirs in the Late Oligocene-Early Miocene Asmari Formation in the Dezful Embayment of SW Iran are characterized by low matrix permeability, and effective drainage is dependent on the occurrence of open fractures. Limited information on fracture orien- tation and fracture density is available from core and borehole image data, and high-quality/high- resolution three-dimensional seismic is often lacking in this area. Well and core data do not contain information on important fracture parameters like length distribution, crosscutting relationships, fracture density v. lithology and bed thickness. The understanding of fracture distri- bution and formation in the region and their effects on fluid flow has been greatly improved by the use of outcrop analogue data. Exposures of the Asmari Formation in the Khaviz Anticline are in close vicinity to the giant hydrocarbon fields. The Khaviz Anticline has a similar geometry and structural history to the major hydrocarbon fields in the area, and represents an excellent analogue for these. Two types of fracture features were observed: diffuse fracturing and fracture swarms. The diffuse fractures form networks and comprise structures grouped into four fractures sets, which are the typical for this type of anticline. Two orthogonal fracture sets are oriented parallel and perpendicular to the fold axis, and two conjugate fracture sets are oblique to the fold axis with their obtuse angle intersecting the trend of the fold axis. The fractures are typically stratabound, sub-perpendicular to bedding and commonly about the bounding stratigraphic surfaces. To a large extent the density and height of fractures in the Asmari Formation are controlled by the mechan- ical stratigraphy, which is controlled by the depositional environment and cycles. These outcrop data have been essential in the generation of discrete fracture network (DFN) models and the population of the fracture properties in the reservoir models.

The Asmari Formation has been studied in the subsurface at the Bibi Hakimeh, Marun and Ahwaz oilfields and in an outcrop section from the Khaviz anticline. It consists of approximately 400 m of cyclic platform limestones and dolostones with subordinate intervals of sandstone and shale. The method of Sr-isotope stratigraphy is well suited for dating these strata because of the rapid rate of change of marine Sr-87/Sr-86 during Asmari deposition (roughly 32-18 Ma) and the common presence of well-preserved macrofossils.Profiles of age against depth in the four areas show a decrease from higher stratigraphic accumulation rates in the lower Asmari to lower rates in the middle to upper part of the formation. There is also a trend towards less open-marine depositional conditions and increasing early dolomitization and anhydrite abundance above the lower part of the formation. These changes reflect the dynamics of platform progradation across the areas studied, from early deposition along relatively high accommodation margin to slope settings to later conditions of lower accommodation on the shelf top.Ages of sequence boundaries are estimated from the age-depth profiles at each locality, providing a framework for stratigraphic correlation. Asmari deposition began in early Rupelian time (34-33 Ma) in the Bibi Hakimeh area, when the studied areas to the NW were accumulating basinal marl facies. Progradation of the platform across the Marun and Ahwaz areas took place in mid-Chattian time (27-26 Ma) and somewhat later (26-25 Ma) in the more basinward Khaviz area. Depositional sequences have durations of 1-3 Ma, whereas component cycles represent average time intervals of 100-300 Ky.Sr analyses of most dolomite, anhydrite and celestite samples plot close to or below the macrofossil age-depth trend for each locality, indicating formation from waters preserving seawater Sr-87/Sr-86 approximately contemporaneous with or slightly younger than the time of sediment deposition. Local deviations from this trend are interpreted as indicating episodes of seepage-reflux and also a contribution of Sr from non-marine sources during formation of the Gachsaran cap rock anhydrite.

A compilation of average porosity and permeability data for petroleum reservoirs in the Permian–Triassic Khuff Formation and the Jurassic Arab Formation shows that most Khuff reservoirs have an average porosity of less than 12%, whereas most Arab reservoirs have an average porosity of 12–26%. Higher porosity correlates with shallower depth, suggesting that burial diagenesis is the main cause of the overall porosity difference between these units. Deeper burial of Khuff reservoirs is inferred to have resulted in greater porosity loss by chemical compaction and associated cementation. A broad correlation also exists between average porosity and average permeability, suggesting that deeper burial and the resulting porosity decrease are also a primary cause of the lower permeabilities of the Khuff reservoirs. In addition to greater burial depth, however, a combination of depositional and early diagenetic factors is also reflected in the lower average porosity and permeability values of the Khuff reservoirs. Khuff strata were deposited on an extensive, poorly circulated, very low-relief shelf and consist in large part of interbedded mudstones and grainstones having relatively fine grain size, with major amounts of depositional calcium sulfate present. Arab reservoirs were deposited under better circulated conditions near platform margins facing deep, intracratonic basins and, thus, have coarser, more grain-dominated fabrics and lesser overall content of chemically precipitated grains, calcium sulfate, and dolomite. Khuff deposits were likely composed of less stable mineralogy than Arab sediments because the Late Permian was a time of aragonite seas, whereas the Late Jurassic was a time of calcite seas. The combined result of these factors is that Arab reservoirs are characterized by greater preservation of primary depositional pore types, more coarsely crystalline dolomite fabrics, and lesser plugging by anhydrite. Finally, a possible factor affecting the average porosity and permeability values is petroleum composition, which is gas in most Khuff reservoirs and oil in Arab reservoirs. Lower economic cutoff values for gas production would favor inclusion of low-permeability zones in Khuff reservoirs, thus reducing average reservoir values. Two main aspects of these results are innovative. This is the first time that porosity and permeability values for either Khuff or Arab reservoirs have been examined regionally. Second, the conclusion that thermal exposure is the primary control on average porosity and permeability in these units is consistent with previous work from other carbonates, but is new for the Middle East.

Dolomitisation is an important factor controlling reservoir quality in the Asmari Formation in many producing fields in SW Iran. Dolostones have higher average porosities than limestones. Petrographic and geochemical studies have been used to determine the causes of Asmari dolomitisation at the Bibi Hakimeh and Marun fields and at the Khaviz anticline. The formation is generally characterized by a large-scale trend of upward-decreasing accommodation. Basal strata were deposited under relatively open-marine, high-energy conditions, whereas the Middle to Upper Asmari succession was deposited in relatively protected settings with more frequent evidence of exposure and evaporitic conditions. There is a general upward increase in the abundance of both anhydrite (occurring as nodules and cement) and dolomite. Two main types of dolomite fabric are recognised, reflecting the textures of the precursor limestones: (1) finely crystalline pervasive dolomite (commonly

Fluctuations in climate, sea level and sedimentation rates, in addition to the neotectonic activity, during the geological evolution of the Tigris–Euphrates delta (in the last 10,000 years) had resulted in the deposition of various sedimentary units. Previously, five main stratigraphic units, with other sub-units, have been identified by the author during the study of the Holocene deltaic successions of Lower Mesopotamia and as based upon the results of petrological, geochemical, palaeontological and radiometric analyses of his PhD dissertation. Each unit has been produced through various depositional and diagenetic processes in addition to the dominant climate. Such processes together have been clearly recorded in the forms of either the authigenic minerals occurring in each sequence, particularly the Ca–Mg carbonates, evaporites and clay minerals, the biological activities represented by shell remains of molluscs, foraminifers and ostracods, or the preservation of organic matters within organic-rich layers. This review discusses the impact climatic changes had on the accumulated sedimentary facies during the Holocene evolution of the Tigris–Euphrates delta. Arid climate dominated the study area in the early Holocene after a long period of the wetter conditions of Pleistocene. Such a climatic change has resulted in the formation of gypcretes rich in palygorskite and dolomite occurring within the calcareous fluvial-plain muds, similar to the modern fluvial plain deposits. However, the sediments were highly admixed with coarser sandy deposits of playa and aeolian sources in the western desertic margins, and with older reworked sands of Zagros foothills to the Northeast of Lower Mesopotamia. During the mid-Holocene marine invasion, when the climate became wetter as well, brackish-water/marine sedimentary sub-units were deposited, overlying the previous fluvial plain deposits. The deposition started with a transitional sub-unit flourishing over the older early Holocene gypcrete deposits signaling the marine transgression. The best preservation of organic matters occurred beneath this sub-unit. This sub-unit, which is also characterised by microcrystalline mixing zone dolomite, was followed by the deposition of another sub-unit rich in molluscs and foraminifera that collected during the transgressive period. After about 2000 years of maximum flooding (i.e. high stand period), a regressive tidal flat unit was deposited ending the marine/brackish-water deposition and signaling another climatic change towards a more arid setting around 4000 years ago. These climatic conditions are still continuous in the area and reflect the petrology of the modern salt-covered fluvial plain deposits of these rivers. However, marsh/lacustrine deposits and environments remained, covering some vast lowland parts of the fluvial plains, but further inland to the north of the present-day northern Gulf coasts. Complex implications of neotectonic activities, sea-level fluctuations and differential sedimentation rates in addition to the climatic changes during Holocene have resulted in the formation and preservation of these unique marshlands, which are still covering most parts of the ancient Tigris–Euphrates–Karun delta.

A stratigraphic scheme for the Paleozoic of the Southwestern Desert of Iraq is proposed based upon the review of recently published data from several deep wells in the western part of the country and from outcrops in other regions in Iraq. The main formations are described in terms of facies distribution, probable age, regional thickness, and correlations with the well-reported Paleozoic successions of the adjacent countries (e.g. Jordan and Saudi Arabia), as well as with the Thrust Zone of North Iraq. The Paleozoic depositional and tectonic evolution of the Western and Southwestern Deserts of Iraq, particularly during Cambrian, Ordovician and Silurian, shows marked similarity to those of eastern Jordan and northern Saudi Arabia. However, local lithological variations, which are due to Late Paleozoic Hercynian tectonics, characterize the Upper Paleozoic sequences. The Lower Silurian marine “hot” shale, 65 meters thick in the Akkas-1 well in the Western Desert, is believed to be the main Paleozoic source rock in the Western and Southwestern Deserts. Additional potential source rocks in this region could be the black shales of the Ordovician Khabour Formation, the Upper Devonian to Lower Carboniferous Ora Shale Formation, and the lower shaly beds of the Upper Permian Chia Zairi Formation. The main target reservoirs are of Ordovician, Silurian, Carboniferous and Permian ages. Similar reservoirs have recently been reported for the Western Desert of Iraq, eastern Jordan and northern Saudi Arabia. In addition, two main regional seals (Lower Silurian shales and Permian evaporites) of northeastern Arabia extend over most of the Southwestern Desert, together with several other local seals. These considerations render the unexplored Paleozoic Southwestern Desert of Iraq prospective.

This paper reports on the sedimentology and stratigraphy of the mid-Cretaceous Mishrif Formation, one of the principal carbonate reservoirs in Central and Southern Iraq. The Cenomanian Mahilban, Maotsi and Fahad Carbonate Formations of Central Iraq are the lateral chronostratigraphical equivalents of the Mishrif and underlying Rumaila Formations of Southern Iraq. Together, these units represent a single mid-Cretaceous carbonate succession in the Mesopotamian Basin.

Organic-geochemical studies of sediments at the surface and in shallow boreholes in the Tigris-Euphrates Delta of SE Iraq are integrated in this paper with sedimentological and stratigraphic studies. Limited organic preservation was found to have occurred in surface sediments from marshland areas (“Ahwar”), although organic-rich sediments were observed within older Holocene borehole sequences. Reeds of Phragmites and Typha sp. are the main source of organic material in both the surface and the underlying sediments in this area. Petrographic studies indicated that this material consists mainly of “immature” components such as woody structures, algal amorphous and herbaceous matter, with some marine structures and pollen. The principal factor controlling organic-matter preservation in this arid deltaic setting seems to have been the comparatively high rate of sedimentation, particularly in the early-middle Holocene. Most of these organic-rich sediments (5–15% TOC) are overlain by middle Holocene brackish/marine sediments of the transgressive Hammar Formation. The organic-rich sediments are generally older than these transgressive sediments (i.e. 6,000–9,000 years BP, according to their location within the study area).

The Khasib, Tanuma and Sa'di Formations represent a complete sedimentary cycle of the Turonian-Lower Campanian in the Mesopotamian Basin. This carbonate sequence comprises many shale/marl units alternating with some porous and fractured carbonate units which are highly-productive reservoir units in some oilfields. The microfacies and lithostratigraphic analyses of the whole sequence reveal almost the same depositional environments, excluding the faunal diversity and fossils types. Middle shelf is the main depositional environment for the whole sequence. However, influence of the outer shelf or open marine is quite obvious throughout, with the planktonic faunal abundance, particularly in some levels of the Khasib Formation (at the base of the sequence) and the Sa'di Formation (at the top). Also, the detrital input during shallowing of the basin was high in some time intervals, mainly during the deposition of the Tanuma Formation. The thickness maps of the three formations studied coupled with the stratigraphy and electro-facies analysis explain the distribution and palaeogeography during the depositional history of the three formations. Accordingly, the main basin for the whole sedimentary cycle showed different sub-basins during the deposition of each formation due to tectonic instability. The mineralogical XRD analysis shows that the detrital input was calcareous and consists of calcite and dolomite (about 70%); quartz and clay minerals are the main insoluble residues, but do not exceed 35%. Almost the same mineral content was detected in the carbonate sediments of the three formations studied. As a result the mineralogical analysis was of limited utility in facies identification. On the other hand, kaolinite and illite are the main clay minerals distinguished in the whole sequence. This assemblege indicates a dominantly humid climate during the entire depositional history. However, the clay petrography, through SEM analysis, reveals that some of the illite is of diagenetic origin while most of the kaolinite is detrital and resisted burial diagenesis.

Holocene sedimentation rates have been calculated for the lacustrine/deltaic deposits of Lower Mesopotamia. The calculations depend on the radiocarbon dating of five organic-rich borehole samples and four near-surface shelly samples. Rates between 1 and 1.8 mm/year were dominant throughout the Holocene from 8400 years BP until about 3000 years BP . During the later stage of the Holocene, rates of 0.4 mm/year were not exceeded. The role of near-surface overburden compaction was excluded in these calculations. Near-surface overburden compaction is an effective mechanical process, particularly for muddy deposits such as those of the Tigris-Euphrates Delta. However, such compaction is usually ignored by most petroleum geologists down to >100 m depth. A geotechnical approach was developed to measure the relative amount of this compaction and to correct the above-mentioned sedimentation rates geotechnically. Correction curves were established, based on laboratory consolidation tests, for the three dominant sedimentary types sand, silt and clay fractions. In addition, a general correction curve was derived based on the general distribution of these three types of sediment in the Holocene succession of the Tigris-Euphrates Delta. Each curve shows the relation between the percentage of thickness reduction, due to mechanical compaction, and depth. Thickness reductions of up to >25% have been determined, particularly within the first 5 m of the sedimentary column. Below this depth interval, the slope of the curves reduces down to 15 m depth and more. When the previously mentioned sedimentation rates were recalculated using the established curves they increased to 1.3–2.2 mm/year. Based on these latter calculations, it is essential for petroleum geologists to reassess the concept of near-surface overburden compaction.

In Iraq the Upper Permian Chia Zairi Formation, together with the overlying Lower Triassic Mirga Mir Formation, forms the chronostratigraphic equivalent for the Khuff Formation of eastern Arabia and the Dalan and Kangan formations of Iran. The Chia Zairi Formation extends from upper-north Ora outcrops near the Turkish border to southern Iraq (wells West Kifil-1 and Diwan-1), thus covering central and eastern parts of the country. It is absent in the far western deserts at the Jordanian border and some areas near the Syrian border (i.e. Khelsia High region and south of Jabal Sinjar) due either to non-deposition or erosion. Lithologically, the Chia Zairi Formation is composed of both carbonates and siliciclastics, and the carbonate proportion increases northward and eastward. However, the formation is deeply buried in most regions except from the northwestern and potentially southwestern region too, where, based on seismic, it is estimated to be around 3–5 km deep.

Lithological analysis of the Lower Fars Formation in southern Iraq was carried out through the study of cuttings and cores, where available, and electric logs of 26 selected boreholes. The formation exhibits a complex repetition of various lithofacies, namely, limestone, dolomite, anhydrite/gypsum, halite, marl and siliciclastics. Four depositional cycles, with different thicknesses, were identified in the study area. The depositional environment of the formation is reconstructed as being a semi-closed marginal basin dominated by saline tidal flats receiving siliciclastic sediments. More than one Recent sabkha type could be the analogue of the studied sequence.

Palygorskite has been widely reported as a clay phase in the Recent sediments of the northern Arabian Gulf (e.g. Aqrawi & Sadooni, 1988). It is usually thought to have originated from the sediments and soils of the Mesopotamian flood plains introduced by northwesterly (Shamal) winds (e.g. A1-Bakri et al., 1984). A1-Rawi et al. (1969) and Eswaran & Barzanji (1974) have detected palygorskite within Mesopotamian soils. This paper describes the occurrence of palygorskite in the Recent sediments of southern Mesopotamia and its possible origins will be discussed.

The Cenomanian-early Turonian Mishrif Formation reservoir of the Mesopotamian Basin accommodates more than one third of the proven Iraqi oil reserves within rudist-bearing stratigraphic units. Difficulty in predicting the presence of reservoir units is due to the complex palaeogeography. Extensive accumulation of rudist banks occurred along an exterior shelf margin of the basin along an axis that runs from Hamrin to Badra and southeast of that, with interior margins around an intrashelf basin. Buildups were stacked or sometimes shingled as thicker shallowing-up cycles of several smaller-scale accommodation cycles. As a result, each field shows different combinations of pay zones, barriers and seal geometries. The sequence stratigraphic analysis led to three complete 3rd order sequences being distinguished. Eustatic sea level changes controlled development of the sequence stratigraphy. Tectonism primarily defined the sites of platform development that complicated the architectural heterogeneity of the depositional sequences.

The middle Cretaceous Mishrif Formation contains a widespread carbonate succession of Cenomanian-Early Turonian age in the Mesopotamian Basin of southern Iraq. More than one third of the proven Iraqi reserves are found within the rudist-bearing carbonate reservoirs of the Mishrif Formation. Rudist facies coincide with the crestal areas of many fields in the region, particularly in those anticlines that show evidence of synsedimentary structural growth. However, other structures have also proven to be non-productive at this level because of the presence of tight or microporous offshore facies instead of rudist-bearing reservoir facies. Difficulty in predicting reservoir distribution is due to the complex palaeogeography. It would appear that following drowning of the uniform and extensive Albian carbonate platform, the Mishrif Formation was established along a NW-oriented zone that is largely bounded by the East Baghdad and Hamrin-Makhul structures and their continuation towards Iran (as the Bala Rud zone). To the southwest of this zone, the intra-shelf basin carbonates of the Rumaila Formation were deposited. Whereas to the northeast, the sub-basinal carbonates of the Dokan Formation were deposited. Across the Mishrif Formation facies belt, sequences show stacked or sometimes shingled geometries. As a result, each field shows different combinations of pay zones, barriers and seal geometries. Detailed palaeogeographical reconstruction on a systems tract basis can be used to construct play fairway maps for exploring this carbonate system, particularly with respect to finding stratigraphic traps. Many existing anticlines, tested by one or more wells that are thought to be ‘dry’, may turn out to have productive facies along strike, or down-dip on the flanks. In addition, pure stratigraphic traps may be found in relatively undeformed ‘synclinal’ areas between the major anticlines. However, exploring such trap types will require 3-D seismic to pin-point tune the positions of the external and internal shelf margins via the application of high-resolution sequence stratigraphy.

Abstract The Asmari Formation (Oligocene-early Miocene) is the most important reservoir in the oilfields in the Zagros foreland of the Dezful Embayment of SW Iran. The carbonate reservoirs of the Asmari Formation are characterized by low matrix permeability, and effective drainage is dependent on the occurrence of open fractures. The fractures have been formed during the Zagros Orogeny since late Miocene, which is also responsible for the geometry and formation of NW-SE trending anticlinal traps. Dolomitisation is another factor controlling reservoir qualities as dolostones in general have higher porosities than limestones. The fractures are typically stratabound and sub-perpendicular to bedding. The density and dimension of fractures are controlled by the mechanical stratigraphy, which is controlled by the depositional environment and cycles. In the platform top sequence, the Dunham's (1962) texture classification appears to be the most important factor controlling the Fracture Intensity (FI) rather than the mechanical layer thickness. Mud supported textures (mudstone and wackestone) have higher FI than grain supported (packstone and grainstone). The fracture density is high in the vicinity of faults. Two main types of dolomite fabric are recognised; finely crystalline pervasive dolomite ( General Geology The Dezful Embayment of SW Iran is one of the rich oil provinces in Middle East, see Fig. 1. The structural and stratigraphic features of Dezful Embayment have been investigated continuously since mid-last century (e.g. Haynes and McQuillan, 1974; McQuillan, 1985; Blanc et al., 2003, Wennberg et al. 2006 & 2007, Ehrenberg et al 2007). Most oilfields in this region are asymmetric anticlines having NW-SE trend as in Fig. 1.

A compilation of average porosity and permeability data for Cretaceous petroleum reservoirs of the Middle East reveals important differences between the two main tectonic provinces. The Arabian Platform is characterized by inverse correlation of average porosity with burial depth in both carbonates and sandstones, whereas the Zagros Fold Belt (almost exclusively carbonates) has distinctly lower porosity and no depth correlation. These contrasts are suggested to reflect the fact that Arabian Platform strata are mostly near their maximum burial depth, whereas Zagros strata have experienced varying uplift and erosion following maximum burial in mid-Tertiary time. The carbonate reservoirs show no correlation between average porosity and average permeability, probably because of wide differences in the dominant pore types present, and permeabilities tend to be much higher for sandstones than for carbonates. Existence of the Arabian Platform porosity–depth correlation, despite an apparently wide diversity of depositional settings and early diagenetic porosity modifications among the individual reservoirs, illustrates and confirms some fundamental generalities about how burial diagenesis controls the overall porosity evolution of reservoir rock bodies. Although porosity commonly shows enormous small-scale heterogeneity in both carbonates and sandstones, the average pre-burial porosity of larger stratigraphic intervals tends to be very high. Burial diagenesis progressively destroys this porosity by chemical compaction and associated (stylolite-sourced) cementation. Thus, all portions of the affected rock body move toward the zero limit as depth increases, although the rates of porosity occlusion vary greatly, depending on rock fabric and early diagenesis. Average reservoir porosity therefore tends to correlate inversely with maximum burial depth, regardless of initial lithological heterogeneity.

A Middle Eastern Cretaceous rudist-bearing carbonate reservoir is chosen for this study. This limestone sequence (about 55 m [180 ft] thick) consists of tight argillaceous mudstones and wackestones at the base, grading upward into a more porous section dominated by bioclastic grainstones and packstones. Applying sequence-stratigraphic concepts, the reservoir is classified into two main sequences. In addition, the upper zone is further divided into two sequences of smaller accommodation cycles. Each of the latter includes a distinctive reservoir type (i.e., dominated by either barrier or shoal facies). The lower main sequence is considered as one unified nonreservoir cycle in the study area of the field. The three-dimensional seismic and sequence-stratigraphic analyses are used in conjunction with designing a deterministic structural model. A three-layer model is used, each layer representing a stratigraphic sequence previously recognized. Describing the facies and flow units of carbonate reservoirs for reservoir-simulation purposes is a critical task that needs careful study of both the depositional textures and the diagenetic overprints in a sequence-stratigraphic framework. Eight main depositional facies are recognized for facies-modeling purposes. However, only two of them are high-quality reservoirs, dominated by grain-supported textures. The reservoir architecture is generated using a combination of grid- and object-based simulation techniques to accurately reproduce the facies- and reservoir-type distributions. Petrophysical parameters porosity and permeability are stochastically simulated within facies bodies using a Gaussian cosimulation algorithm. Well data are used as hard information that must be honored, whereas acoustic impedance information is used to define porosity maps, which are used as conditioning trends in the simulation. These trends are believed to represent the effects of diagenesis. Statistical analysis of the relationship between acoustic impedance and porosity allowed us to use impedance-derived porosity trends as conditioning data in petrophysical simulation. These porosity trends, adjusted to the range of data values from the wells, are believed to represent the effects of diagenetic activity. Using these trends alters the resultant data distribution compared with the distribution as inferred from well data. This is a consequence of the assumption that these porosity trends do reflect diagenesis and, in addition, does so not only at the wells but in the interwell areas as well.