ABSTRACT The lateral transition from carbonate platforms to intra-shelf basin in Aptian – Early Albian times is documented along a regional transect in the excellent exposures of the Zagros Mountains. An integrated dataset, including carbonate carbon-isotope curves, and ammonite and planktonic foraminifera biostratigraphy constrains the regional correlations, and forms the basis for an age revision of the Kazhdumi and Dariyan formations. Of particular importance in this study is the Kuh-e-Bangestan section, wherein a continuous succession of Aptian – Albian intra-shelf basinal deposits was used to erect a detailed ammonite and planktonic foraminifera biostratigraphic zonation scheme, in combination with a carbon-isotope curve and organic-matter measurements. Key observations are: (1) the oldest Kazhdumi intra-shelf deposits are of Early Aptian age ( D. deshayesi Zone), indicating a time-equivalent initiation of the Kazhdumi and neighbouring intra-shelf Bab Basin. (2) The presence of well-developed organic matter-rich sediments in the N. nolani and H. jacobi zones is interpreted as equivalent to part of the OAE1b set. (3) The presence of an exceptionally well-developed Upper Aptian – Lower Albian succession (80 m thick) shows a clear positive carbon-isotope excursion and a faunal crisis, with the turn-over of the planktonic foraminifera assemblage (only small and low diversity Globigerinelloides and Hedbergellids ) and the total absence of ammonites. A composite carbon-isotope curve is proposed based on sections measured in both the platform and basin settings. This curve deserves attention because it has an expanded Upper Aptian - Lower Albian section, which is well dated (ammonites, planktonic foraminifera and orbitolinids).

ABSTRACT A regional sequence-stratigraphic model is presented for the Barremian to Lower Albian sedimentary systems of southwest Iran, based on seismic-scale outcrop sections, and constrained by new biostratigraphic and chemostratigraphic age dating (ammonites, planktonic forams, orbitolinids and carbon and oxygen isotope curves). This study proposes a fundamental revision of the geometrical relationships between classically used lithostratigraphic units, and demonstrates the relative influence of both eustatic sea-level fluctuations and regional tectonic control on sedimentation. These new insights have significant implications for both the understanding of the sedimentation patterns, as well as the architecture of the mid-Cretaceous petroleum systems in the studied region. The following depositional sequences are defined: (1) two third-order sequences of Barremian age, which have a flat-bedded stratigraphic architecure and consist of low-angle, mixed carbonate-siliciclastic ramp systems (Gadvan Formation). The maximum flooding surface of the Arabian Plate (AP) Bar 2 Sequence has been dated with the occurrence of the short-range index fossil Montseciella arabica . (2) an Aptian second-order Supersequence which shows a geometrical evolution from a flat-bedded architecture to a carbonate platform to organic-rich intra-shelf basin topography and corresponds to part of the Dariyan Formation and the Kazhdumi Formation. This evolution can be subdivided in three phases which are related to the Aptian third-order sequences defined in the UAE and Oman: the early transgression (Apt 1 sequence), the late transgression (Apt 2) and the highstand (Apt 3 and 4). The maximum flooding occurred in, or just after, the D. deshayesi ammonite zone. (3) an Upper Aptian – Lower Albian second-order Supersequence, represents the continuation of the carbonate platform to organic-rich intra-shelf basin system established in the previous supersequence. It corresponds to the Upper Dariyan and Kazhdumi formations and the ‘Burgan equivalent’ informal unit. During the initial lowstand sedimentation (Apt 5) was confined to the intra-shelf basin, and during the transgression and highstand (Apt 6, Alb 1-2) only part of the exposed carbonate platforms were flooded, with facies varying from orbitolinid limestones along the northern margin to condensed siliciclastics along the southern margin. Sedimentation was controlled by the eustatic sea-level variations, the influx of siliciclastics and the activation of regional fault systems such as the Kazerun Fault and the Hendijan Fault. The fault activiation caused the differentiation between the southern coastal Fars area, with relatively less accommodation, and the northern Khuzestan and Lurestan area, which subsided more rapidly. Due to the high-resolution biostratigraphic control the sedimentation patterns in these two tectonically different-behaving areas could be compared and depositional sequences defined. The time control also allowed comparison with the sequences defined in Oman, Qatar and the UAE, which are for the Barremian and Aptian very similar. The Upper Aptian – Lower Albian Supersequence is well documented in Iran and in Qatar, whereas in the UAE and Oman only the lowstand of this system is documented.

ABSTRACT A cyclostratigraphic framework is proposed for the Arabian Plate’s (AP) Upper Barremian – Aptian transgressive-regressive (T-R) depositional sequences, which calibrates their absolute age and attempts to correlate them to the global sequences in the geologic time scale GTS 2004. The cyclostratigraphic framework is constrained by a substantial biostratigraphic, chemostratigraphic and sequence-stratigraphic dataset now available for the Arabian Plate (GeoArabia Special Publication 4), which is an improvement from the three radiometric age estimates available for this interval in GTS 2004. The proposed new orbital time scale is based on the axioma that the Earth’s long-eccentricity cycles (ca. 405 ky), here called stratons , are the best-expressed basic building blocks of the stratigraphic record. The 405-ky orbital scale is interpreted in the Arabian Plate and globally as follows: Late Barremian cycles WM3–WM7 (WM for Wadi Mu’aydin type section in Oman) correspond to stratons 316–312 (ca. 127.9–125.9 Ma) and form together Sequence AP Barremian 2. This correlates to global sequences Bar5 and Bar6 in GTS 2004. An Early Aptian Hiatus over the Arabian Plate (between cycles WM7 and WM8) and a coeval lowstand wedge at the Neo-Tethyan margin correspond to stratons 311 and 310 (ca. 125.9–125.1 Ma) which are interpreted as a short-lived Early Aptian Glaciation. The Barremian/Aptian boundary is here proposed at the base of the wedge (ca. 125.9 Ma). The wedge is correlated to global sequence Ap1 which started at 125.0 ± 1.0 Ma in GTS 2004. Early Aptian cycles WM8 and WM9 correspond to stratons 309 and 308 (ca. 125.1–124.3 Ma) and form together Sequence AP Aptian 1. This is correlated with global sequence Ap2 with the Nannoconid Crisis in its later part. The preferred position of lowermost Aptian AP MFS K70 is within cycle WM9 (mid straton 308: 124.5 Ma), which matches the global MFS Ap2 at ca. 124.4 Ma in the D. oglanlensis zone in GTS 2004. Early Aptian cycles WM10–WM12 correspond to stratons 307–305 (ca. 124.3–123.1 Ma) and form together Sequence AP Aptian 2. This correlates to the TST of global sequence Ap3. Early Aptian cycles WM13–WM16 correspond to stratons 304–301 (ca. 123.1–121.4 Ma) and form together Sequence AP Aptian 3. This is correlated to the HST/RST of global sequence Ap3. Lower Aptian AP MFS K80 is variably positioned in Wadi Mu’aydin by authors and not resolved here. Straton 304 (WM13) at ca. 122.9 Ma is predicted as an MFI and possibly correlated to global MFS Ap3 (ca. 123.0 Ma, ca. D. weissi / D. deshayesi boundary in GTS 2004). Eight early Late Aptian clinoforms, defined in subsurface Abu Dhabi, are interpreted as the expression of stratons 300–293 (ca. 121.4–118.2 Ma). Together these form Sequence AP Aptian 4, which correlates almost precisely to global sequence Ap4 and most of the E. subnodosocostatum zone. Nine Late Aptian clinoforms, recognized in subsurface Oman and Abu Dhabi, together with possibly three unresolved sequences correspond to stratons 292–281 (ca. 118.2–113.3 Ma), and form Sequence AP Aptian 5. This sequence is correlated to global sequence Ap5 and the uppermost part of the E. subnodosocostatum , P. melchioris and N. nolani zones, as dated in the Kazhdumi Basin in southwest Iran. This interval is interpreted as a long-lasting Late Aptian Glaciation. Uppermost Aptian Sequence AP Aptian 6 is defined in subsurface Qatar (lower Nahr Umr Formation) and in southwest Iran (Kazhdumi Formation) and corresponds to stratons 280–278 (ca. 113.3–112.1 Ma). It correlates to the Aptian part of global sequence Ap6, H. jacobi zone and Jacob and Killian oceanic anoxic events, and implies that the Aptian/Albian boundary probably occurs at the top of Straton 278 (112.1 Ma) as consistent with GTS 2004 (112.0 ± 1.0 Ma).

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 The giant fields of the Zagros fold belt of southwestern Iran fall within the greater context of the Arabian-Iraq-Persian basin and contain cumulative recoverable reserves estimated at 87 billion barrels of oil and 514 trillion cubic feet of gas. The regional geology of the area comprises a wedge of Paleozoic to Holocene sediments, 10–15 km (6–9 mi) or more in thickness, supported on a mobile Eocambrian salt economic basement. Forming a classic carbonate–evaporite sequence, the succession contains prolific source-reservoir combinations and effective seals of integrity. Located on the eastern subducting boundary of the Arabian plate, hydrocarbon generation has been largely controlled by the Neogene Zagros orogenic event. A number of potential source sequences have been recognized. With one exception, all are typically of an organic-rich, argillaceous, lime-mudstone litho-textural type. Most of the giant oil accumulations in Asmari (Oligocene-Miocene) and Bangestan Group (Upper Cretaceous) reservoirs have a common provenance in the Kazhdumi Formation of Early Cretaceous (Albian) age. The Pabdeh (Paleogene), lower Garau–Gadvan (Lower Cretaceous, Neocomian), and Sargelu (Middle Jurassic) formations contribute less to overall reserves, having been the source for selected subordinate Asmari, Bangestan, and Khami Group (Upper Jurassic-Lower Cretaceous) reservoirs only. Gas in Permian-Triassic reservoirs has a provenance in Ordovician-Silurian siliciclastics.

ABSTRACT During the Mesozoic Era, episodes of siliciclastic input onto the dominantly carbonate Arabian shelf form important elements of petroleum plays, forming proven and potential reservoirs, source rocks, and seals. This chapter examines the temporal and spatial extent of these siliciclastic episodes. It then compares them against known tectonic, climatic, and eustatic events affecting the Arabian plate that may have been acting independently or coincidently to control siliciclastic input by means of hinterland uplift, influence on denudation and run off, incision, and creation of sediment pathways and accommodation space. Particularly important phases of siliciclastic input occur in (1) the Early Triassic (Olenekian Sudair shale) coincident with major eustatic lowering, an episode of humid climate and rifting on the northern part of the Arabian plate; (2) Late Triassic (late Norian initial Minjur Sandstone) coincident with East Mediterranean rifting, a humid episode and a major eustatic sea-level fall; (3) Middle Jurassic (early Bajocian initial Dhruma Sandstone) coincident with localized uplift and a humid climate and immediately postdating a eustatic sea-level fall in the Aalenian; (4) Early Cretaceous (late Valanginian–Barremian Zubair sandstone) postdating a Valanginian eustatic lowering and coincident with humid climate and uplift in northern and western Arabia; (5) Mid-Cretaceous (latest Aptian–middle Albian Burgan Sandstone) coincident with Arabian shield uplift, humid climate, and a eustatic low. Other episodes of siliciclastic input also occur, although they tend to be more localized. Important seals are formed during the progradation of siliciclastic systems “poisoning” carbonate shelves or during transgression when distal pro-delta siliciclastic systems retreat back across the shelf, capping up-systems tract fluvial or shelfal sandstones, or when they are located above major unconformities, capping carbonate reservoirs. Siliciclastic reservoirs include the well-known and prolific fluvial and paralic sandstones that contribute, for example, to the Burgan field in Kuwait and to the Zubair and Nahr Umr reservoirs of the northern Gulf. Lowstand sands (both lowstand deltas and slope and basin gravity flow deposits) form viable, but underexplored, reservoir targets. Source rocks may be deposited in front of prograding delta systems linked to high nutrient supply and water stratification caused by freshwater overhang, leading to anoxia and preservation of organic matter. A well-known example is the Kazhdumi Formation of the Iranian Zagros. A better understanding of the fundamental controls on siliciclastic input onto the Arabian plate will enable better predictions of these key petroleum play elements and a better understanding of the subsurface risk associated with their occurrence.

Abstract A unique geological setting that included a long-lived shallow-water, often reefal, carbonate barrier along the edge of South Tethys and a nearly thousand kilometer wide back-barrier shallowwaterIntracratonic Sea occurred during much of Mesozoic time in what is now SW Iran. This barrier hampered water circulation between South Tethys and the Intracratonic Sea. This setting remained very sensitive to sea-level variations and climatic conditions and resulted in the accumulation of excellent source rocks in localized depressions when anoxic conditions prevailed. This paper describes the functioning of five petroleum systems through time, in terms of the distribution and organic composition of source rocks, evolution of their maturity through time, geometry of drains and reservoirs, and trap availability at the time of migration. It also addresses the relative timing of trap formation versus oil expulsion from the source rocks. For the older systems, namely Paleozoic (Llandoverian source rocks), Middle Jurassic (Sargelu), Late Jurassic (Hanifa–Tuwaiq Mountains/Diyab), and Early Cretaceous (Garau), oil and/or gas expulsion occurred before the Middle Miocene; that is, before the onset of the main phase of Zagros folding. Thus, oil migrated along gently dipping ramps toward large low-relief regional highs and salt-related structures. Oil and gas later re-migrated to the nearest Zagros anticlines. In such cases, oil was trapped far away from the kitchen where it was formed. In sharp contrast to the earlier petroleum systems, oil expulsion occurred almost everywhere in the Dezful embayment after the onset of the Zagros folding for the Middle Cretaceous to Early Miocene system (Kazhdumi, Pabdeh source rocks). Oil migrated vertically toward the closest anticlines through a system of fracturing. A comparison was made between the amount of oil expelled from the source rocks, as calculated by modeling, and the initial oil-in-place discovered in fields. As the result of this short distance migration, oils can be directly linked to the source rocks which generated them using oil-oil and oil-source rock pyrolysate correlation based upon d13C and biomarkers. Although there are excellent source rocks in all five petroleum systems, three of these systems have a limited impact on the Iranian reserves. Two of them, Middle Jurassic (Sargelu) and Early Cretaceous (Garau), contain almost no reservoirs associated with the source rocks. Most of their oil could not be expelled and was cracked in situ to pyrobitumen and gas. The kitchens in the Late Jurassic system (Hanifa–Tuwaiq Mountains/Diyab) are peripheral to Iran. Oil migrated to the SW edge of the Gavbendi/Qatar Arch and accumulated in saltrelated structures prior to the deposition of efficient caprocks, resulting in accumulations of heavy oil and bitumen. The Paleozoic petroleum system caused the accumulation of huge gas reserves (750+ TCF in Iran) in the Permo-Triassic carbonates of southern Fars and its contiguous offshore. Oil and later gas were expelled from Llandoverian source rocks and migrated toward the Gavbendi/Qatar Arch where a single gas field was formed with a peripheral oil leg prior to the Zagros folding. Oil was progressively cracked into pyrobitumen and either light oil or gas. Part of the gas re-accumulated in later-formed Zagros anticlines. The Middle Cretaceous to Early Miocene system (Kazhdumi, Pabdeh source rocks) caused the impressive gathering of oil fields that represent approximately 8% of global oil reserves in the Dezful embayment, an area of only 60,000 km2 (37500 mi2). Oil migrated vertically from Kazhdumi (Albian) and Pabdeh (Late Eocene to Oligocene) source rocks into Sarvak (Cenomanian) and Asmari (Early Miocene) carbonates, which were capped by the Gachsaran evaporites. Modeling indicates that 90% of the oil originated from the Kazhdumi. Oils are grouped into families based upon carbon isotopic composition and biomarkers. Correlation between pyrolysates and oils confirms the geological and modeling assumptions used to explain the current location of the oil (and gas) fields.

Abstract In the current Zagros Fold Belt of Iran and in its contiguous offshore, five petroleum systems caused an impressive gathering of oil and gas fields that represent some 8% and 15% of global oil and gas reserves, respectively. Almost all the oil fields are located in the relatively small Dezful Embayment, which extends over 60 000 km 2 , whereas most of the gas fields are concentrated in Central and Coastal Fars and in the contiguous offshore area. This paper describes the functioning of the various petroleum systems through time, each petroleum system having its own specificity, and reconstructs the succession of events that explains the current location of the oil and gas fields and the reservoirs in which oil and/or gas accumulated. In addition to the classical description of the petroleum systems (distribution and organic composition of the source rocks, evolution of their maturity through time, geometry of drains and reservoirs, and trap availability at the time of migration), the influence of tectonic phases (Acadian, Hercynian, Late Cenomanian to pre-Maastrichtian, and Late Miocene to Pliocene Zagros phases) on the various systems are discussed. As the time of oil and/or gas expulsion from the source rocks is necessary to reconstruct migration paths and to locate the traps available at the time of migration, extensive modelling was used. The timing of oil or gas expulsion was compared with the timing of tectonic events. For the older systems, namely the Palaeozoic (Llandovery source rocks), Middle Jurassic (Sargelu), Late Jurassic (Hanifa–Tuwaiq Mountain–Diyab) and Early Cretaceous (Garau), oil and/or gas expulsion occurred before the Zagros folding. Oil migrated over long distances, according to low-angle geometry, towards large-scale low-relief regional highs and salt-related structures. In the current Zagros Fold Belt, oil and gas remigrated later to the closest Zagros anticlines. In contrast, for the prolific Middle Cretaceous to Early Miocene System (Kazhdumi, Pabdeh), oil expulsion occurred almost everywhere in the Dezful Embayment after the onset of the Zagros folding. Oil migrated vertically towards the closest anticlines through a system of fractures. A comparison was made between the oil expelled from the source rocks, as calculated by the model, and the initial oil in place discovered in the fields. Oils were grouped into families based upon isotopic composition (carbon and sulphur), and biomarkers. Correlation between pyrolysates and oils verifies the origin of the oils that was proposed to explain the current location of the oil (and gas) fields.