ABSTRACT We investigated the sequence stratigraphic architecture of Neogene deposits in the Zagros region using 12 measured stratigraphic sections and performing strontium isotope measurements on 10 samples. We combined our data with 44 previous published Sr isotope measurements and five paleomagnetostratigraphic sections in order to place the foreland deposits in a chronostratigraphic framework. Two chronostratigraphic charts were created, and transgressive-regressive system tracts were identified in the western and eastern sectors of the Zagros. These results show that the southward migration of the Zagros fold-and-thrust belt led to broadly upward-coarsening, progradational sequences due to progressive uplift and seaward (south-westward) migration of the Zagros fold-and-thrust belt. In addition, we show distinct differences in the sequences between the eastern and western sectors. It can be inferred that, in the strike-parallel direction, the Zagros foreland basin did not respond in a simple manner to tectonics and global sea-level changes during the Neogene. The different architecture of the sequences and mismatch of Neogene sedimentary architecture with global sea-level curves imply different orogen-parallel evolutionary histories and strong impact of tectonics on the basin evolution as compared to eustasy. Finally, we present evidence for a retrogradational sequence related to a period of enhanced deformation and related crustal thickening during the middle Miocene in the eastern Zagros.

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.

Abstract The Zagros Mountains are situated along the NE margin of the Arabian plate and are the product of complex deformation which began in Late Cretaceous time as a result of the collision between the Arabian and Central Iranian plates. During Pliocene time, deformation increased when plate convergence was accelerated by the opening of the Red Sea. This stimulated the migration of a deformation front from the collision zone towards the SW into the undisturbed Zagros basin and led to the creation of the Zagros Mountain Belt. The type and distribution of the deformation in the Zagros are controlled mainly by plate velocity, which is linked to the anticlockwise rotation of the Arabian plate around a pole in Syria, and the regional stratigraphy. The sedimentary cover and the underlying metamorphic basement decouple along an important detachment horizon, the Hormuz Salt Formation, and the uneven thickness and distribution of this salt plays a key role in determining the geometry of the deformation belt. Analysis of the distribution and geometry of the folds provides evidence for the southwestwards migration of the deformation front into the Arabian plate. The analyses are consistent with field evidence for serial folding, which indicates that each fold takes c. 600 ka to develop fully, and with the model of a southwestward advancing deformation front driven by the processes of serial folding and footwall collapse.

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.