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 Huge hydrocarbon potential still exists in Iran, even after prolific production for many decades. Currently, Iran is operating about 28 drilling rigs (23 of them onshore) and is producing about 3.6 million barrels of oil per day and about 3 trillion cubic feet of gas per day from approximately 1200 wells in 39 fields. Since oil was discovered at Masjid-i-Suleiman in 1908, exploration has extended onshore and offshore to several provinces and regions of southwestern Iran. Oil production from the Zagros fold belt dominates Iranian hydrocarbon production. Zagros reservoirs are in Jurassic, Cretaceous, and Tertiary carbonates and siliciclastics. Despite decades of exploration and production, future exploration and field- development opportunities appear to abound, now aided by new technologies and tools.

ABSTRACT The lower–middle Cambrian boundary transition in Iran consists of the upper lower Cambrian Shale and Quartzite units of the Lalun Formation (nearly all siliciclastics) and the overlying lower middle Cambrian Member 1 carbonates (dominantly carbonates) of the Mila Formation, the facies and stratigraphy of which reflect deposition on an extensive ramp platform in the northern passive margin of Gondwana. This paper focuses on facies and sequence stratigraphic analyses of the boundary interval to document the unconformable boundary on the Quartzite unit, which may record the late early Cambrian global Hawke Bay (Toyonian) regression, and to define depositional sequences for regional and global correlation. The Shale unit unconformably overlies the fluvial red beds of the Lalun Sandstone unit. The unconformity is marked by a pebbly chert arenite containing black chert clasts and reworked caliche pisoids in places, and it is coeval with the black chert conglomerate at the base of the Shale unit equivalent in east-central Iran. The Shale unit conformably underlies the Quartzite unit, the base of which is marked by a change in depositional trend, but where no evidence for an unconformity is recognized. This unit includes two reef horizons composed of dolomitized individual and compound metazoan buildups capped by planar to wavy stromatolite. The upper contact of the Quartzite unit marks a regional unconformity and the abrupt appearance of shallow-marine carbonates of the Mila Formation. The unconformity is characterized by a distinctive dark reddish brown- to red-weathering horizon, in which most of the sand grains are altered to a hematitic matrix and the sand content decreases toward the top of the profile. Close to the unconformable boundary, in a short stratigraphic interval (~3 m), an open-marine thrombolite reef zone capped by oolithic limestone is recognized near the base of the Mila Formation. The basal Mila boundary thrombolites, widespread development of stromatolite reefs, oncoids, and ooids, and absence of metazoan reefs in the middle Cambrian Member 1 of the Mila Formation may indicate a stressed ecosystem in the aftermath of the Hawke Bay extinction event and reef crisis similar to basal Triassic deposits. Four depositional sequences (third-order cycles) are recognized in the late early Cambrian–early middle Cambrian interval that are similar to the south China sequences. There is a good correlation between the unconformable boundaries recognized in this study and the major global sea-level falls reported for the late early Cambrian and early middle Cambrian intervals. The unconformity-bounded upper lower Cambrian fluvial red bed succession that overlies the lower Cambrian shallow-marine carbonates signifies the first pause in the Cambrian transgression. It correlates with the major global lowstand at the base of the Toyonian Stage and is here referred to as the early Toyonian Hawke Bay regression. The conspicuous unconformity on the Quartzite unit that underlies the basal transgressive package and the boundary thrombolite reef zone near the base of the Mila Formation indicate a major hiatus that is interpreted to correspond to the culmination of the global late early Cambrian Hawke Bay regression (Sauk I-II unconformity) and the Hawke Bay extinction event. The event resulted in the demise of metazoan reef builders and the emergence of microbialite reefs, oncoid, and ooid facies in the middle Cambrian basal members of the Mila Formation.

Abstract Early to mid Palaeozoic marine phytoplankton are represented by acritarchs and associated forms, which had a global distribution from the early Cambrian to the early Carboniferous (Mississippian). Palaeozoic phytoplankton assemblages show varying degrees of cosmopolitanism and endemism through time. A high degree of cosmopolitanism was evidently characteristic of the Cambrian and much of the Late Ordovician, Silurian and Devonian, but provincialism was more marked in the Early Ordovician and Hirnantian (latest Ordovician), the latter at a time of major palaeoenvironmental perturbations. Distribution patterns of Palaeozoic phytoplankton are attributed to a number of interacting factors, including palaeolatitude, palaeotemperature, oceanic circulation patterns, the disposition of continents, differentiation between oceanic and cratonic (distal–proximal) assemblages, and sedimentary environments and facies. There are indications that biogeographical ranges of taxa shift over time. Moving our understanding of Palaeozoic phytoplankton biogeography forward requires (1) targeted investigation of regions and time periods for which no or little data exist, (2) quantitative analysis of data to investigate how similarity between regions varies through time and how this might correlate with other datasets such as carbon isotope stratigraphy or sea-level, and (3) rigorous application of well-defined time slices to compare coeval assemblages, at least within the limits of resolution.