The timing of the orogenic events associated with the closure of South Tethys significantly influenced the generation, migration and entrapment of petroleum in the Zagros Foldbelt of Iran. This influence was particularly important in the Dezful Embayment, which is one of the world's richest oil provinces, containing some 8% of global oil reserves in an area of only 60,000 sq. km. In the Dezful Embayment, oil and associated gas occur in two carbonate reservoirs ‐ the Sarvak Formation of Cenomanian to Turonian age, and the Oligocene ‐ Early Miocene Asmari Formation, sealed by the evaporites of the Gachsaran Formation. The oil and associated gas are trapped in large “whaleback” anticlines which formed during the Neogene Zagros orogeny. Two excellent source rocks, the Albian Kazhdumi Formation and the upper part of the Pabdeh Formation (Middle Eocene to Early Oligocene), supplied the Asmari and Sarvak reservoirs and with them form the Middle Cretaceous to Early Miocene Petroleum System. This system was found to be independent of older petroleum systems. Two particular problems are addressed in this paper. The first is the relative timing of trap formation versus oil expulsion from the source rocks. If oil expulsion occurred prior to Zagros folding, the oil would have migrated along gently dipping ramps towards the Persian (Arabian) Gulf and Southern Iraq, and would have been trapped a long way from the source kitchen. By contrast, if oil expulsion took place when the whaleback anticlines already existed or had at least begun to develop, the oil generated would have moved almost vertically towards the nearest anticline. Secondly, we assess the type of heatflow to be used for modelling. This could be either variable or constant, depending on the stability or instability of the Arabian Platform and on subsidence variations during source rock maturation. Our conclusions can be summarized as follows. First, the paroxysmal phase of Zagros folding commenced in the Dezful Embayment towards the end of the Middle Miocene around 10 Ma ago and continued throughout the Late Miocene and Pliocene. Second, bearing in mind the remarkable stability of the Arabian Platform for some 260 Ma, during which there was almost continuous gentle subsidence between the Permian transgression and the Early Miocene, a constant heatflow was used for modelling. Burial profiles and maturity indices, such as vitrinite reflectance and Rock‐Eval parameters, demonstrate that the Kazhdumi and Pabdeh source rocks reached the onset of oil expulsion during deposition of the Agha Jari Formation between 8 and 3Ma, depending upon the location. This chronology means that oil migrated from source rocks into preexisting Zagros structures. Therefore, oil migrated over short distances to nearby traps within well‐defined drainage areas, the geometry of which can be deduced from seismic data. Moreover, the Zagros folding induced prominent fracturing which can be observed both at outcrop and in wells. This fracturing, which affects lime‐stones as well as marls, enhanced...
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.
More than 1500 trillion cubic feet (Tcf) of gas reserves have been discovered in Permo‐Triassic carbonates sealed by thick Triassic anhydrites in the Zagros Foldbelt (SW Iran), the southern part of the Gulf (Iran, Qatar, and Abu Dhabi) and Saudi Arabia. This paper discusses the origins of this gas in terms of source rock distribution and thermal maturation through time (as indicated by modelling), regional variations in thermal maturation (as indicated by cumulative isopachs), and long‐range migration and accumulation of hydrocarbon prior to the Zagros orogeny. The sequence of events leading to the present‐day distribution of gas accumulations is reconstructed in detail. The only important source rocks so far identified in the Late Proterozoic to Late Triassic succession are organic‐rich, radioactive shales which are dated as Llandoverian (early Silurian). Oil generation began in the Middle Jurassic in areas of greatest subsidence, while the gas window was reached locally as early as the Middle Cretaceous. Huge volumes of oil, then of gas, accumulated in a few major regional highs and salt‐related structures prior to the Zagros orogeny. Part of the gas was lost during Zagros folding as some of the anticlines were breached, and another portion, possibly associated with light oil, remigrated into unbreached Zagros anticlines. Among critical parameters essential to the appraisal of the numerous Permo‐Triassic prospects present in Lurestan, Fars and in the Iranian Offshore, three are discussed in this paper, namely (i) the location of prospects in comparison to pre‐Zagros regional highs and to reconstructed pre‐Zagros gas accumulations; (ii) the characteristics of potential reservoir intervals in the Dalan/Kangan Formation; and (iii) the extent of the Dashtak evaporitic seal. The distribution of surface oil, bitumen and gas seepages together with indirect hydrocarbon indications provides an additional exploration tool.
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