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Zagros Fold Belt
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 km2, whereas most of the gas fields are concentrated in Central and Coastal Fars and in the contiguous offshore area.
Fig. 1 Location map of the oil and gas fields in the Zagros foothills and contiguous offshore (after Bordenave & Hegre 2005).
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.
Fig. 2. Schematic stratigraphy and source rocks–reservoir–seal relationship for the Dezful Embayment and neighbouring areas. The main source rocks are indicated by green flags and marginal ones by white flags; reservoirs (Res), and seals are also indicated (after Bordenave & Burwood 1995).
Two types of petroleum systems charged the Iranian Zagros and contiguous offshore: (1) the systems using long-distance migrations along gently dipping ramps towards regional highs, and saltrelated structures where oil and/or gas accumulated; in the Zagros Fold Belt, oil and/or gas was later re-accommodated in Zagros anticlines; (2) the systems using vertical migration to the large-scale Zagros anticlines according to their drainage. The four older petroleum systems (Palaeozoic, Middle Jurassic, Late Jurassic and Early Cretaceous) belong to the first type, whereas the Middle Cretaceous to Early Miocene System belongs to the second. For this reason the age dating of the oil expulsion is extremely important for comparison with the onset of the Zagros folding.
Among the petroleum systems, the Middle Jurassic and the Early Cretaceous Systems were less important for the charge of the Iranian oil reserves because their source rocks, even excellent ones, were not or poorly connected with reservoirs. In contrast, almost the entirety of the oil reserve comes from the Middle Cretaceous to Early Miocene System. In the same way, the gas reserves originated almost entirely from the Palaeozoic System.
The Palaeozoic Petroleum System formed by Llandovery source rocks and Dalan and Kangan carbonate reservoirs is sealed by the thick massive anhydrite of the Dashtak Formation. As the result of the north–south residual high that extended between Central Fars and the Qatar Arch before the Llandovery transgression, and the Hercynian uplift followed by intense erosion, Llandovery source rocks are confined in two basins, one extending from Oman to Abu Dhabi and to the north of Bandar Abbas, and another including Saudi Arabia, west of Qatar, Iraq, Jordan, and most probably the Dezful Embayment. The Palaeozoic System does not generally interfere with younger petroleum systems, at least in areas where the Dashtak evaporites are well developed. In contrast, in the Bandar Abbas coastal area, gas probably of Llandovery origin found its way upward to younger reservoirs.
Fig. 3. The Zagros Fold Belt and contiguous offshore areas: location of the gas fields assumed to be related to the Palaeozoic Petroleum System (after Bordenave 2008).
Oil, and later gas, expelled from the Llandovery kitchens migrated over long-distance paths towards gentle regional highs, located either in Fars and contiguous offshore (Gavbendi–Qatar High, South Bushehr High, North Bandar Abbas High) or in Lurestan (South Samand High), where they accumulated. Early salt-related structures situated on migration paths also trapped large amounts of gas. Oil accumulation began during the Middle Jurassic. The gas window was reached in increasing areas from the Early Cretaceous onwards. Progressively, larger amounts of gas invaded the upper part of the highs, pushing the oil down-dip in the form of peripheral oil legs. The existence of early oil accumulations was confirmed by major detrital bitumen deposited during the Cenomanian, as the result of the onset of the pre-Maastrichtian orogenic phase, and during the Early Miocene. Part of the gas moved at later date into the large-scale Zagros folds formed during the Late Miocene to Pliocene. The reconstruction of the gas pools that were formed prior to the Zagros orogeny remains one of the main keys to be used to evaluate the potential of the current Zagros anticlines, as only anticlines connected with pre-Zagros accumulations have trapped gas.
Fig. 4. Extension and thickness of the Early Llandovery source rocks (cut-off TOC ¼ 2% or 1508 API gamma ray), before the erosion that followed the Hercynian phase (after Bordenave 2008).
In the Middle Jurassic Petroleum System, Sargelu source rocks, deposited in the Mesopotamian Depression, are limited above and below by thick evaporitic layers. As the result of the absence of reservoirs, a large part of the oil generated has not been expelled and was cracked in situ into pyrobitumen and gas. However, close to the edge of the depression, some oil migrated laterally towards the porous platform facies of the Surmeh–Khami Formations, and accumulated on regional highs such as the Kuh-e Mish-Kharg High before the Zagros folding. Eventually, some oil moved into Zagros anticlines.
Oils of Buzurgan and Djebel Fauqi on the Iraqi side of the Iran–Iraq border were probably charged by the Sargelu source rocks, as they are compatible with the Sargelu pyrolysate. Some Sargelu oil could have supplemented Kazhdumi oil at Cheshmeh Khush and Susangerd, and Garau oil at Chillingar– Garangan, Sulabedar, and Kharg.
In the Late Jurassic Petroleum System, the Hanifa–Tuwaiq Mountain–Diyab source rocks accumulated in the two sides of the Qatar Arch. In the Mesopotamian Depression they corresponded to the condensed Naokelekan Formation. In Iran, oil migrated from the Hanifa–Tuwaiq Mountain– Diyab kitchens towards both the Gavbendi and South Bushehr Highs, and accumulated in saltrelated structures located on the southern edge of the highs, such as F, FB and 3H. At the time of migration (Eocene), no efficient caprock was available and the oil lost most of its lighter fractions.
Therefore, heavy oil and bitumen remained trapped in porous zones from the Arab zone to sometimes up to almost the sea floor. The large Mand Zagros anticline probably collected the oil accumulated prior to the Zagros folding in the South Bushehr High. In the Early Cretaceous Petroleum System, similarly to the Sargelu, the Garau source rocks are not connected with reservoirs in the centre of the Lurestan Depression, and a good part of the Garau oil could not migrate and was cracked in situ. However, on the SW edge of Lurestan, two middle-sized fields, Sarkan and Maleh Kuh, were charged by the Garau source rocks.
In the Dezful Embayment, Garau oil was drained by the thin, but porous, limestone of the Khalij Member of the Gadvan Formation. Oil and gas were found in this layer in all the deep wells drilled there. However, the reserves are relatively small. Some oil migrated laterally through the shelf edge, and accumulated in north–south regional highs such as in the Kuh-e Mish–Kharg High. The oil of the Chillingar– Garangan, Sulabedar, and Kharg fields probably originated from Garau source rocks, where they supplemented the oil that originated from Naokelekan– Sargelu source rocks.
In the Middle Cretaceous to Early Miocene Petroleum System, two main source rocks, Kazhdumi and Pabdeh, charged the Asmari and Bangestan reservoirs, capped by the Gachsaran evaporites. Up to 300 m of outstanding Kazhdumi source rocks accumulated in an intrashelf silled depression that corresponded almost exactly to the current Dezful Embayment. The Kazhdumi source rocks probably extended towards the High Zagros and the Shiraz area. Pabdeh sources were deposited in a NW–SE trough extending from Lurestan to the Shiraz area, where a maximum of 200 m of excellent source rocks accumulated. In addition, in the NE of the Gulf (Binak Trough), the argillaceous Ahmadi Member of the Sarvak Formation represents anoxic conditions and includes up to 100 m of source rocks that generated an additional charge to the Asmari and Bangestan reservoirs.
The onset of the Zagros folding occurred in the NE part of the Dezful Embayment after the end of the deposition of the Mishan Formation, 12.5 Ma ago, and in South Lurestan between 8.1 and 7.2 Ma. Modelling showed that oil began to be expelled from Kazhdumi source rocks between 8 and 3 Ma, for the various producing trends of the Dezful Embayment, after the onset of the Zagros folding. The only exception corresponded to the Karanj and Parsi area, where expulsion commenced between 18 and 12 Ma.
The Middle Cretaceous to Early Miocene Petroleum System provides an example of a system for which the onset of oil expulsion from source rocks occurred after the beginning of the formation of the Zagros large-scale anticlines. Oil migrated almost vertically to the closest anticlines, enhanced by fracturing developed in reservoirs, as well as in more argillaceous intervals, in the crestal part of the high-relief anticlines. Oil that accumulates in an anticline originates from the source rocks located in its ‘drainage area’. The concept of ‘drainage area’ permits a comparison between the calculated amount of oil expelled from source rocks existing in one anticline, based upon their areal extent and maturity, and its current oil in place (IOIP).
Source of information: Current distribution of oil and gas fields in the Zagros Fold Belt of Iran and contiguous offshore as the result of the petroleum systems. M. L. Bordenave, J. A.Hegre. 2014
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