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 shallow-water Intracratonic 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 remigrated 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 δ¹³C 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 salt-related 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.