The U.S. Geological Survey assessed how much undiscovered, technically recoverable conventional oil and gas may remain in the Zagros Fold Belt of Iran, Iraq, Syria and Turkey.
The three conventional assessment units (AUs) in the Zagros Fold Belt. USGS.
The bottom line
| Resource | Mean estimate | Range (F95–F5) |
|---|---|---|
| Oil | 3,316 million barrels (3.3 billion) | 1,667–5,638 million barrels |
| Gas | 80,258 billion cubic feet (80.3 trillion) | 34,977–147,998 billion cubic feet |
| Natural gas liquids | 1,889 million barrels (1.9 billion) | 827–3,470 million barrels |
F95 means a 95-percent chance of at least that amount; F5, a 5-percent chance. All results are fully risked.
How the fold belt formed
The Zagros Fold Belt formed in the Neogene on the northeastern margin of the ancient continent Gondwana, but its story begins much earlier.
| Time | What happened |
|---|---|
| Proterozoic | Assembling terranes stamped a north–south and northwest–southeast grain on the margin that guided all later deformation |
| Late Neoproterozoic–Early Cambrian | Stretching formed north–south rift basins holding up to 2 km of Hormuz salt, which now lies beneath the Fars area |
| Cambrian–early Carboniferous | A quiet, passive margin gathered sandstones, shales and carbonates, including the organic-rich Silurian Gahkum shales |
| Carboniferous | The Hercynian orogeny squeezed and uplifted the margin, eroding away most Silurian and Devonian rock — unevenly |
| Late Permian–Late Triassic | Opening of the neo-Tethys Ocean flooded the margin, building carbonate platforms and basins with the organic-rich Khuff marls |
| Early Jurassic–Late Cretaceous | Continued subsidence laid down major source rocks: the Jurassic Sargelu, Tuwaiq Mountain, Naokelekan and Hanifa marls and the Albian–Cenomanian Kazhdumi Formation |
| Late Cretaceous | Subduction beneath Eurasia began closing the neo-Tethys and thrust ophiolite sheets onto the margin, forming a proto-foredeep and the first northwest–southeast folds |
| From about 25 million years ago | Collision with Eurasia stacked thrust sheets southwestward, built the Zagros Fold Belt and formed the Mesopotamian foredeep; most source rocks were buried and matured |
| Neogene | Opening of the Gulf of Aden and Red Sea reshaped the folds into today's structures |
Where the oil and gas come from
The USGS defined one Paleozoic–Cenozoic Composite Total Petroleum System: petroleum from several source rocks on the old Gondwanan margin, later buried under Zagros thrust sheets and up to 5 km of foreland-basin sediment. Possible sources include the Gahkum shales and marls of the Sargelu, Tuwaiq Mountain, Naokelekan, Hanifa, Kazhdumi, Derdere, Karabogaz and Paleogene Pabdeh formations; the Kazhdumi shales are a known source in the fold belt.
| Assessment unit | Geologic model |
|---|---|
| Zagros Fars Fold Belt | Oil from Jurassic and Cretaceous basin source rocks migrated into carbonate platform-margin, dolomite, reef and karst reservoirs; gas from the Gahkum shales moved into fractured Mesozoic carbonates in folds and Hormuz salt structures |
| Zagros Central Fold Belt | Oil and gas from Silurian, Jurassic, Cretaceous and Eocene sources moved into folds formed as the neo-Tethys closed; burial beneath the advancing thrusts matured the source rocks. Kazhdumi is the main oil source; most reservoirs are fractured Asmari carbonates sealed by Gachsaran evaporites |
| Zagros South Turkey Fold Belt | Oil from Kazhdumi (and possibly Derdere and Karabogaz) shales and gas from Gahkum shales in carbonate platform-margin, reef and karst reservoirs, plus minor Silurian–Devonian sandstones |
Renewed thrusting linked to the opening of the Red Sea and Gulf of Aden may have fractured carbonate reservoirs and evaporite seals, letting some oil and gas escape across the belt.
Results by unit (means)
| Assessment unit | Oil (million barrels) | Gas (billion cubic feet) | NGL (million barrels) |
|---|---|---|---|
| Zagros Fars Fold Belt | 373 | 448 + 54,940 | 7 + 1,319 |
| Zagros Central Fold Belt | 2,875 | 6,039 + 18,649 | 181 + 373 |
| Zagros South Turkey Fold Belt | 68 | 6 + 176 | 0 + 9 |
| Total | 3,316 | 80,258 | 1,889 |
Where two figures are given, the first is gas or liquids in oil fields and the second in gas fields. Every unit was assigned a probability of 1.0.
The Central Fold Belt holds most of the undiscovered oil; the Fars Fold Belt, most of the gas.
Key assumptions
- Fars: a median of 30 oil fields and 125 gas fields, with gas fields of up to 40,000 billion cubic feet.
- Central: a median of 100 oil fields (oil fields up to 2,000 million barrels) and 50 gas fields (up to 30,000 billion cubic feet).
- South Turkey: a median of 40 oil fields and 10 gas fields, mostly small — median oil fields of 1 million barrels and gas fields of 6 billion cubic feet.
Sources
Based on U.S. Geological Survey Fact Sheet 2024–3048, "Assessment of undiscovered conventional oil and gas resources of the Zagros Fold Belt of Iran, Iraq, Syria, and Turkey," by the Zagros Fold Belt Assessment Team (Christopher J. Schenk and others); a work of the United States government in the public domain.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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