Introduction

The U.S. Geological Survey (USGS) assessed the potential for undiscovered, technically recoverable conventional oil and gas resources within the Zagros Fold Belt in Iran, Iraq, Syria, and Turkey (fig. 1). The following tectonic summary is based upon several sources (Sepehr and Cosgrove, 2004; Carruba and others, 2006; Homke and others, 2009; Pirouz and others, 2011; Saura and others, 2015; Carminati and others, 2016; Koshnaw and others, 2017; Orang and others, 2018; Abdulnaby and others, 2020). The Zagros Fold Belt formed in the Neogene on the northeastern margin of Gondwana, but the tectonic history began with the assembly of Proterozoic terranes that imprinted a regional north–south and northwest–southeast structural grain onto the continental margin. These structural trends affected subsequent Phanerozoic deformation (Al-Husseini, 2000; Ruban and others, 2007). A period of extension along the northeastern margin of Gondwana from late Neoproterozoic to Early Cambrian formed north–south-trending rift basins, in which as much as 2 kilometers of salt of the Hormuz Formation accumulated (Grosjean and others, 2009), which now underlies the Fars area of the Zagros Fold Belt. From the Cambrian to the early Carboniferous, the northeastern margin of Gondwana was passive, which led to the accumulation of extensive sandstones, shales, and platform carbonates, including the organic-rich shales of the Silurian Gahkum Formation. The Hercynian orogeny in the Carboniferous led to regional contraction, uplift, and erosion that removed most Silurian and Devonian rocks from the northeastern margin (Konert and others, 2001), although the level of erosion across the margin was spatially heterogeneous. Late Permian to Late Triassic extension related to the initial opening of the neo-Tethys Ocean led to a regional transgression and the formation of extensive carbonate platforms and intraplatform basins with organic-rich marls of the Permian Khuff Formation. Passive margin subsidence of the northeastern margin from Early Jurassic through Late Cretaceous formed extensive carbonate platforms and intraplatform basins where several major petroleum source rocks were deposited, consisting of organic-rich marls of the Jurassic Sargelu, Tuwaiq Mountain, Naokelekan, and Hanifa Formations, and the Albian–Cenomanian Kazhdumi Formation. Most of these source rocks were subsequently buried and thermally matured in the Neogene within the northeastern margin. In the Late Cretaceous, continued subduction of oceanic lithosphere beneath Eurasia led to the initial stages of closure of the neo-Tethys Ocean and to the obduction of ophiolite thrust sheets onto the northeastern margin of Gondwana. Obduction resulted in contraction across the margin, flexural subsidence that formed a proto-foredeep basin, and the formation of northwest–southeast-trending folds and anticlines along inherited lines of structural weakness. The continent-to-continent collision of the northeastern margin of Gondwana with Eurasia began about 25 million years ago with the progressive southwest advancement and stacking of thrust sheets associated with the growing Zagros Fold Belt, and the formation of the Mesopotamian foredeep basin southwest of the thrust belt. Most of the contractional structures of the Zagros Fold Belt formed or were modified at this time. The Neogene opening of the Gulf of Aden and the Red Sea further modified the folds and anticlines into the structures observed in the Zagros Fold Belt today (Mouthereau and others, 2012).

Total Petroleum System and Assessment Units

The USGS defined a Paleozoic–Cenozoic Composite Total Petroleum System (TPS) encompassing petroleum generated from several source rocks deposited on the northeastern margin of Gondwana that were subsequently buried by Zagros thrust sheets and as much as 5 kilometers of foreland basin sediments. Source rocks that may have provided oil and gas to Zagros structures include shales of the Silurian Gahkum Formation and organic-rich marls of the Sargelu and Tuwaiq Mountain Formations, Naokelekan Formation, Hanifa Formations, Kazhdumi Formation, Derdere Formation, Karabogaz Formation, and Paleogene Pabdeh Formation (Beydoun and others, 1992; Opera and others, 2013; Mashhadi and Rabbani, 2015; Baniasad and others, 2017). Organic-rich shales of the Kazhdumi Formation are source rocks in the Zagros Fold Belt (Ala, 1982; Beydoun and others, 1992; Alizadeh and others, 2012; Sfidari and others, 2016; Alipour, 2022), but oil and gas generated from other source rocks are included in the geologic models in this study. Assessment units (AUs) defined within the composite TPS are the Zagros Fars Fold Belt AU, Zagros Central Fold Belt AU, and Zagros South Turkey Fold Belt AU.

The geologic model for the Zagros Fars Fold Belt AU is for oil generated from Jurassic and Cretaceous intrashelf basinal source rocks to have migrated into carbonate platform-margin, dolomite, reef, and karst reservoirs; gas sourced from organic-rich shales of the Gahkum Formation migrated into Mesozoic fractured carbonate reservoirs within northwest–southeast-trending contractional structures and Hormuz salt structures (Bordenave, 2003).

The geologic model for the Zagros Central Fold Belt AU is for oil and gas generated from Silurian, Jurassic, Cretaceous, and Eocene source rocks to have migrated into northwest–southeast-trending contractional structures related to the closure of the neo-Tethys Ocean beginning in the Late Cretaceous. The progressive formation of the southwest-directed thrusts created a foredeep in front of the growing thrust belt, and the related sediment accumulation and thrust loading served to thermally mature source rocks across the northeastern margin. Geochemical analyses show that the major oil source rock in this AU is the Kazhdumi Formation. Oil and gas migrated into a spectrum of carbonate reservoirs, but most reservoirs are fractured carbonates of the Asmari Formation, sealed by evaporites of the overlying Gachsaran Formation.

The geologic model for the Zagros South Turkey Fold Belt AU is for oil generated from shales of the Kazhdumi Formation and possibly from shales of the Derdere and Karabogaz Formations—and gas from shales of the Gahkum Formation—to have migrated into carbonate platform-margin, reef, and karst reservoirs within Zagros contractional structures. Reservoirs also include minor Silurian and Devonian fluvial-deltaic to shelf sandstones.

Renewed movement of thrusts related to the opening of the Red Sea and the Gulf of Aden modified the northwest–southeast-trending contractional structures and led to possible loss of oil and gas throughout the fold belt due to fracturing of carbonate rocks and evaporite seals (Ziegler, 2001).

Each of the AUs was assessed for undiscovered conventional oil and gas resources. The assessment input data for three conventional AUs are summarized in table 1 and in Schenk (2025).

Undiscovered Resources Summary

The USGS quantitatively assessed undiscovered conventional oil and gas resources in three AUs in the Zagros Fold Belt Province (table 2). The estimated mean resources are 3,316 million barrels of oil (MMBO), or 3.3 billion barrels of oil, with an F95–F5 range from 1,667 to 5,638 MMBO; 80,258 billion cubic feet of gas (BCFG), or 80.3 trillion cubic feet of gas, with an F95–F5 range from 34,977 to 147,998 BCFG; and 1,889 million barrels of natural gas liquids (MMBNGL), or 1.9 billion barrels, with an F95–F5 range from 827 to 3,470 MMBNGL.

For More Information

Assessment results are also available at the USGS Energy Resources Program website, https://www.usgs.gov/programs/energy-resources-program.

Zagros Fold Belt Assessment Team

Christopher J. Schenk, Tracey J. Mercier, Phuong A. Le, Andrea D. Cicero, Ronald M. Drake II, Sarah E. Gelman, Jane S. Hearon, Benjamin G. Johnson, Jenny H. Lagesse, Heidi M. Leathers-Miller, and Kira K. Timm