Petroleum systems of the Iranian Zagros Fold and Thrust Belt

Abstract

The Paleozoic, Mesozoic and Cenozoic successions of the Zagros Fold and Thrust Belt (ZFTB) of Iran are endowed with prolific petroleum systems. The Sarchahan―Dalan/Kangan(.) petroleum system is responsible for giant accumulations of gas in the Paleozoic series. The Mesozoic series contains the largest volume of hydrocarbon accumulations in the ZFTB. The main petroleum systems in the Jurassic succession are Sargelu―Surmeh(!), Hanifa―Surmeh(!) and Diyab―Surmeh(!) petroleum systems. The Cretaceous succession holds the Garau―Fahliyan(!), Kazhdumi―Sarvak/Ilam(!) and Middle Sarvak―Mishrif(!) petroleum systems. The Cenozoic series contains the Pabdeh―Asmari(!) petroleum system, which is partially active over limited areas of the ZFTB.Occurrence and evolution of these petroleum systems were basically controlled by the tectono-stratigraphic evolution of the ZFTB. The tectonic history of the ZFTB is characterized by three distinct phases including continental rifting (i.e., Early Permian), passive margin development (i.e., Triassic to Jurassic) and continental collision (i.e., Cretaceous to recent). These phases not only controlled the geometry and spatial distribution of essential elements (i.e., source, reservoir and seal rocks), but also greatly impacted the hydrocarbon generation/migration/accumulation processes associated in each petroleum system.Exploration in the ZFTB has resulted in the discovery of 125 oil fields (with proved reserves of ~157 billion barrels of oil) and 57 gas fields (with natural gas reserves of ~191 trillion cubic feet). The Oligo-Miocene Asmari and the Cenomanian-Santonian Sarvak/Ilam carbonates are the main oil-producing intervals. The Asmari carbonates, hosting >45% of Iranian oil reserves, are the most prolific oil reservoir in the ZFTB (e.g., daily production from the Asmari reservoir of Ahwaz Field reaches 700,000 barrels). On the other hand, more than 90% of gas reserves in the ZFTB are accumulated in the Permo-Triassic Dalan/Kangan carbonates (e.g., at supergiant gas fields including South Pars, Kish, North Pars and Golshan). Asymmetric folds provide the most important type of hydrocarbon traps in the ZFTB and stratigraphic traps remain to be explored in the future.