Almost all the oil production of Iran is concentrated in a small portion of the Zagros orogenic belt, the 40,000 Sq km Dezful Embayment of the Khuzestan province. The accumulation of oil results from a Cretaceous/Tertiary petroleum complex which includes 6 source rock layers of unequal importance, i.e. the basal part of the Garau Formation (Neocomian), the Gadvan (Barremian), the Kazhdumi (Albian), the Ahmadi (Early Cenomanian), the Gurpi (Senonian) and the Pabdeh (Eocene). These layers generated 99% of the onshore Iranian oil reserves, trapped into two main reservoirs, the Asmari (Early Miocene) and Bangestan (Cenomanian) limestones which contained 330 billions barrels of original oil in place, corresponding to more than 7% of the current global reserves. This paper discusses the distribution of Cretaceous source rocks in the Iranian Foothills and the Iranian side of the Gulf, and the geological, oceanological and climatic factors which favored dysoxic to anoxic conditions and the deposition of organic-rich sediments. The impact of each of these source rocks on oil accumulations is also briefly considered. Among Cretaceous source rocks, two of them, the Neocomian Garau deposited over Lurestan, NE Khuzestan and the northeastern part of the Gulf, and the Albian Kazhdumi, deposited in the central part of the Dezful Embayment, have excellent characteristics with initial organic carbon sometimes higher than 10%, Hydrogen Indices (HI) in the 600 g HC/kg C range and maximum thickness in excess of 300 m. Their organic matter is mainly algal with high sulfur content, qualifying them as Its type. They were deposited in well established depressions included in an overall shallow water intracratonic basin communicating with the Southern Tethys Ocean. These depressions became anoxic as the result of a water stratification caused by the conjunction of high stand waters, humid climate and the existence of a sill separating the depression from the open sea. On the contrary, the Gadvan and Gurpi Formations were deposited in poorly defined depressions where dysoxic conditions prevailed in few limited areas only, i.e. SE Khuzestan and NW Fars for the Gadvan and in a narrow NW-SE trough extending from Lurestan to North Fars for the Gurpi. In these areas, Gadvan and Gurpi have only marginal potentials, TOC values in the 1-2% range and HI varying from 150 to 400 g HC/kg C. The impact of the various source rocks is extremely variable. In Lurestan, rich Garau potential source rocks are seldom associated with valid traps. The basinal ＇＇Oligostegina＇＇ tight facies of the Sarvak remains most often associated with Garau facies, and the excellent Asmari reservoir is breached in the Lurestan structures as the result of the Lurestan uplift during Pliocene. Therefore, only few small fields are related to Garau source rocks, accumulated in the Sarvak and Ilam, when deposited under a high energy facies in SE Lurestan. In Central and Southern Dezful Embayment, very prolific thick mature Kazhdumi source rocks associated with excellent reservoirs, very large structures and the efficient Gachsaran cap rock, formed one of the most efficient Petroleum Systems, which would account for the accumulation of more than 95% of the onshore Iranian oil in place. An impressive gathering of giant fields, namely Ahwaz, Agha Jari, Bibi Hakimeh, Gachsaran, Mansuri, Marun and Rag-e Sefid, located in an area of 40 000 Sq km, are originated from the Kazhdumi source rocks. The contribution of other Cretaceous source rocks, i.e. Gadvan and Gurpi, is assumed to be negligible, because of their poor characteristics and their limited areal extent, The Gurpi is immature or marginally mature in most of the study area, with the exception of the NE Khuzestan and the deeper synclinorial areas of Central Khuzestan, where it may have generated very limited amounts of oil.