ABSTRACT
A more extensive and deeper water basin persisted from middle Jurassic to middle Cretaceous times around the head of the Gulf, where shales and argillaceous limestones are of source rock quality. Shales also filled temporary anoxic basins in the Gulf and in Iran during the Cretaceous. The Jurassic and Lower Cretaceous source rocks are presently within the oil generation window over much of the region: it is apparently only over the north - south high of the Qatar Arch that they are immature. The Middle Cretaceous is marginally mature over most of the Gulf; in south-west Iran, however, the Kazhdumi Formation has reached peak generation and even the uppermost Cretaceous has locally yielded oil. Carbonates provide the majority of the reservoirs of the region. It is where these deteriorate somewhat in quality, in Kuwait and neighbouring areas, that they are largely replaced by clean sands in the Middle Cretaceous and in the Miocene.
