The discovery and development of the enormous oil resources of the Arabian-Iranian basin have provided the supplies necessary to meet world oil requirements during the post-World War II period. Underlying the Arabian-Iranian basin is a deep basement of Precambrian rock that outcrops on the Western Arabia plateau. The outcrops are Precambrian massifs, blocks of intensely deformed igneous and metamorphic rocks. The relatively large number of oil reservoirs in the region implies extensive vertical migration of petroleum from a more limited occurrence of rich source rocks. In the north-eastern part of Saudi Arabia the basal Cretaceous unit locally contains zones that appear to be source rocks. The geologically younger trend parallels the northwest-southeast axes of the Iranian onshore Tertiary fields, and the older trend parallels the north-south axes of the Arabian Mesozoic fields. There is an enormous gas potential in the Arabian-Iranian basin. The potential for gas associated with the major structures is huge.