Ghawar field is located about 50 mi. inland from the western shore of the Persian Gulf, in Hasa Province, Kingdom of Saudi Arabia. The field was discovered in 1948 by a wildcat well at Ain Dar. Other wildcats, Haradh (1949), 'Uthmaniyah (19 51), and Shedgum (1952), proved to be subsidiary closures of the same great anticline and parts of the same oil field. Oil-finding has been primarily through structure drilling, although surface mapping furnished initial clues and gravity mapping proved helpful. The oil field is a structural accumulation along at least 140 mi. of the N.-S.-trending En Nala anticlinal axis. The fold is simple in the S., develops low marginal crestal closures in the center, and is subdivided into 2 adjacent anticlines in the N.. Seven crestal closures have been found, but oil appears to be continuous from Ain Dar [25 degrees 59'N. 49 degrees 23'E.] to Haradh [24 degrees 08'N. 49 degrees 05'E.]. Ghawar oil is Upper Jurassic, occurring in the shallow-water carbonate sediments of the upper Jubaila and lower Arab formations. It is confined above by the lowest of the 3 anhydrite members of the Arab formation. Only minor shows of oil have been found in higher parts of the Arab. The most prolific production is derived from calcarenites in which much original pore space survives. Other rocks making up the reservoir are fine-grained limestone, calcarenitic limestone, dolomitic limestone, and dolomite. Partial dolomitization, usually porphyroblastic, is extensive. The oil accumulation, conservatively estimated at 875 sq. mi. in area, has a maximum vertical oil column of about 1,300 ft. Gravities range from 36 degrees API in the N . to 33 degrees API in the S. The crude is undersaturated, with saturation pressures decreasing to the S. The oil-water contact rises to the S. The reservoir water ranges from 240,000 ppm. total solids in the N. to 38,000 ppm. in the S. On the W. side of the structure elevations of the contact are higher, and salinities of the water are lower than on the E. Incipient growth of the Ghawar structure apparently started in the Lower Cretaceous, but the structure in its present form first became apparent in truncation at an unconformity between the early and late Upper Cretaceous (roughly post-Cenomanian and pre-Maastrichtian). Growth continued into the middle Eocene with minor disturbances possibly continuing to the Miocene. No fundamental structural disturbance, except perhaps regional tilting, seems to have taken place during and after Miocene time. The source of Ghawar oil apparently is in the Arab and Jubaila formations. The oil volume forces an assumption of substantial lateral migration. Temporary accumulation of the oil in a stratigraphic trap prior to its localization in the present oil field is the preferred hypothesis.