The Sacha oil field in the Ecuadorian Oriente was discovered in early 1969. Production began in July 1972, and at the end of 1980 had exceeded 190 million bbl. Drilling through 1980 had resulted in 91 oil wells and 2 dry holes. Estimated original primary recoverable reserves surpass 632 million bbl. The field is on a very low-relief anticline about 17.5 mi (28 km) long and averaging 4 mi (6.5 km) wide. Vertical closure amounts to 200 ft (60 m) and there are 41,000 acres (16,600 ha.) of areal closure on top of the principal reservoir. The Hollin Formation, the basal Cretaceous sandstone, is the principal reservoir, having produced 80% of the oil through 1980 and containing about 68% of the original reserves.--Modified journal abstract.