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Since 1977, crude oil reserves have been primarily sustained by the extension and development of existing fields (called field growth, reserves growth, or the EIA preferred term: proved ultimate recovery appreciation; see the Proved Ultimate Recovery section later in this chapter) rather than the discovery of new oil fields. Only 8 percent of reserves additions since 1976 were booked as new field discoveries. Proved ultimate recovery appreciation is the sum of net revisions and adjustments, extensions, and new reservoirs in old fields. Since 1977, the largest component of proved ultimate recovery appreciation for crude oil is upward revisions and adjustments, which accounted for 62 percent of all crude oil reserves additions. The 18,393 million barrels of total discoveries accounted for the remaining 38 percent of reserves additions.