PETROLEUM GEOLOGY/Reserves 333 Figure Creaming curves for the UK sector of the North Sea showing the impact of reserves creep on the older fields The upper curve includes all fields that had been discovered and were producing by 1998 The symbols represent estimates of the same 25 fields made at different times (1980, 1985, 1990, and 1998) The figure shows the change in estimated size of these discoveries The full line shows the addition of a further 100 discoveries made up to 1998 and which are currently in production Figure Published estimates of world ultimate oil recovery (trillions of barrels) This chart shows the various published estimates of ultimate oil recovery through time Earliest estimates made in the 1940s suggested an ultimate recovery of around 0.5 trillion barrels However, the average estimate since Halbouty (1981) has been around 2.0 trillion barrels USGS, United States Geological Survey oil recovery factor is around 35% (Figure 5), and that the current resource base of reserves is around trillion barrels, original oil in place would be nearly trillion barrels Therefore, every 1% increase in global oil recovery would lead to an additional 55 billion barrels of oil being produced Moving the average global recovery factor up to 45%, a not unrealistic target, could increase global recoverable reserves by around 550 billion barrels Over the past 50 years, estimates of global recoverable oil reserves have risen annually, except for two small falls in the late 1970s, despite annual increases in oil production (Figure 6) This has led to a great deal of attention being paid to reserve reporting by