The world's 932 giant oil and gas fields are considered those with of ultimately recoverable oil or gas equivalent. Geoscientists believe these giants account for 40 percent of the world's petroleum reserves. They are clustered in 27 regions of the world, with the largest clusters in the Persian Gulf and Western Siberian Basin. The past three decades reflect declines in discoveries of giant fields. The years 2000–11 reflect an upturn in discoveries and appears on track to be the third best decade for discovery of giant oil and gas fields in the 150-year history of modern oil and gas exploration.