Most oil is produced from giant oil fields found before the 1970s and is collectively termed conventional oil. This oil will continue to dominate supply until well past peak production. Some nonconventional oil is already in production and will be increasingly important after peak. At that time a discontinuity will mark the onset of a decline in the supply of this fuel that has powered the world economy for most of the twentieth century. To obtain an accurate estimate of the oil reserves, it is necessary to determine how much oil has been found and when it was found. The amount discovered at any reference date comprises how much had been produced to that date (cumulative production) and how much remains to be produced from known fields (reserves).