The evolution of decline rates of oil fields over past decades includes the impact of new technologies and production techniques and clearly shows that the average decline rate for individual giant fields is increasing with time. These factors have significant implications for the future, since the most important world oil production base – giant fields – will decline more rapidly in the future. In roughly mid 2004, total world oil production ceased to expand. Instead, new production has only succeeded in keeping world oil production relatively flat. An analysis by Cambridge Energy Research Associates estimated that the weighted decline of production from all existing world oil fields was roughly 4.5%. Andrew Gould, CEO of Schlumberger, stated that an accurate average decline rate is hard to estimate, but an overall figure of 8% is not an unreasonable assumption. The International Energy Agency (IEA) came to the conclusion in 2008 that the average production-weighted decline rate worldwide was 6.7% for post-peak fields which means that the overall decline rate would be less, since many fields are not yet in decline. Modeling future field behavior is done by extrapolating the historical production data with an exponential decline curve. This does not take dramatic deviations into account and assumes that declines will continue approximately exponentially. This leads to a somewhat optimistic extrapolation. The decline rate of a field is affected by introduction of new technology, investments, changes in strategies and other factors affecting production. Decline curves can be made much more detailed and complicated (e.g. hyperbolic). The decline of smaller fields is equal to or greater than those of the giants. A detailed study of fields showed that giants declined at an average of 13%, while the small fields, condensate, and NGL declined at 20% or more. A small field requires fewer wells to fully develop; hence it is more easily depleted. A large field requires many more wells, often widely separated, so it is typically depleted more slowly. High depletion rates, which are common in small fields, have been shown to strongly correlate with high decline rates.