heavy crude oil, as well as renewable energy sources. It is thought by many, including energy economists such as Matthew Simmons, that prices could continue to rise indefinitely until a new market equilibrium is reached at which point supply satisfies worldwide demand. Timothy Kailing, in a 2008 Journal of Energy Security article, pointed out the difficulty of increasing production in mature petroleum regions, even with vastly increased investment in exploration and production. By looking at the historical response of production to variation in drilling effort, he claimed that very little increase of production could be attributed to increased drilling. This was due to a tight quantitative relationship of diminishing returns with increasing drilling effort: As drilling effort increased, the energy obtained per active drill rig was reduced according to a severely diminishing power law. This analysis suggested that even an enormous increase of drilling effort was unlikely to lead to significantly increased oil and gas production in a mature petroleum region like the United States. A prominent example of investment in non-conventional sources is seen in the Canadian oil sands. They are a far less cost-efficient source of heavy, low-grade oil than conventional crude; but when oil trades above $60/bbl, the tar sands become attractive to exploration and production companies. While Canada's oil sands region is estimated to contain as much "heavy" oil as all the world's reserves of "conventional" oil, efforts to economically exploit these resources lag behind the increasing demand of recent years. Until 2008, CERA (a consulting company wholly owned by energy consultants IHS Energy) did not believe this would be such an immediate problem. However, in an interview with The Wall Street Journal, Daniel Yergin, previously known for his quotes that the price of oil would soon return down to "normal", amended the company's position on 7 May 2008 to predict that oil would reach $150 during 2008, due to tightness of supply. This reversal of opinion was significant, as CERA, among other consultancies, provided price projections that were used by many official bodies to plan long-term strategy in respect of energy mix and price. Other major energy organisations, such as the International Energy Agency (IEA), had already been much less optimistic in their assessments for some time. In 2008, the IEA drastically accelerated its prediction of production decline for existing oilfields, from 3.7% a year to 6.7% a year, based largely on better accounting methods,