In October 2009, the USGS updated the Orinoco tar sands (Venezuela) recoverable "mean value" to , with a 90% chance of being within the range of 380-, making this area "one of the world's largest recoverable oil accumulations". Despite the large quantities of oil available in non-conventional sources, Matthew Simmons argued in 2005 that limitations on production prevent them from becoming an effective substitute for conventional crude oil. Simmons stated "these are high energy intensity projects that can never reach high volumes" to offset significant losses from other sources. Another study claims that even under highly optimistic assumptions, "Canada's oil sands will not prevent peak oil", although production could reach by 2030 in a "crash program" development effort. Moreover, oil extracted from these sources typically contains contaminants such as sulfur and heavy metals that are energy-intensive to extract and can leave tailings, ponds containing hydrocarbon sludge, in some cases. The same applies to much of the Middle East's undeveloped conventional oil reserves, much of which is heavy, viscous, and contaminated with sulfur and metals to the point of being unusable. However, high oil prices make these sources more financially appealing. A study by Wood Mackenzie suggests that by the early 2020s all the world's extra oil supply is likely to come from unconventional sources. Production The point in time when peak global oil production occurs defines peak oil. Some believe that the increasing industrial effort to extract oil will have a negative effect on global economic growth, leading to demand contraction and a price collapse, thereby causing production decline as some unconventional sources become uneconomical. Some believe that the peak may be to some extent led by declining demand as new technologies and improving efficiency shift energy usage away from oil. Worldwide oil discoveries have been less than annual production since 1980. World population has grown faster than oil production. Because of this, oil production per capita peaked in 1979 (preceded by a plateau during the period of 1973–1979). The increasing investment in harder-to-reach oil as of 2005 was said to signal oil companies' belief in the end of easy oil. While it is widely believed that increased oil prices spur an increase in production, an increased number of oil industry insiders believed in 2008 that even with higher prices, oil production was unlikely to increase significantly. Among the reasons cited were both geological factors as well as "above ground"