According to the IEA World Energy Outlook 2008, p. 261:

Output of natural gas liquids — light hydrocarbons that exist in liquid form underground and that are produced together with natural gas and recovered in separation facilities or processing plants — is expected to grow rapidly over the Outlook period. Global NGL production is projected to almost double, from 10.5 mb/d in 2007 to just under 20 mb/d in 2030.

One can see from IEA's chart of World Oil Production by Source that the growth of natural gas liquids, or NGLs, is being depended on as a significant contributor to total world oil production:

World Oil Production by Source, Reference Scenario, shown as Figure 11.1 on page 250 of IEA WEO 2008.
In this post, I will document that there is good reason to believe that the IEA WEO 2008 projections in the reference scenario overshoots the likely world production of NGLs by as much as 35 - 50 % by 2030.

One way of estimating expected NGLs is as a ratio to natural gas production, representing the wetness or dryness of gas. One would expect this ratio to decline over time, based on what normally has been observed from fields, areas and regions with good quality data. Instead, the IEA is forecasting that this ratio will increase in the future.