This paper discusses the uncertainties that exist in estimating the remaining ultimately recoverable resources of oil globally including the reasons for these and, where possible, how they may be mitigated, resolved, or reduced in future assessments. The encompassing and ambiguous terms 'conventional oil' and 'unconventional oil' are disaggregated into the numerous categories of oil that exist within each. These categories are investigated individually in order to identify the specific uncertainties by which they are influenced and affected. A key finding is that there are intrinsic uncertainties within every category of oil. Future assessments of global oil resources and projections of oil production should hence acknowledge these issues, explain or assess the effects that they have on results, and present ranges in any estimates produced or provided. An initial estimate is made of the technically recoverable resources of the light tight oil often called 'shale oil': oil found in low permeability shale formations requiring stimulation to be extracted. These resources are estimated to range on a global scale between 150 and 508 billion barrels with a central estimate of 278 billion barrels. (C) 2012 Elsevier Ltd. All rights reserved.