Abstract
We review the methods used to estimate subsurface risk and volumetric uncertainty over the exploration and production
lifecycle. Whilst probabilistic techniques are essential to decision-making, we show they have limitations which must be
understood. In frontier situations they are hard to justify and non-technical factors weigh heavily in decisions. As more
information becomes available, probabilistic estimates are more justifiable, but results can be very sensitive to small changes
in input parameters and to other factors. The nature of the transition to development varies. In simple accumulations, once
an economic volume has been discovered, exploration risk is replaced by development execution risk. In the case of extensive
reservoir complexes, the risk of not finding an economic volume may persist. In both situations, uncertainties relating to
reservoir detail, fluid properties, and natural drive mechanisms become critical. Production data add to the complexity, as do
interactions with external factors such as surface facilities. Resolving uncertainty at this stage, therefore, requires techniques
to integrate and interpret large amounts of data. Given the subjective nature of much of the input into evaluations, likely