Abstract
This paper shows how to incorporate uncertainty on whether two horizons being considered for drilling are correlated or independent. Prior to actual drilling there is no way to be sure of the state of the two horizons in relation to each other. We show that this uncertainty automatically adds an uncertainty to the mean assessed resource estimate and that this uncertainty can dominate over uncertainties brought about by imperfect knowledge of the success probabilities and potential resources of each of the individual horizons, as well as the distributions chosen to represent such uncertainties, and the nominal values of the parameters and their skewness of dynamic range around the nominal values. The main aim is to show which factors dominate in assessments of uncertainty of potential resources. In this way one can focus on narrowing the relevant parameter uncertainties causing the largest contributions without the need to spend time and effort in addressing other parameter uncertainties that would help but little, if at all, without the main focus on the important contributors to uncertainty of the mean resource assessment.