Complex hydrocarbon traps are those in which a number of different trapping elements have to work favourably and simultaneously in order to allow access to the full hydrocarbon volume potential of the prospect. The probability of success varies across the prospect and the volume distribution. A consistent assessment of the probability of success relative to the volume uncertainty of hydrocarbon traps is essential for unbiased prospect characterisation and vital for decision making, portfolio management and for delivering predicted value. It is relatively straightforward to assess the probability of geological success and volume uncertainty of simple anticlines or domal structures. Unfortunately, simple four-way closures are usually drilled early in the exploration of a basin or play and become increasingly less common with exploration maturity. As a consequence, prospects available for drilling in mature exploration areas are typically complex traps, i.e. they possess a combination of different trapping elements. In such cases, trapping of the full volume potential requires that many geological elements work concurrently. Complex traps are often perceived to carry a comparatively lower probability of geological success. However, the introduction of additional factors in a multiplicative chance estimation model may not reflect the true probability of finding hydrocarbons at the prospect location. Evaluations which involve multiplying additional chance factors may lead to an under-estimation of the probability of geological success and an over-estimation of the hydrocarbon volume. A solution to this problem is to calculate the probability of occurrence of each possible success scenario or outcome. This paper describes a straightforward method for assessing volume uncertainty in complex traps which is independent of the model or method used to estimate the probability of geological success. A faulted four-way closure is used as an example for the detailed description of the suggested workflow.