This paper discusses the assessment methodology of geologic probabilities of success of drillable prospects determined by petroleum exploration geologists. The commonly accepted industry standard assessment methods suggest the probability evaluation of key components of oil and gas accumulation: source rock, reservoir, seal rock and trap, and migration and timing. On the assumption that the risked events resulting in the assembly of the key components are geologically and eventalgebraically independent, the geological probability is computed as the product of the probabilities of each. Without challenging the overall correctness of the approach, this study argues that the presumed independency does not always apply. For these cases, probability evaluation of the actually and truly independent geologic and hydrodynamic processes is advised.