Online Journal for E&P Geoscientists

Norman, Charles D.^*1
(1) GeoKnowledge, LaGrange, GA.

Probabilistic assessment of an exploration prospect includes estimation of the prospect’s chance of success, roughly the probability that the prospect will be a discovery. The process of estimating the chance of success is known as “risking.” Prospects are risked by considering the chance that each geologic element required for the accumulation of hydrocarbons is present or adequate. The individual elements are known as risk factors. Traditionally, two risk factors are used to assess the adequacy of the potential trap: structure and seal.

Adequacy of structure, sometimes referred to as “trap geometry,” is defined as the presence of a trap of an area and height sufficient to contain an accumulation of the minimum size necessary for “success.” Adequacy of seal is defined as the presence of top seal and/or fault seal adequate to support a column of the minimum size necessary for “success.”

While consideration of two risk factors is a valid approach for the assessment of most structural traps such as four-way anticlines and fault-dependent closures, it is not adequate for the assessment of stratigraphic traps. Proper risking of stratigraphic traps requires four trap-related risk factors: presence of top seal, presence of base seal, stratigraphic merging of top- and base seals, and trap extent or geometry.

Presence of top seal: The presence of a seal above the reservoir that will support the minimum column height.

Presence of base seal: The presence of a seal below the reservoir that will support the minimum column height. In the updip portion of stratigraphic traps, the sealing ability of the bed below the reservoir is just as important as the sealing ability of the overlying bed.

Stratigraphic merging: The top seal and base seal beds must merge together to create the updip stratigraphic trap. If they do not merge, a trap does not exist. The potential stratigraphic trap becomes essentially a thief zone that carries hydrocarbons out of the prospect.

Trap extent: The resulting trap must be of sufficient thickness and area to contain the minimum hydrocarbon volume required for “success.” Areally-limited traps, or very thin traps, may not contain sufficient hydrocarbons to be considered a discovery.

Failure to consider these additional risk factors may cause overestimation of the prospect’s chance of success, leading to incorrect investment decisions.