Importance of Assessing Risk and Volume Relationships in Multiple-Target Exploration Prospects

Norman, Charles D.

Probabilistic assessment of multiple-target exploration prospects should consider three types of geologic relationships between the targets: risk dependency, parameter correlation, and hydrocarbon communication. Definition of these relationships should be a fundamental activity within the geologic evaluation. Failure to evaluate these relationships will result in incorrect assessments of risk and volume.

Targets are also known as zones, reservoirs, segments, or compartments. Targets within an exploration prospect may represent separate stratigraphic intervals, fault blocks, depositional bodies, facies within a depositional body, or traps. Targets are defined by a unique combination of risk and volumetric parameters. They are assessed individually, then aggregated to create the overall prospect assessment. The aggregation must include definition of the geologic relationships between the targets in order to properly assess a prospect's probability of success and success case volume.

Risk dependency defines relationships in the targets' probabilities of success. Targets that share a risk dependency are more likely to succeed together or fail together. Risk dependency impacts both the prospect's overall probability of success and the prospect's success case volume. It is a critical aspect of the geologic evaluation. Evaluations that do not consider risk dependencies will overestimate the prospect's probability of success and underestimate the success case volume.

Parameter correlations define relationships between the targets' volumetric parameters. Targets within the same reservoir interval may have similar net thicknesses and porosities. Targets within the same trap may have similar structural areas and gas-oil-ratios. Parameter correlation impacts the range of the potential success case volumes within the prospect. Failure to consider parameter correlation may contribute to success case P10/P90 ratios for the prospect that are unreasonably narrow.

Hydrocarbon communication refers to spilling or leaking of hydrocarbons between targets in geologic time, as opposed to during production. Communication may result in shared hydrocarbon-water contacts, or migration of hydrocarbons between segments. The assumption that each target will fill individually, and will have a unique hydrocarbon-water contact, usually results in overestimation of prospect volumes.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013