Dependent Risk Calculations in Multiple-Prospect Exploration Evaluations
Authors: Wang B. , Kokolis G. P. , Rapp W. J. , Litvak B. L. 
Published: December 2007

To evaluate the risk (chance of success) of a multiple-prospect exploration opportunity, two types of information must be assessed: (1) the initial risks of each prospect and (2) the dependency of those risks among prospects. Risk dependency may cause individual prospect risks to increase or decrease as other prospects in the opportunity are drilled. High risk dependency and prior successes tend to result in prospect risks more favorable than the initially assessed risk values while risk independence would keep the risks unchanged despite prior successes or failures. In the past, most simple analyses have considered prospect risks either to be independent or 100% dependent (so-called "shared risks"). Furthermore, when dependent risk was considered, the same dependency tended to be used for all geologic controls (including structure, seal, reservoir and charge). A more accurate and realistic view results when individual risks are viewed as partially dependent, with different geologic controls having different degrees of dependence; e.g., the seal dependency may be very weak while the hydrocarbon-charge dependency is very strong. Separate consideration of the degree of dependence for each of the geologic controls allows for a more objective assessment of the overall risk dependency between prospects.

The principles outlined above also apply when evaluating the risk of an exploration prospect that consists of multiple potentially prospective geologic zones. The risk of any particular zone being successful is often dependent on the success or failure of other zones in that same prospect.

This paper presents a method to calculate partially dependent risks for multiple-prospect and multiple-zone evaluations. Different dependencies of geologic controls are handled separately in the calculations and then combined to generate overall dependent geologic risks. The degree of assumed dependency controls the trend of increasing or decreasing prospect risk. The method described can also be used to calculate dependent commercial risks if resource size distributions and commercial threshold sizes are provided.

In dependent-risk calculation, it is critical to ensure the weighted-average dependent risk of each prospect, which is equal to the summation of all calculated success probabilities, is identical to the initially assessed risk. In other words, the average risk of a prospect never changes regardless of drilling order and regardless of the failure or success of other prospects. This rule of "risk conservation" must be honored to avoid over or under estimating prospect risks. Violation of this rule can substantially change the prospective resource assessment, resulting in misleading project economics, and therefore potentially inappropriate economic decisions

This paper provides detailed calculation procedures to generate dependent risks employing the principles described above. An example risk evaluation of a 3-prospect lease block is provided to demonstrate the methodology. A 3-zone single prospect example is also shown.