A drilling prospect with several potential pay zones is modeled. Analytical arguments involving conditional probabilities reveal that geological dependence has no bearing on either the expected number of successful layers or the expected combined reserves. Monte Carlo simulation sheds light on the dispersion of the reserve distribution. A case study using an offshore Louisiana prospect with three layers illustrates the principles described. The general approach outlined in this model of layered pay zones can be applied to other types of prospects having multiple components that may be related.

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