This brief note describes the probabilistic structure of the Arps/Roberts (A/R) model of petroleum discovery. A model similar to the A/R model is derived from probabilistic propositions demonstrated to be similar to the Barouch/ Kaufman (B/K) model, and also demonstrated to be similar to the Drew, Schuenemeyer, and Root (D/S/R) model. This note attempts to elucidate and to simplify some fundamental ideas contained in an unpublished paper by Barouch and Kaufman.1This note and its predecessor paper4does not attempt to address a wide variety of statistical approaches for estimating petroleum resource availability.3Rather, an attempt is made to draw attention to characteristics of certain methods that are commonly used, both formally and informally, to estimate a petroleum resource base for a basin or a nation. Some of these characteristics are statistical, but many are not, except in the broadest sense of the term.