Regional supply of potential oil and gas can be assessed by projecting growth of known fields, by extrapolating discovery rates in partially drilled sedimentary rocks, and by geologically analyzing undrilled sedimentary rocks. The probable supply from future growth of existing fields is estimated by projecting past year-to-year revision ratios that reflect the known pattern of reserve additions. The possible supply from new fields in partially drilled rocks is estimated by extrapolating historical discovery rates expressed as oil-equivalent barrels found per foot of new-field wildcat drilling. (This method applies only to maturely explored areas and depths where the discovery rate is generally declining.) The speculative supply from new fields i undrilled areas or depths is estimated by geologic analysis and comparison. As one example, after assessing the probable and possible gas, we estimate the deep speculative gas potential of south Louisiana by multiplying predicted volumes of sandstones, in cubic miles, by related yields, in cubic feet of gas per cubic mile of sand. The results of each assessment are presented as a probability curve showing the existence chance for each amount (or more) of potential gas. The range of values on the curve reflects the combined uncertainties in the input factors of our estimate. Basic assumptions, interpretations, and quantities are specified so that validity may be checked by others. We account for geologic risk and any other anomalies affecting the outcome. Where feasible, each different play, area, or category is assessed separately using different approaches if necessary. Assessment curves are then added by Monte Carlo simulation. Our ultimate assessment goal is to make realistic judgments based on geologic fundamentals and experience.