Abstract: |
Economic analyses are often highly sensitive to accumulation size, thereby influencing when and at what cost a potential resource will become available. For established plays, it may be expedient to base predictions of future field sizes on previous discoveries. In most cases, the historical size distribution is inappropriate to apply to the remaining potential within an established play because the largest accumulations tend to be discovered early in the exploration process. This intentional sampling bias must be accounted for when projecting sizes of future discoveries, as does the tendency to underreport the ultimate recoverable volume of hydrocarbons early in the life of a field. Exploration can be represented as a sampling process that both is size biased (creamed) and truncated. One can in cases where this representation is appropriate obtain an unbiased estimate of the resource base population (parent population) parameters by correcting for the bias (un-creaming).This can be achieved by explicitly modeling the relation between the sample parameters and the population parameters. Field size data from the Gulf of Mexico shelf were used to explore the relationship between discovered and future field sizes. Using Beta distributions and distributions with the same uncreaming property as assumed parent populations, it is possible to simulate discovery sequences that closely match the historical development of the basin. A discovery sequence can be approximated using lognormal distributions across multiple stages of exploration and over a wide distribution of discovery sizes. These observations suggest that one cannot impute the shape of the underlying parent distribution from the size distribution of past discoveries. An advantage, however of using a Beta distributed parent population is that the creaming bias is represented by a single parameter that can be estimated from the discovery sequence and subsequently used for an unbiased estimation of the parent population that potentially can incorporate a larger number of small fields than the lognormal distribution, and thus may significantly impact play economics. |
