The Norwegian Petroleum Directorate's assessment of the undiscovered resources of the Norwegian Continental shelf—background and methods

Abstract

In February 1993 the Norwegian Petroleum Directorate presented their recent assessment of the undiscovered hydrocarbon resources of the Norwegian shelf. The assessment is based on play analysis. Much effort was put into defining the different plays, in mapping the geographical limits of each play, and compiling data on their critical volume parameters. Each volume parameter was defined as a size distribution with a minimum, expected and maximum value. These parameters were used as input in the computation of the total resources in each play. The basic assumption behind this computation is that the relative field size distribution for the finite number of accumulations in each play may be fitted to a lognormal distribution. The estimate of the total undiscovered resources in each play is given as a probability distribution and is presented as a set of a low (P 95 ), a mean and a high (P 5 ) estimate. The estimated distribution for each play was aggregated by histogram summation to give the total probability distribution for the undiscovered resources of the whole of the Norwegian shelf. The computations were performed with the computer program FASPUM which was developed by R.A. Crovelli and H. Balay at the United States Geological Survey. Examples from selected plays are given. A major concern in the analysis was the calibration of input data and the risk factor associated with each parameter. We found the number of drillable prospects within each play to be the most difficult parameter to assess. At the same time, this parameter has a great impact on the final result. The risk analysis on both play level and prospect level was done according to the NPD internal standard procedure. The plays fall into two different categories; those confirmed by discoveries and those that are conceptual and not yet confirmed by discoveries. The risk associated with the non-confirmed plays is difficult to assess because it requires a subjective calibration against the risk-level in the confirmed plays. The calibration problems and the risk analysis are discussed.