ABSTRACT A prototype expert system for formation evaluation is introduced and discussed, beginning with the rationale for its development, and presenting briefly qualitative assessments and tests of its performance. The background and definition of petrophysical formation evaluation is clarified, and the specific geological frame of the Permian Southern Basin of the North Sea, in which the system operates, is presented, with indications of future developments. The different modules of the system are discussed in some detail, and the interaction between modelling, advice-giving, and the user interface is clearly shown to be a crucial factor. Of particular importance is the ability of the system of presenting the reasons for its choice of solution in a clear text and in a user-intelligible fashion. Tests and performance of the system are discussed from a qualitative viewpoint, rather than after rigorous benchmark tests. This is considered appropriate at this stage, since the system is by no means in its final configuration. The conclusion has been reached by Britoil that the system has demonstrated that the application of expert systems design and technology to petrophysical formation evaluation is both feasible and desirable, and further development would therefore be beneficial.