Unique gas chromatographic fmgerprints of oil from individual reservoir units can be obtained when careful attention is paid to sample collection and analytical technique. The comparison and correlation of these fmgerprints can be used to evaluate reservoir continuity, identify well production problems and assign the relative amounts of production to specific intervals when production is commingled. This use of fluid geochemistry is a useful accompaniment to the more traditional reservoir geology and production engineering tools, such as well logs and well tests. The result is a more complete understanding of reservoir characteristics. The chromatographic technique can be applied in the early stages of field development as well as the later stages of reservoir management. This paper describes the technique of reservoir typing through gas chromatography and illustrates its use with several geologic and engineering case histories from Angola and the Gulf of Mexico.