The index of refraction of petroleum is believed to be a useful criterion of its commercial value. Methods of determining the refractive index of crude oils with the Abbe refractometer are described and previously published data on the relation between refractive index and specific gravity of crude oils and distillation fractions are discussed. Available data on Venezuelan crudes are shown graphically and demonstrate an approximate linear relation between refractive index and specific gravity (or A.P.I. gravity).

Refractive index has some advantages over specific gravity as a means of grading oils. It is particularly adapted to the preliminary evaluation of oil in sands encountered during drilling because only a minute quantity of oil is needed for an accurate determination. Finally, it is a valuable aid in identifying oils of similar provenance and in correlating oil-bearing horizons between different wells.