Before the use of gas chromatography-mass spectrometry and biomarkers, correlation of locations' geology was used to find how different formations relate to each other and to their environment. Oil-oil correlations (comparing petroleum to other oil found locally or in other areas) and oil-source correlations (comparing petroleum and its source) were performed; infrared spectrometry, refractive indices, solvent extractable organic matter, compound class distribution, and elemental analysis are all methods of doing oil-source correlations.