Crude oils and rock samples from carbonate basins in France (Aquitaine), Guatemala, Iraq, and Tunisia/Libya were analyzed by the usual methods of organic geochemistry. The studies were based mainly on computerized gas chromatography-mass spectrometry, with special emphasis on mass fragmentography. Several classes of biological markers were used for the purpose of characterizing the environments of deposition and for correlating crude oils with source rocks. The following specific features commonly were observed in the oil and rock samples from these carbonate formations: (1) the predominance of normal alkanes with even carbon numbers, usually more pronounced in the rocks than in the oils; (2) a ratio of pristane to phytane mostly below 1; (3) the presence in significant c ncentrations of the higher homologs of the hopane triterpanes, up to C₃₅ (more important in rocks than in oils); (4) usually small amounts of steranes (C₂₁-C₂₃ and C₂₇-C₂₉ ranges), with predominance of non-rearranged structures; and (5) commonly an unusually high concentration of individual isoalkanes (C₂₃) and cyclohexylalkanes (C₂₁).

These molecular criteria partly reflect an important microbial contribution, which is usually more pronounced in highly anoxic environments. Furthermore, they have been used successfully in correlations among crude oils and between crude oils and their source rocks. The results agree with other geochemical and geological data.

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