The crude oils in lower Tuscaloosa Cretaceous reservoirs in Mississippi and Alabama can be divided into groups on the basis of their chemical compositions. One of these groups appears indigenous to the lower Tuscaloosa interval. The oils in this group, all in unfaulted structural and stratigraphic traps, are located in south-central and southwestern Mississippi, where the lower Tuscaloosa has been subjected to the deepest burial and greatest diagenetic influence. The remaining group of oils in the lower Tuscaloosa are commonly contained by faulted structures. They are situated

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where secondary migration routes across formational boundaries may have been created or where younger and/or older source beds have been brought into contact with the reservoir sandstones.

The chemical compositions of the crude oils were determined primarily by chromatographic and mass-spectrometric methods. Most useful were the analyses of the individual light-hydrocarbon components, the isoprenoid isoparaffins, the sterane naphthenes, the saturate and aromatic hydrocarbon compound types, and the stable carbon isotope ratios.

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