The crude oils in lower Tuscaloosa Cretaceous reservoirs in the central Gulf Coast fall into two 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 in south-central and southwestern Mississippi, where the lower Tuscaloosa has been subjected to the deepest burial and greatest diagenetic influence. The second group of oils commonly is in lower Tuscaloosa reservoirs on faulted structures. The oils of this group are trapped where secondary migration routes across formational boundaries may have been created, or where younger and/or older source rocks 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 sterane naphthenes, the saturate and aromatic hydrocarbon compound types, and the stable carbon-isotope ratios.