Recurrent Appearance of Source-Rock Facies in Cretaceous to Eocene Carbonate Series of Tunisia
B. Tissot, R. Pelet, B. B. Furollet, J. L. Oudin, 1984. "Recurrent Appearance of Source-Rock Facies in Cretaceous to Eocene Carbonate Series of Tunisia", Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks, James G. Palacas
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Oil production in Tunisia is obtained from a carbonate series of mainly shallow-water facies comprising Middle to Upper Cretaceous, Paleocene and Eocene formations interbedded with the reservoirs. Several organic-rich beds with good petroleum potential appear in the series; however, they are of limited geographic extent, which results in poor chronostratigraphic correlation from well to well. The deposition of these beds seems to have depended on the local appearance of anoxic conditions in an otherwise constant environment. The most plausible model, consequently, is a shallow, consistently oxygen-depleted basin with eutrophic planktonic growth in which anoxic conditions could occur in slightly depressed topographic hollows that varied both in place and time.
The character of the organic matter of these beds exhibits features that correspond well with the model suggested. The chloroform extract, rich in NSO (nitrogen, sulfur, and oxygen) compounds and especially in resins, generally shows a strong, even predominant, n-alkane distribution and a high phytane-pristane ratio. In some interesting but uncommon cases this picture is reversed, however. The chemically isolated, chloroform-insoluble fraction, studied by Rock-Eval pyrolysis, exhibits characteristics typical of type II organic matter that originates from marine planktonic remains. All these features show little variation with geologic age. Correspondingly, the oils in the reservoirs are all of similar composition independent of reservoir age. The composition of the oil is compatible with the concept of generation from the organic beds previously considered. Consequently, a single oil type appears to have been generated by distinct source beds corresponding to the recurrent appearance of the same organic facies.