Hydrocarbons in phosphatized and non-phosphatized sediments from the phosphate basin of Gafsa

Abstract

The sedimentary layers in the phosphate Basin of Gafsa show interlayering of phosphatized and less phosphatized beds (shales, marls, limestone and cherts). 1. (1) The n-alkane distribution of the chloroform extracts and the pyrochromatograms obtained from the related kerogens are very similar in all the strata, suggesting that the organic matter which accumulated in these strata had a similar origin, marine planktonic. 2. (2) The occurrence of some significant fossil molecules in poorly phosphatized sediments (chert) including C-ring monoaromatic steroids (and benzohopanoids) confirms the autochthonous marine origin and the immature character of the organic matter and reflects intensive bacterial anaerobic activity. The lack of these fossil molecules in phosphorites is essentially due to the more oxidative conditions occurring during phosphatogenesis. In contrast the presence of these fossil molecules in humic acid fractions from phosphorites, reflects a specific property of humic substances to incorporate hydrocarbons. From petrological studies, humic acids and incorporated fossil molecules are thought to be preserved because they were trapped inside phosphorite pellets or ooids at an early stage of diagenesis. 3. (3) The chromatograms of hydrocarbons and pyrochromatograms for the humic compounds which can be extracted from each of the strata are very similar to those of the non-humic organic matter of the same samples. This suggests that hydrocarbons have been incorporated by the humic acids at an early stage of diagenesis. The amount of such humic-associated hydrocarbons can reach 20% of the humic acid weight. 4. (4) The amount of hydrocarbons in the bulk sediment confirms that the conditions favourable, or not favourable, to phosphate concentrations are a result of variation in Eh during deposition and not variation in organic matter type.