Radhwani, and 4 co-authors reported in 2018 that Saouef lignite layers and carbonaceous shales are studied in order to reconstruct the origin and depositional environment of organic matter (OM), as well as to evaluate their economic potential. The study is based on organic compositions, bulk geochemical parameters, biomarker data, and carbon isotope composition of OM (δ13C). Lignite samples, collected next to the Saouef village, were macroscopically identified as stratified matrix coal or as humic coal.
Rock Eval pyrolysis and the palynofacies analysis indicate the predominance of Type-III-IV kerogen (OM). The depositional environment fluctuated between terrestrial environments with a low water table to the limnic environment. The OM is derived from higher land plant remains (e.g. wood, root, bark) from a mixed vegetation including conifers and angiosperms, and emerged macrophytes. The dominance of huminite over liptinite and intertinite maceral groups, along with the phytoclasts, confirms the woody origin of the OM. A high bacterial activity is evidenced by the abundance of hopanoids in the extractable OM as well as by the amorphous OM content. The high measured TOC values exceeding 20 wt% in some samples may give an economic value of the organic layers of Saouef Formation. However, a Tmax < 435 °C and Rr < 0.5% induce the immaturity of the samples despite the presence of expelled bitumen in one sample.
The sedimentary organic matter of the Saouef Formation in northeast Tunisia constitutes lignite/carbonaceous layers on top of each prograding parasequence of a river delta. The petrographic, palynofacies and geochemical results of the organic matter suggest a marginal dysoxic to anoxic basin. The n-alkanes and the steroids biomarkers data
suggest the origin of OM predominantly from higher land plants and emerged macrophytes. Biomarker data further indicate the varying contributions of conifers and angiosperms to peat-formation. The high abundances of hopanoids indicate a high bacterial activity in the mire responsible for gelification of the plant tissues. The δ13C data of TOC are consistent with values found in middle Miocene lignite deposits.The observed variation of δ13C values might be related to the fluctuation of climatic parameters.
The palynofacies analysis, the Rock Eval and the biomarker (n-alkanes and terpenoids distributions) data suggest a kerogen (OM) Types III/IV with the potential to generate gas. However, Tmax mostly doesn't exceed 435 °C and the huminite reflectance averages 0.34% Rr only characterizing the Saouef coaly layers as immature (“lignite”).
Biomarker maturity parameters also confirm the immaturity of the Saouef organic matter.
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