Petrographic, palynofacies and geochemical characteristics of organic matter in the Saouef Formation (NE Tunisia): Origin, paleoenvironment, and economic significance

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.