Total organic carbon (TOC) determination, Rock-Eval pyrolysis, extractable organic matter content (EOM) fractionation, gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) analyses, were carried out on 79 samples from eleven outcrop cross sections of the Bahloul Formation in central and northern Tunisia. The TOC content varied between 0.23 to 35.6%, the highest average values (18.73%, 8.46% and 4.02%) being at the east of the study area (at Ain Zakkar, Oued Bahloul and Dyr Ouled Yahia localities, respectively). The Rock-Eval maximum pyrolysis temperature (Tmax) values in the 424-453°C range delineated a general east-west trend increase in the organic matter (OM) maturity. The disparity in hydrogen index (HI) values, in the range 114-824 mg hydrocarbons (HC) g -1 TOC, is relevant for the discrepancy in the level of OM preservation and maturity among localities and samples. The n-alkane distributions, maximizing in the C17 to C20 range, are typical for a marine planktonic origin, whereas pristine/phytane (Pr/Ph) average values in the 1-2 range indicate an oxic to suboxic depositional environment. Pr/n-C17 and Ph/n-C18 ratios in the 0.38-6.2 and 0.68-3.25 range, respectively, are consistent with other maturity indicators and the contribution of specific bacteria to phytol as a precursor of isoprenoids. The thermal maturity varies between late diagenesis to main-stage of petroleum generation based on the optic and the cis-trans isomerisation of the C29 sterane [20S/(20S+20R) and 14b(H),17b(H)/(14b(H),17b(H)+14a(H),17a(H)), respectively] and the terpane [18a(H) 22,29,29,29,: Ts/(Ts+Tm)] ratios. The Bahloul OM is represented by an open marine to estuarine algal facies with a specific bacterial contribution as revealed by the relative abundance of the aaa-20R C27 (33-44%), C28 (22-28%) and C29 (34-41%) steranes and by the total terpanes/total steranes ratio (1.2-5.33). These results attested that the Bahloul OM richness was controlled both by an oxygen minimum zone induced by high productivity and restricted circulation in narrow half graben structures and around diapirs of the Triassic salt.

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The Albian organic-rich successions of the lower part of the Fahdene Formation (Albian to Cenomanian, Tunisia) were studied using sedimentology (analysis of carbonate contents and observation of thin sections), bulk organic geochemistry (Rock-Eval pyrolysis), and molecular biomarker distributions. The selected outcrops cover different structural domains from western central Tunisia (Jebel Hamra) to the Diapir zone or the Tunisian Trough (Koudiat Berkouchia, Jebel Ghazouan and Ain Asker from the southwest to the northeast, respectively). Total Organic Carbon values between 0.17 and 3.43% evidenced the record of the Albian oceanic anoxic events OAE1b and OAE1c in this southern Tethys margin. Rock-Eval data and molecular biomarkers (n-alkanes, steranes and terpanes) revealed thermally mature organic matter (OM) consisting of a mixed Type II/III OM (marine planktonic and continental origins). The abundant tricyclic terpanes, C30 17α(H) Diahopanes, and the regular decrease of homohopanes (C31 to C35) in the terpane series, were also indicative of a high contribution of bacterial OM that had been developed under stagnant dysoxic/anoxic environments.

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