Organic Geochemistry of the Cenomanian-Turonian Bahloul Formation Petroleum Source Rock, Central and Northern Tunisia

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 degrees 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 114-824mg 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 14(H),17(H)/(14(H),17(H)+14(H),17(H)), respectively] and the terpane [18(H)22,29,30-Trisnorneohopane/(18(H)22,29,30-Trisnorneohopane+17(H)22,29,30-Trisnorhopane): 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 -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.