Organic Geochemistry of the <scp>C</scp>enomanian–<scp>T</scp>uronian <scp>B</scp>ahloul <scp>F</scp>ormation Petroleum Source Rock, Central and Northern <scp>T</scp>unisia

Abstract

Abstract Total organic carbon ( TOC ) determination, R ock‐ E val 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 B ahloul F ormation in central and northern T unisia. 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 A in Z akkar, O ued B ahloul and D yr O uled Y ahia localities, respectively). The R ock‐ E val maximum pyrolysis temperature ( T max) 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 C 17 to C 20 range, are typical for a marine planktonic origin, whereas pristine/phytane ( P r/ P h) average values in the 1–2 range indicate an oxic to suboxic depositional environment. Pr/n‐ C 17 and P h/n‐ C 18 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 C 29 sterane [20 S /(20 S +20 R ) and 14β( H ),17β( H )/(14β( H ),17β( H )+14α( H ),17α( H )), respectively] and the terpane [18α( H )22,29,30‐Trisnorneohopane/(18α( H )22,29,30‐Trisnorneohopane+17α( H )22,29,30‐ T risnorhopane): T s/( T s+ T m)] ratios. The B ahloul OM is represented by an open marine to estuarine algal facies with a specific bacterial contribution as revealed by the relative abundance of the ααα‐20 R 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 B ahloul 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 T riassic salt.