Hassene Affouri
University of Sfax, Tunisia
Title: Organic matter in the Albian, Cenomanian-Turonian and Ypresian petroleum source rocks in central and northern Tunisia
Biography
Biography: Hassene Affouri
Abstract
Total organic carbon (TOC) determination, Rock-Eval pyrolysis, extractable organic matter content (EOM) fractionation, as well as gas chromatography-mass spectrometry (GC-MS) analyses, were carried out on samples from outcrop cross sections of the Albian (Lower Fahdene Formation), Cenomanian-Turonian (Bahloul Formation) and Ypresian (Bou Dabbous Formation) in central and northern Tunisia. At the Albian age organic-rich facies were deposited corresponding to a global ocean anoxic event called \"OAE1b\". In northern Tunisia, the Albian sediments are represented by limestones and marly limestones rich in organic matter (OM) and correspond to the base of the Fahdene Formation (Albian-Cenomanian). In central and northern Tunisia, the Cenomanian-Turonian (C/T) boundary succession is represented by thinly laminated carbonates with some marly beds assigned to the Bahloul Formation. This formation was deposited during a so-called ocean anoxic event 2 (OAE2). The paleogeography of the Lower Eocene (Ypresian) basin in Tunisia was characterized by the deposition of various facies that had been named formations which include the Bou Dabbous Formation. It is one of the four lateral equivalents of the Metlaoui Group and consists of micrite and marls with abundant planktonic foraminifera (Globigerina). The total organic carbon (TOC) values and Rock-Eval analysis showed that these Formations had preserved Type II/III marine and continental organic matter (OM). Locally, TOC values are very high. These values are reliable to the structural framework during the Albian, the C/T and the Ypresian epochs. The Rock-Eval maximum pyrolysis temperature (Tmax) values in the ~ 420-460°C range delineated a general east-west and north-west trend increase in the OM thermal maturity. The disparity in hydrogen index (HI) values was relevant for the discrepancy in the level of OM preservation and maturity among localities and samples. The Albian, C/T and Ypresian OM are rich in biological markers or biomarkers such as alkanes, steranes and terpanes. These were used to assess the degree of thermal maturity and to reconstruct the OM precursors and their deposition environments. 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 values in the 0.38-6.2 and 0.68-3.25 range, respectively, are consistent with other maturity indicators and denote the contribution of specific bacteria to phytol as a precursor of isoprenoids (Pr and Ph). The thermal maturity varies between the late diagenesis to the main-stage of petroleum generation based on the optic and the cis-trans isomerisation of the C29 sterane and the terpane [18ï¡(H)22,29,30-Trisnorneohopane/ (18ï¡(H)22,29,30-Trisnorneohopane + 17ï¡(H)22,29,30-Trisnorhopane) : Ts/(Ts+Tm)] ratios. Furtheremore, these OM are represented by an open marine to estuarine algal facies with a specific bacterial contribution as revealed by the relative abundance of the ï¡ï¡ï¡-20R C27, C28 and C29 steranes and by the abundance of the tricyclic terpanes (T-C21 to T-C29) and the hopane compounds for all the samples. Locally, the good preservation of OM in the Albian facies and in the Bou Dabbous Formation has been evidenced by the good conservation of the C34 and C35 homohopanes compounds and the high abundance of gammacerane in central compared to the northern part of Tunisia. These features are characteristics of a highly bacterial contribution to the organic supply and extremely anoxic deposition environments. Finally, the OM richness in these studied Formations was controlled both by an oxygen minimum zone induced by high productivity and restricted circulation in narrow grabens structures.