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

Abstract

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.

Cretaceous oceanic anoxic events (OAEs) recorded in the northern margin of Africa as possible oil and gas shale potential in Tunisia: An overview

“…It is mainly represented in Hodna basin, Aurès-Kef trough (eastern Algeria), onshore northwestern Tunisia, the Gulf of Hammamet-Gabes, and locally in the Negrine-Gafsa trough (see Figure 21 for location; Soua et al 2009). Well and outcrop data confirm these organic-rich provinces, and the exact boundaries have been clearly traced in earlier studies (Layeb 1990;Maamouri et al, 1994b;Saidi et al 1997;Soua and Tribovillard 2007;Soua et al 2009Soua et al , 2011bSoua 2011;Affouri et al 2013;Layeb et al 2013), especially north of the Gulf of Gabes domain ). Dealing with the Soua and Tribovillard (2007) model, the palaeogeographic setting suggests that distribution of the organic-rich facies represents the impingement of an OMZ onto the southern Tethyan margin and upwelling conditions where the organic-rich facies is associated with siliceous deposition (Soua 2013b).…”

The role of Domanik deposits in the formation of oil pools in the central areas of the Volga–Ural petroleum province (Buzuluk depression)

“…The ratio of the ααα 20R C 27 and C 29 regular steranes also characterizes the facies composition of the source OM (Moldowan et al, 1985). Moreover, the higher proportion of the C 29 homologue in oils may be due to the activities of specific bacteria capable to synthesize these compounds (Affouri et al, 2013;Freese et al, 2008;Granthan, 1986;Volksman, 1988). It is evident from Fig.…”

The Albian oceanic anoxic events record in central and northern Tunisia: Geochemical data and paleotectonic controls

“…Furthermore, we suggest that the C30 17α(H)-diahopane could be the typical biological marker of the Albian OM in our study. To our knowledge, this compound has never been detected in other Tunisian petroleum source rocks such as the Cenomanian-Turonian Bahloul Formation (OAE2) (Affouri et al, 2013) or the Ypresian Bou Dabbous Formation (Affouri and Montacer, 2014).…”