Aptian–Albian shale oil unconventional system as registration of Cretaceous oceanic anoxic sub-events in the southern Tethys (Bir M’Cherga basin, Tunisia)

Abstract

The Bir M'Cherga basin (North-east Tunisia), with about 600 km 2 area, had recorded four Middle Cretaceous source rocks well stratigraphically correlated with the four known oceanic anoxic sub-events: OAE1a, OAE1b, OAE1c, and OAE1d. Variety of lithology, thickness and organic richness had characterized these source rocks. The sedimentary tectonic analysis, the petrology and geochemistry study established the petroleum system of these source rocks. Basin formation began early in the Barremian-Aptian interval by synsedimentary tectonics reactivating old basement faults. During the Aptian-Albian, the formed basin had a depocenter that recorded thicker black shales while its NW and SE edges remained raised under the Triassic halokinetic activities. The evolution of the sedimentary filling during this period generated two sedimentary cycles corresponding to two filling second-order fining and thickening upwards sequences. The black shales that constitute these source rocks are formed between subtidal and external platform environment and are interbedded with juxtaposed high organic rich layers and poor ones. The rich organic facies consists of dark shale that constitutes the source rock. The poor organic beds formed by light grey and nodular limestones constitute an intra host reservoir. Thereby, petroleum system consists in an "unconventional oil shale hybrid systems with a combination of juxtaposed organic-rich and organic-lean intervals associated to open fractures". The kerogen is essentially amorphous, with marine planktonic origin and low ligneous organic matter contribution. This organic material of dark facies had been well preserved in an anoxic environment with little or no energy. Light grey limestones were of oxic-to-sub-oxic environment. The stage of the thermal evolution for these source rocks provided by IH/T max diagram is of the "oil window". The average transformation ratio (TR) is estimated as 45% suggesting thus black shales are oil shale resources which still close about untransformed 55% of its hydrocarbon generating potential. The expulsion and release of oil into these source rocks are proven by the observation of hydrocarbons filling micro-cracks and by the variable values of the oil saturation index OSI ranging from 0 to 138%. The latter exceeds 100% near the paleo-high reliefs indicating two "oil crossover" areas attributed to the high degree of oil source rock saturation and accumulation of hydrocarbons considered ideal for hydraulic fracturing. This oil crossover is a consequence of secondary migration into black shale source rock, achieved by various faults created during the distensive phase that were reactivated again several times.

Geochemistry and palynology of the upper Albian at the Abu Gharadig Basin, southern Tethys: Constraints on the oceanic anoxic event 1d

“…The first recognition of the OAE 1d (the Breistroffer level) was by Bréhéret () based on geochemical evaluation of upper Albian sediments of the Vocontian Basin, southern France, having an overall total organic carbon (TOC) content of 1–2 wt%. Further studies confirmed the global distribution of the OAE 1d subevents; for instance, studies from the northwestern Tethys (in the Vocontian Basin, SE France, Bornemann et al, ; Bréhéret, ; Giorgioni et al, ; and in the Pialli Italian Apennines, Coccioni, ), the southern Tethys (in Tunisia, Ben Fadhel, Layeb, Hedfi, & Ben Youssef, ; Talbi, Lakhdar, Smati, Spiller, & Levey, ), the northwestern Atlantic (in Blake Nose Plateau, Watkins, Cooper, & Wilson, ; Wilson & Norris, ), the United States (Scott, Formolo, Rush, Owens, & Oboh‐Ikuenobe, ), and southwestern Venezuela (Rodríguez‐Cuicas, Montero‐Serrano, & Garbán, ). Generally, the OAE 1d is characterized by: (a) a low to moderate positive carbon excursion (0.5–2‰; Bornemann et al, ; Watkins et al, ; Wilson & Norris, ) and TOC contents (1–2 wt%; Bornemann et al, ; Bréhéret, , ; Rodríguez‐Cuicas et al, ); and (b) being a product of stratification collapse in the water column especially in the western tropical Atlantic (Wilson & Norris, ).…”

“…The reported OAE 1d within the upper part of the Kharita Formation can be correlated with the Mouelha black shale Member from northern Tunisia (Ben Fadhel et al, ), where the former authors identified the OAE 1d based on organic matter‐enrichment (TOC content between 1.8 and 2.44 wt%). To the northwest of Tunisia, Talbi et al () focused their study on the geochemical and petroleum potentiality of the four subevents of OAE 1. They indicated that OAE 1d was dominated by a high TOC content averaged by 2.02 wt%, except for one section that had 13.91 wt% TOC content, which the authors attributed to reworking of Triassic TOC‐rich outcrop.…”

Sequences, discontinuities and water stratification in a low-energy ramp: the Early Albian sedimentation in central Tunisia

“…Since the pioneer work by Burollet (1956), Dubourdieu (1956) andM'Rabet (1981), the stratigraphy and sedimentology of the Hameima Fm and its equivalents have been studied by Chihaoui (2009), Chihaoui et al (2010) andBen Chaabane et al (2019), based on the ammonite biostratigraphic framework by Latil (2011). The Lower Fahdene Fm and its equivalent have been mainly studied for biostratigraphic purposes (Vila et al 1995;Zghal et al 1997;Chihaoui et al 2010;Latil 2011;Ben Fadhel et al 2014), for potential source-rocks analyses (Khalifa et al 2018;Talbi et al 2019) or for synsedimentary tectonics (e.g. Martinez et al 1991;Arfaoui et al 2011;Jaillard et al 2013;Masrouhi et al 2014).…”