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The flooding of the Lower Jurassic shelf in the North Gondwana Palaeomargin during the early Toarcian occurred on a fragmented and irregular topography affected by differential subsidence—due to the activity of listric faults along the North-South Axis of Tunisia—that favoured lateral changes in facies and thickness at a kilometric scale. The onset of Toarcian sedimentation (Polymorphum ammonite Zone, NJT5c nannofossil Subzone) in two adjacent sections was characterised by the deposition of limestones under high-energy conditions. The Châabet El Attaris section was located in a depressed sub-basin, and recorded restricted environmental conditions owing to water stagnation and an oxygen-depleted sea-bottom. Therefore, dark mudstones developed, with increased TOC contents and enhanced accumulation of redox-sensitive elements. The sedimentation of limestones bearing gutter cast structures is related to gravity flows probably linked to storm activities. These processes favoured the remobilization of sediments at the sea floor, as well as oxygen input to bottom waters, as shown by the record of trace fossils including Zoophycos, Ophiomorpha, and secondarily, Chondrites and Diplocraterion. The thinly interbedded dark mudstones are locally rich in thin-shelled bivalves that re-colonised the sea bottom after the sedimentation of these high-energy deposits, and subsequently underwent mass mortality related to the return of oxygen-depleted conditions. The Kef El Hassine section is located in the upper part of a tilted, less subsident block, as indicated by its reduced thickness compared with the Châabet El Attaris section; the absence of dark mudstones implies oxic conditions. The Polymorphum Zone consists of limestones showing evidence of sedimentation under high-energy conditions, along with hardgrounds. The occurrence of Zoophycos (deep-tiers) in the upper part of some limestone beds of the Polymorphum Zone is linked to minor erosive processes. The top of the high-energy sequence—below the deposits of a marly interval corresponding to the Levisoni Zone—is interpreted as a hardground given the high content of belemnites and Arenicolites, some of them boring on the eroded Zoophycos and Thalassinoides. This study shows that the sedimentary expression of the Jenkyns Event is not uniform across Tunisia, supporting the importance of local conditions in determining the development of anoxic conditions.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5510162

Geochemical data indicate that the Late Silurian-Early Devonian Fegaguira Formation is the main source rock that has been charged the Ordovician and the Triassic reservoirs in the Chotts basin, South of Tunisia. Its good to excellent geochemical characteristic and its convenient maturity range allow it to be nominated as a good shale reservoir.The reservoir assessment was based essentially on geochemical and mineralogical elements including: (1) source-rock distribution, thickness, organic richness, maturity level, petroleum generation and migration; (2) organic facies distribution, quality and kerogen transformation; and (3) mineralogical assessment, clay type and quartz richness. A geochemical database consisting of 224 cuttings samples of the Fegaguira Formation was used to determine its geochemical characteristics including mineralogy. The examined shales show different thickness distribution that decreases towards the south east. They were subdivided into three distinct units according to their diagraphic responses, lithology and organic richness. The two upper units exhibit good total organic carbon averaging 5%, an original hydrogen index averaging 560 mg HC/g of TOC suggesting that the Fegaguira Formation was originally rich in type II organic matter (oil and gas prone kerogen), maturity ranges from 0.7 to 1.1 % R 0 , transformation ratio is about 0.78% and quartz richness up to 16%. Detailed framework studies in petroleum geochemistry, lithostratigraphy and mineralogy combined with conventional modeling were used to estimate the unconventional oil and gas resources of the Fegaguira Formation. The important organic content, the carbon transformation ratio, the gas derived both from kerogen cracking and oil-to-gas cracking, including the calculated porosity and the quartz percentage suggest that the Feguiguira Formation is a promising reservoir.

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