Abstract 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 – owing 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 characterized 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 total organic carbon 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 seafloor, 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-colonized 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.

Shallow to deeply penetrating bioturbation by organisms on carbonate shelves can alter the original depositional texture of carbonate sediments, rearrange and modify the primary porosity and permeability patterns, and effectively increase the overall flow properties in multiple intervals. To explore the impact of bioturbation on reservoir quality and its spatial and vertical patterns, this study examined sedimentologically, ichnologically, and geostatistically ubiquitous bioturbated strata throughout outcrops of the Middle Jurassic Tuwaiq Mountain Formation and Upper Jurassic Hanifa Formation in central Saudi Arabia. Each lithofacies within the studied intervals had an ichnofabric index (ii) range from nonbioturbated (ii1) to beds completely homogenized by bioturbation (ii6). Most important was the occurrence of laterally extensive (>5 km) Glossifungites Ichnofacies, which represent firmgrounds with ii2 to ii5. These Glossifungites Ichnofacies are composed of complex and deep, three-dimensional Thalassinoides burrow networks (TBN) in mud-dominated lithofacies. These TBN have pore systems that consist of (1) open and partially open macropores (size of several centimeters), and (2) interparticle and moldic pores within the burrow filling, which consists of peloids, skeletal grains, and coated grains in a grain-dominated packstone texture. The TBN pore system, which typically penetrates the entire extent of the mud-dominated bioturbated beds, provides permeability pathways in an otherwise less permeable medium. Outcrop data and three-dimensional models suggest that these permeable pathways can contribute to overall reservoir flow in three ways: (1) TBN beds contribute to the overall reservoir flow as a single flow unit if bound above and below by impermeable beds (e.g., lateral flow in vertical well). (2) TBN breach the bed boundaries and, thus, connect above and below into more porous, more permeable grainy beds, providing overall reservoir connectivity for the carbonate reservoir and contributing to vertical and lateral flow. (3) TBN beds connect otherwise laterally compartmentalized reservoirs and contribute to vertical flow. Controls on the lateral and vertical variability of the TBN in the study area can be attributed to changes in water chemistry of the depositional environments, which are likely linked to global and local controls. This spatial and temporal relationship impacts the lateral and vertical distribution of flow properties of TBN strata in bioturbated reservoirs. Understanding such relationships is critical for secondary and tertiary recovery of oil by water flooding because such relationships can provide a prediction about the trend of vertical and lateral flow properties.