Abstract Micritic sediments containing dark, discrete, organic-rich burrows, situated in a light grey background carbonate mud, were deposited over a broad geographical area in deep-shelf, bathyal and basinal environments in the western margin of the Tethys Ocean during the Early and Middle Jurassic. These hemipelagic deposits represent a distinct depositional regime marked by low-energy, soft-bottom and only locally dysoxic environments. Still, it is unclear whether the trace-fossil assemblages occurring in these deposits pertain to a network of several community types – the ichnotaxa differing from basin to basin – or to a single community of environmentally broad-ranging, burrow-producing species. Lower Jurassic trace-fossil assemblages are found in the Western Carpathians and in the Subbetic, Betic Cordillera: that is, in basins separated by more than 2000 km in their original palaeogeographical areas. The stereotypical Chondrites and Zoophycos trace-fossil assemblages that occur in the analysed deposits share two ichnogenera of distinctive morphology ( Lamellaeichnus and Teichichnus ). Agglutinated foraminifera Bathysiphon occurs together with the described trace-fossil assemblage and determines the epibenthic palaeoenvironmental conditions. In the Western Carpathians, a Lamellaeichnus -dominated assemblage alternates with a Zoophycos -dominated assemblage in small, metre-scale cycles in the upper Pliensbachian, and the proportion of the Zoophycos assemblage increases stratigraphically upwards, probably owing to reduced basin ventilation during the early Toarcian. Within the southern Iberian palaeomargin, represented by the Betic Cordillera, Zoophycos is scarce in the facies.

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.