Ichnology as a tool in carbonate reservoir characterization: A case study from the Permian – Triassic Khuff Formation in the Middle East

ABSTRACT In the Upper Permian to Lower Triassic Khuff Formation in the Arabian Gulf, a vast shallow-marine carbonate platform developed broad facies belts with little significant changes in the lithofacies. However, trace fossil assemblages and ichnofabrics, in combination with sedimentological observations, serve in subdividing this platform and in distinguishing sub-environments. From proximal to distal, these are sabkha and salina, tidal flat, restricted lagoon, open lagoon, platform margin, shoreface/inner ramp, slope/outer ramp and basin/deeper intra-shelf. In this way, changes in relative sea level can be better reconstructed and guide the sequence stratigraphic interpretation. Meter-scale shallowing-upward cycles dominate the succession and, in addition to conventional methods, bioturbation, trace fossil assemblages and tiering patterns aid in interpreting subtidal, lower and upper intertidal and supratidal portions of these peritidal cycles. Bioturbation (and cryptobioturbation) have an impact on the primary reservoir quality before diagenetic processes overprint the deposits. For instance, deposit-feeders (such as vermiform organisms) introduce a certain amount of mud and decrease porosity and permeability considerably, whereas others like the Zoophycos-producers fill their dwellings with ooid grains and turn a mudstone from a barrier to a flow unit. This novel study demonstrates the value of ichnological information in carbonate reservoir characterization and the significance of trace fossil analysis in facies interpretation, reservoir zonation and the impact of bioturbation on the reservoir quality.