Palaeoenvironmental control on multi-scale vertical and lateral heterogeneities within a Triassic - Jurassic carbonate ramp system (Musandam Peninsula)

Abstract

Sedimentologic heterogeneities in carbonate systems are caused by various processes during and after deposition and can have a profound influence on reservoir properties relevant for hydrocarbon recovery or carbon storage. This study is focussed on examining and quantifying sedimentologic heterogeneities on an equatorial, Triassic-Jurassic, Middle Eastern, epeiric carbonate ramp (Musandam Peninsula, UAE and Oman) at the regional scale (~ 45 km), the ‘inter-well’ scale (~ 1.9 km) and the metre- to tens of metres scale (~120 m). The stratigraphic ages of the examined strata were refined using a combination of the existing biostratigraphic framework and chemostratigraphic analysis (stable carbon and radiogenic strontium isotopes). Results suggest that the regional architecture and carbonate factory evolution across the Triassic- Jurassic boundary was mainly governed by relative sea level changes and not affected by ocean acidification in the equatorial region. Lithofacies types were deposited and are preserved as facies mosaics on the shallow-marine carbonate ramp, despite an overall ‘layer-cake’ type bedding pattern. Geostatistical analysis reveals that facies have a probability of 60 % of being laterally discontinuous over distances between 220 and 1100 m on the shallowmarine carbonate ramp. The comparison with other preserved facies mosaics as well as with modern analogous systems suggests that the large amount of lateral facies variability, also observed within individual beds at the metre-scale, is a result of the combination between locally heterogeneous carbonate production as well as of coeval mixing and re-sedimentation processes. The combination of laterally heterogeneous facies types and vertical exposure stages is likely the result of constant autocyclic mixing and re-shuffling of grains within the shallow-marine EOD and allocyclic changes in relative sea level. This study highlights the capability of neritic carbonate successions to archive palaeoceanographic changes in deep time as well as to preserve small-scale facies mosaics.