From regional to bed-scale sedimentological heterogeneities on a Triassic - Jurassic carbonate ramp

Triassic and Jurassic carbonates across the Middle East host large amounts of hydrocarbons, but the degree of sedimentary heterogeneity across different scales in carbonates varies significantly. In this study we investigate the regional evolution of a carbonate ramp across the Late Triassic to Early Jurassic on the Musandam Peninsula (UAE and Oman) in a series of wadis, 45 km apart. Lithostratigraphy as well as carbon isotope stratigraphy were used to correlate the sections on a regional scale, and this work revealed that the broad environment of deposition was continuous over the region throughout the studied time interval. However both the conventional reservoir facies (oolitic grainstones) and the unconventional reservoir facies types (marls and shales) are mostly discontinuous across the study area. We have also investigated smaller scale (<1 km, or inter-well scale) facies heterogeneities using seven closely spaced Middle Jurassic sections logged across a 1900 x 1200 x 120 m fault-bounded outcrop, which offers a pseudo-3D view of the stratigraphy. The main conclusion at the inter-well scale is that lithofacies types deposited on the subtidal part of the carbonate ramp show a high degree of lateral and vertical variability, especially in the shallow-water, high-energy ramp despite a largely ‘layer-cake’ bedding pattern. When we zoom in to investigate individual bedsets we see similar facies merging patterns. We attribute this large amount of lateral heterogeneity, facies merging and interfingering as result of currents and waves in the overall shallow part of the carbonate ramp. If facies have contrasting petrophysical properties in subsurface reservoirs, this has important implications for hydrocarbon production from shallow-water carbonate units.