Sedimentological and isotopic heterogeneities within a Jurassic carbonate ramp (UAE) and implications for reservoirs in the Middle East

Abstract

Carbonate rocks are major hydrocarbon reservoirs in the Middle East and across the world. Capturing the spatial distribution and dimensions of carbonate sedimentary facies is thus of interest for reservoir modelling and for the understanding of process sedimentology. Here we present data from a Middle Jurassic carbonate ramp outcropping on the Musandam Peninsula (Wadi Naqab, Northern United Arab Emirate of Ras-Al-Khaima), which serves as an analogue for the hydrocarbon bearing units in the subsurface of the Middle East with respect to stratigraphic age, lithofacies types, vertical stacking patterns and palaeogeographical setting. Seven closely spaced sections were logged across a 1900 × 1200 × 120 m fault-bounded outcrop offering a pseudo-3D view of the stratigraphy. Sections were correlated bed-by-bed by walking key surfaces and/or by using photopanels, to produce a detailed lithofacies map. Stable isotopes (carbon and oxygen) were measured from micrite samples in order to provide chemostratigraphic constraints. Ten lithofacies types were identified in thin sections and hand specimens, ranging from marls and mudstones to grainy oncolites and peloidal-ooidal grainstones. Beds are organised in a ‘layer-cake’ architecture, but individual bed thicknesses vary laterally. Lateral merging between different lithofacies types within beds is very common, resulting in a complex facies mosaic at the scale of the outcrop. Lithofacies types have a maximum probability of 40% of being continuous between logged sections with the highest frequencies of lateral variations occurring in the open, shallow part of the ramp, where maximum wave and current energy are indicated by facies. The vertical stratigraphic stacking pattern of the succession is characterised by metre-scale shoaling upwards parasequences. These are frequently capped by hardgrounds, interpreted as composite surfaces with depleted carbon and oxygen isotope values being evidence for frequent exposure during relative sea-level lowstand, followed by flooding and formation of a condensed hardground. The carbon isotope curves show a large amount of vertical ‘isotope heterogeneity’ reflected by meteoric diagenesis around hardground surfaces, and these are not always traceable laterally between the closely spaced neritic sections due to bias in sampling. However, if considering only the positive envelop of the carbon isotope curve and thus filtering out the meteoric diagenetic events, carbon isotope chemostratigraphy becomes possible and confirms a Bajocian age for the section. The main conclusion of the study is that maximum heterogeneity of facies at the scale of 100 m or less is expected in an environment of deposition with high wave energy. This has important implications for reservoir applications since rock facies often template flow units.