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Carbonate oil reservoirs are sometimes regarded with apprehension in the petroleum industry since it can be difficult to predict the quality of, and ensure high recovery factors from, this rock family. Particular problems are the complex and heterogeneous nature of porosity in carbonate rocks, often leading to large ranges in permeability for any given porosity, and the organization of carbonate successions most commonly as vertically heterogeneous, but laterally persistent, layers.Important issues that arise time and again in carbonate reservoir description include (a) predicting reservoir quality at inter-well scales and in uncored wells, (b) recognizing problematic high-permeability layers, (c) determining the permeability component to allocate to fractures and connected vug systems, and (d) populating reservoir models with representative physical parameters. Because porosity in carbonate rocks generally presents as diverse and heterogeneous, conventional core plugs are seldom representative of large rock volumes and significant issues remain in terms of the scale-compatibility of the various datasets for measured physical parameters that are used in carbonate reservoir description.Many of the world's largest carbonate reservoirs were discovered and developed shortly after the Second World War and are now showing signs of maturity, expressed variously as poor pressure support, water or gas breakthrough and stranded resources. The proportion of the world's ‘conventional’ petroleum that is reservoired in carbonate rocks is commonly estimated at around 50–60% and many large carbonate reservoirs are likely to have a production lifetime beyond 50 years. It is no coincidence then that the petroleum industry has been the primary source of funding of and promotion of research into carbonate rocks and depositional systems, often with impacts extending well beyond oil and gas exploitation.

Summary This paper presents a case-study of an investigation into a Middle Eastern carbonate reservoir, the Mishrif Formation of eastern offshore Abu Dhabi, and its lateral equivalent, the Shilaif Member of the Salabikh Formation. The work represents a major part of a sedimentological/seismic stratigraphic study, the aims of which were to model reservoir facies and geometry, and to predict areas of best reservoir potential. The study is based on interpretation of cores, cuttings and wireline logs from 30 wells distributed over some 200 km 2 . The Mishrif of the study area forms part of an extensive Cenomanian carbonate platform. It represents deposition on a local leeward (i.e. shelfward) margin which prograded westwards into an intrashelf (the Shilaif) basin. Six lithofacies associations were recognized in the Shilaif and Mishrif, representing lateral and vertical transitions from silt-grade basinal pelagic-foraminiferal wackestones (I), through a platform margin sequence (II) which coarsens upwards to coarse rudist/ostreiid shoal (III) and biostromal (IV) packstones and grainstones. Shoal and biostromal facies are overlain in the east by bedded back-shoal packstones (V) and nodular micritic platform-lagoonal (VI) sequences. A significant unconformity exists at the top of the Mishrif. Three stages of diagenesis have been recognized in Mishrif limestones: early (micritization, submarine cementation); unconformity-related (extensive leaching, freshwater cementation); and burial diagenesis (stylolitization, fracturing, burial cementation, neomorphism). Leaching and burial cementation were the most important diagenetic events and significantly overprint the generally simple variations in reservoir character due to original depositional facies distribution. Most favourable porosity and permeability values occur in coarse-grained shoal, upper-slope, and biostromal sediments (upper II, III, IV) where exposed and leached at the final platform margin, whereas to the east, in the platform interior, these facies have suffered severe burial cementation. The slight regional eastwards dip of the Mishrif reservoir, its westwards pinch-out into basinal facies, and the presence of a proved top-seal fulfil the requirements for stratigraphic trapping of hydrocarbons, with the best reservoir facies advantageously situated. A well drilled to test this play encountered the predicted sequence but found only residual oil. This may be due to imperfect lateral or bottom reservoir sealing.

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Tubiform fossils conventionally referred to Serpula cf. advena Salter and species of Spirorbis Lamarck from the British Lower Limestone Shales and Border Group (Lower Carboniferous) are re‐examined. They occur in peritidal carbonate environments of schizohaline aspect. These fossils superficially resemble calcareous polychaete tubes but have skeletal characters, including molluscan wall structure, numerous internal septa, and protoconch, which indicate that they represent a new group of substrate‐attached, disjunctly coiled gastropods. They resemble archaeogastropods in internal morphology of the skeleton but show parallels in external form and occurrence with the extant Vermetidae. There are two principal modes of occurrence: (1) erect tubes forming intertidal biostromes associated with non‐skeletal algal laminites, and (2) prostrate discoidal tubes encrusting subtidal skeletal stromatolites or occasionally forming larger irregular bioherms. These biostromes and bioherms are comparable in structure to Recent vermetid reef developments.