Carbonate rocks and petroleum reservoirs: a geological perspective from the industry

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.