Abstract
Miocene reef complexes are well exposed in the Alicante-Almeria region of southeast Spain. Predominant reservoir lithofacies are (1) in the off-reef environments (i.e., fore-reef slope deposits and overlying shallow platform sediments), a porous leached microsucrosic to spherulitic dolomite with mostly moldic and intercrystalline porosity and good to very good matrix permeability, and (2) in the reef-core framework, (A) a tight, karstified (i.e., fissured), coarsely crystalline dolomite with poor matrix reservoir potential but good to very good fissure porosity with darcy permeability and (B) a porous, leached, coarsely crystalline dolomite with vuggy matrix porosity and good matrix permeability. The most significant conclusion in terms of reservoir geology is that these reefs do not have a homogeneous reservoir quality. Each rock lithofacies has different geometries and capillary characteristics. In volumetric terms, the most significant reservoir unit in such reefs would be the fore-reef slope deposits. Assuming typical values for reservoir properties, such fore-reef deposits could contain 75% of the total hydrocarbons in comparable reservoirs. However, the best productivities could be obtained from fissured reservoirs, that is, from the reef-core framework, although low production rates might be required to avoid premature water or gas breakthrough along highly permeable channel conduits of karstic origin.
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