Reservoir Geological Characterization of Miocene Reef Outcrops from the Western Mediterranean Basins of Southeastern Spain

J. M. Dawans

Miocene reef complexes are well exposed in the Alicante-Almeria region of southeast Spain. Exposures of these dolomitized and karstified reefs are at such a scale (up to 150 m thick, 1.5-2 km in lateral extent) that they form good analogs of comparable subsurface examples. The reefs comprise three distinct depositional bodies: (1) the fore-reef slope deposits (up to 100 m thick and 1.5-2 km in lateral extent), (2) the reef-core framework (up to 30 m thick and 0.5-1 km in lateral extent), and (3) a sequence of shallow carbonate platform deposits (up to 30 m thick and 1-2 km in lateral extent) which overlies the other two depositional units.

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 addition, potential permeability baffles/barriers occur within the reefs, and they could restrict cross flow or even cause reservoir compartmentalization if laterally extensive. The major controls on reservoir quality are variations in original lithology and extent/type of subsequent diagenesis.

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.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.