Karst and Mixing-Zone Porosity in the Amposta Marino Field, Offshore Spain

P. L. Wigley, J. D. Bouvier, M. Dawans

The offshore Amposta Marino field, located southeast of the Ebro delta, produces from the karstified Lower Cretaceous Montsia Limestone. The field is a buried-hill structure overlain by marine clastic sediments. During drowning, the buried hill was surrounded by sea water and thus represents the typical setting in which mixing-zone corrosion is a major porosity-creating process in modern carbonates (e.g., Yucatan Peninsula, Bahamas).

Mixing-zone corrosion in a coastal setting differs from freshwater karstification in that it creates an essentially horizontal zone of well-connected matrix and cave porosity. This high-porosity zone is best developed on the flanks of a structure and, ideally, has a rim-like distribution.

On 3-D seismic, we recognize a subhorizontal reflector that cuts across the gently dipping Amposta structure. This reflector was proven by seismic-to-well matching to represent a highly porous interval. Mapping of this reflector showed that it occurs somewhat discontinuously around the western margin of the structure and fairly continuously around the eastern margin, forming a clear "rim" feature around the flanks of the buried hill. This rim-shaped, high-porosity zone is significantly more continuous than would be expected for a freshwater karst system and is consistent with an origin through mixing-zone corrosion processes. Its distribution, as well as reservoir performance data and the geologic history of the field, indicate that the actual porosity system in the Montsia reservoir s most probably the product of an originally compartmentalized porosity network, i.e., a meteoric karst system of which the lateral communication was greatly enhanced as a result of subsequent mixing-zone corrosion during a major transgression.

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