Abstract: Porosity in Late Miocene Reefs, Southern Spain

A. K. Armstrong, Parke D. Snavely, Warren O. Addicott

Seacliff exposures, 40 km east of Almeria, southeastern Spain, contain late Miocene reefs and patch reefs formed of milleporids, Scleractinia, calcareous algae, and Mollusca. The reef cores are as much as 65 m thick, and several hundred meters wide. Fore reef talus beds areally extend to 1,300 m and to 40 m thickness. The reefs and reef breccias are composed of calcitic dolomite. The reefs lie on volcanic rocks that have a K/Ar date of 11.5 m.y. and, in turn, are overlain by the upper Miocene Vicar Formation.

In the reef cores and fore reef breccia beds porosity is both primary and postdepositional. Primary porosity is of three types: (a) boring clams in the Scleractinia coral heads, cemented reef rocks, and breccias; (b) intraparticle porosity within the milleporids, corals, Halimeda plates, vermetid worm tubes; and (c) interparticle porosity between bioclastic fragments and broken reef breccia. Postdepositional moldic porosity was formed by the solution of aragonitic material such as molluscan, milleporid, and Scleractinia coral fragments.

Resulting pores range from a few micrometers to 30 cm. Extensive intercrystalline porosity from dolomitization of calcium carbonate bioclasts and micritic matrix formed dolomite rhombs sized between 10 and 30µ. Some porosity reduction has occurred by incomplete and partial sparry calcite infilling of interparticular, moldic, and intercrystalline voids.

The high porosity and permeability of these reefs make them important targets for petroleum exploration in the western Mediterranean off southern Spain.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC