ABSTRACT: Secondary Porosity Development in Neogene/Quaternary Slope Carbonates, Great Bahama Bank, Linked to Retarded Aragonite Diagenesis in Marine Pore Waters

MALIVA, ROBERT G., PETER K. SWART, and ROBERT N. GINSBURG, University of Miami, Miami, FL

The core CLINO (2173 ft) taken through the Neogene-Quaternary prograding platform margin of Great Bahama Bank has abundant secondary porosity formed by the dissolution of aragonitic peloids and fossil fragments (mostly Halimeda sp. and mollusks). Large parts of the distal slope packstone and grainstone facies of the lower part of the core (below 1300 ft) contain from 5 to 45% moldic porosity in addition to intergranular and intragranular macroporosity and microporosity. The molds are supported by either granular calcite cement, partially recrystallized plastically deformed peloids (= pseudo-matrix), or, less commonly, sucrosic dolomite.

Aragonite dissolution and neomorphism were retarded in the lower parts of CLINO because the limestones remained in a marine phreatic environment during early diagenesis and were not exposed to meteoric water. The survival of aragonite beyond the main porosity-reducing diagenetic events was crucial for secondary porosity development and preservation. Had the aragonite been dissolved during early diagenesis, the resulting molds would have been vulnerable to compactional collapse and calcite cementation. Early neomorphism of the aragonite to calcite would have made the formerly aragonite grains resistant to leaching. The marine pore waters that were responsible for aragonite preservation during early diagenesis became undersaturated with respect to aragonite later during diagenesis, poss bly resulting from the oxidation of organic matter by sulfate, causing the secondary porosity formation. The core CLINO demonstrates how the relative timing of diagenetic events can have a profound effect on reservoir quality.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)