Porosity Controls in Late Jurassic Algal Reefs, Mississippi Salt Basin

Ezat Heydari, Clyde H. Moore

Reefs associated with high-rise salt structures that were active during Late Jurassic deposition (Smackover-Haynesville) have been the target of numerous deep tests in the Mississippi Salt basin. One such test, in Wayne County, Mississippi, encountered a 24-m reef. The reef exhibited no porosity or permeability, while sequences above (21 m) and below (24 m) were dolomitized, porous, and permeable.

The reef sequence consists of a facies mosaic of encrusting to columnar massive red coralline algae, laminated to stromatolitic blue-green algae(?) with associated pelleted internal sediments and other features characteristic of modern framework reefs. The reef complex exhibits a strong early marine diagenetic overprint consisting of bladed to fibrous magnesian calcite(?) cements and botryoidal masses of magnesian calcite or aragonite. The lack of discernible freshwater diagenesis and the nature of the reef framework seem to indicate deeper water conditions for reef development and subsequent early diagenesis.

The associated dolomites have relict texture indicating that they were originally grainstones, perhaps derived from the reef itself.

In the reef sequence, the geologic setting (relatively deep water), early marine cementation, and encrusting nature of the reef-formers produced a non-porous rock which was not susceptible to early dolomitization. However, the associated porous grainstones allowed active circulation of dolomitizing fluids (marine water?), leading to total dolomitization and a favorable reservoir facies.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.