Abstract

Diagenesis is a major control of reservoir quality within carbonate rocks. In many cases, perhaps the majority of them, diagenesis is as significant as primary depositional fabric in regulating porosity and permeability in limestone and dolostone (cf., Bathurst, 1975).

The Smackover Formation is an important producer of hydrocarbons along much of the Gulf of Mexico and in southwestern Alabama, several small production areas have been developed. The Smackover Formation is pervasively dolomitized throughout this area of Alabama including the Appleton Field in Escambia County. Here, four textural varieties of dolomite are recognized in eight cores examined as part of a carbonate diagenesis study. Dolomite types are distinguished on the basis of petrographic character (crystal size, shape and turbidity), fabric preservation, localization, and luminescence properties. All but the last phase (Type 4) formed during relatively early diagenetic events. The first phase (Type 1) may be partially synsedimentary.

Cathodoluminescence suggests that the first variety of dolomite was derived from fluids of hypersaline character. Types 2 and 3 dolomites were likely formed from mixed water (hypersaline and meteoric). Contrasts in petrographic character of the dolomite varieties and of the porosity developed within carbonate units of the Smackover Formation in the Appleton Field were due to differences in nucleation rate, growth rate, and intensity of concurrent limestone dissolution. Type 4 dolomite was hydrothermal in origin (c. <250 °C) and formed during late-stage burial. It occurred simultaneously with fracturing and pressure solution and was postdated by a final phase of fracture-filling, luminescent calcite spar.

Dedolomitization is common in several areas of the cores examined. In most instances, dedolomitization appears to have been induced by fluids moving along fine fractures less than 50 µm wide. Brecciation and annealing of Types 2 and 3 dolomite by later Type 4 dolomite also occurs in some areas in the vicinity of the Appleton Field.