Understanding the processes and products of carbonate diagenesis is essential to exploration for, and optimum development of, hydrocarbon reservoirs in carbonate rocks. Much (and perhaps most) cementation and formation of secondary porosity (except fractures) in carbonates occurs at relatively shallow depths in one of four major diagenetic environments: the vadose zone, meteoric phreatic zone, mixing zone, and marine phreatic zone. Each of these zones may be divided into several parts on the basis of rate of water movement and saturation of the water with respect to calcium carbonate.

Most carbonates are deposited in and begin their diagenetic history in the marine phreatic environment. This zone may be divided into two end members of a continuous spectrum: a zone of relatively little water circulation in which micritization and minor intragranular cementation occur, and a zone of good water circulation near the sediment/water interface of shelf margins or the upper shoreface in which extensive intergranular and cavity-filling cementation occur. Fibrous aragonite and micritic Mg-calcite are the dominant cements.

With subaerial exposure, fresh water will replace sea water in the pores of shallow-water carbonates, and a zone of mixed fresh and marine waters may form. In long-lived mixing zones, dolomite may form if the water is of relatively low salinity, whereas bladed Mg-calcite may form if the water is relatively marine. Active water circulation in the mixing zone, which may be caused by seasonal rainfall, is necessary for dolomitization or cementation.

Diagenesis in the freshwater phreatic environment may involve leaching in the zone of solution, neomorphism of grains accompanied by extensive intergranular calcite cementation in the active saturated zone, or neomorphism of grains without cementation in the stagnant saturated zone. Syntaxial overgrowths on echinoderm fragments and interlocking crystals of equant calcite that coarsen toward pore centers are typical of cementation in the active freshwater phreatic zone.

The freshwater vadose environment is the zone with both air and meteoric water in the pores and may be divided into the zone of solution and the zone of precipitation. CO₂ from the atmosphere and soil contributes to solution which generally occurs near the soil zone and forms vugs, molds, and etched grains. When the water becomes saturated with respect to calcite, evaporation or CO₂ loss may cause precipitation of fine equant calcite in the form of pendant and meniscus cements. Grains may be altered to calcite, particularly in humid climates, and caliche crusts may be produced by evaporation and/or biologic (generally algal) factors.

Climate plays an important role in early diagenesis if subaerial exposure occurs. In arid climates, cementation in freshwater environments may be limited and primary intergranular porosity may be preserved. In humid climates, little primary porosity is likely to escape cementation, but significant amounts of secondary moldic and vuggy porosity may form.

Interpretation of diagenesis in carbonates is complicated by the fact that diagenetic environments may change many times in the history of a carbonate rock. By recognizing the processes leading to the formation or preservation of porosity, and the distribution of diagenetic

End_Page 461------------------------------

environments in which those processes acted, the distribution of porosity in subsurface carbonates can often be predicted.