Carbonate Diagenesis

The three diagenetic realms in which porosity modifications (e.g., dissolution, cementation, compaction) take place are the marine, meteoric, and subsurface environments. The meteoric environment—with its dilute waters, easy access to CO2, and wide range of saturation states with respect to carbonate phases—has high potential for porosity modification, including destruction by cementation and generation of secondary porosity by dissolution. Modern shallow-marine environments are particularly susceptible to porosity destruction by cementation due to high levels of supersaturation of marine waters relative to metastable carbonate minerals. Decreasing saturation with depth can lead to development of secondary porosity by dissolution of aragonite. In the geologic past, shallow-marine waters were often undersaturated with respect to aragonite. The subsurface environment is marked by loss of porosity through compaction and related cementation. Thermal maturation and degradation of hydrocarbons and the slow flux of basinal fluids during progressive burial drive later porosity modification by cementation and modest local dissolution. Recognition and differentiation of the porosity modification history of carbonate rocks is aided by a number of analytical tools. Petrography enables us to reconstruct the sequence of relatively timed diagenetic events responsible for porosity modifications. Trace element and stable isotope analyses of cements and dolomites provide insight into the types of waters involved in these events. Two-phase fluid inclusions are used to estimate temperatures of cement or dolomite formation and the composition of precipitating or dolomitizing fluids. The definitiveness of trace element analysis is often limited by uncertainties in distribution coefficients, temperature fractionation effects, or low concentration values. Two-phase fluid inclusion studies also pose significant problems (e.g., stretching of inclusions during burial, recognition of primary inclusions, and accuracy of pressure corrections). Therefore, these tools should be used to provide constraints on assessing environments of diagenetic events, within an appropriate petrographic/geologic framework. The continuing development of new instruments and techniques (e.g., the ion probe, clumped isotope analysis) holds great promise for the future of geochemical analyses in diagenetic studies.