In Part II of this manual, we present case studies of reservoirs that formed in each of the major dolomitization environments. Supratidal-sabkha, reflux, marine-meteoric mixing zone, and burial dolomite reservoirs are each discussed in separate sections. The examples were selected to best illustrate the ways in which geochemistry was used successfully in conjunction with stratigraphic, structural, petrographic, and facies information to identify the origin of the dolomite reservoir. We believe that these examples will serve as useful templates for the explorationist who wishes to use geochemical data to evaluate dolomite petroleum reservoirs and to determine which dolomite model best fits his or her reservoir or play. A summary of the geochemical criteria for recognition of each of these dolomite types is presented in Table 2.
Generation of hypersaline brines by evaporation of seawater near the surface of sabkhas and supratidal flats (Fig. 38) results in precipitation of anhydrite, which increases the Mg/Ca ratio of the residual brine (Fig. 39). The lateral and vertical movement of this brine results in dolomitization of supratidal and underlying intertidal and sub tidal sediments.
Shoaling upward cycles topped by tidal flats or supratidal evaporites produce reservoirs that follow the shorelines of basins, lagoons, and epeiric seas. Repeated transgressions and regressions result in multiple wedges of subtidal and intertidal sediments beneath and within impermeable, often anhydride supratidal facies (Fig. 40; Shinn, 1983).