Carbonate Reservoirs

The overarching diagenetic drive during progressive burial of carbonate rocks is toward the loss of porosity through mechanical and chemical compaction (the latter consisting of pressure solution plus related cementation). The passive margin diagenetic regime is marked by relatively rapid burial with steadily rising temperatures and pressures. Once a mechanically stable grain framework is achieved, the effective stress from sediment loading can eventually suffice to cause chemical compaction. Early cementation, the presence of organic frameworks, overpressuring, dolomitization, and especially the filling of reservoir pores with oil all act to retard the onset and efficiency of chemical compaction. Aggressive pore fluids, the presence of metastable mineral phases, and admixtures of siliciclastics or other insolubles tend to accelerate the process. Catagenesis of organic matter in source rocks yields aggressive formation waters capable of calcite dissolution just prior to hydrocarbon maturation. But despite evidence of local late secondary porosity generation, theoretical considerations lead to the conclusion that such evidence represents limited and local porosity rearrangement. Hydrocarbon-filled deep carbonate reservoirs experience progressive loss of porosity with increasing depth, due to precipitation of pyrobitumen as circumgranular linings, and also—during very deep burial—by renewed precipitation of calcite cement (with carbon derived from destruction of methane) and resumption of mechanical and/or chemical compaction. The active margin diagenetic regime is characterized by rapid movement of large volumes of warm-to-hot basinal fluids, mobilized by tectonism, through complex subsurface conduits. Reactions between expelled fluids and conduit carbonates include recrystallization of earlier dolomites and calcites, replacement dolomitization, dissolution of calcite, dolomite and/or evaporites, and sulfide mineralization. Ascending hydrothermal fluids associated with wrench faults and rifting may result in fault-localized leaching of calcite, dolomitization, and dolomite cementation capable of creating or enhancing “hydrothermal dolomite” reservoirs. The most abundant diagenetic product is saddle dolomite. The postorogenic diagenetic regime is characterized by topographically driven meteoric recharge into deeply buried aquifers. There is minimal impact on carbonate porosity unless meteoric waters are dissolving anhydrite/gypsum. Where this occurs, a chemical drive can promote dissolution of dolomite and precipitation of calcite, accompanied by porosity enhancement. In the absence of evaporite dissolution, meteoric waters equilibrate with carbonate aquifers and minimal porosity modification occurs downstream. Crossplots of porosity versus thermal maturity appear to possess porosity predictive capability. Carbonate reservoirs are relatively prone to souring at depth; hydrocarbons in sour reservoirs can be partially or entirely consumed by destructive redox reactions with sulfur in the forms of H2S and S0. The initiation and extent of these reactions depend upon the availability of reactant sulfur (from evaporites and organosulfur compounds) and dissolved iron (derived from Fe-rich siliciclastics, if proximal).