Carbonates are calcareous sediments composed of shredded and reworked remains of shelly invertebrate animals and calcareous algae, microscopic lime-secreting organisms (foraminifera and algae), and coated fecal pellets mixed with lime mud matrix. In addition to these shallow-water sediments, deep ocean bottoms are covered with limey organic microscopic debris (pelagic lime mud). Carbonate facies that make hydrocarbon reservoirs have been formed in clear, shallow, subtropical waters within the photic zone. They consist mainly of aragonite and Mg-rich calcite, which are stable only in marine saline water. Present-day depositional environments of shallow carbonates have been studied worldwide, e.g., Bahamas, Florida shelf, Arabian Gulf, Yucatan, and Red Sea.
Deposition of carbonate sediment follows a consistent sequential pattern. A halo of about eight sediment types is formed as a carbonate ramp, and then a rimmed platform is established. This occurs in a clear, warm marine environment on an evenly downsloping sea floor around a gentle uplift that furnishes a minimum amount of siliciclastics. The environments range in sequence from deep offshore muds and turbidites, to debris flows on the slope, reef tracts or oolitic lime sand shoals at the platform margin, to deposition of lime mud sediments in low-energy, open-marine, backreef shelf or interior shelf lagoons with restricted circulation, and finally to shoreward tidal flats or evaporitic salt marshes and ponds. Both interior shelf deposits and their subsequent alteration are controlled by climate (arid versus humid). This predictable pattern of depositional environments is valuable in discerning carbonate facies progression. Since slope debris flow, reef rock, and lime sand grainstones often retain some of their originally high porosity, it is important to map them in sequence and to locate them with reference to structures. Even the poorly permeable lime muds of the shelf lagoons can be improved by preferential dolomitization and can engender the Mg-rich fluids necessary for subsequent dolomite formation.
Porosity in carbonates is thus depositionally controlled but is also related to diagenetic alteration to which carbonate sediment is susceptible, including (1) cementation in marine environments, (2) presence of fresh meteoric water, (3) presence of warm, hypersaline burial fluids, (4) replacement of original aragonite and high-Mg calcite with crystals of low-Mg calcite, (5) leaching of aragonitic shells, and (6) brecciation, fracturing, and vein and stylolite formation. Most carbonate diagenesis reduces original porosity. Limestones are good reflecting horizons and have high velocity when well cemented. Reservoir discernment and prediction of trends on noncemented sediment or dissolution and fracturing of limestone and dolomite is at present challenging. Isotope geochemistry and fluid inclusion studies appear to help geoscientists in exploration of carbonate reservoirs.