Miocene carbonate reservoirs worldwide were deposited in a wide spectrum of climatic regimes. Diagenetic pathways and reservoir quality of these Miocene carbonates appear to have been primarily controlled by the prevailing climatic regime. Two end-members are here discussed: 1) humid-oceanic tropical to subtropical settings, and 2) arid, M-locked temperate to subtropical settings.

In humid-oceanic, tropical/subtropical settings (i.e., Miocene carbonates in Southeast Asia and early to middle Miocene carbonates in the Western Mediterranean), meteoric involvement is essential in the development of economic reservoirs. Porosity distribution and evolution are dependent on depositional trends and sequence boundaries. Transgressive carbonates are mostly tight because of their relatively fine-grained textures, intense compaction, and isolation from meteoric water influence. In contrast, porosity is best developed immediately beneath type-I sequence boundaries in highstand carbonate buildups where the effects of meteoric-water leaching and karstification are most intense. Laterally, types and values of porosity change rapidly from one facies to another. Moldic and vuggy porosity is best developed in reef core and peri-reef facies because of the abundance of metastable skeletal grains. Lagoonal and inter-reef sediments are commonly mud-supported with predominant intercrystalline and chalky microporosity. Off-reef/basinal facies are mostly tight because of the common mud-size matrix material and intense compaction. Calcite cementation is a common porosity obliterating process, occurring in both meteoric and burial environments. Dolomite occurs only locally and may have been related to different types of mixing of marine and meteoric waters, oceanic groundwater pumping or warm fluids derived from basinal compaction. In essence, porosity generation for these humidoceanic, tropical-subtropical Miocene carbonates was largely associated with subaerial dissolution processes although hydrothermal corrosion and fracturing in the subsurface also produced significant quantities of porosity locally.

In comparison, in arid, evaporitic settings (i.e., early and middle Miocene carbonates in the Middle East and late Tortonian-Messinian carbonates in the Mediterranean), more limited recharge of fresh groundwaters minimized both leaching of metastable skeletal components and karst processes. Instead, sea-level fall and lowstand commonly resulted in evaporitic conditions. With the ensuing marine transgression, mixing of hypersaline basinal brine and normal sea water would cause extensive dolomitization and associated leaching of metastable skeletal components, particularly along the platform margins. As the marine transgression proceeded, processes of dolomitization and dissolution could also have occurred within the platform-interior carbonates. This would depend on the volume of basinal hypersaline brines and the degree to which they were diluted by normal sea-water. Consequently, a large volume of moldic, vuggy and intercrystalline porosity were created. Calcite cements are rare because the dissolved calcium carbonates were incorporated into dolomites. Under shallow burial conditions, the most common porosity-obliterating process is the widespread anhydrite cementation. In some cases, both the primary porosity and early-generated secondary porosity were almost entirely occluded by anhydrite cements. With continued burial, the porosity of these carbonates was restored to 15-30% through fracturing and late corrosion of anhydrite cements, micritized gains and matrix. The corrosive fluids responsible for such a large-scale late corrosion are believed to have been associated wth source-rock maturation or basinal shale compaction. Porosity generation of these arid, subtropical- and temperate-type carbonates was essentially associated with early dolomitization, skeletal aragonite dissolution, and late corrosion of anhydrite cements and fine-grained sediments.

Diagenetic pathways in these two end members of climatic settings are clearly controlled by the balance of evaporation/rainfall and related paleooceanographic factors. The well-studied pathways in humid-oceanic, tropical/subtropical carbonates are predominantly controlled by early fresh water diagenesis and depositional facies. In arid, land-locked settings, less-publicized diagenetic processes related to basinal evaporitic conditions override the effects of the early fresh-water diagenesis.

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