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Submarine cementation commonly forms a narrow zone of low permeability within a carbonate unit that may act as a diagenetic seal over potential reservoir facies. Although the process of submarine cementation still is not clearly understood, it does appear to be a near-surface, rock or sediment/water interface phenomenon. The diagenetic model proposed here involves the effect of submarine cementation on previously lithified carbonates, such as submerged relict shelf-margin buildups (e.g., drowned reefs, ooid shoals) or previously subaerially exposed formations (e.g., dune ridges) that were submerged by later sea level rise. These deposits generally have pronounced topographic relief (visible on seismic), good reservoir geometries, and high internal porosity of either prima y or secondary origin.
Petrologic studies on examples of both of these situations--a submerged early Holocene barrier reef off Florida and a 175-km (110-m) long submerged Pleistocene eolian ridge in the Bahamas--show that their exposed surface and uppermost facies (0-1 m, or 0.3 ft, below top) are further infilled and cemented, creating an extensively lithified, low porosity/low permeability zone or "diagenetic cap rock." Quantitative mineralogic studies of occluding cements reveal an exponential reduction in porosity while moving upward into the seal zone. Submarine cements effectively infill and form a surficial permeability barrier that acts to impede further diagenesis and porosity reduction within underlying potential reservoir facies.
To form this diagenetic seal only requires that the original carbonate buildup be resubmerged for some brief period of time prior to subsequent burial by sediments. If buildup accumulation later resumes without intermediate sediment burial--a common stratigraphic situation--the diagenetic seal would represent a disconformity separating two similar facies.
The early formation of a diagenetic cap rock lends support to models of early hydrocarbon migration and emplacement. Prediction and recognition of submarine diagenetic seals will aid in exploration and development of obvious buildup reservoirs as well as subtle intraformational traps.
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