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Abstract

Burial diagenesis of platform carbonates is a very complex process occurring over tens of millions of years, encompassing several different tectonic settings, exhibiting many diagenetic products and involving waters of diverse origins and compositions. To facilitate understanding of these processes, burial diagenesis of platform carbonates is divided into three hydrotectonic realms: (1) passive margin burial diagenesis, (2) collision margin burial diagenesis, and (3) post-orogenic burial diagenesis.

The passive margin burial diagenetic realm, exemplified by the northern Gulf of Mexico, is characterized by extensional tectonics, growth faulting, a relatively uniform subsidence rate, slow upward flow of compaction-driven fluids, and increases in temperature, pressure and salinity of pore waters with burial depth. The passive margin burial diagenetic realm can be divided into three subrealms: (1) pre-oil window diagenesis (<100°C), (2) oil-window diagenesis (100-140°C) and (3) gas-window diagenesis (140-200°C). Quantitative analyses of Upper Jurassic calcite ooid grainstones that have experienced all 3 subrealms indicate that, of the original 45% porosity, an average of 12% can be filled by carbonate cement, whereas 33% is lost by compaction and pressure solution. Unless a well-developed convection system exists, low HC03 contents of formation waters and slow fluid flow rates in passive margin settings suggest that most carbonate cements are derived locally from pressure solution of host carbonates. In passive margin settings, Mg2 + contents of burial calcite cements should initially increase and then decrease with burial. This trend develops because initial increases in Mg2+ distribution coefficient (DMg2+) with temperature outweigh decreases in the mMg2 V mCa2+ ratio of pore waters, resulting in increases in Mg2+ contents of calcite cements. But later, continued lowering of mMg2+/mCa2+ ratios reverses this trend. Data suggest that Sr2+ contents of calcite cements, initially very low, increase with temperature during deep burial. Wide variations in mFe^/mCa2* and mMn2+/mCa2+ ratios of formation waters indicate that concentrations of Fe2+ and Mn2+ in carbonate cements are of limited value beyond local scales. The 8,80 compositions of calcite cements first decrease and then increase during passive margin burial. This trend develops because temperature-dependent calcite-water fractionation produces an initial decrease in 8180 values of cements; but later, progressive increases in 8,80 values of formation waters overcome temperature fractionation effects, resulting in increases in 8,sO composition of carbonate cements. The 8,3C values of carbonate cements can follow either one of two paths. In the absence of hydrocarbons, 8I3C compositions of pore waters are buffered by host carbonates and change little with burial. Where hydrocarbons are present, their decarboxylation at temperatures above 150°C promotes precipitation of carbonate cements with low 8,3C values. Therefore, two burial 8180-dl3C trends are generated: (1) a “C trend” when organic-derived C02 is not available and (2) a “D trend” when hydrocarbon destruction generates significant amounts of CO,.

The collision margin burial diagenetic realm, exemplified by the Ouachita collision belt, is characterized by compressional tectonics, thrust faulting, variable uplift/subsidence rates and episodic focused expulsion of tectonic fluids toward the craton. Three subrealms are recognized based on the intensity and types of diagenetic alterations: thrust belt diagenetic zone, foreland diagenetic zone, and craton margin diagenetic zone. The more important carbonate diagenetic events in collision margin settings are: (1) extensive pressure solution and fracture-fi11 carbonate cementation, (2) Mississippi Valley-type (MVT) mineralization, and (3) K-feldspar and magnetite precipitation. As much as 50% (by volume) of carbonates can dissolve by tectonic pressure solution and be available to precipitate in tectonic fractures. H2S produced by thermochemical sulfate reduction in deep carbonate reservoirs during passive margin settings is expelled during collision margin diagenesis to react with base metal-rich fluids in shallow depths leading to the formation of MVT ore deposits. Sulfide mineralization generates significant amounts of acids which can dissolve syndepositional dolomites and reprecipitate them as burial dolomites. K-feldspars form when K+-rich pore waters (generated by K-feldspar dissolution during passive margin diagenesis) are flushed to shallow parts of the basin.

The post-orogenic burial diagenetic realm, exemplified by the Madison aquifer in the U.S. midcontinent, is characterized by a lack of tectonic activity, dominance of topographically-driven fluid flow and high fluid flow rales. Rainwater charged with soil C02 enters aquifers in highland recharge areas dissolving carbonates and evaporites along its flow paths. Temperature, pressure, alkalinity, pH, Ca2 +, Mg2+, Cl+ and Na+ concentrations of groundwater increase and Eh decreases toward the discharge area. Important diagenetic processes in this realm are evaporite and carbonate dissolution, dedolomitization, calcite precipitation and bacterial sulfate reduction.

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