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Field investigations in the area of Grand Isle, Louisiana, indicate the presence of active carbonate precipitation. Modern shoreline facies are being cemented by high-magnesium calcite, ranging from 10 to 50 mole % magnesium carbonate. Cementation occurs between the beach and marsh environments as surface crusts or laminae. The environment is characterized by seasonal high temperature (up to 50°C), high salinity (40 to 90^pmil), supratidal stromatolites and organic decomposition.
After burial, lithified crusts undergo chemical alteration, as indicated by a decrease in magnesium carbonate content (20 to 35 mole %). The cement eventually undergoes dissolution and reprecipitation, forming a well-cemented sandstone. Rip-up clasts of these sandstones are found along Gulf Coast beaches where a transgressive sequence occurs. These clasts are typically cemented by high-magnesium calcite with 10 to 15 mole % magnesium carbonate. Carbon isotope analyses of rip-up clasts indicate that dissolution and reprecipitation processes are methane-derived due to organic decomposition.
Buried crusts may be preserved in the subsurface and form an effective barrier to fluid migration. Where cementation is extensive, porosity is occluded on the marsh-side of the barrier-island sequence. Cement distribution is, therefore, a primary controlling agent of hydrocarbon migration and subsequent entrapment. Migration of hydrocarbons from organic-rich marsh sediments would be prevented by the early formation of the permeability barrier. Cementation, then, would account for accumulation of hydrocarbons on the marsh side of the barrier-island sequence and the formation of a stratigraphic (or "diagenetic") trap.
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