Abstract

Petrographic, isotopic, and fluid-inclusion evidence from normally and overpressured sand-stones of the lower Tuscaloosa Formation (Upper Cretaceous) in the Gulf Coast documents quartz-overgrowth precipitation at 90 degrees C or less, calcite cement precipitation at approximately 100 and 135 degrees C, and prismatic quartz cement precipitation at about 125 degrees C. Textural evidence suggests that carbonate cement dissolution occurred before the second phases of calcite and quartz precipitation, and was followed by precipitation of grain-rimming chlorite and pore-filling kaolinite. Geochemical calculations demonstrate that present-day lower Tuscaloosa Formation water from 5500 m depth could either dissolve or precipitate calcite cements in model simulations of upward water flow. Calcite dissolution or precipitation depends on PCO 2 variability with depth (i.e., whether there is one or two-phase flow) or on the rate of generation of CO 2 with depth. Calculations suggest that 10 5 -10 6 rock volumes of water are required to flow through the section to precipitate 1-10% calcite cement. Compaction analysis suggests that late-stage compaction occurred in normally pressured sandstones after dissolution of carbonate cements, but was hindered in overpressured sandstones despite the presence of high porosity. These results document the inhibition of compaction by overpressured fluids and constrain the timing of pressure seal formation. Modeling results demonstrate that the proposed paragenesis used to constrain timing of pressure seal formation is feasible, and that most of the cement diagenesis occurred before the pressure seal became effective as a permeability barrier.

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