DIAGENESIS, COMPACTION, AND FLUID CHEMISTRY MODELING OF A SANDSTONE NEAR A PRESSURE SEAL - LOWER TUSCALOOSA FORMATION, GULF-COAST

Petrographic, isotopic, and fluid-inclusion evidence from normally and overpressured sandstones of the lower Tuscaloosa Formation (Upper Cre taceous) in the Gulf Coast documents quartz-overgrowth precipitation a t 90 degrees C or less, calcite cement precipitation at approximately 100 degrees and 135 degrees C, and prismatic quartz cement precipitati on at about 125 degrees C. Textural evidence suggests that carbonate c ement dissolution occurred before the second phases of calcite and qua rtz precipitation, and was followed by precipitation of grain-rimming chlorite and pore-filling kaolinite. Geochemical calculations demonstr ate that present-day lower Tuscaloosa Formation water from 5500 m dept h could either dissolve or precipitate calcite cements in model simula tions of upward water flow. Calcite dissolution or precipitation depen ds on P-CO2 variability with depth (i.e., whether there is one or two- phase flow) or on the rate of generation of CO2 with depth. Calculatio ns suggest that 10(5)-10(6) rock volumes of water are required to flow through the section to precipitate 1-10% calcite cement. Compaction a nalysis suggests that late-stage compaction occurred in normally press ured sandstones after dissolution of carbonate cements, but was hinder ed in overpressured sandstones despite the presence of high porosity. These results document the inhibition of compaction by overpressured f luids and constrain the timing of pressure seal formation. Modeling re sults demonstrate that the proposed paragenesis used to constrain timi ng of pressure seal formation is feasible, and that most of the cement diagenesis occurred before the pressure seal became effective as a pe rmeability barrier.