Relationships Among Carbon Dioxide, Pore-Fluid Chemistry, and Secondary Porosity, Texas Gulf Coast: ABSTRACT

Abstract

Sequences of diagenetic minerals associated with secondary porosity show striking similarities. The formation of quartz overgrowths on detrital quartz grains is followed generally by carbonate cementation. The dissolution of this carbonate is the main secondary porosity-forming event, which commonly precedes kaolinite precipitation and iron-rich carbonate cementation. In the Texas Gulf Coast, oxygen isotopic analyses provide temperature estimates of authigenic phases that predate and postdate secondary porosity development: quartz, >= 80°C (176°F); kaolinite, >= 70°C (158°F); albite, 100°-150°C (212°-302°F); late carbonate, > 100°C (212°F). These data suggest that secondary porosity in the Tertiary Gulf Coast form at temperatures of about 100° ± 25°C (212° ± 45°F). Correlations among calcite saturation indices in pore fluids, abnormally high permeabilities, and mole % CO2 in natural gases of the Eocene Wilcox Group imply a strong interrelationship between CO2 and secondary porosity development in clastic reservoirs. The CO2 content of gases varies systematically with both the reservoir age and temperature, which suggests a kinetic control on generation. The amount of CO2 in natural gases increases rapidly at approximately 100°C (212°F); this coincides with a rapid increase in the ratio of secondary to primary porosity in associated sandstones. Stable isotopic analyses of carbonate cements indicate a strong component of organically derived carbon and therefore cycling of carbon between inorgan c and organic systems. The type, amount and distribution of organic matter, and early carbonate in both shales and sandstones control the quantity of CO2 available for generating secondary porosity. End_of_Article - Last_Page 478------------