FROM WHENCE THE CO2? INVESTIGATING THE ROLE OF CLAY-CARBONATE REACTIONS DURING DIAGENESIS AS A POTENTIAL SOURCE OF CO2 IN THE PARADOX BASIN, SE UTAH

The Colorado Plateau contains a number of large carbon dioxide reservoirs. A number of normal and reverse faults throughout the Paradox Basin (SE Utah) dissect north-northwest trending anticlines giving rise to a series of footwall reservoirs with fault-dependent columns. Numerous CO₂-charged springs and geysers are associated with these faults. Several localities near Green River, Utah have been documented as experiencing prolonged periods of CO₂ discharge as evidenced by large accumulations of travertine, cold water seeps, springs, and geysers attributed to fault seal failure. The Farnham Dome in the northern Paradox Basin is also characterized by extensive calcite veining. Despite numerous studies performed in this region it is still not clear how CO₂ was generated in the first place. Previous workers used stable isotope and noble gas geochemistry in order to elucidate the origins of CO₂ within the Paradox Basin and the greater Colorado Plateau physiographic province. The majority of these authors conclude CO₂ in the NE Paradox Basin is primarily crustal in origin. CO₂ in the southern Colorado Plateau is thought to have formed as a result of magmatic activity.

Here we explore dissolution of calcite in the presence of kaolinite as one possible CO₂-generating mechanism. We use the Geochemist's Workbench to trace reaction paths and stable isotopes of fluids and minerals during a period of burial beginning in the middle Pennsylvanian and ending in the Oligocene. pCO₂ values recorded as a function of temperature are comparable to values obtained from sediments in the Gulf Coast and waters from geothermal wells in Iceland. We set the δ¹³C of the initial fluid (Pennsylvanian seawater) to be 3‰; the value of water at 25°C in equilibrium with calcite having a δ¹³C = -1‰. Similarly, δ¹⁸O for the initial fluid is set to 23‰; the value for a fluid in equilibrium with calcite having a δ¹⁸O = -11‰. Fluid compositions are plotted on activity diagrams as more saline fluids are introduced into the rocks during diagenesis in order to trace silicate reaction progress. Finally, we allow the resulting fluid to mix with Mesozoic-type aquifer groundwater and degas along a sliding fugacity path. We compare the resulting stable isotope compositions of calcite and CO₂(g) generated in our model to samples collected in the NE Paradox Basin and Uinta Basin.