157 Geochemistry (Posters)

Tuesday, 6 November 2012
9:00 AM - 6:00 PM
Hall B

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

Paper number: 157-17
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

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.