NATURAL CO2 DISCHARGE AND IMPLICATIONS FOR SEQUESTRATION

Long-term effects of injecting CO₂-charged fluids into the subsurface are poorly understood. Elevation of pore pressures and reaction of minerals within a CO₂ reservoir may affect trap integrity. Predicting the capture potential of faulted reservoirs requires knowledge of fault-sealing mechanisms, which are functions of fault-rock properties and fault-zone architecture both within the reservoir and overlying strata.

The Colorado Plateau contains a number of large carbon dioxide reservoirs some of which leak and some of which do not. Several normal faults within the Paradox Basin (SE Utah) dissect the Green River anticline giving rise to a series of footwall reservoirs with fault-dependent columns. Numerous CO₂-charged springs and geysers are associated with these faults. This study seeks to identify regional sources and subsurface migration of CO₂ to these reservoirs and the effect(s) faults have on trap performance.

Geochemical analyses of fault rocks, calcite veins, and travertine samples from sites near Green River, Utah, have been used to determine the source of carbon isotopes from sedimentary derived carbon and deeply sourced CO₂. XRF and XRD data taken from several transects across the normal faults are consistent with mechanical mixing and fluid-assisted mass transfer processes within the fault zone. δ¹³C range from -6‰ to +10‰ (PDB); δ¹⁸O values range from +15‰ to +24‰ (VSMOW). In modeling the mixing curves for CO₂-bearing fluids, we assume δ¹³C_carbonate = -1.5‰, typical of the Honaker Trail limestone, δ¹³C_organic = -28‰, and a deep source with δ¹³C_endogenic = -5‰. Modeling results predict one-half of TDIC is derived from dissolution of carbonates, one-quarter from other sedimentary sources, and one-quarter from external sources (e.g., Tertiary volcanic activity and/or lower crustal CO₂). This is supported by drill-stem test data, well-water analyses, and structural analyses within the Paradox Basin from other studies. Water chemistry of springs and geysers are intermediate in composition between Na-Cl-SO₄ and Ca-Mg-HCO₃ end members typical of the deep Paleozoic and shallower Jurassic Navajo aquifers, respectively.