STRUCTURE AND STABLE ISOTOPE SYSTEMATICS OF THE FARNHAM DOME, SE UTAH

The Paradox Basin, SE Utah, hosts large accumulations of hydrocarbons and CO₂. Many reservoirs are found in crests of north-northwest trending anticlines often having four-way fault seal closure. Thus the Paradox Basin provides a unique opportunity to explore issues related to CO₂ sequestration. Farnham Dome (FD) is a ~25 km² area in east-central Utah featuring a natural CO₂ reservoir. The FD is a Laramide-related anticline formed in part by re-activation of older reverse faults. CO₂ found primarily in the Jurassic Navajo sandstone is believed to have formed from thermal decomposition of Paleozoic age carbonates. It is believed that the reservoir has remained sealed for at least 10 million years. Questions still remain as to the source of extensive calcite deposits within FD. Calcite deposits are found on fault surfaces and as fracture fill. Previous workers propose leaks developed during Sevier deformation with subsequent calcite precipitation eventually sealing the leaks. Alternatively, calcite precipitated from groundwater- sourced in the Uinta Basin to the north- that migrated along a detachment zone in the Carmel Formation.

This study seeks to determine whether the presence of calcite in the FD is related to leakage of CO₂ due to seal failure or precipitation associated with faulting during deformation. Approximately 120 calcite samples were collected in the field and from one core penetrating the Mesozoic and Paleozoic reservoirs. Preliminary δ¹³C and δ¹⁸O data cluster from -5.57 to -7.37‰ and from 17.96 to 18.59‰, similar to syndeformational veins found in the Green River and Moab areas to the southeast. Structural data document a complex fracture system related to fold geometry and bedding. Sub-vertical calcite-filled fractures bisect NW-SE trending sets of conjugate shear fractures with a strike-slip sense of displacement perpendicular to bedding are common on the fold limbs. Conjugate sets of reverse and normal faults trending parallel to the fold axis and at high angles to bedding, compactions bands, and en echelon vein arrays are found in association with areas of the fold where deformation is greatest. We interpret clustering of stable isotope data, association of calcite with shear sets, and structural data as indicating calcite precipitation from a single fluid reservoir during deformation.