AN INVESTIGATION OF THE IMPACT OF MAGMATIC-HYDROTHERMAL ALTERATION ON CO2-RESERVOIRS IN THE LEADVILLE LIMESTONE, SOUTHWEST COLORADO

In the Four Corners region about eight hundred and twenty-six billion cubic feet of CO₂ have been extracted, mostly from the Mississippian Leadville Limestone. Understanding the sources of CO₂, as well as the primary and diagenetic features of these strata is important to constrain the history and evolution of these reservoirs. Regional Cenozoic intrusive masses may have played a critical role in the origin of CO₂ deposits and properties of the reservoir rock; interpreting the role of the intrusions on the development and production of CO₂ reservoirs is critical.

This study focuses on a well-exposed transect in the Leadville Limestone leading away from a 26 Ma granite stock, over an eight mile distance near Silverton, Colorado. Field and petrographic studies define seven carbonate facies within the Leadville Limestone, in addition to a silt and sand karst infill of the Pennsylvanian Molas Formation. All units were exposed to thermometamorphism and hydrothermal alteration.

Preliminary field data reveal extensive silication and dolomitization in the Leadville Limestone, controlled by both facies and fractures. Coarser-grained facies tend to show a greater degree of post-burial dolomitization; in peloidal grainstones, euhedral dolomite rhombs are non-fabric selective. However in peloidal to crinoidal grainstone facies proximal to the intrusive mass, the formation of epidote-calcite-wollastonite-garnet skarn via high temperature metamorphism has caused extensive reduction in porosity and permeability. Migration of magmatic-hydrothermal fluids took place primarily along fractures which provided pathways to specific facies that then underwent differential alteration. Adjacent to fractures, clusters of 1-2mm dolomite crystals occur; similar to hydrothermal dolomite reservoir facies in the Lisbon Field of southeastern Utah (Eby et al., 2009).

Impacts of heat and hydrothermal fluids on rock types are currently being investigated with petrographic, carbon-oxygen-strontium isotope data, and fluid inclusion analyses. These data will help understand the role of magmas on the evolution, porosity, permeability and potential CO₂ traps in the Leadville Limestone. These data will provide insight into origins and migration of CO₂ within existing reservoirs.