ESTIMATING THE STORAGE CAPACITY OF A POTENTIAL CO2 RESERVOIR THROUGH ANALYSIS OF PALEO-MIGRATION OF CO2 IN AN EXPOSED ANALOG

Injection of carbon dioxide (CO₂) at depth into stacked saline aquifers separated by low permeability shale units has been proposed as a viable method to reduce the amount of anthropogenic CO₂ released into the atmosphere (Allis et al, 2003). Large anticlines in central Utah are natural analogs for a class of injection sites. The shale-siltstone sections may act as hydrostratigraphic trapping mechanisms which can be utilized as natural reservoirs, capable of storing large volumes of injected CO₂. In most cases migration of CO₂ to the surface is inhibited by the presence of a low permeability top seal, but in some cases the integrity of the top seal is compromised by the presence of fractures and faults. Evidence of paleo-fluid migration through pore space and along fractures and faults is preserved in many formations throughout Utah in the form of altered host rock, and calcite mineralization in Mode I fracture systems and in deformation-band faults. In these areas the transition from sandstone to siltstone or shale along fractures and faults show a continuum of alteration across the transition. Microscopic analysis of altered matrix, mineralized fractures, and deformation banding within the Jurassic Glen Canyon and San Rafael Groups provide insight into fluid migration within a reservoir during a large influx of CO₂. Identification of primary and secondary flow features on the microscopic scale enhances understanding of features observed on a mesoscopic scale, and allow for partial reconstruction of an exposed paleo-reservoir at a macroscopic scale. The degradation of pore space through cementation of minerals on the microscopic scale can be used to correlate varying degrees of degradated pore space to areas exhibiting a higher degree of faulting or fracturing which allow for increased fluid migration. Partial reconstruction of an exposed reservoir in which leakage has occurred allows for estimations of maximum reservoir storage capability prior to failure along dominant faults and fracture zones. The correlated data provides insight into the storage capacity of a potential CO₂ sequestration site.