Structural Controls on Leakage from a Carbon Dioxide Reservoir in Southeastern

Utah, USA.

Ben Dockrill 1, Zoe K. Shipton 1, and James P. Evans 2.

1 Department of Geology, Trinity College Dublin, Ireland

2 Department of Geology, Utah State University, Logan, USA

dockrilb@tcd.ie

Concerns about global warming have prompted the investigation of carbon dioxide sequestration in underground reservoirs as an acceptable short-term solution to lower greenhouse gas emissions. Natural carbon dioxide reservoirs in the Colorado Plateau provide a natural laboratory for studying factors that influence these storage systems. The risk of significant leakage along faults is a potential concern for CO2 sequestration. A regional scale study of a presently leaking carbon dioxide reservoir is underway in the northwestern Paradox Basin, southeastern Utah. The reservoir contains a west-north-west trending set of clay-rich normal faults that cut siliclastic sediments folded into an open, gently north plunging salt-cored anticline. The faults are well-defined brittle shear zones externally bound by slip zones and characterised by lenses of siltstone and sandstone enclosed in a matrix of gouge interspersed with further slip zones. Localised along the fault traces are evidence of past and present fluid flow with CO2-charged geysers and springs, tufa deposits and zones of iron oxide reduction in high permeability sandstones. All these features are confined either to the immediate footwall where the fault cuts the anticline crest or to locations where uncapped wells have penetrated the reservoir. These field observations indicate that CO2-rich fluids migrated up-dip towards the crest of the anticlines and pooled in structural highs before leaking along the clay-rich fault zones. Three-dimensional modelling is now being undertaken to test this working hypothesis and understand the effects of faults on fluid migration in the regional system.