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Mechanical Stratigraphy and Stress History of Cap-rocks Analysis of Exhumed Analogs in Central and South-eastern Utah and Implications for CCS

Elizabeth S. Petrie and James P. Evans
Utah State University

Top-seal failure of subsurface waste storage systems such as those proposed for the mitigation of anthropogenic CO2 accumulation can occur when pre-existing optimally oriented fault and fracture systems are reactivated or when new fractures are induced due to increased fluid pressures. The presence of discontinuities in seal lithologies affects their mechanical and hydrogeologic properties; migration of fluids or gas through mm- to cm-scale discontinuity networks can result in focused fluid flow within and across a caprock. We examine the mechanical and fracture stratigraphy of Paleozoic and Mesozoic analogues of failed cap-rocks exposed in central and south-east Utah to understand the nature and distribution of fluid flow pathways in various sealing lithologies. Each seal type has experienced a unique depositional and tectonic history, all are heterolithic, low permeability (0.001 to 0.12 D), and show evidence of fluid flow across the cap-rock through open-mode and shear fractures. We combine outcrop analysis with the unique loading history and resultant uniaxial strain model at each locality to understand the timing of fracture initiation and paleo-tectonic stress orientation, if it differs from the current dominant crustal stress orientation. Burial history models evaluated in this study suggest that most formations reach a maximum burial depth > 1.6 km and experience an overburden stress of up to 50 MPa. As lithostatic load increases with burial depth the potential for initiation of natural hydrofractures increases because the excess pressure above the hydrostatic gradient required for failure decreases. Once zones of weakness have been established within the cap-rock they exist as loci for future deformation and fluid flow.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013