FAILURE OF CAPROCK SEALS - MECHANICAL STRATIGRAPHY, STRESS HISTORY AND TENSILE FAILURE ANALYSIS OF EXHUMED ANALOGS

The varied sedimentologic and tectonic histories of terrigenous caprock seals and their inherent mechanical properties control the distribution and morphology of permeable fractures within these lithologic units. The migration of fluids or gas through mm- to cm-scale discontinuity networks comprised of bedding planes, stratigraphic interfaces, and vertical fracture networks can result in focused fluid flow which compromises seal integrity. We examine four failed caprock seals, Paleozoic and Mesozoic analog formations in Utah that exhibit evidence for subsurface fluid flow via permeable fracture networks, as evidenced by mineralized extension and shear fractures. We identify features in outcrop that suggest failure due to extensional-shear and hydraulic extension and subsequent fluid flow. Tensile rock strength, derived from indirect tensile strength tests range from 2.3 MPa in siltstone to 11.5 MPa in calcareous shale, and are lithology dependent. Burial history models suggest that the caprock seal analogs reached a maximum burial depth greater than 1.6 km and experienced an overburden stress of up to 70 MPa. Analysis of the evolution of the pore-fluid factor, λ_v, through time shows changes in expected failure mode of extensional shear or hydraulic extension, and the failure type depends on a combination of mechanical rock properties and differential stress at different points in the burial history. With increasing lithostatic load, λ_v decreases for all intact rock scenarios, however differences in mechanical properties due to lithology can inhibit formation of extensional shear failure by increased pore fluid pressure alone. When these datasets are combined with simple three-dimensional finite element modeling, we can predict how the fine-scale mechanical stratigraphy of the seal influences the potential for failure due to localized strain accumulation at mechanical boundaries.