NATURAL FRACTURE ANALYSIS NEAR THE JURASSIC NAVAJO/CARMEL CONTACT ALONG THE EAST FLANK OF THE SAN RAFAEL SWELL, UTAH: ANALOGS FOR SEAL BYPASS AT A RESERVOIR-SEAL INTERFACE

Meso-scale faults and fractures in the subsurface contribute to seal bypass in fine-grained, low-permeability lithologies allowing reservoir fluid such as oil, natural gas, carbon dioxide (CO₂), or wastewater to leak to the surface. We evaluate the variability and distribution of transmissive faults and fractures in reservoir and caprock lithologies on the east flank of the San Rafael Swell near the interface of the Jurassic Navajo Sandstone and its proposed seal, the Carmel Formation. Natural outcrop exposures of the Navajo-Carmel interface are analogous to potential sequestration sites and are displayed in zones of varying structural complexity. By combining meso- and micro-scale observations, we can determine how fracture density, style of deformation, and structural diagenesis vary within each setting, allowing seal failure.

Transmissive fractures are observed within three settings and are distinguished by the presence of mineralized fracture apertures and alteration halos. The three settings include fold-related curvature, the steeply dipping monoclinal fold limb, and conjugate faults. Mesoscopic methods of fracture population mapping were conducted at field areas in the three settings. The techniques include scan line measurements and 3D outcrop fracture modeling to determine how fracture densities and attitudes differ. Thin sections were made from multiple sites within each area to determine how fracture-related structural diagenesis affected the petrophysical properties of the reservoir and caprock.

Structural setting and timing relationships play an important role in determining the distribution of transmissive fractures. Using scan line data, we determine the mean fracture orientations for each site, distinguishing mineralized and altered fracture patterns. We also determine fracture density and fracture spacing distributions for two generations of fractures which relate to regional and localized stress. Using petrographic analyses, we identify multiple generations of dissolution and cementation which provide evidence for fluid migration through fractures at depth.