Characteristics of bleaching around major structures in a reservoir-cap rock system, SE Utah, USA

Abstract

Bleached rocks are commonly formed from CO2-saturated water leakage to the surface. This provides an opportunity to understand the controlling factors of fluid flow in the subsurface. This study examined the relationships between bleaching patterns and geologic features or elements. We analyzed bleaching characteristics of multiple-layered sedimentary rocks around two major faults (Moab and Salt Wash faults) and fold axes (Green river anticline system) based on field observations and quantitative measurements (scanline survey and permeability) in the exhumed reservoir-cap rock systems in SE Utah, USA. The results showed that strongly bleached layers of sedimentary rock have a higher density of deformation bands compared with unbleached layers. This is consistent with the general property of deformation bands that frequently develop in layers with higher porosity and permeability. Although almost all fault zones act as conduits for fluid flow, some fault zones filled with clay-rich gouges could impede fluid flow. In addition, the internal sealing characteristics of the layer boundaries such as bedding planes could be important factors as they can act either as a pathway or a barrier for lateral fluid flow depending on the existence of filling materials such as calcite or kerogen. Our research may be useful for assessing fluid leakage in oil reservoirs or CO2 sequestration sites located in a reservoir-cap rock system of sedimentary basins.