Deformation bands formed during soft-sediment deformation: Observations from SE Utah

Several types of syndepositional deformation structures contain strain localization structures known as disaggregation bands. Abundant field examples from Utah show that such bands can be related to vertical movements linked to loading and fluid expulsion, forming a pre-tectonic set of strain localization structures in deformed sandstones that can easily be overlooked or misinterpreted as tectonic structures in petroleum reservoirs. Plug measurements and thin-section investigations show that they have little or no influence on fluid flow. In contrast, disaggregation bands formed as a response to tectonic stress at higher confining pressures (depths) in the same lithology show up to 3–4 orders of magnitude reduction in permeability. This makes it important to distinguish between synsedimentary and tectonic deformation bands. They should also be separated because only bands formed in relation to tectonic stress can be used to predict nearness to important faults and to assess the extent of faulting in a reservoir.