Petrophysical Constraints on Deformation Styles in Aztec Sandstone, Southern Nevada, USA

Abstract

— Adjacent stratigraphic units that have undergone an identical deformation history often show variability with regard to deformation style. We present one such example and attribute the variability in deformation to variations in host rock properties. The Aztec sandstone of southern Nevada has two distinct zones of deformation style, the Lower and Upper structural domains. The Lower domain has deformed predominantly by opening mode fractures, whereas the Upper domain has deformed predominantly by deformation band faulting. Within a Transitional domain, deformation band abundance increases toward the Upper domain. We use petrophysical data (ultrasonic velocity, elastic moduli, grain and bulk density, helium porosity, and gas permeability) to distinguish among the host rocks of the differently deformed domains. The laboratory results of 29 samples of Upper, Transitional, and Lower domain Aztec sandstone impart the following petrophysical distinctions among the stru ctural domains: (1) the Lower and Transitional domains show similarly high VP and VS and are both well cemented; (2) the Transitional and Upper domains show similarly high porosity and permeability; and, (3) the Upper domain is poorly cemented. We demonstrate that: (1) the nature of intergranular cement controls VP and VS; and, (2) based on the petrophysical properties we have examined, deformation style is controlled by porosity.