Linking elastic and petrophysical properties associated with deformation bands

Deformation bands are common structures in faulted porous sandstones and are interpreted to represent zones where petrophysical properties within the band are different from the host rock. Porosity within the deformation band may be enhanced or reduced depending on the band type. Previous studies show that porosity has a negative correlation with rock velocity. This study investigates the relationship between velocity and porosity of sandstones that contain zones of deformation bands. It aims to provide the connection between petrophysical and elastic properties associated with deformation bands. This work incorporates outcrop studies as well as field and laboratory measurements of Jurassic Entrada sandstone in north-central New Mexico. Thin section observations of the deformation bands from the study area show that the bands consist of mostly quartz grains and have the same mineralogical composition as the surrounding rock. The porosity within the band is reduced relative to the surrounding rock. The decrease in size and sorting of the grains within the band is also observed. Previous workers, who have described the deformation bands with similar properties, have explained the formation of these deformation bands through the process of cataclasis which involves grain crushing. To test this assumption, an investigation of the pressure required to crush a quartz grain from published laboratory experiments along with the estimation of regional stresses at the time of the deformation bands' formation in this study area were conducted. From these estimations, it was concluded that the pressures generated during the formation of deformation bands in the study area are insufficient to cause cataclasis. Therefore, an alternative explanation of low-porosity deformation bands' formation under low pressure conditions is proposed in this work. Modeling of rock velocity variation with porosity was performed using the core samples measurements from the study area. The obtained models were used to estimate porosity from acquired outcrop velocity data. Unlike laboratory data, some of the measured outcrop velocities exhibit anisotropy, meaning the velocity depends on the measurement orientation, associated with the dominant strike direction of deformation bands. Therefore, a correction for the anisotropy observed in these field measurements was applied to the above models.