Petrophysical Properties and Sealing Capacity of Fault Rock, Aztec Sandstone, Nevada

Abstract

To refine flow models for sand-dominated fault rock, we present petrophysical data of host and fault rock samples from the eolian Aztec Sandstone, Valley of Fire State Park, Nevada, that has been deformed by strike-slip faults formed by progressive shearing along joint zones. The data include bulk mineralogy, porosity, permeability, grain-size distribution, and mercury-injection capillary pressure measurements of 40 host, fragmented, and fault rock samples. To investigate the impact of shear strain on fault zone properties, three sample localities with average shear strains of 28, 63, and 80 were investigated (25–160-m [82–525-ft] slip). No bulk mineralogical changes caused by fault zone cementation or mineral alteration were detected when comparing host and fault rock. Fault rock permeability is one to three orders of magnitude lower than median host rock permeability. Porosity reductions are less pronounced and show considerable overlap in values between the sample suites. Some fault rock samples appear to have dilated with respect to median host rock porosity. Median grain sizes for fault rock samples range from 3 to 51 m, which is as much as two orders of magnitude reduction from host rock median grain sizes. There appears to be a lower limit of median grain size of 3 m for fault rock samples irrespective of average fault shear strain. Fault rock capillary injection pressures range from one to almost two orders of magnitude higher than the host rock equivalent. For standard fluid properties, calculated maximum sealable hydrocarbon column heights range between 10 and 70 m (33 and 230 ft) of gas and 20–120 m (66–400 ft) of oil. These petrophysical data show that faults formed by shearing of joints in high-permeability, sand-prone systems will act as significant barriers to fluid flow during reservoir production and might be capable of sealing small to moderate hydrocarbon columns on an exploration timescale as well, assuming adequate continuity of the fault rock over large areas of the fault.