ALBERT

Description: We explore the influence of mechanical deformation in natural sands through experiments on water-saturated samples of quartz sand. Stresses, volumetric strain, and microseismicity (or acoustic emission, AE) rates were monitored throughout each test. Deformation of quartz sand at low stresses is accommodated by granular flow without significant grain breakage, whereas at high stresses, granulation and cataclastic flow are dominant. Sands deformed under isotropic conditions show compactive strains with an inverse power-law dependence of macroscopic crushing strength on mean grain size. Triaxial compression at high effective pressures produces compactive strain and a high AE rate associated with considerable particle-size reduction. Triaxial compression at low effective pressure produces dilatant granular flow accommodated by grain boundary frictional sliding and particle rotation. On the basis of experiment results, we predict the evolution of porosity and macroscopic yield strength as a function of depth for extensional and contractional basins. Sand strength increases linearly with depth for shallow burial, whereas for deep burial, strength decreases nonlinearly with depth. At subyield stresses, porosity evolves as a function of applied mean stress and is independent of distortional stress. Our predictions are in qualitative agreement with observations of microfracture density obtained from laboratory creep-compaction experiments and with natural sandstones of the Gulf of Mexico basin. Mechanical deformation contributes as much as a 30% increase to fluid pressure evolution, which has particular application to sedimentary systems that display zones of fluid overpressure. Furthermore, deformational strains cannot be fully recovered during uplift, erosion, and unloading of a sedimentary basin. Stephen Karner is a scientist at the Idaho National Laboratory and was previously a postdoctoral researcher at Texas A&M University. He received his B.Sc. (honors) degree from Flinders University of South Australia (1987), his M.A. degree from Queens College, City University of New York (1993), and his Ph.D. from the Massachusetts Institute of Technology (1999). His research interests include rock deformation, fault mechanics, granular mechanics, diagenesis, and geothermal energy.Judith Chester received a B.S. degree in geology from the University of California at Los Angeles and an M.S. degree and a Ph.D. in geology from Texas A&M University. She has been a faculty member at Texas A&M University for the past eight years. Her research focuses on fracture and faulting in the continental crust, specifically directed at understanding petroleum reservoir systems and the physics of earthquakes. Frederick Chester employs experimental rock deformation and field study to understand the brittle deformation of rock. He received a B.A. degree in geophysics from the University of California at Santa Barbara and an M.S. degree in geology and a Ph.D. in geophysics from Texas A&M University. He has been on the faculty at Texas A&M for the past eight years. Andreas Kronenberg is an associate of the Center for Tectonophysics and the Ray C. Fish Professor in the Department of Geology and Geophysics, Texas A&M University. He received his B.S. degree in geology from the University of California at Los Angeles (1977) and his Ph.D. of geology from Brown University (1983). His research interests include the mechanical properties of Earth materials, structural geology, and mineral physics. Andrew Hajash received his B.S. degree in geology (1969) and his M.S. degree in geophysics (1970) from Florida State University. He received his Ph.D. in geology (1975) from Texas A&M University and has been on the faculty there for the past 29 years. His research centers on experimental water-rock interactions in open systems, including the chemical and mechanical aspects of solution-transfer creep.