Reports: AC8
47221-AC8 Petrophysical Controls on Fault Damage Zone Width and Character: As Important as Displacement?
We are testing the hypothesis that variations in grain contact strength explain variations in fault damage zone characteristics in sandstones. We proposed to test this hypothesis in faulted, variably cemented, quartz-rich sandstones. Parameters such as porosity and cement mineralogy, location, and quantity in country rock and damage zone samples will be related to grain contact strength through measurement of P and S wave velocities using ultrasonic methods and controlled confining and pore pressures in the laboratory. Compressional and shear velocities can be used to directly compute elastic moduli that are closely related to grain contact strength in porous sedimentary rocks. The advantage of this approach is that it provides a non-destructive method of detecting framework strength for even relatively weak grain contacts. If appropriate upscaling methods are applied, these ultrasonic velocity data also can provide a link between the results of this study and exploration data such as sonic logs and seismic reflection studies of reservoirs.
Because we were unable to identify an appropriate student for this research project until nearly a year into the funding period, we have been working on it for just over a year. Progress includes training and development of our graduate student and an undergraduate assistant (Samuel Harms), completion of our field study, and refinement of our research strategy. During the 2008-2009 academic year, M.S. student John Schneider divided his time between class work and research, ending the year with a 3.85 GPA. He completed courses in chemistry, rock mechanics, and advanced structural geology as well as a range of seminars in geophysics, upper crustal deformation, and the tectonic history and geology of the San Andreas fault, which culminated in a 9-day field trip. He also attended a short course on interpretation of seismic data run by a cohort of petroleum geologists in Fall, 2008. Research included a literature search in addition to time spent learning to use equipment needed to prepare a suite of samples collected in August, 2008 from Teapot Dome, Wyoming that will ultimately be used for ultrasonic velocity, porosity, and microstructural analyses. PI Goodwin presented some preliminary results of this field study at the Geological Society of America annual meeting in Houston in October, 2009.
In late June, John and PIs Goodwin and Tobin visited a range of potential field sites and selected several fault zones in southern Utah for detailed study. John subsequently worked for several weeks collecting samples and data on fault damage zone widths and the character and spacing of minor faults within damage zones at these sites. One key site, the Big Hole fault, is a deformation band fault zone that contains two generations of deformation bands. The older structures show no evidence for grain size reduction, and we infer that they were formed prior to lithification; the younger structures are shear deformation bands, in which grain and pore size is demonstrably reduced. The Moab fault and a smaller, unnamed fault juxtapose sandstones with highly variable clay content; both individual deformation bands and the damage zone are thinner where clay content is higher, but deformation band density is greater in beds with less clay. Collectively, damage zone structures in both Wyoming and Utah show greater variability than anticipated, leading us to expand our research plan to include ultrasonic velocity measurements of these different structures in addition to undeformed host rock.
John is currently in Houston, spending the Fall, 2009 semester as an intern at Chevron, temporarily slowing our research effort. We requested and received a one-year, no-cost extension to our grant, and expect to request one more extension to accommodate John’s industry experience.
