SAFOD: 2004 sample analysis workshop Report to NSF; November 2004 James P. Evans, Dept. of Geology, Utah State Univ., Logan, UT 84322-4505, jpevans@cc.usu.edu Ben van der Pluijm, Dept. of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, vdpluijm@umich.edu Teng-fong Wong, Department of Geosciences, State University of New York at Stony Brook Stony Brook, NY 11794-2100, Teng-fong.Wong@stonybrook.edu (back to main site)

Executive summary

The NSF-sponsored workshop on sample analysis from the SAFOD core, cuttings, and fluids was held Oct. 8-9, 2004 in San Jose, California. About thirty scientists representing US academic institutions, the US Geological Survey, several foreign institutions and industry attended to craft a set of recommendations regarding the analyses and handling of rock and fluid samples, and to foster communication among researchers in this topic area. We present a summary of the workshop outcomes to personnel at the NSF and to the general research community. This summary includes suggested scientific projects and a roadmap for future conferences and efforts to maximize what we may learn from SAFOD.

Introduction

The scientific objective of the San Andreas Fault Observatory at Depth (SAFOD) portion of EarthScope is to directly study the physical and chemical processes that control deformation and earthquake generation within an active plate-bounding fault zone. The SAFOD drill site is located on a segment of the San Andreas Fault that moves through both aseismic creep and repeating microearthquakes. A central component to the SAFOD project is the sampling and analysis of fault rock, which offers a unique opportunity to study natural fault materials from a continental seismogenic zone. Questions that can be addressed from the study of natural fault rock included in the EarthScope proposal were:

• What are the mineralogy, deformation mechanisms, and constitutive properties of the fault rocks?
• Why does the fault creep?
• What are the strength and frictional properties of recovered fault rocks at realistic in-situ conditions of stress, fluid pressure, temperature, strain rate, and pore fluid chemistry?
• What determines the depth of the shallow seismic to aseismic transition?
• What is the nature and extent of chemical water-rock interaction and how does this effect fault zone rheology?
• What are the composition and origin of fault-zone fluids and gasses?
• Is fluid chemistry relatively homogeneous, indicating pervasive fluid flow and mixing, or heterogeneous, indicating channelized flow and/or fluid compartmentalization?
• What are the physical properties of fault zone materials and country rock (seismic velocities, electrical resistivity, density, porosity)?
• How do physical properties from core samples and downhole measurements compare with properties inferred from surface geophysical observations?

The workshop on analysis of rocks and fluids from SAFOD was held in San Jose, California, October 8-9, 2004, and brought together 28 active researchers from a variety of fields who have committed to work on or have expressed an interest in the study of San Andreas fault rock. Given overlapping interests and approaches and the time line for coring and fluid sampling activities in 2005 and 2007, we felt it is critical to coordinate/develop a research approach that complements individuals’ PI activities, that strengthens the communications between the project managers and the science community, that avoids duplication of effort, that identifies possible gaps in what science might be proposed for rock and fluid analysis, and that creates an environment to foster new proposed work in the upcoming years.

We thank the National Science Foundation for support for this workshop, and we thank Steve Hickman and Mark Zoback for their time spent at the workshop. We also congratulate the project managers for successfully completing Phase 1 of the project, and for successfully retrieving ~20 m of core from the borehole that will be used for preliminary study.

The workshop achieved its original objectives, and planned for the future. We visited the SAFOD drill site [Figure 1] and had the opportunity to examine effects from the 28 Sept. M 6.0 Parkfield earthquake. We developed a list of research topics critical to addressing the scientific questions outlined above, we discussed in detail operational aspects of drilling, coring, and fluid sampling, and we helped foster communication among researchers who have been funded or are seeking funding to examine rocks and fluids from SAFOD.

Scientific Objectives Developed by Workshop Participants

The workshop broke out into two research interest groups to define specific objectives and operational suggestions for the handling and analyses of the core, cuttings and fluids from SAFOD. These groups were loosely categorized as the geochemistry/fluids group and the physical properties group, and we met in plenary session to discuss the outcomes and suggestions for activities. Below is an unranked list of topics that the group felt should be examined with the core, cuttings and fluids. The analyses that workshop participants felt would help answer the questions enumerated above include:

Characterization on Site or Near Real-time Off-site

· Mineralogy (XRD) and petrography (optical) of cuttings and core

· Downhole petrophysical measurements (e.g., Logging While Drilling)

· In-situ measurements of fluid pressure and permeability

· Mesostructural core description (e.g., basic lithology, core condition, fluid content, fracture orientation, density, distribution, cross-cutting relationships; surface markings; veins and vein fillings; foliations)

· Multi-sensor track physical property logging (natural gamma logs)

· Bulk magnetic susceptibility

· Stress relaxation measurements

· Continuous sampling for fluid chemistry (major ions)

Characterization in Laboratory

· Core reorientation (from magnetic remanence methods and/or FMI logs)

· Microstructural properties (including particle- and pore-size distribution, and textural analyses, microstructures, nature of mineral distribution); possibly CT scan on selected cores

· Density and porosity properties

· Thermal and electrical properties

· Magnetic properties (anisotropy of susceptibility, magnetic mineralogy)

· Frictional strength and rheological properties

· Permeability and poromechanical properties

· Seismic velocities and anisotropy; anelastic and attenuation behavior

· Detailed core description (building on on-site description)

· Elemental analysis (ICP) of bulk rock and minerals

· Fluid chemistry (minors, stable isotopes); fluid inclusion in vein minerals (microthermometry, chemical analysis and structural control)

· CO₂, CH₄, noble gas analyses (from pressurized fluid samples)

· Microbial activity and organic-C analysis

· Vein and fluid inclusion chemistry (major and minor elements, stable isotopes)

· Dating of host minerals and fault rock (U/Pb. Ar, ESR, TL dating) – these might be experimental

The participants felt very strongly that a number of observations of core and sampling of fluids should be made at or near the drill site, and that close communication should occur between scientists responsible for sample characterization and those responsible for operations of SAFOD. Ideally, portions of a science team should be available at the drill site to make real-time observations. This will establish a baseline database regarding the condition of core in the state nearest to in situ conditions, make the critical observations regarding the nature of the sampled rocks, and will provide data to the drilling and coring operators that might be critical for operational decisions. An informal review of other drilling projects through fault zones shows that this might be an optimal plan for managing the samples and drilling operations. We feel this is critical for the success of the analytical portion of the SAFOD effort, and brings the geological and geochemical communities directly into the SAFOD project.

The workshop participants also provided feedback on the core and sample handling and subsequent analysis to Steve Hickman, and we anticipate that these will be communicated to NSF. The participants respect and appreciate the policy considering the interests of the general community. The participants also felt that on-site science team members responsible for initial observations are critical for adequate core handling and subsequent analyses. This arrangement would significantly enhance the “value” of the core and success of subsequent analyses.

The plan for the future

The workshop participants, its organizers, and SAFOD co-chief scientists Steve Hickman and Mark Zoback felt the workshop was highly successful in identifying critical observations to be made on core and fluids, developing a core handling procedure at the drill site and establish curation procedures, and helped foster communication among workers who will work on the core, cuttings and fluids. To build on the success of this first workshop and to continue in developing science teams to work on the materials, we feel there is a need to have a sample analysis workshop in Spring 2005, perhaps in conjunction with the EarthScope meeting in New Mexico. This will allow workers to present preliminary results from analyses, to identify gaps in work being done, to help formulate projects before the 2005 proposal submission round, to provide an open venue to allow scientists to present results and to open the project up to a broad range of investigators, and to demonstrate to the broader EarthScope community the progress of the SAFOD project to date.

List of participants

Conveners Jim Evans Utah State University Ben van der Pluijm University of Michigan Teng-fong Wong SUNY Stony Brook SAFOD Co-Chief Scientists Steve Hickman U. S. Geological Survey Mark Zoback Stanford University Participants Ann Blythe University of Southern California Anne-Marie Boullier Grenoble, France Roland Burgmann University of California, Berkeley Fred Chester Texas A&M University Judith Chester Texas A&M University Nick Christensen University of Wisconsin David Goldsby Brown University Laurel Goodwin University of Wisconsin

Miriam Kastner University of California, San Diego David Kirschner Saint Louis University David Lockner U. S. Geological Survey Diane Moore U. S. Geological Survey Tullis Onstott Princeton University Ze’ev Reches University of Oklahoma Ernest Rutter University of Manchester Toshi Shimamoto Kyoto University John Solum University of Michigan ShengRong Song (TCDP) National Taiwan University Hidemi Tanaka University of Tokyo Harold Tobin New Mexico Tech Tom Torgersen University of Connecticut Peter Vrolijk ExxonMobil Upstream Research Lawrence Warr University of Strasbourg

Figure 1. SAFOD core, cuttings and fluids workshop participants at the SAFOD site (October 8, 2004).

Last update: November 30, 2004