Nanoscale porosity in SAFOD core samples (San Andreas Fault)

https://doi.org/10.1016/j.epsl.2010.10.040Get rights and content

Abstract

With transmission electron microscopy (TEM) we observed nanometer-sized pores in four ultracataclastic and fractured core samples recovered from different depths of the main bore hole of the San Andreas Fault Observatory at Depth (SAFOD). Cutting of foils with a focused ion beam technique (FIB) allowed identifying porosity down to the nm scale. Between 40 and 50% of all pores could be identified as in-situ pores without any damage related to sample preparation. The total porosity estimated from TEM micrographs (1–5%) is comparable to the connected fault rock porosity (2.8–6.7%) estimated by pressure-induced injection of mercury. Permeability estimates for cataclastic fault rocks are 10 21–10 19 m2 and 10 17 m2 for the fractured fault rock. Porosity and permeability are independent of sample depth. TEM images reveal that the porosity is intimately linked to fault rock composition and associated with deformation. The TEM-estimated porosity of the samples increases with increasing clay content. The highest porosity was estimated in the vicinity of an active fault trace. The largest pores with an equivalent radius > 200 nm occur around large quartz and feldspar grains or grain-fragments while the smallest pores (equivalent radius < 50 nm) are typically observed in the extremely fine-grained matrix (grain size < 1 μm). Based on pore morphology we distinguish different pore types varying with fault rock fabric and alteration. The pores were probably filled with formation water and/or hydrothermal fluids at elevated pore fluid pressure, preventing pore collapse. The pore geometry derived from TEM observations and BET (Brunauer, Emmett and Teller) gas adsorption/desorption hysteresis curves indicates pore blocking effects in the fine-grained matrix. Observations of isolated pores in TEM micrographs and high pore body to pore throat ratios inferred from mercury injection suggest elevated pore fluid pressure in the low permeability cataclasites, reducing shear strength of the fault.

Research Highlights

► Transmission electron microscopy (TEM) was used to describe nanometer-sized pores in SAFOD core samples. ► The total porosity estimated from TEM micrographs ranges between 1 and 5%. ► BET and mercury injection data indicate low permeability and pore blocking effects.

Introduction

The mechanical behavior of faults depends strongly on the interplay of fluids and damaged fault rocks (Hickman, 1991, Hubbert and Rubey, 1959). Local variation of porosity and fault zone permeability may influence fluid flow and effective pressure, affecting fault mechanics (e.g. Blanpied et al., 1992, Byerlee, 1993, Janssen et al., 2004, Rice, 1992, Schulz and Evans, 1998, Sibson et al., 1975). Laboratory studies and observations of exhumed fault zone rocks indicate that porosity and permeability reduction by compaction or fracture healing may induce high pore fluid pressure, influencing faulting and fault stability (e. g. Faulkner and Rutter, 2001, Hickman et al., 2007, Rice, 1992). Although studies of exposed fault rocks continue to provide important results about the interaction between porosity, fluid flow and fluid pressure, the available information is limited because exhumed fault rocks were altered during exhumation, obscuring fault-related mineral assemblages and textures (Solum and van der Pluijm, 2004).

Core samples from the San Andreas Fault Observatory at Depth (SAFOD) borehole provide a unique possibility to study the microstructures of fresh fault rocks of an active plate-bounding fault from seismogenic depth. A first microstructural study of SAFOD core samples yielded porosity values of 0–18%, with an average porosity of 3% for less deformed shale (Blackburn et al., 2009). Unfortunately, the interpretation of pore origin remains difficult because the applied methods (SEM combined with image-processing, using thresholding techniques) did not allow to distinguish between porosity formed in-situ and pore space formed during core recovery and sample preparation (see also Desbois et al., 2009). To our knowledge permeability data of SAFOD core samples is not yet available.

Here, we present an analysis of submicron pores. Since pores with diameters < 1 μm are not visible in optical thin sections we used transmission electron microscopy (TEM) imaging. In addition, common techniques of porosity determination, such as mercury porosimetry or the BET gas adsorption methods, were used to measure the connected rock porosity, pore volume and pore surface areas of our samples. Porosity data were used to estimate permeability. Different pore types are related to sample mineralogy and fabric. Porosity, permeability and pore structure data (i.e. surface area, pore size distribution and pore volume) are used to characterize pore spaces. We discuss the results in terms of fault evolution and compare our observations with those on core material from the Chelungpu Fault drilling Project (TCDP) in Taiwan (e.g. Song et al., 2007) and the Nojima Fault drilling program in Japan (e.g. Shimamoto et al., 2001).

Section snippets

Geological setting

The San Andreas Fault (SAF) is a 1.300 km-long transform fault forming the boundary between the northwestward moving western Pacific plate and the eastern North American Plate (Fig. 1). The SAFOD drill site is located in central California at the transition between the creeping segment of the SAF to the North and the Parkfield segment (Fig. 1a). The geology of the SAFOD drill site (Fig. 1b) is characterized by the presence of arkosic sedimentary rocks on the southwestern side of the fault and

Samples

We analyzed microstructures of four samples from SAFOD phase III cores (S1, S2, S3 and S4; see also Photographic Atlas of the SAFOD Phase 3 Cores 2010, for detailed descriptions of cores). The samples were recovered from different core sections located close to or at some distance to zones of active deformation (Fig. 1c). The mineralogical composition of all samples is documented in Table 1. All depth reported for our samples are measured depth (MD) and be synchronized to the Phase 2

TEM

TEM was performed using a FEI Tecnai G2 F20 X-Twin transmission electron microscope (TEM/AEM) equipped with a Gatan Tridiem energy filter, a Fishione high-angle annular dark field detector (HAADF) and an energy dispersive X-ray analyzer (EDX). In general, contrast in HAADF images depends on chemical composition (Z-contrast imaging) and sample thickness. Porosity is always imaged as dark contrast. In TEM bright field images porosity is imaged as bright contrast because of absent diffraction

TEM observations of pores

Pore space is commonly subdivided into primary and secondary porosity (Choquette and Pray, 1970). Primary porosity results from depositional voids between grains and particles and secondary porosity forms during burial and diagenesis due to dissolution and/or fracturing. Here, we distinguish (1) four in-situ pore types (I–IV) describing pore spaces likely formed during deformation of the samples but prior to coring and (2) two pore types (V–VI) with unclear origin. Apparently, one part of

Discussions and conclusions

In spite of significant differences in the measured mass of TEM (ng) and MIP samples (1.5 g), the porosity estimates from TEM images and MIP are in close agreement. This suggests that TEM micrographs yield a representative image of microstructures and porosity. For sample S1, TEM based porosity estimates are likely too small due to the presence of larger pores not adequately represented in the TEM micrographs.

The significant adsorption–desorption hysteresis loops in BET isotherms for samples

Acknowledgements

We thank Andreas Hendrich for helping with the drafting of figures, Stefan Gehrmann for sample preparation, Rudi Naumann for XRD analyses and Anja Schreiber for TEM foils preparation using FIB technique. This work was funded by DFG grant JA 573/4-1. Ben van der Pluijm and an anonymous reviewer provided very constructive comments and suggestions that helped improve this paper. Special thanks are addressed to the SAFOD science team for sampling and support.

References (63)

  • J. Byerlee

    Model for episode flow of high-pressure water in fault zones before earthquake

    Geology

    (1993)
  • BlackburnE.D. et al.

    A microstructural study of SAFOD gouge from actively creeping San Andreas Fault Zone; implications for shear location models

    EOS Trans. AGU

    (2009)
  • M.L. Blanpied et al.

    An earthquake mechanism based on rapid sealing of faults

    Nature

    (1992)
  • BoullierA.M. et al.

    Microscale anatomy of the 1999 Chi-chi earthquake fault zone

    Geochem. Geophys. Geosyst. (G3)

    (2009)
  • K.K. Bradbury et al.

    Mineralogical and textural analyses of drill cuttings from the San Andreas Fault Observatory at Depth (SAFOD) boreholes: initial interpretations of fault zone composition and constraints on geological models

    Geospehere

    (2007)
  • S. Brunauer et al.

    Adsorption of gases in multimolecular layers

    J. Am. Chem. Soc.

    (1938)
  • T.-M.N. Chen et al.

    Laboratory characterization of permeability and its anisotropy of Chelungpu Fault rocks

    Pure Appl. Geophys.

    (2009)
  • P.W. Choquette et al.

    Geologic nomenclature and classification of porosity in sedimentary carbonates

    AAPG Bull.

    (1970)
  • DesboisG. et al.

    Imaging porosity at pore scale in SAFOD samples using the combination of Ar-beam sectioning and SEM microscopy: preliminary results

  • Desbois, G., Urai, J.L., Kukla, P.A., 2009. Morphology of pore space in clay stones — evidence from BIB/FIB ion beam...
  • DibbleeT.W. et al.

    Regional geologic map of San Andreas and related faults in Carrizo Plain, Temblor, Caliente and La Panza Ranges and vicinity, California: a digital database

  • D.R. Faulkner et al.

    Can the maintenance of overpressured fluids in large strike-slip fault zones explain their apparent weakness?

    Geology

    (2001)
  • GratierJ.L. et al.

    Pressure solution as a mechanism of creep and sealing in active faults: evidence from the SAFOD samples

    EOS Trans. AGU

    (2009)
  • S.J. Gregg et al.

    Adsorption, Surface Area and Porosity

    (1982)
  • HickmanS.

    Stress in the Lithosphere and the strength of active faults

    Rev. Geophys.

    (1991)
  • HickmanS. et al.

    Stress orientations and magnitudes in the SAFOD pilot hole

    Geophys. Res. Lett.

    (2004)
  • S. Hickman et al.

    Structure and properties of the San Andreas Fault in Central California: recent results from the SAFOD experiment

    Sci. Drilling

    (2007)
  • HickmanS. et al.

    Structure and composition of the San Andreas Fault in central California: recent results from SAFOD sample analyses

    EOS Trans AGU

    (2008)
  • S. Hickmann et al.

    Introduction to special section: preparing for San Andreas Fault Observatory at Depth

    Geophys. Res. Lett.

    (2004)
  • HoldsworthR.E. et al.

    The microstructural character and evolution of fault rocks from the SAFOD core and potential weakening mechanisms along the San Andreas Fault

    EOS Trans. AGU

    (2009)
  • M.K. Hubbert et al.

    The role of fluid pressure in the mechanics of overthrust faulting I: mechanics of fluid-filled porous solids and its application to overthrust faulting

    Geol. Soc. Am. Bull.

    (1959)
  • Cited by (0)

    View full text