Nanoscale porosity in SAFOD core samples (San Andreas 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.
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