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192   ROCK PHYSICS ANALYSIS OF SHALE RESERVOIRS

            45° to the bedding plane (e.g., Vernik and Nur, 1992) or on   with  TOC content and maturity indicator measurements
            one sample in at least three directions (e.g., Dewhurst and   such as hydrogen index (HI) and vitrinite reflectance (R ).
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            Siggins, 2006; Wang, 2002).                          Bocangel et al. (2013) reported dynamic elastic  moduli,
              These studies of source rocks established a foundation for   TOC content, and maturity of the  Wolfcamp Shale from
            the recent studies of ORSs as reservoir rocks but certainly   Midland Basin. Patrusheva et al. (2014) studied static and
            did not answer all the questions that these challenging uncon­  dynamic moduli of the Mancos Shale with known  TOC
            ventional reservoirs raise.  The ultimate ambitious goal of   content. A number of authors (e.g., Dewhurst et al., 2008;
            rock physics is to predict physical properties of overburden   Hornby, 1998; Johnston and Christensen, 1995; Wang, 2002)
            and reservoir rocks from their seismic response with at least   studied shales as seals with no relation to the organic content.
            a few well points (Avseth et al., 2005). ORSs play all roles in   These ultrasonic measurements are performed on dry and
            the unconventional reservoirs, sometimes serving simulta­  saturated shales, drained or with controlled pore pressure.
            neously as reservoir, seal, and source rocks, and must be   Some  of  these  shales  still  comprise  significant  content  of
            comprehensively investigated for key properties such as   organic matter  but without sufficient  information on its
            VTI anisotropy, velocity–porosity and porosity–permeability   fraction, texture, or maturity. Comparison of rock physics
            relations, fracture‐induced azimuthal anisotropy, and so on.   attributes of the organic‐rich and organic‐lean seal shales is
            Establishing  correlations  of seismic  velocities,  VTI,  HTI   interesting as these shales exhibit similar rock physics prop­
            and orthorhombic anisotropy, attenuation and other seismic   erties, namely, they are highly anisotropic, almost imperme­
            attributes with total organic carbon (TOC) content, organic   able, and might be a source of abnormal pore pressure.
            matter maturity, hydrocarbon saturation, and permeability is   Here we bring together some published data on ultrasonic
            practically important and controlled laboratory rock physics   experiments on shales. We complement the ORS database
            experiments are indispensable here.                  with ultrasonic velocities obtained at different saturation
              This chapter reviews major developments in rock physics   conditions as they can shed some light on the effects of sat­
            of ORSs and indicates the main outstanding questions. First,   uration on elastic properties of shales as well as on the effects
            we bring together available published laboratory measure­  of variations in inorganic matrix mineralogy on elastic prop­
            ments on shales including those shales whose TOC content   erties of ORSs. Table 9.1 contains published information on
            is unknown. Second, we review experimental and theoretical   saturation state,  TOC content, and maturity indicators of
            studies of anisotropic elastic properties of ORSs in connec­  shales used in this study.
            tion with their TOC fraction, partial saturation, and maturity.
            We do not specifically look at the shale microstructure and
            how it relates with the maturation state of the shale as this   9.3  ORGANIC MATTER EFFECTS
            topic is covered in another chapter of this book. However,   ON ELASTIC PROPERTIES
            we pay special attention to the effects of microstructure and
            maturity on elastic parameters of ORSs. Then application of   The ORS dataset covers a broad range of  TOC fractions
            the findings of rock physics modeling for predicting the   (from 0 to 20.1%) and maturity levels (HI = 1 ÷ 692 and
            seismic response of ORSs will be assessed from recent   R   = 0.38 ÷ 3.5%). Elastic moduli broadly decrease with
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            seismic surveys. Finally, an attempt to estimate orientation   the increase of TOC content (Fig. 9.1). The fact that ORSs
            of vertical fracture sets permeating Bakken Shale from   are strongly anisotropic and the  Thomsen’s anisotropy
            amplitude versus offset and azimuth (AVOAz) data will be   parameters broadly increase with the increase of kerogen
            discussed.                                           volumetric fraction (Fig. 9.1) was also pointed out by Vernik
                                                                 and coauthors (Vernik, 1994;  Vernik and Landis, 1996;
                                                                 Vernik and Liu, 1997; Vernik and Nur, 1992). This strong
            9.2  LABORATORY MEASUREMENTS                         anisotropy of  ORSs was  explained with  the fact  that the
            ON SHALES: AVAILABLE DATASETS                          kerogen forms lenticular beds in inorganic matrix and might
                                                                 be  or  not  be  load  bearing  depending  on  its  fraction  and
            Controlled laboratory measurements on ORS samples are     maturation degree.
            crucial as, initially they link velocities in shales with their   A number of theoretical approaches for quantitative mod­
            TOC content and maturity and, secondly, they are the best (if   eling of elastic properties of shales have been developed
            not the only) way to verify theoretical predictions. Despite   (e.g., Carcione et al., 2011; Sayers, 2013a; Vernik and Nur,
            their ubiquity and two decades of research, shales remain the   1992). These models tackle the problem of how the TOC
            least experimentally studied sedimentary rocks. To the best   content affects elastic properties of ORSs. The answer to this
            of our knowledge, the extensive research of Vernik and coau­  question is not trivial. Hashin and Shtrikman upper and
            thors (Vernik, 1993; Vernik and Liu, 1997; Vernik and Nur,   lower bounds (Hashin and Shtrikman, 1963) give a range of
            1992) comprises most of the published laboratory ultrasonic   elastic moduli of mixture if the microstructure of the constit­
            measurements on organic‐rich shales that are complemented   uents is not known. But this range for ORS is quite broad as
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