Page 212 - Fundamentals of Gas Shale Reservoirs
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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 ).
0
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
0
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