Page 147 - Fundamentals of Gas Shale Reservoirs

WELL LOG ANALYSIS OF GAS SHALE RESERVOIRS 127
The density of the organic matter is low (typically limestone and dolomite, but they should be clearly distin­
1.1–1.4 g/cm ) compared to the matrix density (2.6–2.8 g/ guished using (the) GR log. It is believed that organic‐rich
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cm ) of the shale layers. Due to this low density value, the shales have low photoelectric values compared to normal
presence of organic matter can decrease the measured bulk shales due to the low PEF values of kerogen (Boyer et al.,
density of the formation. Moreover, high levels of gas 2006), but there are many mineralogical complexities, and
content can reduce the bulk density of the gas shale layers. tracking PEF changes versus organic matter is not possible
The presence of pyrite (FeS ) and siderite (Fe CO ) found in most of the time.
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the organic‐rich shale can elevate formation density. The
most favorable environments for siderite formation are 6.4.1.6 Sonic Log The sonic log measures the speed of
reducing freshwater systems (potential environment for ker­ sound waves in rocks. Numerical studies suggest that in situ
ogen type I), while pyrite commonly occurs in marine sedi­ rock parameters such as mineral composition and TOC, as
ments (potential environment for kerogen type II) (Passey well as the interaction among them, can significantly
et al., 2010; Lim et al., 2004). influence the sound wave velocities of the organic‐rich
The density log can give a qualitative indication for esti­ rocks. The presence of organic matter in gas shale rocks
mating the thermal maturity of the gas shale layers as well reduces both the density and the compressional and shear
(Labani and Rezaee, 2012). For example, in some wells of wave velocities, and hence the acoustic impedance, while
the Perth Basin, there is a decreasing trend for density log increasing the velocity anisotropy (Zhu et al., 2011). Besides
responses with increasing thermal maturity in the potential that, the presence of gas and high clay‐bound water, which is
gas shale layers. Although this relationship is not so strong, common in shales, can decrease the sonic wave velocity.
it seems compatible with what occurs in the gas shale layers The main application of acoustic measurements for gas
during thermal maturity evolution. By increasing thermal shale evaluation is to provide (the) mechanical properties for
maturity in the organic‐rich shale layers, the following gas shale reservoirs. Full waveform sonic log (shear and
changes may happen: compressional) can be used for determining the Poisson’s
ratio, Young’s modulus, shear modulus, bulk modulus, yield
• Changes in the type of saturated fluid from brine to gas strength, and compressive strength, all of which are impor­
• Changing of the heavier components of hydrocarbon tant for determining the brittle shale intervals (i.e., favorable
into the lighter ones and finally dry gas intervals for hydraulic fracturing) (Grieser and Bray, 2007;
• Generation of porosity in the organic matter due to its Alexander et al., 2011). Cross‐dipole shear sonic log can be
used for determining velocity anisotropy of the gas shale
thermal transformation (Loucks et al., 2009) formations. Velocity anisotropy is an important parameter
• Increase in pore pressure due to mineral transformation that is of interest in geomechanical applications related to
(smectite to illite) and hydrocarbon generation reservoir characterization. A high level of velocity anisot­
ropy is primarily due to the lenticular distribution of kerogen
All of these transformations can result in decreasing density and preferred orientation of clay mineral parallel to the
of the formation with increasing thermal maturity. It is worth bedding plane (Vernik and Milovac, 2011). Velocity anisot­
mentioning that sometimes the use of the density log and ropy can give an idea of the formation permeability due to
NPHI log as a thermal maturity indicator is not possible. For the higher crack density accompanied with the laminated
example, the presence of heavy minerals could increase the organic matter.
density and hide the decreasing effect of thermal maturity.
Therefore, it could be said that conventional logs can only 6.4.1.7 Pulsed Neutron Mineralogy Log Petrophysical
be used for thermal maturity estimation if the lithology of evaluation of unconventional reservoirs mainly depends on
the formation does not vary significantly over the interval determining the mineralogy of the shale layers. The pulsed
of interest. neutron mineralogy tool is a kind of unconventional log for
determining the mineralogy of the formation. This tool,
6.4.1.5 Photoelectric Factor Log Photoelectric factor accompanied with a natural GR spectroscopy tool, can deter­
(PEF) is a kind of density tool that measures the PEF mine the concentrations of elements available in the matrix
absorption of a formation. The photoelectric absorption of the gas shale layers including aluminium, carbon, calcium,
index is used principally for lithological determination. This iron, gadolinium, potassium, magnesium, sulfur, silicon,
log is mainly controlled by mean atomic number of the thorium, titanium, and uranium (Pemper et al., 2006). Each
formation. It is slightly influenced by formation porosity; mineral in the matrix requires a specific amount of each
however, the effect is not enough to hinder correct matrix element based on stoichiometry. Currently, the following
identification when dealing with simple lithologies (one‐ minerals can be quantified by pulsed neutron mineralogy
mineral matrix). PEF log is little affected by the fluid in the along with the spectral GR: illite, smectite, kaolinite, chlorite,
pores. Shales have photoelectric values somewhere between glauconite, apatite, zeolites, halite, anhydrite, hematite,