SPWLA 53rd Annual Logging Symposium, June 16–20, 2012
A NEW CAPTURE AND INELASTIC SPECTROSCOPY TOOL TAKES
GEOCHEMICAL LOGGING TO THE NEXT LEVEL
R. J. Radtke, Maria Lorente, Bob Adolph, Markus Berheide, Scott Fricke, Jim Grau, Susan
Herron, Jack Horkowitz, Bruno Jorion, David Madio, Dale May, Jeffrey Miles, Luke Perkins,
Olivier Philip, Brad Roscoe, David Rose, and Chris Stoller, Schlumberger
(Odom et al., 2008), and innovating on a traditional
design (Galford et al., 2009).
Copyright 2012, held jointly by the Society of Petrophysicists and Well Log
Analysts (SPWLA) and the submitting authors.
This paper was prepared for presentation at the SPWLA 53rd Annual Logging
Symposium held in Cartagena, Colombia, June 16-20, 2012
Existing spectroscopy tools suffer from several
technical shortcomings. The first of these is
measurement precision, which is inversely proportional
to the square root of logging speed. The precision of
measured elements is a function of count rate, and
currently the maximum count rate is limited by the
available scintillator, photomultiplier tube (PMT), and
electronics technologies. Precision and accuracy of
elemental concentrations are also compromised at
borehole temperatures because, for the scintillation
detectors currently in use, the spectral resolution and
performance degrade rapidly with increases in
temperature. Tools using a BGO detector, for example,
require some form of downhole temperature control and
have a limitation of time at temperature. Lastly, systems
using PNGs must cleanly separate the capture and
inelastic spectra, which can be very challenging.
ABSTRACT
The increasing complexity of today’s reservoirs
demands an accurate understanding of formation
composition and mineralogy. This is particularly true
for unconventional reservoirs, in which quantification
of both mineralogy and organic carbon is critical for
resource
evaluation.
The
new
geochemical
spectroscopy tool described here combines the
advantages of inelastic and capture gamma ray
spectroscopy, opening new avenues for detailed
description of complex reservoirs. Capitalizing on
advances in technology, the new service provides
higher precision and improved accuracy for the analysis
of key elements in rock formations and simultaneously
offers a standalone quantitative determination of total
organic carbon (TOC). The measurements are offered at
faster logging speeds. Eliminating the americiumberyllium (241AmBe) radioisotopic source makes
combination with traditional measurements a much
more attractive and viable logging option for both
conventional and unconventional markets.
These shortcomings are largely overcome by the tool
described in this paper. The new tool improves the
precision and accuracy of all elements traditionally
measured by capture spectroscopy. In addition, it
successfully integrates capture and inelastic gamma ray
spectroscopy, making it possible to measure carbon
accurately, account for inorganic carbon from carbonate
minerals, and determine total organic carbon (TOC),
which is essential for the evaluation of many
unconventional plays such as shale gas and shale oil.
Merging capture and inelastic data also significantly
improves precision, accuracy, and interpretation
consistency. This is particularly true for magnesium, a
key element for differentiating calcite from dolomite.
The tool’s measurements, including TOC, do not
require a calibration to core or the use of complex,
local, empirical, single- or multiple-tool interpretation
models.
INTRODUCTION
Geochemical logging was introduced over 30 years ago,
starting with a wireline tool based on a pulsed-neutron
generator (PNG) and a thallium-doped sodium iodide
(NaI(Tl)) scintillation detector (Hertzog, 1980). The
emergence of new scintillation detectors led to tools
based on gadolinium oxyorthosilicate (GSO) (Scott et
al., 1991) and bismuth germanate (BGO) (Herron and
Herron, 1996a), both of which are still in active use.
Ruggedization of PNG technology enabled the
introduction of nuclear spectroscopy to logging-whiledrilling (LWD) environments (Weller et al., 2005).
Recent developments include combining capture and
inelastic spectroscopy (Pemper et al., 2006; Herron et
al., 2011), introducing new scintillation detectors
This paper provides details on the technology that
makes these measurement breakthroughs possible and
shows several field examples highlighting the
performance.
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SPWLA 53rd Annual Logging Symposium, June 16–20, 2012
TOOL DESCRIPTION
The scintillator also produces much more light per
incident photon than other materials. The light yield is
approximately 50% larger than NaI(Tl), which has
historically been considered the benchmark for high
light output in a scintillator. High light output translates
directly into improved spectral resolution, because the
counting statistics associated with the light collected
from the scintillator is one of the main factors that
determine resolution.
The basic layout of the tool is shown in Figure 1. The
outer diameter is 114 mm (4.5 in.), which is smaller
than that of previous generation openhole spectroscopy
tools. It is rated to 138 MPa (20,000 psi) and 175°C.
The measurement section contains a deuterium-tritium
(d-T) PNG, which emits 14-MeV neutrons. The
neutrons interact with the formation to produce gamma
rays that are detected by a scintillation detector.
Tungsten shielding reduces the direct passage of
neutrons and gamma rays from the generator to the
detector. The housing near the detector is surrounded
with a thermal neutron shield containing boron to
reduce the number of capture gamma rays produced
from the tool.
Table 1 Summary of some essential room temperature
performance parameters of scintillators used for
downhole spectroscopy applications
The combination of a state-of-the-art scintillation
detector, a high-output pulsed neutron generator, and a
very fast pulse processing system opens up new
possibilities in downhole spectroscopy logging. The
following sections provide details on the technological
advances that underlie the unprecedented tool
performance.
Property
NaI(Tl)
BGO
Density
(g/cm3)
3.67
7.13
5.29
Effective Atomic
Number
50.8
75.2
46.9
Primary Decay
Time (ns)
230
300
25
8.2
61
Light Yield
43
(photons/keV)
Source: Lecoq et al., 2006.
1.8
LaBr3:Ce
Relative light yield
1.6
Fig. 1 Measurement section of the spectroscopy tool.
1.4
1.2
NaI(Tl)
1
0.8
0.6
0.4
BGO
0.2
New Spectroscopy Detector - The tool uses a large
cerium-doped lanthanum bromide (LaBr3:Ce) gamma
ray detector (Van Loef et al., 2001; Saint Gobain
Crystals, 2006; Stoller et al., 2011), which is coupled to
a state-of-the-art high-temperature photomultiplier.
LaBr3:Ce is a very fast scintillator with high light
output and excellent spectral resolution. In addition,
LaBr3:Ce
has
outstanding
high-temperature
performance with only a minimal loss in light output
and resolution at temperatures up to 200°C.
LaBr3:Ce
0
0
50
100
150
200
Temperature (°C)
Fig. 2 Temperature dependence of scintillator light
yield for LaBr3:Ce, NaI(Tl), and BGO. LaBr3:Ce has
higher light output over the entire range of logging
temperatures, unlike NaI(Tl) and BGO.
Of great importance in a logging tool, the performance
of LaBr3:Ce is maintained at high temperatures. This
can be seen by comparing the light output of LaBr 3:Ce,
NaI(Tl), and BGO as a function of temperature (Figure
2). While the light output of BGO is already low at
25°C, it drops significantly with small temperature
increases. Above 60°C, the light output is too low to
make acceptable logging measurements. This makes it
necessary to use thermal protection in BGO-based
A comparison of the properties of LaBr 3:Ce with other
scintillators commonly used in downhole spectroscopy
applications is shown in Table 1. The decay time of the
light from the scintillator is an order of magnitude
faster than NaI(Tl) and BGO. The fast decay makes
very high counting rates possible and consequently
improves measurement precision.
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SPWLA 53rd Annual Logging Symposium, June 16–20, 2012
spectroscopy tools and limits the available time in most
boreholes for useful measurements. This limitation
excludes long-duration operations such as drillpipe
conveyance or tractoring. In contrast, LaBr3:Ce
maintains its high light output over the entire
temperature range.
clearly sharper and better defined. This allows any
measured spectrum to be more easily decomposed into
its component parts, resulting in improved precision of
spectroscopic answers.
As described previously, a decrease in light output
translates into undesirable spectral degradation. The
elemental standard spectra for BGO (Figure 3, orange
curves) show significant broadening and loss of
definition as temperature increases from room
temperature to the BGO maximum operating
temperature of 60°C. On the other hand, the standard
spectra from LaBr3:Ce (Figure 3, blue curves) are
almost identical between room temperature and the
much higher temperature of 150°C.
New-Generation Pulsed Neutron Generator - Neutrons
are produced by a new, high-performance d-T PNG
developed to meet the needs of this advanced
spectroscopy tool. The PNG eliminates the need for a
radioisotopic source such as 241AmBe, thus reducing
operational, transportation, and safety risks.
The high neutron output of the PNG (capable of 3 × 108
neutrons/s nominal and higher) enables the tool to make
the best use of the fast LaBr3:Ce scintillator by
providing a very high counting rate and consequently
better measurement precision and faster logging speeds.
This output is about a factor of 8 higher than that of
present radioisotope sources.
The PNG is designed to allow clean separation of
inelastic and capture gamma rays, improving the quality
of both measurements. A necessary condition for this
separation is a well-defined, repeatable neutron burst
shape, which enables optimum timing of the inelastic
and capture measurements and a capture measurement
uncontaminated by inelastic gamma rays. To ensure
that this condition is met, the PNG uses a hot cathodebased Minitron* neutron tube. The hot cathode
technology produces a crisp 8.0-µs burst with rise and
fall times faster than 400 ns (Figure 4). The timing of
the burst is stable and predictable, making it possible to
begin clean capture spectra acquisition very close to the
burst for maximum count rates.
Fig. 3 Elemental standard spectra measured with
LaBr3:Ce and BGO detectors at room temperature
(upper panel) and at higher temperatures (lower
panel).
The PNG has been designed for reliable operation over
extended periods at temperatures up to 175°C. The
technology builds on what was developed for the
successful LWD PNG (Weller et al., 2005). Extensive
test facilities were constructed and used to evaluate
multiple PNG design generations. To date, over 3,600
LaBr3:Ce has spectral resolution unmatched in a
downhole tool. An example is shown in the top panel of
Figure 3, where elemental standard spectra for
LaBr3:Ce are compared with those of BGO at room
temperature. The features in the LaBr3:Ce standards are
* Mark of Schlumberger.
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SPWLA 53rd Annual Logging Symposium, June 16–20, 2012
job-equivalent runs at maximum environmental
conditions have been completed with several dozen
PNG units. The extensive test results, accumulated over
several years of development, were used for continuous
design improvements to provide high reliability and
performance at tool introduction.
decomposition are called yields. The elemental
sensitivities relate the yields to the elemental weight
fractions which are of interest; these sensitivities are
largely a function of neutron capture cross section.
Fig. 4 Neutron burst shape used for spectroscopy.
Electronics - The combination of the fast LaBr3:Ce
scintillator and the high-output PNG produces a very
high counting rate, often in excess of 2,500,000
counts/s. Processing such fast signals requires the use
of specialized electronics. The electronics must provide
excellent spectroscopy performance and outstanding
pile-up rejection to limit spectral distortion due to
nearly coincident gamma ray arrivals.
Fig. 5 Schematic of the spectroscopy acquisition
system. Signals from the detector pass through an
integrator and analog-to-digital converter (ADC) and
are then processed to form the pulse-height spectrum.
The pulse height analyzer system that accomplishes
these tasks is shown schematically in Figure 5. Gamma
rays are detected in the large LaBr3:Ce scintillator
described previously, which is coupled to a hightemperature spectroscopy photomultiplier with an
integrated high-voltage supply and preamplifier. The
signals from the preamplifier enter an integrator. The
integrated signal is digitized by an analog-to-digital
converter (ADC) and processed to obtain the pulse
height, which is accumulated in a pulse height
histogram that provides a spectrum of counts versus
pulse height.
Characterization for this tool relies primarily on
extensive measurements in real and laboratory
formations. Many of the formations used were designed
specifically for geochemical applications. In addition to
the elements conventionally measured by capture (H,
Na, Cl, K, Ti, Cr, Ni, Ba, and Gd), inelastic (C, O), or
both types of spectroscopy (Mg, Al, Si, S, Ca, and Fe),
the characterization includes other elements, such as
Mn and several other metals (see Table 2). Figure 6
shows some of the formations used to measure
elemental standards and sensitivities.
Mathematical modeling has been employed to optimize
the design of the tool for both capture and inelastic
spectroscopy and to construct representative formations
for characterization. The modeling is based on the
industry-standard Monte Carlo N-Particle (MCNP)
transport code (Pelowitz, 2008). The MCNP code was
adapted to simulate spectroscopy measurements by
adding the capability to record detected gamma rays by
the isotope and nuclear reaction that created them and
Tool Characterization - To translate the spectra
acquired by the tool into petrophysically relevant
elemental concentrations, standards and sensitivities are
needed. Elemental standards represent the distinct
spectral signature of each element detected with the
tool. As described in the next section, measured spectra
can then be represented by a linear combination of the
standards after correction for environmental and
electronic factors. The coefficients from the
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SPWLA 53rd Annual Logging Symposium, June 16–20, 2012
by the region where they were produced. Converting
the gamma ray response to a pulse height spectrum is
accomplished through the GAMRES code (Evans,
1981), which was tailored with the measured nonlinear
light output of the LaBr3:Ce scintillator. The modeling
results are in very good agreement with measured
spectra. However, any simulation of this kind will
suffer from inaccuracies in even the most up-to-date
neutron-gamma cross sections and in realistically
simulating light collection in the scintillator, PMT
response, and pulse processing. Experimental
measurements for derivation of elemental standards and
sensitivities thus remain the procedure of choice for
obtaining unbiased spectroscopy answers.
Fig. 6 Some of the geochemical formations used in the
tool characterization.
Table 2 List of some elements that can be determined
through capture or inelastic gamma ray spectroscopy.
Element
Description
Capture
Inelastic
Al
Ba
C
Ca
Cl
Cu
Fe
Gd
H
K
Mg
Mn
Na
Ni
O
S
Si
Ti
Aluminum
Barium
Carbon
Calcium
Chlorine
Copper
Iron
Gadolinium
Hydrogen
Potassium
Magnesium
Manganese
Sodium
Nickel
Oxygen
Sulfur
Silicon
Titanium
√
√
√
√
√
√
√
√
√
√
√
√
√
√
√
√
√
√
√
√
Nuclear Interactions, Gamma Ray Measurements, and
Data Acquisition - The well-defined burst of the PNG
creates a population of high-energy (14-MeV) neutrons
around the tool which interact and moderate to thermal
energies on the scale of microseconds. These neutrons
induce the emission of gamma rays from nuclei in the
formation, borehole, and tool via two primary
interactions: inelastic scattering and thermal neutron
capture. In the case of inelastic scattering, which can
occur only above specific neutron energy thresholds, a
portion of the incident neutron energy is transferred to
the target nucleus, which de-excites by emitting gamma
rays at one or more characteristic energies. In thermal
neutron capture, neutrons near equilibrium temperature
are absorbed by a target nucleus, which de-excites by
emitting a different set of characteristic gamma ray
energies. These “fast” and “slow” interactions are
illustrated in Figure 7. The gamma rays themselves
undergo scattering as they travel toward the detector,
where they deposit some or all of their remaining
energy. The principle of the geochemical logging
measurement is that the total detected gamma ray
spectrum can be deconstructed based on each element’s
characteristic standard spectrum, as discussed below.
√
√
√
√
√
PHYSICS AND INTERPRETATION
Transforming the spectra acquired by the tool into
petrophysical quantities is an involved process. In brief,
neutrons are emitted from the PNG and produce gamma
rays. The pulse height spectra of the detected gamma
rays are recorded as a function of time relative to the
pulsing sequence of the PNG. Each spectrum is
decomposed into a linear combination of standard
spectra from individual elements. The coefficients of
the linear combination are converted to elemental
weight fractions, which may be further analyzed to
produce mineralogy. These steps are described in more
detail in the following sections.
Elements that are typically found in the downhole
environment exhibit a wide range of cross sections for
thermal neutron capture. One advantage of inelastic
interactions is that they are less affected by
environmental factors including the presence of
elements such as chlorine, which have high thermalcapture cross sections and can significantly reduce the
population of thermal neutrons available for capture by
elements of interest in the formation. Table 2 presents a
list of some of the elements that can be measured
through capture or inelastic gamma ray spectroscopy.
For some elements, capture and inelastic gamma rays
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SPWLA 53rd Annual Logging Symposium, June 16–20, 2012
are available, but both are not necessarily used or useful
for the determination of elemental concentrations.
Spectra-to-Yields Processing - The purpose of spectrato-yields processing is to obtain an accurate suite of
relative elemental yields based on decomposition of the
recorded neutron-induced gamma ray spectra. An
inherent assumption is that the measured spectra can be
correctly represented by a linear combination of known
elemental standard spectra. Given this assumption, the
desired yields can be produced by a linear weighted
least-squares (WLS) fit of the measured spectrum with
a set of standard spectra within a specified energy
range. However, temperature variations and electronics
effects cause spectral distortions, which must be
compensated for by transforming the measured
spectrum’s energy calibration to match that of the
standards. The energy resolution of the standards also
must be matched to that of the measured spectrum. This
methodology is formulated as a nonlinear least-squares
problem and solved utilizing the Levenberg-Marquardt
method (Gill et al., 1981; Grau and Schweitzer, 1987,
1989). With the measured spectrum at the proper
energy calibration and the standards adjusted to match
the resolution of the measured spectrum, the elemental
relative yields can be derived using a linear WLS
method.
Fig. 7 Conceptual sketches of the two primary neutrongamma interactions in geochemical logging, showing a
fast neutron undergoing inelastic scattering (top) and
thermal neutron capture (bottom)
The timing sequence of the PNG and its connection
with the spectral data is shown in Figure 8. The
inelastic spectrum is acquired during an 8.0-s-long
neutron burst in a time gate denoted SBUR. The capture
spectra come from early (SEAR) and late (SLAT)
capture gates immediately following the burst plus a
capture gate (STAU) that follows a series of 50 bursts.
The STAU gate is also used for a formation neutron
capture cross-section measurement (). This sequence
is repeated 62 times, after which a quiescent period of
approximately 8 ms is used to acquire a background
spectrum (SBKG) to measure radiation from the tool,
formation activation, and background radiation. The
roles of these gates in the processing are described in
the next section. The cycle takes 125 ms to complete
and is repeated 3 times to form each spectroscopy data
frame. The spectral data channels are recorded and used
by the acquisition system for gain regulation of the
PMT, then depth-gated and written out for further
processing.
Major benefits of the well-defined neutron burst are that
there is no contamination by inelastic reactions during
the “capture” gates and that capture data can be
collected very soon after the end of the burst.
Fig. 8 Timing sequence used for data acquisition.
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SPWLA 53rd Annual Logging Symposium, June 16–20, 2012
The spectra-to-yields processing is performed
independently on appropriate time gate spectra to
produce capture and inelastic relative yields. For
capture processing, spectra from the three capture gates
(SEAR, SLAT, and STAU) are summed to create a
total capture pulse height spectrum. The net capture
spectrum to be decomposed is obtained by subtracting
the background spectrum (SBKG) while properly
accounting for duty factors and counting losses. For
inelastic processing, the contributions from capture
must first be removed. This is accomplished by
subtracting the early capture gate (SEAR) from the
burst gate (SBUR) using a subtraction factor
determined from neutron-capture physics, duty factors,
and counting losses. The result is a net inelastic
spectrum and associated variance (Figure 8).
C and Mg are of particular value. An important
limitation of the inelastic physics is that its set of yields
is not complete enough to form a closure model.
Instead, the normalization factor that converts these
inelastic yields into elemental concentrations is derived
by requiring that the derived dry weights of some of the
elements, which occur in both inelastic and capture
measurements, are consistent. The method ensures that
all the resulting dry weights are mutually consistent for
elements with both capture and inelastic yields. As a
consequence the analysis gains the enhanced precision
of the inelastic Mg measurement and achieves better
accuracy overall.
A second benefit of the inelastic analysis is the
determination of a total carbon concentration. By using
common association factors for carbonate minerals, the
amount of inorganic carbon present can be quantified
and subtracted from the total inelastic carbon to
compute TOC (Herron and LeTendre, 1990). The
combined analysis of inelastic and capture
measurements is part of the generational breakthrough
represented by this tool.
By definition, the elemental yields are “relative” in that
the sum of either the capture or the inelastic yields for
each spectrum is separately equal to unity. Elemental
relative yields are a function of the volumetric
proportion of an element in the measurement region, as
well as the sensitivity of the tool to each element. These
yields are the starting point for determining quantitative
elemental concentrations and mineralogical volumes.
Dry-Weight Elements to Minerals - Mineralogy and/or
lithologic fractions can be derived from elemental
concentration logs through either a sequential
processing method such as SpectroLith* (Herron and
Herron, 1996a, 1996b) or through an inversion
approach (Mayer and Sibbit, 1980; Quirein et al., 1986;
Cannon and Coates, 1990; Peeters and Visser, 1991).
Yields to Dry-Weight Elements - The conversion of
relative spectral yields from neutron capture into
absolute elemental concentrations is accomplished via a
modified geochemical oxides closure model (Grau and
Schweitzer, 1989; Grau et al., 1989) or through an
iterative inversion technique such as ELAN* elemental
log analysis (Quirein et al., 1986). Oxides closure
models take advantage of common mineralogical
associations to relate the concentration of unmeasured
elements to the concentration of measured elements.
For capture elements, the weight fraction (Wi) of
element i at a given depth is a function of a closure
normalization factor (F) determined at that depth, the
measured relative yield (Yi ) of element i at that depth,
and the tool’s sensitivity (Si) to element i for capture
reactions:
The SpectroLith technique is based on the derivation of
empirical relationships between accurate elemental
concentrations and mineral concentrations. A highquality core database of sedimentary rock samples
characterized by dual-range Fourier transform infrared
(DRFT-IR) mineralogy (Herron et al., 1997) and
chemical analyses has been used to develop and evolve
the processing algorithms. Chemical analysis
measurement techniques include X-ray fluorescence,
sulfur by combustion infrared detection technique
(LECO), TOC by elemental analyzer, and trace
elements primarily by induction-coupled plasma mass
spectrometry. In combination, these techniques measure
virtually all of the elements that influence nuclear well
logs or are of interest for formation evaluation. The
DRFT-IR library includes more than 50 mineral
standards representing 28 minerals.
Wi = F(Yi /Si). ............................................... (1)
The normalization factor F is determined at each depth
by solving the simple closure relation:
F (AiYi/Si) = 1, ........................................... (2)
where Ai is a factor accounting for all the unmeasured
elements that are associated with element i.
The quantitative elemental concentrations and
processed lithology can be used directly with other log
data in petrophysical interpretation programs to provide
more accurate petrophysical answers (Mayer and
The inelastic measurement offers a complementary,
independent set of yields from which to extract
elemental concentrations. The inelastic sensitivities to
7
SPWLA 53rd Annual Logging Symposium, June 16–20, 2012
Sibbit, 1980; Quirein et al., 1986; Cannon and Coates,
1990; Peeters and Visser, 1991).
fractions must therefore be validated to provide
confidence in any mineralogy or other formation
properties derived from them.
TOOL APPLICATIONS
Figures 10-13 provide four examples that compare the
elemental weight fractions measured by the tool with
those analyzed from core. Each figure displays a
mineralogy column and elemental concentrations of
silicon, calcium, iron, magnesium, sulfur, potassium,
aluminum, titanium, manganese, and total organic
carbon. The log data are displayed as solid black lines
with shaded areas representing the uncertainties; core
data are displayed as red dots; all values are presented
in weight fraction. The core data are used for
validation only; they are not used for calibration of
elemental concentration logs in any example.
The tool performance described in this paper has been
validated by extensive field testing performed in several
locations, mostly in North America, and covering the
main unconventional plays in the United States and
Canada.
Over 50 successful jobs were performed accumulating
more than 500 hours of tool operation with no PNG or
other hardware failures.
Repeatability and Precision – An example of tool
repeatability is shown in Figure 9. The left eight tracks
show two logging passes made at 900 ft/h. The curves
overlay very well in all cases, even for the historically
challenging measurement of Mg and the new
quantification of C. The right eight tracks compare two
passes acquired at 3,600 ft/h. Here, the repeatability is
slightly worse, as expected, but the major rock-forming
elements Si, Ca, Fe, and even Mg would still produce a
good quantitative lithological description of the
formation at much faster logging speeds than existing
tools.
Each of the examples comes from a different basin in
North America. The agreement between core and log
elemental concentrations is very good across a wide
variety of lithologies. Of particular interest is the new
log of TOC, which closely matches the organic carbon
weight fraction measured on core samples. This
information can be extremely valuable when evaluating
organic-rich shale formations as well as conventional
reservoirs.
Examples of the accuracy of the TOC measurement are
given in Figure 14, which presents logs from four wells
in unconventional resource plays from North America.
TOC ranges from 0 to 12 weight percent (displayed as
weight fraction in the figures). Log and core data agree
well over the entire dynamic range in both water-based
and oil-based mud systems. In oil-based muds, TOC
can be biased unless the contribution to the C signal
from the oil in the borehole is properly removed. The
logs were acquired at speeds between 600 and
1,200 ft/h, considerably faster than traditional carbon
logging.
Measurement precision is a function of the tool design,
logging speed, depth averaging, and logging
environment. Borehole diameter and salinity, formation
porosity and salinity, mud and formation composition,
and neutron output are all contributing factors. A tool
planner is available to estimate the precision of
elemental concentrations and interpreted properties
such as matrix density or lithology for a given
environment. The algorithms used in the tool planner
are based on MCNP modeling that was benchmarked to
experimental data. The recommended logging speed
will depend on the required precision for a given
measurement of interest. As suggested by Figure 9, if
only basic mineralogy is needed and wellsite efficiency
is critical, logging speeds of up to 3,600 ft/h may be
achieved. A slower logging speed may be required for
more demanding applications such as carbon
quantification, precise dolomite determination, or
logging in high-salinity borehole fluids.
DISCUSSION
Enabled by recent technological advancements, the
measurement capabilities of the new elemental
spectroscopy tool represent an important step beyond
the previous generation of tools. Early field results
highlight the considerable potential of this new service
that enables a range of answer products and
applications not possible with previous tools or tool
combinations.
Accuracy – The elemental weight fractions measured
by the tool can be used to evaluate the mineral
composition of a formation. Interpretation of log data to
determine porosity, permeability, saturation and
geomechanical properties generally depends on
mineralogy. The accuracy of the elemental weight
For the first time, the precision and accuracy of
measured Mg and S enable a solution for mineralogy in
carbonates at standard wireline logging speeds.
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SPWLA 53rd Annual Logging Symposium, June 16–20, 2012
Logging for TOC becomes a reality for unconventional
resource plays such as shale gas, shale oil, and in situ
thermal processing of oil shales. Improvements in the
precision and accuracy of other measured elements
such as Na and K and quantification of new elements
such as Mn permit a more thorough and accurate
mineralogy interpretation.
Transactions of the 31st Annual SPWLA Logging
Symposium, 2427 June 1990, Paper V.
Evans, M. L., 1981, A computer model for calculating
gamma-ray pulse-height
spectra for logging
applications, LA-UR-81-400, Los Alamos National
Laboratory, Los Alamos, New Mexico.
Galford, J., Truax, J., Hrametz, A., and Haramboure,
C., 2009, A new neutron-induced gamma ray
spectroscopy
tool
for
geochemical
logging,
Transactions of the SPWLA 50th Annual Logging
Symposium, 21–24 June 2009, The Woodlands, Texas,
USA, Paper X.
Operational aspects also distinguish the new tool. The
improved precision of the measurement translates
directly into faster logging. The electronic source
means that no additional radioisotope source or logging
runs are required to obtain geochemical information.
The higher temperature rating of the detector allows
very long jobs at temperature as with drillpipe or tractor
conveyance. The reduced tool diameter enables logging
in slimmer boreholes. The tool is also combinable with
most wireline openhole services, including triple
combo, nuclear magnetic resonance (NMR), dielectric,
sonic, and imaging tools.
Gill, P. E., Murray, W., and Wright, M. H., 1981,
Practical Optimization, Academic Press.
Grau, J., and Schweitzer, J. S., 1987, Prompt γ-ray
spectral analysis of well data obtained with NaI(T1) and
14 MeV neutrons, Nuclear Geophysics, Vol. 1, No. 2,
157–165.
The combination of all these features allows the
simultaneous acquisition of neutron, density, resistivity,
NMR, and capture and inelastic spectroscopy in
demanding environments with a single toolstring. The
availability of real-time analysis of the measurements
supports enhanced real-time log quality control and intime decision-making.
Grau, J., and Schweitzer, J. S., 1989, Elemental
concentrations from thermal neutron capture gammaray spectra in geological formations, Nuclear
Geophysics, Vol. 3, No. 1, 1–9.
Grau, J. A., Schweitzer, J. S., Ellis, D. V., and Hertzog,
R. C., 1989, A geological model for gamma-ray
spectroscopy
logging
measurements;
Nuclear
Geophysics, Vol. 3, No. 4, 351359.
Last but not least, the tool is a key step in eliminating
radioisotope sources. The benefits in connection with
health, safety, and the environment and with the costs
of transportation, security, and liability are clear.
Herron, M. M., Matteson, A., and Gustavson, G., 1997,
Dual-range FIR mineralogy and the analysis of
sedimentary formations, Proceedings of the 1997
Society of Core Analysts, 7–10 September, Calgary,
Paper SCA-9729.
ACKNOWLEDGMENTS
The authors thank Energen Resources Corporation and
Whiting Oil and Gas Corporation for supporting field
testing of this tool in their wells and for allowing the
use of their well log data. The authors also gratefully
acknowledge Saint-Gobain Crystals, particularly Peter
Menge and the engineering team, for their contributions
in developing the detector technology described in this
paper. Lastly, we thank the many people on the
Schlumberger development team in Sugar Land,
Princeton, and Cambridge for bringing this tool from
concept to reality. None of the results reported here
would have been possible without their expertise,
dedication, and hard work.
Herron, M. M., Grau, J. A., Herron, S. L., Kleinberg, R.
L., Machlus, M., Reeder, S. L., Vissapragada, B.,
Burnham, A. K., Day, R. L., and Allix, P., 2011, Total
organic carbon and formation evaluation with wireline
logs in the Green River oil shale, SPE 147184, SPE
Annual Technical Conference and Exhibition, 30
October–2 November 2011, Denver, Colorado, USA.
Herron, S. L., and Herron, M. M., 1996a, Quantitative
lithology: An application for open and cased hole
spectroscopy, Transactions of the SPWLA 37th Annual
Logging Symposium, 16–19 June, New Orleans,
Louisiana, USA, Paper E.
Herron, S. L., and Herron, M. M., 1996b, Quantitative
lithology: Open- and cased hole applications derived
from integrated core chemistry and mineralogy
database, Geological Society Special Volume on Core-
REFERENCES
Cannon, D. E. and Coates, G. R., 1990, Applying
mineral knowledge to standard log interpretation,
9
SPWLA 53rd Annual Logging Symposium, June 16–20, 2012
Log Integration, Geological Society of London Meeting
on Core-Log Integration, 1718 September, Burlington
House, Piccadilly, London.
models, Transactions of the 27th Annual SPWLA
Logging Symposium, 913 June, Paper DD.
Saint Gobain Crystals, 2006, BrilLianCe scintillators
performance summary.
Herron, S. L., and Herron, M. M., 2000, Application of
nuclear spectroscopy logs to the derivation of formation
matrix density, Transactions of the SPWLA 41st
Annual Logging Symposium, 4–7 June, Dallas, Texas,
USA., Paper JJ.
Scott, H. D., Stoller, C., Roscoe, B. A., Plasek, R. E.,
and Adolph, R. A., 1991, A new compensated throughtubing carbon/oxygen tool for use in flowing wells,
Transactions of 32nd Annual SPWLA Logging
Symposium, 16–19 June, Midland, Texas, USA, Paper
MM.
Herron, S. L., and Le Tendre, L., 1990, Wireline source
rock evaluation in the Paris Basin, AAPG Studies in
Geology #30, Deposition of Organic Facies (AAPG,
Tulsa), p 57-71
Stoller, C., Adolph, B., Berheide, M., Brill, T.,
Clevinger, P., Crary, S., Crowder, B., Fricke, S., Grau,
J., Hackbart, M., Herron, S., Jorion, B., Lorente, M.,
Madio, D., Miles, J., Philip, O., Radtke, R. J., Roscoe,
B., Shestakova, I., Ziegler, W., and Menge, P. R., 2011,
Use of LaBr3 for downhole spectroscopic applications,
IEEE Nuclear Science Symposium and Medical
Imaging Conference (NSS/MIC), 23–29 October,
Valencia, Spain, 191–195.
Hertzog, R. C., 1980, Laboratory and field evaluation
of an inelastic-neutron-scattering and capture gamma
ray spectrometry tool, SPE Journal, Vol. 20, No. 5,
October, 327–340.
Lecoq, P., Annenkov, A., Gektin, A., Korzhik, M.,
Pedrini, C., 2006, Inorganic Scintillators for Detector
Systems: Physical Principles and Crystal Engineering
(Springer-Verlag, Berlin), 23–26.
Van Loef, E. V. D., Dorenbos, P., van Eijk, C. W. E.,
Kraemer, K., and Guedel, H. U., 2001, High-energyresolution scintillator: Ce3+ activated LaBr3, Applied
Physics Letters, Vol. 79, 1573–1475.
Mayer, C., and Sibbit, A., 1980, GLOBAL, a new
approach to computer-processed log interpretation, SPE
9341, SPE Annual Technical Conference and
Exhibition.
Weller, G., Griffiths, R., Stoller, C., Allioli, F.,
Berheide, M., Evans, M., Labous, L., Dion, D., and
Perciot, P., 2005, A new integrated LWD platform
brings next-generation formation evaluation services,
Transactions of the SPWLA 46th Annual Logging
Symposium, 26–29 June, New Orleans, Louisiana,
USA., Paper JJ.
Odom, R., Dahlin, T., Dolliver, D., Elizondo, E., Seeds,
E., Starnes, J., and Wilson, R., 2008, Design and initial
field-test results of a new pulsed-neutron logging
system
for
cased
reservoir
characterization,
Transactions of the SPWLA 49th Annual Logging
Symposium, 25–28 May 2008, Edinburgh, Scotland,
UK, Paper O.
ABOUT THE AUTHORS
Peeters, M., and Visser, R., 1991, A comparison of
petrophysical evaluation packages: LOGIC, FLAME,
ELAN, OPTIMA, and ULTRA, Log Analyst, JulyAugust 1991, 350357.
R. J. Radtke is a Principal Tool Physicist at
Schlumberger’s Houston Formation Evaluation
Integration Center (HFE) in Sugar Land, Texas. He has
been involved in design, characterization, and algorithm
development for nuclear LWD and wireline tools since
joining Schlumberger in 1999. He graduated from The
University of Chicago in 1994 with a PhD in physics.
Pelowitz, D. B., Editor, 2008, MCNPX User’s Manual,
Version 2.6.0, LA-CP-07-1473, Los Alamos National
Laboratory, Los Alamos, New Mexico.
Pemper, R., Sommer, A., Guo, P., Jacobi, D., Longo, J.,
Bliven, S., Rodriguez, E., Mendez, F., and Han, X.,
2006, A new pulsed neutron sonde for derivation of
formation lithology and mineralogy, SPE 102770, SPE
Annual Technical Converence and Exhibition, 24–27
September 2006, San Antonio, Texas, USA.
Maria Lorente is Wireline Product Champion for newgeneration nuclear tools, based in Sugar Land, Texas.
She started her career as part of the engineering team in
France and then moved to wireline field operations
working in various assignments in the Middle East and
Latin America. She holds a BS degree in electrical
engineering from Universitat Politècnica de València,
Spain, and an MS degree in electrical engineering from
Supélec, France.
Quirein, J., Kimminau, S., LaVigne, J., Singer, J., and
Wendel, F., 1986, A coherent framework for
developing and applying multiple formation evaluation
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SPWLA 53rd Annual Logging Symposium, June 16–20, 2012
Bob Adolph is an Engineering Advisor at
Schlumberger’s Princeton Technology Center (PTC) in
Princeton, New Jersey. Interests include wireline and
LWD nuclear measurements, especially gamma ray
spectroscopy, pulsed neutron technology, electronics,
and signal processing. He joined Schlumberger in 1979
and holds a BS in electrical engineering from Rice
University.
a PhD in physics from the University of Pittsburgh in
1996.
Dale May graduated in 1979 with a BS in physics from
Texas Tech University, Lubbock, Texas. He joined
Schlumberger in Midland, Texas, in 1980, as a field
engineer. He has held positions as a field engineer,
specialist engineer, sales engineer, log analyst, and
center manager and is currently an Advisor
Petrophysicist for Schlumberger Oilfield Services.
Markus Berheide is a Principal Research Scientist in
the Sensor Physics department at Schlumberger-Doll
Research (SDR) in Cambridge, Massachusetts. His
background is in nuclear measurements and enabling
technologies. He holds a Doctorate in physics from
Ruhr-Universität Bochum (Germany).
Jeffrey Miles is a Senior Research Scientist at SDR. He
received a PhD in physics from the Massachusetts
Institute of Technology in 2007. His interests include
the modeling of all aspects of nuclear physics in the
oilfield, with emphasis on neutron-gamma spectroscopy
and algorithms for fast modeling and inversion.
Scott Fricke is a Principal Tool Physicist at HFE. He
has been involved in software development,
interpretation, and design of nuclear logging tools. He
received a PhD in theoretical nuclear physics from the
University of Minnesota in 1985.
Luke Perkins is a Principal Engineer in the generators
development group at PTC. He received his PhD in
nuclear engineering from U.C. Berkeley and has
worked since 1997 in Manufacturing, Sustaining and
Research and Engineering positions within the oilfield
radiation generators product line, including leading the
development and commercialization of the EcoScope*
pulsed neutron generator.
Jim Grau is a Scientific Advisor at SDR. He has been
involved for over 30 years in all aspects of borehole
elemental analysis using nuclear spectroscopy
techniques, including tool design, data acquisition
software, and spectral analysis techniques. Jim received
a PhD in experimental nuclear physics from Purdue
University in West Lafayette, Indiana.
Olivier Philip is a Principal Engineer in the detector
development group at PTC. He received a PhD in
nuclear engineering from Texas A&M University. He
worked on nuclear tool design in the Schlumberger
Sugar Land Product Center from 1996 to 2000. Since
2000, he has worked on photomultiplier and detector
design and development for several downhole
applications.
Susan Herron is a Scientific Advisor in the Sensor
Physics department at SDR. Her research interests
include nuclear spectroscopy applications, mineralogy,
and petrophysics. She holds a PhD in Geological
Sciences from State University of New York at Buffalo.
Jack Horkowitz is a Petrophysics Advisor with
Schlumberger working at HFE on a number of projects
including
spectroscopy
processing,
integrated
interpretation products and LWD calipers. Jack joined
Schlumberger in 1995 and holds a PhD in geology from
the University of South Carolina. He is a past President
of SPWLA (2006–07).
Brad Roscoe is a Scientific Advisor and the Nuclear
Program Manager at SDR. His areas of interest include
nuclear detectors, nuclear sources, gamma-ray
spectroscopy, and measurement integration. He earned
a PhD in nuclear engineering in 1981 from the
University of Illinois at Urbana-Champaign.
David Rose is Principal Petrophysicist and Manager of
Interpretation Engineering for Nuclear Answer
Products at HFE. He holds a Bachelors in geophysics
from the Colorado School of Mines.
Bruno Jorion is a Principal Engineer (Electrical) in the
Engineering department HFE. He received a Diplôme
d'Ingénieur from l'Institut Supérieur d'Electronique du
Nord (ISEN, France) and has been working in several
nuclear and NMR tool designs.
Chris Stoller is a Scientific Advisor at PTC. He joined
Schlumberger in 1986 and has worked on a variety of
nuclear tool projects in wireline and LWD. He received
a PhD in physics from the Swiss Federal Institute of
Technology in Zurich in 1976.
David Madio is a Senior Petrophysicist for
Schlumberger at HFE. His interests include mineralogy,
neutron porosity, and magnetic resonance. He received
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SPWLA 53rd Annual Logging Symposium, June 16–20, 2012
ADDITIONAL FIGURES
900 ft/h
3,600 ft/h
Fig. 9 Example of elemental weight fractions repeatability at two different logging speeds, 900 ft/h in the left eight
tracks and 3,600 ft/h in the right eight tracks. The well is located in Central Texas and was drilled with an 8-in. bit
size and filled with fresh water. This interval consists of alternating zones of very pure limestone where Ca
approaches 0.4, the value of calcite; dolomite where Mg approaches 0.13, the value in dolomite; and a small
amount of sandstone (Si = 0.47 in quartz and about 0.3 in feldspars). Limestone and dolomite are easily
distinguishable from the Ca and Mg concentrations even at 3,600 ft/h. The very low concentrations of Fe, S, K, and
Al are also properly reproduced in the log.
12
Zone A
SPWLA 53rd Annual Logging Symposium, June 16–20, 2012
Zone B
x100
Zone C
x125
Zone D
x150
x175
Fig. 10 Field example from a well in North Dakota. Zones A, B and C are part of the Bakken formation, and zone D
corresponds to the Three Forks formation. The figure compares elemental weight fractions measured by the tool
(black lines) to those derived from core analysis (red points). The yellow shaded areas along the black lines
represent the uncertainties (which are very small in this example). The track to the left of the depth track shows the
mineralogy, and the track to the right of the depth track shows an enhanced core photograph. Overall there is a
very good match between the measurements and the core, especially the total organic carbon (TOC) measurement
(far right track) ranging from nearly zero (zones B and D) to more than 12 weight percent (zones A and C). The
core photograph shows the highly laminated nature of this formation, which can cause occasional scatter when
comparing log data to standard core plug data. The logging speed was 600 ft/h.
13
SPWLA 53rd Annual Logging Symposium, June 16–20, 2012
x100
x150
x800
x850
x900
Fig. 11 Field example from Western Canada showing two intervals from the same well. The figure compares
elemental weight fractions measured by the tool (black lines) to those derived from core analysis (red points). Due
to the low salinity mud system, the bit size, and a logging speed of 1,080 ft/h, the measurement statistical
uncertainties are less than the thickness of the lines. The track to the left of the depth track shows the mineralogy.
There is very good agreement between core and log for all elemental concentrations and total organic carbon
(TOC).
14
SPWLA 53rd Annual Logging Symposium, June 16–20, 2012
x100
x300
x500
Fig. 12 Field example from West Texas showing three intervals from the same well. The figure compares elemental
weight fractions measured by the tool (black lines) to those derived from core analysis (red points). The yellow
shaded areas along the black lines represent the uncertainties. The track to the left of the depth track shows the
mineralogy. Overall there is a very good match between the measurements and the core. The logging speed was 900
ft/h.
x050
x450
Fig. 13 Field example from the Southeastern United States showing two intervals from the same well. The figure
compares elemental weight fractions measured by the tool (black lines) to those derived from core analysis (red
points). The yellow shaded areas along the black lines represent the uncertainties. The track to the left of the depth
track shows the mineralogy. Note the change in elemental concentrations and the good agreement between log and
core data for all elements in both the clay-bearing and quartz-rich formations. The logging speed was 1,800 ft/h.
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SPWLA 53rd Annual Logging Symposium, June 16–20, 2012
x075
x000
x000
x000
x100
x100
x050
x125
x100
x100
x200
x150
x150
x200
x300
x175
x200
x300
x400
x200
x250
x400
x500
x225
x300
x500
x250
x600
x350
Water-Based Mud
Oil-Based Mud
Fig. 14 Comparison of total organic carbon (TOC) measured by the new tool and derived from core from four wells
in unconventional formations throughout North America. For each well, the first track shows the total carbon (TC,
in red) derived from the inelastic measurement and the total inorganic carbon (TIC, in black) computed from
carbonate minerals using capture spectroscopy elements. The separation between the two curves is TOC, which is
displayed in the second track with core data (red dots) for validation.
16