CAPTURE SPECTROSCOPY BASICS
induced gamma ray
logs record concentrations of individual chemical elements derived
from the characteristic energy levels of gamma rays emitted by a
nucleus that has been activated by neutron bombardment. Pulsed
neutron spectroscopy and elemental capture spectroscopy are other
common names for this kind of log. Chlorine, oxygen activation,
aluminum activation, and carbon oxygen logs also fall into this
category, as well as the reservoir saturation tool (RST) since it combines
a carbon oxygen log with a normal (non-spectrographic) pulsed
neutron log into one device.
These tools were designed to assist in assessing hydrocarbon content
of the reservoir and were run through casing, where invasion was not
usually an issue.
Many of these tools are now obso;ete but may be found in older well
files. Understanding their uses and limitations may provide useful
clues to reservoir evaluation.
The geochemical log (GST) and its successor the elemental capture
spectroscopy log (ECS) were primarily intended for mineral
identification. The GST and ECS can be run in either cased or open
incarnation of an elemental capture spectroscopy log was the
chlorine log. The recorded curve was a measure of the concentration
of chlorine in the formation. High chlorine meant salt water in high
to moderate porosity. Low chlorine meant hydrocarbon or fresh water
or low porosity. By using the porosity log, we could sort out low
porosity but sorting hydrocarbon from fresh water required local
knowledge. This tool was rare, usually run through casing but open
hole examples exist.
CARBON OXYGEN LOG
The next incarnation of an elemental
capture spectroscopy log was the carbon/oxygen log. It presented
a log curve of carbon/oxygen capture ratio, a fraction (C/O) and
silicon/calcium capture ratio (Si/Ca). High C/O indicate
hydrocarbons as opposed to water and high Si meant sandstone as
opposed to carbonate rocks. Shale and mudstone should have low
C/O and low Si/Ca, except silty shale could have Moderate Si/Ca
ratios. Count rates were measured in counts per minute instead
of counts per second, so the tool had to be run very slowly as
a through casing tool. It worked best in high porosity.
PULSED NEUTRON SPECTROSCOPY LOG
Pulsed neutron spectroscopy log
is a wireline log of the yields of different elements in the
formation; measured using induced gamma ray spectroscopy with a
pulsed neutron generator. The elemental yields are derived from
two intermediate results: the inelastic and the capture
spectrum. The inelastic spectrum is the basis for the
carbon-oxygen log, and can also give information on other
elements. The capture spectrum depends on many elements, mainly
hydrogen, silicon, calcium, iron, sulfur and chlorine.
Since the elemental yields give information only on the relative
concentration of elements, they are normally given as ratios,
such as C/O, Cl/H, Si/(Si + Ca), H/(Si + Ca) and Fe/(Si + Ca).
These ratios are indicators of oil, salinity, lithology,
porosity and clay, respectively. The main purpose of the log is
to determine lithology, the principal outputs are the relative
yields of silicon, calcium, iron, sulfur, titanium and
gadolinium. The yields give information only on the relative
concentration of these elements. To get absolute elemental
concentrations, it is necessary to calibrate to cores, or, more
often, use a model such as the oxide-closure model.
The depth of investigation of the log
is several inches into the formation. It can be run in open or cased
hole. Pulsed neutron spectroscopy logs were introduced in the mid
1970s after a decade or more of investigation.
geochemical log is a more recent incarnation and was run in
cased hole as the GST tool and in open hole as the GLT tool
It is a log of elemental
concentrations from which the geochemistry of the formation may
be derived. Several different logs provide information on
elemental weight concentrations: natural gamma ray spectroscopy,
elemental capture spectroscopy or pulsed neutron spectroscopy,
and aluminum activation. The combination of all of their outputs
is known as a geochemical log, since it provides information on
most of the principal elements found in sedimentary rocks.
Raw log curves for a GST log
As for the
pulsed neutron spectroscopy log, absolute concentrations can be
derived by calibration to core or by using a model such as the
oxide-closure model. The absolute elemental concentrations can
then be converted into mineral concentrations using a model that
defines what minerals are present. The first complete
geochemical logs were run in the mid 1980s.
oxide-closure model for converting relative elemental yields from a
pulsed neutron spectroscopy log to absolute weight concentrations
uses the assumption that the sum of all oxides in the rock matrix is
1.00. The model is based on the observation that, with few
exceptions, sedimentary minerals are oxides, so that the sum of the
dry weight percent of all oxides must be 100%. The weight percent of
an oxide can be calculated from the dry weight percent of the cation
by knowing the chemical formula.
The absolute dry
weight fraction, W, of element i is given by:
1: Wi = F * Yi / Si
F = unknown normalization factor
Yi = measured spectral gamma ray yield
Si = tool sensitivity to that element, measured in the laboratory.
The dry weight
fraction of the oxide is then:
2: Qi = F * Xi * Yi / Si
Qi = the oxide association factor, given by the chemical formula.
Since the sum of all Wi equals 1.00, it is possible to calculate F
and determine each Wi .
ELEMENTAL CAPTURE SPECTROSCOPY LOG
The elemental capture spectroscopy (ECS)
log is the current version of activation logging. Unlike earlier
versions it does not use a pulsed neutron source but uses
instead a standard
americium beryllium (AmBe) neutron source
and a large bismuth germanate (BGO) detector to measure relative
elemental yields based on neutron-induced capture gamma ray
spectroscopy. The primary elements measured in both open and
cased holes are for the formation elements silicon (Si), iron
(Fe), calcium (Ca), sulfur (S), titanium (Ti), gadolinium (Gd),
chlorine (Cl), barium (Ba), and hydrogen (H).
processing uses the 254-channel gamma ray energy spectrum to
produce dry-weight elements, lithology, and matrix properties.
The first step involves spectral deconvolution of the composite
gamma ray energy spectrum by using a set of elemental standards
to produce relative elemental yields. The relative yields are
then converted to dry-weight elemental
concentration logs for the elements Si, Fe, Ca, S, Ti, and Gd
using the oxides closure method. Matrix properties and
quantitative dry-weight lithologies are then calculated from the
dry-weight elemental fractions using empirical
relationships derived from an extensive core chemistry and
The outputs are dry-weight lithology fractions
anhydrite + gypsum from S and Ca
QFM (quartz + feldspar + mica)
Matrix properties (from elements)
matrix grain density
matrix thermal and epithermal neutron
■ Integrated petrophysical analysis
■ Clay fraction independent of gamma ray, spontaneous potential, and
■ Carbonate, gypsum or anhydrite, pyrite, siderite, coal, and salt
fractions for complex reservoir
■ Matrix density and matrix neutron values for more accurate porosity
■ Sigma matrix for cased and open hole sigma saturation analysis
■ Mineralogy-based permeability estimates
■ Quantitative lithology for rock properties modeling and pore pressure
prediction from seismic data
■ Geochemical stratigraphy (chemostratigraphy) for well-to-well
■ Enhanced completion and drilling fluid recommendations based on clay
versus carbonate cementation
■ Coalbed methane bed delineation, producibility, and in situ reserves
RESERVOIR SATURATION LOG
The reservoir saturation tool (RST) is
a combination of a modern carbon oxygen log and a standard
pulsed neutron log.
The dual-detector spectrometry system of the
through-tubing reservoir saturation tool enables
the recording of carbon and oxygen and dual burst thermal decay
time measurements during the same trip in the well.
carbon/oxygen (C/O) ratio is used to determine the formation oil
saturation independent of the formation water salinity. This
calculation is particularly helpful if the water salinity is low
or unknown. If the salinity of the formation water is high, the
dual burst thermal decay time measurement is used. A combination
of both measurements can be used to detect and quantify the
presence of injection water of a different salinity from that of
the connate water.
■ Formation evaluation behind casing
■ Sigma, porosity, and carbon/oxygen measurement in one trip in the
■ Water saturation evaluation in old wells where modern open hole logs
have not been run
■ Measurement of water velocity inside casing, irrespective of wellbore
angle (production logging)
■ Measurement of near-wellbore water velocity outside the casing
■ Formation oil volume from C/O ratio, independent of formation water
■ Flowing wells (in combination with an external borehole holdup sensor)
■ Capture yields (H, Cl, Ca, Si, Fe, S, Gd, and Mg)
■ Inelastic yields (C, O, Si, Ca, and Fe)
■ Three-phase borehole holdup
■ PVL* Phase Velocity Log
■ Borehole salinity
■ SpectroLith lithology indicators Nuclear
Weight percent curves from the ozide model for a GLT log.
Computed lithology from oxide model, including porosity and
hydrocarbon saturation from C/O ratio.
Alternate analysis for lithology and chromostratigraphy.