INELASTIC AND CAPTURE SPECTROSCOPY  BASICS
Activation, or induced gamma ray spectroscopy, 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), the Litho-Scanner, and Pulsar logs.
 

The geochemical log (GST) and its successor the elemental capture spectroscopy log (ECS) were widely used for hydrocarbon and mineral identification. The GST and ECS could be run in either cased or open hole. Slow logging speed reduced their early acceptance by industry but the modern versions of spectroscopy logs are run at normal speeds due to much improved detector design.

Historically, cased hole tools analyzed carbon/oxygen ratios from the high-energy neutron inelastic scattering spectra.  In contrast, open hole tools were initially constructed to evaluate lower energy nuclear capture interactions.  In fact, the tools could be run in either open or cased holes.
The tools utilize one of two source types: chemical refers to an AmBe source with an average neutron output of 4.2 MeV (ECS), or pulsed neutron generator (PNG), which bombards the formation at 14.1 MeV.  The newest versions, LithoScanner and Pulsar (Schlumberger) deliver near simultaneous data gathering from both inelastic and capture spectra, meaning better inputs for petrophysical interpretation.    

These tools, their predecessors and successors are described here in chronological order so you can see the evolution of tool capability and to help you assess the value of the data in your well files.

Chemical sources emit neutrons in the slow range; PNG tools emit energy in the fast range.

CHLORINE LOG (CLT)
The first 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 (C/O)
The next incarnation of an elemental capture spectroscopy log was the carbon/oxygen log. It presented a log curve of carbon/oxygen ratio, a fraction (C/O) and silicon/calcium 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. A C/O log is incorporated in most recent versions of induced spectroscopy tools. The principle behind the C/O log is inelastic neutron scattering.

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.

This log should not be confused with pulsed neutron or thermal decay time (TDT) logs, which measure quite different rock properties.

While chemical source tools emit slow neutrons, meaning only elements from the capture spectrum are counted, the PNG's 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. Early tools were physically long, expensive and required complex interpretation models.

INDUCED GAMMA RAY SPECTROSCOPY LOG aka GEOCHEMICAL LOG (GST)
The induced gamma ray spectroscopy log, also called the  geochemical log, is a more recent  spectroscopy incarnation and was run in cased hole as the GST tool and in open hole as the GLT tool (Schlumberger terminology). It is a log of elemental concentrations from which the geochemistry of the formation may be derived.
 

Raw log curves for a GST log    

The words "geochemical log" as used here should not be confused with the same term used to describe laboratory procedures to determine quite different chemical properties of rocks.

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.

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.

The 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

Where:
  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:Oi = F * Xi * Yi / Si

Where:
 Oi = the oxide association factor, given by the chemical formula.

Since the sum of all Oi equals 1.00, it is possible to calculate F and determine each Wi .

ELEMENTAL CAPTURE SPECTROSCOPY LOG (ECS)
The elemental capture spectroscopy (ECS) log was the next 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 germinate (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).

Wellsite 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 mineralogy database.  The ECS tool was a mere 10.2 feet long, beginning the trend to shorter, slimmer tools with an increased logging speed, making them more attractive to industry.

The outputs are dry-weight lithology fractions (from elements)
  ■  total clay
  ■  total carbonate
  ■ anhydrite + gypsum from S and Ca
  ■  QFM (quartz + feldspar + mica)
  ■  pyrite
  ■  siderite
  ■  coal
  ■  salt

Matrix properties (from elements)
  ■  matrix grain density
  ■  matrix thermal and epithermal neutron
  ■  matrix sigma.

Applications
  ■ Integrated petrophysical analysis
  ■ Clay fraction independent of gamma ray, spontaneous potential, and density neutron
  ■ Carbonate, gypsum or anhydrite, pyrite, siderite, coal, and salt fractions for complex reservoir
analysis
  ■ Matrix density and matrix neutron values for more accurate porosity calculation
  ■ 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 correlation
  ■ Enhanced completion and drilling fluid recommendations based on clay versus carbonate cementation
  ■ Coalbed methane bed delineation, producibility, and in situ reserves estimation

RESERVOIR SATURATION LOG (RST)
The reservoir saturation tool (RST) is a combination of a modern carbon oxygen log and a standard pulsed neutron log. The dual-detector spectroscopy 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.

The 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.

Applications
  ■ Formation evaluation behind casing
  ■ Sigma, porosity, and carbon/oxygen measurement in one trip in the wellbore
  ■ 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 (remedial applications)
  ■ Formation oil volume from C/O ratio, independent of formation water salinity
  ■ 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

LITHO-SCANNER LOG

The current generation lithology tool is called LithoScanner (Schlumberger terminology).  ECS’s chemical AmBe source was replaced with a high-output pulsed neutron generator (PNG) and the BGO detector superseded by a LaBr3:Ce scintillator having 3 times better resolution.  These design changes increase the neutron output by a factor of ~7 over a chemical source.  Inelastic interactions are separated from capture data due to source timing; a pulse-height histogram, or spectra, is generated during and after each neutron burst.  Some elements, such as carbon and oxygen, have a signature in just one spectra; others are present in both, meaning some elements are measured with greater precision while other elements are newly quantified.  Twenty-one elements making up eight mineral groups are output, including magnesium, which allows the differentiation of dolomite from calcite, and aluminum for computing clay volume. Inorganic carbon is measured and subtracted from total inelastic carbon, giving a stand-alone TOC.  This is key to computing kerogen volume in shale gas plays.  Additionally, elements such as copper can be used for metal ore assays.

Importantly, unlike a chemical source tool, the Pulsed Neutron Generator does not emit radiation unless activated, making it safer to use and more environmentally friendly.  The slim 4.5” OD tool has a depth of investigation of 7 to 9” with a vertical resolution of 18” and a logging speed up to 3600 ft/hour. 

Outputs:

■ Elemental Yields

■ Elemental Weight Fractions:  Al, Ba, Br, C, Ca, Cl, Cu, Fe, Gd, H, K, Mg, Mn, Na, Ni, O, P, S, Si, Sr, Ti

■ Total Organic Carbon (TOC)

■ Dry Weight Mineral Concentrations – Anhydrite, Clay, Calcite, Coal, Dolomite, Evaporite, Pyrite, QFM, Siderite

■ Matrix Properties (Density, Neutron and Sigma)

Applications:

■ Shale Gas Reservoirs (Kerogen Volume and Hydrocarbon Saturation from TOC)

■ Mechanical Properties

■ Facies Identification

■ Sequence Stratigraphy and Clay Typing

PULSAR LOG (PNX-A)

2015 saw the introduction of a new measurement:  FNXS, or Fast Neutron Cross-Section, which detects neutrons induced from fast neutron inelastic scattering.   The Schlumberger tool, Pulsar, combines high neutron outputs with multiple detectors in a 1.73” OD tool, designed to be run in cased holes through tubing.  The tool can be operated rig-less and is a good choice for horizontals, or wells with instability issues. 

The major application is the detection of gas zones:  the fast neutron cross section values are similar for both matrix and water but read much lower in gas.  This is presented in an FNXS TPHI curve overlay which shows better resolution than the open hole density neutron gas crossover method.  Unique to cased-hole interpretation, it is no longer necessary to have open hole density logs. 

The tool logging speed varies according to the acquisition mode, from 200 ft/hour in inelastic capture mode, up to 3600 ft/hour in inelastic gas, sigma and HI mode. 

Outputs:

■ Sigma

■ HI Hydrogen Index

■ TPHI Thermal Neutron Porosity

■ FNXS Fast Neutron Cross-Section

■ Inelastic and Capture Elemental Yields

■ Carbon/Oxygen Ratio

■ Total Organic Carbon (TOC)

Applications:

■ Standalone Formation Evaluation (bypassed zones, depleted reservoirs, wells with no OH or ancient logs and Carbon Storage reservoirs)

■ Differentiate and quantify gas zones from liquid-filled porosity or tight zones

■ Water saturation regardless of salinity

■ Low-resistivity pay evaluation

■ Well-to-well correlation and sequence stratigraphy

■ Detection of water entry and flow behind casing

EXAMPLE LOGS

Weight percent curves from the oxide 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.

ECS Log showing volume track with mineral concentrations.

LithoScanner mineralogy showing increased elemental precision:  carbonates separated into calcite and dolomite; chlorite from total clay.

Pulsar Log:  simulated log response modeled in sand/shale sequence, comparing RHOB/TPHI with FNXS/TPHI overlays,
 

Examples of CO2 detection and quantification at current reservoir condition. Different gas indicators are presented, including Sigma, Neutron count rates and porosity, Fast Neutron Cross Section, and its deviation from Fast Neutron Cross Section of matrix components in presence of gas. SIGMA, TPHI, FNXS end points calculated based on gas density and composition. Lithology and porosity are measured based on induced gamma ray spectroscopy combined to TPHI and FNXS, eliminating the need for open hole logs.

showing better gas and clay response.