Natural Gamma and Spectral Gamma Borehole Logging

Basic Concept

The natural-gamma and spectral-gamma borehole logging methods can passively measure the ambient-gamma radioactivity produced by the decay of radioisotopes within subsurface materials. The term “natural” is used mainly to distinguish between these passive-gamma methods and other active-source logging techniques such as gamma-gamma density logging. Deviations in the amount and type of gamma radioactivity are primarily attributed to variations in geochemistry and, thus, can be used to interpret changes in mineralogy and lithology.

The natural-gamma (or total-gamma) log measures the total-gamma emissions in the borehole per time, and data are typically output as counts per second (CPS). Because of its relative simplicity and cost, total-natural gamma is one of the first borehole logs collected and sometimes collected with multiple sondes (i.e., borehole tools). The natural-gamma log has two common uses: 1) determining basic lithology and correlating stratigraphy and 2) depth correcting other borehole logs.

The spectral-gamma log measures gamma emissions as a function of energy emitted and can be used to estimate the chemical identities of the radioactive-source materials. Spectral-gamma logging is labor intensive because data are collected either with very slow logging rates or stationary measurements at fixed depths. The spectral-gamma method also requires post processing for quantitative analysis, the results of which can be used for detailed interpretations of mineral composition, clay-typing, and lithology.

Theory

Radioactive isotopes (or radioisotopes) are forms of chemical elements with nuclei that contain an imbalanced ratio of protons to neutrons that causes nuclear instability. Seeking energetic stability, these nuclei spontaneously emit radiation causing the radioisotopes to decay over time. The initial (i.e., parent) radioisotopes are chemically altered to become daughter (or progeny) isotopes that may also have unstable nuclei. Radioactive decay continues until a stable form is reached, and often occurs successively in decay chains.

Alpha-, beta-, and gamma radiation are the three types of radiation originating from unstable nuclei during radioactive decay and have different physical properties and behaviors. Gamma radiation (or a “gamma ray”) is a type of electromagnetic radiation consisting of gamma photons, which are elementary particles that exhibit characteristics of particles and high-frequency waves. Gamma photons readily penetrate and travel through numerous materials (e.g., steel, polyvinyl chloride (PVC) casing), which allows for their application in many boreholes (Keys, 1989).

In nature, gamma photons originate almost entirely from potassium-40 (⁴⁰K) and the extensive decay chains of thorium (Th) and uranium (U). Thorium and uranium decay chains result in stable lead (Pb) isotopes, and potassium-40 decays directly into argon and calcium isotopes, both of which are stable. These three (i.e., KUT) radioisotopes and their daughter products constitute 98% of the naturally occurring-radioactive elements and exist in varying proportions depending on minerology and geochemistry (Young, 1980).

The most radioactive of the common sediments are shales and the natural-gamma log was initially used to produce “shale logs” that identified shaley layers (Mussett, 2000). Generally, shales/clays, sands, and gravels/hard rocks respectively produce high, intermediate, and low total-gamma counts. However, some metamorphic and igneous rocks are rich in uranium and thorium, for example, and local geology may require consideration. That said, the natural-gamma method is useful for simplistic lithologic differentiation, especially within unconsolidated sediments (Williams and others, 1993).

The decay of the KUT radioisotopes and each individual member of their decay sequences results in the emission of gamma photons with distinct and characteristic energy levels. By measuring the energy of each gamma photon, the spectral-gamma method can be used to determine the types and relative concentrations of radioactive sources. This differentiation, which can be conducted by spectral stripping, window modeling, or basis of spectra analysis, allows for advanced geochemical, mineralogical, and lithological distinction and characterization.

Typically, both gamma tools detect gamma radiation using a scintillation detector, which is a radiation-detecting crystal composed of sodium iodide (NaI) or cesium iodide (CsI). As a high energy gamma photon strikes the crystal, the crystal becomes ionized and releases an electronic pulse in the form of light. The light pulses are counted and timed by the gamma tools, and total-gamma radiation is determined by the pulses detected per unit time (i.e., counts per second).

The scintillation crystal within a typical spectral-gamma tool is optically coupled to a photomultiplier tube (i.e., sensitive light detector) that amplifies the light pulse. Additionally, the photomultiplier tube outputs a current pulse that is energetically proportional to the photon and relates gamma CPS to gamma-photon energy. However, in order to record and separate the full natural-gamma spectrum, the spectral-gamma tool measures between 0 to 3 megaelectronvolts (MeV) using 256, 512, or 1024 channels.

Note that not all gamma tools are equal. Some spectral-gamma tools eliminate the Compton and photoelectric effects, which produces lower total-gamma counts relative to other tools that do not consider such effects. Additionally, the counts per second can be calibrated to American Petroleum Institute units (APIu). API units are a logging industry standard that allows natural gamma logs to be directly comparable regardless of the size or type of the detection crystal (Belknap and others, 1959).

Applications

The natural- and spectral-gamma tools can collect depth-dependent data with high vertical resolution in variously constructed boreholes (e.g., open; air-, water-, or mud filled; PVC- or steel cased). Though the properties of borehole-construction materials can affect gamma-photon penetration, qualitative interpretation of natural-gamma data does not require consideration of well construction. However, information of borehole diameter, casing type and thickness, and sediment saturation is required for model-derived quantitative analysis of spectral-gamma data.

The gamma-photon emission rate is subject to a counting uncertainty (i.e., statistical variations over time). While the average rate over a long period of time is constant, the short-term rate observed will vary. Increasing the measurement time reduces uncertainty, and it is important to collect spectral-gamma data while the tool is at fixed depths or trolling very slowly. The trolling rate is determined by the expected level of radioactivity and decreases with decreased gamma emissions (Young, 1980).

Natural- and spectral-gamma logs can produce “total gamma-ray” logs that display the total-gamma emissions in CPS or APIu as a function of depth. Basic spectral-gamma tools count the number of each potassium, uranium, and/or thorium energy window in real time to produce “KUT” logs. In such, gamma counts are separated according to their energy ranges, which indicate relative variations in potassium, uranium, and/or thorium. However, basic “KUT” logs do not capture a complete spectrum or calculate radioisotope concentrations.

More advanced spectral-gamma tools are used to determine concentrations of radioisotopes as a function of depth below a reference point. Using a conversion model, these advanced “KUT” logs show potassium in weight percent (wt%) and uranium and thorium in parts per million (ppm) relative to the total-gamma log. Plots of stationary-spectral data show the total emissions in CPS relative to each channel (MeV) within the energy spectrum and allow for more detailed chemical concentration analysis.

Successful data interpretation requires tool calibration in a thermally stable and controlled environment in which measurements of known gamma counts and isotope concentrations are collected. Additionally, spectral stripping is done concurrently with calibration. Spectral stripping determines the relation between the observed effects that the test materials have on the gamma spectrum and the known standards for each individual element. Only by comparing measured data to calibrated logs can qualitative analysis of total-gamma emissions and radioisotope concentrations be possible.

Variations in the amount of gamma radioactivity can qualitatively indicate changes in subsurface mineralogy, and, though the application of natural-gamma logging is limited, it is widely implemented in numerous studies. Alternatively, spectral-gamma logging has the potential for detailed chemical analysis of subsurface materials, as it can estimate the identities and concentrations of radioactive-source materials. Such capabilities allows the spectral-gamma borehole method to be useful in the following:

Identification/differentiation of subsurface lithology/minerology/geochemistry
Correlation of stratigraphy
Mapping of lithologic facies
Identification of fracture zones
Identification of water-rock interactions
Clay typing
Study of palaeoclimate/paleoenvironment

Examples/Case studies

Amartey, E.O., Akiti, T.T., Armah, T., Osae, S., and Agyekum, W.A., 2017, Integrating gamma log and conventional electrical logs to improve identification of fracture zones in hard rocks for hydrofracturing: a case study from Ghana: Applied Water Science, v. 7, no. 3, p. 1091-1098, doi:10.1007/s13201-016-0450-z.

Abstract: Hydrofracturing of low-yielding boreholes in hard rocks is a widely used technique in Africa for improvement of yield, thus making them qualified for installation of a hand-pump for domestic water supply. However, the success rate of the hydrofracturing campaigns seems not to be that high as generally claimed by contractors. One reason amongst others might be that the selection of zones for hydrofracturing in the individual borehole is based on pre-hydrofracturing investigation using conventional electrical logs only. Thereby, the zones selected are the occurring resistivity minima interpreted as weak zones with some fracturing. However, resistivity minima can also be caused solely by lithological reasons, which then in most cases could have been seen on a gamma log as corresponding increased gamma radiation. The advantages of using gamma logging in combination with conventional electrical logging technique for prediction of fractured zones in basement rocks is illustrated by investigations of three low-yielding boreholes located in different geological environments in crystalline basement rocks in Ghana.

Asfahani, J., 2002, Phosphate Prospecting Using Natural Gamma Ray Well Logging in the Khneifiss Mine, Syria: Exploration and Mining Geology, v. 11, no. 1-4, p. 61-68, doi:10.2113/11.1-4.61.

Abstract: Natural gamma ray well logging, an effective tool in geophysical prospecting, is used to investigate the radioactive and phosphatic layers in the Khneifiss mine in Syria. The interpretation of the gamma ray measurements, using numerical methods of analysis developed previously and applied successfully in some phosphatic areas in Syria, make it possible to define precisely the phosphate thickness from place to place in the study area. This technique has been successfully applied while studying seven boreholes in the area. Sixty-three core samples from phosphatic layers in the boreholes have been analyzed, using gamma ray spectrometry for the determination of P2O5 , U, Th, and K. Good correlation between P2O5 content and U concentration has been found. The total count gamma logs correlate reasonably well with the U core analysis, suggesting that radioactive equilibrium exists in the U decay series. These gamma logs can be therefore used effectively to quantitatively map the distribution of P2O5 and U. The characteristics of both subsurface phosphatic sand and phosphatic rocks have been investigated and outlined using a statistical approach. The affinity of uranium to some trace elements such as V, Sr, Cu, and Ni has been verified using correlation matrices of these elements.

Chen, Z., Zha, M., and Jin, Q., 2004, Application of Natural Gamma Ray Logging and Natural Gamma Spectrometry Logging to Recovering Paleoenvironment of Sedimentary Basins: Chinese Journal of Geophysics, v. 47, no. 6, p. 1286-1290, doi:10.1002/cjg2.616.

Abstract: We apply natural gamma ray (GR) curves and natural gamma spectrometry (NGS) curves to the Member 3 of Shahejie Formation from the well Niu‐38. The result indicates that the GR curves can not reflect the change of paleoenvironment well, but the curves of uranium (U) and the ratio of thorium to uranium (Th/U) from NGS are consistent with the change of paleoenvironment and have good correlation with the contents of carbon organic, which suggests that U and Th/U are better parameters for paleoenvironment than GR. In the quiet and reductive condition U and Th/U are much applicable, and with the conditions having more oxidation their capability of application weakens. So the application of GR curves to the recovering of paleoenvironment should be together with NGS curves, and the NGS curves should be used as criterion.

Cripps, A.C. and McCann, D.M., 2000, The use of the natural gamma log in engineering geological investigations: Engineering Geology, v. 55, no. 4, p. 313-324, doi: 10.1016/S0013-7952(99)00085-X.

Abstract: The natural gamma log is widely used in both cased and uncased boreholes to identify changes in lithology down the length of a borehole. This is particularly important in the ground investigation process where 100% core recovery is often not achieved in the borehole programme. In this paper the use of the gamma log to provide additional information from the ground investigation boreholes is examined and illustrated by a number of case histories. It is shown that the gamma log can be used to study both the lithological variations within an individual borehole and changes in geological structure across a construction site. It is also demonstrated that a geological formation or sequence can be classified in terms of its natural gamma signature, which enables the geologist to observe its presence over large distances on a regional basis. The importance of the calibration of the gamma sonde and the use of the correct logging speed is emphasised in the paper. The successful calibration of the log against a quarry wall using a known observed geological structure as a reference is also described together with the use of the resulting information to obtain a geological log for boreholes drilled behind the quarry face. Its use in extrapolating between boreholes to detect and trace the presence of a particular geological formation to estimate its vertical and lateral extent for possible extraction is also discussed.

Davies, S.J. and Elliott, T., 1996, Spectral gamma ray characterization of high resolution sequence stratigraphy: examples from Upper Carboniferous fluvio-deltaic systems, County Clare, Ireland: Geological Society, London, Special Publications, vol. 104, p. 25-35, doi: 10.1144/GSL.SP.1996.104.01.03.

Abstract: The application of high resolution sequence stratigraphy requires the ability to recognize key surfaces which record fluctuations in relative sea-level. In sub-surface studies, gamma ray logs have been used to identify maximum flooding surfaces, but their full potential has not been realized. Gamma ray profiles produced using a portable spectrometer on exposed Upper Carboniferous fluvio-deltaic deposits in western Ireland reveal that key surfaces and systems tracts can be characterized more comprehensively and recognized with greater confidence if spectral gamma ray data (K, U, Th and their respective ratios) are used in conjunction with traditional total count data. Maximum flooding surfaces can be distinguished from lesser flooding surfaces by a distinctive U peak (> 5 ppm) and low Th/U ratio (< 2.5). Erosional unconformities and their associated incised valley fills are characterized by consistently low total counts (40–50 cps) and high Th/K ratios (> 6). Laterally correlative interfluves are represented by distinctive palaeosols that can be clearly identified in spectral gamma ray data by their anomalously low K content (<0.4%) and exceptionally high Th/K ratio (> 17). Finally, the stacking pattern of parasequence sets can be identified using the trends of Th/K ratios from sandstones in successive parasequences. These results have widespread implications for the recognition of high resolution sequence stratigraphic signatures in the stratigraphic record, with particular reference to the subsurface analysis of fluvio-deltaic deposits.

Šimíček, D. and Bábek, O., 2015, Spectral gamma-ray logging of the Grès d'Annot, SE France: An outcrop analogue to geophysical facies mapping and well-log correlation of sand-rich turbidite reservoirs: Marine and Petroleum Geology, v. 60, p. 1-17, doi:10.1016/j.marpetgeo.2014.10.010.

Spectral gamma-ray (GRS) logging is a powerful tool in cyclo- and sequence stratigraphy of carbonate depositional systems. In siliciclastic systems with low chemical maturity, the multi-component character of sediment complicates the interpretation of such logs. This work focuses on understand better the relationships between GRS logs and siliciclastic facies stacking patterns, using GRS measurements combined with lithology, modal composition and geochemistry. This study is focused on outcrops in the St. Antonin–Annot–Grand Coyer–Chalufy areas of the Oligocene Grès d´Annot Formation (SE France). The studied siliciclastics reveal moderately high total radioactivity and mean concentrations of K: 3.8 %, U: 4.5 ppm and Th: 13 ppm. U and Th are predominantly contained within heavy minerals and so tend to be concentrated in mudstones and heterolithic sand-mud facies whereas sandstone and conglomerate facies have slightly higher levels of K due to higher contents of K-feldspars and micas. The abundance of minerals containing radioactive elements and the relatively low compositional contrast between different facies are considered the main reasons why the standard gamma-ray measurements provide only a limited proxy for lithology in this case. Two complete regressive-to-transgressive cycles were identified based on combined Th and Th/K logs and facies stacking patterns. The cycle boundaries are marked by shifts towards coarse-grained facies, abrupt increase of U and Th concentrations and Th/K ratio within the proximal deposits at the St. Antonin section. In the distal Chalufy section, the response of the gamma-ray proxies to facies shifts is reversed. These boundaries, interpreted as basal surfaces of forced regression, can be correlated with prominent erosional surfaces at the Annot section. GRS logging is a sensitive method capable of indicating changes of genetic depositional units in sand-rich turbidite systems. However, detailed facies analysis, mineralogy and geochemistry producing the radioactive signal are necessary to correctly interpret such logs.

Schnyder, J., Ruffell, A., Deconinck, J., and Baudin, F., 2006, Conjunctive use of spectral gamma-ray logs and clay mineralogy in defining late Jurassic–early Cretaceous palaeoclimate change (Dorset, U.K.): Palaeogeography, Palaeoclimatology, Palaeoecology, v. 229, no. 4, p. 303-320, doi:10.1016/j.palaeo.2005.06.027.

Abstract: Detrital clay mineralogy is controlled by weathered source rock, climate, transport and deposition that in turn influence the spectral gamma-ray (SGR) response of resultant sediments. Whilst a palaeoclimate signal in clay mineralogy has been established in some ancient successions, the SGR response remains contentious, largely because the data sets have yet to be collected at the same or appropriate vertical scales to allow comparison. In addition, the influence of organic matter on SGR is not always considered. Here, we present clay mineralogical, total organic carbon (TOC) and SGR analyses from the late Jurassic and early Cretaceous of the Wessex Basin, a period of previously documented palaeoclimate change. The aim of this paper is to estimate the sensitivity of SGR as palaeoclimatic tool, SGR and clay mineral data having been collected at the same sample points, making this one of the most rigorous comparison of clay mineral and SGR to date. Overall, the correlation between high thorium/potassium or thorium/uranium and kaolinite associated with a well-established palaeoclimate change shows that elevated thorium may be used as a proxy for humid palaeoweathering, as suggested by few previous studies.

References

Asfahani, J., 2002, Phosphate Prospecting Using Natural Gamma Ray Well Logging in the Khneifiss Mine, Syria: Exploration and Mining Geology, v. 11, no. 1-4, p. 61-68, doi:10.2113/11.1-4.61.

Belknap, W.B., Dewan, J.T., Kirkpatrick, C.V., Mott, W.E., Pearson, A.J., and Rabson, W.R., 1959, API Calibration Facility for Nuclear Logs, in Proceedings, Drilling and Production Practice: New York, NY, American Petroleum Institute, p. 289-317.

Cripps, A.C. and McCann, D.M., 2000, The use of the natural gamma log in engineering geological investigations: Engineering Geology, v. 55, no. 4, p. 313-324, doi:10.1016/S0013-7952(99)00085-X.

Keys, S.W., 1989, Borehole Geophysics Applied to Ground-Water Investigations: Dublin, Ohio, National Water Well Association, 313 p.

Mussett, A.E. and Khan, M.A., 2000, Looking Into The Earth: An Introduction to Geological Geophysics: New York, Cambridge University Press, 470 p.

Schnyder, J., Ruffell, A., Deconinck, J., and Baudin, F., 2006, Conjunctive use of spectral gamma ray logs and clay mineralogy in defining late Jurassic–early Cretaceous palaeoclimate change (Dorset, U.K.): Palaeogeography, Palaeoclimatology, Palaeoecology, v. 229, no. 4, p. 303-320, doi:10.1016/j.palaeo.2005.06.027.

Williams, J.H., Lapham, W.W., and Barringer, T.H., 1993, Application of electromagnetic logging to contamination investigations in glacial sand and gravel aquifers: Ground Water Monitoring and Remediation Review, v. 13, no. 3, p. 129–138.

Young, J.H., 1980, Spectral Gamma-Ray (KUT) Borehole Logging, in SPE Annual Technical Conference and Exhibition, September: Dallas, Texas, Society of Petroleum Engineers, 12 p., doi:10.2118/9465-MS.