Natural Gamma Ray Spectroscopy Research Articles

Chemical cyclicity and correlation of Lower Lias mudstones using gamma ray logs, Yorkshire, UK

Natural gamma-ray spectrometry on coastal sections from Redcar and Robin Hood's Bay permits the radioactive characterization of five different mudstone units and the construction of an outcrop gamma-ray log for the Lower Lias. Correlation of these five units with their equivalents in the gamma-ray log of the Felixkirk Borehole, located 40 km to the west, has been established. Statistical comparison of tie points in the composite coastal section shows there to be a stratigraphic gap of 10 m between the two localities relative to the continuous section in the Felixkirk Borehole. The distribution of Th (ppm), K (%) and U (ppm) are generally highly correlated with the total radioactivity. However, anomalies do occur: a peak in the Th/K ratio can be related to an iron-oolitic horizon marking a condensed sequence in the early/mid-Sinemurian; a sharp fall in the Th/K ratio followed by high U values coincides with the early-Pliensbachian transgression. Comparison with mineralogical (clays) and chemical (Si/Al and Al/K ratio, V, Total Organic Carbon) profiles of the same section allows for an evaluation of the significance of radioactive element distribution in terms of grainsize, run-off and oxygenation indicators. A spline curve fitted through the total radioactivity data emphasizes low frequency cyclicity (wavelengths of between 10 and 30 m) in the outcrop section. These cycles appear to be correlatable with cycles of comparable wavelengths in the low-pass filtered gamma-ray log of the Felixkirk Borehole. Sedimentological evidence confirmed high radioactivity peaks correspond to transgressive phases. The gamma-ray signal of the Felixkirk borehole has also been analysed for high frequency cyclicity. With an overall rate of accumulation of about 13m/Ma variation in the Milankovitch band is marked by units 5 m or less in thickness. Evidence for eccentricity, obliquity and precession cycles has been found both in borehole and in outcrop. A typical shift from lower to higher frequencies occurs consistently in transgressive phases, and is related to increased sedimentation rates.

Geological Applications of the Spectral Gamma-Ray Logging Tool in Kansas

The natural gamma-ray spectroscopy tool subdivides the total gamma-ray count measured in subsurface formations into contributions from potassium (%), thorium (ppm), and uranium (ppm). Examples of spectral gamma-ray logs are common in the Chase Group of southwestern Kansas but are also found in a variety of other stratigraphic sections across Kansas. The most common applications are in an improved estimation of shale content based on thorium and potassium and the recognition of localized uranium-rich fractures in otherwise clean carbonates. The thorium/potassium ratio is a useful general indicator of relative potassium enrichment and allows broad distinctions of illitic and micaceous facies from sandstones and shales richer in kaolinite and/or chlorite. The basal Cretaceous unconformity is sometimes highlighted dramatically by this ratio, where the potassium felspar content of the Permian Cedar Hills Sandstone contrasts with the overlying Cheyenne Sandstone. Thorium-potassium crossplots are helpful in recognizing the pattern of distinctive clay mineral associations. The thorium/uranium ratio is an excellent indicator or redox potential associated either with the original depositional environment or subsequent diagenesis. Patterns of marine transgression and regression are readily interpreted from spectral gamma-ray logs run in Permian and Cretaceous sections. In other cases, low Th/U ratios appear to reflect diagenesis, suchmore » as in some hot regressive limestones in the Lansing Group, where uranium may have been enriched by ancient groundwater regimes.« less

Natural gamma-ray spectrometry in hydrocarbon-bearing sandstones from the Norwegian Continental Shelf

Abstract Concentrations of thorium (Th), uranium (U) and potassium (K) are measured using natural gamma-ray spectrometry (NGS) logs. The precision of NGS measurements in sandstones varies: K measurement is in general moderately good; Th measurement varies with the service company involved; U measurement is unreliable. Concentrations of Th, U and K in standard tables of minerals used commonly in log evaluation are not representative of the actual elemental concentrations found in sandstone minerals. Th/K cross-plots are shown to be unsuitable for the identification of clay minerals in sandstones. Illite cement cannot under normal geological circumstances be identified from the K content of the rock. Authigenic kaolinite has no particular affinity for concentration of Th. Th in sandstones is concentrated predominantly in detrital heavy minerals with up to 200f the total Th present in silt-sized and finer authigenic REE-phosphates and Th salts. Variation in the K content of micas from some Jurassic sandstones demonstrates the need for using formation-specific chemical data for the minerals of interest (micas, feldspars, clay minerals) as input parameters in normative calculations. Appropriate statistical control of the validity of Th, U and K contents for specific minerals is required.

An investigation into nuclear log responses of North Sea Jurassic sandstones using mineralogical analysis

Abstract The results of laboratory mineralogical analyses have been compared with downhole log responses to determine the possibility of detecting individual minerals using the gamma-ray, natural gamma-ray spectroscopy, formation density and litho-density logs. Cored sequences from the Murchison, Thistle, Kittiwake, Gannet and Tartan Fields, North Sea, are used as case examples. K-feldspars, micas, detrital clays (illite, and possibly kaolinite) and phosphatic cements are shown to produce a measurable response on the gamma-ray and natural gamma-ray logs. Authigenic kaolinite did not impart radioactivity to the sandstones in the examples studied. The photoelectric absorption index ( Pe ) was found to be particularly sensitive to Fe-bearing minerals and baryte. A number of problems constrain full characterization of the mineralogy of a sandstone: specific detection of quartz and plagioclase feldspars is not possible by nuclear logs, distinction between minerals is problematical when a log responds to several minerals simultaneously, and calibration for minerals with a variable composition such as ferroan dolomite and mixed-layer illite-smectites is difficult.

Ancient subaerial exposure and freshwater leaching in sandstones

Research Article| December 01, 1990 Ancient subaerial exposure and freshwater leaching in sandstones D. Emery; D. Emery 1BP Exploration Company, Ltd., Britannic House, Moor Lane, London EC2Y 9BU, England Search for other works by this author on: GSW Google Scholar K. J. Myers; K. J. Myers 1BP Exploration Company, Ltd., Britannic House, Moor Lane, London EC2Y 9BU, England Search for other works by this author on: GSW Google Scholar R. Young R. Young 1BP Exploration Company, Ltd., Britannic House, Moor Lane, London EC2Y 9BU, England Search for other works by this author on: GSW Google Scholar Author and Article Information D. Emery 1BP Exploration Company, Ltd., Britannic House, Moor Lane, London EC2Y 9BU, England K. J. Myers 1BP Exploration Company, Ltd., Britannic House, Moor Lane, London EC2Y 9BU, England R. Young 1BP Exploration Company, Ltd., Britannic House, Moor Lane, London EC2Y 9BU, England Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1990) 18 (12): 1178–1181. https://doi.org/10.1130/0091-7613(1990)018<1178:ASEAFL>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation D. Emery, K. J. Myers, R. Young; Ancient subaerial exposure and freshwater leaching in sandstones. Geology 1990;; 18 (12): 1178–1181. doi: https://doi.org/10.1130/0091-7613(1990)018<1178:ASEAFL>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We present an example of leaching by meteoric water in the Jurassic Magnus Sandstone Member of the northern North Sea. The interpretation is based on information from several independent techniques, including downhole natural gamma-ray spectrometry, detailed petrographic analysis, and cross-sectional time-slice reconstructions from seismic data. The initial lines of evidence that prompted further investigation into freshwater involvement in the Magnus Sandstone Member were brackish formation-water compositions and a potassium depletion shown by the gamma-ray spectrometry log, in sandstone beneath the unconformity at the base of the Cretaceous section. Subsequent detailed petrographic study confirmed that the potassium depletion had been brought about by potassium feldspar dissolution, resulting in precipitation of kaolinite and creation of secondary porosity, the latter also revealed by conventional core-analysis porosity and permeability measurements. Finally, cross-sectional time-slice reconstructions from a regional seismic line across the Magnus oilfield strongly suggested subaerial exposure of the Magnus Sandstone Member in Early Cretaceous time and consequent freshwater leaching of the Magnus Sandstone Member and generation of the unconformity at the base of the Cretaceous. The combination of several lines of evidence from the Magnus field supports the process of subaerial exposure and freshwater leaching to create secondary porosity in sandstones. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Natural gamma ray spectroscopy applied to borehole measurements

Current techniques for performing natural gamma-ray spectroscopy measurements in borehole environments are described. The data were obtained using the system known as the compensated spectral natural gamma tool, which uses the complete energy spectrum to provide information on the borehole environment in addition to concentrations of potassium (K), uranium (U) and thorium (Th) in the formation. Elemental concentrations are estimated from count rates in 13 selected energy ranges (windows) using the method of weighted-least-squares, which incorporates real-time compensation for borehole effects. The use of a low-atomic-number toolcase allows gamma rays in the low-energy, photoelectrically sensitive part of the spectrum to be transmitted to the detector. The system gain stabilizer operates in a coincidence mode so that formation gammas incident on the detector can be separated from those which originate in the /sup 241/Am stabilization source. The low-energy data are processed into ratios of Compton and photoelectric-window count rates. Two examples demonstrate the presentation of the logs and the quality of the results.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Clay and Framework Mineralogy, Cation Exchange Capacity, Matrix Density, and Porosity from Geochemical Well Logging in Kern County, California: ABSTRACT

Elemental concentrations of several inorganic elements were determined in a continuous basis with depth using the Schlumberger gamma-ray spectrometry (GST) and natural gamma-ray spectrometry (NGS) logs in a Santa Fe Energy Company well in the Kern Front field in Bakersfield, California. Logs of Al, Si, Ca, K, Fe, Ti, and non-pore H were processed by a matrix multiplication procedure, used previously for a Venezuelan well, into estimated abundances of quartz, feldspar, calcite, ilmenite, and the clay minerals kaolinite, illite, and smectite. A total of 64 core-plug samples were analyzed for elemental content by x-ray fluorescence and neutron activation analysis and for mineralogy by bulk and < 4 ..mu.. clay x-ray diffraction analyses. Log-derived elemental concentrations and mineral abundances show good agreement with the core values. Cation exchange capacity (CEC) is estimated from the abundances of the clay minerals and typical values for these clays. The CEC log agrees well with CEC values determined on the core samples. The mineral abundance logs also permit an estimation of the average matrix density as a function of depth, derived from typical grain densities for each mineral phase. This matrix density log, combined with the bulk density log, yields a porosity log that comparesmore » well with over 200 measurements on core.« less

A Kalman filtering approach to natural gamma ray spectroscopy in well logging

This paper describes an application of (adaptive) Kalman filtering to a geophysical subsurface estimation problem. The NGT® is a sonde designed to detect the natural gamma rays of various energies emitted from a formation by the radioactive nuclei of potassium (K) and the thorium (Th) and uranium-radium (U) series. Using a minicomputer at the surface, the (Th,U,K) concentrations along the borehole have to be estimated on-line from the detection of the gamma rays in five energy windows. The standard technique in the logging industry has been to compute the elemental concentrations at a given depth using only the observed counting rates at the same depth. The resulting estimates have fairly large statistical errors which have limited the application of the NGT in computer reservoir evaluation. In this paper, it is shown that a Kalman filter based on a dynamical model of the (Th,U,K) vertical variations can produce real-time estimates that are readily usable on a quantitative basis. The paper focuses on the usual critical issues in applying Kalman filtering to real data, namely modeling, adaptivity, and computational aspects.

Reservoir Facies Zonation Using Wireline Logs: ABSTRACT

Sedimentary rocks can be described an distinguished from others not only by their lithology, geometry, sedimentary structures, and fossil content but also by their overall response on the electric logs--their FACIOLOG processing zones together those sections of a lithological sequence which have a comparable suite of electric log responses to give an zonation. Geologists have traditionally used electric logs as a basis for when drawing up composite (lithological) columns. FACIOLOG processing now offers the potential of treating all the logs, including the dipmeter, on an objective and quantitative basis. The is made by cross-plotting all the log responses on a multi-dimensional set of axes and then using cluster analysis to identify locally dense areas. Each cluster represents a series of depth intervals with a similar suite of log responses (an electrofacies). The degree of similarity of the various clusters is then expressed in the form of a dendrogram, and the complete well section is displayed with each level assigned to its own particular electrofacies. How closely does the electrofacies zonation correspond to the more conventional lithofacies zonation? Generally there is good agreement because electric logs, especially with new services such as the litho-density and natural gamma ray spectrometry tools, respond to the basic mineralogy of the rock matrix as well as the fluid content. FACIOLOG processing also incorporates the high resolution information from the dipmeter, which corresponds to the basic sedimentology. Because of the usually good match between the and the lithological zonations in cored sequences (especially in siliclastic sequences), FACIOLOG processing can be used to extrapolate the results of core analysis into those sections in the well where there is no core. A new 22 in. (56 cm) wide presentation format allows all the logs, the dipmeter curves (with GEODIP or CLUSTER tadpoles), and the itself to be used as a basis for integrating all the information acquired when a well was drilled. Lithological descriptions from cuttings and cores or other stratigraphic information can easily be integrated onto the flexible format. Presenting all the logs together and having the additional advantage of the quantitative is clearly an aid to well-to-well correlation. With data banks, specific zones may now be automatically traced across End_Page 504------------------------------ complete reservoirs. Identifying zones with a consistent and continuous log response greatly aids manual log interpretation. It also provides a valid means of data reduction for the first passes of a computer-processed interpretation because an interpretation model may be tested by treating a limited, yet representative, number of points. Cross-plot interpretation can also be simplified by using the type on the z-axis, and displaying averaged log values for each electrofacies. The FACIOLOG approach is designed not to compete with conventional facies analysis but to put electric logs into a framework which the geologist can easily integrate with his own studies and thereby squeeze the maximum amount of geological information out of them. End_of_Article - Last_Page 505------------

Significance of Gamma-Ray Spectroscopy for Evaluating Shallow Gas Reservoirs from Bowdoin Dome, Montana: ABSTRACT

Natural gas is currently being produced from shallow gas reservoirs of Late Cretaceous age in the Bowdoin dome area, north-central Montana. Clay-rich rocks of the Mosby Sandstone Member of the Belle Fourche Shale, the Greenhorn Formation, and the Carlile Shale are the principal reservoirs. Laterally equivalent rocks are major producers of gas in southeastern Alberta. The Mosby consists of composite bedsets of lenticular to wavy-bedded sandstone and dark-gray shale. The Greenhorn, which unconformably overlies the Mosby, is as much as 40 ft (12 m) thick and consists of black, organic carbon-rich (as much as 9% organic carbon) shales, bentonites, and calcarenties consisting mainly of inoceramid prisms. Lateral equivalents of the Greenhorn are probably the principal source be s for oil occurrences farther west in the Montana disturbed belt. The Carlile Shale overlies the Greenhorn with apparent conformity and is lithologically similar to the Mosby. However, the Carlile mudstones contain more smectitic mixed-layer clays than those of the Mosby. Exploration and production of hydrocarbons from these clay-rich rocks present significant problems in log interpretation and in well completion and treatment procedures. In an effort to address these problems, a detailed geochemical, petrologic, mineralogical, and stratigraphic study was undertaken to characterize the gas-productive formations at Bowdoin dome and to evaluate the usefulness of natural gamma-ray spectroscopy for interpreting geology and reservoir quality. The gamma-ray spectral log resolves the total gamma-ray spectrum into the three most common components of radiation--gamma rays from 40K, U-series decay, and Th-series decay. In principle, the log can be used to determine clay content, clay composition, source-bed richness, and lithology. Results of this inv stigation suggest that the log successfully estimated organic-carbon content, identified bentonites, and located the unconformity at the base of the Greenhorn. In addition, subtle differences in clay composition between the Belle Fourche and Carlile Shales were apparent. These differences in clay mineralogy have important implications for interpreting the irreducible water saturation and water sensitivity of the formations and thus for the choice of drilling and completion techniques. Detailed analyses of individual core samples provide much more accurate information about single point samples than did interpretations of single points from the spectral log. However, analyses of individual core samples only distinguish mineralogical or geochemical deviations with frequencies greater than two times the sampling interval. In contrast, the log provided a running average capable of resolving relatively small-scale fluctuations and displayed considerable precision and accuracy when calibrated to laboratory analyses. In conclusion, gamma-ray spectroscopy provides a potentially important tool for the investigation of unconventional gas reservoirs and source beds in clay-rich and organic carbon-rich rocks. End_of_Article - Last_Page 468------------

Sedimentology of Tight Gas Sandstones: Well Log and Core Evaluation: ABSTRACT

A cooperative study with the Delta Drilling Company (Tyler, Texas) provided a unique correlation of experimental tools (induced gamma-ray spectroscopy, digital sonic) and commercial tools (natural gamma-ray spectroscopy, high-resolution dipmeter) with petrophysical/geologic measurements on a whole rock core from the tight gas sandstones of the Upper Jurassic Cotton Valley Group in east Texas. For example, a SARABAND suite of logs and dipmeter analyses processed according to the major genetic units of sedimentation (barrier bar, tidal delta) demonstrates the dynamic conditions of fluvial and shallow-marine (tidal) systems. Induced and natural gamma-ray spectroscopy with a SARABAND presentation yields a stratigraphic analysis of the clay/non-clay fractions and the lithology. Of growing economic importance, fracture containment boundaries to hydraulic fracturing can be predicted from a mechanical properties log--a combination of SARABAND and digital sonic. The tight gas sandstones are characteristically well-laminated and bedded lithic sandstones with low porosities (< 10%) and low permeabilities (< 0.1 md). The intergranular pores are lined with diagenetic minerals--quartz overgrowths and calcite, and are filled, lined, and/or bridged with nonexpandable illite, chlorite, and illite/chlorite mixed-layer clay minerals. The depositional paleoenvironment of the lower section of the Cotton Valley Group is interpreted as a sequence of shallow-marine, organically burrowed, layered, foreshore-shoreface deposits comprising the above-mentioned major genetic sedimentation units, the barrier bar and the tidal delta. End_of_Article - Last_Page 972------------

THE LITHO-DENSITY* LOG

The Litho-Density Log (LDT) measures a new formation parameter, Pe, the effective photoelectron cross-section. This measurement is made at the same time as the measurement of bulk density, ρb. The equipment is com bined with Gamma Ray and Compensated Neutron Loqqinq (CNL*) devices.Pe is a function of Z. the effective atomic number. The Z of a formation determines its chemical content and therefore its lithology.The effect of gas on the Pe measurement is almost neglible so it becomes a good lithology Indicator in non-shaly gas-bearing reservoirs. As a result, it provides a means for confirming porosity determinations made with corrected bulk density and CNL porosity data.The radioactive logging parameters for a number of sedimentary minerals have recently been computed. With this knowledge and the addition of the Pe measurement it is possible to discriminate between tight streaks and heavy minerals such as siderite.A number of clays, such as glauconite, may be identifiable with cross-plot methods. Knowledge of the presence of glauconite, which is thought to be formed in shallow marine waters, can be useful in establishing depositional environments.If a Natural Gamma Ray Spectroscopy (NGS*) Log is available, additional cross-plots are proposed which will enhance the identification of clay mineralsSeveral of the proposed interpretation techniques are illustrated with data from Australian and other well logs.*Mark of Schlumberger