Now that we know how the tools work, we are ready to discuss how gamma ray logs are used in log analysis. Gamma ray logs are used primarily to distinguish clean, potentially productive intervals from probable unproductive shale intervals. Spectral gamma ray logs are most useful in identifying the following. Density the litho density tools ldt use a chemical gamma ray source and two or three gamma ray detectors. Gamma rays from the formation are counted in a detector system. Shales and clays are responsible for most natural radioactivity, so the gamma ray log often is a good indicator of such.
Sedimentary facies interpretation of gamma ray gr log as basic well logs in central and lower indus basin of pakistan. Sedimentary facies interpretation of gamma ray gr log as basic well logs in central and lower indus basin of pakistan article pdf available november 2016 with 3,173 reads how we measure reads. The geologist, log analyst, or reservoir engineer needs to be thoroughly familiar with the. Gamma rays have longer penetrations than either alpha or beta rays, and can be measured by simple counter devices, such as a geiger counter. To make it work, you need to have gamma ray log, resistivity logs, density and neutron logs plotted on the same log plot respectively. Many elements are naturally radioactive as a result of. The computed gamma ray or th curve can be used to evaluate the clay content where radioactive minerals are present.
Geophysical characterization and borehole geophysical. What is commonly known as the gamma log, is actually differentiated into the 1 gamma ray log, and 2 the spectral gamma ray log. Gammaray logging has evolved from a qualitative estimation of the variation of natural radioactivity with depth in a borehole, using a geigermuller detector, to a sophisticated quantitative logging technique which yields information on the identification and amount of the radioelements present, as well as data on the likely geologic history of mobilisation of these radioelements in. In this case, the other tracks most often include resistivity, density, neutron or sonic logs fig.
Jun 29, 2012 log log plot or derivative plot in well test analysis. Note high resistivity upper and lower shales, which are the source rock for the oil in the sand. Crains petrophysical handbook natural and spectral gamma. This starts out exactly the same as the standard gross gamma ray count log. It is a wireline logging method used in mining, mineral exploration, waterwell drilling, for formation evaluation in oil and gas well drilling and for other related purposes. A major application was to solve north sea log interpretation problems in micaceous sands. Spontaneous potential sp and gamma ray logs are discussed first, as their primary use is correlation and their primary interpretive target is gross lithology the distinction between reservoir and nonreservoir. These include resistivity logs, as well as the creation of density, sonic, and other types of logs. It is an alternate to the analysis of cores, sidewall samples and cuttings. The latest variant on the gamma ray log is the spectral gamma ray log. The various openhole services offered by schlumberger are discussed in some detail, together with essential methods of interpretation and basic applications. Basically, a formation with high gamma ray reading indicates that it is a shaly or shale, when the low gamma ray reading indicates a clean formation sand, carbonate, evaporite, etc.
Gamma ray gr record intensity of radioactive source clay minerals as major component presented in mineralogical composition of rock section. Example of a spectral gamma ray log track 1 on a resistivity log on low resistivity, radioactive bakken sand 4 ohmm in best sand. Shales and clays are responsible for most natural radioactivity, so gamma ray log often. Gamma ray logging is a method of measuring naturally occurring gamma radiation to characterize the rock or sediment in a borehole or drill hole. The aim of gamma ray logging is to measure natural gamma radiation emitted by the. The radioactive particles like uranium, potassium, thorium are very fine grains. Gamma ray log lithology log measures natural radioactivity uses a scintilometer geiger counter potassium k, uranium u, thorium th, phosphorous p k abundant in clay shales unaffected by fluids high u reducing environ. Illustrates the location map of the study area as enclosed in a square data collection the data used in the present study consists of a suite of well logs. System components gamma ray control panel the pulse counting circuitry controls the operation of the lcd located in the center of the panel. Fluvial stratigraphic architecture in outcrop and its. It looked similar to an sp log and was easy to use in correlating zones from well to well. Neutron porosity acoustic magnetic resonance t2 distribution general log matrix responses. The earliest detectors on logging tools were geiger counters, but these have been replaced by scintillation detectors in most modern tools. These are real shales with gamma ray readings between 250 and 500 api units.
The standard gamma ray and the gamma ray minus the uranium component are also presented. The counts recorded on a standard gamma ray log are the sum of all gamma rays, most of which are emitted by potassium40 and isotopes in the uranium and thorium series. There is an implied or assumed relationship between grain size or change in grain size fining upwards and the wireline log gamma ray shape. Natural gamma ray spectrometry tool ngt, are given by basile et al.
Overview gamma ray log is aimed at measuring the natural radioactivity of the formation, being passive measurement method. The spectral gammaray log geological log interpretation. The instrument at the surface is designed to accept the output of a scintillation detector. Clay minerals are formed from the decomposition of igneous rock.
Petrophysical evaluation of shaly sand reservoirs in. It was updated by indiana geological survey igs personnel on may 9, 2012 to add new data and to include existing locations of wells with sample only designations rotary cuttings or cores that were formerly excluded. Most interpretations of spectral gamma ray logs focus on the relationships between the three elemental concentrations. In the field of wireline logging, there are a number of different records and reports that must be generated. Crains petrophysical handbook natural and spectral.
Its just a common sense, stepbystep approach in interpreting any well logs. The measurement scale of the gamma ray log is in api american petroleum institute units, accepted as the international reference standard that allows consistent comparisons to be made between a wide variety of gamma ray counting devices. Neutron porosity logs measure the number of pores in drilled section. Measurement of natural emission of gamma rays in oil and gas wells are useful because shales and sandstones typically have different gamma ray levels.
These advantages, coupled with the increase in drilling costs, have made logging of freshwater wells more cost effective. Established oilfield geophysical log interpretation methods have been applied to the analysis of freshwater aquifers and indicate that interpretations depend on reliable empirical data. Results and interpretation log facies and depositional environments gamma ray log shapes and results from core analysis were used to define the log facies and depositional environments in this study. The ngs natural gamma ray spectrometry tool uses fivewindow spectroscopy to resolve the total gamma ray spectra into k, th, and u curves. Caliper and natural gamma logs of well bk2202, bridgetown township, northern bucks county. Gamma ray log shamshad ppt materials chemical substances. Older tools were recorded in counts or as equivalent weights of radium per ton. Different types of rock emit different amounts and different. The first gamma ray logs were run by lane wells in 1936. The gamma log often is used to define lithology and correlate geologic units between boreholes. Gamma ray tool the gamma ray log typical applications determination of reservoir thickness lithology indicator correlation between wells estimation of shale volume principle of measurement the gr log is a recording of the count rate of at gamma emissions from rock formations adjacent to the borehole. As with all ussr logs, the interpretation and correction of these logs to api units is not straightforward. Pdf geological interpretation of spectral gamma ray sgr.
Pdf the study analyses gammaray logging data ngam collected by the attarat power company apco. However, there is an added level of sophistication. Gamma ray log can only differentiate sandstones from siltstone or mud, but cannot. Analysis of gamma ray diagrams of downhole logging of wells demonstrated the layeringrelated distribution of elevated radioactive elements and proved the reliability of the stratigraphic scheme.
Gamma ray ngrt log 2 spectral density sdl 3 compensated neutron dsn log 4 caliper log 5 spontaneous potential sp log and 6 resistivity log figure 1. To detect, recognize and evaluate radioactive minerals. In this enhancement to natural gamma ray logging, the energy levels of incoming gamma rays are counted in a series of energy windows, and an algorithm converts the energy spectrum to count rates for potassium %, thorium ppm, and uranium ppm. The measurement can be made in both openhole and through casing. The gamma ray can be an excellent estimator of shale volume in areas with little uranium and where radioactive salts are associated primarily with clay minerals. Whereas the spectral gamma ray log is able to differentiate between the different radioactive elements k. Unlike the gamma ray log, a density log must be run without the casing and varies with borehole diameter. Pdf on jan 1, 2016, jolanta klaja and others published geological interpretation of spectral gamma ray sgr logging in selected boreholes find, read and cite all the research you need on. The gamma ray log is the simpler of the two and only measures the total radioactive count rate in api units. Log interpretation principlesapplications schlumberger. In calculating these ratios, schlumberger uses the following cutoffs.
Its measure the intensity of natural radioactivity of rocks. Pdf sedimentary facies interpretation of gamma ray gr log. Gr log is used to predict the varying lithology in borehole by measuring the spontaneous emission of gr radiation. Correlation between gamma ray data and architectural elements shows that determining stratigraphic variability from subsurface gamma ray log is not so reliable. Spectral gamma ray log the amount of each individual element k,u,th contributing to this radioactivity by the wavelengths of their characteristic gamma emissions. Photoelectric, compton scattering, twin production. The results have been an increase in waterwell logging activity and a subsequent increased demand for better interpretation principles as applied to groundwater problems. Overview well log definition and its importance fundamentals of qualitative and quantitative log interpretation formation evaluation with well logs processing and interpretation of well log data conclusion 3.
The time constant and logging speed therefore influence both the bed resolution and the amount of noise recorded on the gamma ray log. The energy levels of the gamma rays are characteristic of their source isotope and form distinctive spikes on a gamma ray emission spectrum, although in practice these spikes. In most stratigraphic and petroleum geological applications, the gamma ray log is used. The radiations from three elements k,u,th in combination. Well log the continuous recording of a geophysical parameter along a borehole produces a geophysical well log.
Geophysics is applied to societal needs, such as mineral resources, mitigation of natural hazards and environmental protection. Radioactive isotopes of elements continuously decay to more stable forms and emit radiation of several types. A study was made of the natural gamma ray, neutronneutron and resistivity logs in the production part of well kj32. Gamma ray log interpretation southwest exploration services. The section is nearly a thousand feet in length and the gamma ray scale has been expanded to 400 api units to accommodate the higher radioactive black shales. It was hailed as a great advance over the sp log because its value does not depend on mud or formation water resistivity. While the gamma ray log traditionally has been used primarily for welltowell correlation, it also plays a role in quantitative log analysis. Tied to lithology, porosity, and fluid content density log dphi % porosity rhob bulk density gcm3 porosity. Petrophysical well log analysis for hydrocarbon exploration. The count rate is also smoothed by the logging speed of the tool as it is raised through the borehole. Sedimentary facies interpretation of gamma ray gr log as. How to easily read a well log in 5 minutes or less using. Well log analysis for reservoir characterization aapg wiki.
The formation evaluation gamma ray log is a record of the variation with depth of the natural radioactivity of earth materials in a wellbore. Pdf sedimentary facies interpretation of gamma ray gr. Analysis of the gamma ray logs indicates that the log trend fall mostly into four categories namely. The gamma ray log was recorded for an upper pennsylvanian section of limestones and shales from an exploration well in wabaunsee county, kansas. In the analysis of spectral gamma ray data, use is commonly made of the ratios of the abundances of the main radioactive sources. In formulating the discussion, the primary log curves from the seismicstratigraphic tool string, and the lithoporosity tool strings are. Through this log we can determine the caving size and wash outs. Gamma ray logging well logging is a geophysical prospecting technique which measures the physical properties of rocks with a sensor positioned in a borehole.
Natural radioactivity is characterized by the constant and spontaneous emission of highly energized electromagnetic waves such as gamma rays due to the decay of unstable isotopes of potassium k40, thorium th232, and uranium u238. For example, the gammaray spectral log distinguishes the three naturally occurring radioelements, thorium, uranium and potassium, while the gross count gamma. From the change in gamma ray response with depth, geologist s may infer the deposition al environment. The measurement is used to locate shale beds and quantify shale volume. Method the two most frequently used methods of estimating shale volume from the gamma ray log are the linear and the nonlinear estimators. Two detectors count the number of returning gamma rays which are related to formation electron density for most earth materials, electron density is related to formation density through a constant returning gamma. Rocks and most type of soils emit gamma ray gr in varying amount. In particular, thk and thu are often used for petrophysical interpretation and log correlation. Basic well log interpretation chemistry physical sciences. Gamma log records the natural gamma radiation emitted from rocks penetrated by the borehole. Sonic log correlate seismic data in time to well data in depth density log tool emits gamma rays detects returning scattered gamma rays gamma ray absorption is proportional to rock density measures density. The reinterpreted log was primarily based on the gamma ray log readings, then revised based upon significant variations in selfpotential.
The influence of the radioactive minerals was clearly identified from the spectral gamma ray log, which gives high thorium and potassium value in the sand zone figure 5. The depth of investigation is a few inches, so that the log normally measures the flushed zone. Open hole wireline logging gr api gamma ray tool the gamma ray log typical applications determination of reservoir thickness lithology indicator correlation between wells estimation of shale volume principle of measurement the gr log is a recording of the count rate of at gamma emissions from rock formations adjacent to the borehole. Interpretation of gammaray logs of the stratified oil shale seam in the attarat um ghudran deposit, central jordan article pdf available in oil shale 334. The gamma ray log is combinable with all tools, and is almost always used as part of every logging combination run because of its ability to match the depths of data from each run.
In this paper, i will present the principal logging results from odp leg 159 in light of the statistical analysis. It later become known as the gamma ray log, and was the first through casing method for analyzing a formations lithologic properties. Pdf interpretation of gammaray logs of the stratified. The emitting elements of primary gamma radiations include potassium 40, uranium, and thorium which are associated with rocks forming minerals in variable amount.
A log of the total natural radioactivity, measured in api units. Caliper log caliper log use to measure the hole size. Pdf interpretation of gammaray logs of the stratified oil. The api standard is set by the primary calibration test pit at the university of houston where a. Pdf interpretation of gammaray logs of the stratified oil shale. Sedimentary facies interpretation of gamma ray gr log as basic well logs in central and lower indus basin of pakistan adeel nazeera, shabeer ahmed abbasib, sarfraz hussain solangib a pakistan petroleum limited, islamabad, pakistan b center for pure and applied geology, university of sindh, jamshoro, pakistan article info. These data points are then plotted on a logging form to obtain the graphical log needed for interpretation. How to create lithology logs from gamma ray log duration. Borehole gamma ray data are used in both qualitative and quantitative log interpretation.
1430 1358 1040 1394 1042 1039 1576 1413 1303 1055 1182 452 309 936 1184 669 82 1491 239 252 93 794 262 1395 903 865 599 183