Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
  • G01T1/02—Dosimeters
  • G01T1/10—Luminescent dosimeters
  • G01T1/11—Thermo-luminescent dosimeters

Definitions

• This invention relates to a method of exploration for minerals and petroleum and in particular to a method of exploration for mineral and petroleum using artificial thermoluminescence analysis methods.
• This invention 5 further includes a method of determining the proximity of uranium and petroleum deposits utilising thermoluminescence analysis techniques.
• Thermoluminescence describes the emission of light caused by thermal activation of trapped excess electrons and their corresponding electron deficient sites (holes). Activation may lead to a recombination of electron and hole which will result in the emission of a 5 quantum of light.
• Ionizing radiation entering a crystal is capable of dislodging electrons from their atomic positions, thus creating free electrons and holes (sites which have lost an electron). Most ' electrons and holes recombine almost 0 immediately but in non-conducting minerals a small percentage of the holes and excess electrons may be trapped on lattice defects and impurities.
• a well known hole 5 trap is a silicon site in which Al 3+ has been substituted for Si 4+ . Electrons can also be trapped, and this usually occurs on vacant oxygen sites where
• the intensity and shape of the quartz glow curve depend on a number of factors such as lattice vacancies and impurities capabe of acting as traps, respective crystal field energies, and trap densities and charge occupancies.
• the charge occupancy rate is a function of, and is affected by external by, external physical effects where the term external physical effect refers to external influences, such as further ionizing radiation or heat. As the charge occupancy affects the strength of the TL signal, TL has in the past been used as a dosimeter.
• thermoluminescence analysis techniques can be utilised for petroleum maturation determinations.
• the purpose of artificial thermoluminescence (TL) in petroleum maturation is to determine the palaeotemperatures of sediments (usually sandstone or carbonates) in a potential oil-bearing basin.
• TL thermoluminescence
• Such determinations rely on the fact that the basic TL glow curves may alter in shape and intensity as a function of temperature.
• precisely how differing temperatures affect the TL glow curve within a sedimentary sequence then such information can be used to indicate past sediment temperatures. Since petroleum matures in a narrow temperature window (100° to 180°C) a knowledge of the past temperatures is obviously useful. Therefore it -is an object of this invention to provide a method of exploration for minerals or petroleum using a thermoluminescence analysis technique.
• a method of exploration for minerals or petroleum using thermoluminescence analysis of a crystaline sample to determine the proximity of the sample to a mineral or petroleum deposit comprises first irradiating the sample with gamma-radiation, said irradiation being sufficient to substantially fill the crystal lattice traps within the sample, heating the sample from ambient temperature to an elevated temperature, and measuring the intensity of luminescent radiation at a plurality of glow peak temperatures, and relating the said luminescent intensities to luminescent intensities of reference samples, said reference samples having been subjected to known degrees of an external physical effect, so as to determine the extent of external physical effect to which the sample has been subjected, wherein the degree of external physical effect indicates the proximity of the sample to a mineral or petroleum deposit.
• Thermoluminescence can be used in two ways in geological studies.
• the first relates to charging or filling of available traps by ionizing radiation. As the radiation dose is increased the TL intensity also increases.
• This is the principle applied in TL dating, in particular in archaeology and in general to sediments in which samples have initially been bleached by UV light from the sun.
• the second type of application follows from the changes in the number of available traps caused by the external physical effects such as ionizing radiation or palaeotemperatures. These changes, expressed in the range of glow curves shown in Figure 1, reflect the total external physical effect to which the sample has been subjected, which includes the past and present external physical effects.
• TL analysis in for example uranium exploration, over conventional techniques such as geochemistry, soil radon surveys, alpha metering and radiometrics, is that these processes rely on detection of the uranium or its stored products close to the mineralization.
• TL is a measure of past as well as present radiation dose it does not rely on proximity to the mineralization in order to allow detection of radiation sensitization or damage to the host quartz lattice.
• progressive cumulative deposits such as a Tertiary role front type deposit
• TL has the capability of tracing radiation effects over a distance of kilometres from the present ore position.
• Such a technique is obviously useful in the presence of arid and deeply weathered environment where past radiation effects, which cause the TL anomalies, have been completely removed from the surface or from the sample.
• a method of exploration for petroleum using thermoluminescence analysis of sediment samples to determine the palaeotemperatures of the sediment samples comprises testing sediments of known thermal regimes using thermoluminescence techniques and comparing the results to sediment samples of unknown thermal regimes in order to establish the palaeotemperatures of sediment samples.”
• the TL analysis technique begins with the preparation of a suitable sample.
• the object of sample preparation is to extract grains of monomineralic quartz and to irradiate them to a level required to fill the maximum number of crystal lattice traps.
• Samples of rock, drill core, chips or sand are suitable. These are mechanically crushed and ground then sieved to remove any size fraction between 30 and 150 mesh BSS. This grain size is chosen to minimise the effect of alpha radiation relative to beta and gamma radiation in the TL process. All samples are then ultrasonically washed to remove any dust or dirt adhering to their outer surface, and then washed with acetone and left to dry. As minor amounts of accessory minerals such as feldspar and zircon may still be present, each sample is passed through a franz electromagnet separator with currents in excess of 1.2 amps and a reverse slope of 1°. Quartz, being bimagnetic, is forced into the magnetic stream of the separator, whereas feldspar and zircon being non-magnetic are taken by gravity into the non-magnetic stream. The resultant separate is approximately 99% pure quartz.
• a critical point in the TL process is the filling, by radiation, of vacant lattice traps. Any radiation dose will produce some artificial TL, but optimal results are achieved when all or most of the lattice traps are filled. This ensures that comparisons between samples reflect the differing trap densities (which are themselves the result of exposure to radiation from mobile uranium).
• a radiation dose between 5 x 10 5 rads and 10° rads of cobalt-60 gamma radiation is used.
• Samples are then wrapped in alumium foil to protect them from direct light (which can cause the artificial TL process to begin) and are left for 24 to 72 hours to allow any phosphorescence or radioluminescence to decay away.
• quartz grains are then placed on a 1 cm diameter stainless steel disc which is placed on a heating strip in a standard Littlemore scientific equipment TL apparatus.
• the - heating rate is approximately 1.23°C per second.
• the oven gas used is high purity nitrogen and the photomultiplier setting is approximately 1364 volts. After measurement the samples are weighed and a number of parameters calculated, i.e. glow peak temperatures, intensities, percentages and ratios of intensities.
• the major advantage of artificial TL is that cumulative radiation effects on the quartz can be related to an increase in intensity of firstly, the low temperature glow peak (LT) followed by a decrease after it surpasses an optimal level.
• the middle temperature glow peak (MT) begins to increase until it too reaches an optimum intensity related to radiation, after which it also then decreases.
• the high temperature glow peak (HT) continues to increase.
• Intensities alone are usually of limited value in uranium exploration as many fluctuations occur. For these reasons ratios of glow peaks or percentages of one glow peak (usually the HT peak) as a proportion of the total are used to monitor proximity to uranium mineralization. These latter parameters do not suffer the same fluctuation as intensities alone.
• Figure 1(a) shows a glow curve for quartz which has not been exposed to more than background amounts of radiation. Quartz has several glow peaks. The exact temperature at which they occur depends on the heating rate. For this reason, and for simplicity (as this is a representation of expected glow curve variations rather than actual glow curve variations) only three glow curve peaks will be shown in subsequent diagrams rather than all known glow peaks of quartz. The three glow peaks will be referred to as the low temperature, middle temperature and high temperature glow peaks (LT, MT, HT).
• Sensitizaion is expected to begin at approximately 5 x 10 5 rad gamma radiation.
• the LT glow peak increases in intensity, such that it obscures all other glow peaks.
• the increase in LT peak intensity may be two or three orders of magnitude relative to the initial intensity. This is indicated in Figure 1(b).
• Equivalent gamma dose should be higher than 10 10 rad; perhaps as high as 10 11 or 10 12 rad gamma radiation.
• glow curves such as Figure 1(e) are almost invariably associated with uranium mineralization or in close proximity to uranium mineralization.
• FIG. 2 is a plot of high temperature (HT) peak percentage on the X-axis and some other variable on the Y-axis.
• the variable on the Y-axis will depend on the stage of radiation sensitization and/or desensitization of the project area though may be low temperature (LT) peak intensity, middle temperature (MT) peak intensity (as with many Middle Proterozoic Sandstones) or a ratio of the low and middle temperature glow peaks.
• a quartz sample At low or background radiation doses a quartz sample will have a very low HT peak percentage and a large LT or MT peak intensity. Such a sample will therefore slot in the upper left hand corner of Figure 2.
• the LT or MT peak intensity will decrease while the HT peak intensity (and hence HT peak percentage) will increase.
• the sample position will then move down from the upper left hand corner of Figure 2 through the increasing sensitization field.
• the LT and/or MT glow peaks will have both decreased in intensity whilst only the HT peak continues to increase (as in Figure 1(e).
• This sample conforming to a strongly radiation damaged sample, would plot in the lower right hand corner of Figure 2.
• most uranium ore samples from a variety of case studies have been found to plot in the lower right hand corner of such variation diagrams in accordance to theory.
• TL in petroleum maturation is to determine the palaeotemperatures of sediments in a potential oil-bearing basin. Such determinations rely on the fact that basic TL glow curves may alter in shape and intensity as a function of temperature.
• An older (e.g. Permian) basin such as the Cooper Basin in South Australia, may be a better basin for calibration.
• Part of the basement of the Cooper Basin is known to contain very radioactive granites, creating a high heat flow. These granites are old (+ 1500 million years) and contain quartz with a homogenous TL glow curve corresponding to what one would call high radiation damage in analogy to quartz from the vicinity of uranium deposits. Basal sedimentary material was apparently derived from this source and contains quartz with the same TL characteristics as those of the radioactive granites.
• a further way of using quartz as a palaeothermometer is to compare several quartz fractions from a given area whose temperature is known from vitrinite reflectance, organic chemistry or illite (clay) crystallinit , e.g. material containing radiation damaged quartz, igneous quartz and fragments of vein chert quartz. If these quartz fragments can be recognized microscopically and their annealing characteristics are different, palaeothermal information may be obained which is not as sensitive to the age of the source material.
• annealing implies return to first or lower order kinetics of the untrapping mechanism, comparison of different glow peaks of differently sensitised material (or radiation-damaged material) will lead to an applicable method of palaeothermometry once the age of the sediment is known.
• the first step is to test the TL behaviour of common minerals found in sedimentary basins (quartz, calcite, apatite, zircon, feldspar, etc.) against the number of time/temperature conditions in an attempt to generalize annealing behaviour. This may be accomplished in three main ways :

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• Investigating, Analyzing Materials By Fluorescence Or Luminescence (AREA)

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• 1987
  • 1987-01-22 CA CA000527954A patent/CA1268560A/en not_active Expired - Fee Related
  • 1987-01-23 ZA ZA87513A patent/ZA87513B/xx unknown
  • 1987-01-23 WO PCT/AU1987/000017 patent/WO1987004528A1/en not_active Application Discontinuation
  • 1987-01-23 JP JP87500904A patent/JPH01502394A/ja active Pending
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  • 1987-01-23 EP EP19870900785 patent/EP0291506A4/en not_active Withdrawn
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