WO1987004528A1

WO1987004528A1 - Minerals and petroleum exploration using thermoluminescence

Info

Publication number: WO1987004528A1
Application number: PCT/AU1987/000017
Authority: WO
Inventors: Mark Bret Mcewen Hockman; Peter Johannes Maria Ypma
Original assignee: Luminis Pty. Ltd.; Australian Commercial Research & Development Limit
Priority date: 1986-01-24
Filing date: 1987-01-23
Publication date: 1987-07-30
Also published as: CA1268560A; ZA87513B; DK498987D0; EP0291506A1; BR8707542A; DK498987A; JPH01502394A; EP0291506A4

Abstract

A method of exploration for minerals (e.g. Uranium) and petroleum using thermoluminescence analysis of a crystalline sample to determine the proximity of the sample to a mineral or petroleum deposit. The method comprises first irradiating the sample with gamma-radiation so as to substantially fill crystal lattice traps within the sample, heating the sample from an ambient temperature to an elevated temperature, measuring the intensity of luminescent radiation at a plurality of glow peak temperatures and relating the luminescent intensities to luminescent intensities of reference samples having known characteristics indicative of the proximity of the sample to a mineral or petroleum deposit. The sample may be treated to remove unwanted accessory minerals such as Feldspar and Zircon and may be shielded from direct light to allow phosphorescence and radioluminescence to decay to insignificant levels.

Description

MINERALS AND PETROLEUM EXPLORATION USING THERMOLUMINESCENCE

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.

BACKGROUND OF THE INVENTION

0 Thermoluminescence (TL) 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.

In quartz, the mineral most widely used in TL investigations of uranium deposits, a well known hole 5 trap is a silicon site in which Al³⁺ has been substituted for Si⁴⁺. Electrons can also be trapped, and this usually occurs on vacant oxygen sites where

O 9^ώ— charge i.s mi.ssi.ng. Ini.tially the number of hole traps in quartz may be expected to be larger than the number of electron traps, but as the number of holes and electrons eventually trapped must be equal, the electron trapping mechanism becomes the predominant factor affecting the resultant strength of the TL signal.

These trapped charges can be released by thermal activation which facilitates recombination of the holes and electrons. If recombination occurs at a specific site it may result in the emission of a quantity of visible light. This light can be measured and recorded as a glow peak. As electrons and holes may be trapped on a variety of sites with different crystal field energy, different amounts of thermal activation will be required to release them, and so over a range of temperatures a number of glow peaks are recorded which constitute a glow curve. Quartz typically gives three major glow peaks at approximately 190°C, 260°C, 350°C with subordinate peaks at 150°C and 220°C.

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.

Presently, conventional techniques for the exploration of uranium deposits include techniques such as geochemistry, soil radon surveys, alpha measuring, and radiometrics. These techniques rely on detection of the uranium or its daughter products close to the mineralization.

In addition to techniques in relation to uranium deposits, 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. Such determinations rely on the fact that the basic TL glow curves may alter in shape and intensity as a function of temperature. When it is known 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.

SUMMARY OF THE INVENTION

Since external physical effects also affect trap densities within a crystal, a permanent variation in both intensity and shape of the glow curve may result. This property of "remembering" exposure to past external physical effects may be utilised in mineral and petroleum exploration and can be detected by artificial TL analysis. For example radiation emissions from uranium affect the trap densities in quartz, which results in a permanent variation in both intensity and shape of the TL glow curve. This property of quartz to remember past exposure to radiation may be utilised in uranium exploration and can therefore be detected by artificial TL analysis. Briefly, according to one aspect of this invention, 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. Therefore, the advantage of 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. However, because 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. In the case of 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.

In a further aspect of this invention, 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." DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the invention need not necessarily include the abovementioned details, two embodiments are described hereunder in further detail, but it will be seen that the invention need not be confined or restricted to any one or combination of the further details of these embodiments.

In this first embodiment, a method of exploration for uranium is described. 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 frantz 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 10⁵ rads and 10° rads of cobalt-60 gamma radiation is used.

This should be delivered at dose rates of greater than 2.5 x 10⁵ rads per hour. 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.

Approximately 15 to 20 milligrams of 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 (ATL) 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. During this process 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. At this stage in the cumulative radiation process only the high temperature glow peak (HT) continues to increase. At extremely high levels of radiation damage even the HT will decrease. These changes in glow peak intensities and hence glow peak ratios are schematically shown in Figure 1. 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).

As the radiation dose increases one would expect the process of sensitization to begin. Sensitizaion is expected to begin at approximately 5 x 10⁵ rad gamma radiation. For doses in excess of 5 x 10⁵ rad 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). The sensitization of the LT glow peak

^•7 should continue up to doses in the vicinity of 10 - 10° rads gamma radiation. Beyond these dose levels, one expects that de-sensitization of the LT glow peak could occur. The decease in LT peak intensity is illustrated in Figure 1(c). Figure 1(d) indicates the expected glow curve shape when the MT glow peak is also de-sensitized. The radiation dose at which this occurs is unknown precisely, though it is thought to be in^•the range of 10⁹ - 10¹⁰ rad of gamma radiation. Further radiation will result in the HT peak being the only glow peak to continue increasing in intensity, whilst the LT and MT peaks continue to decrease in intensity (Figure 1(e)).

Equivalent gamma dose should be higher than 10¹⁰ rad; perhaps as high as 10¹¹ or 10¹² rad gamma radiation. Experience has shown that glow curves such as Figure 1(e) are almost invariably associated with uranium mineralization or in close proximity to uranium mineralization.

Samples examined from the natural reactor core at the Oklo uranium deposit exhibited no artificial TL at all. This is illustrated in Figure 1(f) and may represent the end point of radiation damage in quartz as observable by TL.

In order to utilize data from all three glow peaks TL variation diagrams have been developed which are diagrammatic methods of interpretation. They plot one variable on the horizontal axis (most commonly the percentage of the HT glow peak, although the MT may also be used) versus a function of one or both of the other glow peaks on the vertical axis. These can be either the intensity of the LT or MT glow peak, or the percentages of the LT or MT glow peaks, though more commonly and more usefully a ratio of two remaining glow peaks e.g. LT/MT, or LT/HT will be used. This enables comparative radiation effects on individual samples or groups of samples to be observed. An example of such a diagram is shown in Figure 2. A method of incorporating most major data based on the glow curve variations expected from Figure 1 and observed in case studies, involves the use of variation diagrams. A general example of these variation diagrams is shown in Figure 2. Figure 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.

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. As radiation levels to such a sample increase, 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. As radiation continues, ultimately 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. In fact, 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.

In a second embodiment, a method of exploration for petroleum and maturation determinations is described. The purpose of artificial 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.

By experimentally studying the stability of defects through different thermal regimes on a number of different quartz, e.g. synthetic quartz, vein quartz, varius metamorphic quartz and igneous quartz. Once the TL behaviour of each quartz type in response to the different thermal regimes is known then this can be applied to any sedimentary horizon.

An empirical approach would be to use quartz from a sedimentary horizon whose lateral temperature variations are known by means of apatite and vitrinite (a maceral) reflectances.

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. These basal sands have been faulted up and down and underwent different temperature changes, as shown by vitrinite reflectant studies and thermal regimes which produce dry gas (mature), no gas (over-mature), or wet gas or oil (sub-mature). Quartz material the source of which can be demonstrated, is ideal for an empirical of experimental investigation.

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. If 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 rationale behind the use of TL as a palaeothermometer is that different glow peaks will respond differently to varying cumulative time/temperature conditions, i.e. glow peaks in a sample subjected to a temperature in the oil maturation zone (say 130 degrees C) will respond in differing manners. All glow peaks below 130 degrees C. will be completely drained; glow peaks above 130 degrees will be completely or partially drained (depending on the time of annealing) whereas higher temperature glow peaks will not be affected or only partially affected. If the sample is then returned to ambient temperatures prevailing, then all glow peaks will again be recharged, including those below 130 degrees C. The ratio of different glow peak intensities will then therefore give crucial information concerning the prior annealing conditions.

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 :

A. Laboratory annealing studies on different minerals - beginning with different varieties of quartz - to derive kinetic equations describing behaviour of TL glow peaks with time and temperature. Such studies will also be compared with annealing studies of naturally occurring samples from sedimentary basins to assess the validity of extrapolation of results to geologically meaningful time periods. Samples similar to those naturally occurring ones already calibrated by the Fission Track Laboratory, University of Melbourne may be useful in this respect.

B. Collection of samples from a variety of time/ temperature annealing conditions already calibrated by other means can be used to test and refine equations derived from (A).

C. Simulated case study histories examining variations in TL glow curve properties from a variety of sedimentary basins will provide an empirical base for understanding varying thermal annealing conditions in individual basins. These studies will be just as necessary as those of (A) and (B) since different sedimentary basins will have experienced different paleothermometry conditions and since previous TL geothermometry studies have shown that identical glow curves can be generated from different time/temperature annealing conditions.

An alternative to the previous method is to compare annealing induced return to first or lower order untrapping mechanisms of two sedimentary minerals of vastly different annealing characteristics like quartz and calcite. The number of annealing equations to be derived from this approach will successfully deal with unknowns of detrital mineral derivation, and presedimentary annealing.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A method of exploration for minerals or petroleum using thermoluminescence analysis of a crystalline sample to determine the proximity of the sample to a mineral or petroleum deposit comprising: 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, 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.

2. A method of exploration for minerals or petroleum according to claim 1 wherein before the sample is irradiated with gamma-radiation, accessory minerals are substantially removed from the sample.

3. A method of exploration for minerals or petroleum according to claim 1 or claim 2 wherein after the sample is irradiated with gamma-radiation, the sample is shielded so as to allow phosphorescence or radio- luminescence to decay to insignificant levels.

4. A method of exploration for minerals or petroleum according to any one of the preceding claims wherein the level of gamma-radiation is between 5 x 10⁵ and 1 x

10° rads, delivered at a dose rate of greater than 2.5 x 10⁵ rads per hour.

5. A method of exploration for minerals or petroleum according to any one of the preceding claims wherein the sample is crushed prior to irradiating with gamma- radiation to give particle sizes of between 30 and 150 mesh.

6. A method of exploration for uranium using thermo¬ luminescence analysis of a quartz sample to determine the proximity of the quartz sample to a uranium deposit, comprising the following sequential steps: (i) treatment of the quartz sample to substantially remove feldspar, zircon and other accessory minerals from said sample;

(ii) irradiating the quartz sample with gamma- radiation, said irradiation being sufficient to substantially fill the crystal lattice traps within the (iii) shielding the quartz sample from direct light so as to allow phosphorescence and radio- luminescence to decay to insignificant levels; (iv) heating the quartz sample; (v) measuring the intensity of luminescent radiation at a plurality of glow peak temperatures, and

(vi) relating the said luminescent intensities to luminescent intensities of reference samples, wherein said reference samples have been subjected to known amounts of radiation, so as to determine the amount of naturally occurring radiation to which the sample has been subjected.

7. A method of exploration according to claim 6 wherein the said luminescent intensities of the quartz sample correspond to a low temperature, middle temperature and high temperature glow peak, and wherein the relationship between the high temperature glow peak luminescent intensity and either the low temperature glow peak luminescent intensity or middle temperature glow peak luminescent intensity is compared to said reference samples.

8. A method of exploration according to claim 6 wherein the said luminescent intensities of the quartz sample correspond to a low temperature, middle temperature and high temperature glow peak, and wherein the relationship between the high temperature glow peak luminescent intensity and either a ratio between the low temperature and middle temperature glow peak luminescent intensities or low temperature and high temperature glow peak luminescent intensities is compared to said reference samples.

9. A method of exploration for petroleum using thermoluminescence analysis of a sandstone sample to determine the palaeotemperatures of the sandstone sample comprising the following sequential steps: (i) treatment of the sandstone sample to substantially remove feldspar, zircon and other accessory minerals from said sample;

(ii) irradiating the sandstone sample with gamma- radiation, said irradiation being sufficient to substantially fill the crystal lattice traps within the sample;

(iii) shielding the sandstone sample from direct light so as to allow phosphorescence and radio- luminescence to decay to insignificant levels; (iv) heating the sandstone sample;

(v) measuring the intensity of luminescent radiation at a plurality of glow peak temperatures, and

(vi) relating the said luminescent intensities to luminescent intensities of reference samples, wherein the palaeotemperatures of the reference samples are known, so as to determine the palaeotemperatures of the sandstone samples.

10. A method of exploration according to claim 9 wherein the said luminescent intensities of the sandstone samples correspond to a low temperature, middle temperature and high temperature glow peak, and wherein the relationship between the high temperature glow peak luminescent intensity and either the low temperature glow peak luminescent intensity or middle temperature glow peak luminescent intensity is compared to said reference samples.

11. A method of exploration according to claim 9 wherein the said luminescent intensities of the sandstone sample correspond to a low temperature, middle temperature and high temperature glow peak, and wherein the relationship betweeen the high temperature glow peak luminescent intensity and either a ratio between the low temperature and middle temperature glow peak luminecent intensities or low temperature and high temperature glow peak luminescent intensities is compared to said reference samples.

12. A method of exploration for minerals and petroleum substantially as described and in accordance to and as illustrated in the accompanying drawings.

13. A method of exploration for uranium substantially as described and in accordance to and as illustrated in the accompanying drawings.

14. A method of exploration for petroleum substantially as described and in accordance to and as illustrated in the accompanying drawings.