This invention relates to a method of exploration for petroleum, oil, gas and hydrocarbons using natural thermoluminescence analysis methods (hereinafter abbreviated NTL) . In a previous patent specification No. PCT/AU87/00017, the inventors have disclosed an analysis method using thermoluminescence. In general, 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 electrons and holes which will result in the emission of quanta of light. The present patent application, although different to the abovementioned specification, is a further development of thermoluminescence techniques. External physical effects such as ionizing radiation, mineralisation processes and temperature conditions cause defects within minerals. These defects are capable of trapping electrons and holes (sites which have lost an electron). These electrons and holes are produced by ionising radiation either in nature (NTL) or in the laboratory (artificial thermoluminescence or ATL) . Most electrons and holes recombine almost immediately but in non-conducting minerals a small percentage of the holes and excess electrons may be trapped on lattice defects and impurities. In minerals such as quartz, for example, a material often used in TL investigations of mineral deposits, a well known hole trap is a silicon site in which Al has been substituted

4+ for Si . Electrons can also be trapped, and this usually

2- ocσurs on vacant oxygen sites where 0 charge is missing. Initially the number of hole traps in quartz may be expected t 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 quantum 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 several major glow peaks in the temperature range from room temperature (20 ^* C) to 650 ^* C.

The intensity and shape of the mineral glow curves depend on a number of factors such as lattice vacancies and impurities capable of acting as traps, respective crystal field energies, and trap densities and charged occupancies. Several of these factors, including the charge occupancy rate, are a function of, and are affected by, external physical effects where the term external physical effect refers to external influences such as ionising radiation, mineralising processes or heat. As the charge occupancy affects the strength of the TL signal, dosimeters have in the past made use of TL.

In this invention, the exploration for petroleum, oil, gas and hydrocarbons will use natural TL. The exploration for hydrocarbons using TL will be conducted on naturally occurring rocks, minerals, or combinations of minerals to determine proximity of a sample to a hydrocarbon occurrence, the potential for hydrocarbons to occur in the environment of the sample or the source rock maturation conditions of the sample. Such TL analysis can be carried out on whole rock samples, or samples comprising quartz, feldspars, carbonates, clays, micas, apatite, zircon, sphene and/or other common or accessory rock- forming minerals. The following description relates to the mineral quartz. The NTL method comprises first heating the sample from ambient temperature to an elevated temperature and measuring the glow peak temperature of the major luminescent radiation a

a plurality of wavelengths. The glow peak temperature is then related to the glow peak temperature of reference samples wher the reference samples have been subjected to known and derived degrees of external physical effects wherein the degree of external physical effect on the unknown sample can be deduced to determine the proximity of the unknown sample to petroleum, oil, gas or hydrocarbon occurrences, the potential for petroleum, oil, gas or hydrocarbons to occur in the environmen of the sample or the source rock maturation conditions of the sample.

This invention uses NTL techniques to determine temperature/time annealing conditions that the sample has been subjected to. This enables the maturation conditions of the sample to be determined. If the maturation conditions of the sample are known, an accurate estimate as to the likelihood or potential for hydrocarbons such as petroleum, oil and gas to occur in this sample can be determined.

In its broadest form, this invention relates to a method of exploration for petroleum, oil, gas or hydrocarbons using thermoluminescence analysis of a crystalline sample to determine the proximity of said crystalline sample to a petroleum, oil, gas or hydrocarbon deposit, said method comprising (a) heating said crystalline sample to an elevated temperature, (b) measuring the glow peak temperature of the major luminescent radiation at a plurality of wavelengths, and (c) comparing said glow peak temperature, the temperature of initial rise of the glow curve or the temperature at which a given natural thermoluminescence intensity is reached at any specific wavelength or kinetic parameter with the correspondin measurements for 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 said crystalline sample has been subjected, wherein the degree of external physical effect indicates the proximity of said crystalline sample to a petroleum, oil, gas or hydrocarbon deposit, the potential for petroleum, oil, gas or hydrocarbons to occur in the environment of said crystallin

sample, the source rock maturation conditions of said crystalline sample, or the geothermal history of said crystalline sample.

The use of NTL allows the determination of the environmental temperature that a particular sample has been subjected to over a given geological time period.

Preferred embodiments are described hereunder in further detail, but the invention need not be confined or restricted to the details of these further embodiments. In this first embodiment, samples of rock, drillcore, chips or sand are suitable for analysis. These are gently crushed and then sieved to remove any size fraction between 15 and 425 mesh BSS. Monomineraliσ extractions may be made for NTL analysis by means of standard cleansing, electromagnetic and heavy liquid separation, acid digestion and hand-picking techniques. Monomineralic extractions used for palaeo temperature and maturation studies include quartz, feldspars, carbonates, clays, micas, apatite, zircon, sphene and other common or accessory rock-forming minerals. Multimineralic or whole rock samples may also be used in the process. The NTL curves are gained by heating the sample from ambient temperature to an elevated temperature and measuring the glow peak temperatures (GPT) at a plurality of wavelengths. The GPT of the unknown samples are then compared with the GPT of reference samples. These reference samples are those which have been subjected to known or derived temperature/time annealing conditions and hence the maturation conditions of the unknown sample can be deduced by the comparison. Methods of analysis of GPT which are used in these comparisons include glow peak temperatures, the temperature of initial rise of the glow curve or the temperature at which any given natural TL intensity is reached at any specific wavelength or kinetic parameter.

The radioactive element concentration of each sample may also be measured and taken into account in the assessment of GPT maturation conditions.

Normally the GPT increases with increasing time/ temperature (maturation) as demonstrated in Figures 1-5.

Figures 1-3 describe samples taken from progressively increasing depths within the Otway Group of the Otway Basin in south-eastern Australia. The time of heating for these samples is estimated to be approximately 10-40 million years. Samples are in thermal equilibrium. Figure 1 show the NTL glow curve for a sample at a depth of 3822ft. The GPT occurs at 350"C. Figure 2 shows the NTL glow curve for a sample from a depth of 5760ft. The GPT occurs at 449 ^* C. Figure 3 shows the NTL glow curve for a sample from a depth of 8510ft and the GPT occurs a 486'C. Therefore it can be seen that as the depth (and hence ambient temperature) of a sample increases so does the GPT. Figures 4 and 5 describe samples taken from the Cooper Basin in South Australia. In this basin the period of heating is greater than 100 million years. Figure 4 show the NTL glow curve for a sample from a depth of 4502ft. A glow peak with a GPT of approximately 490 ^* C is present. This GPT value is much higher than that for a sample of similar depth from the Otway Basin which has a shorter heating period. Similarly, Figure 5 shows the NTL glow curve for a sample from a depth of 8745ft. The GPT occurs at 586 ^* C - again at a much higher temperature than a sample of corresponding depth from the Otway Basin.

Given that there are optimum environmental temperature versus geological time relationships in which it is more likel to find oil or gas, as shown in Figure 6, and since the GPT.TL process provides an indication of environmental and/or palaeoenvironmental temperature conditions, then, knowing the age of the sample, its depth or current temperature and/or its period of heating, it is possible to determine the likelihood or potential for hydrocarbons to occur in the sample or their likelihood of having formed and been retained in such samples. These relationships are further illustrated in Figure 7 where the zone of peak oil generation versus depth/current temperature and time of heating are defined in terms of GPT.

In assessing palaeotemperature and maturation conditions, it is also necessary to know whether a sample has been tectonically uplifted during its previous geological history. The GPT method can also be used to determine whether such

uplift has occurred. Figure 8 shows an NTL glow curve for a sample from the Otway Group in the Otway Basin. It has two prominent glow peaks with GPTs at approximately 357 ^* C and 600 ^* C. When the 357"C glow peak is plotted on Figure 7 it shows that this sample has been heated for less than the 10-40 million years of the rest of the Otway Group. If the 600'C peak represents a previous thermal equilibration then a depth of approximately 12000 feet would be required. The sample is presently at a depth of 4000ft. The amount of uplift is therefore 12000 ft - 4000 ft = 8000 ft. This agrees with uplift for this area of the Otway Ranges estimated by other palaeothermometry techniques.

Therefore glow-peak shape, or multiple GPTs, may sometimes be used to determine geothermal history. A knowledge of maturation conditions by GPT techniques enables determination of the potential of the unknown samples to have, produce or contain petroleum, oil and/or gas. The formation of petroleum, oil or gas is critically dependent on the past temperature/time conditions which have prevailed within the sample, and an understanding of the palaeotemperature conditions will clearly indicate the potential for such hydrocarbons to have formed or matured.