WO1983003676A1

WO1983003676A1 - Analysis of kerogens

Info

Publication number: WO1983003676A1
Application number: PCT/GB1983/000114
Authority: WO
Inventors: Secretary Of State For Energy In Her ... The
Original assignee: Bather, John, Maxwell
Priority date: 1982-04-20
Filing date: 1983-04-19
Publication date: 1983-10-27
Also published as: EP0105891A1

Abstract

A method of analysing kerogen which consists of pyrolysing the kerogen, and analysing the pyrolysis vapour produced for its C12 alkane content and optionally its C13 alkane content. The analysis can be used to identify oil and/or gas producing kerogens by measuring the weight ratio of C12 alkane to C13 alkane in the vapour, or by comparing the C12 alkane content of the vapour with that of pyrolysis vapours from non-oil and/or non-gas producing kerogens. The pyrolysis vapour is produced by flash pyrolysis, and is analysed by gas chromatography-quadrapole mass spectrometry. The method may be used to explore for oil and/or gas reserves, by pyrolysing mineral samples taken from various subterranean locations, and analysing any pyrolysis vapours produced therefrom.

Description

Analysis of Kerogens This invention relates to the analysis of kerogen and to the use of the analysis results in the exploration for fossil fuels, particularly oil, and/or natural gas.

Kerogen is the name given to the organic material insoluble in common organic solvents which is found finely dispersed in sedimentary deposits often in combination with soluble organic matter which will generally form a minor amount of the total organic content of the deposit: see for example Durand et al Revue De L'lnstitut Francais De Petrole XXVII 6 866 (1972). Kerogen is formed by catagenesis of once living matter over a period of several million years. In this respect Mclver (7th World Petrol Cong. Proc 1967) has pointed out that kerogen is not simple unaltered detritus but is formed from varying mixtures of simple organic precursors. Mclver also points out that carbohydrates, proteins, lipids etc., at or near the sediment-water interface in the recent environment, are attacked and altered in varying degree by the microbial population. Some of the products of this attack then probably undergo condensation or polymerisation to form kerogen which is substantially resistant to bacterial attack. Different organic precursors generate kerogens of different composition. The Van Krevelen diagram characterises kerogens of different types. Briefly the diagram is a plot of H/C ratio of the kerogen vs. its O/C ratio. Kerogens having a common organic precursor type, eg marine origin, vegetable origin, lie on the same line in the diagram, the diagram having different lines for kerogens from different organic precursor types. All lines converge towards the origin (pure carbon).

Kerogen is broken down by diagenesis over a period of millions of years and the position of a particular kerogen on its line (ie the value of its H/C ratio as compared to its O/C ratio) depends on the diagenetic history of the kerogen, ie the depth at which it is buried and the temperature to which it has been subjected. The lines of the diagram may thus be regarded as evolution paths for the kerogens.

It has long been known that kerogens of particular composition generate oil and/or gas upon breakdown. The first stage appears to be thermal breakdown of the kerogen, gradually at first, then more rapidly to produce hydrocarbons. This is the principal phase of oil formation as designated by Vassoyevich et al (Ihternat. Geology Rev. 1970 121276-1296). At greater depth increased cracking of carbon-carbon bonds occurs and generates light hydrocarbons from kerogen and from previous oil as well. This is the phase of condensate and gas formation.

Commercially viable quantities of oil and/or gas are most likely to be generated from shales that contain rich deposits of organic carbon, ie amounts greater than 2-3% total organic carbon content although Mclver 7th World Petroleum Conference Proc 1967 has pointed out that any shale or fine-grained rock that contains organic matter of any kind is a potential source of hydrocarbons. If the amount of organic matter contained by the shale or rock is a few tenths of a percent, then the rock or shale is a potential source of commercial quantities of oil and gas. Moreover, limits have been defined on the Van Krevelen diagram for the H/C and O/C rations of kerogens which will have been producers of oil and/or gas. Oil and/or gas will have been generated by such kerogens in an equilibrium reaction (thus leaving some of the original kerogen) and will tend to migrate from the kerogen deposits to more favourable geological locations. Consequently kerogen which has been oil or gas producing will be found in the vicinity of the oil and/or gas reservoir and in fact Tissot et al (The American Association of Petroleum Geologists Bulletin 58 500 (1974) ) have pointed out that the chemical and physical study of the insoluble organic matter, or kerogen, is a way to characterise the various types of organic matter and thus to evaluate the oil and gas potential of the formations.

A method of exploring for oil thus consists of sampling shale from the subsurface, to see whether it has an organic richness commensurate with commercial oil and/or gas formation and analysing the kerogen to see whether or not it has been oil producing, thus assessing the likelihood of finding gas or oil in the vicinity.

The methods commonly used for analysing the kerogen, which has generally been previously extracted from the shale sample, include esr studies and measurement of the vitrinite reflectance. These methods are however time consuming and may take two days or so to yield results. The length of time taken for this analysis is a disadvantage when one considers the cost of exploration per unit time since a long delay in providing the results may result in fruitless and expensive exploration operations.

Espitalie et al (Revue De L'lnstitut Francais Du Pet role XXXII 23-43 (1977) have described a method of analysing kerogen in which the kerogen is pyrolysed and the pyrolysis vapour is analysed by mass spectrometry to define a "hydrogen index" and an "oxygen index" for the kerogen which equate to its H/C and O/C values respectively on the Van Krevelen diagram, thus giving a measure of whether the kerogen has been oil and/or gas producing. We have conducted studies on kerogen samples which have been taken from the vicinity of oil and/or gas fields and which from the conventional analysis methods discussed above are known to have been oil and/or gas producing. The results have been compared with those obtained for kerogens which are known not have been oil and/or gas producing. The studies have involved pyrolysing the various kerogen samples and analysing the gaseous pyrolysis product for its alkane composition. We have found that the pyrolysis vapour from the oil and/ or gas producing kerogens displays an enhanced concentration of C₁₂ alkane. According to the present invention there is provided a method for the identification of oil and/or gas producing kerogens comprising pyrolysing a kerogen to produce a pyrolysis vapour, and analysing the pyrolysis vapour for its C₁₂ alkane content to determine whether or not the kerogen has been oil and/or gas producing. The invention also provides a method of exploring for oil and/or gas comprising taking a mineral sample from a subterranean location, pyrolysing any kerogen present in the sample to produce a pyrolysis vapour, and analysing the pyrolysis vapour for its C₁₂ alkane content to determine whether or not the kerogen has been oil and/or gas producing. The analysis information is then used to determine the likelihood of the presence of oil and/or gas in the vicinity of the location from which the sample was taken.

We believe that the C₁₂ component of the pyrolysis vapour which is enhanced in kerogens which has been oil and/or gas producing is the C₁₂ n-alkane but we do not rule out the possibility that one or more branched C₁₂ alkanes constitute all or part of the C₁₂ fraction. Other alkanes in the pyrolysis vapour, which may range up to C₂₄ or more, are we believe n-alkanes. One preferred method of determining whether or not the C₁₂ alkane component of the pyrolysis vapour is sufficiently enhanced for the kerogen to have been oil and/or gas producing is to determine the weight ratio of the C₁₂ alkane content of the vapour relative to the C₁₃ alkane content thereof. In kerogen samples which are known, from other methods, to have been oil and/or gas producing, we have found this ratio to be greater than 1, whereas in kerogens known not to have been oil and/or gas producing the ratio is less than 1. Determination of the C₁₂ : C₁₃ weight ratio in the pyrolysis vapour thus provides a method of determining whether the kerogen has been oil and/or gas producing.

By taking mineral samples, known to contain non-oil and/or non-gas producing kerogens, from a series of subterranean locations at substantially the same geographical location (eg by drilling down through geological strata from a single well-head), and pyrolysing and analysing the kerogen in each sample under the same conditions, we have found that the C₁₂ alkane content of the pyrolysis vapour from the kerogen in each sample remains fairly constant. Furthermore we have found that the C₁₂ alkane content of the pyrolysis vapour from kerogens which are known to be oil and/or gas producing and which are derived from a subterranean mineral samples also taken from substantially the same geographical location as the kerogens known no to be oil and/or gas producing, will be at least 1.5 and more usually at least 5 times the C₁₂ alkane content of the non-oil and/or non gas producing kerogens. Consequently given a knowledge of the C₁₂ alkane levels normally found in the pyrolysis vapours of non-oil and/or non-gas producing kerogens it is possible readily to tell whether the C₁₂ alkane fraction of the pyrolysis vapour of a particular kerogen being sampled from a subterranean location at substantially the same geographical location is sufficiently enhanced for the kerogen to have been oil and/or gas producing.

The analysis results obtained on a kerogen sample by the method of the invention should be considered in terms of the organic content of the mineral sample to assess the chances of commercially viable quantities of oil and/or gas having been produced from the kerogen. If a favourable result is obtained, then full scale exploration using the conventional expensive exploration methods may commence to determine the location of the oil and/or gas. We are unable to explain with any certainty the reason as to why oil and/or gas producing kerogens give rise to a pyrolysis vapour with an enhanced C₁₂ alkane concentration. It is however known that as.pointed out above, the breakdown of kerogen to form oil and/or gas is an equilibrium reaction so that a sample of kerogen which has produced oil and/or gas will contain remaining kerogen with the potential for oil and/or gas formation. We believe that the pyrolysis produces from this latter type of kerogen a vapour of similar composition to oil, which is also rich in C₁₂ alkanes and it is for this reason that our analysis method will indicate whether or not a particular kerogen sample has or has not been a producer of oil and/or gas.

The pyrolysis is preferably a flash pyrolysis, in order to prevent the thermal reformation of active organic molecules formed during pyrolysis from interfering with the accuracy of the results obtained from the analysis. For example, at the high temperatures required for the pyrolysis of kerogen, aromatic molecules such as benzene, naphthalene and phenanthrene may react with alkane radicals to produce substituted aromatics, thus possibly reducing the amount of alkanes which may subsequently be detected during the analysis. This problem is overcome by preferably maintaining the temperature of the kerogen at a maximum, pyrolysis temperature for no more than about 200 μs. We have found that the minimum period of time the kerogen must be maintained at its pyrolysis temperature to ensure sufficient pyrolysis vapour is generated, is about 90 μs. Accordingly, the flash pyrolysis is preferably conducted by heating the kerogen to its pyrolysis temperature for a period of 10 μs, holding the pyrolysis temperature of the kerogen for 90 μs, and then removing the source of pyrolysis heating. Flashpyrolysis is most preferably effected at the optimum temperature for generation of the gaseous pyrolysis product. This optimum temperature may be determined using Differential Scanning Calorimetry which is a process for determining the temperature, at which thermal decomposition occurs when the sample is heated. Generally the pyrolysis temperature will be in the range of 500-650ºC.

The preferred method of analysing the gaseous pyrolysis product is by gas chromatography . The column of the chromatograph may be a glass tube (eg 25m long with an internal diameter of 0.1 mm and 0.1 mm thick walls) and having its walls internally coated with a silicone oil stationary phase of several molecular layers thickness. Preferably the oven temperature of the chromatograph will initally be in the region of 50°C and programme temperature control will be used to raise this temperature by 10°C per minute so that all products are eluted from the column. Helium is a suitable carrier gas.

The chromatograph used by us is a Girdel Series 32. Gas chroraotography allows the use of small kerogen samples.

Such samples can then be analysed using a fine bore pyrolysis tube surrounded by an electrical heating coil and located in a stream of carrier gas for passing the gaseous pyrolysis product on to the column. A CDS 190-Pyroprobe is suitable. The analysis result may be provided by a conventional recorder associated with the detection system of the gas chromatograph. More preferably however a gc-ms technique is employed, preferably using quadrapole mass spectrometry, and a computer programmed to display the relative amounts of the alkanes, and their identity, from the chromatographic column.

In any analysis of the chromatographic results, the amount of C₁₂ alkane present in the pyrolysis vapour may be determined by assessment of the area under the peak for this fraction. In assessing areas under peaks, eg when comparing amounts of C₁₂ and C₁₃ alkanes in the pyrolysis vapour, it is necessary to take account of the fact that the base line of the chromatogram is not linear, this being due to asphaltenes and napthenes which are present in the pyrolysis vapour. Techniques for determining the relative areas of peaks in chromatograms with non-linear base lines are of course well established. A suitable quadropole mass spectrometer for use in conjunction with the gas chromatograph is the Nermag R10-10B.

The analysis may be performed on kerogen which has been extracted from mineral components of the sample. A suitable extraction method is given by Mclver (loc cit), in which method the mineral component is first crushed, mixed with 1M aqueous HCL to dissolve out carbonates present in the component; thai mixed with 11M aqueous HF to dissolve out the silicates, and is finally subjected to zinc/HCL reduction to remove the sulphide and so leave kerogen as the sole remaining solid constituent. preferably, however, the kerogen is pyrolysed from rock or shale samples directly, thus greatly simplifying the analysis procedure. It is an advantage of the present invention that direct pyrolysis of rock or shale samples containing kerogen does not generally affect the accuracy of the analysis results. Rock or shale samples are first crushed, and then milled in a high speed rotary mill in a liquid suspension of 0.05M aqueous HCI, to produce particles that will pass through a 120 British Standard mesh. The particles are then dried in a vacuum oven at 30°C. The samples are milled to ensure their surface area is sufficiently high to allow a rapid release of pyrolysis vapour when the samples are subjected to flash pyrolysis, and we have found that this release is enhanced if a small quantity of HCI is used in the liquid suspension during milling to break ddwn the carbonate in the samples. The invention is illustrated by reference to Figs 1-4 of the accompanying drawings. Fig 1 shows the chromatogram of the pyrolysis vapour (maximum pyrolysis temperature 610°C) obtained from a rock sample containing kerogen known, from conventional methods, to have been oil and/or gas producing. Fig 2 shows a chromatogram for a pyrolysis vapour generated under the same conditions but for a rock sample containing kerogen known not to have been oil and/or gas producing. The various peaks in the chromatograms of Figs 1 and 2 are annotated to show the various alkane peaks. Figs 3 and 4 respectively show the ratios of the areas under the peaks shown in the chromatograms of Figs 1 and 2. It will be seen from Fig 3 that the amount of C₁₂ alkane present in the pyrolysis vapour (as represented by the area under the C₁₂ alkane peak) is substantially greater than the C₁₃ alkane amount. The reverse is true for the non-oil and/or gas producing kerogen, as will be seen from Fig 4. The advantage of the present invention is that it provides a rapid method for the analysis of kerogen to determine whether or not it has been oil and/or gas producing. With the use of gc-ms techniques as discussed above results are available within seconds of commencing the analysis run. This has obvious advantages when one considers the time and expense which may be wasted continuing the exploration of oil whilst waiting for a kerogen analysis by conventional methods.

Claims

1. A method for the identification of oil and/or gas producing kerogens comprising pyrolysing a kerogen to produce a pyrolysis vapour and analysing said vapour to determine whether or not the kerogen has been oil and/or gas producing, characterised in that the pyrolysis vapour is analysed for its C₁₂ alkane content.

2. A method according to claim 1 characterised in that the C₁₂ alkane content of the pyrolysis vapour is subsequently compared with the C₁₂ alkane content of a second pyrolysis vapour, produced by a non-oil and/or non-gas producing kerogen, to determine their weight ratio.

3. A method according to claim 1 characterised in that the pyrolysis vapour is also analysed for its C₁₃ alkane content, and the C₁₂ alkane content of the pyrolysis vapour is subsequently compared with the C₁₃ alkane content of the pyrolysis vapour to determine their weight ratio. km A method according to claim 1 characterised in that the pyrolysis vapour is produced by flash pyrolysis.

5. A method according to claim 4 characterised in that the maximum temperature of the kerogen during the flash pyrolysis is maintained for between 90 and 200 micro-seconds.

6. A method according to claim 5 characterised in that the maximum temperature is the temperature at which thermal decomposition of the pyrolysis vapour, as determined by Differential Scanning Calorimetry, occurs.

7. A method according to claim 1 characterised in that the pyrolysis vapour is analysed by gas chromatography.

8. A method according to claim 7 characterised in that the pyrolysis vapour is analysed by gas chromatography - quadrapole mass spectrometry.

9. A method of exploring for oil and/or gas comprising taking a mineral sample from a subterranean location, pyrolysing any kerogen present in said sample to produce a pyrolysis vapour, and analysing said vapour to determine whether or not the kerogen has been oil and/or gas producing, characterised in the pyrolysis gas is analysed for its C₁₂ alkane content.

10. A method according to claim 9 characterised in that the C₁₂ alkane content of the pyrolysis gas, produced from any kerogen present in the mineral sample, is subsequently compared with the C₁₂ alkane content of a second pyrolysis vapour, produced by a non-oil and/or non-gas producing kerogen from a mineral sample taken from a second subterranean location at substantially the same geographical location, to determine their weight ratio.

11. A method according to claim 9 characterised in that the pyrolysis gas, produced from any kerogen present in the mineral sample, is also analysed for its C₁₃ alkane content, and the C₁₂ alkane content of the pyrolysis vapour is subsequently compared with the C₁₃ alkane content of the pyrolysis vapour to determine their weight ratio.