ISOTOPE ANALYSIS

Abstract

The invention provides apparatus and methods for determining the isotope ratio of a sample. The apparatus comprises a dynamically heated chamber (1); a reactor (4), wherein an outlet of the dynamically heated chamber is coupled to a reactor inlet; an isotope ratio spectrometer (6), wherein an outlet of the reactor is coupled to a spectrometer inlet; such that a gas flow path is provided from the dynamically heated chamber to the isotope ratio spectrometer; wherein the apparatus includes at least one selective gas trap (3,5) in the gas flow path, the gas trap being configured to selectively and reversibly trap one or more gases present in the gas flow in use.

Claims

1. An apparatus comprising; a dynamically heated chamber; a reactor, wherein an outlet of the dynamically heated chamber is coupled to a reactor inlet; an isotope ratio spectrometer, wherein an outlet of the reactor is coupled to a spectrometer inlet; such that a gas flow path is provided from the dynamically heated chamber to the isotope ratio spectrometer, wherein the apparatus includes at least one selective gas trap in the gas flow path, the gas trap being configured to selectively and reversibly trap one or more gases present in the gas flow in use.

2. The apparatus of claim 1, wherein a first selective gas trap is included in the gas flow path between the reactor and the spectrometer.

3. The apparatus of claim 2, wherein the first selective gas trap is configured to trap one or more gases selected from the group consisting of carbon monoxide, carbon dioxide, sulphur dioxide, nitrogen and hydrogen.

4. The apparatus of claim 3, wherein the first selective gas trap is configured to trap carbon dioxide.

5. The apparatus of claim 2, wherein a second selective gas trap is included in the gas flow path between the dynamically heated chamber and the reactor.

6. The apparatus of claim 3, wherein a second selective gas trap is included in the gas flow path between the dynamically heated chamber and the reactor.

7. The apparatus of claim 4, wherein the second selective gas trap is included in the gas flow path between the dynamically heated chamber and the reactor.

8. The apparatus of claim 5, wherein the second selective gas trap is configured to trap hydrocarbon gases.

9. The apparatus of claim 5, wherein said hydrocarbon gases comprise hydrocarbons substituted with sulphur, oxygen, nitrogen, and combinations thereof.

10. The apparatus of claim 6, wherein the second selective gas trap is configured to trap hydrocarbon gases; and wherein said hydrocarbon gases further comprise hydrocarbons substituted with sulphur, oxygen, nitrogen, and combinations thereof.

11. The apparatus of claim 7, wherein the second selective gas trap is configured to trap hydrocarbon gases; and wherein said hydrocarbon gases further comprise hydrocarbons substituted with sulphur, oxygen, nitrogen, and combinations thereof.

12. The apparatus of claim 1, wherein the at least one selective gas trap comprises a sorbent material that selectively sorbs one or more gases present in the gas flow in use.

13. The apparatus of claim 12, wherein the selective gas trap additionally comprises a heater, such that the trap can be heated in use to desorb trapped gas.

14. The apparatus of claim 1, wherein the reactor comprises a catalyst, which catalyses one or more of (a) oxidation of hydrocarbons and carbon monoxide to carbon dioxide, (b) oxidation of sulphur-containing compounds to sulphur dioxide, (c) conversion of nitrogen-containing compounds to nitrogen, (d) reduction of hydrocarbons to hydrogen gas and carbon monoxide, (e) pyrolysis of hydrocarbons to hydrogen gas.

15. The apparatus of claim 1, wherein the isotope ratio spectrometer differentiates isotopes by mass spectrometry or spectroscopy.

16. A method of determining isotope ratios in fractions of a sample, the method comprising the sequential steps of; (i) dynamically heating the sample to thermally differentiate fractions of the sample; (iii) selective reaction of the step (i) output to produce a reacted gas; and (v) completing isotope ratio spectrometry wherein the method additionally comprises; (ii) selectively and reversibly trapping components of the step (i) output prior to step (iii); and/or (iv) selectively and reversibly trapping components of the reacted gas prior to step (v).

17. A method according to claim 16, wherein step (iii) comprises one or more of the following reactions; (a) oxidation of hydrocarbons and carbon monoxide to carbon dioxide, (b) oxidation of sulphur-containing compounds to sulphur dioxide, (c) conversion of nitrogen-containing compounds to nitrogen, (d) reduction of hydrocarbons to hydrogen gas and carbon monoxide, (e) pyrolysis of hydrocarbons to hydrogen gas.

18. A method of claim 16, wherein the sample comprises rock.

19. The method of claim 17, wherein the sample comprises rock.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0059] FIG. 1 is a schematic representation of the gas flow path according to one embodiment of the invention.

DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS

[0060] The term “hydrocarbon” as used herein, includes organic compounds formed of carbon and hydrogen, optionally substituted with one or more heteroatoms including sulphur, nitrogen and oxygen. “Non-hydrocarbons” encompasses compounds that are not substantially formed of both carbon and hydrogen, and specifically includes CO, CO.sub.2, NO.sub.x (particularly NO.sub.2), H.sub.2, N.sub.2 and SO.sub.X (particularly SO.sub.2).

[0061] According to one embodiment, thermogravimetric analysis of rock is completed using a Weatherford® Source Rock Analyzer (SRA) 1.

[0062] 50-100 milligram-sized sample aliquots are heated under an anoxic atmosphere in a small crucible to give three initial fractions, termed herein as

[0063] S1 (“volatile” hydrocarbons),

[0064] S2 (“cracked” or “pyrolyzed” hydrocarbons),

[0065] S3 (“freely desorbed” CO.sub.2), and

[0066] Air is then introduced to the crucible atmosphere to oxidize (combust) the remaining organic matter to yield fraction S4, containing as CO and CO.sub.2 (“combustible” matter).

[0067] The SRA is a quantitative instrument and is equipped with a calibrated flame ionization detector (FID, for hydrocarbons in S1 and S2) and two calibrated infrared detectors (IRs, for detection of CO and CO.sub.2 in S3 and S4). This quantitative analysis allows a qualitative (e.g. relative hydrocarbon type and degree of saturation) and semi-quantitative (e.g. total organic carbon content and maturity) assessment of the source rock.

[0068] Stable isotope data provides valuable extra data, allowing for determination of various features of the source rock, including but not limited to (a) whether the oil inventoried in the S1 fraction is genetically related to the kerogen in the S2 fraction by comparison of their δ.sup.13C signatures, (b) the impact of sulphur content on kerogen-to-oil conversion kinetics via their respective sulphur content and δ.sup.34S compositions, and (c) the impact of drilling mud overprinting to the S1 fraction by comparing the covariance of its δ.sup.13C composition with that of S2 and aliquots of drilling mud themselves.

[0069] In one embodiment, as shown in FIG. 1, a heated transfer line 2 connects the SRA 1 output to a hydrocarbon trap 3. The transfer line is 2 a metal tube (e.g. stainless steel, copper, nickel) that is wrapped in heating tape or wound with resistive heating wire and insulated. The heated transfer line 2 is heated to a temperature of about 100-400° C. A thermocouple is connected to the tube to monitor the temperature.

[0070] The hydrocarbon trap 3 can be a thermally insulated metal (e.g. stainless steel, brass, nickel, etc.) or glass tube. A resistive heating wire is wound around the trap to allow the trap to be heated up to about 350° C. The tube is filled with a sorbent material that specifically traps hydrocarbons at a given temperature, whilst not retaining CO.sub.2 or other analyte gases. The sorbent can be based on a metal oxide, a polymer, or a molecular sieve; the particle dimensions of the sorbent can range from 120 to 28 mesh. The trap temperature can be adjusted from about −180° C. to about 350° C. at a rate of up to 200° C./min, such that in use, sorbed hydrocarbons can be rapidly released in a single short burst or pulse into the reactor 4. The trap 3 outlet may be connected to the reactor 4 directly, or via another heated transfer line 2.

[0071] In an alternative embodiment, the heated transfer line 2 connects the SRA 1 output directly to the reactor 4 input (and trap 3 is not present).

[0072] The reactor 4 may comprise a quartz, nickel or alumina tube, which can be kept empty or be loaded with a catalyst. The catalyst may comprise transition and/or noble metal (e.g., copper, platinum, nickel, chromium, palladium, etc.) wires or powder, either alone or in various mixtures with each other (such as copper, nickel and platinum wires intertwined). In alternative embodiments, the reactor 4 includes a second inlet for the introduction of either an oxidizing or a reducing reagent gas to synthesize an oxidation or reducing catalyst, respectively, in situ. The reactor 4 can be heated up to 1500° C. by use of a furnace to effectuate catalytic oxidation (producing CO.sub.2 and/or SO.sub.2), sequential catalytic oxidation-reduction (producing N.sub.2), singular catalytic reduction (producing H.sub.2 and CO), or pyrolysis (producing H.sub.2).

[0073] The reactor 4 outlet is coupled to a downstream gas trap 5. The coupling may be a direct connection, or may be via a heated transfer line 2. In some embodiments, a water trap may be provided in the gas flow path between the reactor 4 and the downstream gas trap 5. The downstream trap 5 is similar in structure to the hydrocarbon trap 3 described above. However, the sorbent material is selected to specifically trap CO and/or CO.sub.2 and/or SO.sub.2 and/or N.sub.2 and/or H.sub.2. In use, this traps analyte gas which can either be generated in the SRA itself, or be the result of the reactions occurring in the reactor.

[0074] The downstream gas trap 5 outlet is coupled to the isotope ratio spectrometer 6 inlet, either directly or via a heated transfer line 2. The IRS 6 may be an isotope ratio mass spectrometer (IRMS) or a spectroscopy-based system.

[0075] In embodiments, the apparatus may additionally comprise gas flow control units such as tee pieces (T-pieces) and selective valves. These can be employed to control gas flow along the gas flow path, thereby allowing precise and accurate measurements to be taken; the particular units employed and their location within the flow path will depend on their desired function, as will be apparent to the skilled person. In one example embodiment, valves are provided in the gas flow between the downstream gas trap 5 and the IRS 6, such that non-analyte gases are diverted around the IRS (i.e. out of the gas path). This advantageously prevents the IRS from being saturated (which can lead to non-linear IRS behaviour, as discussed above). In such embodiments, a helium stream may be connected to the valves, whereby when non-analyte gases are being diverted around the IRS, the helium stream passes through the IRS. This can be effected using a four-port valve, or two 3-port valves in series.

[0076] In use, a flow of volatilized hydrocarbons and non-hydrocarbon gases is evolved from a rock sample in the SRA 1. This passes into the heated transfer line 2. As described above, the apparatus may or may not contain a hydrocarbon trap 3. The hydrocarbon trap 3 (where present) is connected to the reactor 4 either directly or via an additional heated transfer line 2. If the trap 3 is in place, hydrocarbons (including those containing sulphur and nitrogen) are trapped and focused at a cool temperature, whilst all other non-hydrocarbon gases including CO and CO.sub.2 pass through the trap. After a pre-defined time or event-based trigger, the trapped hydrocarbons are released into the reactor 4 by rapidly heating the trap 3 to maintain the pneumatic cohesion (i.e. narrow peak) of the desorbed sample. If the trap 3 is not in place, then the whole stream including hydrocarbons passes through the reactor 4. Oxidized and/or reduced non-hydrocarbon gaseous products then exit the reactor 4 and are trapped and concentrated on the gas trap 5. After a pre-defined time or event-based trigger, the gas trap 5 is heated releasing trapped analyte gases (CO.sub.2 and/or SO.sub.2 and/or N.sub.2 and/or H.sub.2) into the Isotope Ratio Spectrometer 6.

[0077] The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.