GB2286671A - Sample preparation for kerogen analysis - Google Patents

Abstract

A kerogen sample containing palynomorphs, is microwave heated in the presence of an oxidising reaction medium for a period of time sufficient to bleach the palynomorphs and/or to reduce the level of any amorphous organic matter present which interferes with later identification of palynomorphs. Utility is in treating samples from oil drilling to identify strata. Oxidation agent in reaction medium may be HNO3. Kerogen source may also be coal.

Description

PREPARATION OF SAMPLES FOR KEROGEN ANALYSIS This invention relates to the preparation of samples for kerogen analysis, and more particularly to an improved technique for the rapid preparation of such samples.

In this specification kerogen is defined as sedimentary organic matter that is insoluble in organic solvents and mineral acids.

In conventional oil-drilling techniques, it is necessary to take samples, known as "hot-shots", at regular intervals to determine the identity of the stratum through which the drill bit is passing. The samples collected are treated with hydrofluoric acid to remove silicates, leaving behind kerogen, typically fossilised pollen and plant spores, which can be identified to provide an indication of the age of the stratum from which the sample was taken.

When preparing samples for palynological and other types of analysis it is often necessary to oxidise the kerogen liberated from sediments by hydrofluoric acid digestion in order to make dark or thick walled palynomorphs sufficiently translucent for microscopic examination.

When processing samples rich in amorphous organic matter (AOM), for example those obtained from Kimmeridge Clay type facies of northwestern Europe, it is also necessary to minimise the levels of AOM present in order to concentrate and easily distinguish for identification any palynomorphs which may be present (Figure 1, 1).

AOM occurs in two forms, dispersed and clumped.

Dispersed AOM can be simply removed by washing it through a suitable sized sieve, while clumped AOM cannot and often incorporates palynomorphs. In order to remove clumped AOM and release any palynomorphs that may be present, extended and frequently repeated oxidative treatment is usually required. This can take up to four days in some cases to obtain clean samples which can be identified using a microscope. Oxidative treatment enables unwanted AOM to be selectively removed by virtue of the fact that the majority of AOM is more susceptible to oxidation than sporopollenin which forms the walls of many palynomorphs.

There are some problems with existing treatment methods. For example when samples rich in AOM are treated with an oxidising agent such as Schulze's solution (70% nitric acid supersaturated with potassium chlorate), the initial intensity of the reaction rapidly declines due to a build up of insoluble oxidation products (humic acids) at the surfaces of the remaining kerogen which limit further oxidation. These humic acids may be removed by subsequent treatment of the sample with 2% potassium hydroxide solution (KOH) after each oxidation stage, but repeated KOH treatment is often seen to cause palynomorphs to swell and fragment.

The problems of limited oxidation and repeated KOH treatment can be overcome by using a repeated "overnight" treatment whereby each sample is repeatedly oxidised with fresh Schulze's solution for 24 h periods until the optimum level of oxidation is obtained.

Whichever method is chosen, it will be apparent that these are very time-consuming procedures, which need to be carried out under carefully controlled conditions. In many cases drilling has to be delayed until the results of the sample evaluation are available, which can be extremely costly.

In the analysis of coals it is also necessary to oxidise kerogen samples to examine the fossil content thereof. Such treatments have also hitherto been extremely time consuming.

The use of heating has been proposed in geochemistry to speed up the total dissolution of rock samples in fuming nitric acid, but in palynology and coal analysis oxidation is always carried out at room temperature, possibly because the heated reaction medium would be potentially explosive, or because noxious fumes are given off by heating, or because it has hitherto been considered necessary to use conditions in which the nitric acid oxidation process proceeds sufficiently gently that the palynomorphs are not too rapidly degraded.

In palynology, microwave heating has been confined to the drying of spore and pollen samples, and as an aid to hydrofluoric acid digestion of silicates.

The prior art also recognises that the reaction products of nitric acid treatment vary depending upon the concentration of the acid. Hitherto it has been believed to be necessary to use fuming nitric acid at room temperature to obtain the required oxidative effect.

From the above it will be seen that there is a need for a method for the more rapid preparation of palynology samples, which preferably also can obviate some of the difficulties involved in the use of harsh reagents such as fuming nitric acid and Schulze's solution.

In accordance with the invention, it has now surprisingly been found that not only can the oxidation reaction time can be substantially reduced by the use of microwave heating, but that clean palynomorphs can thereby be obtained in certain circumstances using milder reaction conditions.

The present invention provides a method for the preparation of samples for kerogen analysis in which kerogen, containing palynomorphs, is subjected to microwave heating in the presence of an oxidising reaction medium for a period of time sufficient to bleach the palynomorphs and/or to reduce the level of amorphous organic matter present.

The invention is particularly applicable to Jurassic and Carboniferous samples, which contain a high proportion of bituminous residues. Such strata occur in North Sea locations and in many other parts of the world where oil drilling is carried out.

One method by which the microwave heating can be carried out is by containing the reaction medium in a pressure relief vessel and placing the vessel in a microwave oven. For oxidation purposes pressurised system microwave heating can be achieved, for example, by using a plurality of 120ml capacity pressure relief type teflon PFA digestion vessels, heated separately at full power in a CEM Corp MDS-81 600W microwave oven fitted with a rotating carousel.

Preferably, however, there is used a focused microwave system in which a waveguide directs the microwave onto the reaction medium. In this method the reaction medium is preferably contained in a microwave transparent vessel and the microwaves are directed toward a lower part of the vessel, allowing a reflux action to take place in an upper part thereof. A focused microwave system of this type operates at atmospheric pressure, thus eliminating the need for a pressure relief vessel to contain the reaction medium. Suitable focused microwave heating systems are available from Prolabo and sold under the trademark Microdigest 301 and Microdigest 401.

The oxidising medium can comprise any suitable oxidising agent or combination of oxidising agents, which may for example include oxygen, ozone, hydrogen peroxide, potassium permanganate, chromic acid and nitric acid.

The oxidising medium may comprise a solvent, usually water, and should have a pH appropriate for the oxidising agent or combination used. The preferred oxidising agent is nitric acid.

The volume of oxidising agent compared to the volume of the vessel in pressurised systems needs to be maximised to ensure complete oxidation of samples.

Reductions in volume can result in under-oxidation of samples either due to the oxidising agent either becoming exhausted or reduced to such a low concentration that it becomes ineffective. In pressurised system microwave heating the maximum volume currently possible is 100ml, governed by the manufacturer's recommended safe maximum volume for pressure vessels. However, focused microwave systems can permit the use of larger vessels.

The concentration of oxidising agent is important.

Experimentally in pressurised system microwave heating 17.5% nitric acid has proved to be the most effective, for example made by diluting 70% nitric acid 1:4 with distilled water (use of tap water is not recommended as it has been found to effect the efficiency of the technique). Higher concentrations of oxidising agent (35 and 70%) were found to reduce the selectivity of oxidation, ie although assemblages were obtained which were relatively "clean" in terms of AOM, the number of palynomorphs observable had been reduced.

In the focused microwave heating method, fuming nitric acid has been found to be the most effective oxidising agent. Schulze's solution has been found in general to be too reactive, but it may be applicable to sampled which are thermally mature and therefore require bleaching in addition to removal of AOM. Hydrogen peroxide may be suitable at higher power settings, which would have safety and environmental benefits. Another oxidising agent which may be useful is 2% potassium hydroxide saturated with potassium permanganate.

The quantity of kerogen used needs to be kept relatively constant in the pressurised system (approximately 2ml of moist kerogen per 100ml of 17.5% nitric acid) if the level of oxidation obtained for a particular sample is to be repeated or improved upon.

For AOM-rich samples, sufficient kerogen is usually obtained after HF digestion of approximately 20g of crushed rock or cuttings for several oxidation attempts.

If the reaction is repeated with less kerogen, the sample may be over-oxidised. Over-oxidation is possibly due to a relative reduction in the amount of more oxidation susceptible AOM within the sample, which while present acts as a buffer and is selectively oxidised before the palynomorphs. conversely, if the reaction is repeated using a larger quantity of kerogen the sample will tend to be under-oxidised due to a relative increase in AOM within the sample.

Sample heating times vary and need to be established by experiment. Suitable times can vary from one minute to one hour, but experience suggests that eight minutes is sufficient for the majority of samples in the case of the pressurised system and two to five minutes in the case of the focused microwave system. Oxidation levels may be adjusted by simply increasing or decreasing the amount of oxidation time, as in standard oxidation techniques. Where longer oxidation is required fresh kerogen should be used each time. If only a slight increase in level of oxidation is required, further treatment with standard techniques is recommended.

Microwave power settings may also be used to vary the heating time, and particularly in the case of the focused microwave system, the reactivities may be substantially increased at high power levels. This is because the microwave field is constant rather than "pulsed" as in the pressurised system.

Further reagents may be added to the reaction medium as required. These can include "boiling point elevators" for example "persates" for hydrogen peroxide, and nitrates for nitric acid, or metal catalysts such as Fe/Cu, all to increase the rate of reaction. A similar effect can be obtained by the addition of suitable electrolytes which promote increased dielectric loss and ionic conduction.

The presence of such reagents may permit the use of more dilute reaction media thereby increasing selectivity, improving safety and reducing costs.

The invention is illustrated by the following examples: EXAMPLE 1 Three samples of kerogen obtained after HF digestion of dark grey mudstone ditch cuttings recovered from Kimmeridge Clay facies from the North Sea, were oxidised using both the "overnight" and the microwave techniques referred to below. Those treated using the "overnight" technique were repeatedly oxidised for 24 h periods.

After each 24 h period a portion of the sample was removed, treated with KOH in an ultrasound bath for 30 s, stained and prepared as strew mount slides. The samples oxidised using the microwave technique were heated for varying lengths of time using fresh kerogen each time.

All samples were then treated with KOH and ultrasound, stained and similarly mounted on slides.

Microwave heating was carried out using up to 100ml of reaction medium (17.5% w/v aqueous nitric acid, approximately 2ml of kerogen) in a 120ml capacity pressure relief type Teflon PFA digestion vessel, at full power in a CEM Corp MDS-81 600W microwave oven fitted with a rotating carousel.

After one 24 h period of "overnight" oxidation all three samples still contained large clumps of AOM and very few visible palynomorphs (Figure I, 2). After two 24 h periods all three samples contained less AOM and higher numbers of palynomorphs, though the majority of these were still partly concealed by AOM (Figure I, 3).

After three 24 h periods all three samples contained large numbers of palynomorphs relatively free of AOM, but levels of AOM were still considered too high to be acceptable (Figure I, 4). After four 24 h periods all three samples still contained some AOM, but were considered to have been sufficiently oxidised, as further oxidation would have proved detrimental to the palynomorphs already released from the AOM. Some dinoflagellates were beginning to show signs of overoxidation by becoming paler in colour (Figure I, 5). (A method for further reducing quantities of AOM, without subjecting palynomorphs to further harsh oxidation, involves treating samples with 2% KOH supersaturated with potassium permanganate with the aid of an ultrasonic bath).

Using the microwave technique, heating times were increased from 5 to 11 min by 3 min increments; 8 min was judged to be the optimum time. Further fresh kerogen was then oxidised for 7 and 9 min, but 8 min was still judged to be the optimum, with all three samples showing a substantial decrease in the levels of AOM they contained and improvement in the condition of palynomorphs compared to those oxidised using the "overnight" technique (Figure I, 6, 7). All three samples also showed no apparent differences in palynomorph diversity.

The microwave technique has also been used to treat a number of AOM-rich Carboniferous and Devonian samples, which using standard oxidation techniques required only relatively short oxidation times (10 min). In the case of one Devonian sample, the assemblage was "cleaned" of AOM in only 10 min with 70% nitric acid. However, large amounts of unstructured cuticle material still persisted after oxidation. The same sample was then oxidised using the microwave technique. After 8 min oxidation the sample was also clean of AOM, and contained none of the cuticle seen within the standard sample. These samples have since been logged in order to check the validity of the microwave technique with no decrease in spore diversity being reported.

EXAMPLE 2 The method of Example 1 was repeated except that microwave heating was carried out using a reaction medium consisting of 10 to 50ml of fuming nitric acid and 0.2g of kerogen in a Prolabo Microdigest 401 focused microwave digester. Very good assemblages of palynomorphs were obtained in from two to 5 minutes using a 10% power setting.

Similar good results were obtained using 0.2g of coal in place of the kerogen.

The focused microwave heating system has a number of important advantages over the pressurised vessel microwave heating system as follows: 1 In the focused microwave heating system the correct volumes of reagents can be added automatically at a controlled rate prior to heating. This reduces chemical handling and improves safety, and is particularly important with iron pyrite-rich samples. The system lends itself to automation, and for example it may be possible for up to 16 samples to be digested, oxidised, neutralised and diluted with reagent evaporation between each step. Samples requiring different regimes may be catered for by the preselection of stored programmes.

2 Vapour collection and neutralisation is safer in focused microwave systems. The danger of rupture or venting of the pressure vessel due to the presence of carbonate, reactive clays or metal fragments during HF/HC1 digestion is reduced.

3 The focused microwave system permits multistage reactions, for example, oxidation with successive reagents followed by cooling/dilution by automatic addition of water. Successive addition also permits complexing of HF with boric acid following HF digestion of silicates.

4 The use of non-pressurised vessels allows rapid evaluation of sample progress without the need for lengthy cooling, opening and resealing.

5 The reproducibility of sample oxidation due to the focused nature of the microwave field is greatly improved. This is difficult to achieve in pressurised systems due to the need for venting and the variability of the microwave field.

6 The use of higher capacity, tall vessels in focused microwave systems allows the treatment of larger samples and provides space for any adverse reaction.

7 The focused microwave system gives more thorough digestion of silicates and carbonates during HF/HCl digestion due to better reagent circulation. there is also less likelihood of insoluble calcium fluoride formation that in pressurised systems.

8 The presence of oil, common in industrial samples, is advantageous due to its dramatic super heating effects which further increase reaction rates.

The reader's attention is directed to all papers and documents which are filed concurrently with this specification and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps or any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

Claims (11)

1. A method for the preparation of samples for kerogen analysis in which kerogen, containing palynomorphs, is subject to microwave heating in the presence of an oxidising reaction medium for a period of time sufficient to bleach the palynomorphs and/or to reduce the level of any amorphous organic matter present.

2. A method according to Claim 1, in which the reaction medium is contained in a pressure relief vessel placed in a microwave oven.

3. A method according to Claim 1, in which the reaction medium is heated using a focused microwave system.

4. A method according to Claim 1 or 3, in which the reaction medium is contained in a vessel at atmospheric pressure.

5. A method according to any of the preceding Claims, in which the oxidising reaction medium comprises an oxidising agent comprising nitric acid.

6. A method according to Claim 2, in which the oxidising medium comprises dilute nitric acid.

7. A method according to Claim 3, in which the oxidising medium comprises fuming nitric acid.

8. A method according to any of the preceding Claims, in which the reaction medium is heated for a period of time of from one to eight minutes.

9. A method according to any of the preceding Claims, in which further reagents are added to the reaction medium to increase the apparent boiling point, dielectric loss, or ionic conduction of the reaction medium.

10. A method according to any of the preceding Claims substantially as described in the Examples.

11. A palynomorph sample prepared using a method according to any of the preceding claims.