CA2183628A1 - Preparation of samples for kerogen analysis - Google Patents

Abstract

A method for the preparation of samples for kerogen analysis in which kerogen, containing palynomorphs, is subjected to microwave heating in the presence of a suitable reaction medium for a period of time sufficient to bleach the palynomorphs and/or selectively to reduce the level of unwanted kerogen present.

Description

wos5l2276l PCT~P95/00558 (~A2 1 83628 PREPARATTON OF SAMPLES FOR KEROGEN A~TYSTS

This invention relates to the preparation of samples for kerogen analysis, and more particularly to an improved t~r~n;que for the rapid preparation of such samples.

In this specification kerogen is defined as sedimentary organic matter that is substantially insoluble in organic solvents and mineral acids other than nitric acid. Kerogen is found, for example in sedimentary rocks and coals.

In conventional oil-drilling techniques, it is necessary to take samples at regular intervals to determine the identity of the stratum through which the drill bit is passing. The samples are scrubbed with detergent or organic solvents, to remove mud and any diesel oil contamination from the drilling operation, and then treated with hydrochloric acid to solubilise cations which may form insoluble fluorides. After ~cAnting off the hydrochloric acid, the samples are treated with hydrofluoric acid (HF) to remove silicates, leaving behind kerogen, typically fossilised pollen and plant spores, which can be identified to provide an indication of the age and the thermal history of the stratum from which the sample was taken. If the samples are required urgently ("hot-shots") they are treated for a short time l-,ONFIRMATION COPY

wo9sl2276l pcT~ps5mos58 CA2 1 ~362~

with hot hydrofluoric acid, whereas routine samples may be subjected to prolonged treatment at room temperature.

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

In addition 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 reduce selectively the levels of AOM and other unwanted kerogen present in order to concentrate and easily distinguish for identification any palynomorphs which may be present (Figure I, 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 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. The treatment is conventionally carried out using strong oxidising agents since oxidative WO95/2276~ 3 ~ ~Pg5/00558 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 saturated 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 to 10% potassium hydroxide solution (KOH) or other alkali 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 partly avoided by using a repeated and lengthy "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, however, it will be apparent that these are very time-consuming procedures, WO95/22761 PCT~P95/00558 ~A21 ~362~3 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 ner~ss~ry to oxidise kerogen samples to examine the fossil content thereof. Such treatments have also hitherto been e~lc ~ly time consuming.

The use of heating has been proposed in geochemistry to speed up the total dissolution of rock samples, 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. Whatever the reason, the heating of samples for kerogen analysis in the precence of a suitable reaction medium for a period of time sufficient to bleach the palynomorphs and/or to reduce the level of unwanted kerogen present has not hitherto been proposed.

The prior art has also recognised that the reaction products of nitric acid treatment vary depending on the WO95/22761 C A 2 1 ~ 3 ~T~P95/00558 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 treatment of palynology samples, either to bleach the palynomorphs, or to reduce selectively unwanted kerogen, which preferably can also obviate some of the difficulties involved ln the use of harsh reagents such as fuming nitric acid and Schulze's solution.

In palynology, microwave heating has been confined to the drying of modern spore and pollen samples. It is also known, in geochemical analysis, in order to dissolve completely non-fossiliferous silicate rock samples, to treat the samples by HF digestion in a pressurised microwave heating system. Finally, in paleopalynology, it has been proposed to carry out HF digestion of sedimentary rock samples in a pressurised microwave heating system to release palynomorphs, but this suggestion has not been pursued commercially due to difficulties in implementation.

In accordance with the invention, it has now surprisingly been found that not only can the reaction time for ble~ching the palynomorphs and/or selectively reducing the level of unwanted kerogen can be WO95/22761 C A 2 1 8 3 6 2 8 PCT~Pg5/00558 substantially reduced by heating, but that clean palynomorphs can thereby be obtained in certain circumstances using milder reactants.

The present invention provides a method for the preparation of a sample for kerogen analysis, in which kerogen, cont~i~;ng palynomorphs, is sub~ected to microwave heating in the presence of a suitable reaction medium for a period of time sufficient to bleach the palynomorphs and/or selectively to reduce the level of unwanted kerogen 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. Pressurised system microwave heating can be achieved, for example, by using a plurality of 120ml capacity pressure relief type teflon PFA digestion vessels, or a plurality of reflux vessels, heated separately at full power in a CEM Corp MDS-81 600W
microwave oven fitted with a rotating carousel.

WO 9~/22761 ~ A ~ 1 8 3 ~3EP95/00558 ~183~oa8 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 contAine~ 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.

A suitable reaction medium is one that is capable of solubilising AOM and unwanted kerogen and ble~hing the palynomorphs, and can, for example, be an oxidising medium, although this is by no means essential.

The reaction medium can, for example, comprise a 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 of greater than 50%, and preferably greater than 90% concentration.

WO95/22761 ~ 2 1 ~ 3 6 2 8 PCT~P95/00558 Other reaction media which can be used include, for example, nitrating media, such as, for example, nitric acid of less than 50%, and preferably less than 25%
concentration.

The volume of reaction medium compared to the volume of the vessel in pressurised systems needs to be maximised to ensure complete treatment of samples.
Reductions in volume can result in under-exposure of samples either due to the reaction medium becoming exhausted, or reduced to such a low concentration that it becomes ineffective. In pressurised system microwave heating the maximum volume currently possible for pressure relief-type digestion vessels is lOOml, governed by the manufacturer's recommended safe maximum volume for pressure vessels. However, the use of reflux vessels, and the use of focused microwave systems can permit larger vessels to be employed.

The concentration of the reaction medium is important. Experimentally in pressurised system microwave heating 17.5% nitric acid has proved to be the most effective, made, for example, by diluting 70% nitric acid l: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 nitric acid (35 and 70%) were found to reduce the selectivlty of treatment, ie although assemblages were obtained which Wo95/22761 ~ 21 ~ 36 2 8 PCTtEP95tOOS58 were relatively "clean" in terms of AOM, the number of palynomorphs observable had been reduced.

In the focused microwave heating method, however, nitric acid of greater than 50% concentration has been found to be the most effective reaction medium.
Schulze's solution has been found in general to be too reactive, but it may be applicable to samples which are thermally mature and therefore require ble~ch~ng 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 lOOml of 17.5%
nitric acid) if the level of treatment obtained for a particular sample is to be repeated or improved upon.
For AOM-rich samples, sufficient kerogen is usually obt~;ne~ after HF digestion of approximately 20g of crushed rock or cuttings for several treatment attempts.
If the reaction is repeated with less kerogen, the sample may be over-exposed. Over-reaction may possibly be 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 woss/22761 C A 2 1 8 3 6 2 ~ PCT~P95/00558 palynomorphs. Conversely, if the reaction is repeated using a larger quantity of kerogen the sample will tend to be under-exposed 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. Treatment levels may be adjusted by simply increasing or decreasing the reaction time, as in standard oxidation techniques.
Where longer treatment is required fresh kerogen should be used each time. If only a slight increase in level of treatment is required, further oxidation 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 WO95/22761 PCT~P95/00~58 C~2 1 ~36~

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 prec~nce 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:

Three samples of kerogen obtained after HF digestion of dark grey mudstone ditch cuttings recovered from Kimmeridge Clay facies from the North Sea, were treated using both the "overnight" and the microwave techniques referred to below. Those treated using the Novernight"
technique were repeatedly treated 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 treated 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.

.

WO95/22761 ~ 3 6 2 PCT~/00558 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 r-~pAC;ty pressure relief type Teflon PFA digestion vessel, at full power in a CEM Corp MDS-81 600W microwave oven fitted with a rotating carousel.

The results of microscopic illustrated in Figures 1 to 7 of the accompanying Drawing.

Figure 1 shows a slide-mounted sample before treatment. After one 24 h period of "overnight"
treatment all three samples still contained large clumps of AOM and very few visible palynomorphs (Figure 2).
After two 24 h periods all three samples cont~in~ less AOM and higher numbers of palynomorphs, though the majority of these were still partly concealed by AOM
(Figure 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 4). After four 24 h periods all three samples still contained some AOM, but were considered to have been sufficiently treated, as further reaction would have proved detrimental to the palynomorphs already released from the AOM. Some dinoflagellates were beginning to show signs of over-reaction by beco~;ng paler in colour (Figure 5). (A
method for further reducing quantities of AOM, without .

WO95/22761 ~ ~ ~1836~8 PCT~P95/00558 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 technigue, heating times were increased from 5 to ll min by 3 min increments; 8 min was judged to be the optimum time. Further fresh kerogen was then reacted 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 conta~e~
and improvement in the condition of palynomorphs compared to those reacted using the "overnight" technique (Figures 6, 7 show representative palynomorphs). All three samples also showed no apparent differences in palynomorph diversity.

The microwave t~chn; que 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 (l0 min). In the case of one Devonian sample, the assemblage was "cleaned" of AOM in only l0 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 WO95/22761 C~ 2 1 8 3 6 2 8 PCT~P95/00558 have since been logged in order to check the validity of the microwave technique with no decrease in spore diversity being reported.

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 (98%) and 100.2g 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.

15Similar good results were obtained using 0.2g of coal in place of the kerogen.

Microwave heating of the reaction medium is found to increase the rate of reaction of the kerogen treatment and surprisingly has also been found to increase the selectivity of the reaction. Without wishing to be bound to any particular theory, this is believed to be due to the particular mode of heating provided by microwave energy. It has been found, for example, that when a sample produced by a microwave heating treatment is compared with a sample produced by heating a similar reaction medium by conventional means, the sample heated by microwave treatment has a much higher species wossl2276l C~ 21 8 36 2`8 PCT~P95/00558 diversity with a much higher proportion of retained bisaccate pollens and pteridophyte spores.

The focused microwave heating system has a number of important advantages over the pressurised vessel microwave heating system as follows:

l 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, treated, 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/HCl digestion is also obviated.

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 WO95/22761 C~ 2 ~ ~ 3 6 28 PCT~P~5/00558 complexing of HF with boric acid following HF digestion of silicates as describe in our copending UK Patent Application No. (agents ref. P31071GB) entitled "Improvements in or Relating to the Preparation of Samples for Palynological Analysis" and filed concurrently herewith.

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

The reproducibility of sample treatment 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 than in pressurised systems.

wos5l2276l PCT~P95/00558 8 The presence of oil, common in industrial samples, is advantageous due to its dramatic super heating effects which further lncrease reaction rates.

The invention can be applied to suitable kerogen samples however prepared, but preferably the kerogen samples are prepared using the HF digestion method described and claimed in our copending UK Patent Application No. (Agents ref. P31071GB) entitled "Improvements in or Relating to the Preparation of Samples for Palynological Analysis" and filed concurrently herewith. The entire disclosure of the aforesaid UK patent application is incorporated herewith by reference.

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.

WO95/22761 C A 2 1 ~ 3 6 2 ~ PCT~P95/00558 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.

The invention is not restricted to the deta~ls of the foregoing embodiments. This invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims (12)

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 a suitable reaction medium for a period of time sufficient to bleach the palynomorphs and/or selectively to reduce the level of unwanted kerogen 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 reaction medium is an oxidising reaction medium.

6. A method according to Claim 2, in which the reaction medium comprises nitric acid of less than 50%
concentration.

7. A method according to Claim 3, in which the reaction medium comprises nitric acid of greater than 50%
concentration.

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.

12. A method for the preparation of a sample for palynological analysis, in which kerogen, containing palynomorphs, is subjected to heating in the presence of a suitable reaction medium for a period of time sufficient to bleach the palynomorphs and/or to reduce the level of unwanted kerogen present.