Note: Descriptions are shown in the official language in which they were submitted.
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~
11
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:
FXAMPLE 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 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
14
have since been logged in order to check the validity of
the microwave technique with no decrease in spore
diversity being reported.
5EXAMPLE 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 (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
16
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
CA2 1 83628
17
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
18
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.
