Field of the invention
[0001] The present invention relates to a method for
identification of molecular tracers for secondary oil
5 migration. More particularly, the present invention
relates to a method for simultaneous isolation and
identification of molecular tracers such as
benzocarbazoles and benzo[b]naphthothiophenes from crude
oils and/or rock extracts.
10
Background of the invention
[0002] Hydrocarbon reservoirs are formations of rock in
which oil and natural gas have accumulated. In
15 subsurfaces, oil and gas are released from source rocks,
having high organic carbon content, and migrate to
reservoir rocks that have sufficient porosity and
permeability. Migration occurs in two stages, namely,
primary and secondary migration. Primary migration is the
20 process by which hydrocarbons are expelled from a source
rock into an adjacent permeable carrier bed. In primary
migration, oil and gas migrate together as a single
liquid phase due to high pressure in the source rock. In
secondary migration, oil and gas separates from the
25 single liquid phase and migrates through carrier beds
into a reservoir rock which is sealed by the cap rock so
that the hydrocarbons are not lost.
[0003] Secondary oil migration is critical as the
30 direction of oil migration plays an important role in the
discovery of new oil reservoirs. Information on the
secondary oil migration pathway is also crucial for
tracking location of the source rock of hydrocarbons. It
has been observed that obtaining the information on
3
secondary oil migration through seismic or other remotesensing methods poses numerous challenges.
[0004] Through extensive research it has been established
5 that the direction of secondary oil migration can be
traced using certain biomarkers present in crude oils
and/or rock extracts. For example, certain NSO (nitrogen,
sulphur and oxygen) containing compounds that are present
in trace concentrations in crude oils and/or rock
10 extracts have a great potential in tracing secondary oil
migration pathway.
[0005] Specifically, pyrrolic nitrogen compounds, such as
alkylcarbazoles and benzocarbazoles (BCs) are used as
15 biomarkers for tracing secondary oil migration pathway.
Also, Dibenzothiophenes (DBTs) and
benzo[b]naphthothiophenes (BNTs) are important sulfurcontaining heterocyclic aromatic compounds in crude oils
and sedimentary rock extracts, which are used as
20 biomarkers for tracing secondary oil migration pathway.
These biomarkers undergo systematic fractionations due to
the interaction with rocks and solid organic matter in
the migration pathway. A quantitative method for
isolation and identification of biomarkers such as
25 benzocarbazoles and benzo[b]naphthothiophenes provides an
understanding of the secondary oil migration pathway.
However, it has been observed that, various isomers of
benzocarbazoles and benzo[b]naphthothiophenes in crude
oils and/or rock extracts get masked by other abundant
30 biomarkers during Gas Chromatography – Mass Spectrometry
(GCMS) analysis and therefore accurate results cannot be
obtained for tracing secondary oil migration pathway.
4
[0006] Further, existing methods for identification of
benzocarbazoles and benzo[b]naphthothiophenes involve
sampling in two separate aliquots. Conventionally, the
samples in two separate aliquots are subjected to two
5 separate isolation processes, thereby necessitating an
additional sample and thereby adding to the cost and
duration of the biomarker separation process.
Conventional methods involving two separate aliquots are
not effective, since benzocarbazoles and
10 benzo[b]naphthothiophenes are obtained in two separate
fractions using two different methods. Yet further,
conventional biomarker separation techniques are not user
friendly and are time consuming.
15 [0007] In light of the above drawbacks, there is a need
for an improved method for simultaneous isolation and
identification of benzocarbazoles and
benzo[b]naphthothiophenes from crude oils and/or rock
extracts for tracing secondary oil migration pathway to
20 discover new oil reservoirs. Also, there is a need for an
improved method for accurate and precise tracing of
secondary oil migration pathway. Yet further, there is a
need for a method which provides specific biomarker
separation which is user friendly, less time consuming
25 and cost effective.
Summary of the invention
[0008] In various embodiments of the present invention, a
30 method for simultaneous isolation and identification of
benzocarbazoles and benzo[b]naphthothiophenes is
provided. The method comprises the steps of a)
introducing an analyte to a chromatographic column having
a stationary phase comprising alumina and silica, (b)
5
eluting the analyte with a first mobile phase, a second
mobile phase and a third mobile phase successively and
(c) collecting one or more compounds eluted from the
analyte through the chromatographic column in different
5 mobile phases, wherein eluted NSO compounds in the third
mobile phase are enriched with benzocarbazoles and
benzo[b]naphthothiophenes, wherein the first mobile phase
is petroleum ether with a boiling point of 40-60oC, the
second mobile phase is benzene, and the third mobile
10 phase is a mixture of dichloromethane and methanol in a
ratio of 20:80 %v/v.
[0009] In an embodiment of the present invention, the
analyte is a deasphaltened crude oil or bitumen adsorbed
15 onto activated silica. In another embodiment, the ratio
of alumina and silica in stationary phase is 1:1.
[0010] In an embodiment of the present invention, the
stationary phase comprising alumina and silica are
20 activated for a duration of 12hrs at a temperature of
110oC. The ratio of volume of first mobile phase to
second mobile phase to third mobile phase is 1:1:1.
Brief description of the drawings
25
[0011] The present invention is described by way of
embodiments illustrated in the accompanying drawings
herein:
30 [0012] Figures 1(a) and 1(b) are graphical
representations depicting successful isolation of isomers
of benzocarbazoles without coelution of other biomarker
compounds in the region of benzocarbazoles, in accordance
with an embodiment of the present invention;
6
[0013] Figures 2(a) and 2(b) are graphical
representations depicting successful isolation of isomers
of benzo[b]naphthothiophenes without coelution of other
5 biomarker compounds in the region of
benzo[b]naphthothiophenes, in accordance with an
embodiment of the present invention;
[0014] Figures 3(a) and 3(b) are graphical
10 representations of isolation of benzocarbazoles in
comparison with the conventional methods, in accordance
with an embodiment of the present invention;
[0015] Figures 4(a) and 4(b) are graphical
15 representations of isolation of benzo[b]naphthothiophenes
in comparison with the conventional methods, in
accordance with an embodiment of the present invention;
and
20 [0016] Figure 5 illustrates an isopleth map of molecular
parameters of benzocarbazoles and
benzo[b]naphthothiophenes, in accordance with an
embodiment of the present invention.
25 Detailed description of the invention
[0017] The present invention discloses a method for
isolation and identification of benzocarbazoles and
benzo[b]naphthothiophenes simultaneously, in accordance
30 with an embodiment of the present invention. The present
invention provides for separation of benzocarbazoles and
benzo[b]naphthothiophenes from other molecular tracers of
secondary oil migration present in crude oils or rock
extracts, thereby rendering the method of present
35 invention effective and reliable.
7
[0018] The disclosure is provided to enable a person
having ordinary skill in the art to practice the
invention. Exemplary embodiments herein are provided only
for illustrative purposes and various modifications will
5 be readily apparent to persons skilled in the art. The
general principles defined herein may be applied to other
embodiments and applications without departing from the
spirit and scope of the invention. The terminology and
phraseology used herein is for the purpose of describing
10 exemplary embodiments and should not be considered
limiting. Thus, the present invention is to be accorded
the widest scope encompassing numerous alternatives,
modifications, and equivalents consistent with the
principles and features disclosed herein. For purposes of
15 clarity, details relating to technical material that is
known in the technical fields related to the invention
have been briefly described or omitted so as not to
unnecessarily obscure the present invention.
20 [0019] In various embodiments of the present invention,
the method for simultaneous isolation and identification
of benzocarbazoles and benzo[b]naphthothiophenes
comprises the steps of a) introducing an analyte to a
chromatographic column having a stationary phase,
25 (b)successively eluting the analyte with a first mobile
phase, a second mobile phase and a third mobile phase and
(c) collecting one or more compounds eluted from the
analyte through the chromatographic column. In an
exemplary embodiment of the present invention, the
30 compounds eluted from the analyte through chromatographic
column in the third mobile phase comprises of NSO
compounds enriched with benzocarbazoles and
benzo[b]naphthothiophenes. In an embodiment of the
8
present invention, the method further comprises analyzing
the compounds eluted from chromatographic column for
identification of benzocarbazoles and
benzo[b]naphthothiophenes.
5
[0020] In an exemplary embodiment of the present
invention, the chromatographic column is wet-packed using
petroleum ether (40-60°C) with alumina and silica. The
stationary phase of the present invention comprises of
10 alumina and silica. In an exemplary embodiment of the
present invention the ratio of alumina and silica is 1:1.
In an exemplary embodiment of the present invention, the
first mobile phase is petroleum ether with a boiling
point of 40o-60oC. In an exemplary embodiment of the
15 present invention, the second mobile phase is benzene. In
an exemplary embodiment of the present invention, the
third mobile phase is a combination of dichloromethane
and methanol in a ratio of 20:80 %v/v. The exact amount
of the stationary phase and the mobile phase depends on
20 the amount of analyte i.e., deasphaltened crude oil
and/or rock extract. Typically, 0.5g of analyte requires
25g alumina and 25g silica as stationary phase, and 150mL
of each of the mobile phases, viz., petroleum ether with
boiling point of 40o-60oC, benzene and dichloromethane:
25 methanol (20:80 %v/v).
[0021] In an exemplary embodiment of the present
invention, a method for simultaneous isolation and
identification of benzocarbazoles and
30 benzo[b]naphthothiophenes comprises the steps of a)
introducing an analyte to a chromatographic column having
a stationary phase comprising alumina and silica,
(b)eluting the analyte with a first mobile phase
9
comprising of petroleum ether (40-60°C) under conditions
such that one or more compounds that are aromatics or NSO
compounds in the analytes are retained in the stationary
phase, and one or more compounds that are saturates such
5 as n-alkanes, branched alkanes and cycloalkanes from the
analyte are eluted, (c) eluting the one or more retained
compounds from the analyte by applying a second mobile
phase comprising benzene under conditions such that one
or more compounds are retained in stationary phase and
10 aromatics from analyte are eluted, (d) eluting the one or
more retained compounds from the analyte by applying a
third mobile phase comprising dichloromethane and ethanol
in a ratio of 20:80, under conditions such that
benzocarbazoles and benzo[b]naphthothiophenes are eluted
15 and (e) collecting compounds eluted from the analyte
through chromatographic column.
[0022] In this exemplary embodiment of the present
invention, the column used is a glass column having a
20 length of 70cm and an internal diameter of 1.2cm. The
analyte is deasphaltened crude oil or bitumen adsorbed
onto activated silica. For this, minimum quantity of
silica, that had been activated overnight at 110°C, is
added to deasphaltened crude oil or bitumen and the
25 mixture is stirred vigorously to homogenize. The
stationary phase is activated overnight at a temperature
of 110oC to remove any adsorbed moisture. The first
mobile phase is petroleum ether with boiling point of
40o-60oC. In an exemplary embodiment of the present
30 invention, the ratio of volume of first mobile phase to
second mobile phase to third mobile phase is 1:1:1.
10
[0023] In various embodiments of the present invention,
the compounds eluted through the above-mentioned steps
(a) to (e) comprises of NSO compounds enriched with
benzocarbazoles and benzo[b]naphthothiophenes, which are
5 isolated simultaneously. The simultaneous isolation of
benzocarbazoles and benzo[b]naphthothiophenes is based on
relative affinity of the compounds in the analytes
between the mobile phase and the stationary phase.
Relative affinity is based on selective non-covalent
10 interactions between the analyte and the mobile and
stationary phases. Therefore, quantity and composition of
mobile phase and stationary phase is crucial for ensuring
accurate isolation of benzocarbazoles and
benzo[b]naphthothiophenes, thereby enabling tracing of
15 secondary oil migration pathway.
[0024] In an embodiment of the present invention, the
eluted polar fractions enriched with benzocarbazoles and
benzo[b]naphthothiophenes are processed for simultaneous
20 identification of benzocarbazoles and
benzo[b]naphthothiophenes. In an embodiment, polar
fractions obtained after column chromatography were
analyzed on Gas Chromatography Mass spectrometer (GCMS
Perkin Elmer Clarus 500) using fused silica capillary
25 column DB-5MS (30m x 0.25 mm x 0.25μm) and Helium as
carrier gas. GC oven temperature was programmed as
follows: Initial oven temperature was kept at 40°C for 1
min, increased at rate of 2°C/min up to 300°C with final
hold time of 20 minutes.
30
[0025] In an embodiment of present invention,
benzocarbazoles interact with solid organic or mineral
phases, via hydrogen bonding with unpaired electrons on N
11
atom, in a petroleum carrier system. Therefore, more rodshaped benzo[a]carbazole gets preferentially removed from
oils onto carrier bed phases during migration as compared
to the sub-spherical benzo[c]carbazole. Thus, in crude
5 oils, with increasing migration distance, there is
enrichment of benzo[c]carbazole to benzo[a]carbazole
indicating the pathway of secondary oil migration.
Molecular migration parameter, [a]/([a]+[c])-BC
(BC=benzocarbazole), decreases from source rock to
10 reservoir along the migration pathway, which can be used
to trace new hydrocarbon reservoirs.
[0026] In an embodiment of the present invention,
benzo[b]naphthothiophenes which are thiophene-class
15 aromatic compounds also exhibit strong absorption or
adsorption interaction with solid organic or mineral
phases, via hydrogen bonding, due to similar molecular
skeleton as that of carbazole. Therefore, in crude oils,
with increasing migration distance, enrichment of
20 benzo[b]naphtho[1,2-d]thiophene to benzo[b]naphtho[2,1-
d]thiophene is observed indicating the pathway of
secondary oil migration. Molecular migration parameter,
[2,1] BNT/ ([2,1] BNT + [1,2] BNT) decreases from source
rock to reservoir along the migration pathway which can
25 be used to trace new hydrocarbon reservoirs. As shown in
Figure 5, isopleth map of molecular parameters of
benzocarbazoles and benzo[b]naphthothiophenes reveals an
overall oil migration, filling points, and preferred
filling pathway orientation.
30
[0027] Advantageously, the method of the present
invention is less time consuming, economical and does not
employ multiple chemicals. In addition, the method of
12
present invention requires only a single aliquot of
sample for analysis, thereby eliminating the need for
larger sample size for simultaneous isolation and
identification of benzocarbazoles and
5 benzo[b]naphthothiophenes.
[0028] The disclosure herein provides for examples
illustrating the process for simultaneous isolation and
identification of benzocarbazoles and
10 benzo[b]naphthothiophenes in accordance with an
embodiment of the present invention. The examples used
herein for such illustration are intended merely to
facilitate an understanding of ways in which the
embodiments may be practiced and to further enable those
15 of skill in the art to practice the embodiments.
Accordingly, following examples should not be construed
as limiting the scope of the embodiments herein.
Working Examples
20 Example 1
[0029] A glass column (1.2 cm internal diameter, 70 cm
length) was wet-packed using petroleum ether with boiling
point of 40-60°C, with 25g alumina and 25g silica, that
25 had been activated overnight at 110°C. 0.5g of
deasphaltened oil or bitumen was adsorbed on activated
silica. For this, minimum quantity of silica, that had
been activated overnight at 110°C, was added to
deasphaltened crude oil or bitumen and the mixture was
30 stirred vigorously to homogenize. This homogenized
mixture was then introduced at the top of the
chromatographic column and successively eluted with three
mobile phases. The first mobile phase was 150mL petroleum
13
ether having boiling point of 40-60°C, which eluted
saturates from the column. The second mobile phase was
150mL benzene which eluted aromatics from the column. The
third mobile phase was 150mL dichloromethane: methanol
5 (20:80, %v/v) which eluted NSO compounds from the column.
The NSO fraction so obtained was enriched in
benzocarbazoles and benzo[b]naphthothiophenes.
[0030] Polar NSO fractions obtained after column
10 chromatography were analyzed on Gas Chromatography Mass
spectrometer (GCMS Perkin Elmer Clarus 500) using fused
silica capillary column DB-5MS (30m x 0.25 mm x 0.25μm)
and Helium as carrier gas. Gas chromatography oven
temperature was programmed as follows: Initial oven
15 temperature was kept at 40°C for 1 min, increased at
2°C/min up to 300°C with final hold time of 20 minutes.
[0031] Figures 1(a) and 1(b) depict successful isolation
of isomers of benzocarbazoles as there was no coelution
20 of other biomarker compounds in the region of
benzocarbazoles. The y-axis depicts the relative
abundance of these isomers i.e., benzo[a]carbazole is
most abundant followed by benzo[c]carbazole and
benzo[b]carbazole is the least abundant isomer.
25
[0032] Figures 2(a) and 2(b) depict successful isolation
of isomers of benzo[b]naphthothiophenes as there is no
coelution of other biomarker compounds in the region of
benzo[b]naphthothiophenes. The y-axis depicts the
30 relative abundance of these isomers i.e.,
benzo[b]naphtho[2,1-d] thiophene is the most abundant
isomer.
14
[0033] The present invention offers an improved method
for isolation and identification of benzocarbazoles and
benzo[b]naphthothiophenes simultaneously whose presence
were masked by other abundant markers in conventional
5 methods. Figures 3(a) and (b) represent a graphical
representation of isolation of benzocarbazoles in
accordance with an embodiment of present invention in
comparison with conventional methods. As seen in figure
3a, mass fragmentogram at m/z 217 depicts isolation and
10 identification of various isomers of benzocarbazoles from
crude oils and/or rock extracts with present invention.
Figure 3b illustrates mass fragmentogram at m/z 217 using
a conventional method depicting that the benzocarbazoles
are not properly isolated and identified.
15
[0034] Figures 4(a) and (b) represent graphical
representations of isolation of benzo[b]naphthothiophenes
in accordance with an embodiment of present invention in
comparison with conventional methods. Figure 4a
20 illustrates mass fragmentogram at m/z 234 which depicts
isolation and identification of various isomers of
benzo[b]naphthothiophenes from crude oils and/or rock
extracts with the present invention. Figure 4b
illustrates mass fragmentogram at m/z 234 using a
25 conventional method which depicts that the
benzo[b]naphthothiophenes are not properly isolated and
identified.
[0035] Figure 5 is the isopleth map of molecular
30 parameters of benzocarbazoles and
benzo[b]naphthothiophenes which reveals an overall oil
migration, filling points, and preferred filling pathway
orientation. As shown in figure 5, with increasing
15
migration distance, there is enrichment of
benzo[c]carbazole to benzo[a]carbazole indicating the
pathway of secondary oil migration. Thus, molecular
migration parameter, [c]/([c]+[a])-BC
5 (BC=benzocarbazole), increases from source rock to
reservoir along the migration pathway. In the above
isopleth map of figure 5, the ratio [c]/([c]+[a])-BC
increases from southwest to northeast direction, thereby
suggesting oil filling from southwest direction.
10
[0036] While the exemplary embodiments of the present
invention are described and illustrated herein, it will
be appreciated that they are merely illustrative. It will
be understood by those skilled in the art that various
15 modifications in form and detail may be made therein
without departing from the scope of the invention.

We claim:
1) A method for simultaneous isolation and identification
of benzocarbazoles and benzo[b]naphthothiophenes, the
5 method comprising the steps of:
introducing an analyte to a chromatographic column
having a stationary phase comprising alumina and
silica;
10
eluting the analyte with a first mobile phase, a second
mobile phase and a third mobile phase successively; and
collecting one or more compounds eluted from the
15 analyte through the chromatographic column, wherein
eluted NSO compounds are enriched with benzocarbazoles
and benzo[b]naphthothiophenes, wherein the first mobile
phase is petroleum ether with a boiling point of 40-
60oC, the second mobile phase is benzene, and the third
20 mobile phase is a mixture of dichloromethane and
methanol in a ratio of 20:80 %v/v.
2) The method as claimed in claim 1, wherein the analyte
is a deasphaltened crude oil or bitumen adsorbed on to
25 activated silica.
3) The method as claimed in claim 1, wherein the ratio of
alumina and silica in the stationary phase is 1:1.
30 4) The method as claimed in claim 1, wherein the
stationary phase comprising alumina and silica are
activated for a duration of 12hrs at a temperature of
110oC.
17
5) The method as claimed in claim 1, wherein the ratio of
volume of first mobile phase to second mobile phase to
third mobile phase is 1:1:1.
5 6) The method as claimed in claim 1, wherein the method
comprises analyzing the compounds eluted from the
chromatographic column for the isolation and
identification of benzocarbazoles and
benzo[b]naphthothiophenes.