BIT_ESS2.WPD DRAFT 19 November 1998 Classification, Petrographic Expression, and Reflectance of Native Bitumen Jeff Quick Utah Geological Survey Introduction Native bitumen is naturally occurring, solid organic material that originates, with few exceptions, from material expelled by sedimentary organic matter during catagenesis. Note that, in this text, the word bitumen is used to mean "native bitumen" rather than the common meaning of organic matter extracted from rocks with organic solvents. Native bitumens, especially those in developed commercial deposits, are often named after people or places. These names have subsequently been applied to bitumens found in other localities. In other instances, otherwise similar occurrences have been given different names. Accordingly, the names given to native bitumens vary; a glossary is appended. This text reviews nomenclature and criteria used in bitumen classification systems, and also examines the petrographic expression and optical properties of bitumens observed through he reflected light microscope. Classification of Bitumens Historically, the classification of native bitumen developed for marketing purposes or for technical reasons related to their use as fuels and paving materials, or in the manufacture of preservatives, resins, lacquers and paints. The emergence of the petroleum and associated
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BIT_ESS2.WPD DRAFT 19 November 1998
Classification, Petrographic Expression, and Reflectance of Native Bitumen
Jeff Quick Utah Geological Survey
Introduction
Native bitumen is naturally occurring, solid organic material that originates, with few
exceptions, from material expelled by sedimentary organic matter during catagenesis. Note that,
in this text, the word bitumen is used to mean "native bitumen" rather than the common meaning
of organic matter extracted from rocks with organic solvents.
Native bitumens, especially those in developed commercial deposits, are often named
after people or places. These names have subsequently been applied to bitumens found in other
localities. In other instances, otherwise similar occurrences have been given different names.
Accordingly, the names given to native bitumens vary; a glossary is appended. This text reviews
nomenclature and criteria used in bitumen classification systems, and also examines the
petrographic expression and optical properties of bitumens observed through he reflected light
microscope.
Classification of Bitumens
Historically, the classification of native bitumen developed for marketing purposes or for
technical reasons related to their use as fuels and paving materials, or in the manufacture of
preservatives, resins, lacquers and paints. The emergence of the petroleum and associated
BIT ESS2.WPD DRAFT 19 November 1998
petrochemical industry greatly diminished the economic significance of the bitumen industry.
Consequently, the classification of bitumens became largely academic. Ironically, renewed
interest in the recognition, genesis and classification of native bitumen has emerged to meet the
needs of the petroleum industry. This is not surprising since most bitumens are related to some
aspect of the origin, migration, entrapment or destruction of petroleum. Genetic relationships
between bitumens and metallic ores, the behavior of bitumens during ore processing, as well as
bitumen occurrences in geothermal systems has also contributed to the renewed interest in
bitumen classification.
An early classification system of native bitumen was begun by Herbert Abraham in 1918
with the publication of the first edition of Asphalts and Allied Substances. His system is fully
explained in the 5th (1945) edition of the text and remains largely unchanged in the 6th and final
(1960) edition of this monumental work. Abraham's classification uses a physicochemical
approach and is based on:
1. physical properties (consistency at room temperature, streak, fusibility),
2. empirical behavior (solubility in carbon disulfide, fixed carbon) and,
3. chemical properties (oxygen and wax content).
Hunt and others, (1954) graphically illustrated Abraham's classification in a simple chart (figure
1) that has been widely cited and modified. Nowhere in Abraham's text is his classification so
elegantly presented; this suggests that figure 1 implies a certainty of classification that Abraham
deliberately avoided. Although Abraham provides a table listing distinguishing characteristics of
"bituminous substances"(simplified in Table 1), examination of this table shows that these
J. Quick 2
BIT ESS2.WPD DRAFT 19 November 1998
characteristics correspond to analytical data on an as-received, rather than mineral-matter-free
basis. Ignoring the effect of mineral dilution on assay results precludes the precise thresholds
and diagnostic criteria of a rigorous classification.
Solubility in Carbon Disulfide
soluble insoluble
Bitumen
liquid
Non-Bitumen
solid
fusible
Petroleum
difficultly fusible fusible infusible
Mineral Wax Asphalt Asphaltite
1 Pyrobitumen
oxygen free oxygen
containing
Asphaltic Pyrobitumen
Non-Asphaltic Pyrobitumen
1. All Crudes
2. Oil Seeps
3. Ozocerite
4. Montan Wax
5. Hatchettite
6. Scheererite
7. Bermudez Pitch
8. Tabbyite
9. Liquid Gilsonite
10. Argulite
11. Gilsonite
12. Grahamite
13. Glance Pitch
14. Wurtzilite
15. Elaterite
16. Albertite
17. Impsonite
18. Ingramite
19. Peat
20. Lignite
21. Coal
Figure 1. Abraham's classification of naturally occurring hydrocarbons according to Hunt and
others (1954).
J. Quick 3
BIT_ESS2.WPD DRAFT 19 November 1998
Table 1. Synoptic table of distinguishing characteristics of native bitumens (modified from
Figure 2. Classification of organic matter in geological formations advocated by King and others
(1963). Note the initial distinction of sedimented (syngenetic) and derived (epigenetic) organic
matter.
Following the ideas set out by King and his co-workers, Hunt (1979) revised his earlier
effort (figure 1) to recognize the importance of an initial genetic distinction; his revised
classification is shown in figure 3. Besides this initial genetic distinction, figure 3 differs from
his earlier effort (figure 1) by the use of the atomic H/C ratio to divide pyrobitumens into
metamorphosed pyrobitumens (low H/C) and polymerized, unmetamorphosed pyrobitumens
(high H/C). He also narrows the classification by omitting petroleum.
J. Quick 5
BIT ESS2.WPD DRAFT 19 November 1998
Natural Bitumens and Coal
I
Allochthonous
CS2 soluble X
CS9 insoluble
Bitumen
fusible • '
Mineral Wax -T-
Asphalt
ozocerite scheererite
athabasca trinidad tabbyite
difficultly fusible
Asphaltite
gilsonite grahamite glance pitch
(manjak)
1 Pyrobitumen
H/C> 1 H/C<1
elaterite ingramite wurtzilite albertite
impsonite anthraxolite
Autochthonous
Coal
Sapropelic
cannel coal boghead coal
(torbanite) (coorongite)
Humic
peat lignite bituminous anthracite
Figure 3. Classification of natural bitumens and coals after Hunt (1979). H/C is atomic
hydrogen to carbon ratio.
Arguably, differentiating sedimented and derived organic matter is not entirely objective.
For example, King and his co-workers note that the initial genetic distinction shown in figure 2 is
interpretive and is based on a knowledge of the mode of occurrence gained from hand specimens
and field relations. Although Hunt (1978) points out that elevated H/C and (N+S)/0 ratios can
be used to differentiate bitumens from coals (figure 4) he states this should be used together with
"other methods such as microscopic examination". Indeed, the necessity of using additional
diagnostic methods is evident in figure 4 where impsonite and anthraxolite are indistinguishable
from coal based solely on elemental analysis. In the present author's experience, microscopic
examinations should be augmented by etching of the specimen (Pontolillio and Stanton, 1994) to
reveal underlying botanic structure which is not always visible in high rank coals. Other useful
indicators of native bitumen include high vanadium or nickel contents ( ) as well as distinctive
optical textures observed using slightly crossed nicols and the polarized light microscope.
J. Quick 6
BIT ESS2.WPD DRAFT 19 November 1998
Figure 4. Differentiation of humic and sapropelic coals from native bitumen according to
atomic ratios (from Hunt, 1978). H/C is the atomic hydrogen to carbon ratio; (N+S)/0 is the
nitrogen plus sulfur to oxygen ratio. Elemental data are expressed on a mole percent
(presumably organic) basis where C+H+N+O+S=100.
The classification suggested by King and others (1963) is shown in figure 5. This
classification is noteworthy since it accounts for the effect of mineral dilution on assay results;
volatile matter is on a dry-ash-free basis (daf), and percent atomic carbon on a dry-ash-sulfur-
nitrogen-free basis (dasnf). Solubility in carbon disulfide is used to make the final determination
in instances where when an unknown sample plots on, or close to, the classification line.
Although solubility used in figure 6 is on a mineral containing basis, bitumens used to establish
the system were hand-picked, floated, or demineralized. Accordingly, expression of solubility on
a daf basis is justified where this system might be used. King and his co-workers note that a
J. Quick 7
BIT ESS2.WPD DRAFT 19 November 1998
certain amount of overlapping between the boundaries shown in figure 5 can be anticipated and
(p.53) "to eliminate indecision in such instances priority should be given to either the volatile
matter or the atomic percent carbon depending on which parameter is more sensitive for the area
of the curve being considered".
30 40 50 60 70 80 90 100 Carbon (mole percent)
Figure 5. Classification of native bitumens after King and others, 1963 (p.53). Examination of
King's data indicates: 1) volatile matter is on a daf basis; 2) Carbon is mole percent where
C+H+0= 100 mole percent, and oxygen is estimated by difference where weight percent O=100-
(C+H+N+S) all on an dry, ash-free basis, S refers to organic sulfur; and 3) solubility in carbon
disulfide corresponds to solubility of the organic fraction which can be approximate by
calculation to a daf basis.
J. Quick 8
BIT ESS2.WPD DRAFT 19 November 1998
Examination of figures 1 and 5 shows the classifications suggested by King and others
(1963) and by Hunt and others (1954) include petroleum whereas petroleum is omitted from
Hunt's later effort (figure 3). Although the inclusion of petroleum in a bitumen classification is
logical from a scientific basis, a means to clearly differentiate petroleum is required. Lacking
such criteria causes problems from an industrial or regulatory perspective given different uses
and production techniques. Accordingly, Meyer and De Witt (1990) modified the classifications
shown in figures 1 and 3 to both include and clearly differentiate petroleum (crude oil) from
natural bitumens and coals. This modified classification is shown in figure 6 where bitumens are
distinguished by a viscosity greater than 10,000 cP, and reservoir bitumens are shown to have
variable solubilities in carbon disulfide.
Natural Bitumens and Coal
r Allochthonous
Viscosity <10,000 cP »
Crude Oil
Viscosity >10,000 cP
Natural Bitumen
CS2 soluble X
CS2 insoluble
Soluble Natural Bitumen
fusible
Mineral Wax
ZLL
Natural Asphalt
ozocerite scheererite
:n
"L Pyrobitumen
Reservoir Bitumen
difficultly fusible H/C > 1
Asphaltite
athabasca trinidad lake tabbyite
gilsonite grahamite glance pitch
(manjak)
H/C<1
elaterite ingramite wurtzilite albertite
impsonite anthraxolite shungite
1 Autochthonous
Coal
Sapropelic (anaerobic)
i
cannel coal boghead coal
(torbanite) (coorongite)
Humic (aerobic)
_r peat lignite bituminous anthracite
J. Quick 9
BIT ESS2.WPD DRAFT 19 November 1998
Figure 6. Classification of natural bitumens crude oil and coals presented by Meyer and De Witt
(1990).
Examination of the classification systems shown in figures 1,3,5 and 6 shows that, with
the exception of King's classification (figure 5), bitumens can be classified into two main groups
according to solubility in carbon disulfide. Remarkably, none of these classification systems
provide unambiguous, solubility thresholds. Indeed, since nearly all bitumens exhibit some
solubility (Table 1) a solubility threshold is needed to make this distinction. The significance of
this omission is clearly demonstrated by the work of Orhun (1969) who tried to use Abrahams
system to classify some Turkish bitumens. Orhun's observations (figure 7) show that
Abraham's classification fails to classify substances of intermediate solubilities in carbon
disulfide. Orhun called these bitumens of intermediate solubility "substances between asphaltite
and asphaltic pyrobitumen".
King and others (1963) use 45 percent solubility1 to differentiate the pyrobitumen
albertite from the asphaltite grahamite in figure 5. They state (p.53): "Abraham also considered
the solubility of grahamite to be greater than 45 percent, and this distinction has been followed".
Apparently, they followed the Abrahams synoptical table of distinguishing characteristics, partly
summarized in Table 1, where the minimum solubility of grahamite on mineral containing basis
is 45 percent. As noted by Ohrun (1969), Abraham indicates that grahamite is characterized by
solubilities of 90 to 100 % on a mineral free basis. Thus, although the stated reason for the 45
1 Although not stated by King and his co-workers, examination of their data and methods suggests that their solubility thresholds are essentially on a dry, mineral free basis.
J. Quick 10
BIT_ESS2.WPD DRAFT 19 November 1998
percent solubility limit might be questioned (and its reporting basis unclear), the use of such a
threshold does allow for a comprehensive classification of native bitumens.
GILSONTTE GLANCE PITCH
GRAHAMITE Sikeftikan
Gercus Harbol Kasrok
Avgamasyano.l Seguruk
Avgamasya trench 7 Herbis
Milli WURTZILITE
ALBERTTTE IMPSONTTE
Nivekara Kaluk-Sivit
Besiri Ceffane-Tahtadizgehi
Gundukiremo Seridahli
i 1 1 1 1 1 r 0 10 20 30 40 50 60 70 80 90 100
Solubility in Carbon Disulfide (daf)
Figure 7. Solubility in carbon disulfide of native bitumens from Turkey (black bars) compared
to characteristics of bitumens according to Abraham (stippled bars) (from Ohrun, 1969). Note
the continuous range of solubilities from near 0 to near 100 percent.
The above discussion should make clear that the classification systems discussed so far
are more conceptual than practical. Besides the inherently subjective distinction required to
establish a syngenetic or epigenetic origin for an unknown specimen, these systems generally
J. Quick 11
BIT ESS2.WPD DRAFT 19 November 1998
lack the rigorous criteria required for a useful classification. Undoubtedly, analytical thresholds
might be established and standard methods specified such that the general categories and
nomenclature in shared by these systems might be preserved. However, it is worth considering
other criteria besides solubility and fusibility that could serve a classification system.
Furthermore, the use of carbon disulfide should be discouraged given the exceptionally toxic and
flammable nature of this solvent. Towards this end, Jacob's (1981) conceptual diagram showing
the origin and maturation paths native bitumens (figure 8) coupled with Jacob and Wehner's
diagnostic criteria for these materials (Table 2) is worthwhile.
J. Quick 12
BIT ESS2.WPD
asphalt rich in asphaltene
gilsonite
glance pitch
grahamite
DRAFT
crude oils
19 November 1998
V heavy liquid
paraffin
\ * ozokerite
light
epi-impsonite
meso-impsonite
kata-impsonite
hs of the various native bitumens (from Jacob, 1980, p.215)
napthene-rich asphalt
V wurtzilite
V albertite
Fig
ure
8.
Illu
stra
tion
of
the
orig
ins
and
mat
urat
ion
pat
J. Quick 13
BIT_ESS2.WPD DRAFT 19 November 1998
Table 2. Microscopic criteria useful to classify dispersed native bitumen (modified from Jacob
andWhehner, 1981, p.21)
Mineral Wax ozokerite
Pyrobitumens wurtzilite albertite impsonite
epi-impsonite meso-impsonite kata impsonite
Asphalts
Asphaltites gilsonite glance pitch grahamite
reflectance a
< 0.02
< 0.10 0.10- 0.70
-0.70- 2.00 2.00 3.50
> 3.50
0.07- 0.11 0.11-0.30 0.30- 0.70
fluorescence b
9.00-
0.10-<
< < <
0.40-
0.05-0.05-
<
50.00
2.00 0.10
0.02 0.01 0.01
4.00
0.40 0.20 0.05
solubilityc
soluble
insoluble insoluble
insoluble insoluble insoluble
soluble
soluble soluble
soluble or insoluble
softening temperatured
30 - 90
no flow no flow
no flow no flow no flow
< 104
104--164 104 - 164 164 287
notes: a. mean random reflectance oil immersion, may be calculated from values obtained using water immersion. b. fluorescence intensity at 546 nm where a masked uranyl glass standard (10 um diameter, Wild Leitz Co.)
equals 1.00 intensity units. c. observed solubility in immersion oil (or when cleaning specimen with petroleum ether). d. observed hot-stage softening temperature (degree Celcius).
Geochemical classification systems
more.
J. Quick 14
BIT_ESS2.WPD DRAFT 19 November 1998
Petrographic Expression of Bitumens
We should keep clear the difference between petrographic nomenclature (descriptive) and
petrologic nomenclature (interpretive). Furthermore, like current ICCP maceral nomenclature, a
useful petrographic classification of bitumens should be largely independent of thermal maturity.
Granular bitumen, (protobitumen, prebitumen)
Granular bitumen is the most common kind of bitumen in samples that are within the oil
window. It has a granular texture comparatively low reflectance. This material is called
"prebitumen" by Jacob and Hiltmen, "protobitumen" by Bertrand and "granular bitumen" by
Landis and Castano. None of these authors observed a consistent relationship between the
reflectance of this granular form of bitumen and that of vitrinite. However, since granular
bitumen is not present in immature or post mature rocks, it's presence indicates that the host rock
is mature.
Bertrand observed that protobitumen is intimately associated with hydrogen rich kerogen
from which it is thought to be derived. In some instances, a direct transformation of amorphous
type II kerogen into granular bitumen, appears possible.
Homogeneous bitumen (migrabitumen)
Granular bitumen often grades, either gradually or abruptly, into a higher reflecting
bitumen with a homogeneous texture. Bertrand (1993) and Jacob and Hiltman (1985) measure
J. Quick 15
BIT ESS2.WPD DRAFT 19 November 1998
the reflectance of this homogeneous material which they call "migrabitumen". Importantly,
Bertrand notes that the designation "migrabitumen" does not imply an allochthounous origin,
whereas Jacob's (1990) use of the term is less restrictive and includes solid hydrocarbons that
may have migrated for several kilometers. Landis and Castano measured the reflectance of
material they call "homogeneous bitumen" which is similar to the petrographic description of
migrabitumen used by both Bertrand (1993), and Jacob and Hiltman (1985). Landis and Castano
specifically exclude reflectance measurements on solid hydrocarbons that fill voids (moldic
bitumen) reasoning that this material may have been deposited from a migrated liquid phase and
it's reflectance may not correlate with the thermal maturity of the host rock.
Bertrand notes that broad, sometimes bimodal, bitumen reflectance histograms commonly
result where both granular bitumen (protobitumen) and homogeneous bitumen (migrabitumen)
are measured. Landis and Castano clearly document this phenomena by presenting stacked
histograms for granular and homogeneous bitumen. Bertrand notes that "when a change of
thermal alteration produces a confusion between the protobitumen and the migrabitumen the
resulting solid bitumen continues to be identified as migrabitumen." Multiple generations of
homogeneous hydrocarbon may also occur. In these instances Jacob (1990) observed that only
the lowest reflecting bitumen population shows a consistent relationship with vitrinite
reflectance.
Optical Properties of Native Bitumens
J. Quick 16
BIT_ESS2.WPD DRAFT 19 November 1998
The relationship between vitrinite reflectance and native bitumen reflectance is of
significance where an estimate of thermal maturity is desired. Optical anisotropy of bitumens is
of less immediate practical significance but may relate to the origin and genesis of different
bitumens. The following discussion examines some published reports on these topics.
Jacob and Hiltman (1985) observed:
Rvil=0.618*Rbit. + 0.40.
whereas inspection of the data presented by Landis and Castano (1995) shows:
RV1I = 0.897* Rbit +0.415
where; Rvil = mean random reflectance of vitrinite, and,
Rbit = mean random reflectance of native bitumen.
These relationships are compared in Figure 9. Jacob and Fliltmans correlation shows that
native bitumen has a lower reflectance than associated vitrinite below 1% Rvit. and a higher
reflectance than associated vitrinite above 1.0% Rvit. Their equation was based on about 30 data
points distributed between 0.35 and 2.0%R(vit). Landis and Castano observed that the
reflectance of native bitumen is less than the reflectance of vitrinite below about 4.0 %Rvit and is
greater than vitrinite above 4.0 %Rvit. The reason for the significant difference between the two
correlation scales shown in Figure 9 is uncertain. Nonetheless, results presented by Bertrand
(1993) and Riediger (1993) suggest a possible explanation.
J. Quick 17
BIT ESS2.WPD DRAFT 19 November 1998
1=1
o O
O
<D
0
-
-
-
-
-
1 1
/ y /y 'y
1 — 1 — 1 — 1 — 1 —
Landis and Castano (1995)
lacob and Hiltman (1985)
/ y
y
— i — i — i — i —
y y
y
— i — , — , — T — , , | ,
0 1 2 3 4 5 Mean Reflectance of Bitumen
Figure 9. Comparison of the
relationship between mean random bitumen reflectance and mean random vitrinite reflectance
suggested by Jacob and Hiltman (1985) and Landis and Castano (1995).
Bertrand (1993) examined the relationship between bitumen reflectance and vitrinite
reflectance using about 600 samples from 4 geologic provinces in Canada. In those samples that
lack vitrinite an equivalent vitrinite reflectance was calculated from measured zooclast
reflectance. Bertrands results show small differences between the basins but significantly
different bitumen - vitrinite correlations for different host rock lithologies. Examination of their
equations and figures show the following general relationships for:
Limestone Rvit = 1.15 * Rbit +0.114
Shale Rvil = 0.858 * Rbl, + 0.452
and, Sandstone Rvit = 0.949 * Rbit + 0.315
J. Quick 18
BJTJESS2.WPD DRAFT 19 November 1998
where Rvit. = mean random reflectance of vitrinite, and Rbit = mean random reflectance of native
bitumen. These relationships are illustrated in Figure 10. Bertrand shows that bitumens
occurring in sandstones exhibit the greatest variation between different geologic provinces and
suggests that they are the least reliable indicators of maturity. However, bitumens occurring in
shales and especially limestones show more consistent relationships with vitrinite reflectance and
can be used to estimate thermal maturity.
J
'B 'G A "5 " > o ti
3 3
Pi B l
o -a
a 1 o
o-
/
/
— i — i — i — i —
p
/ / ^
/J/
/
7 ^
— — — limestone
1 ' ' • '
Shal
- Sand stone
• • • •
0 1 2 3 4 5
Mean Random Reflectance of Migrabitumen
Figure 10. Comparison of the relationship between mean vitrinite reflectance and bitumen
reflectance in limestones, shales and sandstones. Constructed from data presented by Bertrand
(1993).
Riediger (1993) measured the reflectance of homogeneous bitumen occurring in the
Lower Jurassic Nordegg member of the western Canadian basin. Samples from 22 wells
J. Quick 19
BIT_ESS2.WPD DRAFT 19 November 1998
distributed throughout the basin were examined. Equivalent vitrinite reflectance values were
extrapolated from know vitrinite reflectance gradients according to sample location and depth.
Examination of Reidiger's data shows that:
R _ i A (-0.1571) * R (0.2815) r v vi t . — lyJ ^ b i t .
where Rvit. = mean random reflectance of vitrinite, and Rbit = mean random reflectance of native
bitumen. Thus, native bitumen in the Nordegg member shows a lower reflectance than
associated vitrinite below 0.6% reflectance and a higher reflectance than associated vitrinite
above 0.6% reflectance. Figure 11 compares the relationship between bitumen reflectance and
vitrinite reflectance observed by Riediger with both Jacob and Hiltman's (1985) correlation and
Landis and Castano's (1995) correlation.
'c
>
o
o
-
.
-
-
-
-
/
— 1 — 1 — 1 — 1 —
1 1 I
Landis and Castano (1995)
Jacob and Hiltman (1985)
Riediger (1993)
•
— i — i — i — i —
s
/
*
•
/
/
/ /
0 1 2 3 4 5
Reflectance of Bitumen
Figure 11. Comparison of the relationship between mean random bitumen reflectance and mean
random vitrinite reflectance suggested by Riediger (1993), Jacob and Hiltman (1985), and Landis and Castano (1995).
J. Quick 20
BIT_ESS2.WPD DRAFT 19 November 1998
Riediger notes that the Nordeg member is rich in sulfur and sources high-sulfur, low API
oils. The distinctive relationship between bitumen and vitrinite reflectance in the Nordegg
member is probably related to the thermally labile type IIS kerogen which from which it is
derived. Hence the maturation path of bitumen reflectance appears to varies according to the
composition of the bitumen which is determined by the composition of the organic material from
which it is derived. This explanation is also consistent with the Betrtand's observation the
bitumen/vitrinite reflectance relationship varies according to the lithology of the host rock.
Discussion
It appears that a single, universally applicable, relationship between bitumen reflectance
and vitrinite reflectance does not exist. If differences between the various bitumen - vitrinite
reflectance scales are due to different types of bitumen, then any correlation between bitumen
and vitrinite reflectance must account for the type of bitumen. Ideally, bitumen type should be
established using petrographic criteria.
Data presented by Potter et al., (1993) further attests to the significance of the type of
bitumen where bitumen is used to establish thermal maturity. Figure 12 shows the increase of
bitumen reflectance with depth in samples taken from a single drillhole. The reflectance of four
types of bitumen, designated A, B, C, and D, are shown to increase with increasing depth of
burial. Potter and her co-workers note that the two lower reflecting types of bitumen (A and B)
are isotropic and that type A shows visible fluorescence. Types C and D were observed to be
J. Quick 21
BIT_ESS2.WPD DRAFT 19 November 1998
visibly anisotropic. Although these bitumen types were distinguished according to morphology,
their observations suggest that both optical anisotropy and fluorescence intensity could be used
to establish bitumen type. Importantly, both of these parameters can be objectively measured
and quantified.
1000
2000-
oo
. 4—>
Q
3000
4000-
• o A O
D n O
. • O • o
0 ° A A
A
O
• O A O • O A
• OA no • O A
o
A
O
bitumen types
•
o
A
O
A
B
C
D
DO • • • O A O • O A
• O A
DO A
0.5 1 % Reflectance
Figure 12. Variation of bitumen reflectance with depth for 4 different kinds of bitumen observed
in a single well. Data from Potter and others, 1993.
J. Quick 22
BIT_ESS2.WPD DRAFT 19 November 1998
Anisotropy of bitumens
The optical anisotropy of bitumen has special importance for bitumens occurring in
hydrothermal systems. Elevated optical anisotropy of bitumen in hydrothermal systems has been
attributed to the high heating rates associated with hydrothermal mineralization. This idea is
based on comparison of bitumen anisotropy with the anisotropy of vitrinite artificially matured at
different heating rates as well as evaluation of published reflectance data for various bitumen
occurrences (Goodarzi et al., 1993, Goodarzi, 1984). Figure 13 shows the relationship between
bireflectance (max - min reflectance in polarized light) and percent mean maximum reflectance
for both native bitumen, (normal burial heat flow), and heat-affected bitumen, (anomalously high
heat flow).
o
-*—> o
B | 4 -
6 3
S 2
native bitumens matured under a normal burial
gradient V
heat affected
bitumens
r T- ' 2
r T - ' 5 6
Bireflectance
Figure 13. Cross plot showing the reflectance - bireflectance relationship for natural bitumens
and heat affected bitumens. (Constructed from relationships presented by Goodarzi, 1984)
J. Quick 23
BIT ESS2.WPD DRAFT 19 November 1998
According to Khorsani and Michelsen, 1993 bitumens that occur in hydrothermal veins
shows enhanced bireflectance. They suggest that a cross-plot (figure 14) can be used to
distinguish bitumen matured under regional geothermal gradients (low anisotropy) and bitumen
developed in hydrothermal systems (high anisotropy). As with most generalization, exceptions
can occur (two of which are plotted on figure 14).
Normal Response of Bitumen,
7 \v <a 6 o c a
u
§4 P
E 3 c a
% 2 Native Bitumen in Hydrothermal Systems
Isotropic Bitumen Associated with
0 Hydrothermal Gold
0 1 2 3 4 5 6 Bireflectance
Figure 14. Cross plot showing high bireflectance for typical bitumens in hydrothermal systems
(Khorsani and Michelsen, 1993) annotated with some unusual data for bitumen in gold deposits.
J. Quick 24
BIT ESS2.WPD DRAFT 19 November 1998
In some instances, bitumen in hydrothermal systems shows remarkably low anisotropy
(figure 14). These uncommon (?) occurrences might be explained by the composition of the
bitumen. During pyrolysis of coal and petroleum pitch abundant sulfur and/or oxygen can inhibit
the development of optical anisotropy (Marsh 1989). These elements scavenge hydrogen that
would otherwise be used to stabilize free radicals formed during thermal bond breakage.
Without hydrogen, the radicals readily form cross-links which prevent the aromatic fragments
moving into their preferred, aligned orientation. The result is an optically isotropic, condensed
macromolecule where the aromatic units are randomly oriented. Although this mechanism is
well-cited in literature dealing with the manufacture of carbon materials, I have found no
publication that shows it also occurs in natural systems. Nonetheless, the lack of preferred
orientation of the aromatic units is suggested for an optically isotropic bitumen observed in some
hydrodrothermal gold deposits (figure 14). In one of these deposits, petrographic examination
has shown an unusual instance where dendritic gold appears to have nucleated on the bitumen
surface ( ).
References
Abraham, H., 1960, Asphalts and Allied Substances, sixth edition, volume 1, Van Nostrand,
Princeton, 370p.
Bell, G., and Hunt J. M., 1963, Native bitumens associated with oil shales: in, Organic