Influences of TOC on Pore Structure of Shale from Montney
Formation, West-Central Alberta, Canada
Tianyu Xia
1
, Zhaohui Xia
2
, Penghui Su
2*
,Liangchao Qu
2
,Wei Ding
2
and Yunpeng Hu
2
1
The Middle School Affiliated to Beijing Jiaotong University, Beijing 100044, China;
2
PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China.
Key words: Tight gas, TOC, pore structure, thermal evolution
Abstract: Shale gas has become one of the key points for the development of unconventional oil and gas. To better
understand the influence factors of TOC in shale from Montney formation, total organic carbon (TOC),
Rock-Eval analysis and saturation measurements were conducted to quantitatively analyze the influence
factors of TOC. The results show that the kerogen type in the study block is gas-prone. Porosity and
permeability have good correlations with TOC for high-maturity samples, demonstrating that organic pores
are well developed in studied shale. The relationship between TOC and T
max
is not obvious in low maturity
samples, whereas the relationship between TOC and T
max
show a negative correlation for the samples with
high maturity. Organic matter begins to transform into hydrocarbon before T
max
reaches a certain level. Gas
saturation increases with the rise of TOC. Bulk density decreases with the increase of TOC. In general, TOC
has a great influence on the pore structure of shale and the hydrocarbon-generating transformation of
kerogen and could well characterize the gas content and thermal evolution of the reservoir.
1 INTRODUCTION
Shale gas is one of the most important types of
unconventional oil and gas resources (Shao et al.
2017; Wei X et al. 2018; Sayed et al. 2017). Shale
gas usually refers to natural gas with low economic
value in low-permeability shale reservoir that have
no natural capacity and can be produced through
large-scale fracturing or special process technologies
(Holditch, 2006; Ji et al. 2016). Shale reservoirs are
mostly distributed in the center of the basin or deep
in the basin structure, showing a large area of
continuous distribution. Permeability, formation
pressure, water saturation and porosity are the most
important evaluation parameters for shale gas (Wang
et al. 2012; Chukwuma et al. 2018; Zou et al. 2012).
In order to form a shale gas sweet spot, the
effective source rock thickness should be greater
than a certain value. TOC is also an important
parameter for the evaluation of shale and the
selection of sweet spots. The development of shale
pores is closely related to the organic matter (Curtis
et al. 2010). Organic matter forms nanoscale pores
in the process of maturation and evolution (Slatt and
O'Brien, 2011; Loucks et al. 2012). The higher the
content of TOC, the larger the specific surface area
of the core (Wang et al. 2015), indicating that
organic carbon is an important factor in controlling
the pore development of the reservoir. The effect of
TOC on the pore structure in shale has been not
fully studied (Milliken et al. 2013; Mastalerz et al.
2013).
The major goals of this article are to understand
the physical properties, thermal evolution, fluid
composition and bulk density of the shale through
various experiments. The relationships between
TOC and pore structures were discussed to
understand the effect of TOC on pore structure. The
relationships between TOC and T
max
, gas saturation
and bulk density were established to explore the
influence factors of TOC. This study are of great
importance for deepening the understanding of TOC
in shale gas reservoirs.
Xia, T., Xia, Z., Su, P., Qu, L., Ding, W. and Hu, Y.
Influences of TOC on Pore Structure of Shale from Montney Formation, West-Central Alberta, Canada.
In Proceedings of the International Workshop on Environment and Geoscience (IWEG 2018), pages 541-546
ISBN: 978-989-758-342-1
Copyright © 2018 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
541

Table 1: Pore structure and Geochemical parameters of studied shale samples.
Samples
De
p
th Porosit
y
Permeabilit
y
Bulk densit
y
TOC Tmax
Production
Index
Gas Saturation
m % mD
g
/cc % °C %
1 2139.10 11.32 0.001770 2.62 1.07 370 0.32 0.71
2 2142.00 9.13 0.001060 2.60 1.55 317 0.62 1.54
3 2147.10 6.80 0.000066 2.55 3.25 342 0.58 1.59
4 2148.60 5.67 0.000018 2.60 4.43 351 0.73 2.02
5 2151.00 3.30 0.000012 2.53 3.89 491 0.48 2.14
6 2152.00 2.77 0.000006 2.55 3.57 489 0.42 1.35
7 2154.00 3.21 0.000009 2.56 3.52 490 0.33 1.77
8 2155.80 2.44 0.000008 2.54 3.80 496 0.32 1.48
9 2157.10 3.20 0.000008 2.53 3.03 486 0.49 2.21
10 2157.80 5.09 0.000011 2.52 3.04 319 0.64 3.40
11 2161.80 3.28 0.000027 2.51 5.87 496 0.41 1.99
12 2162.70 2.17 0.000012 2.58 3.18 516 0.61 1.11
13 2165.40 1.45 0.000006 2.64 1.70 506 0.58 0.55
14 2166.60 2.34 0.000007 2.56 5.07 502 0.47 1.03
15 2167.20 3.02 0.000012 2.43 4.35 529 0.65 1.74
16 2168.80 2.45 0.000010 2.71 0.67 370 0.84 0.52
17 2172.10 1.18 0.000001 2.76 0.50 326 0.92 0.39
18 2176.50 2.14 0.000036 2.74 0.90 427 0.66 0.52
19 2182.80 1.81 0.000003 2.75 1.00 571 0.77 0.25
20 2186.60 1.41 0.000006 2.73 1.55 311 0.89 0.26
2 SAMPLES AND METHODS
The Montney Formation is a late Triassic mainly
fine-grained unit, deposited in marine environments,
with a large-scale distribution of the reservoir
bodies. The unit is up to 280 meters thick and the
facies are shaley in the north and west areas, silty in
the center and become coarser (sandy) in western
Alberta. The brittle quartzic content is about 40%,
making it is easy to improve the physical properties
of the reservoir through fracturing.
Routine experiments were performed on 20
samples from Montney formation, west-central
Alberta, Canada. Retort saturation measurements
were performed on core samples. Porosity, grain
density and fluid saturation were calculated from a
representative portion of the crushed sample and the
whole sample was used for bulk density
determination. Rock Eval analysis is a standardized
method for describing the thermal maturities, types
and hydrocarbon generation potential of organic
matter in sedimentary rocks. Saturation
measurements with retort method were conducted
on samples. The parameters obtained in this study
are listed in Table 1.
3 RESULTS AND DISCUSSION
3.1 Effect of TOC on Pore Structure
Parameters
Figure 1: Relationship between porosity and permeability.
The permeability ranges from 0.000001 mD to
0.001770mD, with an average value of 0.000152
mD. The porosity ranges from 1.2 % to 11.7 %, with
IWEG 2018 - International Workshop on Environment and Geoscience
542

an average value of 4.3%. This shale gas reservoir
can be characterized as low porosity, low
permeability and shows strong heterogeneity. The
porosity shows a good exponential relation with the
permeability (Figure 1), with the correlation
coefficient of 0.7842.
The relationship between TOC and organic pores
has been widely studied (Curtis et al. 2012; Chen
and Xiao, 2014), and the effect of TOC on porosity
was not fully studied. The number of organic pores
generally increases with the rise of TOC, whereas
the relationship between TOC and porosity varies in
different sedimentary reservoir (Milliken et al. 2013;
Mathia et al. 2016). Lai found that the pore space is
mainly composed of intergranular pores,
intragranular pores and microfractures in shale from
Kelasu thrust belt (Lai and Wang, 2015). Organic
nanopores and microfractures formed by the
migration of fluid in kerogen are related to the
development of TOC. Figure 2 depicts the
relationship between TOC and porosity. For the
samples with low maturity, the relationship between
TOC and porosity is not obvious, whereas the
relationship between TOC and porosity have a good
positive correlation for the samples with high
maturity. The result demonstrates that the difference
exists in pore types between low-maturity and high-
maturity samples. Organic pores or microfractures
formed by the migration of fluid in kerogen
developed in high-maturity samples, whereas
organic pores are not well developed in low-
maturity samples. Permeability has the similar law
as porosity, as shown in Figure 3. In general, pore
types and their distribution determine the
relationships between TOC and pore structure
parameters.
3.2 Relation between TOC and
Maturity
Maturity of organic matter not only reflects the
conversion of organic matter into oil and gas, but
also characterizes the development of pores in the
reservoir (Chen and Xiao, 2014). The relationship
between maturity of organic matter and pore
structure is complex because thermal evolution will
not only cause changes in the porosity of organic
matter, but also cause the conversion of clay
minerals (Chen and Xiao, 2014).The relationship
between TOC and organic pores and the relationship
between the maturities of thermal evolution have
been widely discussed. However, the relationship
between the maturity of thermal evolution and TOC
was rarely studied. T
max
is an important parameter to
evaluate the maturity of organic matter. Organic
matter begins to transform into hydrocarbon before
T
max
reaches a certain level. The kerogen of samples
in this study is in a high-mature stage and achieves
high level conversion when T
max
is greater than
450℃ (Figure 4). Figure 4 also shows that the
kerogen type in the study block is gas-prone.
Figure 2: Relationship between TOC and porosity.
Figure 3: Relationship between TOC and permeability.
Influences of TOC on Pore Structure of Shale from Montney Formation, West-Central Alberta, Canada
543

Figure 4: The determination of kerogen type based on the
correlation between T
max
and PI.
Figure 5: Relationship between TOC and T
max.
Figure 5 shows that for the samples with low
maturity, the relationship between TOC and T
max
is
not obvious, whereas the relationship between TOC
and T
max
show a weak negative correlation for the
samples with high maturity. Along with the
enhancement of thermal evolution, organic matter
generates hydrocarbons and forms pores, thus the
amount of organic matter decreases, which is the
reason for the decrease of TOC with the rise of T
max
in mature stage. The hydrocarbon generation of
kerogen is related to various factors, such as mineral
composition, kerogen type and compaction (Curtis
et al. 2012). In the immature stage, organic matter is
not converted to hydrocarbon. The abundance of
organic matter is mainly related to the depositional
conditions of the reservoir in the low maturity stage,
which is the reason for the weak correlation between
TOC and T
max
. Therefore, no obvious correlation
exists between TOC and T
max
in immature stage.
3.3 Relation between TOC and Gas
Saturation
The above discussion shows that organic matter
begins to transform into hydrocarbon before T
max
reaches a certain level. In this study, the T
max
of
some samples is less than 400, and is in the
immature stage. Figure 6 shows that all samples
have gas saturation. The gas in immature samples
may be transported from the mature samples, which
already have more organic matter transformation.
Perhaps because the kerogen type in the study block
is gas-prone and begins to transport into gas at lower
degrees of thermal evolution, which could be
demonstrated by Figure 6. In general, for high-
maturity samples, the gas saturation increases with
the rise of TOC and the enhancement of thermal
evolution. The obvious correlation between TOC
and gas saturation was not depicted in Figure 6 for
low-maturity samples. This further confirms that the
gas in the low-maturity samples was migrated from
the nearby high-maturity samples.
Figure 6: Relationship between TOC and gas saturation.
3.4 Relation between TOC and Density
Milliken found that when the TOC is less than 5.6%,
the porosity shows an increase with increasing TOC
content, while there is a certain negative correlation
between porosity and TOC when the TOC is greater
than 5.6% (Milliken et al. 2013). The inhibitory
effect of high TOC content on organic pores may be
due to mechanical compaction or microcomponents
of non-hydrocarbon generation contained in organic
matter. The high TOC reservoirs have stronger
ductile characteristics and are more susceptible to
compaction under formation conditions. Figure 7
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544

shows that bulk density decreases with the rise of
TOC, indicating that the higher the organic matter
content, the smaller the density of the reservoir.
Higher organic matter content means greater effect
of the compaction on the reservoir, but high organic
matter generally predicts a decrease in density.
Figure 7: Relationship between TOC and bulk density.
4 CONCLUSIONS
In this paper, 20 samples in shale from Montney
formation, west-central Alberta, Canada were
subjected to various experiments to understand the
effect of TOC on pore structure and the influence
factors of TOC. The following conclusions have
been obtained.
• The reservoir in this study is characterized by
low porosity, low permeability and shows
strong heterogeneity. For high-maturity
samples, TOC has a good positive correlation
with porosity and permeability.
• The TOC is affected by thermal evolution,
organic hydrocarbon generation and mineral
composition. When T
max
is greater than a
certain level, thermal evolution will have an
impact on TOC.
• The gas in the low-maturity samples was
migrated from the nearby high-maturity
samples. The gas saturation increases with the
rise of TOC and the enhancement of thermal
evolution for high-maturity samples.
• Bulk density decreases with the rise of TOC,
indicating that the higher the organic matter
content, the smaller the density of the
reservoir.
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