Organic matter pore characteristics of over-mature marine black shale: a comparison of organic fractions with different densities

Abstract

Organic matter pores are considered to be the most important type of pore for preserving hydrocarbon gases in shale gas reservoirs. The organic matter in each over-mature marine shale sample was separated into two organic fractions with densities of greater than and less than 1.25 g/cm³, and then their molecular compositions and pore characteristics were quantitatively evaluated using solid state ¹³C-nuclear magnetic resonance (NMR) and gas (N₂ and CO₂) adsorption analyses, respectively. The results revealed that aromatic carbon is the dominant molecular composition of the over-mature organic matter in the Lower Cambrian Niutitang shale. During the over-mature stage, the organic fractions with densities of greater than and less than 1.25 g/cm³ have no significant differences in molecular composition. The organic fractions with densities of greater than and less than 1.25 g/cm³ do have significant differences in their organic pore characteristics. In contrast to the high density organic fraction, the low density fraction contained abundant micropores and lacked mesopores and macropores. The organic pore structures of the different occurrence states of organic matter were significantly different. The C/O of organic matter in different occurrence states are obviously different, which proves that the organic pore structure is closely related to both the occurrence state and density of the organic matter. However, these relationships are still unclear and require further study.

Graphical abstract

Keywords

pore characteristics / organic matter / black shale / Early Cambrian / Yangtze Block

1 Introduction

As an unconventional natural gas reservoir, shale has an ultrafine pore size and low porosity and permeability compared with conventional sandstone reservoirs (Clarkson et al., 2012; Milliken et al., 2013; Hu et al., 2018; Feng et al., 2020; Zhang et al., 2020; Wang et al., 2022a). The ultrafine pore size of shale is not only determined by its fine grain size but also greatly affected by the organic matter it contains. Previous studies have demonstrated that the organic matter makes the largest contribution to the porosity (> 60%) of shale reservoirs, especially for organic-rich shale reservoirs, while the contributions of the minerals are usually small (Slatt and O’Brien, 2011; Milliken et al., 2013; Ji et al., 2019; Dong et al., 2021; Wang et al., 2021; Xia et al., 2021).

In addition to the thermal maturity, the type of organic matter is a significant controlling factor in the development of organic matter pores in shale. In general, pores are more developed in types I and II kerogen than in type III kerogen owing to the high hydrocarbon generation potentials of types I and II (Loucks et al., 2012; Liu et al., 2022). Using scanning electron microscopy and reflected-light microscopy, Liu et al. (2022) determined that secondary pores develop in the solid bitumin or pyrobitumin after hydrocarbon expulsion, while terrigenous organic matter, including vitrinite and inertinite, can host primary cellular pores but lack secondary pores. It is noteworthy that over-mature marine shale usually only contains type I kerogen, and its macerals are difficult to identify based on the fluorescence properties. For this type of shale, using scanning electron microscopy, Hong et al. (2020) qualitatively determined that migrated bitumen is much more porous than primary kerogen, and organic matter pores with circular or irregular shapes are well developed in bitumen but are rare in spherical organic matter and graptolites. However, the quantitative characteristics and differences in the pores in the different organic fractions in over-mature shale are still unknown. The main difficulty is the separation of the organic fractions in this type of shale because the fluorescence extractability and the solubility of the organic matter are weak during the over-mature stage.

Density is a significant physical property of organic matter, and it has been successfully used to separate the macerals in coal (Dyrkacz et al., 1984; Guo et al., 2013; Yan et al., 2019). However, the separation of the organic fractions in over-mature marine shale using density has been poorly studied. Pyrobitumen and kerogen residue are the main organic matter fractions in over-mature marine shale and belong to sapropelite. The density of the sapropel group is less than 1.25 g/cm³. However, some organic matter fractions are mixed and symbiotic with other minerals (such as organic clay complex, mutually wrapped organic matter, etc.) in shale, which leads to the density of organic matter fractions in this part being much higher than that of sapropelite (Xu et al., 2018; Zhang et al., 2019). It should be noted that the density of 1.25 g/cm³ was selected because the average density of the organic matter in the Niutitang black shale is about 1.25 g/cm³. Thus, the organic matter in the shale samples was separated using this density threshold to obtain the light and heavy fractions of the organic matter. Using scanning electron microscopy, recent studies have determined that the pore structures of pyrobitumen and kerogen residue are significantly different (Hong et al., 2020; Wang et al., 2020; Xie et al., 2021), but the quantitative pores characteristics of these fractions are still unknown.

In this study, two typical black shale samples from the Lower Cambrian Niutitang Formation were selected. The organic matter fractions in these samples were separated using a density threshold of 1.25 g/cm³. The molecular compositions and pore characteristics were quantitatively evaluated using solid state ¹³C-nuclear magnetic resonance (NMR) and gas (N₂ and CO₂) adsorption analysis, respectively. The goal of this study was to explore the relationships between the pore characteristics, occurrence states, and density of the organic matter in over-mature marine shale.

2 Materials and methods

2.1 Sample collection and preparation

Two fresh black shale samples were collected from the GZW section in north western Guizhou Province and the XZL section in northern Guizhou Province (Fig.1(a)). In both sections, the sampling location was near the Ni-Mo polymetallic layer, and the representative stratigraphic columns are shown in Fig.1(b). The mineral compositions, total organic carbon (TOC) content, and equivalent vitrinite reflectance (R_o) of these shales were measured at the China University of Petroleum, and the results are presented in Tab.1. The qualitative characteristics of their minerals, organic matter, and pores were observed using a scanning electron microscope, and several images are presented in Fig.2 and Fig.3.

2.2 Kerogen separation

We select black shale samples with high TOC for kerogen separation, and first the shale samples were crushed into powder with size of 200 mesh using a ball milling instrument. The kerogen in these shale samples were isolated following the Chinese national standard “Isolation method for kerogen from sedimentary rock (GB/T 19144–2010 (2010))”. About 10 g of kerogen were purified using chloroform for 72 h, and the kerogen was wrapped in tinfoil after being naturally dried in a fume hood. The detailed procedure of kerogen isolation is shown in Fig.4.

Flotation liquid with a density of 1.25 g/cm³ was prepared using the following method. 1) Iron metatungstate (i.e., the heavy liquid) was poured into a beaker, and 2) a mixture of ethyl alcohol and water with a volume ratio of 1:2 was added to the beaker to dilute the heavy liquid until its density was 1.25 g/cm³.

The kerogen powders and flotation liquid were mixed in a centrifuge tube with a volume ratio of 1:20, and then the mixture was centrifuged at 5000 r/min for 30 min. After this, the sample was let stand for 10 min, and the supernatant was filtered through a 0.22-μm nylon membrane to retrieve the kerogen fraction with a density of less than 1.25 g/cm³ (labeled as GZW-1 and XZL-1). The remainder of the sample was washed using distilled water and was filtered through a 0.22 μm nylon membrane to retrieve the kerogen fraction with a density of greater than 1.25 g/cm³ (labeled as GZW-2 and XZL-2).

2.3 Experimental methods

The kerogen fractions (GZW-1, XZL-1, GZW-2, and XZL-2) were subsequently subjected to solid-state ¹³C-NMR analysis at the Hefei Institutes of Physical Science, Chinese Academy of Sciences, and low-pressure gas (including N₂ and CO₂) adsorption measurements at the China University of Petroleum.

¹³C-NMR analysis technology is widely used in organic chemical analysis. It is an effective method to analyze the structure of organic molecules. With the development of ¹³C-NMR techniques such as high-power decoupling, cross-polarization, magic angle rotation and sideband suppression, the application of ¹³C-NMR spectrum in kerogen structure analysis is increasing (Gao et al., 2017; Liu et al., 2021; Wang et al., 2022b). Its advantages are mainly that the samples are not destroyed in the analysis and it can be used for purposes after the experiment. The chemical shift is wider, and the resolution is higher than that of hydrogen spectrum. Quantitative information on a variety of functional groups can be obtained. The solid-state ¹³C-NMR experiments were performed using a Bruker AVANCE III 600 spectrometer with a resonance frequency of 119.20 MHz. ¹³C magic-angle spinning (MAS) NMR spectra with high-power proton decoupling were recorded using a 4-mm probe with a spinning rate of 12 kHz, a π/2 pulse length of 3.60 μs, a 1H decoupling strength of 80 kHz, and a recycle delay of 2.0 s. The chemical shifts of the ¹³C were externally referenced to tetramethyl silane (TMS).The low-pressure gas (N₂ and CO₂) adsorption measurements were conducted using an ASAP 2460 automatic specific surface area and pore size apparatus. N₂ adsorption can be used to characterize the 1.70 to 300 nm pores (Chen et al., 2017), mainly including mesopores and macropores, whereas CO₂ adsorption is an effective method of micropore characterization (Okolo et al., 2015). Based on the gas adsorption measurements, the Brunauer-Emmett-Teller (BET) theory (Wang et al., 2015) was used to calculate the total specific surface area. The micropore volume and the micropore specific surface area were calculated using density function theory (DFT) (Klimakow et al., 2012; Han et al., 2016), and the meso-macropore volume and meso-macropore specific surface area were calculated using the Barrett-Joyner-Halenda (BJH) theory (Joyner et al., 1951).

3 Results and discussion

3.1 Compositions and lithofacies

According to the lithofacies division scheme of Ning et al. (2021), the mineral compositions of platform, slope and basin were collected for lithofacies analysis (Fig.5). As shown in Fig.5, argillaceous shale and siliceous shale are predominant in the inner shelf. A portion of siliceous shale and a small amount of mixed shale with carbonate shale belong to the slope region. Siliceous shale is predominant in the deep basin Clay minerals and quartz have a strong relationship with organic matter (Xia et al., 2020; Yuan et al., 2021). Considering these, the most representative siliceous and argillaceous shale from the inner shelf were selected in this study. The mineral compositions and TOC contents of the two shale samples are presented in Tab.1. Both samples are organic-rich shales with TOC contents of 8.78% (sample GZW) and 6.60% (sample XZL), and they are in the over-mature stage of thermal evolution. Shale sample GZW is rich in quartz, and shale sample XZL is rich in clay mineral. As a result, the GZW shale is siliceous shale, and sample XZL is argillaceous shale.

3.2 Structural characteristics of different organic fractions

The ¹³C-NMR spectra were fitted with Gaussian curves using the Origin 7.5 software. Fig.6(a) shows the raw curves and the compositions of the different types of carbon in the different organic fractions of shale sample XZL. In both the XZL-1 and XZL-2 fractions, the aromatic carbon region contains a peak, while both the aliphatic carbon and carbonyl carbon regions are flat (Fig.6(a)), indicating that aromatic carbon is the dominant type of carbon in both fractions.

As is shown in Fig.6(b), XZL-1 contains a large proportion of protonated aromatic carbon (96.82%), and the remaining 3.18% is bridgehead aromatic carbon. In contrast to XZL-1, the protonated aromatic carbon content of XZL-2 is higher (99.22%), and it contains 0.59% bridgehead aromatic carbon and 0.19% oxygen connected to quaternary carbon.

Previous research shows that 1) during the immature stage, kerogen is generated by bacterial action and condensation polymerization because the C-H bonds of aliphatic in kerogen release and aliphatic-intermolecular pores occur; 2) from low-mature to mature stage, asphaltenes, liquid hydrocarbon, and residual kerogen are generated from thermal-depolymerization, and the cracking of asphalt generate liquid hydrocarbon and marginal-oil pores (liquefied pores); 3) during the high to over-mature stage, hydrocarbon transforms into dry gas, and pores (including intermolecular pores in aromatic and gas pores) generated in organic matter because of the release of C-H bonds in kerogen aromatics. The isomerization and demethylation of aromatic compounds during this stage have a good coupling effect on the change of specific surface area of organic matter (Miao et al., 2017; Gao et al., 2018; Wang et al., 2019). Other studies have reported that the aromatic carbon content of organic matter increases and the aliphatic carbon content decreases during thermal evolution, and as a result, aromatic carbon is enriched in over-mature organic matter (Liu et al., 2017; Miao et al., 2017; Craddock et al., 2018; Xiao et al., 2021). Shale sample XZL is in the over-mature stage, with R_o value of 2.70%, and thus the results are consistent with those of previous studies. In addition, our experiments revealed two other phenomena. 1) There is a small difference in the molecular compositions of the organic fractions with different densities in the over-mature marine shale. 2) The organic fractions with different densities underwent different processes during the thermal evolution of the shale.

The higher proportion of aromatic carbon and lack of aliphatic carbon in XZL-1 compared with XZL-2 indicates that the low density organic matter fraction evolved faster than the high density fraction during the thermal evolution. Previous studies on the relationships between the chemical compositions, density, and thermal evolution of the macerals in coal revealed that the hydrocarbon generation potential is in the order of exinite > vitrinite > coal > fusinite during thermal evolution, while both the hydrogen content and density follow the sequence of fusinite > coal > vitrinite > exinite (Chang et al., 2008; Liu et al., 2008; Zhao et al., 2010). Our experimental results suggest that the organic fractions with different densities have different hydrocarbon generation potentials and underwent a different thermal evolution process, even though these fractions belong to a single maceral (sapropelite). The development of secondary pores is controlled by the hydrocarbon generation potential of the organic matter (Hong et al., 2020; Liu et al., 2022).

3.3 Pore characteristics of different organic fractions

The isotherm N₂ adsorption/desorption curves of the organic fractions are shown in Fig.7(a). Not all of the curves contain distinct inflection points in the low relative pressure section (P/P₀ < 0.8), and the curves increase sharply in the high relative pressure section, which is characteristic of a type III curve according to the International Union of Pure and Applied Chemistry (IUPAC).

The characteristics of the micropores were analyzed via CO₂ isothermal adsorption, and the adsorption curves of the organic fractions are presented in Fig.7(b). XZL-1 and GZW-1 have much higher CO₂ adsorption capacities than XZL-2 and GZW-2, indicating that the low density organic fraction has a higher CO₂ adsorption capacity than the high density organic fraction.

Based on the N₂ adsorption data, the pore volumes in the diameter range of 1.48–120 nm were calculated. The pore volumes of XZL-1, XZL-2, GZW-1, and GZW-2 are 0.0495 cm³/g, 0.2537 cm³/g, 0.0099 cm³/g, and 0.1180 cm³/g, respectively, indicating that the high density organic fractions make a larger contribution to the volumes of the mesopores and macropores than the low density organic fractions. The size distribution of the 1.48–120 nm diameter pores (Fig.7(c)) shows that all of the fractions have multiple peaks, such as at 25 nm, 35 nm, 50 nm, 70 nm, and 90 nm. It should be noted that XZL-2 and GZW-2 have larger pore volumes and surface areas than XZL-1 and GZW-1 in the entire range (Fig.7(c) and Fig.7(e)). The results imply that the organic fractions with different densities have significantly different pore characteristics, and the high density fraction contains abundant mesopores and macropores.

The pore volume and surface area in the diameter range of 0.36–1.11 nm was calculated from the CO₂ adsorption data. Both the volumes and surface areas of the low density fractions (XZL-1: volume of 0.0185 cm³/g and surface area of 64.44 m²/g; GZW-1: 0.0130 cm³/g and 44.886 m²/g) are much larger than those of the high density fractions (XZL-2: 0.0009 cm³/g and 2.87 m²/g; GZW-2: 0.0019 cm³/g and 13.36 m²/g) (Fig.7(d) and Fig.7(f)). This indicates that the micropores are more enriched in the low density fractions, which is opposite to the distribution of the mesopores and macropores based on the N₂ adsorption data.

Xie et al. (2020) discovered that low density organic fractions which have lots of telalginite and lamalginite macerals (1.06–1.23 g/mL) generated much more light and heavy hydrocarbons than the high density organic fractions which have a lot of detrovitrinite (1.26–1.30 g/mL) in oil shale. Low density organic fractions are characterized by greater amounts of aliphatic compounds, whereas high density organic fractions have larger amounts of aromatic compounds. It is the essence of the evolution of different types of organic matter pores that aromatic carbon rearrangement and aliphatic chains and carbon-heteroatom bond were broken in kerogen. Kerogen transforms from a disordered structure to a graphite crystal structure in the course of thermal evolution. The condensation reaction leads to the closing of organic matter pores. In recent years, through thermal maturity experiments, many scholars have found when thermal maturity is in the high to over-mature stage, kerogen and pyrobitumen had more micropores, and the porosity of organic matter were decreased. (Liu et al., 2013; Wang et al., 2016a; Tenger et al., 2021; Xu et al., 2021; Xu et al., 2022a; Xu et al., 2022b). Based on the above, the low density organic matter fractions (XZL-1 and GZW-1) have more micropores and the high density fractions (XZL-2 and GZW-2) have more mesopores and macropores, which shows the evolution degree of the low density fractions is higher than that of the high density fractions.

3.4 Implication in reservoir evaluation of over-mature marine shale

The above discussion demonstrates that the organic fractions with various densities in the over-mature marine shale have significantly different pore characteristics and molecular compositions. Thus, it is important to determine whether these organic fractions have markedly different occurrence states.

In this study, sample XZL is argillaceous shale with a clay mineral content of 59.20% (Tab.1). In this sample, most of the organic matter is combined with clay minerals to form organo-clay composites (Fig.2 and Fig.8(b)), and the remaining organic matter is distributed in the intergranular pores and fractures or is dispersed in the matrix. The weight ratio between XZL-2 and XZL-1 is about 1.68:1, indicating that about two-fifths of the organic matter in sample XZL has a density of less than 1.25 g/cm³.

Sample GZW is a siliceous shale with a quartz content of 58.60%, and most of the organic matter in this sample is distributed in the intergranular pores between the quartz particles (Fig.2 and Fig.8(b)). The proportion of the high density organic fraction is 81.75%, and only a small part of the organic matter has a density of less than 1.25 g/cm³.

Ye et al. (2009) and Fan et al. (2010) used the separation of organic matter components and fluorescence analysis to discover the relationship between organic matter density and organic matter. It is difficult to use fluorescence analysis to distinguish organic matter in the over-mature marine shale. Scanning electron microscopy is the most effective way to identify organic matter; recent studies have found that the organic pore structures of the different occurrence states of organic matter are significantly different. The main differences are 1) pyrobitumen is much more porous than solid kerogen (Hong et al., 2020; Wang et al., 2020; Zhang et al., 2020); 2) amorphous pyrobitumen is rich in mesopores, while spherical pyrobitumen is mainly rich in macropores (Zhang et al., 2020; Xie et al., 2021); 3) in over-mature marine shale, organic matter is weak in fluorescence and solubility, making it difficult to accurately distinguish the types of microscopic components. So Zhang et al. (2017, 2019) divided the organic matter in the Niutitang over-mature marine shale into four types according to their occurrence states (they are banded, massive, filled, and mutually wrapped organic matter); and 4) organic matter pores are better preserved in siliceous shale than in argillaceous shale because siliceous shale has a rigid skeleton composed of quartz particles (Wang et al., 2016b; Dong et al., 2021; Gao et al., 2021; Xie et al., 2021). These phenomena can also be observed in Fig.2. Although the experiments conducted in this study cannot directly reflect the relationship between the occurrence state and density of organic matter, it is obvious that the organic pore structure is closely related to both the occurrence state and the density of the organic matter.

The scanning electron microscope images acquired in this study show that the banded and filled organic matter do not contain pores, while the massive and mutually wrapped organic matter is rich in pores (Fig.2). From the qualitative evaluation of scanning electron microscope images, it can be seen that the mutually wrapped organic matter develops mesopores and macropores (Fig.2(b) and Fig.2(c)). The massive organic matter usually develops micropores and mesopores (Fig.2(d) and Fig.2(e)). We also found that different types of pores developed in different organic matter components in the Niutitang samples from other regions (Fig.3). The development of organic matter pores are consistent with Fig.2. According to EDS analysis (Fig.9), the C/O of organic matter in different occurrence states are obviously different, which proves that the types of organic matter are different and density of the organic fraction are different. The organic matter pores developed by different occurrence states of organic matter are of different sizes, which is almost consistent with the pore relationship of organic components with different density, but we need more evidence to prove it.

In contrast to shale sample GZW, which is rich in quartz, shale sample XZL has a much higher clay mineral content (Tab.1). In shale sample XZL, most of the organic matter is mutually wrapped with clay minerals, while most of the organic matter is massive in shale sample GZW and is surrounded by quartz, indicating that the main occurrence states of the organic matter in these shales are different. In addition, shale sample GZW has a much higher proportion of low density organic matter than shale sample XZL (Fig.8(a)). Does this imply that the densities of the organic matter in the intergranular pores and the organic matter combined with the clay minerals are different? This is an interesting phenomenon, and it will provide new methods for reservoir evaluation of over-mature marine shale if the real relationships between the density, occurrence, and pore structure can be determined.

4 Conclusions

1) Aromatic carbon is the dominant molecular composition of the over-mature organic matter in the Lower Cambrian Niutitang shale. During the over-mature stage, the organic fractions with densities of greater than and less than 1.25 g/cm³ do not have significantly different molecular compositions.

2) The organic fractions with densities of greater than and less than 1.25 g/cm³ have significantly different organic matter pore characteristics. In contrast to the high density organic fraction, the low density fraction contains abundant micropores and lacks mesopores and macropores.

3) The organic pore structures of the organic matter with different occurrence states are significantly different. The organic pore structure is closely related to both the occurrence state and density of the organic matter. However, this relationship is still unclear and requires further study.

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