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Pore characteristics of different organic matter in black shale: A case study of the Wufeng-Longmaxi Formation in the Southeast Sichuan Basin, China
Marine and Petroleum Geology 111 (2020) 33–43
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Research paper
Pore characteristics of different organic matter in black shale: A case study of the Wufeng-Longmaxi Formation in the Southeast Sichuan Basin, China
T
Wentao Zhanga,b,c,∗, Wenxuan Hua, Tenger Borjiginb,c, Feng Zhua a
Nanjing University, Nanjing, Jiangsu 210000, China Wuxi Research Institute of Petroleum Geology, Sinopec Petroleum Exploration & Production Research Institute, Wuxi 214000, China c State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Wuxi 214000, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Gas shale Organic pore Broad ion beam-scanning electron microscopy Wufeng-Longmaxi Formation Sichuan Basin
Organic matter (OM) pores are recognized to be the most important pore type in gas shales in the Ordovician Wufeng-Silurian Longmaxi Formation in the Sichuan Basin, China. However, the effects of the type and origin of organic matter on pore development in OM remain controversial. In this study, detailed scanning electron microscope (SEM) analysis of siliceous shales was conducted for core samples from the Wufeng-Longmaxi Formation, taken from five shale gas wells in the Southeast Sichuan Basin. In addition to bitumen, several types of morphologically distinct organic macerals were identified. Due to the high maturity (vitrinite reflectance (Ro) > 2.2%) and compaction of the sample, confirming the origin of some OM types was difficult. However, each OM type is characterized by distinct pore features: (1) the pores in bitumen are normally well developed, with circular or irregular shapes; (2) spherical kerogen contains uneven pores, with irregular and angular pores at the periphery and few pores in the core; (3) algal fragments may have angular pores, the distribution and structure of which are influenced by the initial structure of the organic matter; (4) bacteria-like aggregates represent the accumulation of numerous microbial granules, and contain residual pores; (5) graptolites are relatively tight and usually have few or no pores. The findings indicate that bitumen and algal fragments are the most favourable hydrocarbon-generating organisms for OM pore development. Notably, organic matter originating from different biological sources can influence the extent, shape, and structure of the pores. Some organic pores are recognized as remained pores of sedimentary stage, which may be previously regarded as pores formed during the gas generation period.
1. Introduction Many studies have been performed on the classification of nanoscale pores in gas shales in recent years. The pores are normally subdivided into interparticle pores, intraparticle pores, organic matter pores, and micro-fractures (Loucks et al., 2012; Yu, 2013; Mathia et al., 2016; Nie et al., 2017). It is widely believed that organic pores are the most significant pore type in gas shale (Loucks et al., 2009, 2012; Curtis et al., 2012; Milliken et al., 2013). The porosity of gas shale is often positively correlated with the total organic matter content (TOC), confirming the significant contribution of OM pores to the overall porosity (Milliken et al., 2013; Loucks et al., 2012). It has been indicated that organic pores may account for up to 60% of the total pore volume in some shale reservoirs (Tian et al., 2013). Additionally, OM pores often have good connectivity and tend to absorb gas rather than water, making them more important for shale gas exploration (Nie and
∗
Jin, 2016). However, the existing classifications do not account for more detailed division of organic pores. In fact, organic pores in organic matter such as bitumen and kerogens are highly heterogeneous. Pores develop differently in organic particles and even in different parts of a single type of organic matter (Curtis et al., 2012; Milliken et al., 2013; Löhr et al., 2015). Using the point counting method based on the scanning electron microscopy (SEM) image to calculate the porosity, Loucks et al. (2009, 2012) found that the porosity of different organic particles ranges from 0% to 40%. The occurrence of pores in organic matter is not only related to the thermal maturity, but is also influenced by the type of organic matter. Some studies indicate that the pores are well developed in Type II kerogens, but are not developed in Type III kerogens (Loucks et al., 2012). Loucks and Reed (2014) proposed several criteria for distinguishing migrated organic matter from in-situ organic matter. Löhr et al. (2015) studied the characteristics of organic
Corresponding author. Nanjing University, Nanjing, Jiangsu 210000, China. E-mail address: [email protected] (W. Zhang).
https://doi.org/10.1016/j.marpetgeo.2019.08.010 Received 1 April 2019; Received in revised form 29 July 2019; Accepted 6 August 2019 Available online 08 August 2019 0264-8172/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. (a) Location of analyzed samples. (b) Vertical profiles of the study formations.
calculated by model analysis, and the results indicate that organic pores account for about 31–62% of the total pores due to the variances in the total organic matter content (Tian et al., 2013). Previous SEM studies revealed the existence of strong heterogeneity during pore development in different organic macerals in this area, but the relationship between organic macerals and pore evolution is still not clear. On the basis of previous studies, the Wufeng-Longmaxi shales in the Southern Sichuan Basin are investigated herein through SEM analysis by monitoring the variations in the organic pore structure. Several types of organic matter are identified on the basis of the morphology, size, and relation with minerals, and the characteristics of the pores for each type of organic matter are studied to deepen our understanding of the effect of organic matter types on pore development.
pores in samples of different maturities and revealed that the pores differ greatly in large and dispersed amorphous organic matter, and pores are also present between organic particles in some colonial algae and spherical organic aggregates, but are not found in woody organic matter and Tasmanian algae. Nie et al. (2018) studied non-polished samples of Wufeng-Longmaxi shale in China by scanning electron microscopy, and highlighted the pore characteristics of three kinds of organic particles, i.e. algae, bitumen, and fossil fragments. Nie et al. (2017) reported that algae had a structured pore network compared to bitumen. Mathia et al. (2016) described five types of organic pores: bubble-like pores, sponge-like pores, complex pores, “pendular” pores, and terrestrial maceral pores. This kind of classification mainly reflects the shape and size of the pores, but is not related to the type of organic matter. Yan et al. (2015) concluded that organic pores can be roughly divided into three types: organic pores in pyrolysis bitumen, linear organic pores along the mineral boundary, and kerogen residual pores, based on analyses of the Longmaxi Formation shale in the Sichuan Basin. However, the method used to evaluate bitumen and kerogen was not described in the study. It is generally believed that the formation of micro- and nano-pores in organic matter is related to the pyrolysis of organic matter (Loucks et al., 2012; Curtis et al., 2012; Bernard et al., 2012; Mathia et al., 2016). In immature black shale, the interparticle pores between inorganic minerals are predominant, while there are few micro-pores in organic matter. At the mature stage, especially after entering the gas generation window, pyrolysis enables partial conversion of organic matter into oil and gas, thus forming a large number of micro-pores in the organic matter. In fact, the original pores and structure of organic matter can affect the evolution of the organic pores (Löhr et al., 2015). Black shale from the top of the Ordovician Wufeng Formation to the Silurian Longmaxi Formation (Wufeng-Longmaxi Formation) in the Sichuan Basin of China has been proven to have great potential for shale gas exploration. The Fuling shale gas field targeting this interval has additional proven reserves of shale gas of 1607.5 × 108 m3 (Jin et al., 2016). Many studies have been conducted on the pores in Wufeng-Longmaxi gas shale with focus on the pore type, structure, and controlling factors (Zou et al., 2010; Jiao et al., 2014; Shan et al., 2015; Cao et al., 2015; Yan et al., 2015; Hu et al., 2017; Zhao et al., 2017; He et al., 2017). The proportions of organic and inorganic pores were
2. Geological setting From the Late Ordovician to Early Silurian, with the force of the Caledonian movement, several uplifts including the Chuanzhong uplift, Qianzhong uplift, and Jiangnan Xuefeng uplift, were formed in the Upper Yangtze Platform (Guo et al., 2014; Huang et al., 2018). This caused a rise in the sea-level, and thus relatively low-energy and anoxic conditions dominated in the southeast Sichuan Basin, leading to the deposition of thick, organic-rich shale in this area (Chen et al., 2004). With continual sedimentation from the Permian to late Cretaceous period, the black shale was buried as deep as 5 km underground and entered the gas generation stage. Thereafter, it was uplifted and endured denudation, reaching a current burial depth of 2–3 km (Guo and Liu, 2013). The Wufeng-Longmaxi black shale in the Southeast Sichuan Basin, with a total thickness of 80–110 m (Fig. 1), is the most promising target bed for shale gas development in South China (Liang et al., 2008). The Wufeng Formation in the bottom is only about 5–10 m thick and is mainly composed of black shale, with a high carbonaceous and siliceous content. In the top of the Wufeng Formation, there is a shell limestone layer less than 1 m thick in the study area. The Longmaxi Formation is in comfortable contact with the underlying stratum. The lower part of the Longmaxi Formation mainly comprises black shale, which is similar to the shale in the underlying layer. In the upper layers, the carbonaceous and siliceous content is lower and the content of clay mineral is 34
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Table 1 Total organic carbon (TOC) content, vitrinite reflectance (Ro), and mineral content of samples. Well
JY1
JY2 JY41-5
YZ1
a b
Sample
J1-1 J1-2 J1-3 J1-4 J1-5 JY2-23 JY2-25 41-5-1 41-5-2 41-5-3 41-5-4 41-5-5 YZ1-1 YZ1-7 YZ1-10 YZ1-12 YZ1-13 YZ1-15 YZ1-16 YZ1-17
Depth (M)
2359 2364 2383.93 2404 2413 2561 2566 2525.13 2555.19 2563.09 2607.17 2618.15 4459.1 4480 4499.35 4506.07 4508.05 4513.15 4515.15 4516.04
TOC%
3.04 3.12 3.58 4.22 3.56 3.23 3.44 0.38 1.98 1.36 3.92 4.71 1.14 2.35 2.41 7.22 8.07 5.44 5.01 4.63
Ro %
2.2–3.06
Mineral composition%
a
Quartz
Clay
Feldspar
Carbonate
Others
34 34 38 49 57
43 41 40 34 25
12 11 8 8 7
9 11 11 6 6
2 3 3 3 5
29 45 38 47 35 33 50 33 67 69 79 67 38
63 42 41 30 48 58 33 41 22 22 17 25 50
6 8 8 7 8 6 8 13 6 5 1 4 8
0 3 10 7 2 0 3 5 2 1 2 2 1
2 2 3 9 7 3 6 8 3 3 1 2 3
2.46–2.7b
3.14
Guo et al. (2014). Nie et al. (2018).
3. Samples and methods
include radiolarians, sponges, foraminifera, and brachiopods. Siliceous mineral and clay are the main minerals in the samples according to Xray diffraction analysis (Table 1). In addition to some clastic sedimentary grains, the silica in shale also originates from radiolarian-related biological silica (Lu et al., 2016).
3.1. Samples
3.2. Method
The core samples of black shale studied herein were taken from the Wufeng-lower Longmaxi Formation of five shale gas wells (JY1, JY2, JY41-5, YZ1) in the eastern and southeastern parts of the Sichuan Basin (Fig. 1). Wufeng-Longmaxi shale has a relatively high organic carbon abundance, with a TOC greater than 2% in most samples (Table 1). Shale in the lower stratum has a higher TOC than that in the upper stratum, and the 20–30 m interval in the lower part has a TOC above 4%, being the major pay zone. The equivalent vitrinite reflectance (Ro) of the shale ranges from 2.2% to 3.14%, suggesting a stage of high maturity to over-maturity. The organic matter is mainly humic–sapropelic (Type I-II1) (Guo and Liu, 2013; Dai et al., 2014). Reflected light petrography analyses show that the main organic matter is dispersed amorphous organic matter (Fig. 2), which could be solid bitumen or remnants of kerogens after degradation. The black shale contains abundant graptolites, which are richer in the lower strata. Other fossils
Sub-samples from the drill cores were cut using a low-speed microdiamond saw and then pre-polished with silicon-carbide paper. Thereafter, the samples were polished using a broad ion beam (BIB) polisher (Leica EM RES102 Ion Beam Milling System) to remove about 100 μm of rock from the pre-polished surface of each shale. The BIB polisher was operated at 6 kV, and the samples were treated for 4 h to produce a planar surface of about 1 cm2 that was suitable for pore investigation using SEM. A Helios 650 field-emission scanning electron microscope (FE-SEM) was used to observe the surfaces of the shale samples perpendicular to the bedding. No metal or carbon was sprayed on the surfaces of the samples; thus, the real surfaces could be observed, enabling evaluation of the nano-pores. In the secondary electron (SE) images, siliceous minerals display brighter colours due to charge effect; in contrast, pyrite appears darker because of its high electrical conductivity; organic matter and pores appear the darkest, while the edges of some
higher. The upper part of the Longmaxi Formation comprises gray siltstone, indicating that the sedimentary facies changed from the deepwater shelf to shallow water shelf (Guo and Zhang, 2014).
Fig. 2. Organic petrology of the samples. AOM: amorphous organic matter; Z: zooclasts. 35
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4.2. Types of kerogen and related pores
organic pores may appear bright because of the charge effect. The atomic number plays a dominant role in the appearance of the BSE images, where pyrite and other heavy minerals are the brightest, quartz is darker and organic matter and pores are the darkest.
4.2.1. Spherical kerogen organic matter Spherical kerogen is found in many samples, and is very unique in terms of the special features compared with other organic matter. (a) The shape is spherical (with circular cross-sections) (Fig. 4A and B), while some are hemispherical (Fig. 4C) or ellipsoidal (Fig. 4D) due to compaction and other effects, with diameters ranging from 2 μm to more than 10 μm. (b) They have clear edge outlines, generally with a two-layer structure; the organic matter in the core has smooth edges, the porous organic matter at the periphery is in close contact with the spherical organic matter in the core, and the organic matter at the periphery is bounded by the surrounding minerals. (c) Sporadic automorphic–semiautomorphic phosphate minerals are visible in the organic matter (bright white spots in organic matter, Fig. 4). (d) Some spherical organic matter is surrounded by minerals such as silica (Fig. 4G and H). With obvious differences between the core and the periphery, the micro-pores in the spherical kerogen organic matter are non-uniform. Moreover, the pore development is also distinct for different types of organic matter. Generally, there are three cases. First, small pores with diameters < 20 nm are observed in the core, while dense pores with diameters > 20 nm are developed at the periphery; there is gradual transition from the core to the periphery (Fig. 4A and B). Second, a small number of pores are locally developed in the core (which is relatively tight), while the pores are well developed at the periphery; a clear boundary exists between the core and the periphery (Fig. 4C–F). Third, the tight spherical organic matter is surrounded by minerals such as silica, which is thought to be caused by the early diagenetic metasomatism of peripheral organic matter (Fig. 4G and H). The pores are consistent in shape and size. The section of a single pore appears as an irregular polygon or deformed polygon, and the pore size is generally dozens of nm. The pores in the peripheral organic matter are well developed and evenly distributed, reflecting similar chemical properties, but may be significantly different from those in the core. The pore characteristics of the organic matter at the periphery are quite different from those in the core, thus enabling the recognition that the organic matter at the periphery is bitumen generated by kerogen in the core. After careful observation, we still believe that these two parts are the same constituents of original kerogen. If the organic matter at the periphery is bitumen formed later, then the surrounding minerals should be in extensive contact with the organic matter in the core. However, this phenomenon is not observed. On the contrary, mineral particles such as quartz only penetrate into the organic matter at the periphery, but seldom enter the organic matter in the core. Fig. 5B shows that both the organic matter in the centre and that at the periphery of the lower part of kerogen are deformed simultaneously by the extrusion of minerals, which indicates a greater possibility that the organic matter in the centre and at the periphery have been coexistent since the early stage. The biological origin of the spherical organic matter is still uncertain. Previous studies reveal that the spherical kerogen in this interval is algae cystocarp, acritarchs, and single-cell planktonic algae (Nie et al., 2018; Pang et al., 2018), but the size of the spherical organic matter in previous studies is tens to hundreds of μm, which is much larger than the spherical organic matter mentioned herein. A and B: Small pores are developed in the core of spherical organic matter, which is in contrast to the surrounding porous organic matter. The white bright spots in the organic matter are apatite mineral. C, D, E and F: Only a small number of pores are developed locally in the core, which is clearly bounded by the surrounding porous organic matter. The pores are irregular and angular. G and H: The pores are not developed in the core, and the organic matter is enclosed by siliceous minerals. A: sample JY2-25, B: YZ1-16, C: YZ1-15, D:JY2-23, E: JY2-23, F: JY2-25, G:YZ1-13, H: YZ1-12.
4. Results As mentioned above, at the highly mature to post-mature stage, most kerogens have endured strong compaction and physiochemical changes, and are mixed with absorbed residual oil. Hence, it is difficult to differentiate solid bitumen from the kerogen matrices. However, identification may be achieved on some of them on the basis of the shape, interior structure, and distribution. For instance, OM within zooclast cavities should be bitumen, while some units with special shapes could be kerogen matrices, although there may be residual absorbed oil. Typical bitumen and four kinds of kerogen were distinguished in this study. Evaluation of the pore structures could help us understand how the parent materials influence pore development.
4.1. Bitumen and related pores Bitumen is one of the main organic macerals in black shale. Compared with in-situ kerogen, it is classified as migrated organic matter. Loucks and Reed (2014) proposed several criteria for distinguishing bitumen from kerogen. After observation of Silurian highmaturity shale in the Sichuan Basin, we conclude that typical bitumen and kerogen can be distinguished on the basis of three aspects. First, bitumen has no specific shape and fills in the intergranular pores or cracks of minerals, so it takes the pore shape as its own shape, while kerogen mostly contacts minerals in granular form and may have a defined shape. Second, bitumen has a uniform internal texture, and its pores are uniform or random, while some kerogens appear heterogeneous with biological structures. Third, minerals around bitumen display cementation (for example, quartz particles show automorphic characteristics due to secondary enlargement), while kerogen usually makes direct contact with sedimentary particles (Fig. 3). Bitumen in Silurian black shale in the Sichuan Basin is usually distributed in the intergranular pores of granular minerals (Fig. 3A, B, E, and F), inter-clay pores (Fig. 3D), or intragranular pores (Fig. 3C). The pore shapes of bitumen are consistent with these pore types. The size of such organic matter is generally less than 20 μm, mostly within 10 μm (Fig. 3). Pores are widespread in bitumen. According to the characteristics, the pores can be divided into two types: (1) irregular pores (Fig. 3A, C, and E) and (2) bubble-like pores. The former are uniformly distributed throughout organic matter bitumen with high pore density; the pores are similar in size (generally less than 50 nm), but with irregular shapes. The latter are nearly circular or elliptical and pores with different sizes ranging from 10 nm to more than 100 nm coexist (Fig. 3B, D, and F). Some super-large pores with sizes of up to hundreds of nm or even 1 μm may be formed occasionally. Such super-large pores are proposed to be complex pores formed by the interconnection of multiple pores after enlargement (Mathia et al., 2016). Microscopically, the edges of the pores are zigzag like a coastline, forming a hairy-edged structure. The original outlines of multiple pores can be observed in some macro-pores (shown by the arrows in Fig. 3D). Bitumen is mainly distributed in intergranular pores (A, B, D–F) or intragranular pores (C) of minerals. Quartz minerals in contact with bitumen often have automorphic shapes (arrows in Fig. A). Pores in bitumen are of two different types:irregular pores (Fig. A, C, E) and bubble-like pores (Fig. B, D, F). A: sample JY1-4, B: sample JY1-3, C: sample YZ1-17, D: sample YZ1-10, E: sample JY1-5, F: sample JY1-4.
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Fig. 3. SEM images of bitumen-related pores.
the clay, and clay was later formed when the formation fluid flowing into the channels reacted with the rock during burial. In the majority of such types of organic matter, the pores are relatively well developed. The pores are irregular and angular, with a uniform size (10–50 nm) (Fig. 5B and D). They often correspond to high plane porosity (about 20%). In the high magnification images, the organic matter shows a fibrous structure; the pores occur as gaps between fibres and are therefore well interconnected. In addition, the pores are mainly located between fibrous clay minerals. The closer the pores are to the clay minerals, the less developed they become. This also shows that the chemical composition is different for different parts of the organic matter. Hydrocarbon generation is difficult in the region closer to the channels. A (sample YZ1-16) and C (sample YZ1-16): Organic matter is nodular and has no uniform shape, with sizes ranging from tens to hundreds of μm. The edge intersects with minerals. The quartz at the edge is mainly isomorphic. Fibrous illite clay is found in the organic matter. The distribution of clay has a certain structure. B and D: Magnification of A and C, where pores are well developed. They are irregular and angular, with a uniform size. Pores are relatively underdeveloped in the region near clay. Another type of algal fragment differs from the above-mentioned type in two respects. First, it is clean inside without clay. Second, it has
4.2.2. Algal fragments One type of algal fragment is characterized by an irregular and amorphous bulk shape, generally larger than 20 μm (up to 100 μm), and is often surrounded by minerals such as quartz. Clay minerals with a fibrous shape are distributed in the algal fragments (Fig. 5). Despite the interpenetration between the edge and surrounding minerals, this type of algal fragment is considered as a kerogen rather than migrated bitumen based on three considerations. First, if it is bitumen filling intergranular pores, filling would have occurred after entering the oil-generating window. In this diagenetic stage, the intergranular pores of tens to hundreds of μm are too large relative to the mineral particles of generally around 5 μm; thus, the minerals can hardly form supporting structures to prevent compaction of the pores. Secondly, secondary enlargement of quartz is rarely seen in mineral particles at the edge of organic matter, whereas secondary enlargement is a common diagenetic phenomenon for quartz at the edge of pores. Third, the pores in organic matter are consistent in shape and size–generally irregular angular pores, and no circular pores are observed; the distribution of clay minerals in organic matter shows certain structural characteristics (Fig. 4B and D). There is usually no clay inside the organic matter, thus the microscopic clay here should be formed after burial. It is reasonable to speculate that there were some channels or fractures at the location of
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Fig. 4. Pore characteristics in spherical kerogen organic matter.
and C, respectively.
a neat overall outline, which distinguishes it from bitumen. The shape of such organic matter is irregular and angular. Its size is mostly less than 20 μm. This type of algal fragment has a relatively neat outline, and some silica intrusion can be seen (Fig. 6E and F). Such organic matter has well-developed pores with high plane porosity, where the pores are irregular and angular with a uniform size and good connectivity. The organic matter has unique shapes and neat outlines (A, C, E, and F). Pores in OM are numerous with an irregular and angular shape and similar size (B and D). A: sample YZ1-16, C: sample YZ1-16, E: sample YZ1-16, F: sample 41-5-5. B and D are magnifications of the boxes in A
4.2.3. Bacteria-like aggregates This organic matter comprises small aggregates of quasi-spherical bacteria (Fig. 7). Pellets are easily recognized in Fig. 6B; however, in most situations, the pellets are merged together due to compaction, making it hard to determine the original morphology (Fig. 6a, c, d). These aggregates may have been regarded as whole kerogen or bitumen in previous studies, but the structure of the pellets can still be seen through detailed observation (top left corner of Fig. 6a). The diameter of the pellets ranges from 300 to 400 nm. Affected by compaction, some 38
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Fig. 5. Potential benthic algal fragments and pores.
pellets are flattened to form flat spheres (Fig. 6B). Fig. 6D shows that the surface of the pellets is not smooth, and there are many burrs. It is common for some intergranular pores to be filled with siliceous mineral during long geological periods (Fig. 7A, B and D). Bacteria-like particles have few pores, and all the observed pores are among pellets. The pore shape is irregular and angular with a pore size of 10–50 nm. These pores are significantly different from the bubblelike pores in bitumen. The residual intergranular pores are randomly distributed because they are mainly controlled by the influence of fluid activity and related to the connectivity of intergranular pores. The pores in Fig. 6A and C are more common in our samples, and are partial residual pores retained after the inter-compression of pelletoid aggregates under compaction. In this case, the original morphology of the pellets is unidentifiable. Sometimes, intergranular pores may be enlarged due to hydrocarbon generation (Fig. 6D). Those later-formed pores are preferentially distributed along the contact boundary of the bacteria-like particles. Intergranular pores among bacteria-like particles can often be compacted (A and C) or filled with siliceous minerals (B, D), and only a small amount of residual pores were retained. A: sample 41-5-3, B: sample YZ1-7, C: sample YZ1-1, D: sample JY2-23.
developed in graptolite, but are well developed in the organic matter between pyrite grains. D: Graptolite inside is pyritized. Pores are not developed in graptolite, but are well developed in the organic matter between pyrite grains. E, F: SEM image of graptolite. A: sample 41-5-1; B: sample 41-5-2; C, D: sample 41-5-4. E: sample 41-5-2; F: sample JY223. The existence of micro-pores in some graptolites was reported in a previous study (Tenger et al., 2017), but the pores are relatively undeveloped in most of the graptolites observed herein (Fig. 8). This situation is related to the chemical properties of graptolite. Graptolites preserved in rock are the group skeletons secreted by graptolites, and are thought to be composed of collagen (Crowther, 1981). They mainly consist of aromatic compounds with aliphatic groups. They have a lower hydrogen index (20–540 mg g−1) and a higher oxygen index (10–60 mg g−1), making them Type II to III organic matter (Bustin et al., 1989; Briggs et al., 1995). Therefore, the weak hydrocarbon generating capacity of graptolites is the main reason the underdeveloped pores. Another phenomenon is that the organic matter filling the associated pyrite is porous (Fig. 8C and D). This part of organic matter may be filled bitumen. 5. Discussion
4.2.4. Graptolite Large numbers of graptolites were observed in the target formation, and they are also considered as the markers of this formation. Environmental scanning electron microscopy and in-situ energy spectrum analysis show that they mainly exist in the form of flattened carbonaceous films that are essentially parallel to the beddings (Tenger et al., 2017). The section width is usually 2–3 μm and the films are often segmented and generally associated with clustered pyrite aggregates. Some pyrite aggregates are adjacent to the graptolites (Fig. 7A−C), and some are inside the graptolites. The spaces between the pyrite grains are also filled with organic matter. Previous studies and numerous fossil observations reveal that there is a close relationship between graptolite and pyrite; the graptolites can even be completely pyritized (Underwood and Bottrell, 1994). A-C: The banded graptolites coexist with pyrite. Pores are not
5.1. Pore development in bitumen According to the SEM observation, pores are usually well developed in bitumen. Obviously, such pores are formed in the gas-generating stage. In terms of the formation mechanism, it is generally believed that the pores are formed by the precipitation of gaseous hydrocarbons generated during the secondary cracking of residual oil (Bernard et al., 2012; Curtis et al., 2012; Loucks et al., 2012; Chen and Xiao, 2014; Mathia et al., 2016). For the pores in bitumen, preservation of in-situ hydrocarbon gas is an important controlling factor, in addition to maturity. The tiny intergranular pores and high capillary pressure of shale minerals not only partially block the migration of bitumen, trapping it in adjacent pores, but also impede the migration of hydrocarbon gas generated by residual organic matter and keep it in situ. If gas can move 39
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Fig. 6. Another type of potential algal fragment and pores.
developed pores, followed by spherical organic matter. Bacterial-like aggregates retain only part of the original pores, and graptolites and other animal fossils have only a few pores. As the chemical composition varies at different parts of kerogen, the organic pores formed later are somewhat heterogeneous. A typical example is the spherical organic matter shown in Fig. 4, where the pores are rarely or not developed in the core, but are well developed at the edge. Essentially, the chemical composition differs for the two parts. For example, the epidermal tissue outside the cells may contain more fat, making it easier to generate hydrocarbons. In addition, kerogen itself has a certain texture, which also affects the formation and distribution of pores. In the algal fragments shown in Fig. 5, there are some fibrous clays that form vein-like structures. Although there are no pores locally in the organic matter adjacent to clay, there are more pores in the region far from clay. In the magnified view, the organic matter appears fibrous as a whole, much like the image of paper under a scanning electron microscope. The pore morphology, affected by the original texture, is generally irregular and angular, and there are no circular bubble-like pores. The pore size is also uniform, and no coexistence of pores with greatly different sizes is observed in bitumen. Not all the pores in kerogen are formed by late gas generation from pyrolysis; some of the pores are pre-existing pores, mainly from bacteria-like colonies (Fig. 7). Löhr et al. (2015) also noticed in his study
freely in rocks, the porous structure in the organic matter will not be formed. Given the homogeneous character of bitumen (Vandenbroucke and Largeau, 2007), the pores in it should be uniform. The pore distribution in bitumen mentioned above basically conforms to this expectation (Fig. 3). However, the distribution is not absolutely uniform, but there should not be a clear boundary for pore development in the same bitumen particle. For example, the pores in Fig. 3D are not uniform in size and distribution, but are generally randomly dispersed in the range of the size of organic particles. Morphologically, the pores in bitumen constitute two types: (1) irregular pores with spongy uniform distribution, and (2) circular or elliptical pores. The former is often very densely distributed as a whole, with a relatively uniform size (generally tens of nm). The latter has sizes ranging from 10 nm to more than 100 nm; pores of different sizes often coexist. In some zones, pores are interconnected to form large complex pores (Fig. 3D). It is proposed that the mechanism of formation of these two types of pores is related to different hydrocarbon generation processes.
5.2. Pore development in kerogen The SEM images show that the extent of pore development is different for organic macerals. Algal fragments contain the most 40
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Fig. 7. Bacteria-like aggregate and pores.
morphological differences. Each type of organic matter has unique features that are distinct from those of the other types, and pore development in the organic matter is highly heterogeneous. Specifically, pores are universally and uniformly developed in bitumen and algal fragments; on the other hand, pores are partially developed in spherical organic matter and bacterial-like aggregates, but are not developed in graptolites. Pores in organic matter are mainly formed during gas generation from pyrolysis, and are also affected by the original composition and texture of the organic matter. The composition of bitumen is relatively uniform; thus, the pores in bitumen are uniform and have circular or angular shapes. The organic matter with a fibrous texture or lattice texture mainly contains angular pores, which may be related to the original texture of the organic matter. In the spherical organic matter, the pores in the core are significantly different from those at the edge, which may be caused by the difference in the original chemical composition. In addition to the pores of thermogenic origin, others arise from early construction, mainly due to microbial (e.g. bacteria) reworking of original organic matter, such as the pelletoid aggregates mentioned herein. Most of the pores are compacted during the late burial process, with only partial remnants. The types and sources of some organic matter are undistinguishable due to overly high maturity and compaction deformation. Nevertheless, the identification of several types of typical organic matter and the related pore distributions is meaningful for better understanding the formation, evolution, and preservation of shale gas. Graptolites contribute significantly to the TOC of rocks, but have limited capacity for hydrocarbon generation and contain no pores.
that, there are some initial pores remain. For a given sample in our study, the pores in bacteria-like colonies are most easily filled with quartz, while few minerals are found in bitumen pores. This also suggests that the pores in the spherical organic matter were formed earlier, unlike the pores in bitumen that are formed by late gas generation from pyrolysis. Moreover, pellets with sizes of 300–400 nm were observed in the enlarged view. These pellets are even smaller than diatoms, and are strongly thought to be bacteria based on their shapes (namely, organic fragments were reworked by bacteria in the early stage). Such pores are mainly angular, with sizes ranging from tens to hundreds of nm. In most cases, the earlier-formed pores are deformed or disappear due to compaction, except in local areas where clear outlines of intergranular pores can be observed. 5.3. Preservation of OM related pores The effects of compaction on shale pores were discussed in previous studies (Fishman et al., 2012; Loucks et al., 2012; Milliken et al., 2013; Chen et al., 2016). As mentioned above, most of the early pores disappear with increased maturity, and only a part of the pores are preserved. On one hand, this may be attributed to the compaction during burial (Fishman et al., 2012; Milliken et al., 2013). The increase in the plasticity of organic matter with rising temperature results in a further decrease in the number of intragranular pores. On the other hand, in the hydrocarbon generation stage, some bitumen remains in kerogen and thus blocks the pores (Löhr et al., 2015). However, there's no distinct evidence showing that the pores formed in mature stage get deformed or orientated. That's because pore formation postdates most compaction. Rigid quartz minerals formed stable framework, preventing the organic pores from deformation and collapse (Lu et al., 2016).
Acknowledgement This work is financially supported by the National Natural Science Foundation of China (41690133) and National Science and Technology Major Project (2017ZX05036002). We thank the research institute of Jianghan Oilfield Company of SINOPEC for providing samples and data access for our experiments.
6. Conclusions Based on SEM analysis of the Wufeng-Longmaxi Formation black gas shale in the Sichuan Basin, China, we systematically classified the organic matter into five types (including bitumen) according to the 41
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Fig. 8. SEM images of graptolites and pores.
References
Fishman, N.S., Hackley, P.C., Lowers, H.A., Hill, R.J., Egenhoff, S.O., Eberl, D.D., Blum, A.E., 2012. The nature of porosity in organic-rich mudstones of the upper Jurassic Kimmeridge clay formation, North sea, offshore united kindom. Int. J. Coal Geol. 103, 32–50. Guo, T., Liu, R., 2013. Implications from marine shale gas exploration breakthrough in complicated structural area at high thermal stage: taking Longmaxi Formation in well JY1 as an example. Nat. Gas Geosci. 24 (4), 643–651. Guo, T., Zhang, H., 2014. Formation and enrichment mode of Jiaoshiba shale gas field, Sichuan Basin. Pet. Explor. Dev. 41 (1), 28–36. Guo, X., Hu, D., Wen, Z., Liu, R., 2014. Major factors controlling the accumulation and high productivity in marine shale gas in the Lower Paleozoic of Sichuan Basin and its periphery: a case study of the Wufeng-Longmaxi Formation of Jiaoshiba area. Chin. Geol. 41 (3), 893–901. Huang, H., He, D., Li, Y., Li, J., Zhang, L., 2018. Silurian tectonic-sedimentary setting and basin evolution in the Sichuan area, southwest China: Implications for palaeogeographic reconstructions. Mar. Pet. Geol. 92, 403–423. He, Z., Nie, h., Zhao, J., Liu, W., Bao, F., Zhang, W., 2017. Types and origin of nanoscale pores and fractures in Wufeng and Longmaxi shale in Sichuan basin and its periphery. J. Nanosci. Nanotechnol. 17, 6626–6633. Hu, H., Hao, F., Lin, J., Lu, Y., Ma, Y., Li, Q., 2017. Organic matter-hosted pore system in the Wufeng-Longmaxi (O3w-S1l) shale, Jiaoshiba area, eastern Sichuan Basin, China. Int. J. Coal Geol. 173, 40–50. Jiao, K., Yao, S., Liu, C., Gao, Y., Wu, H., Li, M., Tang, Z., 2014. The characterization and quantitative analysis of nanopores in unconventional gas reservoirs utilizing FESEM–FIB and image processing: an example from the lower Silurian Longmaxi Shale, upper Yangtze region, China. Int. J. Coal Geol. 128–129, 1–11. Jin, Z., Hu, Z., Gao, B., Zhao, J., 2016. Controlling factors on the enrichment and high productivity of shale gas in the Wufeng-Longmaxi Formations, southeastern Sichuan Basin. Earth Sci. Front. 23 (1), 1–10. Liang, D., Guo, T., Chen, J., Bian, L., Zhao, J., 2008. Some progresses on studies of hydrocarbon generation and accumulation in marine sedimentary regions, southern
Bernard, S., Horsfield, B., Schulz, H., Wirth, R., Schreiber, A., Sherwood, N., 2012. Geochemical evolution of organic-rich shales with increasing maturity: a STXM and TEM study of the Posidonia Shale (Lower Toarcian, northern Germany). Mar. Pet. Geol. 31, 70–89. Briggs, D., Kear, A., Baas, M., 1995. Decay and composition of the hemichordate rhabdopleura: Implications for the taphonomy of graptolites. Lethaia 28 (1), 15–23. Bustin, R., Link, C., Goodarzi, F., 1989. Optical properties and chemistry of graptofite periderm following laboratory simulated maturation. Org. Geochem. 14 (4), 355–364. Cao, T., Song, Z., Wang, S., Xia, J., 2015. A comparative study of the specific surface area and pore structure of different shales and their kerogens[J]. Sci. China Earth Sci. 58 (4), 510–522. Chen, J., Xiao, X., 2014. Evolution of nanoporosity in organic-rich shales during thermal maturation. Fuel 129, 173–181. Chen, S., Han, Y., Fu, C., Zhang, H., Zhu, Y., Zuo, Z., 2016. Micro and nano-size pores of clay minerals in shale reservoirs: implication for the accumulation of shale gas. Sediment. Geol. 342, 180–190. Chen, X., Rong, J., Li, Y., Boucot, A., 2004. Facies patterns and geography of the Yangtze region, South China, through the ordovician and silurian transition. Palaeogeogr. Palaeoclimatol. Palaeoecol. 204, 353–372. Crowther, P., 1981. The fine structure of graptolite periderm. Spec. Pap. Palaeontol. 26 (93), 389–396. Curtis, M.E., Cardott, B.J., Sondergeld, C.H., Rai, C.S., 2012. Development of organic porosity in the Woodford Shale with increasing thermal maturity. Int. J. Coal Geol. 103 (23), 26–31. Dai, J., Zou, C., Liao, S., Dong, D., Ni, Y., Huang, J., Wu, W., Gong, D., Huang, S., Hu, G., 2014. Geochemistry of the extremely high thermal maturity longmaxi shale gas, southern sichuan basin. Org. Geochem. 74, 3–12.
42
Marine and Petroleum Geology 111 (2020) 33–43
W. Zhang, et al.
Wufeng Longmaxi shale of the Sichuan basin, southwest China. Sci. Rep. 8, 1–11. Pang, Q., Hu, G., Jiao, K., Tan, X., Liu, H., Ye, Y., Yan, S., Zhao, D., 2018. Characteristics of Organic Pores and Composition of Bio-Precursors in the Wufeng and Longmaxi Formation Shales. Southern Sichuan Basin, China. Shan, Z., Wang, X., Wang, Q., 2015. Micropore structure characteristics of the silurian Longmaxi Formation shale in Southeast Sichuan province. J. Geol. 39 (4), 552–555. Tenger, B., Shen, B., Yu, L., et al., 2017. Mechanisms of shale gas generation and accumulation in the ordovician Wufeng-Longmaxi Formation, Sichuan basin, SW China. Pet. Explor. Dev. 44 (1), 69–78. Tian, H., Pan, L., Xiao, X., Wilkins, R., Meng, Z., Huang, B., 2013. A preliminary study on the pore characterization of Lower Silurian black shales in the Chuandong Thrust Fold Belt, southwestern China using low pressure N2 adsorption and FE-SEM methods. Mar. Pet. Geol. 48, 8–19. Underwood, C., Bottrell, S., 1994. Diagenetic controls on multiphase pyritization of graptolites. Geol. Mag. 131 (3), 315–327. Vandenbroucke, M., Largeau, C., 2007. Kerogen origin, evolution and structure. Org. Geochem. 38, 719–833. Yan, J., Jia, X., Shao, D., Zhang, Y., Zhang, T., 2015. Characterization of organic matterhosted pores by SEM method and their formation mechanisms for shales of Longmaxi Formation, Sichuan basin. Nat. Gas Geosci. 26 (8), 1540–1546. Yu, B., 2013. Classification and characterization of gas shale pore system. Earth Sci. Front. 20 (4), 2111–2220. Zhao, J., Jin, Z., Jin, Z., Geng, Y., Wen, X., Yan, C., 2017. Nano-scale pore characteristics of organic-rich Wufeng and longmaxi shales in the Sichuan basin, China. J. Nanosci. Nanotechnol. 17, 6721–6731. Zou, C., Dong, D., Wang, S., Li, J., Li, X., Wang, Y., Li, D., Cheng, K., 2010. Geological characteristics and resource potential of shale gas in China. Pet. Explor. Dev. 37 (6), 641–653.
China (Part 1): distribution of four suits of regional marine source rocks. Mar. Orig. Pet. Geol. 13 (2), 1–16. Loucks, R.G., Reed, R.M., Ruppel, S.C., Jarvie, D.M., 2009. Morphology, genesis, and distribution of nanometer-scale pores in siliceous mudstones of the Mississippian Barnett shale. J. Sediment. Res. 79, 848–861. Loucks, R.G., Reed, R.M., Ruppel, S.C., Hammes, U., 2012. Spectrum of pore types and networks in mudrocks and a descriptive classification for matrix-related mudrock pores. AAPG (Am. Assoc. Pet. Geol.) Bull. 96, 1071–1098. Loucks, R.G., Reed, R.M., 2014. Scanning-Electron-Microscope petrographic evidence for distinguish organic-matter pores associated with depositional organic matter versus migrated organic matter in mudrocks. GCAGS J. 3, 51–60. Löhr, S.C., Baruch, E.T., Hall, P.A., Kennedy, M.J., 2015. Is organic pore development in gas shales influenced by the primary porosity and structure of thermally immature organic matter? Org. Geochem. 87, 119–132. Lu, L., Qin, J., Shen, B., Tenger, B., Liu, W., Zhang, Q., 2016. Biogenic origin and hydrocarbon significance of siliceous shale from the Wufeng-Longmaxi formations in Fuling area, southeastern Sichuan Basin. Pet. Geol. Exp. 38 (4), 460–465. Mathia, E.J., Bowen, L., Thomas, K.M., Aplin, A.C., 2016. Evolution of porosity and pore types in organic-rich, calcareous, Lower Toarcian posidonia shale. Mar. Pet. Geol. 75, 117–139. Milliken, K.L., Rudnicki, M., Awwiller, D.N., Zhang, T., 2013. Organic matter-hosted pore system, Marcellus formation (Devonian), Pennsylvania. AAPG (Am. Assoc. Pet. Geol.) Bull. 97, 177–200. Nie, H., Jin, Z., 2016. Rock and cap rock controls on the upper ordovician Wufeng Formation–lower silurian Longmaxi Formation shale gas accumulation in the Sichuan basin and its peripheral areas. Acta. Geol. Sin-Engl. 90, 1059–1060. Nie, H., Zhang, J., Jiang, S., 2017. Types and characteristics of the lower silurian shale gas reservoirs in and around the Sichuan basin. Acta Geol. Sin.-Engl. 89, 1973–1985. Nie, H., Jin, Z., Zhang, J., 2018. Characteristics of three organic matter pore types in the
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Research paper
Pore characteristics of different organic matter in black shale: A case study of the Wufeng-Longmaxi Formation in the Southeast Sichuan Basin, China
T
Wentao Zhanga,b,c,∗, Wenxuan Hua, Tenger Borjiginb,c, Feng Zhua a
Nanjing University, Nanjing, Jiangsu 210000, China Wuxi Research Institute of Petroleum Geology, Sinopec Petroleum Exploration & Production Research Institute, Wuxi 214000, China c State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Wuxi 214000, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Gas shale Organic pore Broad ion beam-scanning electron microscopy Wufeng-Longmaxi Formation Sichuan Basin
Organic matter (OM) pores are recognized to be the most important pore type in gas shales in the Ordovician Wufeng-Silurian Longmaxi Formation in the Sichuan Basin, China. However, the effects of the type and origin of organic matter on pore development in OM remain controversial. In this study, detailed scanning electron microscope (SEM) analysis of siliceous shales was conducted for core samples from the Wufeng-Longmaxi Formation, taken from five shale gas wells in the Southeast Sichuan Basin. In addition to bitumen, several types of morphologically distinct organic macerals were identified. Due to the high maturity (vitrinite reflectance (Ro) > 2.2%) and compaction of the sample, confirming the origin of some OM types was difficult. However, each OM type is characterized by distinct pore features: (1) the pores in bitumen are normally well developed, with circular or irregular shapes; (2) spherical kerogen contains uneven pores, with irregular and angular pores at the periphery and few pores in the core; (3) algal fragments may have angular pores, the distribution and structure of which are influenced by the initial structure of the organic matter; (4) bacteria-like aggregates represent the accumulation of numerous microbial granules, and contain residual pores; (5) graptolites are relatively tight and usually have few or no pores. The findings indicate that bitumen and algal fragments are the most favourable hydrocarbon-generating organisms for OM pore development. Notably, organic matter originating from different biological sources can influence the extent, shape, and structure of the pores. Some organic pores are recognized as remained pores of sedimentary stage, which may be previously regarded as pores formed during the gas generation period.
1. Introduction Many studies have been performed on the classification of nanoscale pores in gas shales in recent years. The pores are normally subdivided into interparticle pores, intraparticle pores, organic matter pores, and micro-fractures (Loucks et al., 2012; Yu, 2013; Mathia et al., 2016; Nie et al., 2017). It is widely believed that organic pores are the most significant pore type in gas shale (Loucks et al., 2009, 2012; Curtis et al., 2012; Milliken et al., 2013). The porosity of gas shale is often positively correlated with the total organic matter content (TOC), confirming the significant contribution of OM pores to the overall porosity (Milliken et al., 2013; Loucks et al., 2012). It has been indicated that organic pores may account for up to 60% of the total pore volume in some shale reservoirs (Tian et al., 2013). Additionally, OM pores often have good connectivity and tend to absorb gas rather than water, making them more important for shale gas exploration (Nie and
∗
Jin, 2016). However, the existing classifications do not account for more detailed division of organic pores. In fact, organic pores in organic matter such as bitumen and kerogens are highly heterogeneous. Pores develop differently in organic particles and even in different parts of a single type of organic matter (Curtis et al., 2012; Milliken et al., 2013; Löhr et al., 2015). Using the point counting method based on the scanning electron microscopy (SEM) image to calculate the porosity, Loucks et al. (2009, 2012) found that the porosity of different organic particles ranges from 0% to 40%. The occurrence of pores in organic matter is not only related to the thermal maturity, but is also influenced by the type of organic matter. Some studies indicate that the pores are well developed in Type II kerogens, but are not developed in Type III kerogens (Loucks et al., 2012). Loucks and Reed (2014) proposed several criteria for distinguishing migrated organic matter from in-situ organic matter. Löhr et al. (2015) studied the characteristics of organic
Corresponding author. Nanjing University, Nanjing, Jiangsu 210000, China. E-mail address: [email protected] (W. Zhang).
https://doi.org/10.1016/j.marpetgeo.2019.08.010 Received 1 April 2019; Received in revised form 29 July 2019; Accepted 6 August 2019 Available online 08 August 2019 0264-8172/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. (a) Location of analyzed samples. (b) Vertical profiles of the study formations.
calculated by model analysis, and the results indicate that organic pores account for about 31–62% of the total pores due to the variances in the total organic matter content (Tian et al., 2013). Previous SEM studies revealed the existence of strong heterogeneity during pore development in different organic macerals in this area, but the relationship between organic macerals and pore evolution is still not clear. On the basis of previous studies, the Wufeng-Longmaxi shales in the Southern Sichuan Basin are investigated herein through SEM analysis by monitoring the variations in the organic pore structure. Several types of organic matter are identified on the basis of the morphology, size, and relation with minerals, and the characteristics of the pores for each type of organic matter are studied to deepen our understanding of the effect of organic matter types on pore development.
pores in samples of different maturities and revealed that the pores differ greatly in large and dispersed amorphous organic matter, and pores are also present between organic particles in some colonial algae and spherical organic aggregates, but are not found in woody organic matter and Tasmanian algae. Nie et al. (2018) studied non-polished samples of Wufeng-Longmaxi shale in China by scanning electron microscopy, and highlighted the pore characteristics of three kinds of organic particles, i.e. algae, bitumen, and fossil fragments. Nie et al. (2017) reported that algae had a structured pore network compared to bitumen. Mathia et al. (2016) described five types of organic pores: bubble-like pores, sponge-like pores, complex pores, “pendular” pores, and terrestrial maceral pores. This kind of classification mainly reflects the shape and size of the pores, but is not related to the type of organic matter. Yan et al. (2015) concluded that organic pores can be roughly divided into three types: organic pores in pyrolysis bitumen, linear organic pores along the mineral boundary, and kerogen residual pores, based on analyses of the Longmaxi Formation shale in the Sichuan Basin. However, the method used to evaluate bitumen and kerogen was not described in the study. It is generally believed that the formation of micro- and nano-pores in organic matter is related to the pyrolysis of organic matter (Loucks et al., 2012; Curtis et al., 2012; Bernard et al., 2012; Mathia et al., 2016). In immature black shale, the interparticle pores between inorganic minerals are predominant, while there are few micro-pores in organic matter. At the mature stage, especially after entering the gas generation window, pyrolysis enables partial conversion of organic matter into oil and gas, thus forming a large number of micro-pores in the organic matter. In fact, the original pores and structure of organic matter can affect the evolution of the organic pores (Löhr et al., 2015). Black shale from the top of the Ordovician Wufeng Formation to the Silurian Longmaxi Formation (Wufeng-Longmaxi Formation) in the Sichuan Basin of China has been proven to have great potential for shale gas exploration. The Fuling shale gas field targeting this interval has additional proven reserves of shale gas of 1607.5 × 108 m3 (Jin et al., 2016). Many studies have been conducted on the pores in Wufeng-Longmaxi gas shale with focus on the pore type, structure, and controlling factors (Zou et al., 2010; Jiao et al., 2014; Shan et al., 2015; Cao et al., 2015; Yan et al., 2015; Hu et al., 2017; Zhao et al., 2017; He et al., 2017). The proportions of organic and inorganic pores were
2. Geological setting From the Late Ordovician to Early Silurian, with the force of the Caledonian movement, several uplifts including the Chuanzhong uplift, Qianzhong uplift, and Jiangnan Xuefeng uplift, were formed in the Upper Yangtze Platform (Guo et al., 2014; Huang et al., 2018). This caused a rise in the sea-level, and thus relatively low-energy and anoxic conditions dominated in the southeast Sichuan Basin, leading to the deposition of thick, organic-rich shale in this area (Chen et al., 2004). With continual sedimentation from the Permian to late Cretaceous period, the black shale was buried as deep as 5 km underground and entered the gas generation stage. Thereafter, it was uplifted and endured denudation, reaching a current burial depth of 2–3 km (Guo and Liu, 2013). The Wufeng-Longmaxi black shale in the Southeast Sichuan Basin, with a total thickness of 80–110 m (Fig. 1), is the most promising target bed for shale gas development in South China (Liang et al., 2008). The Wufeng Formation in the bottom is only about 5–10 m thick and is mainly composed of black shale, with a high carbonaceous and siliceous content. In the top of the Wufeng Formation, there is a shell limestone layer less than 1 m thick in the study area. The Longmaxi Formation is in comfortable contact with the underlying stratum. The lower part of the Longmaxi Formation mainly comprises black shale, which is similar to the shale in the underlying layer. In the upper layers, the carbonaceous and siliceous content is lower and the content of clay mineral is 34
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Table 1 Total organic carbon (TOC) content, vitrinite reflectance (Ro), and mineral content of samples. Well
JY1
JY2 JY41-5
YZ1
a b
Sample
J1-1 J1-2 J1-3 J1-4 J1-5 JY2-23 JY2-25 41-5-1 41-5-2 41-5-3 41-5-4 41-5-5 YZ1-1 YZ1-7 YZ1-10 YZ1-12 YZ1-13 YZ1-15 YZ1-16 YZ1-17
Depth (M)
2359 2364 2383.93 2404 2413 2561 2566 2525.13 2555.19 2563.09 2607.17 2618.15 4459.1 4480 4499.35 4506.07 4508.05 4513.15 4515.15 4516.04
TOC%
3.04 3.12 3.58 4.22 3.56 3.23 3.44 0.38 1.98 1.36 3.92 4.71 1.14 2.35 2.41 7.22 8.07 5.44 5.01 4.63
Ro %
2.2–3.06
Mineral composition%
a
Quartz
Clay
Feldspar
Carbonate
Others
34 34 38 49 57
43 41 40 34 25
12 11 8 8 7
9 11 11 6 6
2 3 3 3 5
29 45 38 47 35 33 50 33 67 69 79 67 38
63 42 41 30 48 58 33 41 22 22 17 25 50
6 8 8 7 8 6 8 13 6 5 1 4 8
0 3 10 7 2 0 3 5 2 1 2 2 1
2 2 3 9 7 3 6 8 3 3 1 2 3
2.46–2.7b
3.14
Guo et al. (2014). Nie et al. (2018).
3. Samples and methods
include radiolarians, sponges, foraminifera, and brachiopods. Siliceous mineral and clay are the main minerals in the samples according to Xray diffraction analysis (Table 1). In addition to some clastic sedimentary grains, the silica in shale also originates from radiolarian-related biological silica (Lu et al., 2016).
3.1. Samples
3.2. Method
The core samples of black shale studied herein were taken from the Wufeng-lower Longmaxi Formation of five shale gas wells (JY1, JY2, JY41-5, YZ1) in the eastern and southeastern parts of the Sichuan Basin (Fig. 1). Wufeng-Longmaxi shale has a relatively high organic carbon abundance, with a TOC greater than 2% in most samples (Table 1). Shale in the lower stratum has a higher TOC than that in the upper stratum, and the 20–30 m interval in the lower part has a TOC above 4%, being the major pay zone. The equivalent vitrinite reflectance (Ro) of the shale ranges from 2.2% to 3.14%, suggesting a stage of high maturity to over-maturity. The organic matter is mainly humic–sapropelic (Type I-II1) (Guo and Liu, 2013; Dai et al., 2014). Reflected light petrography analyses show that the main organic matter is dispersed amorphous organic matter (Fig. 2), which could be solid bitumen or remnants of kerogens after degradation. The black shale contains abundant graptolites, which are richer in the lower strata. Other fossils
Sub-samples from the drill cores were cut using a low-speed microdiamond saw and then pre-polished with silicon-carbide paper. Thereafter, the samples were polished using a broad ion beam (BIB) polisher (Leica EM RES102 Ion Beam Milling System) to remove about 100 μm of rock from the pre-polished surface of each shale. The BIB polisher was operated at 6 kV, and the samples were treated for 4 h to produce a planar surface of about 1 cm2 that was suitable for pore investigation using SEM. A Helios 650 field-emission scanning electron microscope (FE-SEM) was used to observe the surfaces of the shale samples perpendicular to the bedding. No metal or carbon was sprayed on the surfaces of the samples; thus, the real surfaces could be observed, enabling evaluation of the nano-pores. In the secondary electron (SE) images, siliceous minerals display brighter colours due to charge effect; in contrast, pyrite appears darker because of its high electrical conductivity; organic matter and pores appear the darkest, while the edges of some
higher. The upper part of the Longmaxi Formation comprises gray siltstone, indicating that the sedimentary facies changed from the deepwater shelf to shallow water shelf (Guo and Zhang, 2014).
Fig. 2. Organic petrology of the samples. AOM: amorphous organic matter; Z: zooclasts. 35
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4.2. Types of kerogen and related pores
organic pores may appear bright because of the charge effect. The atomic number plays a dominant role in the appearance of the BSE images, where pyrite and other heavy minerals are the brightest, quartz is darker and organic matter and pores are the darkest.
4.2.1. Spherical kerogen organic matter Spherical kerogen is found in many samples, and is very unique in terms of the special features compared with other organic matter. (a) The shape is spherical (with circular cross-sections) (Fig. 4A and B), while some are hemispherical (Fig. 4C) or ellipsoidal (Fig. 4D) due to compaction and other effects, with diameters ranging from 2 μm to more than 10 μm. (b) They have clear edge outlines, generally with a two-layer structure; the organic matter in the core has smooth edges, the porous organic matter at the periphery is in close contact with the spherical organic matter in the core, and the organic matter at the periphery is bounded by the surrounding minerals. (c) Sporadic automorphic–semiautomorphic phosphate minerals are visible in the organic matter (bright white spots in organic matter, Fig. 4). (d) Some spherical organic matter is surrounded by minerals such as silica (Fig. 4G and H). With obvious differences between the core and the periphery, the micro-pores in the spherical kerogen organic matter are non-uniform. Moreover, the pore development is also distinct for different types of organic matter. Generally, there are three cases. First, small pores with diameters < 20 nm are observed in the core, while dense pores with diameters > 20 nm are developed at the periphery; there is gradual transition from the core to the periphery (Fig. 4A and B). Second, a small number of pores are locally developed in the core (which is relatively tight), while the pores are well developed at the periphery; a clear boundary exists between the core and the periphery (Fig. 4C–F). Third, the tight spherical organic matter is surrounded by minerals such as silica, which is thought to be caused by the early diagenetic metasomatism of peripheral organic matter (Fig. 4G and H). The pores are consistent in shape and size. The section of a single pore appears as an irregular polygon or deformed polygon, and the pore size is generally dozens of nm. The pores in the peripheral organic matter are well developed and evenly distributed, reflecting similar chemical properties, but may be significantly different from those in the core. The pore characteristics of the organic matter at the periphery are quite different from those in the core, thus enabling the recognition that the organic matter at the periphery is bitumen generated by kerogen in the core. After careful observation, we still believe that these two parts are the same constituents of original kerogen. If the organic matter at the periphery is bitumen formed later, then the surrounding minerals should be in extensive contact with the organic matter in the core. However, this phenomenon is not observed. On the contrary, mineral particles such as quartz only penetrate into the organic matter at the periphery, but seldom enter the organic matter in the core. Fig. 5B shows that both the organic matter in the centre and that at the periphery of the lower part of kerogen are deformed simultaneously by the extrusion of minerals, which indicates a greater possibility that the organic matter in the centre and at the periphery have been coexistent since the early stage. The biological origin of the spherical organic matter is still uncertain. Previous studies reveal that the spherical kerogen in this interval is algae cystocarp, acritarchs, and single-cell planktonic algae (Nie et al., 2018; Pang et al., 2018), but the size of the spherical organic matter in previous studies is tens to hundreds of μm, which is much larger than the spherical organic matter mentioned herein. A and B: Small pores are developed in the core of spherical organic matter, which is in contrast to the surrounding porous organic matter. The white bright spots in the organic matter are apatite mineral. C, D, E and F: Only a small number of pores are developed locally in the core, which is clearly bounded by the surrounding porous organic matter. The pores are irregular and angular. G and H: The pores are not developed in the core, and the organic matter is enclosed by siliceous minerals. A: sample JY2-25, B: YZ1-16, C: YZ1-15, D:JY2-23, E: JY2-23, F: JY2-25, G:YZ1-13, H: YZ1-12.
4. Results As mentioned above, at the highly mature to post-mature stage, most kerogens have endured strong compaction and physiochemical changes, and are mixed with absorbed residual oil. Hence, it is difficult to differentiate solid bitumen from the kerogen matrices. However, identification may be achieved on some of them on the basis of the shape, interior structure, and distribution. For instance, OM within zooclast cavities should be bitumen, while some units with special shapes could be kerogen matrices, although there may be residual absorbed oil. Typical bitumen and four kinds of kerogen were distinguished in this study. Evaluation of the pore structures could help us understand how the parent materials influence pore development.
4.1. Bitumen and related pores Bitumen is one of the main organic macerals in black shale. Compared with in-situ kerogen, it is classified as migrated organic matter. Loucks and Reed (2014) proposed several criteria for distinguishing bitumen from kerogen. After observation of Silurian highmaturity shale in the Sichuan Basin, we conclude that typical bitumen and kerogen can be distinguished on the basis of three aspects. First, bitumen has no specific shape and fills in the intergranular pores or cracks of minerals, so it takes the pore shape as its own shape, while kerogen mostly contacts minerals in granular form and may have a defined shape. Second, bitumen has a uniform internal texture, and its pores are uniform or random, while some kerogens appear heterogeneous with biological structures. Third, minerals around bitumen display cementation (for example, quartz particles show automorphic characteristics due to secondary enlargement), while kerogen usually makes direct contact with sedimentary particles (Fig. 3). Bitumen in Silurian black shale in the Sichuan Basin is usually distributed in the intergranular pores of granular minerals (Fig. 3A, B, E, and F), inter-clay pores (Fig. 3D), or intragranular pores (Fig. 3C). The pore shapes of bitumen are consistent with these pore types. The size of such organic matter is generally less than 20 μm, mostly within 10 μm (Fig. 3). Pores are widespread in bitumen. According to the characteristics, the pores can be divided into two types: (1) irregular pores (Fig. 3A, C, and E) and (2) bubble-like pores. The former are uniformly distributed throughout organic matter bitumen with high pore density; the pores are similar in size (generally less than 50 nm), but with irregular shapes. The latter are nearly circular or elliptical and pores with different sizes ranging from 10 nm to more than 100 nm coexist (Fig. 3B, D, and F). Some super-large pores with sizes of up to hundreds of nm or even 1 μm may be formed occasionally. Such super-large pores are proposed to be complex pores formed by the interconnection of multiple pores after enlargement (Mathia et al., 2016). Microscopically, the edges of the pores are zigzag like a coastline, forming a hairy-edged structure. The original outlines of multiple pores can be observed in some macro-pores (shown by the arrows in Fig. 3D). Bitumen is mainly distributed in intergranular pores (A, B, D–F) or intragranular pores (C) of minerals. Quartz minerals in contact with bitumen often have automorphic shapes (arrows in Fig. A). Pores in bitumen are of two different types:irregular pores (Fig. A, C, E) and bubble-like pores (Fig. B, D, F). A: sample JY1-4, B: sample JY1-3, C: sample YZ1-17, D: sample YZ1-10, E: sample JY1-5, F: sample JY1-4.
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Fig. 3. SEM images of bitumen-related pores.
the clay, and clay was later formed when the formation fluid flowing into the channels reacted with the rock during burial. In the majority of such types of organic matter, the pores are relatively well developed. The pores are irregular and angular, with a uniform size (10–50 nm) (Fig. 5B and D). They often correspond to high plane porosity (about 20%). In the high magnification images, the organic matter shows a fibrous structure; the pores occur as gaps between fibres and are therefore well interconnected. In addition, the pores are mainly located between fibrous clay minerals. The closer the pores are to the clay minerals, the less developed they become. This also shows that the chemical composition is different for different parts of the organic matter. Hydrocarbon generation is difficult in the region closer to the channels. A (sample YZ1-16) and C (sample YZ1-16): Organic matter is nodular and has no uniform shape, with sizes ranging from tens to hundreds of μm. The edge intersects with minerals. The quartz at the edge is mainly isomorphic. Fibrous illite clay is found in the organic matter. The distribution of clay has a certain structure. B and D: Magnification of A and C, where pores are well developed. They are irregular and angular, with a uniform size. Pores are relatively underdeveloped in the region near clay. Another type of algal fragment differs from the above-mentioned type in two respects. First, it is clean inside without clay. Second, it has
4.2.2. Algal fragments One type of algal fragment is characterized by an irregular and amorphous bulk shape, generally larger than 20 μm (up to 100 μm), and is often surrounded by minerals such as quartz. Clay minerals with a fibrous shape are distributed in the algal fragments (Fig. 5). Despite the interpenetration between the edge and surrounding minerals, this type of algal fragment is considered as a kerogen rather than migrated bitumen based on three considerations. First, if it is bitumen filling intergranular pores, filling would have occurred after entering the oil-generating window. In this diagenetic stage, the intergranular pores of tens to hundreds of μm are too large relative to the mineral particles of generally around 5 μm; thus, the minerals can hardly form supporting structures to prevent compaction of the pores. Secondly, secondary enlargement of quartz is rarely seen in mineral particles at the edge of organic matter, whereas secondary enlargement is a common diagenetic phenomenon for quartz at the edge of pores. Third, the pores in organic matter are consistent in shape and size–generally irregular angular pores, and no circular pores are observed; the distribution of clay minerals in organic matter shows certain structural characteristics (Fig. 4B and D). There is usually no clay inside the organic matter, thus the microscopic clay here should be formed after burial. It is reasonable to speculate that there were some channels or fractures at the location of
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Fig. 4. Pore characteristics in spherical kerogen organic matter.
and C, respectively.
a neat overall outline, which distinguishes it from bitumen. The shape of such organic matter is irregular and angular. Its size is mostly less than 20 μm. This type of algal fragment has a relatively neat outline, and some silica intrusion can be seen (Fig. 6E and F). Such organic matter has well-developed pores with high plane porosity, where the pores are irregular and angular with a uniform size and good connectivity. The organic matter has unique shapes and neat outlines (A, C, E, and F). Pores in OM are numerous with an irregular and angular shape and similar size (B and D). A: sample YZ1-16, C: sample YZ1-16, E: sample YZ1-16, F: sample 41-5-5. B and D are magnifications of the boxes in A
4.2.3. Bacteria-like aggregates This organic matter comprises small aggregates of quasi-spherical bacteria (Fig. 7). Pellets are easily recognized in Fig. 6B; however, in most situations, the pellets are merged together due to compaction, making it hard to determine the original morphology (Fig. 6a, c, d). These aggregates may have been regarded as whole kerogen or bitumen in previous studies, but the structure of the pellets can still be seen through detailed observation (top left corner of Fig. 6a). The diameter of the pellets ranges from 300 to 400 nm. Affected by compaction, some 38
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Fig. 5. Potential benthic algal fragments and pores.
pellets are flattened to form flat spheres (Fig. 6B). Fig. 6D shows that the surface of the pellets is not smooth, and there are many burrs. It is common for some intergranular pores to be filled with siliceous mineral during long geological periods (Fig. 7A, B and D). Bacteria-like particles have few pores, and all the observed pores are among pellets. The pore shape is irregular and angular with a pore size of 10–50 nm. These pores are significantly different from the bubblelike pores in bitumen. The residual intergranular pores are randomly distributed because they are mainly controlled by the influence of fluid activity and related to the connectivity of intergranular pores. The pores in Fig. 6A and C are more common in our samples, and are partial residual pores retained after the inter-compression of pelletoid aggregates under compaction. In this case, the original morphology of the pellets is unidentifiable. Sometimes, intergranular pores may be enlarged due to hydrocarbon generation (Fig. 6D). Those later-formed pores are preferentially distributed along the contact boundary of the bacteria-like particles. Intergranular pores among bacteria-like particles can often be compacted (A and C) or filled with siliceous minerals (B, D), and only a small amount of residual pores were retained. A: sample 41-5-3, B: sample YZ1-7, C: sample YZ1-1, D: sample JY2-23.
developed in graptolite, but are well developed in the organic matter between pyrite grains. D: Graptolite inside is pyritized. Pores are not developed in graptolite, but are well developed in the organic matter between pyrite grains. E, F: SEM image of graptolite. A: sample 41-5-1; B: sample 41-5-2; C, D: sample 41-5-4. E: sample 41-5-2; F: sample JY223. The existence of micro-pores in some graptolites was reported in a previous study (Tenger et al., 2017), but the pores are relatively undeveloped in most of the graptolites observed herein (Fig. 8). This situation is related to the chemical properties of graptolite. Graptolites preserved in rock are the group skeletons secreted by graptolites, and are thought to be composed of collagen (Crowther, 1981). They mainly consist of aromatic compounds with aliphatic groups. They have a lower hydrogen index (20–540 mg g−1) and a higher oxygen index (10–60 mg g−1), making them Type II to III organic matter (Bustin et al., 1989; Briggs et al., 1995). Therefore, the weak hydrocarbon generating capacity of graptolites is the main reason the underdeveloped pores. Another phenomenon is that the organic matter filling the associated pyrite is porous (Fig. 8C and D). This part of organic matter may be filled bitumen. 5. Discussion
4.2.4. Graptolite Large numbers of graptolites were observed in the target formation, and they are also considered as the markers of this formation. Environmental scanning electron microscopy and in-situ energy spectrum analysis show that they mainly exist in the form of flattened carbonaceous films that are essentially parallel to the beddings (Tenger et al., 2017). The section width is usually 2–3 μm and the films are often segmented and generally associated with clustered pyrite aggregates. Some pyrite aggregates are adjacent to the graptolites (Fig. 7A−C), and some are inside the graptolites. The spaces between the pyrite grains are also filled with organic matter. Previous studies and numerous fossil observations reveal that there is a close relationship between graptolite and pyrite; the graptolites can even be completely pyritized (Underwood and Bottrell, 1994). A-C: The banded graptolites coexist with pyrite. Pores are not
5.1. Pore development in bitumen According to the SEM observation, pores are usually well developed in bitumen. Obviously, such pores are formed in the gas-generating stage. In terms of the formation mechanism, it is generally believed that the pores are formed by the precipitation of gaseous hydrocarbons generated during the secondary cracking of residual oil (Bernard et al., 2012; Curtis et al., 2012; Loucks et al., 2012; Chen and Xiao, 2014; Mathia et al., 2016). For the pores in bitumen, preservation of in-situ hydrocarbon gas is an important controlling factor, in addition to maturity. The tiny intergranular pores and high capillary pressure of shale minerals not only partially block the migration of bitumen, trapping it in adjacent pores, but also impede the migration of hydrocarbon gas generated by residual organic matter and keep it in situ. If gas can move 39
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Fig. 6. Another type of potential algal fragment and pores.
developed pores, followed by spherical organic matter. Bacterial-like aggregates retain only part of the original pores, and graptolites and other animal fossils have only a few pores. As the chemical composition varies at different parts of kerogen, the organic pores formed later are somewhat heterogeneous. A typical example is the spherical organic matter shown in Fig. 4, where the pores are rarely or not developed in the core, but are well developed at the edge. Essentially, the chemical composition differs for the two parts. For example, the epidermal tissue outside the cells may contain more fat, making it easier to generate hydrocarbons. In addition, kerogen itself has a certain texture, which also affects the formation and distribution of pores. In the algal fragments shown in Fig. 5, there are some fibrous clays that form vein-like structures. Although there are no pores locally in the organic matter adjacent to clay, there are more pores in the region far from clay. In the magnified view, the organic matter appears fibrous as a whole, much like the image of paper under a scanning electron microscope. The pore morphology, affected by the original texture, is generally irregular and angular, and there are no circular bubble-like pores. The pore size is also uniform, and no coexistence of pores with greatly different sizes is observed in bitumen. Not all the pores in kerogen are formed by late gas generation from pyrolysis; some of the pores are pre-existing pores, mainly from bacteria-like colonies (Fig. 7). Löhr et al. (2015) also noticed in his study
freely in rocks, the porous structure in the organic matter will not be formed. Given the homogeneous character of bitumen (Vandenbroucke and Largeau, 2007), the pores in it should be uniform. The pore distribution in bitumen mentioned above basically conforms to this expectation (Fig. 3). However, the distribution is not absolutely uniform, but there should not be a clear boundary for pore development in the same bitumen particle. For example, the pores in Fig. 3D are not uniform in size and distribution, but are generally randomly dispersed in the range of the size of organic particles. Morphologically, the pores in bitumen constitute two types: (1) irregular pores with spongy uniform distribution, and (2) circular or elliptical pores. The former is often very densely distributed as a whole, with a relatively uniform size (generally tens of nm). The latter has sizes ranging from 10 nm to more than 100 nm; pores of different sizes often coexist. In some zones, pores are interconnected to form large complex pores (Fig. 3D). It is proposed that the mechanism of formation of these two types of pores is related to different hydrocarbon generation processes.
5.2. Pore development in kerogen The SEM images show that the extent of pore development is different for organic macerals. Algal fragments contain the most 40
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Fig. 7. Bacteria-like aggregate and pores.
morphological differences. Each type of organic matter has unique features that are distinct from those of the other types, and pore development in the organic matter is highly heterogeneous. Specifically, pores are universally and uniformly developed in bitumen and algal fragments; on the other hand, pores are partially developed in spherical organic matter and bacterial-like aggregates, but are not developed in graptolites. Pores in organic matter are mainly formed during gas generation from pyrolysis, and are also affected by the original composition and texture of the organic matter. The composition of bitumen is relatively uniform; thus, the pores in bitumen are uniform and have circular or angular shapes. The organic matter with a fibrous texture or lattice texture mainly contains angular pores, which may be related to the original texture of the organic matter. In the spherical organic matter, the pores in the core are significantly different from those at the edge, which may be caused by the difference in the original chemical composition. In addition to the pores of thermogenic origin, others arise from early construction, mainly due to microbial (e.g. bacteria) reworking of original organic matter, such as the pelletoid aggregates mentioned herein. Most of the pores are compacted during the late burial process, with only partial remnants. The types and sources of some organic matter are undistinguishable due to overly high maturity and compaction deformation. Nevertheless, the identification of several types of typical organic matter and the related pore distributions is meaningful for better understanding the formation, evolution, and preservation of shale gas. Graptolites contribute significantly to the TOC of rocks, but have limited capacity for hydrocarbon generation and contain no pores.
that, there are some initial pores remain. For a given sample in our study, the pores in bacteria-like colonies are most easily filled with quartz, while few minerals are found in bitumen pores. This also suggests that the pores in the spherical organic matter were formed earlier, unlike the pores in bitumen that are formed by late gas generation from pyrolysis. Moreover, pellets with sizes of 300–400 nm were observed in the enlarged view. These pellets are even smaller than diatoms, and are strongly thought to be bacteria based on their shapes (namely, organic fragments were reworked by bacteria in the early stage). Such pores are mainly angular, with sizes ranging from tens to hundreds of nm. In most cases, the earlier-formed pores are deformed or disappear due to compaction, except in local areas where clear outlines of intergranular pores can be observed. 5.3. Preservation of OM related pores The effects of compaction on shale pores were discussed in previous studies (Fishman et al., 2012; Loucks et al., 2012; Milliken et al., 2013; Chen et al., 2016). As mentioned above, most of the early pores disappear with increased maturity, and only a part of the pores are preserved. On one hand, this may be attributed to the compaction during burial (Fishman et al., 2012; Milliken et al., 2013). The increase in the plasticity of organic matter with rising temperature results in a further decrease in the number of intragranular pores. On the other hand, in the hydrocarbon generation stage, some bitumen remains in kerogen and thus blocks the pores (Löhr et al., 2015). However, there's no distinct evidence showing that the pores formed in mature stage get deformed or orientated. That's because pore formation postdates most compaction. Rigid quartz minerals formed stable framework, preventing the organic pores from deformation and collapse (Lu et al., 2016).
Acknowledgement This work is financially supported by the National Natural Science Foundation of China (41690133) and National Science and Technology Major Project (2017ZX05036002). We thank the research institute of Jianghan Oilfield Company of SINOPEC for providing samples and data access for our experiments.
6. Conclusions Based on SEM analysis of the Wufeng-Longmaxi Formation black gas shale in the Sichuan Basin, China, we systematically classified the organic matter into five types (including bitumen) according to the 41
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Fig. 8. SEM images of graptolites and pores.
References
Fishman, N.S., Hackley, P.C., Lowers, H.A., Hill, R.J., Egenhoff, S.O., Eberl, D.D., Blum, A.E., 2012. The nature of porosity in organic-rich mudstones of the upper Jurassic Kimmeridge clay formation, North sea, offshore united kindom. Int. J. Coal Geol. 103, 32–50. Guo, T., Liu, R., 2013. Implications from marine shale gas exploration breakthrough in complicated structural area at high thermal stage: taking Longmaxi Formation in well JY1 as an example. Nat. Gas Geosci. 24 (4), 643–651. Guo, T., Zhang, H., 2014. Formation and enrichment mode of Jiaoshiba shale gas field, Sichuan Basin. Pet. Explor. Dev. 41 (1), 28–36. Guo, X., Hu, D., Wen, Z., Liu, R., 2014. Major factors controlling the accumulation and high productivity in marine shale gas in the Lower Paleozoic of Sichuan Basin and its periphery: a case study of the Wufeng-Longmaxi Formation of Jiaoshiba area. Chin. Geol. 41 (3), 893–901. Huang, H., He, D., Li, Y., Li, J., Zhang, L., 2018. Silurian tectonic-sedimentary setting and basin evolution in the Sichuan area, southwest China: Implications for palaeogeographic reconstructions. Mar. Pet. Geol. 92, 403–423. He, Z., Nie, h., Zhao, J., Liu, W., Bao, F., Zhang, W., 2017. Types and origin of nanoscale pores and fractures in Wufeng and Longmaxi shale in Sichuan basin and its periphery. J. Nanosci. Nanotechnol. 17, 6626–6633. Hu, H., Hao, F., Lin, J., Lu, Y., Ma, Y., Li, Q., 2017. Organic matter-hosted pore system in the Wufeng-Longmaxi (O3w-S1l) shale, Jiaoshiba area, eastern Sichuan Basin, China. Int. J. Coal Geol. 173, 40–50. Jiao, K., Yao, S., Liu, C., Gao, Y., Wu, H., Li, M., Tang, Z., 2014. The characterization and quantitative analysis of nanopores in unconventional gas reservoirs utilizing FESEM–FIB and image processing: an example from the lower Silurian Longmaxi Shale, upper Yangtze region, China. Int. J. Coal Geol. 128–129, 1–11. Jin, Z., Hu, Z., Gao, B., Zhao, J., 2016. Controlling factors on the enrichment and high productivity of shale gas in the Wufeng-Longmaxi Formations, southeastern Sichuan Basin. Earth Sci. Front. 23 (1), 1–10. Liang, D., Guo, T., Chen, J., Bian, L., Zhao, J., 2008. Some progresses on studies of hydrocarbon generation and accumulation in marine sedimentary regions, southern
Bernard, S., Horsfield, B., Schulz, H., Wirth, R., Schreiber, A., Sherwood, N., 2012. Geochemical evolution of organic-rich shales with increasing maturity: a STXM and TEM study of the Posidonia Shale (Lower Toarcian, northern Germany). Mar. Pet. Geol. 31, 70–89. Briggs, D., Kear, A., Baas, M., 1995. Decay and composition of the hemichordate rhabdopleura: Implications for the taphonomy of graptolites. Lethaia 28 (1), 15–23. Bustin, R., Link, C., Goodarzi, F., 1989. Optical properties and chemistry of graptofite periderm following laboratory simulated maturation. Org. Geochem. 14 (4), 355–364. Cao, T., Song, Z., Wang, S., Xia, J., 2015. A comparative study of the specific surface area and pore structure of different shales and their kerogens[J]. Sci. China Earth Sci. 58 (4), 510–522. Chen, J., Xiao, X., 2014. Evolution of nanoporosity in organic-rich shales during thermal maturation. Fuel 129, 173–181. Chen, S., Han, Y., Fu, C., Zhang, H., Zhu, Y., Zuo, Z., 2016. Micro and nano-size pores of clay minerals in shale reservoirs: implication for the accumulation of shale gas. Sediment. Geol. 342, 180–190. Chen, X., Rong, J., Li, Y., Boucot, A., 2004. Facies patterns and geography of the Yangtze region, South China, through the ordovician and silurian transition. Palaeogeogr. Palaeoclimatol. Palaeoecol. 204, 353–372. Crowther, P., 1981. The fine structure of graptolite periderm. Spec. Pap. Palaeontol. 26 (93), 389–396. Curtis, M.E., Cardott, B.J., Sondergeld, C.H., Rai, C.S., 2012. Development of organic porosity in the Woodford Shale with increasing thermal maturity. Int. J. Coal Geol. 103 (23), 26–31. Dai, J., Zou, C., Liao, S., Dong, D., Ni, Y., Huang, J., Wu, W., Gong, D., Huang, S., Hu, G., 2014. Geochemistry of the extremely high thermal maturity longmaxi shale gas, southern sichuan basin. Org. Geochem. 74, 3–12.
42
Marine and Petroleum Geology 111 (2020) 33–43
W. Zhang, et al.
Wufeng Longmaxi shale of the Sichuan basin, southwest China. Sci. Rep. 8, 1–11. Pang, Q., Hu, G., Jiao, K., Tan, X., Liu, H., Ye, Y., Yan, S., Zhao, D., 2018. Characteristics of Organic Pores and Composition of Bio-Precursors in the Wufeng and Longmaxi Formation Shales. Southern Sichuan Basin, China. Shan, Z., Wang, X., Wang, Q., 2015. Micropore structure characteristics of the silurian Longmaxi Formation shale in Southeast Sichuan province. J. Geol. 39 (4), 552–555. Tenger, B., Shen, B., Yu, L., et al., 2017. Mechanisms of shale gas generation and accumulation in the ordovician Wufeng-Longmaxi Formation, Sichuan basin, SW China. Pet. Explor. Dev. 44 (1), 69–78. Tian, H., Pan, L., Xiao, X., Wilkins, R., Meng, Z., Huang, B., 2013. A preliminary study on the pore characterization of Lower Silurian black shales in the Chuandong Thrust Fold Belt, southwestern China using low pressure N2 adsorption and FE-SEM methods. Mar. Pet. Geol. 48, 8–19. Underwood, C., Bottrell, S., 1994. Diagenetic controls on multiphase pyritization of graptolites. Geol. Mag. 131 (3), 315–327. Vandenbroucke, M., Largeau, C., 2007. Kerogen origin, evolution and structure. Org. Geochem. 38, 719–833. Yan, J., Jia, X., Shao, D., Zhang, Y., Zhang, T., 2015. Characterization of organic matterhosted pores by SEM method and their formation mechanisms for shales of Longmaxi Formation, Sichuan basin. Nat. Gas Geosci. 26 (8), 1540–1546. Yu, B., 2013. Classification and characterization of gas shale pore system. Earth Sci. Front. 20 (4), 2111–2220. Zhao, J., Jin, Z., Jin, Z., Geng, Y., Wen, X., Yan, C., 2017. Nano-scale pore characteristics of organic-rich Wufeng and longmaxi shales in the Sichuan basin, China. J. Nanosci. Nanotechnol. 17, 6721–6731. Zou, C., Dong, D., Wang, S., Li, J., Li, X., Wang, Y., Li, D., Cheng, K., 2010. Geological characteristics and resource potential of shale gas in China. Pet. Explor. Dev. 37 (6), 641–653.
China (Part 1): distribution of four suits of regional marine source rocks. Mar. Orig. Pet. Geol. 13 (2), 1–16. Loucks, R.G., Reed, R.M., Ruppel, S.C., Jarvie, D.M., 2009. Morphology, genesis, and distribution of nanometer-scale pores in siliceous mudstones of the Mississippian Barnett shale. J. Sediment. Res. 79, 848–861. Loucks, R.G., Reed, R.M., Ruppel, S.C., Hammes, U., 2012. Spectrum of pore types and networks in mudrocks and a descriptive classification for matrix-related mudrock pores. AAPG (Am. Assoc. Pet. Geol.) Bull. 96, 1071–1098. Loucks, R.G., Reed, R.M., 2014. Scanning-Electron-Microscope petrographic evidence for distinguish organic-matter pores associated with depositional organic matter versus migrated organic matter in mudrocks. GCAGS J. 3, 51–60. Löhr, S.C., Baruch, E.T., Hall, P.A., Kennedy, M.J., 2015. Is organic pore development in gas shales influenced by the primary porosity and structure of thermally immature organic matter? Org. Geochem. 87, 119–132. Lu, L., Qin, J., Shen, B., Tenger, B., Liu, W., Zhang, Q., 2016. Biogenic origin and hydrocarbon significance of siliceous shale from the Wufeng-Longmaxi formations in Fuling area, southeastern Sichuan Basin. Pet. Geol. Exp. 38 (4), 460–465. Mathia, E.J., Bowen, L., Thomas, K.M., Aplin, A.C., 2016. Evolution of porosity and pore types in organic-rich, calcareous, Lower Toarcian posidonia shale. Mar. Pet. Geol. 75, 117–139. Milliken, K.L., Rudnicki, M., Awwiller, D.N., Zhang, T., 2013. Organic matter-hosted pore system, Marcellus formation (Devonian), Pennsylvania. AAPG (Am. Assoc. Pet. Geol.) Bull. 97, 177–200. Nie, H., Jin, Z., 2016. Rock and cap rock controls on the upper ordovician Wufeng Formation–lower silurian Longmaxi Formation shale gas accumulation in the Sichuan basin and its peripheral areas. Acta. Geol. Sin-Engl. 90, 1059–1060. Nie, H., Zhang, J., Jiang, S., 2017. Types and characteristics of the lower silurian shale gas reservoirs in and around the Sichuan basin. Acta Geol. Sin.-Engl. 89, 1973–1985. Nie, H., Jin, Z., Zhang, J., 2018. Characteristics of three organic matter pore types in the
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