Pore Structure Characterization of the Lower Permian Marine

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Pore structure characterization of the Lower Permian marine-continental transitional black shale in the Southern North China Basin, central China Qian Chen, Jinchuan Zhang, Xuan Tang, Wei Dang, Zhongming Li, Chong Liu, and Xuezhi Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01475 • Publication Date (Web): 02 Nov 2016 Downloaded from http://pubs.acs.org on November 5, 2016

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Pore structure characterization of the Lower Permian marine-continental transitional black shale in the Southern North China Basin, central China Qian Chen a,b, Jinchuan Zhang a,b,*, Xuan Tang a,b, Wei Danga,b, Zhongming Lic, Chong Liuc and Xuezhi Zhangd

a

Key Laboratory of Shale Gas Exploration and Evaluation, Ministry of Land and

Resources, China University of Geosciences, Beijing 100083, China b

School of Energy and Resources, China University of Geosciences, Beijing 100083,

China c

Henan Institute of Geological Survey, Zhengzhou, Henan 450000, China

d

Jinhai Oil Production Plant, Liaohe Oilfield Company, PetroChina, Panjin, Liaoning

124010, China *

Corresponding author: [email protected]

ACS Paragon Plus Environment

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Abstract: The Carboniferous-Permian Taiyuan-Shanxi coal-bearing formations, as the most promising shale gas reservoir in north China, compromise a typical shale gas system developed under transitional environment. To investigate the pore structure of the Taiyuan-Shanxi shale in Southern North China Basin, field emission scanning electron microscope (FE-SEM), low pressure nitrogen adsorption (LNA) and high pressure mercury intrusion (HMI) were applied to characterize the pore type, volume and size distribution of eleven shale samples from Mouye-1 well. Organic matter-hosted pores, interparticle pores and intraparticle pores and shrinkage cracks were observed through FE-SEM images. Compared to matured marine shale, the number of organic matter-hosted pores within the Taiyuan-Shanxi shale is much less probably because of the high content of inertinite. The nitrogen total pore volume ranges from 20.36×10-3 mL/g to 31.23×10-3 mL/g, whereas the mercury total pore volume ranges from 2.1×10-3 mL/g to 6.2×10-3 mL/g. Surface area ranges from 8.66 m2/g to 19.38 m2/g. The pore size distribution curves suggest a significant contribution of the macropore (> 50 nm) to total pore volume. Micropore volume and meso-macropore volume obtained from LNA were found separately associated with plagioclase and dolomite. Larger micron-sized pore volume obtained from HMI shows a positive relationship with quartz and a negative relationship with chlorite. However, these correlations are generally weak. The lack of organic matter-hosted pores highlights the importance of pores associated with inorganic material in the coal-bearing transitional shale. Rather than organic content, mineral content and chemical/mineral transformation during diagenesis play more important role with respect to the pore structure. Inhomogeneous pore abundance caused by different chemical transformation degree among samples may be the reason of the weak correlation between pore volume and shale components’ content.


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1. Introduction The Carboniferous-Permian coal-bearing formations in north China are developed in a transitional environment and known for huge hydrocarbon generation capacity.1 They are the main source rock of conventional and tight gas field in Ordos basin, and also contribute to the coal-derived gas fields in Bohai Bay Basin through secondary hydrocarbon-generating.2-7 In addition, they are also the primary targets of coal-bed methane in North China.8,9 The Taiyuan-Shanxi formations in North China mainly composed of interbedded organic-rich shale, coal, siltstones and sandstones have been acknowledged as promising shale gas strata with distinct characteristics compared to marine gas shale formations which are the preferential shale gas exploitation targets so far.10-16 Based on a recently drilled deep core well (depth> 3000 m) aiming at shale gas resource of Zhongmou region, Southern North China Basin (SNCB) (Figure 1), the organic matter (OM) type, richness and maturity of the Taiyuan-Shanxi shale have been systematically evaluated in a previous article.17 However, knowledge on the pore structure which largely affect how hydrocarbon store and flow in shale is still insufficient. Previous studies have shown that both the free gas content and adsorption gas capacity of shale was separately constrained by pore volume and surface area.18,19 While, other factors such as pore connectivity, pore-throat relationship may influence gas migration behavior.20 Because of the significant role in shale gas accumulation and production, pore structure of shale gas reservoir has been considered as one of the key element in shale gas formation characterization and potential assessment and has been a research hotspot over the past few years.21 Most of the related studies focus on marine shale from North America, South China and Europe.22-30 During these processes, the importance of organic matter-hosted pores which is one of the most remarkable differences between shale and conventional sandstone reservoirs were highlighted.19,23,31 Several fluid injection methods have been used to qualify pore volume and pore size distribution (PSD) of gas shales, revealing that the pore system


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of shale are usually dominated by nanopores.32-34 However, these studies have also shown that the pore structure of shale varied from strata because it is multiple controlled by several factors such as burial depth, mineralogy, organic matter (OM) thermal maturity and type.24,35-37 In addition, differences between the typical marine shale and non-marine shale on pore structure were also found.38-40 Therefore, it is necessary to investigate the pore characteristics of the Lower Permian transitional black shale for a better understanding of their implication for shale gas potential. On the basis of the primary study focusing on organic matter characteristics, pore type, pore volume and PSD of the Taiyuan-Shanxi shale from Mouye-1 well was evaluated in this paper.

2. Samples and Methods 2.1 Samples The geological background of the Lower Permian formations in SNCB has been well documented.17 Eleven shale samples were selected from Mouye-1 well with depth ranging from 2084 m to 2954 m. An adapted map (Figure 1) was presented in this article to show the location, lithology of the Mouye-1 well and sample location. Six samples were from the Taiyuan formation while the other five were from the overlying Shanxi formation. The total organic carbon (TOC, wt%) content of these samples, ranges from 0.44% to 4.24% (Table 1), has been previously measured by a Leco C230 carbon analyzer and also published.17 2.2 Methods 2.2.1 X-ray diffraction (XRD) The mineral contents were determined with quantitative X-ray diffraction (XRD) using a Bruker D8 Discover X-ray diffractometer. The operation and calculation followed the relevant oil industry standard of China (SY/T 5163-2010). Each shale sample was firstly smashed and then centrifugal separated. After that, sample with particle size smaller than 10 µm was used to evaluate the total clay mineral content and other non-clay mineral content, while sample with particle size smaller than 2 µm was used to determine the content of each clay mineral relative to


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the total clay mineral. Eventually, each of the mineral was reported as its weight percent relative to whole rock after normalizing with TOC to 100%. 2.2.2 Field emission scanning electron microscopy (FE-SEM) FE-SEM is a common technique for characterizing pores in shale. The type and geometry of pores can be directly identified through FE-SEM images. In this study, each sample was Ar-ion milled prior to the SEM observation to prevent artificial pores being identified.23,24 Approximate 70 µm thick surface material was removed using a Gatan Ilion II milling system with an accelerating voltage of 6-7 KeV for 4 hours and eventually a flat surface approximate 2.5×105 µm2 was produced for observing. The polished surface was then slightly coated with Au using a Leica EM SCD5000 Sputter Coater to improve surface productivity. Pore observation was conducted with a Hitachi S8010 scanning electron microscope (SEM) under secondary electron (SE) mode and accelerating voltage of 15-20 KeV. SEM images were taken with 7 mm working distance and a maximum magnification of 30,000, allowing for pores as small as ~10 nm (in diameter) to be identified. 2.2.3 Low pressure nitrogen adsorption (LNA) Nitrogen adsorption was used to evaluate the pore volume and pore size distribution (PSD) of the shale samples as suggested.32-34 Each sample was crushed to 20-40 meshes and then degassed at 110 °C under vacuum condition for 12 hours. Nitrogen adsorption was conducted at -196 °C which is the saturated vapor pressure of liquid nitrogen. An Autosorb-iQ2 apparatus (Quantachrome Instruments) was used to record the adsorbed nitrogen volume with relative pressure (P/P0) between 0.005 and 0.99. Adsorption branches were used in evaluating pore volume and PSD in order to prevent the effect of TSE phenomenon.41 The total pore volume was the volume of liquid nitrogen at a relative pressure of 0.99 using the nitrogen density of 0.808g/mL, which refers to total volume of pores smaller than 193.5 nm in diameter.34 The micropore (< 2 nm) volume was calculated from the nitrogen volume at a relative pressure of 0.14 as the Horvath–Kawazoe (HK) model suggested and the


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meso-macropore volume was the difference between the total pore volume and the micropore volume.42 Surface area was calculated using a Micropore BET Assistant available in the Quantachrome Software which was designed based on the modified BET model.43 Because most of the surface area of shale comes from micropore, these two parameters obtained from LNA are almost perfectly correlated. To avoid excessive narrative, only micropore volume was discussed in the following, but it is important to note the conclusions about micropore volume also apply to surface area. PSD of the meso-macropore was determined by using the BJH model in which cylindrical pore geometry was assumed.44 2.2.4 High pressure mercury intrusion (HMI) Mercury intrusion was performed with a Pore Master 60 mercury injecter (Quantachrome Instruments). In preparation, each sample was smashed into small fragments with diameter of several millimeters and 3-4 grams of the smashed fragments for each sample were weighed and then put into a dilatometer. After air-leakage test and evacuation, mercury was firstly intruded into the dilatometer under pressure up to 20 psi to let mercury fill the inter-space between the rock fragments. Then, the dilatometer was transferred to a high-pressure station to intrude mercury into samples with a maximum pressure of 35000 psi. The utilized experimental pressure was converted to pore/throat diameter using the Washburn equation with mercury surface tension of 0.485 N/m and contact-angle of 140°.45 According to the equation, pore/throat between 6 nm to ~10 µm in diameter can be detected. Mercury total pore volume is the total mercury volume intruded into the dilatometer apart from the mercury intruded at low pressure (< 20 psi).

3. Results 3.1 Mineral composition The Taiyuan-Shanxi shale samples were composed mainly of quartz and clay mineral. The quartz content ranged from 28.96% to 48.12%, whereas the clay mineral content ranged from 31.59% to 63.39% and consisted of various amounts of illite,


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kaolinite, illite/smectite and chlorite. Plagioclase content reached up to 6.41% and no K-feldspar was detected. The siderite content ranged from 0% to 6.17%. Dolomite, calcite and pyrite contents were generally lower than 5.0% and most of them were found in the Taiyuan shale. Compared to typical marine shales in North America and South China, the Taiyuan-Shanxi shale studied generally contains more clay and less carbonate mineral (Figure 2). 46,47 Several general relationships were found among the contents of plagioclase and different clay minerals. The kaolinite content increased with increasing chlorite content (Figure 3) and decreasing plagioclase content (Figure 4). The illite/smectite content decreased with increasing illite content (Figure 5). These relationships suggest that the mineral transformation processes play a significant role in the present bulk mineral content of the Taiyuan-Shanxi shale. 3.2 FE-SEM In this paper, pores observed through FE-SEM images were described in terms of their related components. The terminology of the ternary pore type classification scheme where pores are subdivided into organic matter pores (OM pores), interparticle pores (interP pores) and intraparticle pores (intraP pores) was used.24 3.2.1 Pores associated with OM Different from the densely distributed OM pores observed in marine shale, the OM pores observed within Taiyuan-Shanxi shale were much less.23,26 In an individual OM particle, only several isolated pores were observed and the size of them ranged from tens to hundreds of nanometers (Figure 6A-C). More slit-elliptical pores were observed within some OM particles that was combined closely with clay minerals (Figure 6D). However, these pores are probably not traditional OM pores originated from hydrocarbon generation but more likely to be interP pores between clay and OM or intraP pores within clay aggregates.48 Alternatively, these pores may also be shrinkage cracks (Figure 6A and D) formed during dessication. More generally, pores were totally absent in other OM particles in the Taiyuan-Shanxi shale (Figure 6E and


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F). 3.2.2 Pores associated with pyrite The

pyrite

in

the

Taiyuan-Shanxi

shale

occurred

either

as

framboidal-elliptical/amorphous clusters or polygonal individual euhedral crystal (Figure 7A-E). The shape and size of these pyrite aggregates and crystals varied significantly even within one sample (Figure 7A, B and E). Pyrite framboids have been proved formed in strictly dyoxic-anoxic environment and the size of the framboids have implications for the oxygen level of the sedimentary water and sedimentation rate.49-52 Therefore, the variety of the morphology and size indicate the Taiyuan-Shanxi shale was formed in a rather oxic-dyoxic water column most likely come from the coastal transitional environment. The pores related to pyrite formed mainly by stacking of pyrite microcrystals, which indicated these pores are inter-crystal pores within pyrite aggregates. In most of the pyrite aggregates, microcrystals were closely compacted so that these inter-crystal pores were not easy to be found. Compared to loosely compacted pyrite aggregates where the space between crystals was usually occupied by ductile OM (Figure 7B, E and F), pores were more common in pure pyrite aggregates due to the supporting of rigid pyrite crystals (Figure 7A-D). 3.2.3 Pores associated with quartz The pores associated with quartz were mostly slit-like interP pores distributed around the margin of quartz grain (Figure 8A-D). The margins of a grain varied from fully closed, semi-closed to unclosed (Figure 8A-D), which may associated with inhomogeneous compacting or difference in the mechanical property of surrounding materials. Angular or edge support (Figure 8C and D) protected part of the interP pores from being destroyed by compaction. The width of these slit-pores ranged from tens to more than one hundred nanometers, and the length that varied greatly with the size of the quartz grain could reach up to more than ten micrometers. 3.2.4 Pores associated with clay minerals


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The pores associated with clay minerals were either interP pores between clay particles (Figure 9A and B) or intraP pores between clay layers (Figure 9C and D). The interP pores were usually polygonal, whereas the intraP pores were triangle or silt-like in two dimensions. The appearance of the intaP pores suggest they were probably survived from pressure by the support of intersecting clay flakes, which were named “card house” structure.53 The shrinkage cracks/pores between or within clay aggregates were commonly observed and sometimes it is difficult to distinguish them from the actual interP and intraP pores. However, some visual criteria may help to make a quick judgment in some cases. Firstly, the slit-like shrinkage cracks/pores usually tend to be concentrated and show regular arrangements (Figure 9E and F). Secondly, the shrinkage cracks/pores usually have rough edge fitting well with the opposite edge (Figure 9E and F). This feature is probably a result of avulsion by shrinkage. 3.2.5 Pores associated with carbonate The intraP pores within carbonate are generally thought to be associated with dissolution and were abundant in the Taiyuan-Shanxi shale.24,35 Compared to the intraP pores associated with clay mineral, part of the intraP pores within carbonate was more likely to be aligned, highlighting boundary of the grains (Figure 10A). Three different type of intraP pores associated with carbonate (mainly dolomite) were observed: 1) polygonal pores with straight edges (Figure 10B) which may inherited from the inner structure of porous carbonate fossils;29 2) well aligned spherical-elliptical intraP pores only distributed around the grain rims (Figure 10C) and 3) both aligned grain-rim intraP pores and isolated intraP pores within the carbonate grain (Figure 10D-F) as a result of scattered distributed and lower content of carbonate.35 Size of these intraP pores ranged from tens of nanometers to several micrometers and the smaller pores tended to occur as regular rhombuses that agree well with the general geometry of dolomite crystals (Figure 10E and F). The consistency in geometry suggests these rhombic pores came from preferential dissolution of unstable dolomite and can be called moldic intraP pores.24


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3.3 Low pressure nitrogen adsorption (LNA) The IUPAC pore size classification where pores were subdivided into micropore (diameter< 2 nm), mesopore (diameter between 2 nm and 50 nm) and macropore (diameter> 50 nm) was used to illustrate the PSD of shales as recommended in this section.30,54 According to the isotherm classification of IUPAC, the nitrogen adsorption isotherms (Figure 11) of the Taiyuan-Shanxi shale were typical type Ⅱ isotherm representing non-porous or macro-porous material.55 The sharp uptake of the isotherms at a high relative pressure suggests the existence of a great number of larger mesopores and macropores. The hysteresis loops have both H3 and H4 characteristics according to the classification of IUPAC, indicating development of narrow slit-like pores. For all the eleven samples, 13.16 mL/g to 20.19 mL/g (STP) nitrogen was adsorbed at P/P0 of 0.99, corresponding to 20.36×10-3 mL/g to 31.23×10-3 mL/g of total pore volume. The micropore volume ranged from 3.54×10-3 mL/g to 7.87×10-3 mL/g and the meso-macropore volume ranged from 15.03×10-3 mL/g to 24.05×10-3 mL/g. The meso-macropore volume accounts for 69%-83% of the total pore volume. The surface area ranged from 8.66 m2/g to 19.38 m2/g and showed linear positive relationship with micropore volume (not shown in figures). The average pore diameter ranged from 5.88 nm to 10.59 nm. A mutual main peak at ~3 nm was found from two different PSD curves: dV/dD and dV/dlog (D) versus pore diameter (Figure 12). According to previous researches, this peak may be related to pores between the elementary units of smectite or illite/smectite group.37,57 For the pores larger than 3 nm, the dV/dD decreased with increasing pore diameter. A second main peak was identified around 30-50 nm from the dV/dlog(D) versus pore diameter curves, proving a significant contribution of larger mesopores and macropores to pore volume.47,57 3.4 High pressure mercury intrusion (HMI) The mercury total pore volume ranges from 2.1×10-3 mL/g to 6.2×10-3 mL/g and was much smaller than the total pore volume obtained from nitrogen. A general


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uptake was found from the PSD curves (Figure 13) of several samples (JX10, JX16, JX33, JX40, JX42 and JX51) at the pressure corresponding to pores 10 µm in diameter. According to Chalmers et al.33, the end of the uptake may be the threshold between surface partial filling and pore filling, and that’s why only volume of pores smaller than 10µm were calculated in this study. Two general peaks were identified from the PSD curves. The first peak appeared at pores between 2 µm and 6 µm in diameter with a general trough distributed between 100 nm-200 nm. With increasing intrusion pressure, dV/dlog(D) of all the samples increases with decreasing pore diameter with an exception of JX24. The second peak generally located at pores smaller than 10 nm.

4. Discussion 4.1 OM pores One of the most notable characteristics of the Taiyuan-Shanxi shale is the lack of pores within OM. Through FE-SEM observation, the OM pores identified are less than the pores associated with inorganic materials especially for clay and carbonate minerals. However, whether OM is porous in a smaller scale (< 10 nm) is still doubtful due to the resolution limitation of FE-SEM. For this reason, nitrogen adsorption results measuring smaller pores should also be analyzed as a complement to FE-SEM. As a result, the TOC content did not show obvious relationship with the micropore volume (Figure 14), but instead, it has a general negative relationship with meso-macropore volume (Figure 15). The correlations are consistent with the observation, suggesting the less porous OM is not the main contributor to the pore system in Taiyuan-Shanxi shale. The negative relationship between TOC content and mesopore volume may be a result of OM occupying the void space associated with clay minerals.56,57 Small organic particles in the mesopore range are usually very difficult to be identified through SEM because they are usually aggregated with clay minerals (Figure 16A) especially using SE detector. However, signs indicate that they may act as filling in the pores within clay minerals (Figure 16B). The formation of OM pores within shales has been considered as a companion


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of hydrocarbon generation and was both controlled by thermal maturity and OM type.48,58 Although the thermal maturity threshold at which organic pores begin to form is still controversial, the viewpoint that these pores are well developed during the conversion stage of oil generating to gas generation period has been generally accepted.25,59,60 For the OM type, easily degraded marine kerogen or bituminite have been found more likely to be porous than woody kerogen at a similar thermal maturation level.23,35,36,59,61,62 In addition, the abundance and size of OM pores in shale can also be reduced by compaction when the shale has a very high TOC content.31 Since the Taiyuan-Shanxi shale has high thermal maturity (Ro> 3.0%) and low-medium OM richness (TOC< 5.0%), the lack of OM pores therein should probably be attributed to the dominated persistent terrigenous inertinite which is generally highly oxidized because of bacterial attack, wildfire or longer transportation before deposition.17,63-65 As a result, the degradation not only reduces the hydrocarbon generation capacity but also the abundance of OM pores. 4.2 Compositional control on pore volume Whole-scale pore volume determination depends on combining use of gas adsorption and mercury intrusion.32-34 Because of the mechanisms and measurement range differences between the two methods, discrepancy is usually inevitable.37,66,67 In this study, a weak positive relationship was found between the total pore volumes separately obtained by LNA and HIM (Figure 17). That may suggest that many pores distributed between the overlap measurement range between the two methods i.e. 6 nm-193.5 nm. This speculation was also supported by the SEM image and PSD result (Figure 11 and 12) that most of the pores is in the range between tens to hundreds of nanometers. Seeking relationships between components’ content and pore volume of different scale is a common method to determine the compositional control on the pore volume of shale because both LNA and HMI implicates pore size but not pore type. In this study, the nitrogen pore volume was subdivided into micropore (< 2 nm) and meso-macropore (2 nm-193.5 nm). The mercury pore/throat volume was


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subdivided into two intervals: pores between 100 nm-1 µm and between 1 µm-10 µm (Table 2). Reverse relationships were found between micropore volume and plagioclase (Figure 18A) for the Shanxi shale and Taiyuan shale. A weak positive relationship was found between the meso-macropore volume and dolomite content (Figure 18B) of the Taiyuan shale. For the larger pore/throat measured by HMI, 100 nm-1 µm pore/throat volume showed negative relationship with the chlorite content (Figure 18C) and positive relationship with quartz for 1 µm-10 µm pore/throat volume (Figure 18D). When combined with SEM observation, these correlations may preliminary suggest that the micropore and meso-macropore in Taiyuan shale were probably associated with intraP pores caused by dissolution of plagioclase and dolomite, while the larger pores are contribute from interP pores associated with quartz. The negative impact of chlorite on pore volume may as a result of blocking or reducing interP pores by covering the quartz grains.68 Nevertheless, the conclusion is somewhat ambiguous because these correlations for all the samples are weak and no obvious relationship has been found between the pore volume and porous clay minerals. Compaction, chemical (mineral) transformation and OM maturation were found to be the three main processes altering the pore structure after deposition.24 The Taiyuan-Shanxi shale samples was obtained from a core well deeper than 2800 m, indicating that they had went through the fasted compaction period during shallow-intermediate burial stage and the compaction effect on the pores was eased for the whole two formations.24 Less significant effect of compaction and OM pores for the Taiyuan-Shanxi shale highlights the importance of the chemical transformation’s effect on pores associated with inorganic material. Dolomite dissolution was observed through SEM images (Figure 10). The relationships among mineral content (Figure 2-4) were probably caused by feldspar transforming to kaolinite and kaolintie/smectite illitization.69,70 The variation in mineralogy indicates a different degree of chemical transformation, resulting in inhomogeneous distributed pores within a same component (Figure 10C and D), which may be an important reason for the low correlation between the pore volume and the shale components’


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content. Besides, these correlations may also be complicated by the relationships between minerals. Because each of the chemical transformation reactions requires specific ion and acid-base property of pore water, we believe the chemical/mineral transformation depends largely on the variable depositional environment. A further explanation of the transformation is beyond the scope of this paper and will be discussed further with more detailed data.

5. Conclusions FE-SEM, nitrogen adsorption and mercury intrusion were used to characterize the pore system of the Taiyuan-Shanxi shale from Mouye1 well from Northern South China Basin. The relevant conclusions are as follows: (1) The Taiyuan-Shanxi shale has a pore system with less OM pores. Pores within OM were either absent or scattered in the SEM images and no densely distributed pores were observed as previously study have found in matured marine. The TOC content has no positive relationship with neither micropore nor meso-macropore volume, suggesting the OM pores have no dominate control on the pore volume of the Taiyuan-Shanxi shale. The high content of inertinite is thought to be the main reason for the lack of OM pores. (2) IntraP pores were abundant and generally found within carbonate and clay minerals’ particles. Formation of these pores was associated with carbonate dissolution, support of intersecting clay flakes and shrinkage during desiccation. Positive relationships exist between micropore and plagioclase as well as meso-macropore and dolomite for the Taiyuan shale. (3) Quartz is the main contributor to interP pores, although several interP pores were also observed between clay particles. A general raise between 200 nm-10 µm was identified from the mercury PSD curves and was thought to be mainly from the contribution of interP pores. Mercury pore/throat volume was roughly subdivided into pore volumes of 100 nm-1 µm and 1 nm-10 µm and they separately showed a general negative relationship with chlorite and a positive relationship with quartz content. (4) Relationships existed among plagioclase and different clay minerals,


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indicating chemical/mineral transformation degree varied differently for the whole Taiyuan-Shanxi formations. The subsequent inhomogeneous pore distribution and correlations between minerals should probably responsible for the weak correlations found between pore volume and shale components’ content.

Acknowledgments The authors thank the Henan Yukuang Geological Exploration Investment Co., Ltd. (PO-SZ-14153) for samples support and permission to publish this paper.

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Table 1. Depth, Vitrinite reflectance (Ro), TOC and mineral contents of the samples. TOC

Quartz

Plagioclase

Calcite

Dolomite

Siderite

Pyrite

Gypsum

Kaolinite

Illite

Illite/Smectite

Chlorite

0.00

22.38

14.07

17.91

9.59

0.00

0.00

3.53

27.07

25.89

2.35

4.18

0.00

0.00

10.48

19.46

13.47

6.49

6.17

0.00

3.02

0.90

43.40

0.00

0.90

1.28

3.40

0.00

1.20

36.76

0.00

2.00

1.24

1.46

2.06

0.00

9.60

23.04

8.64

6.72

0.00

1.02

1.87

0.00

13.95

32.33

8.24

8.87

1.14

0.75

0.00

7.30

40.14

0.00

4.69

Sample ID

Formation

Depth (m)

Ro (%)

JX10

Shanxi

2827.13

3.35

1.09

33.70

0.00

0.00

0.00

1.25

0.00

JX12

Shanxi

2831.66

-

0.44

34.46

5.04

0.00

0.00

1.21

JX16

Shanxi

2841.88

3.27

2.37

40.15

3.40

0.00

0.00

JX24

Shanxi

2857.02

3.55

4.24

38.20

3.17

0.00

0.00

JX33

Shanxi

2889.34

3.55

2.56

46.39

6.41

0.00

0.00

JX36

Taiyuan

2913.2

-

3.74

38.37

5.14

0.00

JX37

Taiyuan

2917.42

-

0.92

29.35

3.46

0.00

JX40

Taiyuan

2925.92

3.58

1.93

38.98

3.79

0.00

1.29

JX42

Taiyuan

2931.5

3.59

1.75

28.96

2.72

0.00

1.84

0.00

2.02

0.00

14.42

35.74

5.64

6.90

JX46

Taiyuan

2941.28

3.50

2.25

36.35

2.24

1.23

0.79

1.06

0.00

0.00

10.65

19.62

20.18

5.61

JX51

Taiyuan

2953.09

3.46

1.67

48.12

3.38

4.83

3.08

2.82

4.51

0.00

6.00

13.90

7.90

3.79

Weight percentage (wt. %)

17

Note: the TOC and vitrinite reflectance (Ro) data have been published previously .


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Table 2. Surface area, pore volume and average pore diameter of the Taiyuan-Shanxi shales. Nitrogen adsorption Sample ID

Mercury intrusion

Total pore

Micropore

Meso-macropore

Modified BET

(

Pore Structure Characterization of the Lower Permian Marine

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