Pore Structure Characterization of the Lower Permian Marine

Nov 2, 2016 - sandstone and have been acknowledged as promising shale-gas strata with distinct characteristics compared to marine shale-gas formations...
0 downloads 0 Views 9MB Size
Subscriber access provided by University of Idaho Library

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

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

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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.

ACS Paragon Plus Environment

Page 2 of 42

Page 3 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

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

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

ACS Paragon Plus Environment

Page 4 of 42

Page 5 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

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

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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,

ACS Paragon Plus Environment

Page 6 of 42

Page 7 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

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

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 42

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

ACS Paragon Plus Environment

Page 9 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

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

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

ACS Paragon Plus Environment

Page 10 of 42

Page 11 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

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

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

ACS Paragon Plus Environment

Page 12 of 42

Page 13 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

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’

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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,

ACS Paragon Plus Environment

Page 14 of 42

Page 15 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

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.

References (1) Zhang, Z.; Li, C.; Long, S.; Xu, H., Exploration prospect of Upper Palaeozoic natural gas in eastern area of North China. Nat. Gas Geosci. 2006, 17, 330-334 (in Chinese with English abstract). (2) Liu, R.; Xiao, H.; Fan, L.; Zhang, C.; Hao A.; Miao, W., A depositional mode of flood-induced braided river delta in Permian of Ordos Basin. Acta Pet. Sin. 2013, 34, 120-127 (in Chinese with English abstract). (3) He, Z.; Fu, J.; Xi, S.; Fu, S.; Bao, H., Geological features of reservoir formation of Sulige gas field. Acta Pet. Sin. 2003, 24, 6-12 (in Chinese with English abstract). (4) Bian, C. S.; Zhao, W. Z.; Wang, H. J.; Chen, Z. Y.; Wang, Z. C.; Liu, G. D.; Zhao, C. Y.; Wang, Y. P.; Xu, Z. H.; Li, Y. X.; Jiang, L., Contribution of moderate overall coal-bearing basin uplift to tight sand gas accumulation: case study of the Xujiahe Formation in the Sichuan Basin and the Upper Paleozoic in the Ordos Basin, China. Pet. Sci. 2015, 12, 218-231. (5) Xu, H.; Zhao, Z.; Lv, F.; Yang, Y.; Tang, Z.; Sun, G.; Xu, Y., Tectonic evolution of the Nanhuabei Area and analysis about its petroleum potential. Geotecton. Metallog. 2004, 28, 450-463 (in Chinese with English abstract). (6) Zhu, Y.; Qin, Y.; Fan, B.; Sang, S.; Yang, Y.; Jiang, B., Evaluation of the Second Hydrocarbon-generation of the Permo-Carboniferous Source Rocks in Wuging Depression. Earth Science 2004, 29, 77-84 (in Chinese with English abstract).

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(7) Zhao, X.; Jin, Q.; Zhang, L.; Liang, H.; Jin, F., Accumulation conditions and perspectives of coal-derived hydrocarbon of Carboniferous-Permian in Northern Jizhong Depression, Bohai Bay Basin. Pet. Geol. Exp. 2010, 32, 459-464 (in Chinese with English abstract). (8) Tang, S.; Sun, S.; Hao, D.; Tang, D.; Coalbed Methane-bearing Characteristics and Reservoir Physical Properties of Principal Target Areas in North China. Acta Geol. Sin. (Engl. Transl.) 2004, 78, 724-728. (9) Shao, L.; Hou, H.; Tang, Y.; Lu, J.; Qiu, H.; Wang, X.; Zhang, J., Selection of strategic relay areas of CBM exploration and development in China. Nat. Gas Ind. (Chengdu, China) 2015, 35, 1-11 (in Chinese with English abstract). (10) Zhang, J.; Xu, B.; Nie, H.; Deng, F., Two essential gas accumulations for natural gas exploration in China. Nat. Gas Ind. (Chengdu, China) 2007, 27, 1-6 (in Chinese with English abstract). (11) Zhang. J.; Yang, C.; Chen, Q.; Zhao, Q.; Wei, P.; Jiang, S., Deposition and distribution of potential shales in China. Earth Sci. Front. 2016, 23, 74-86 (in Chinese with English abstract). (12) Zou, C.; Dong, D.; Wang, S.; Li, J.; Li, X.; Wang, Y.; Li, D.; Cheng, K., Geological characteristics and resource potential of shale gas in China. Pet. Explor. Dev. 2010, 37, 641-653. (13) Wang, S.; Li, D.; Li, J.; Dong, D.; Zhang, W.; Ma, J., Exploration potential of shale gas in the Ordos Basin. Nat. Gas Ind. (Chengdu, China) 2011, 31, 40-46 (in Chinese with English abstract). (14) Jiang, S.; Xu, Z.; Feng, Y.; Zhang, J.; Cai, D.; Chen, L.; Wu, Y.; Zhou, D.; Bao, S.; Long, S., Geologic characteristics of hydrocarbon-bearing marine, transitional and lacustrine shales in China. Asian J. Earth Sci. 2016, 115, 404-418. (15) Ding, W.; Zhu, D.; Cai, J.; Gong, M.; Chen, F., Analysis of the developmental characteristics and major regulating factors of fractures in marine–continental transitional shale-gas reservoirs: A case study of the Carboniferous–Permian strata in the southeastern Ordos Basin, central China. Mar. Pet. Geol. 2013, 45, 121-133.

ACS Paragon Plus Environment

Page 16 of 42

Page 17 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(16) Guo, S.; Fu, J.; Gao, D.; Li, H.; Huang, J., Research status and prospects for marine-continental shale gases in China. Pet. Geol. Exp. 2015, 37, 535-540 (in Chinese with English abstract). (17) Dang, W.; Zhang, J.; Tang, X.; Chen, Q.; Han, S.; Li, Z.; Du, X.; Wei, X.; Zhang, M.; Liu, J.; Peng, J.; Huang, Z., Shale gas potential of Lower Permian marine-continental transitional black shales in the Southern North China Basin, central China: Characterization of organic geochemistry. J. Nat. Gas Sci. Eng. 2016, 28, 639-650. (18) Ambrose, R. J.; Hartman, R. C.; Campos, M. D.; Akkutlu, I. Y.; Sondergeld, C. In New Pore-scale Considerations for Shale Gas in Place Calculations. Society of Petroleum Engineers Unconventional Conference, SPE 131772, Feb. 23-25, 2010, Fort Wort, TX, USA; Society of Petroleum Engineers: Richardson, TX, USA, 2010; DOI: 10.2118/131772-MS. (19) Ross, D. J. K.; Marc Bustin, R., The importance of shale composition and pore structure upon gas storage potential of shale gas reservoirs. Mar. Pet. Geol. 2009, 26, 916-927. (20) Javadpour, F., Nanopores and Apparent Permeability of Gas Flow in Mudrocks (Shales and Siltstone). J. Can. Pet. Technol. 2009, 48, (8), 16-21. (21) Cao, T.; Song, Z.; Wang, S.; Cao, X.; Li, Y.; Xia, J., Characterizing the pore structure in the Silurian and Permian shales of the Sichuan Basin, China. Mar. Pet. Geol. 2015, 61, 140-150. (22) Wang, F. P.; Reed, R. M., Pore networks and fluid flow in gas shales. Society of Petroleum

Engineers

Annual

Technical

Conference

and

Exhibition,

SPE-124253-MS, Oct. 4-7, 2009, LA, New Orleans, Society of Petroleum Engineers: Richardson, TX, USA, 2009; DOI: 10.2118/124253-MS. (23) Loucks, R. G.; Reed, R. M.; Ruppel, S. C.; Jarvie, D. M., Morphology, Genesis, and Distribution of Nanometer-Scale Pores in Siliceous Mudstones of the Mississippian Barnett Shale. J. Sediment. Res. 2009, 79, 848-861. (24) Loucks, R. G.; Reed, R. M.; Ruppel, S. C.; Hammes, U., Spectrum of pore types and networks in mudrocks and a descriptive classification for matrix-related

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

mudrock pores. AAPG Bull. 2012, 96, 1071-1098. (25) Lu, J.; Ruppel, S. C.; Rowe, H. D., Organic matter pores and oil generation in the Tuscaloosa marine shale. AAPG Bull. 2015, 99, 333-357. (26) Jiao, K.; Yao, S.; Liu, C.; Gao, Y.; Wu, H.; Li, M.; Tang, Z., 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. 2014, 128-129, 1-11. (27) Wang, Y.; Zhu, Y.; Chen, S.; Li, W., Characteristics of the Nanoscale Pore Structure in Northwestern Hunan Shale Gas Reservoirs Using Field Emission Scanning Electron Microscopy, High-Pressure Mercury Intrusion, and Gas Adsorption. Energy Fuels 2014, 28, 945-955. (28) Zhang, J.; Fan, T.; Li, J.; Zhang, J.; Li, Y.; Wu, Y.; Xiong, W., Characterization of the Lower Cambrian Shale in the Northwestern Guizhou Province, South China: Implications for Shale-Gas Potential. Energy Fuels 2015, 29, 6383-6393. (29) Klaver, J.; Desbois, G.; Urai, J. L.; Littke, R., BIB-SEM study of the pore space morphology in early mature Posidonia Shale from the Hils area, Germany. Int. J. Coal Geol. 2012, 103, 12-25. (30) Klaver, J.; Desbois, G.; Littke, R.; Urai, J. L., BIB-SEM characterization of pore space morphology and distribution in postmature to overmature samples from the Haynesville and Bossier Shales. Mar. Pet. Geol. 2015, 59, 451-466. (31) Milliken, K. L.; Rudnicki, M.; Awwiller, D. N.; Zhang, T., Organic matter-hosted pore system, Marcellus Formation (Devonian), Pennsylvania. AAPG Bull. 2013, 97, 177-200. (32) Bustin, R. M.; Bustin, A. M. M.; Cui, X.; Ross, D. J. K.; Murthy, P. V. S. Impact of shale properties on pore structure and storage characteristics. Society of Petroleum Engineers Gas Shale Production Conference, SPE 119892, Nov. 16−18, 2008, Fort Wort, TX, USA; Society of Petroleum Engineers: Richardson, TX, USA, 2008; DOI: 10.2118/119892-MS. (33) Chalmers, G. R.; Bustin, R. M.; Power, I. M., Characterization of gas shale pore

ACS Paragon Plus Environment

Page 18 of 42

Page 19 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

systems by porosimetry, pycnometry, surface area, and field emission scanning electron microscopy/transmission electron microscopy image analyses: Examples from the Barnett, Woodford, Haynesville, Marcellus, and Doig units. AAPG Bull. 2012, 96, 1099-1119. (34) Clarkson, C. R.; Solano, N.; Bustin, R. M.; Bustin, A. M. M.; Chalmers, G. R. L.; He, L.; Melnichenko, Y. B.; Radliński, A. P.; Blach, T. P., Pore structure characterization of North American shale gas reservoirs using USANS/SANS, gas adsorption, and mercury intrusion. Fuel 2013, 103, 606-616. (35) Schieber, J., 2010. Common themes in the formation and preservation of intrinsic porosity in shales and mudstones – illustrated with examples across the Phanerozoic. Society of Petroleum Engineers Unconventional Gas Conference, SPE-132370-MS, Feb. 23-25, 2010, Pittsburgh, PA. USA; Society of Petroleum Engineers: Richardson, TX, USA, 2010; DOI: 10.2118/132370-MS. (36) Curtis, M. E.; Cardott, B. J.; Sondergeld, C. H.; Rai, C. S., Development of organic porosity in the Woodford Shale with increasing thermal maturity. Int. J. Coal Geol. 2012, 103, 26-31. (37) Kuila, U.; Prasad, M., Specific surface area and pore-size distribution in clays and shales. Geophys. Prospect. 2013, 61, 341-362. (38) Fang, C.; Amro, M., Pore structure characteristics of non-marine shale in Ordos Basin China, International Petroleum Technology Conference, IPTC-17419-MS, International petroleum Technology Conference, Jan. 20-22, 2014, Doha, Qatar. DOI: 10.2523/IPTC-17419-MS. (39) Fishman, N. S.; Hackley, P. C.; Lowers, H. A.; Hill, R. J.; Egenhoff, S. O.; Eberl, D. D.; Blum, A. E., The nature of porosity in organic-rich mudstones of the Upper Jurassic Kimmeridge Clay Formation, North Sea, offshore United Kingdom. Int. J. Coal Geol. 2012, 103, 32-50. (40) Wang, G.; Ju, Y.; Bao, Y.; Yan, Z.; Li, X.; Bu, H.; Li, Q., Coal-Bearing Organic Shale Geological Evaluation of Huainan–Huaibei Coalfield, China. Energy Fuels 2014, 28, 5031-5042. (41) Groen, J. C.; Peffer, L. A. A.; Pérez-Ramı́rez, J., Pore size determination in ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

modified micro- and mesoporous materials. Pitfalls and limitations in gas adsorption data analysis. Microporous Mesoporous Mater. 2003, 60, 1-17. (42) Qajar, A.; Daigle, H.; Prodanović, M., Methane dual-site adsorption in organic-rich shale-gas and coalbed systems. Int. J. Coal Geol. 2015, 149, 1-8. (43) Rouquerol, J.; Llewellyn, P.; Rouquerol, F., Is the BET equation applicable to microporous adsorbents? Stud. Surf. Sci. Catal. 2007, 160, 49-56. (44) Barrett, E. P.; Joyner, L. G.; Halenda, P. P., The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen Isotherms. J. Am. Chem. Soc. 2014, 24, 207-216. (45) Washburn, E. W., Note on a Method of Determining the Distribution of Pore Sizes in a Porous Material. Cambridge University Press: Cambridge, 1998; pp 115-116. (46) Loucks, R. G.; Ruppel, S. C., Mississippian Barnett Shale: Lithofacies and depositional setting of a deep-water shale-gas succession in the Fort Worth Basin, Texas. AAPG Bull. 2007, 91, 579-601. (47) Tian, H.; Pan, L.; Xiao, X.; Wilkins, R. W. T.; Meng, Z.; Huang, B., 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. 2013, 48, 8-19. (48) Jarvie, D. M.; Hill, R. J.; Ruble, T. E.; Pollastro, R. M., Unconventional shale-gas systems: The Mississippian Barnett Shale of north-central Texas as one model for thermogenic shale-gas assessment. AAPG Bull. 2007, 91, 475-499. (49) Wilkin, R. T.; Barnes, H. L.; Brantley, S. L., The size distribution of framboidal pyrite in modern sediments: An indicator of redox conditions. Geochim. Cosmochim. Acta 1996, 60, 3897-3912. (50) Wilkin, R. T.; Arthur, M. A.; Dean, W. E., History of water-column anoxia in the Black Sea indicated by pyrite framboid size distributions. Earth Planet. Sci. Lett. 1997, 148, 517-525. (51) Wignall, P. B.; Newton, R., Pyrite framboid diameter as a measure of oxygen deficiency in ancient mudrocks. Am. J. Sci. 1998, 298, 537-552.

ACS Paragon Plus Environment

Page 20 of 42

Page 21 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(52) Gallego-Torres, D.; Reolid, M.; Nieto-Moreno, V.; Martínez-Casado, F. J., Pyrite framboid size distribution as a record for relative variations in sedimentation rate: An example on the Toarcian Oceanic Anoxic Event in Southiberian Palaeomargin. Sediment. Geol. 2015, 330, 59-73. (53) Slatt, R. M.; O'Brien, N. R., Pore types in the Barnett and Woodford gas shales: Contribution to understanding gas storage and migration pathways in fine-grained rocks. AAPG Bull. 2011, 95, 2017-2030. (54) Rouquérol, J.; Avnir, D.; Fairbridge, C.; Everett, D.; Haynes, J.; Pernicone, N.; Ramsay, J.; Sing, K.; Ünger, K., Recommendations for the characterisation of porous solids. Pure & Appl. Chern. 1994, 66, 1739-1758. (55) Sing, K. S. W., Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984). Pure & Appl. Chern. 1985, 57, 603-619. (56) Kuila, U.; McCarty, D. K.; Derkowski, A.; Fischer, T. B.; Topór, T.; Prasad, M., Nano-scale texture and porosity of organic matter and clay minerals in organic-rich mudrocks. Fuel 2014, 135, 359-373. (57) Chen, C.; Hu, D.; Westacott, D.; Loveless, D., Nanometer-scale characterization of microscopic pores in shale kerogen by image analysis and pore-scale modeling. Geochem., Geophys., Geosyst. 2013, 14, 4066-4075. (58) Curtis, J. B., Fractured shale-gas systems. AAPG Bull. 2002, 86, 1921-1938. (59) Bernard, S.; Wirth, R.; Schreiber, A.; Schulz, H.-M.; Horsfield, B., Formation of nanoporous pyrobitumen residues during maturation of the Barnett Shale (Fort Worth Basin). Int. J. Coal Geol. 2012, 103, 3-11. (60) Chen, J.; Xiao, X., Evolution of nanoporosity in organic-rich shales during thermal maturation. Fuel 2014, 129, 173-181. (61) Pommer, M.; Milliken, K., Pore types and pore-size distributions across thermal maturity, Eagle Ford Formation, southern Texas. AAPG Bull. 2015, 99, 1713-1744. (62) Loucks, R. G.; Reed, R. M., Scanning-electron-microscope petrographic evidence for distinguishing organic matter pores associated with depositional organic

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

matter versus migrated organic matter in mudrocks. Gcags Transactions 2014, 3, 51-60. (63) Bustin, R. M.; Canada, G. A. O., Coal petrology its principles, methods, and applications. Geological Association of Canada: 1985. (64) Goodarzi, F., Optically anisotropic fragments in a Western Canadian subbituminous coal. Fuel 1985, 64, (9), 1294-1300. (65) Smyth, M., Organic petrology and clastic depositional environments with special reference to Australian coal basins. Int. J. Coal Geol. 1989, 12, 635-656. (66) Lawrence, G. P., Measurement of pore sizes in fine-textured soils: a review of existing techniques. Eur. J. Soil Sci. 1977, 28, 527–540. (67) Schmitt, M.; Fernandes, C. P.; da Cunha Neto, J. A. B.; Wolf, F. G.; dos Santos, V. S. S., Characterization of pore systems in seal rocks using Nitrogen Gas Adsorption combined with Mercury Injection Capillary Pressure techniques. Mar. Pet. Geol. 2013, 39, 138-149. (68) Chen, Q.; Zhang, J.; Tang, X.; Li, W.; Li, Z., Relationship between pore type and pore size of marine shale: An example from the Sinian–Cambrian formation, upper Yangtze region, South China. Int. J. Coal Geol. 2016, 158, 13-28. (69) Van Keer, I.; Muchez, P.; Viaene, W., Clay mineralogical variations and evolutions in sandstone sequences near a coal seam and shales in the Westphalian of the Campine Basin (NE Belgium). Clay Miner. 1998, 33, 159-169. (70) Huang, S. J.; Huang, K. K.; Feng, W. L.; Tong, H. P.; Liu, L. H.; Zhang, X. H., Mass exchanges among feldspar, kaolinite and illite and their influen ces on secondary porosity formation in clastic diagenesis-A case study on t he Upper Paleozoic,Ordos Basin and Xujiahe Formation, Western Sichuan Depression. Geochimica (Beijing, China) 2009, 38, 498-506 (in Chinese with English abstract).

ACS Paragon Plus Environment

Page 22 of 42

Page 23 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Energy & Fuels

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 .

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Page 24 of 42

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

(