Nanoscale Pore Changes in a Marine Shale: A Case Study Using

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Nanoscale pore changes in a marine shale: A case study using pyrolysis experiments and nitrogen adsorption Shangbin Chen, Zhaoxi Zuo, Tim A Moore, Yufu Han, and Clementine Uwamahoro Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01405 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 4, 2018

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Nanoscale pore changes in a marine shale: A case study using

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pyrolysis experiments and nitrogen adsorption

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Shangbin Chen a, b, Zhaoxi Zuo c, Tim A. Moore d, e, Yufu Han f, Clementine

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Uwamahoro a, b

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a. Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process

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of the Ministry of Education, China University of Mining and Technology, Xuzhou

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221116, China

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b. School of Resources and Geoscience, China University of Mining and Technology, Xuzhou, 221116, China c. School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China

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d. Queensland University of Technology, Brisbane, QLD 4001, Australia

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e. Cipher Consulting Ltd, 6 Stardust Street, Kenmore, Brisbane, QLD 4969, Australia

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f. Organci Geochemistry Section, GFZ German Research Centre for Geosciences,

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Potsdam D14473, Germany

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Abstract: Nanoscale pores have an important role in the accumulation of gas in shale

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gas reservoirs. Indeed, the formation of nanopores is critical for the characterization

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and evaluation of shale reservoir. Moreover, the effect of pyrolysis on the

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modification of nanopores is not clear. Therefore, this paper focuses on pyrolysis and

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nitrogen adsorption experiments to examine nanoscale pore structure and evolution in

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marine shale strata with low total organic carbon (TOC). All the examined samples

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contain micropores, mesopores, and macropores. The results show that the number of

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micropores increased as a result of artificial maturation (i.e. pyrolysis), which resulted

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in a significant increase in the surface area and the total pore volume. The openness of

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the pores significantly increased when the maturity was higher than 2.5%Ro (vitrinite

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reflectance). The 1.5-7.5 nm and 60-70 nm pores are the most pronounced to change

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after pyrolysis. Furthermore, liquid hydrocarbons produced during heating were

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shown to influence pores of approximately 41 nm width. In the over-mature stage (Ro

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= 2.77%), the numbers of pores and pore volume significantly increased during

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pyrolysis. The pore structure of the over-mature shale was different from that of the

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shale during the mature and high maturity stages. Pores less than 20 nm wide nearly

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provided 90% of the surface area and at least 50% of the pore volume. The

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transformation of organic matter from the solid state to the liquid and gas states is

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most closely related to the number of mesopores. The pores with sizes less than 10 nm

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in width have the greatest change in the proportion of surface area to pore volume

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with increasing maturation.

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Keywords: nanoscale pore evolution; pyrolysis; thermal maturity; low TOC marine

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shale; China

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1. Introduction

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The general burial depth of shale gas reservoirs in the United States does not

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exceed 3000 m, and typically ranges between 800 and 2600 m.1-3 Conversely, the

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burial depth of Paleozoic shale gas reservoirs lies between 3000 and 4000 m in the

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Sichuan Basin of China; the maximum burial depth of these reservoirs is believed to

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exceed 7000 m.4, 5 Paleozoic shale gas reservoirs in southern China are generally in


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the high- and over-maturity stages. Unfortunately, the pore structure and capacity of

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these reservoirs are not well understood.6-13 Research has shown that high maturity

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leads to nanoscale pores, which can improve shale gas storage capacity.14-21 Due to

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significant heterogeneity in shale gas reservoir, the mineral composition and pore

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structure vary at a nanometric to macrometric scale.22

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Pore structure in shale gas reservoirs is primarily influenced by the thermal

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maturation processes of organic and inorganic materials. The frequency of pores in

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organic matter varies depending on the level of thermal maturity.23-27 Moreover, the

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pores in organic matter are well developed when the maturity is greater than 0.9% Ro

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(vitrinite reflectance), which indicates relatively high pore volume.28 However, there

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is a non-monotonic evolution trend in pore volume during shale maturation processes

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29

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with increasing maturity.25, 27 The increase in the number of pores within organic

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matter is closely related to the initial hydrocarbon generation and secondary cracking

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of organic matter. As the maturity of organic matter reaches the gas generation

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window, porosity continues to increase and is characterized by isolated and complex

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mesopores and macropores.30

; indeed, the pore size and quantity of organic matter did not significantly increase

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Pyrolysis studies on organic-rich shales showed that a large number of nanopores

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are formed from organic matter as maturity increases.27, 31-41 With increasing pyrolysis

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temperature, the number, volume, and surface area of micropores, mesopores, and

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macropores increase in organic-rich shales.36 The number of nanoscale pores in

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organic-rich mudstone and shale increases with increasing temperature and pressure,


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and the peak temperature is consistent with the yield peak temperature of gaseous and

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liquid hydrocarbons. In contrast to micropores, the number of mesopores and

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macropores decrease with increasing temperature and pressure.40 Therefore, the

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processes concerning the evolution of pores are debatable among researchers.36, 40 The

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variation in number and structure-type of pores less than 100 nm in width is more

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complex than those having a width superior to 100 nm.40 Whether the evolution of

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organic matter is too high obscures other phenomena. In low TOC shales, the change

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in nanoscale pores as a function of thermal maturity is not well understood.

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Consequently, and in order to understand nanoscale pore evolution, low TOC marine

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shale samples were collected and pyrolysis experiment, and nitrogen adsorption

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experiments were conducted.

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2. Materials and methods

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2.1 Samples and geologic background

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The Xiamaling Formation (~1.37Ga), located in North China, is characterized by

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black shales of relatively low thermal maturity (Tmax is 445°C) and has been identified

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as a potential petroleum source rock.42 We selected the Xiamaling Formation marine

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shales from the Zhaojiashan profile in the Xiahuayuan area in Zhangjiakou city, Hebei

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Province in China, which is located in the Yanshan Meso-Neoproterozoic rift basin.

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The Yanshan rift basin is an active tectonic unit on the North China platform, also

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known as the "Yanshan subsidence zone", which has an area of 80,000 km2 (Figure

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1a).43 The Yanshan subsidence zone is divided into 7 structural units including two

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uplifts and five depressions, which are the Shanhaiguan uplift, the Mihuai uplift, the


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Xuanlong depression, the Jingxi depression, the Liaoxi depression, the Jibei

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depression, and the Jidong depression (Figure 1b). The study area is located in the

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Xuanlong depression.

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The strata within the study area belong to the Yanshan stratigraphic division of

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the North China-type stratigraphic area. The stratigraphy, from the Archaean to the

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Cenozoic include the Archaean Qianxi Group, the Middle Proterozoic Changcheng

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System and Jixian System, the Upper Proterozoic Qingbaikou System, the Cambrian

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System of the Paleozoic, the Jurassic System of the Mesozoic, and the Quaternary

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system of the Cenozoic (Figure 1c). The Meso-Neoproterozoic strata are divided into

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12 groups of 3 series (from bottom to top), and overlie the Archean Qianxi Group in

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an angular unconformity, with the top covered by Lower Cambrian strata (Figure

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2a).43 The Xiamaling Formation belongs to the Upper Proterozoic Qingbaikou System,

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which unconformably sits atop the Tieling Formation.44 The Xiamaling Formation

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mainly consists of shale and is divided into four lithological members; moreover, the

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Xiamaling Formation ranges from 135 to 335 m in thickness in the Yanshan region.

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Member 1 is approximately 145 m in thickness and is composed of gray-black shale,

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yellow-green foliated sandy shale, and variegated shale containing iron concretions.

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Member 2 is approximately 66 m in thickness and is composed of purple and green

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mudstone, dark-green and turquoise sandstone, and purple shale with yellow-green

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shale interbedded near the top. Member 3 is about 250 to 280 m in thickness and is

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composed of gray-black siliceous shale in the lower part and interbedded

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yellow-green shale and black shale intercalated with sandstone lenses in the upper


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part. Member 3 is characterized by high TOC content black shales and has been

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identified as potential oil source rocks.42,

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thickness and is composed of gray and dark gray mudstone, laminated limestone, and

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gray-green shale and contains abundant stromatolites (Figure 2b).

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Member 4 is approximately 60 m in

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The Xiamaling Formation was deposited in a marine environment and contains

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type I organic matter with TOC contents between 1% and 5%.45-49 The organic matter

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maturity of the Xiamaling Formation in the Xiahuayuan region is reported to be in the

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range of 0.6%-0.7%,50 0.46%-0.76%,51 and 0.6%;52 the reported values indicate low

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maturity. Therefore, the Xiamaling Formation is suitable as a sample for pyrolysis

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experiments.43, 52

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The original block sample from the third member of the Xiamaling Formation

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was collected from a fresh profile (the Zhaojiashan profile) exposed by excavation

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and not affected by oxidation (Figure 1 and Figure 2). The Zhaojiashan profile shows

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that the Xiamaling Formation in this region is extensively developed. The Xiamaling

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Formation is relatively shallow with a present burial depth of 0-1000 m. Therefore,

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vitrinite reflectance (Ro) for organic matter in the Xiamaling Formation ranges from

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0.6% to 0.7%. However, there is a large igneous body exposed in the study area that

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may have affected the maturity of the organic matter. Eight cylindrical samples, of 2.5

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mm × 50 mm dimensions, were drilled from the collected block sample, which

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were labeled A0 as the original samples. Of the 7 original samples A0 were then

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subjected to different pyrolysis conditions; the products were labeled A1, A2, A3, A4,

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A5, A6, and A7.


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2.2 Experimental analysis and methodology

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The pyrolysis experiments were conducted in the Lanzhou Institute of Geology,

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Chinese Academy of Sciences (LIGCAS). The sample was subjected to different

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lithostatic and hydrodynamic pressures associated with burial by a WYMN-3 HTHP

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instrument (developed by Wuxi Institute of petroleum geology, China Petroleum and

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Chemical Co., LTD). The instrument maintains isothermal heating of the samples by

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automatic pressure compensation and eventual expulsion of hydrocarbons

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immediately after the pyrolysis temperatures are reached. The samples were treated

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for 24 hours at the temperatures and pressures included in Table 1. Upon the

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completion of the experiment, the expelled oil, water, and gaseous hydrocarbons were

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collected and analyzed, as detailed by Zhong et al. 53 In order to ensure full pyrolysis,

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the samples were placed in the reactor for a full 24 hours. The experimental

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parameters were designed considering the current main burial depth and the

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hydrostatic pressure of shale gas reservoir of the Sichuan Basin (Table 1).43, 54, 55 The

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rock and fluid densities are 2.6 g/cm3 and 1.0 g/cm3, respectively. The 7 original

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samples (A0) were heated to 350, 400, 420, 450, 480, 520, and 550°C.

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The Average pore size, pore size distribution, and surface area were analyzed via

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nitrogen adsorption. The tests were carried out using a Quadrasorb surface area and

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pore size analyzer; the Quadrasorb instrument uses the static volumetric method.

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After crushing and splitting, samples were degassed in vacuum with temperatures less

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than 150°C for 4 hours (vacuum to 266.664 Pa). The isothermal adsorption-desorption

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experiment was carried out at -195.8°C (77.3K) with pure nitrogen gas. The


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instrument’s aperture detection ranges from 0.35 to 500 nm; the relative pressure

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ranges from 0.001 to 0.998. Density functional theory (DFT) analysis and multi-point

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Brunauer Emmett Teller (BET) analysis regression models were used to acquire the

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pore size distribution and specific surface.56

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In this study, the investigated geochemical parameters include the organic matter

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richness (present-day TOC) and thermal maturity. The TOC was determined by a

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carbon and sulfur analyzer. Thermal maturity was estimated using optical microscopy

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and spectroscopy. Moreover, vitrinite reflectance is the most common optical method

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used; it is performed through microscopic inspection of kerogen and an analysis of the

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reflectivity of the particles via a photomultiplier.57 The vitrinite-like material

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reflectance (Rom) is derived in the marine sediments below the Permian strata.58 The

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vitrinite-like material reflectance can indicate the Early Paleozoic maturity index.57

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Furthermore, Zhong et al.60 established the correlation between the vitrinite-like

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material reflectance (Rom) and the equivalent vitrinite reflectance (Ro). The correlation

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between Rom and Ro is shown in the following equations:

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Ro = 1.042 Rom + 0.052 (0.30% < Rom < 1.40%)

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Ro = 4.162 Rom - 4.327 (1.40% ≤ Rom < 1.60%)

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Ro = 2.092 Rom - 1.079 (1.60% < Rom < 3.0%)

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Using the above arithmetic correlations, the Rom can be converted into equivalent

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Ro, which has been used for reservoir evaluation. The correlation between maximum

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Rom and random vitrinite-like reflectance (Rran,o) is given by Rom = Rran,o

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58 value)*1.064.

(mean

Laser Raman spectroscopy was performed with a Raman spectroscopy


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RmcRo

= 0.0537×d(G-D)-11.2161 (where

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Senterra instrument, using the equation:

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RmcRo

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the Raman method is more accurate in the high and over- mature stage, 62 and can be

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used as a verification method for optical light microscopy tests.

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3. Results

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3.1 Maturity and total organic matter content

is the value tested from the Laser Raman method). It is important to note that

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The results of vitrinite reflectance and laser Raman spectroscopy are given in

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Table 2. The vitrinite reflectance values of the samples A0, A1, A2, A3, A4, A5, A6, and

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A7 are 0.69%, 0.93%, 1.12%, 1.50%, 1.92%, 2.23%, 2.48%, and 2.77%, respectively.

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Vitrinite reflectance values indicate that A1 and A2 are in the mature thermal stage

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(middle catagenesis stage; 0.7% < Ro < 1.3%), A3 and A4 are in the high maturity

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thermal stage (later period of catagenesis stage; 1.3% < Ro < 2.0%), and A5, A6, and

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A7 are in the over-mature thermal stage (metagenesis stage; Ro > 2.0%).54 The

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indicated levels of thermal maturity suggest that A5, A6, and A7 are in the (dry) gas

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generation window. Moreover, Laser Raman spectroscopy yielded similar results to

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the reflected light microscopy, especially in the high and over-mature thermal stages.

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The TOC values after pyrolysis range from 0.56% to 1.30% (Table 2). The TOC

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values decrease as the Ro values increase; this is explained by the conversion of some

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organic matter into hydrocarbons as a result of heating. Nonetheless, the decrease in

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TOC is not linear with the increase of the pyrolysis temperature.


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3.2 Pore structure parameters

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The values for multi-point BET (MBET) surface area, total pore volume, and

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average pore diameter for A0 are 8.2 m2/g, 0.019 ml/g, and 9.1 nm, respectively. The

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pore structure parameters of the other samples (A1-A7) are given in Table 3. The

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surface area values range from 5.9 to 12.9 m2/g, with an average of 9.6 m2/g. The

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surface area fluctuated in a ‘w’-like pattern with increasing maturity (Figure 3a). The

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total pore volume values range from 0.016 to 0.033 ml/g, with an average of 0.023

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ml/g. The total pore volume, similar to the surface area, also fluctuated in a ‘w’-like

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pattern as a function of increasing maturity (Figure 3b). The pores that formed by

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pyrolysis are mainly related to the changes in the shale composition, especially by the

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evolution of organic matter. The TOC content of the sample is small, so the number of

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pores formed by the organic matter is limited. Similarly, the pore volume formed by

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the hydrocarbon generation effect is restricted due to the constraint of pressure.

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Therefore, the pore volumes of A1-A7 are close to that of A0. The average pore

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diameters range from 7.4 to 12.3 nm, with an average of 9.9 nm. With increasing

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maturity, the average pore diameters also fluctuated in a pattern roughly opposite to

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the trend of the surface area and total pore volume (Figure 3c). Moreover, there is a

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positive linear correlation between the total pore volume and the maximum

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hydrodynamic pressure (R2 = 0.5446) (Figure 3d). The pore volume of A6 is less than

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those of the other samples. The degree of condensation and aromatization of kerogen

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increases and superimposed pressure after the over-mature stage is reached; this

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results in the compaction of the mineral pores and the inhibition of hydrocarbon


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generation, which leads to the reduction in the total pore volume. Then, with further

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increase in pressure, the pores are damaged, and the conduction and adjustment of

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pores, especially mesopores, significantly increase; this causes the total volume of the

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pores to increase again.

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4. Discussion

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4.1 Conversion rate of organic matter

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The Ro increased from 0.69% to 2.77% when the temperature increased from 350

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to 550°C. The organic matter is converted to liquid and gaseous hydrocarbons;

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therefore, the TOC contents of the samples A1, A2, A3, A4, A5, A6, and A7 decreased to

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1.30%, 1.05%, 0.56%, 1.09%, 1.13%, 0.9%, and 0.96%, respectively (Figure 4a).

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From these results, the conversion rate of the organic matter Cn (where n = 1, 2, 3, 4, 5,

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6, and 7) can be defined by:

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Cn = (TOCA0-TOCAn)/TOCA0*100

(1)

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The calculated conversion rates of the seven samples (A1-A7) are thus 5.11%, 23.36%,

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59.12%, 20.44%, 17.52%, 34.31%, and 29.93%, respectively (Figure 4b). The results

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showed the absence of a significant positive linear correlation between conversion

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rate and temperature. Different temperatures and pressure conditions have different

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organic conversion rates. The conversion rate of A3 showed to be the highest

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(59.12%). At this high maturity thermal stage (late catagenesis stage), oil generation

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and oil cracking simultaneously occur; therefore, the residual organic matter in A3 is

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the lowest.

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With increasing thermal maturity, there exists a roughly linear correlation


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between surface area on one hand and TOC (Figure 4c) and TOC conversion rate (Fig.

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4e) on the other. Similarly, there exists a linear correlation between the total pore

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volume on one hand and TOC (Figure 4d) and TOC conversion rate on the other (Fig.

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4f). The increase in the trend of the pore volume is weaker than that of the surface

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area (Figure 4e and f). However, the evolution of organic matter produces a large

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number of micropores15-20, which significantly increases the specific surface area. Due

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to the pressure effect in our case, the increase in pore volume is limited. If the

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parameters of A0 were used as reference values, the TOC conversion rates of A3 and

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A4 would be 59.12% and 20.44%, respectively; moreover, the surface area rates of

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change of A3 and A4 would be 48.90% and 57.63%, respectively. We also defined the

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conversion rate of surface area Surfacen (n = 1, 2, 3, 4, 5, 6, and 7) as:

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Surfacen = (SurfaceA0-SurfaceAn)/SurfaceA0*100)

(2)

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All these characteristics suggest that in the process of organic matter generation, the

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sizes of micropores increase, which results in a disproportionate increase in the

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surface area with respect to the total pore volume.

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4.2 Pore structure characterization

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Pore characteristics can be estimated by using nitrogen adsorption and desorption

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isotherms (Figure 5a-i). All the curves were similar to TypeⅡisotherm according to

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the BET classification. Although there are differences among adsorption isotherms,

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they generally formed reverse “S” shaped curves. At a low-pressure stage (p/p0 =

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0-0.2), the volume of adsorption slowly rises; this indicates that the adsorption

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process is both monomolecular and polymolecular. During the intermediate stage


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(p/p0 = 0.2-0.8), the adsorption volume slowly increases with increasing pressure; this

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suggests that the adsorption process is multimolecular. Finally, during the last stage

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(p/p0 = 0.8-1.0), the adsorption volume sharply rises, but the samples were not close to

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saturation adsorption despite the relative pressure reaching the saturated vapor

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pressure; this implies the presence of mesopores and macropores in the sample.

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Hysteresis loops were detected in all the samples (Figure 5). Hysteresis loops are

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formed from mismatching between adsorption and desorption isotherms in the

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high-pressure stage (p/p0 = 0.4-1.0). Furthermore, hysteresis loops indicate that the

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samples contain open pores. Different hysteresis loop types reflect the shape and

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characteristic of the pore structure. Type A hysteresis is attributed to cylindrical pores;

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type B hysteresis is associated with slit-shaped pores; type C hysteresis is attributed to

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wedge-shaped pores with open ends; type D hysteresis loops result from

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wedge-shaped pores with narrow necks at one or both open ends; and type E

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hysteresis loop is attributed to “ink-bottle” pores.63 With the exception of the presence

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of micropores, the hysteresis loops in all the isotherms close before reaching a relative

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pressure of 0.3 in the desorption process. Indeed, the fact that all the hysteresis loops

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are not closed in our case indicates that the samples contain micropores.63, 64

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All the adsorption isotherms are steep near the saturated vapor pressure and at

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moderate pressure (Figure 5). This feature is similar, but not identical, to the class B

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hysteresis loop provided by De Boer; 61 moreover, it is also similar to the H3-type and

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H4-type hysteresis loops recommended by IUPAC. The characteristics of the

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hysteresis loops indicate that the pore structure of the shale is relatively complex and


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is composed of nanopores with a semi-amorphous structure. Pores have peculiar

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shapes and mainly consist of cylindrical pores with two open ends and parallel-plate

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pores with four open sides. However, the shapes of the pores vary among samples, as

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indicated by the different hysteresis loops. The hysteresis loops of A0 and A1 are

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narrower than those of A4, A5, A6, and A7, which suggest that the wedge-shaped pores

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are changed to slit-shaped pores upon increasing the thermal maturity (Figure 5b and

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f-i).

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The degree of pore openness is positively correlated with the slope of the

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adsorption lines.63 The slope of the adsorption line of all the samples, except A1,

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increased with increasing thermal maturity (Figure 5j). The slope of the adsorption

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line A1 decreased because organic matter is consumed in the process of hydrocarbon

296

generation during maturity; 27, 28, 30 thus, liquid hydrocarbons are mainly produced,

297

which plug some of the pores and consequently limit pore openness. The A7 isotherm

298

showed an increase in the slope (Figure 5j), indicating that there are more open pores

299

in comparison to the other samples. The openness of the pores is significantly

300

increased when the maturity is greater than 2.5%.

301

4.3 Nanoscale pore size distribution

302

Pore volume distribution was obtained by the DFT method (Figure 6). The DFT

303

model is suitable for calculating the pore size distribution in mesopores and

304

micropores.64, 65 Moreover, each peak on the graph represents a proportion of the

305

pores. Five features can be seen in figure 7. Firstly, the peaks are divided into two

306

types, which are consistent and inconsistent peak shapes. The first inconsistent peak


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shape consists of multiple peaks; the peaks are located between 1.5 and 7.5 nm

308

(Figure 6a). The second inconsistent peak shape is located between 60 and 70 nm

309

(Figure 6b). In the other regions, the pore widths and peak shapes were consistent.

310

Secondly, there is a distinct peak at 1.5 nm, indicating that a large number of pores

311

have a width less than 1.5 nm. Thirdly, there are multiple peaks between 2 and 50 nm,

312

indicating that the pores within this range are predominantly present in the samples.

313

Fourthly, A7 has markedly larger pores in comparison to the other samples; indeed, A7

314

indicated an over-mature stage. Furthermore, A7 showed the maximum ordinate

315

change in the pore width range between 9.5 and 70 nm (Figure 6a). Finally, A1’s

316

ordinate is less than that of A0 when pore width was below 41 nm; this changed after

317

41 nm (Figure 6c). All these features suggest the predominance of pores of width less

318

than 10 nm and the high sensitivity of pore width to temperature. Changes in pore

319

structure were observed in the width range of 1.5-7.5 nm and 60-70 nm. The liquid

320

hydrocarbons produced during heating influence the pores with 41 nm width. In the

321

over-mature stage (A7, Ro = 2.77%), the distribution of pore structure does not vary,

322

but the number of pores and pore volume significantly increase. This indicates that the

323

pore structure of the over-mature shale is different from that of the mature and high

324

maturity shale. The peaks indicate that the pores in this region occupy a certain

325

proportion, and the differences in peak numbers and peak maxima can reflect the

326

differences in the distribution of pores at each region. Since the samples A1-A7

327

evolved from the original sample (A0), the pore structures are similar at the

328

macroscopic scale. Nonetheless, there are differences at the microscopic level due to


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329

changes in the composition of materials caused by pyrolysis. Additionally, changes in

330

the pore structure are concentrated in the two regions, 1.5-7.5 nm and 60-70 nm,

331

indicating that the most prominent changes caused by pyrolysis mainly occur in these

332

two pore intervals.

333

4.4 Evolution of nanoscale pore surface area and volume

334

The surface area and total pore volume vary with increasing maturity (Figure 3

335

and Figure 8a and b). The sizes of the micropores, mesopores, and macropores are 50 nm, respectively. The trends in micropore and mesopore surface area

337

are similar to that of the total surface area, while the trend in volume within

338

mesopores is similar to that of the total pore volume (Figure 7a and b). The

339

proportions of micropore surface area range between 8.3% and 13.2% when the

340

maturity is less than 1.2% (i.e. for A0, A1, and A2; Figure 7c). The proportions of

341

micropore surface area range between 30.9% and 36.2% when the maturity is greater

342

than 1.2% (i.e. for A3, A4, A5, A6, and A7). Moreover, the proportion of mesopore

343

surface area varies between 84.7% and 88.9% when the maturity is less than 1.2%.

344

When the maturity is greater than 1.2%, the proportions of mesopore surface area

345

fluctuate between 20.3% and 67.3%. The proportions of macropore surface area do

346

not change when the maturity ranges between 1.1% and 2.8%. Furthermore, there is a

347

correlation between the trends of the proportions of the surface area of micropores

348

and mesopores. However, the distribution of the total pore volume is different from

349

that of the surface area (Figure 7c and d). Mesopores and macropores account for

350

80.9%-85.9% and 10.5%-16.3% of the total pore volume, respectively (Figure 7d).


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These percentages hardly vary with increasing maturity. When maturity is greater

352

than 1.2%, the proportions of micropore volume account for 0.7%-2.1%; when

353

maturity exceeds 1.2%, the proportions of micropore volume increase to 3.9%-5.9%

354

(Figure 7d).

355

To further discuss the variation in the distribution of micropores, mesopores, and

356

macropores in the context of the total surface area and total pore volume, we defined

357

the coefficient K An (n = 1, 2, 3, 4, 5, 6, and 7) as:

358

K An = (proportion of An-proportion of A0/proportion of A0*100)/ conversion rate

359

of TOC An *100

(3)

360

where “proportion A0” is the ratio of pore volume (or surface area) of micropores,

361

mesopores, or macropores to the total pore volume (or surface area) of the sample A0;

362

“proportion An” is the ratio of pore volume (or surface area) of micropores, mesopores,

363

or macropores to the total pore volume (or surface area) of the sample An.

364

The coefficient KAn reflects the consistency of pore change and the

365

transformation of organic matter. Organic matter transformation into one kind of pore

366

is reflected by stable K values. Figure 7 (e and f) shows that the K values of mesopore

367

surface area and volume are the most stable, indicating that organic matter

368

transformation mainly concern mesopores.

369

Distribution histograms (Figure 8a and b) were generated for surface area and

370

pore volume using < 10, 10-20, 20-30, 30-40, 40-50, 50-60, 60-70, and > 70 nm width

371

sizes. The pore surface area is predominantly distributed in pores inferior to 20 nm in

372

size. The proportions of surface area within the < 10 and 10-20 nm pores are in the


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373

range of 68.8%-84.6% and 8.7%-16.8%, respectively; the mean values are 76.0% and

374

13.3% for the pore sizes < 10 nm and 10-20 nm, respectively. Furthermore, the

375

proportions of pore volume range between 27.5% and 44.3% for pore sizes inferior to

376

10 nm (with an average of 34.8%), and between 18.5% and 23.8% for pore sizes

377

10-20 nm (with an average of 21.2%). Interestingly, pores less than 20 nm in width

378

account for approximately 90 and 50% of the total surface area and total pore volume,

379

respectively. During maturation, pores less than 10 nm wide experience the most

380

change. Finally, the correlation between maturation and surface area of pores less than

381

10 nm in width is opposite to that of larger pores (Figure 8c and d).

382

5. Conclusions

383

During heating, the number of micropores increases, which results in a

384

significant increase in the surface area. Isotherm and hysteresis loops indicate that

385

samples contain nanoscale pores as well as micropores, mesopores, and macropores.

386

The number of open pores significantly increases when maturity is greater than 2.5%.

387

Pores between 1.5 and 7.5 nm and between 60 and 70 nm wide are most sensitive to

388

pyrolysis. Liquid hydrocarbons are produced in pores approximately 41 nm wide. In

389

the over-mature stage (Ro = 2.77%), the distribution of pore structure does not vary;

390

yet, the number of pores and pore volume significantly increases, which indicates that

391

the pore structure of the over-mature shale is different from that of the mature and

392

high maturity shales. Micropores and mesopores are the main contributors to surface

393

area; the latter is the main contributor to the total pore volume. Moreover, pores less

394

than 20 nm wide nearly provide 90% of the total surface area and exceed 50% of the


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total pore volume. The transformation of organic matter is notably related to

396

mesopores. The proportions of surface area and pore volume for pores less than 10

397

nm show the most change after pyrolysis. Finally, pores less than 10 nm in width

398

change in size according to thermal maturation, while larger pores change in an

399

opposite manner.

400

Acknowledgements

401

The authors would like to give sincere thanks to the funding agencies that

402

supported this research. This work was supported by the National Natural Science

403

Foundation of China (No. 41772141; No. 41402124), the Fundamental Research

404

Funds for the Central Universities (2017CXNL03), and the Priority Academic

405

Program Development of Jiangsu Higher Education Institutions (PAPD).

406

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