Factors influencing the specific surface areas of argillaceous source rocks

Applied Clay Science 109–110 (2015) 83–94

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Applied Clay Science journal homepage: www.elsevier.com/locate/clay

Research paper

Factors influencing the specific surface areas of argillaceous source rocks Xiaojun Zhu a, Jingong Cai a,⁎, Guoqi Song b, Junfeng Ji c a b c

State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China Shengli Oilfield Company, SINOPEC, Dongying 257015, China Key Laboratory of Surficial Geochemistry, Ministry of Education, Nanjing University, Nanjing 210093, China

a r t i c l e

i n f o

Article history: Received 17 May 2014 Received in revised form 2 February 2015 Accepted 9 February 2015 Available online 26 March 2015 Keywords: Argillaceous source rock Specific surface area Influence factor Significance for unconventional petroleum

a b s t r a c t This paper integrates methods of nitrogen adsorption (N2-BET) and ethylene glycol monoethyl ether (EGME) to present a comprehensive understanding of the specific surface areas (SSAs) of bulk rocks and clay fractions (b 2 μm) in argillaceous source rocks, and employs the X-ray diffraction method to discuss the characteristics and influence factors of SSAs of argillaceous source rocks in order to service for unconventional petroleum exploration and exploitation. The methods of N2-BET and EGME can be used to obtain external surface area (N2-BET SSA) and total surface area (EGME SSA) within argillaceous source rock, respectively, and the difference between the total and the external surface area yields the internal surface area (internal SSA). The internal SSA accounts for over 80% in EGME SSA. Silty mudstone and argillaceous siltstone have larger internal and external surface areas than mud-bearing siltstone. Further, clay minerals (smectite in particular) make the greatest contribution to the internal, external and total surface areas (especially the internal and total surface areas) of source rock, whereas detrital minerals and carbonate minerals have a negative effect on each specific surface area. What is noteworthy is the fact that an abrupt increase in carbonate mineral content results in remarkable decreases in internal and EGME SSAs. These features therefore indicate that the differences in rock constituents (clay, detrital minerals and carbonate minerals) and mineral compositions affect the variations in SSAs characteristics of argillaceous source rocks. By plotting the SSAs vs. depth in bulk rocks and clay fractions, the variation extent of SSAs in clay fractions is better than that in bulk rocks. For either bulk rocks or clay factions, the value of internal SSA/N2BET SSA changes abruptly from 20 in the shallow-part (above 1500 m) to 5–10 in the deep-part (below 2000 m). The evolution of minerals during diagenesis creates variability in the mineral content with burial depth, which causes a subsectional evolution of SSAs in the vertical. As the surfaces within argillaceous source rock are closely correlated with the occurrence of organic matter or hydrocarbon, analyses of the difference between internal surface area and external surface area give the result that the SSAs of argillaceous source rocks are influenced by the factors of rock types, mineral compositions, diagenesis, etc., so attention to specific mineral assemblage and burial depth are of great significance to enhance the success rate of petroleum exploration and development in argillaceous source rocks, particularly in the research of unconventional petroleum systems. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Specific surface area (SSA) is one of the important attributes of sediments or sedimentary rocks. The surface within sediments or sedimentary rocks, which is measured as SSA, is the principal place for the materials of the inorganic phase to interact with other phases such as organic matter, CO2 and nuclear waste (Mayer, 1994a; Kennedy et al., 2002; Iglesias and Quiñones, 2008; Eseme et al., 2012), and it also plays an important role in organic matter accumulation and hydrocarbon occurrence in argillaceous source rocks. Previous studies have shown that the content of total organic carbon (TOC) is positively correlated with SSA in soils, surface sediments and black shale (Weiler and Mills, 1965; Suess, 1973; Tanoue and Handa, 1979; Mayer, 1994b; ⁎ Corresponding author. E-mail address: [email protected] (J. Cai).

http://dx.doi.org/10.1016/j.clay.2015.02.016 0169-1317/© 2015 Elsevier B.V. All rights reserved.

Mayer and Xing, 2001; Kennedy et al., 2002; Kennedy and Wagner, 2011), whereas the fine-grained minerals, which are the principal contributor to the SSAs in sediments and sedimentary rocks, are of extraordinary significance to the organic matter enrichment (Weiler and Mills, 1965; Jenkinson and Rayner, 1977; Bergamaschi et al., 1997; Keil et al., 1998; Mayer, 1999; Dexter et al., 2008). In source rocks, over 70% of organic matters are found to be combined with fine-grained minerals as soluble organic matter (Guan et al., 1998; Cai et al., 2010; Ding et al., 2011), which indicates that the clay fractions in the source rocks are more important to the enrichment of organic matter. In unconventional petroleum systems such as shale gas, in which adsorption and dissociation are the main ways for hydrocarbon occurrence, hydrocarbon is adsorbed on the surface of clay mineral, organic matter and pores with adsorption capacity ranging from 40% to as high as 85% (Curtis, 2002; Mavor, 2003; Montgomery et al., 2005). From the close relationship between organic matter (or hydrocarbon) and SSA (or clay

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fractions), it can be seen that the SSA plays an important role in the organic phase enrichment in sediments or sedimentary rocks. Source rocks or shale include different types of minerals, and different minerals have different crystal attributes, which makes an impact on the SSAs. For instance, smectite with its 800 m2g− 1 of SSA has a large internal surface area, whereas illite with its 30 m2g−1 of SSA only has an external surface area (Murray and Quirk, 1990; Goldberg et al., 2001; Zhu and Cai, 2012). So it is important to focus on the variation in the mineral assemblage in source rocks to characterize the SSAs. Meanwhile, the surface adsorption within the source rock can be divided into physisorption and chemisorption (Wypych and Satyanarayana, 2004; Nilsson et al., 2011). It is generally believed that the physisorption mainly occurs on the external surface and chemisorption predominately on the internal surface. Therefore, distinguishing the characteristics of different SSAs is of great significance for understanding the adsorption characteristics of organic matter and hydrocarbon. In SSA measurement of sediments or sedimentary rocks, different methods such as the solution adsorption method or gas adsorption method can be used separately based on the complexity of the SSA constituent (internal surface area and external surface area) (Cerato and Luteneggerl, 2002; Santamarina et al., 2002; Kaufhold et al., 2010). However, each type of method has its own limitations in the SSA measurement (Yukselen and Kaya, 2006; Arnepalli et al., 2008; Zhu and Cai, 2012). Only by using effective methods can the features of specific surface areas of source rocks be understood systematically. This paper presents a discussion on the features, patterns of development and influence factors of specific surface areas within argillaceous source rocks. Samples of bulk rocks and their clay fractions (b 2 μm) were taken from the Tertiary of well S in Dongying Sag (Bohai Bay Basin, eastern China), and employed methods of physisorption (the nitrogen adsorption method) and chemisorption (the ethylene glycol monoethyl ether method) for SSAs measurement and X-ray diffraction for the content of mineral compositions. The investigation is of great significance to study the characteristics of organic matter and hydrocarbon occurrence in unconventional petroleum systems. 2. Materials and experiments 2.1. Samples A total of 29 core samples of argillaceous source rocks (TOC ranged from 0.03% to 4.01%, with 1.49% on average) have been selected for testing, which were taken by sealed coring at depths of 1000–4000 m in the formations of Guantao (Ng, Miocene), Dongying (Ed, Oligocene) and Shahejie (Es1/2/3, Oligocene) in well S of Dongying Sag, Bohai Bay Basin of eastern China. As there is no unified criterion to distinguish different rock types, many criteria have been established based on different aspects, such as texture, composition and structure (Liu et al., 2001; Jiang, 2003; Hickey and Henk, 2007; Loucks and Ruppel, 2007). In our work, the Bohai Bay Basin in eastern China is a terrestrial basin that is near the source, and thus it contains much more terrigenous detrital minerals. For this study, which is focused on the SSA of minerals, all the samples therefore can be categorized into three rock types based on the content of detrital minerals (Lu and Sang, 2002), which were 25–50% as silty mudstone, 50–75% as argillaceous siltstone, and 75–95% as mud-bearing siltstone. The well S is located on a structural high in the Lijin sub-sag of Dongying Sag. Dongying Sag is one of the secondary tectonic units of the Jiyang depression, which is a Meso–Cenozoic rift basin developed from the paleotopographical background of Paleozoic basement rocks (Zhu et al., 2004). Four geological evolution stages occurred in the Dongying Sag, including the early stage of rifting (the Kongdian Formation), the middle stage of rifting (the lower-middle Shahejie Formation), the late stage of rifting (the upper Shahejie Formation–Dongying Formation), and the post-rifting stage (Neogene) (Zhu et al., 2004). Since the Tertiary, the Sag has developed a Kongdian Formation with intermittent

lacustrine sediments and a Shahejie-4 Formation with salt and saline lacustrine depositions (Zhang et al., 2011). In terms of the samples, the lower Es3 is a deep lake facies; during the sedimentation of the middle Es 3 , the Dongying delta developed rapidly toward the depocenter of the basin; until the late Es3, fluvial-delta depositions covered most of the Sag (Zhu et al., 2004). Afterward, Es2 deposited as a river–delta facies, and Es 1 deposited as a lake deposition (Zhang et al., 2005). The samples were crushed to less than 3 mm and split into several aliquots: The first was an original sample, the second was cleaned by flushing with chloroform solvent for 72 h to remove organic matter (TOC ranged 0.01–2.36% with 0.98% on average after chloroform treatment) and dried naturally, and the third was crushed and ground for the clay fraction (b2 μm) separation using the natural sedimentation method. The chloroform-treated bulk samples and clay fractions were prepared for SSA measurement, and the original samples (bulk rocks and clay fractions) were made for X-ray diffraction. 2.2. Experimental methods 2.2.1. SSA measurement The nitrogen adsorption (N2-BET) method (Brunauer et al., 1938; Santamarina et al., 2002) and ethylene glycol monoethyl ether (EGME) method (Carter et al., 1965, 1986; Cerato and Luteneggerl, 2002) were employed separately to test the source rock and clay fraction to obtain the SSA: N2-BET method: It is used to measure the external surface area (N2-BET SSA) by using the ASAP 2010 M + C automatic SSA instrument of Micromeritics in the USA, under the condition of liquid nitrogen temperature (77 K). The procedure is to take 0.1–0.3 g of sample and put it in the oven with 80 °C for 6 h or longer, then take out the sample and put it in the sample tube for predegassing, set the temperature for the heating bag to 80 °C and degass for not less than 6 h. The sample was weighed again before it was moved to the measurement device, and the test software was run according to the standard measurement program (Webb and Orr, 1997); in addition, the N2-BET SSA of the sample was calculated by the multipoint BET method (thirteen points of relative pressure (P/P 0 ) ranging from 0.05 to 0.35 were collected). EGME method: For the EGME procedure, a vacuum pump (with a vacuum pressure of 609 mmHg) and an electronic analytical balance (with an accuracy of 0.0001 g) were employed. Approximately 1 g of sample was weighed and put in the oven at 80 °C for 6 h or longer. It was then taken out and put in the aluminum tare (with a diameter of 5 cm and a height of 8 mm) and 3 ml EGME solution (analytical grade) was quickly added to the sample with a pipette and swirled gently until the sample was soaked. Afterward, the aluminum tares with mixture were placed in a sealed desiccator with EGME solution, calcium chloride (CaCl2) and phosphorus pentoxide (P2O5) (both of them were analytical grade). The desiccator was evacuated for approximately one hour so as to remove water vapor. More than 8 h later, the tares were weighed after the desiccator was evacuated again; the process of evacuation was repeated, and they were weighed until a constant weight was attained (the mass difference between the two measurements was less than 0.0010 g). Finally, the total surface area (EGME SSA) of the sample was calculated based on the absorbed quantity of EGME molecules (the conversion factor is 2.86 × 10− 4 g/m2). 2.2.2. X-ray diffraction (XRD) X'Pert-MPD diffraction instrument (Philips Corp.) was employed for XRD measurement. The test condition included copper butt, pipe pressure 30 kV, conduit flow 40 mA and scanning speed 2° (2θ) /min. At the beginning of the test, the sample needed to be ground with the grains of 320-mesh. The natural thin section of bulk rock (or clay fraction)

X. Zhu et al. / Applied Clay Science 109–110 (2015) 83–94

was made for the bulk mineral composition detection, and an additional ethylene glycol saturated orientation sheet of clay fraction needed to be made for the measurement of clay mineral composition. The ethylene glycol saturated orientation sheet was made by exposing the oriented sheet in ethylene glycol vapor at 60 °C for 8 h and heating at 250 °C for 2 h as well as 550 °C for 2 h to test the clay mineral composition. The content of each mineral was determined via the analytical standards of minerals content using XRD (SY/T 5163–1995, 1995; SY/T 5983–94, 1995; SY/T 6210–1996, 1996). 3. Results 3.1. SSA It is possible to obtain external surface area (N2-BET SSA) and total surface area (EGME SSA) of argillaceous source rock by using the nitrogen adsorption method and ethylene glycol monoethyl ether method, which can manifest the characteristics of the specific surface areas effectively. 3.1.1. N2-BET SSA According to the results of the N2-BET measurement (Tables 1 and 2), it shows that the test results by N2-BET of 1.6 m2g−1 to be the minimum, 33.7 m2g−1 to be the maximum, and 13.1 m2g−1 on average for the external surface area of bulk samples; and 14.0 m2g−1 to be the minimum, 74.8 m2g−1 to be the maximum, and 39.3 m2g−1 on average for the external surface area of the clay fractions. By comparing the external surface area of bulk samples with that of the clay fractions, the external surface area of each clay fraction was found to be larger than that of the bulk sample. 3.1.2. EGME SSA From the results of the EGME measurement (Tables 1 and 2), it shows the data by EGME of 37.9 m2g−1 to be the minimum, 177.5 m2g−1 to be the maximum, and 111.9 m2g−1 on average for the

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total surface area of bulk samples; and 111.1 m2g−1 to be the minimum, 477.8 m2g−1 to be the maximum, and 248.9 m2g−1 on average for the total surface area of the clay fractions. The internal surface area is obtained from the difference calculation based on the total surface area and external surface area (internal SSA = EGME SSA-N2-BET SSA) (Feller et al., 1992; Liang et al., 2009; Derkowski and Bristow, 2012). The difference calculation resulted in 36.2 m2g−1 to be the minimum, 151.5 m2g−1 to be the maximum, and 97.7 m2g−1 on average for the internal surface area of the bulk samples; and 80.6 m2g−1 to be the minimum, 403.0 m2g−1 to be the maximum, and 209.6 m2g−1 on average for the internal surface area of the clay fractions. The total surface area and internal surface area of each fraction were all greater than those of the bulk sample.

3.2. XRD 3.2.1. Bulk rock compositions It is found from the bulk rock compositions (Table 1, Fig. 1) that the bulk samples mainly contained clay and quartz, followed by plagioclase and calcite, with a small amount of potassium feldspar and dolomite, and minor amount of siderite and pyrite. For the bulk rock compositions, the clay was less than 50%, with 28.8% on average; the detrital minerals (quartz + feldspar), with 48.2% on average, was more than 20%, even as high as 91%. In addition, the content of authigenic nonclay minerals (calcite + dolomite + anhydrite + siderite + pyrite) distributed widely (1–61%) with 23.0% on average, of which the content of carbonate minerals (calcite + dolomite + siderite) varied greatly from 1% to 58%, with 18.9% on average. From the analysis of the bulk compositions in clay fractions (Table 2, Fig. 1), it can be seen that the clay content increased dramatically to over 50%, 85% being the highest and 63.9% as the average, and the other mineral contents decreased. Detrital minerals, with 23.6% of average content, did not contain potassium feldspar. Authigenic non-clay minerals were still dominated by carbonate minerals (12.0% on average) without siderite and pyrite.

Table 1 Lithology, mineral compositions and specific surface areas (SSAs) of bulk rocks. Sample no.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 17 18 19 20 21 22 23 24 25 26 27 28 29

Lithology

Member

Depth (m)

Mud-bearing siltstone Mud-bearing siltstone Mud-bearing siltstone Mud-bearing siltstone Argillaceous siltstone Argillaceous siltstone Silty mudstone Silty mudstone Silty mudstone Silty mudstone Argillaceous siltstone Silty mudstone Silty mudstone Argillaceous siltstone Silty mudstone Silty mudstone Argillaceous siltstone Silty mudstone Silty mudstone Silty mudstone Silty mudstone Silty mudstone Argillaceous siltstone Silty mudstone Silty mudstone Silty mudstone Silty mudstone Silty mudstone

Ng Ng Ed Ed Ed Ed Es1 Es1 Es1 Es1 Es1 Es2 Es2 Es2 Es2 Es2 Esupper 3 Esupper 3 Esupper 3 Esupper 3 Esmiddle 3 Esmiddle 3 Esmiddle 3 Esmiddle 3 Esmiddle 3 Esmiddle 3 Eslower 3 Eslower 3

1308.10 1311.90 1843.89 1845.45 1845.83 1847.25 2426.58 2429.38 2430.48 2432.78 2432.90 2668.08 2668.54 2679.36 2680.72 2773.91 3076.33 3076.51 3289.42 3290.97 3536.79 3538.64 3559.20 3561.21 3645.15 3646.84 3690.80 3691.80

Note: sample no. 16 is not listed for its anomaly.

SSA(m2g−1)

Bulk rock mineral compositions (%) Clay

Quartz

Potassium feldspar

Plagioclase

7 8 22 14 22 24 31 34 30 13 15 38 44 43 55 35 29 24 31 30 45 33 24 46 41 16 32 21

44 24 41 38 45 36 20 26 22 27 28 15 20 38 24 34 36 38 29 27 24 21 53 28 26 20 24 24

22 23 10 7 8 8

25 44 26 40 22 26 8 16 16 16 19 5 8 14 7 11 18

8 9 9 3

5 5 9 6

3 3 3 1 1

4 4 4 4 18 5 6 2 3 4

Calcite

Dolomite

Siderite

Pyrite

2 1 1 1 1 2 1 18 12 10 12 32 20 3 2 10 5 21 28 33 12 12 5 7 23 10 29 41

2 3 37 3 12 20 15 1 1 1 1

3

2 6 1 1 5 3

5 3 1 3 20

2 1 48 8 6

2

7

1

1 3 3 3 2 6 7 1 3 5 1 4 4 4 4 2 3 3 3 3

N2-BET SSA

EGME SSA

1.6 7.4 18.7 11.2 22.9 17.5 16.3 5.0 11.3 9.8 15.2 22.9 21.0 20.6 33.7 15.0 9.3 13.4 6.8 – 13.0 12.8 13.1 18.8 14.4 10.0 10.5 8.9

37.8 86.6 131.1 84.9 152. 1 169.0 124.1 140.3 57.6 60.8 132.6 166.0 154.8 177.5 113.4 78.3 126.3 102.1 153.0 81.5 69.4 155.2 116.3 58.7 113.3 67.8

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Table 2 Mineral compositions and specific surface areas (SSAs) of clay fractions. The lithology refers to the bulk rocks of the clay fractions. Sample no.

Mud-bearing siltstone Mud-bearing siltstone Mud-bearing siltstone Mud-bearing siltstone Argillaceous siltstone Argillaceous siltstone Silty mudstone Silty mudstone Silty mudstone Silty mudstone Argillaceous siltstone Silty mudstone Silty mudstone Argillaceous siltstone Silty mudstone Silty mudstone Silty mudstone Argillaceous siltstone Silty mudstone Silty mudstone Silty mudstone Silty mudstone Silty mudstone Argillaceous siltstone Silty mudstone Silty mudstone Silty mudstone Silty mudstone Silty mudstone

Member

Ng Ng Ed Ed Ed Ed Es1 Es1 Es1 Es1 Es1 Es2 Es2 Es2 Es2 Es2 Es2 Esupper 3 Esupper 3 Esupper 3 Esupper 3 Esmiddle 3 Esmiddle 3 Esmiddle 3 Esmiddle 3 Esmiddle 3 Esmiddle 3 Eslower 3 Eslower 3

Depth (m)

1308.10 1311.90 1843.89 1845.45 1845.83 1847.25 2426.58 2429.38 2430.48 2432.78 2432.90 2668.08 2668.54 2679.36 2680.72 2773.39 2773.91 3076.33 3076.51 3289.42 3290.97 3536.79 3538.64 3559.20 3561.21 3645.15 3646.84 3690.80 3691.80

Bulk rock mineral compositions (%) Clay

Quartz

55 75 82 85 81 72 67 69 59 64 57 56 60 73 74 80 69 70 46 46 49 71 58 57 62 55 56 51 54

16 12 11 9 12 17 10 12 10 9 10 15 18 20 18 14 18 17 28 31 18 20 25 33 28 20 22 25 25

Potassium feldspar

Note: illite:smectite ratio is the smectite proportion in mixed-layer illite/smectite.

Plagioclase 21 11 5 5 6 9 3 4 4 5 6 6 5 5 7 5 6 6 4 3 3 4 5 6 5 3 3 3 3

SSA(m2g−1)

Clay mineral compositions (%) Calcite

1

13 13 14 15 22 16 2

6 19 19 28 4 8 2 3 21 7 19 17

Dolomite

Anhydrite

Siderite

6

2 2

Pyrite

1 1 1 2 20 2 14 8 12 1 1 1 1 1 1 3 1 1 1 1 1 2 1 12 1 1

6

1 2 1

1

1

Mixed-layer illite/smectite

Illite

Kaolinite

Chlorite

Illite:smectite ratio

N2-BET SSA

EGME SSA

90 92 89 82 86 83 75 78 66 71 67 55 55 65 61 65 43 54 36 55 39 23 53 35 39 71 66 63 81

2 2 3 7 3 3 14 12 23 21 21 18 18 13 16 13 20 23 21 32 43 74 36 32 56 22 29 32 15

4 3 4 5 4 7 6 5 6 4 6 15 15 13 13 13 23 13 24 6 11 1 6 14 2 2 2 2 2

4 3 4 6 7 7 5 5 5 4 6 12 12 9 10 9 14 10 19 7 7 2 5 19 3 5 3 3 2

70 75 65 65 65 65 40 40 40 40 40 40 45 40 40 40 40 40 40 20 40 20 20 20 20 20 20 20 20

15.2 59.0 74.8 40.3 73.1 62.4 43.7 26.3 37.3 60.0 54.8 49.2 43.3 40.0 46.5 48.5 42.7 31.1 40.3 14.0 23.8 23.9 30.6 29.5 30.3 17.5 30.6 19.5 30.5

294.4 325.6 477.8 379.7 393.1 429.8 234.7 289.8 261.5 271.9 220.5 211.2 256.7 264.9 269.3 248.4 295.0 218.9 195.3 164.9 165.5 170.3 185.9 193.5 233.8 168.7 132.8 152.1 111.1

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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

Lithology

X. Zhu et al. / Applied Clay Science 109–110 (2015) 83–94

Fig. 1. The distribution of bulk mineral compositions (the average content) in argillaceous source rocks.

3.2.2. Clay minerals From the analysis of the compositions of clay minerals by X-ray diffraction (Table 2, Fig. 2), it is found that the content of mixed-layer illite/ smectite (I/S) and illite was high, and the kaolinite and chlorite content was low. The clay minerals had the greatest portion, as the mixed-layer I/S ranged widely from 23% to 92%, making up more than 40% of each sample except sample No. 22 (3536.79 m) which had a content of 23%, and the overall average content was approximately 63%; the illite content was 2–74%, with 22% on average; the kaolinite content was not more than 25%, and the chlorite content ranged from 2% to 19%, with approximately 7% on average. The illite:smectite ratio in mixed layer I/S ranged from 20 to 75 with an average of 40.

4. Discussion 4.1. The constituent and variation of specific surface areas Because the nitrogen molecule, which is nonpolar, cannot probe into and fill the interlayer micropores of clay minerals completely and because the adsorption of nitrogen is physisorption, the SSA obtained by the N2-BET method refers only to the external crystal surface (Brunauer et al., 1938; Tiller and Smith, 1990; Santamarina et al., 2002; Yukselen and Kaya, 2006); but ethylene glycol monoethyl ether is a polar liquid, and both can be adsorbed on the external edge of minerals and can be exchanged into the interlayers of clay minerals via chemisorption, so the method of EGME can measure both internal and external surface (Carter et al., 1965; Tiller and Smith, 1990; Cerato and

Fig. 2. The relative content distribution of clay minerals in argillaceous source rocks. The white line in the box represents the average content of minerals.

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Luteneggerl, 2002; Yukselen and Kaya, 2006). Therefore, by integrating the methods of N2-BET and EGME, we can obtain the external surface area and total surface area of argillaceous source rocks. The internal surface area can be calculated by combining the test results of the two methods based on the difference calculation (EGME SSA-N2-BET SSA = internal SSA) (Feller et al., 1992; Liang et al., 2009; Derkowski and Bristow, 2012), thus allowing the ability to characterize the SSAs of hydrocarbon source rock and clay fractions systematically and completely. Meanwhile, although the studied sample is comprised of organic matter and inorganic minerals, the minerals are the main body of the sample, and are the dominant contributor to SSAs, especially in the rock with low-TOC (Chiou et al., 1990; Derkowski and Bristow, 2012; Zhu et al., 2013). In view of the constituent of SSAs (Fig. 3), whether it is for bulk samples or clay fractions, the internal surface area is clearly larger than the external surface area, and whether it is internal surface area or external surface area, the SSA of clay fractions is clearly greater than that of bulk samples. This difference of internal and external surface area is much more prominent in clay fractions, indicating that the internal surface and clay fractions make the greatest contributions to the SSAs of argillaceous source rock. By comparing the mean values of SSAs, for the bulk samples, EGME SSA (111.9 m2g− 1 on average) is much larger than N2-BET SSA (13.1 m2g−1 on average), with a fact of nearly 9 times of difference; however, internal SSA (97.7 m2g−1 on average) accounts for 87.2% in EGME SSA, almost 7 times of difference with N2-BET SSA. For clay fractions, EGME SSA (248.9 m2g− 1 on average) is much larger than N2-BET SSA (39.3 m2g−1 on average), with a fact of nearly 6 times of difference; however, internal SSA (209.6 m2g−1 on average) accounts for 84.2% of EGME SSA, almost 5 times of difference with N2-BET SSA. The value of EGME SSA/N2-BET SSA in the clay fraction is lower than that of bulk rock, but the internal SSA still accounts for 80% or more of EGME SSA, which indicates the important position that internal surface and clay fraction have in surfaces within source rock. By comparing SSAs of different types of rocks (Fig. 4), it is found that whether N2-BET SSA or internal SSA, for bulk samples, argillaceous siltstone has the greatest SSA, followed by silty mudstone. Mud-bearing siltstone has the smallest SSA. The trend whereby N2-BET SSA and internal SSA vary with rock types is similar. For clay fractions, mud-bearing siltstone has the largest internal SSA, followed by argillaceous siltstone and silty mudstone, whereas the N2-BET SSA of mud-bearing siltstone is the largest and followed by argillaceous siltstone; the silty mudstone has the smallest N2-BET SSA. This suggests that in bulk rocks, a given content of detrital minerals with clay is better to contribute to SSAs of

Fig. 3. The contrast of SSAs between bulk samples and clay fractions. The white line in the box represents the mean value of SSA; a and b are for bulk rocks; c and d are for clay fractions.

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Fig. 4. The contrast of SSAs among different rock types in argillaceous source rocks.

the rock. A comparison of SSAs of rock types between bulk samples and clay fractions reveals that the change in SSA varies greatly; in particular, the difference in internal SSA is dramatic, which may be correlated with rock constituents and mineral compositions. As seen in Fig. 5, the SSAs in well S evolve regularly with the burial depth, whereby the SSAs are low in the shallow-part (above 1500 m) and the rock type mainly is mud-bearing siltstone. In the mid-part (1500–2000 m), the SSAs increase and the rock type mostly is argillaceous siltstone and silty mudstone. Until the deep-part (below 2000 m), the SSAs decrease again and the rock type is dominated by silty mudstone. The trends of evolution in N2-BET, internal and EGME SSAs are similar: The deeper the depth, the smaller the SSA is. This trend (the extent of variation) is much more notable in the clay fractions, for which the SSAs are the largest in the mid-part and decrease sharply with burial depth. From the value of internal SSA/N2-BET SSA, it can be seen that whether it is the bulk sample or the clay fraction, in the shallow-part the internal SSA is approximately 20 times larger than N2 -BET SSA, and in the deep-part, it is 5 to 10 times larger than N2-BET SSA. In general, the value of the internal SSA/N2 -BET SSA of the bulk samples is 1.4 times larger on average than that of the clay fractions. These features indicate that the characteristics of SSAs in argillaceous source rocks are closely correlated with diagenesis of source rocks, especially in the clay fractions.

4.2. The influence of mineral compositions on the specific surface areas 4.2.1. Rock constituents Argillaceous source rock consists of clay, detrital minerals and authigenic non-clay minerals (Fig. 1). From the comparative analysis, it is found that there are regularities between minerals and SSAs (Fig. 6) that are as follows: With the clay content increase, the values of EGME, N2-BET and internal SSAs can increase simultaneously in the bulk rocks and clay fractions, presenting an excellent positive correlation between clay content and SSAs. When the detrital mineral content increases, the values of EGME SSA and N2-BET SSA can decrease in the bulk rocks and clay fractions, presenting a better negative correlation between detrital mineral content and SSAs. However, the correlation in internal SSA with detrital mineral content appears chaotic. These phenomena indicate that the clay mineral makes the greatest contribution to SSAs and that the detrital minerals have a greater influence on the external surface area but less influence on the internal surface area of the rock. When the content of authigenic non-clay minerals and carbonate minerals is 10% or less, the EGME, N2-BET and internal SSAs of bulk rocks and that of clay fractions exhibit complex variation. Until the content is more than 10%, the N2-BET SSA of the bulk rocks decreases with the increase in the content of authigenic non-clay minerals and carbonate minerals, and the feature is much more obvious when the content is

Fig. 5. The relation between SSAs and burial depth in argillaceous source rocks. The solid is for bulk rocks, and the hollow is for clay fractions.

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Fig. 6. The relations between rock constituents and SSAs in argillaceous source rocks. The primary axis and green solid diamond are for bulk rocks, and the second axis and red solid diamond are for clay fractions.

20–40%. However, it is not evident in the clay fractions that N2-BET SSA varies with the content of authigenic non-clay minerals and carbonate minerals, and the SSAs are stable. The EGME and internal SSAs of bulk rocks and clay fractions decrease with the increase in the content of authigenic non-clay minerals and carbonate minerals. These features indicate that the authigenic non-clay minerals, especially the carbonate minerals, have a negative effect on internal surface area, and a threshold content of carbonate minerals may exist for their effect on external

surface area (the external surface area of bulk rock in particular), to which more attention needs to be paid in future research. 4.2.2. Clay minerals The clay minerals in source rock are composed of different mineral types (e.g., mixed-layer I/S, illite); Figs. 2 and 7 illustrate their different correlations with SSAs. The content of mixed-layer I/S is positively correlated with N2-BET SSA and internal SSA, but the illite content is

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Fig. 7. The relations between clay mineral compositions and SSAs in clay fractions. The left y-axis and solid diamond are for N2-BET SSA, and the right y-axis and hollow diamond are for internal SSA.

negatively correlated with N2-BET SSA and internal SSA, thus resulting in a mutual succession with each other. The relation of N2-BET SSA and internal SSA with the content of kaolinite and chlorite appears chaotic. These features indicate that the mixed-layer illite/smectite contributes the greatest to the SSAs (external, internal and total surface areas), while the contribution of illite is minor. This indicates the variable contribution to the SSAs, which is caused by different attributes of clay mineral. 4.2.3. The influence on SSAs of mineral species and relative content Argillaceous source rock is composed of different minerals, and there is considerable variability in content of different minerals (Tables 1 and 2). The surfaces within source rock consist of the internal surface and external surface, and there are great differences between the internal surface area and the external surface area in different minerals, e.g., detrital minerals and illite only have an external surface area (no more than 10–50 m2g−1), whereas the expandable clay minerals (e.g., smectite) not only have an external surface area but also an internal surface area (the maximum can be as high as 800 m2g−1) (Zhu and Cai, 2012). Therefore, the SSAs of argillaceous siltstone and silty mudstone are much larger than those of mud-bearing siltstone (Fig. 4), which is confirmed by the existing research (Ji et al., 2012a,2012b). However, the clay composition and its content are relative, so the rock dominated by smectite has a large internal SSA, and the rock dominated by illite mainly has N2-BET SSA. Thus, although in bulk rock the SSA of mud-bearing siltstone is smaller than that of silty mudstone, the N2-BET SSA can be increased appropriately as argillaceous siltstone in the clay fraction (Fig. 4). Nevertheless, further analysis has found that the internal SSA is positively correlated with clay content (Fig. 6), which is positively correlated with the content of smectite in particular (Fig. 7); the correlations of N2-BET SSA with detrital minerals (Fig. 6) and illite content (Fig. 7) are much more apparent, which reveals that different types of SSA are controlled by the mineral attributes. These characteristics indicate that the different mineral assemblages are able to affect the ratio of internal to external surface area in hydrocarbon source rocks. Moreover, the authigenic non-clay minerals (carbonate minerals in particular) in argillaceous source rock can result in the

decrease in internal SSA with the increase in their content, but the N2-BET SSA decreases slowly in bulk rocks and remains stable in clay fractions when the carbonate mineral content is 20–40%. If the carbonate mineral content is more than 40%, the N2-BET SSA decreases dramatically. Therefore, carbonate mineral content between 20% and 40% can be regarded as the threshold value to keep N2-BET SSA as a constant. As mentioned above, the rock types and the mineral attributes, especially the content of mineral, have a great influence on the constituents and characteristics of the specific surface areas. 4.3. The influence of diagenesis on the specific surface areas The argillaceous source rocks undergo compaction and mineral transformation during the process of diagenesis (Liu and Zhang, 1992; Armitage et al., 2010), which can influence the variation of the specific surface area and should be observed closely. 4.3.1. Mineral transformation From the relations of rock compositions with burial depth (Fig. 8), in bulk rocks, the clay mineral content tends to increase first and then decrease with burial depth, and detrital mineral content tends to decrease first and then increase with burial depth as well. However, the content of authigenic non-clay minerals, e.g., carbonate minerals and pyrite, increases dramatically below the depth of 2400 m; in particular, the carbonate mineral content can reach to over 50% in the deep-part, which can cause a decrease in SSAs (Fig. 5). For the clay fractions, the content of clay and detrital minerals vary slightly with burial depth, but large amounts of carbonate minerals emerge below 2400 m, indicating that the influence of the effect of the authigenic non-clay minerals on SSAs cannot be neglected. The mineral transformation of different rock types indicates that it also can affect the difference of SSAs (Figs. 4 and 5). Based on the contribution variation of the SSAs from different minerals (Fig. 6), it is believed that mineral transformation during diagenesis is one of the important factors for the evolution of SSAs in source rocks, which results in the remarkable different extent of variation of the SSAs in the depths between the bulk rocks and clay fractions (Fig. 5).

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Fig. 8. The relations between bulk rock minerals and depth in argillaceous source rocks. The solid is for bulk rocks, and the hollow is for clay fractions.

It can be seen in Fig. 9 that the evolution characteristics of clay minerals vary with depth. The content of the mixed-layer I/S decreases from shallow to deep but increases slightly below 3500 m, while the content of illite increases and decreases slightly below 3500 m; both exhibit a negative correlation in content. The content of kaolinite and chlorite at the depth of 2500–3200 m is relatively more than that at the depth of b2500 m and N3200 m. The evolutions of each clay composition are not consistent in the different rock types. Argillaceous siltstone and mud-bearing siltstone successively vary with the depth in content of mixed-layer I/S and illite. For silty mudstone, the content of mixedlayer I/S increases below 3500 m but the content of illite decreases, and this may be attributed to the sudden change in diagenetic environment. This indicates that clay mineral transformation results in different SSAs for different types of rock. In view of the considerable differences of SSA in different clay minerals (Zhu and Cai, 2012) and the inconsistency of the relationships between internal and external surface area and clay compositions (Fig. 6), it is concluded that diagenesis has a great effect on clay mineral transformation and on the variation of the SSAs of source rocks at different burial depths.

4.3.2. Compaction Compaction can cause the structure of clay minerals to be oriented in argillaceous source rocks, and make the crystallization degree of the mixed-layer I/S much more ordered (mainly manifested as the illite: smectite ratio in mixed-layer I/S), we therefore map the relationships between the illite:smectite ratio and SSAs (Fig. 10) to discuss the influence of compaction on SSAs. It can be found that the relations of EGME, N2-BET and internal SSA with the illite:smectite ratio are disorderly in bulk rocks, and regular in clay fractions. With a decrease in the illite: smectite ratio (Fig. 10), the EGME SSA and internal SSA decrease clearly in clay fractions from 111.1 to 477.8 m2g−1, and from 80.6 to 403.0 m2g−1, respectively, and the N2-BET SSA however varies within the range of 14.0–74.8 m2g−1. This indicates that there are variations in different SSAs during the process of compaction, which are much more dramatic in the internal SSA than in the N2-BET SSA; this can influence the occurrence of organic matter or hydrocarbon and the formation of pores. In addition, the effect from the compaction of SSAs in clay fractions is much more prominent than that in bulk rocks, indicating that the extent of variation in bulk rocks is smaller than that in clay

Fig. 9. The relations between clay compositions and burial depth in argillaceous source rocks.

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Fig. 10. The relations between the illite:smectite ratio in mixed-layer I/S and SSAs in clay fractions. In b and d, the left y-axis and green solid circle are for bulk rocks, and the right y-axis and red solid circle are for clay fractions.

fractions (Fig. 5) due to effects of the non-clay minerals (or brittle minerals, e.g., detrital minerals and carbonate minerals). 4.3.3. Vertical response of SSAs during the diagenetic process The compaction, mineral transformation, etc. occur in the process of diagenesis in argillaceous source rocks (Morad et al., 2000; Aplin et al., 2006; Armitage et al., 2010), and there are evident subsection characteristics in the studied samples. In the shallow-part (above 1500 m): The SSA of the bulk rocks is much smaller than that of the clay fractions in mud-bearing siltstone (Fig. 5), which is ascribed to the amount of detrital minerals in bulk rocks and the amount of mixed-layer illite/smectite in clay fractions. The ratio of internal SSA/N2-BET SSA can be as high as 20. This reveals that the internal surface area plays a dominant role in the total surface area during the early stage of compaction. In the mid-part (1500–2000 m): The argillaceous siltstone and silty mudstone are well-developed, and the detrital mineral content decreases while the clay content increases. The SSAs of bulk rocks and clay fractions are the highest comparing with that in the shallow- and deep-part, and the ratio of internal SSA/N2-BET SSA ranges from 5 to 10, which indicates that the proportion of internal SSA in total surface area decreases with the increase in the proportion of N2-BET SSA. In the deep-part (below 2000 m): Although the rock type is predominately silty mudstone and the contents of detrital minerals and clay minerals do not fluctuate greatly, the mixed-layer illite/smectite is negatively correlated with illite, and large amounts of carbonate minerals and pyrite are well-developed (Figs. 8 and 9). Especially below 2400 m, the carbonate mineral content is more than 50% (Fig. 8), and the illite:smectite ratio is reduced to 20 (Table 2), which causes the SSAs of the bulk rocks and clay fractions to decrease. This indicates that the mineral transformation has a great influence on SSAs. From the above analyses, the compaction of argillaceous source rocks alters the structures and superimposed patterns of the clay compositions (Morad et al., 2000; Aplin et al., 2006; Armitage et al., 2010), and the mineral transformation arouses the alterations of mineral attributes and specific surface areas (Murray and Quirk, 1990; Goldberg

et al., 2001; Zhu and Cai, 2012). Particularly the carbonate minerals change the original properties of the rock structure. All these factors effectively control the subsectional characteristics of specific surface areas and the constituents of surfaces (internal surface and external surface) in burial depth, which also can influence the pore formation and organic matter or hydrocarbon occurrence in argillaceous source rocks. 4.4. The significance of petroleum geology The surface within source rocks is an important site for organic matter and hydrocarbon occurrence, and the adsorption attribute that different surfaces make on organic matter and hydrocarbon differs greatly (Mayer, 1994a; Kennedy et al., 2002; Cheng and Huang, 2004; Liu et al., 2013), i.e., chemisorption mainly occurs on the internal surface via chemical bonding, and physisorption occurs chiefly on the external surface via van der Waals' bonding (Wypych and Satyanarayana, 2004; Nilsson et al., 2011; Gasparik et al., 2014). Previous studies indicate that organic matter or hydrocarbon stored in the interlayer domain of clay minerals need more energy to release them than in the pores (Cai et al., 2007, 2010; Liu et al., 2013), indicating that the organic matter or hydrocarbon is adsorbed on the internal surface much more tightly than on the external surface. Although the surfaces within argillaceous source rock consist of internal and external surfaces (Tiller and Smith, 1990; Kennedy et al., 2002; Yukselen and Kaya, 2006; Kennedy and Wagner, 2011), the value of each surface and their ratio (internal surface area/external surface area) are varied in the different rock types (Fig. 4), mineral associations (Fig. 6) and burial depth (Fig. 5). As a consequence, the adsorption mode of organic matter and hydrocarbon in source rocks is accordingly altered in different SSA assemblages. Because of the different adsorption attributes in different surfaces, the organic matter occurred on the surfaces is quite complicated (by comparing the results of TOC–SSA correlation in Mayer, 1994a, 1994b; Mayer and Xing, 2001; Kennedy et al., 2002; Kennedy and Wagner, 2011; Kuila et al., 2012; Ding et al., 2013), and the hydrocarbon generation process of organic matter as well as the stability of hydrocarbon occurrence are definitely affected (Martini et al., 2003; Ji et al.,

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2012b; Liu et al., 2013); in addition, the variation of physico-chemical adsorption can induce great differences in the desorption ability of the adsorbed organic matter and hydrocarbon. As discussed above, this indicates that the silty mudstone and argillaceous siltstone contribute to a large external surface area, and 20–40% of the carbonate minerals can maintain a stable value of external surface area, previous studies also indicate that brittle minerals are favorable for the generation of porosity and cracks in unconventional petroleum systems (Jarvie et al., 2007; Chalmers et al., 2012; Huang et al., 2012; Zou et al., 2013) and the surfaces are the storage sites for adsorbed methane and organic matter (Chalmers and Bustin, 2007; Kennedy and Wagner, 2011; Chalmers et al., 2012). So, we can predict that the silty mudstone and argillaceous siltstone with 20–40% of carbonate minerals, because of their contribution to porosity and storage sites for organic matter and hydrocarbon, can be the best target of unconventional petroleum exploitation in continental basins. Therefore, consideration for variation in the characteristics of internal and external surface areas in argillaceous source rock is meaningful for making an effective exploration scheme. 5. Conclusions To perform the nitrogen adsorption (N2-BET) method and ethylene glycol monoethyl ether (EGME) method for the measurement of SSAs in argillaceous source rocks, it is possible to systematically characterize the constituent of SSAs (including internal and external surface areas) within argillaceous source rocks. Based on the analysis of argillaceous source rocks in rock types, mineral compositions, diagenesis, etc., it can be found that there are various factors that influence the characteristics of specific surface areas: 1) Mineral compositions: Clay minerals (especially smectite, which contains an internal surface and external surface simultaneously) have the greatest contribution to the SSAs, particularly the internal surface area, whereas non-clay minerals (e.g., detrital minerals) basically only contribute to external surface area. When the content of carbonate minerals is more than 40%, there is a strong constraint to SSAs (the internal SSA in particular), and a threshold of carbonate mineral content (20–40%) is found that makes the external surface area stable. 2) Diagenesis: Diagenesis for re-transformation of minerals can cause variability in mineral compositions of source rock; thereby, the vertical variation of subsectional characteristics of SSAs during evolution is formed. The extent of variation in internal surface area is much greater than that in external surface area. 3) Rock types: The SSA of silty mudstone and argillaceous siltstone is larger than that of mud-bearing siltstone, and the internal and external surface areas of silty mudstone and argillaceous siltstone are larger than that of mud-bearing siltstone, which indicates that the ratio of clay to detrital mineral content and the proportion of smectite to illite content in clay greatly affect the SSAs of the rock. The surface within argillaceous source rocks is an important site for the enrichment and occurrence of organic matter and hydrocarbon, the adsorption modes of which include physisorption and chemisorption. The different adsorption properties between the internal surface and external surface result in a different hydrocarbon and organic matter occurrence as well as the different properties of desorption. Therefore, the internal and the external surfaces should be treated differently in the research of the organic- and inorganic-phase in the nano-scale. Meanwhile, the subsectional characteristics of SSAs indicate the boundary of time–space in petroleum exploration, and the proper ratio of internal surface area to external surface area (internal SSA/N2-BET SSA) therefore needs to be concerned. Consequently, fully understanding the characteristics and influence factors of specific surface areas in argillaceous source rocks should be taken into consideration, which is of great significance for further analysis of the organic matter and

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hydrocarbon occurrence and petroleum exploration and development, especially in the unconventional petroleum systems.

Acknowledgments This study received financial support from the National Natural Science Foundation of China (41072089 and 41372130), Major Project of National Petroleum in China (2011ZX05006-001) and Science and Technology Bureau Fund of SINOPEC (P12062). We are grateful to the anonymous reviewers and the Associate Editor, Dr. Robert Schoonheydt, for their constructive comments and suggestions.

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