Characteristics of Lacustrine Shale Reservoir and Its Effect on

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Characteristics of Lacustrine Shale Reservoir and its Effect on Methane Adsorption Capacity In Fuxin Basin Daye Chen, Jinchuan Zhang, Xiaoming Wang, Bo Lan, Zhen li, and Tong Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01683 • Publication Date (Web): 18 Sep 2018 Downloaded from http://pubs.acs.org on September 24, 2018

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Characteristics of Lacustrine Shale Reservoir and its Effect on Methane Adsorption Capacity In Fuxin Basin Daye Chen1, Jinchuan Zhang1 *, Xiaoming Wang2, Bo Lan2, Zhen Li1, Tong Liu1 1. School of Energy Resources, China University of Geosciences (Beijing), Beijing, 100083, China; 2.107 Exploration Team of Northeast CoalField Bureau, Fuxin, 123000, China

Abstract: The Lower Cretaceous Shahai and Jiufotang formation in Fuxin Basin, located northwest of Liaoning Province with stable planar distribution contain abundant shale gas reserves, which are key horizons in the exploration breakthrough of the Mesozoic continental shale gas in north-eastern China. Fuye-1 well was recently drilled, and it is an important shale gas parameter well in the fuxin basin area. A total of 60 shale samples were collected from Shahai and Jiufotang Formation of FuYe-1 well and DY-1 well at the depth of the range 11572762 m. A series of experiments including: organic matter vitrinite reflectance, XRD diffraction analysis, total carbonate content (TOC) measurement, scanning electron microscopy, lowtemperature nitrogen adsorption and methane isothermal adsorption, were conducted through the samples. The final results revealed that in Fuye-1 well, the shale of the Lower Cretaceous Shahai and Jiufotang formation have a maturity of 0.46-1.68%, (average of 0.92%), which is at the

ACS Paragon Plus Environment

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immature-high maturity stage. The kerogens are mostly of type II-III. The mineral components are dominated by clay minerals and quartz, with an average of 21.39% and 30.89%, respectively. The main controlling factors of the methane adsorption capacity are the TOC content, the total clay content and the shale pore structure. Langmuir’s volume (VL) of the shale ranges between 0.17～1.98m3/t, (with an average of 1.21m3/t). The methane adsorption capacity was positively correlated with the content of the total clay minerals and quartz, but varies in different clay minerals. The specific surface area of the shale and the total pore volume were calculated by BET and BJH, the macro-pores ratio of which were negatively correlated with the methane adsorption capacity, whereas the specific surface area and the total pore volume of the mesopores and micropores ratio were positively correlated with the methane adsorption capacity, indicating that the content of the micro-pores and meso-pores is the major contributor to the specific surface area of shale.

Keywords: Shale Reservoir, Pore structure, Methane adsorption, Langmuir’s volume, BET surface area, Continental shale

1. Introduction Shale gas resources in China are abundant. According to the evaluation results of shale gas resources by the Ministry of Land and Resources in 2012, the country's shale gas recoverable resources are 25.08×1012m3, in which the shale gas resources facies are marine, and marinecontinental transitional and continental, and each accounts for one third1. Currently, the marine shale gas in the Sichuan Basin and the shale of the marine-continental transitional facies in the Henan province both have significant breakthroughs2, 3, but the continental shale gas is still in the period of exploration and research4.. In order to achieve


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exploration breakthroughs in the Mesozoic terrestrial shale gas in Northeast China, the Coalfield Geological Bureau of Northeast China implemented a shale gas parameter well in the northwest of Liaoning Province, the FuYe-1 well. This is of great significance to solve the problem of gas shortage in northeast China and to realize unconventional gas breakthrough. Pore type and pore structure are the significant components for a shale reservoir. Generally, adsorption pores can be further subdivided into micropores (﹤ 10 nm) and transitional pores (10–100 nm), and seepage pores are further subdivide into mesopores (100–103 nm) and macropores ( ﹥ 103 nm). Shale porosity, characteristics of pore structure, and mineral composition are three important aspects of analyzing shale reservoir properties5, 6，and they are also closely related to shale gas content and methane adsorption capacity7.. The factors affecting the methane adsorption capacity of shale reservoirs generally include the physical properties of coal, external conditions or geological factors. Among them, the physical characteristics of shale itself include vitrinite reflectance, ash content, moisture content, microstructure and pore structure8, 9. External influencing factors include temperature, ground stress, shale grain size and deformation degree and.so on10, 11. In this paper, based on XRD diffraction, low-temperature nitrogen adsorption and methane adsorption isotherm experiments, the pore characteristics of continental shale in the northwestern Liaoning are characterized, and the influencing factors of the methane adsorption capacity of continental shale are discussed. 2. Geological setting The Fuxin Basin is located in Yan liao platform fold belt, with Sino-Korean Platform to the west and Inner Mongolian Axis to the north. The basin is divided into three second-order structural belts, including western fault terrace zone, central sag zone and eastern fault terrace zone(Figure 1, Figure 2). The third-order structural units are Minjiatun sub-sag, Qinghemen-


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Aiyou structural belt, Yi ma tu sub-sag, Xiaohujia faulted nose, Dongliang structural belt, Haizhou sub-sag and the Haibei faulted nose from south to north, forming a structural pattern of three sags, two uplifts and three faulted noses. They are nearly EW-trending and arranged in parallel and oblique. The arrangement and distribution of the structure are controlled by basement relief and the strike-slip activities of boundary faults12, 13. Fuye-1 well is located in the structural belt of the Meizhou sub-sag(figure 3), Figure 2.During the Early Cretaceous, the Fuxin Basin experienced extension and shrinkage for two times, forming two completed sedimentary cycles. The depositional period of the Jiufotang Formation was the first extensional deep depression period of the basin. Fluvial, fan deltic, deep lake and turbidity deposits were formed in the process of water invasion. The depositional period of Upper Jiufotang Formation was the first shrinkage period of the basin, when the lake basin shrunk and fan delta was extensively developed. The depositional period of the Shahai Formation was basin re-extension and water reinvasion period, when fluvial and deltic sediments were developed in the lower member of the Shahai Formation. Peat swamp facies occurred between shore shallow lake and fan deltic plain. The upper member of the Shahai Formation is dominated by lacustrine mudstone. T The depositional period from the Shahai Formation to the Fuxin Formation was the second shrinkage period of the basin, when the fluvial and swamp facies occurred. During the depositional period of the Late Cretaceous Sunjiawan Formation, the basin was uplifted and large-scaled alluvial fans were formed. The alternating changes of the sedimentary environments of the Fuxin Basin were obviously controlled by the faults in basin margin14. In the early Late Jurassic, in response to the magma upwelling, the volcanic rocks of Yixian Formation


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interbedded with sedimentary rocks were accumulated to a thickness of about 3000m, and after, the crust collapsed and rifted lake was formed.


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Figure.1. The structural outlines of Fuxin Basin showing the locality of FuYe-1well (modified from zhu, 2008)


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Figure 2 Diagram of depositional model of Shahai Formation and shale gas favorable area in Fuxin basin


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Figure 3. Comprehensive column of the Shahai formation and the Jiufotang formation in Fuxin Basin


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3. Materials and Methods 3.1 Samples A total of 43 core shale samples were collected from the Shahai and the Jiufotang formation of FuYe-1 well, 12 samples from the Shahai formation and 31 samples from the Jiufotang Formation. All the samples were carefully packed and sent to the laboratory, which were numbered consecutively from the Shahai formation and the Jiufotang Formation. 3.2. Experimental Methods The TOC contents were analyzed by the instrument of LECO CS744 carbon/sulfur analyzer. The samples, treated by hydrochloric acid solution were crushed to grains less than 100 mesh size, and then 0.1-1g samples were pyrolyzed up to 540℃ for 2 hours with Chinese National standards GB/T 19145-2003, and GB/T 18602-2001, respectively. The Vitrinite reflectance (Ro) was measured under the oil immersion reflected optical light using a Leitz MPV-3 microphotometer, in accordance with the Chinese National Standards GB/T 6948-1998.For each sample, about 40 different vitrinite observation points were randomly selected for measurements and the average data were used. Maceral composition analyses (about 300 points) were performed under transmitted light and fluorescent light using the same instrument of vitrinite reflectance measurement, in accordance with the Petroleum and Natural Gas Industry Standard SY/T5125-2014. X-ray diffraction (XRD) analysis was performed on shale powders (~100 mesh) using a D8 discover diffractor-meter at 40 kV and 30 mA with a Cu radiation. Stepwise scanning rate and sampling frequency was 4_/min and 0.04_ (2q), respectively. For each sample, the semiquantitatively determination for the mineral composition was carried out using the curve area of the major peaks.The low temperature nitrogen adsorption experiment was performed using a Micromeritic TriStar II 3020 surface area and pore size analyzer following Petroleum and


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Natural Gas Industry StandardSY/T 6154-1995. Each sample was sieved to a size between0.28 mm and 0.42 mm for testing under the relative pressure ranging from 0.01 to 0.99 at 77 K. The specific surface area, pore volume and pore structure distribution (PSD) of each sample were obtained using the BET (Brunauer-Emmtt-Teller) equation15, 16, as well as the BJH (BarrettJoyner-Halend method)17,

18

. Shale isotherm adsorption experiments under balanced water

conditions were carried out according to national standard GB/T 19560-2008. Each sample was prepared with a sample size of 0.25mm-0.18mm (60-80 mesh) and a total of about 90-120g. The experimental instrument is 3H-2000 PH analyzer from BeiShiDe technology instrument company (Beijing). The test temperature and the maximum adsorption pressure are 30°C and 20 MPa, respectively 4 Results 4.1 Organic chemistry The total organic carbon content (TOC) of shale samples from the Shahai and Jiufotang formation range from 0.299-3.44%, with an average of 1.59%, and the TOC value mainly distributed from1%to 2%, and greater than 2% (Table 1), indicating that the shale investigated is rich in organic matter (Figure 4). The TOC of the shale change regularly, due to the lake level rise and subsidence. The shale samples of different horizons of the FuYe-1 well show that the TOC in the lower part of the fourth member of the Shahai Formation is the highest, followed by the middle part of Jiufotang formation and the lower part of the Jiufotang formation. Furthermore, the variations of TOC content in the Shahai and Jiufotang formations are closely related to sedimentary environment, and the highest TOC content are distributed in the deepsemi-deep lake environment in the Shahai Formation. According to previous studies conducted on maturity of the organic matter, Ro is less than 0.6%, which is considered to be at immature


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stage, 0.6~1.3% at mature, 1.3~2% at high mature, 2~3% at early over-mature, 3~4% at late overmature5, 19. The shale samples maturity of the Shahai formation and the Jiufotang formation of the Lower Cretaceous from FY-1 well ranges from 0.46-1.68%, with an average of 0.92%, and was at the immature-high mature stage (Table 1, Figure5). Table 1. Test results of TOC and Ro of the shale samples from FY-1 well in Fuxin Basin Sample No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

Depth(m) 1157.15 1157.68 1159.45 1160.29 1161.46 1365 1371 1378 1384 1390 1396 2079.65 2080.4 2081.42 2250.39 2250.91 2363.39 2363.62 2390 2393 2399 2406 2446 2450 2472 2476 2481 2485 2490 2509 2511 2522 2756.38 2756.94 2757.38

Horizons 4th of Shahai formation 4th of Shahai formation 4th of Shahai formation 4th of Shahai formation 4th of Shahai formation 4th of Shahai formation 4th of Shahai formation 4th of Shahai formation 4th of Shahai formation 4th of Shahai formation 4th of Shahai formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation


TOC(%) 3.44

2.54 2.82 2.72 2.05 2.61 2.35 2.01 2.88

0.947 0.41 1.72

Ro(%) 0.573 0.605 0.536 0.534 0.46

0.8 0.73 0.855 0.72 0.573 0.915 0.91

1.61 1.6 0.0299 1.94 1.76 1.6 1.62 1.73 1.6 1.39 2.04 1.72 0.568 0.653 0.446

1.68 1.109 1.294

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36 37 38 39 40

2758.43 2758.93 2759.39 2760.48 2760.97

Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation Jiufotang formation

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0.555 0.564 0.743 1.03

Figure.4 Distribution of TOC contents of samples from well DY-1 in Fuxin Basin


1.42 1.133 1.078 1.348 1.071

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Figure. 5 Relationship between Ro and buried depth of shale samples this study


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Figure. 6 Characteristics of microscopic composition of shale samples organic matter of FuYe1well 1.Sapropelinites and microspores，fluorescence，4th member of Shahai formation 1157.621157.74 m ； 2.Group of microspores ， fluorescence ， 4th member of Shahai formation ， 1157.62-1157.74 m； 3. Sapropelic groundmass ， fluorescence ， 4th member of Shahai formation ， 2250.322250.45 m； 4. Sapropelinites，fluorescence，Jiufotang formation，1157.62-1157.74 m； 5. Huminite，fluorescence，Jiufotang formation，2756.26-2756.50 m； 6. Sapropelinites fluorescence，Jiufotang formation，2760.40.-2760.56vm


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Specifically, the shale of the 4th member of the Shahai Formation is mostly in the immature stage, and the shale of the upper member of Jiufotang formation reaches a mature stage and is at the oil window period, while the lower member of the formation has reached a high maturity stage and is at gas window period.The kerogen type was identified based on the optical characteristics of the different kerogen microstructures. The FuYe-1 shale organic matter micro-component contains a large amount of vitrinite sapropel, inertinite and microspores (figure 6).The vitrinite of the shale samples in the study area is mostly lignin and cellulose of terrestrial plants. It appears as weak fluorescence and brown, rust color, or no fluorescence under fluorescence. It is the structural and unstructured lignocellulosic part of higher plants, and in reflection a large number of sapropel groups can be observed under light, so it can be seen that the type of kerogens in this area are type II~III. The kerogen type in the deep-and semi-deep lake is type II2, while the kerogen type of shale from the environment of shore-shallow lake and delta plains developed- coal is mostly type III. 4.2 Mineralogical compositions Special attention has been to pay the mineral compositions of the shale because the mineral compositions have demonstrated to be one of the major factors influencing the reservoir property20] .The data of mineral percentage of the continental shale samples in this study are listed in table 2. The quartz contents ranges from 4% to 35% (average 21.39%), while the clay contents ranges from the 6% to 52% (average 31%). The minerals mainly include clay, quartz, feldspar, carbonates, and etc. (Table 2; Figure.7a). Clay minerals are mainly composed of illite, illite/smectite mixed layer (I/S), and a small amount of kaolinite and chlorite. The feldspar (potassium feldspar and plagioclase) ranges from 8% to 42% (average 19.5%).The carbonate (calcite and dolomite) ranges from 8% to 76% (averaged 26.6%) (Figure.7b).


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Table 2 Mineralogical compositions of the shale samples from Shahai Formation and Jiufotang Formation in FuYe-1 well Clay mineral content (%) Sample No.

K 2 5 6 7 8 9 10 11 12 15 16 17 19 20 21 22 23 25

Mineralogical composition (%)

Depth/m 1157.68 1161.46 1365 1371 1378 1384 1390 1396 2079.65 2250.39 2250.91 2363.39 2390 2393 2399 2406 2446 2472

C 4 4 2 2 2 1 1 2

I 6 9 2 3 2 3 3 3

1 1 2 2 2 2 2 3

3 2 2 2 3 3

S 15 23 20 21 23 23 20 23 42 37 50 79 21 22 11 20 14 20

I/S %S 75 45 64 45 76 30 74 25 73 20 73 20 2 74 25 72 25 58 60 62 80 50 60 20 50 74 55 74 65 85 65 76 70 81 80 74 55

Clay Quartz Orthoclase Anorthose Calcite Dolomite Pyrite Siderite 41 26 2 7 17 7 41 24 2 10 13 8 2 43 35 2 10 5 4 1 49 30 3 7 5 4 1 1 51 30 2 6 5 4 1 1 52 26 2 7 6 4 2 1 47 33 2 6 5 4 2 1 52 29 2 7 3 5 1 1 6 25 10 24 1 32 2 11 6 3 17 4 59 7 4 2 11 4 72 12 20 7 20 1 40 25 20 7 24 6 18 21 18 7 16 2 33 3 18 18 9 28 1 26 23 13 4 25 1 31 3 23 11 7 35 22 2 34 17 5 14 3 24 3


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Figure7. The contents of total rock minerals and clay minerals for the shale samples Additionally, mostly shale samples also contain a bite of pyrite (average 2%), indicating that shales samples come from anoxic environments of deep and semi-deep lake. Therefore, the shales in the Shahai formation and Jiufotang Formation have a lower content of clay minerals and a higher brittleness index from the other continental shale in China, which is advantageous for the fracturing of shales to some extent. 4.3 Characterization of Pore Morphology by Scanning Electron Microscopy After a detailed observation of SEM images of shale samples, pores type were mainly recognized in shale samples from Shahai Formation and Jiufotang Formation as follows:


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intergranular pores (figure 8 a, b), intragranular pores (figure 8 c, d),and organic pores (figure 8 e, f), which are consider to storage spaces for shale gas

Figure 8.SEM images showing pores types of shale samples Shahai and Jiufotang formations from well DY-1 shale Different pore types and size have different roles to and effects on reservoir capacity and shale-gas production21-23. Therefore, microfractures and micro-pores were greatly developed in the formation of Shahai and Jiufotang, indicating that the shale of that had abundant reservoirspace types and favorable reservoir condition4,9. 4.4 Low pressure nitrogen adsorption Low temperature absorption is widely used to describe the shale pore parameters and pore types by the isotherms. According to the shale samples used in this study, the results show that the BET surface area, total pore volume and average pore size ranges from 0.0174~0.0307×103

cm3/g，4.2367~13.2277m2/g and 7.8753~18.269 nm, with an average of 0.0242×10-3cm3/g,


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7.0785m2/g and 12.166nm respectively (Table 3).The isotherms data for the low-pressure N2 adsorption/desorption analysis from 23 shale samples can be summed up 3 pore types and are illustrated in (Figure 9).The adsorption isotherm at higher relative pressure is inconsistent with the desorption isotherm because of the capillary condensation, resulting in a hysteresis loop. The hysteresis loop is closed at low relative pressure for some samples but open for others samples. The absence of total closure of the hysteresis loop was interpreted as being due to the effect of swelling20, 24-27 (Figure 9b). The pore distribution of this curve is monomodal, and the pores size is mainly distributed around 4 nm; Type III adsorption-desorption isotherm: This type of isotherms is represented by sample19. The isotherm of adsorption and desorption do not coincide, and they are all relatively flat, but the end is steep, and there is no obvious inflection point in the desorption line ( Figure 9c); This type of loop shows that the shale is dominated by parallel plate-like holes that are open around, and there are a few ink-bottle-like holes. The pore distribution of this type of adsorption loop is bimodal, and the pores sizes are mainly distributed between 4nm and 50 nm. The Type I adsorption-desorption isotherm is represented by sample 21. Table 3 Results of Low-temperature Nitrogen adsorption experiment of shale samples this study Sample ID

Burial depth /m

Specific surface area/（ m2·g-1）

Average pore size/ (nm)

Total pore volume/（103 cm3/g）

6

1365

9.1327

94.3333

0.0246

7

1371

8.4253

98.593

0.0244

8

1378

12.4335

79.5175

0.028

9

1384

9.3328

94.8682

0.0256

10

1390

9.8907

88.4905

0.0251

11

1396

13.2277

78.7529

0.0298

19

2390

8.2707

127.9575

0.029

20

2393

6.8502

110.7436

0.0196


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21

2399

4.9183

144.4793

0.0186

22

2406

7.0844

119.0706

0.023

23

2446

6.6388

138.0711

0.0242

25

2472

8.5129

124.7953

0.0289

The adsorption isotherm rises steadily with the relative pressure (P/P0), and it suddenly rises when the relative pressure approaches 1; the adsorption loop is small, and the adsorption isotherm is small。The curves of adoption and desorption almost completely coincide. This type of loop shows that the shale is dominated by a closed hole at one end and contains a few of slitshaped holes (Figure 9a). The pore distribution of these isotherms is unimodal, and the pores are mainly distributed around 50 nm. Type II adsorption-desorption isotherms is represented by sample 10. The adsorption increases steadily with relative pressure, and the desorption isotherm begins to slowly decrease when the relative pressure decreases to reach a sharp drop of 0.5, which was due to the large amount of liquid nitrogen in the ink bottle hole that was once evacuated. This type of adsorption loop indicates that the shale is dominated by thin-necked and wide-bore tubular ink-like holes (Figure 9b). The pore distribution of this curve is monomodal, and the pores size is mainly distributed around 4 nm;


Pore volume (10-3ml.g-1)

Adsorption Desorption

(a)

Pore diameter (nm)


Adsorbed volume (ml.g-1)

Relative pressure (p/p0) Adsorption Adsorption Desorption Desorption

(b)

Relative pressure (p/p0)

Pore diameter (nm)


Adsorption


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Desorption

Relative pressure (p/p0)

(c)

Pore diameter (nm)

Figure.9 The type of nitrogen adsorption-desorption isotherms and pore diameter distribution of shale samples in this study


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Type III adsorption-desorption isotherm: This type of isotherms is represented by sample19. The isotherm of adsorption and desorption do not coincide, and they are all relatively flat, but the end is steep, and there is no obvious inflection point in the desorption line ( Figure 9c); This type of loop shows that the shale is dominated by parallel plate-like holes that are open around, and there are a few ink-bottle-like holes. The pore distribution of this type of adsorption loop is bimodal, and the pores sizes are mainly distributed between 4nm and 50 nm. 4.5 Methane Isothermal Adsorption Experiment of methane adsorption can be calculated by different models; In this study, the Langmuir equation is adopted to characterized the methane adsorption capacity of the shale as it has been similarly done previously by other researchers28, 29. Through the isotherm adsorption test, the experimental data of 10 samples collected from the wellbore 1 were obtained under the experimental conditions of 30°C and the equilibrium moisture of pure methane. Results show that the Langmuir’s Volume ranges between 0.17~1.98m3/t. with an average value of 1.21 m3/t; the Langmuir’s Volume pressure PL is between 0.18 to 1.30 MPa and the average is 0.74 MPa (Table 4, Figure. 10). The difference in the adsorption constants for each sample is larger. According to the characteristics of the adsorption constants and the depth of 2000m, these samples can be divided into two types. Shale VL with a depth of less than 2000m is generally higher than 1.5m3/t, PL is generally more than 0.5 MPa; shale VL is generally less than 1m3/t for burial depths exceeding 2000 m; PL generally does not exceeds 0.5Mpa.The Langmuir’s Volume (VL) shows that the ability of the shale reservoir to adsorb gas. The higher the value, the stronger adsorption capacity of the shale has. The Langmuir’s pressure (PL) reflects the characteristics of gas desorption under different pressure conditions.


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The larger the value, the easier the gas is to desorb in the high pressure section. Therefore, shale reservoirs that are both high in both and Langmuir’s volume and Langmuir’s pressure in the study area are conducive to the storage and extraction of shale gas. Table 4 Results of Isothermal Sorption Tests of shale samples of this work Sample ID

Burial depth (m)

Langmuir's Volume(m3/t)

Langmuir's Pressure(MPa)

6

1365

1.48

0.72

7

1371

1.54

1.08

8

1378

1.91

1.4

9

1384

1.72

1.2

10

1390

1.69

0.97

11

1396

1.99

1.29

19

2390

0.65

0.54

20

2393

0.77

0.73

21

2399

0.44

0.88

22

2406

0.58

0.64

5. Discussion 5.1 Effect of TOC content on methane adsorption capacity The organic geochemical characteristics of shale play an important role in the generation and accumulation of shale gas. Results show that TOC content exerts a strong effect on the adsorption of methane. There is a good positive correlation between the Langmuir’s volume of the shale and the TOC (Figure 7). The higher the organic carbon content, the stronger the adsorption capacity of the shale has, and the TOC contents is one of the important influencing factor of shale adsorption capacity

A positive relationship between TOC content and the


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methane adsorption capacity of organic rich shales has also been observed by other researcher30, 31.

Figure 10Characteristic of isothermal adsorption of shale samples from Fuye-1 well 5.2 Effect of mineral compositions on methane adsorption capacity Clay minerals are considered to be a significant factor influencing the shale gas adsorption capacity32, 33. Results show that there is a significant positive correlation between the clay content, illite content and the Langmuir’s Volume (VL ) (Figure 12 a ,b,). This is due to the fact that the clay minerals possess more crystal layers and porous structures which increase more considerable specific surface area to provide more adsorption site. Compared with clay minerals, brittle minerals have poor specific surface area and usually shows


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poor shale gas adsorption capacity

33, 34

(figure 12c, d); Whereas the feldspar contents and

carbonate contents shows negative correlation with Langmuir’s Volume. Quattz, however, as brittle clay minerals, yet its content show the increase trend of Langmuir’s Volume. It is probably that the shale samples mostly come from the deep and semi-deep lake environment, which has experienced a strong biogenic accumulation and, resulting in being associated with a positive correlation for methane capacity35(Figure 12,e f),. In addition it also probably caused by being related to the development of shale porosity under the support of brittle quartz36.

Figure.11 Relationship between TOC and adsorption constants of shale samples in this study


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Figure. 12 Relationship between clay mineral content and Langmuir's Volume of shale samples from Fuye-1 well 5.3 Effect of pore structure on methane adsorption capacity The pore structure of shale is a dominating factor to the methane capacity. With surface area increasing, the pore size decreases for a given pore volume37.Previous studies show that the pore size distribution controls the methane capacity38, 39.


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Parameters offered by low temperatures nitrogen absorption in this work mainly includes BET surface area, average pore size and pore volume to characterize pore structures. The specific surface area has significant positive correlation with the Langmuir’s volume, while the average pore size show significant negative correlation with Langmuir’s volume, which the correlation coefficients are 0.7776 and 0.89 (Figure 13a).It also can be found that that the increase of microporous and mesopore with the increasing the specific surface, and decrease of macropore with the decrease of Langmuir’s volume, indicating that with the average pore size increasing, the smaller specific surface area are provided, the more disadvantage to the methane adsorption capacity (Figure 13a). Figure 13b shows that there is weak positive trend between pore volume and Langmuir’s volume. Therefore, pore volume from the shales are not the determining factor to the methane capacity.

Figure. 13 Relationship between specific surface area and Average pore sizes and Langmuir's Volume of shale samples


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From the study of the relationship between methane adsorption capacity, the shale BE specific surface area ratio and the total pore volume ratio, results show that the percentage of the shale specific surface area of macropores and the total pore volume are negatively correlated with the methane adsorption capacity ,while the specific surface area and total pore volume of the shale mesoporous ratio, the surface area and percentage of total pore volume of the shale micorpore specific surface area,are positively correlated with the methane adsorption capacity, which indicate that the mescopore and micropores are the main contributors to the surface area of the shale (Figure.14a, b).

Figure. 14 Relationship between percentage of specific surface area and percentage of total pore volume and methane adsorption capacity


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Based on the investigation of the shale reservoirs conditions including pore characterization and methane adsorption capacity, the Member of Shahai and Jiufotang shales reservoir have greater Langmuir volumes, which positive correlation with clay content and quartz which are dominant mineral content for shale reservoir, indicating that the shale reservoir is favorable for shale gas exploration and exploitation. 6 Conclusion (1) The shale maturity of the Shahai Formation and the Jiufotang Formation of the Lower Cretaceous from the FuYe-1Well ranges between 0.46%-1.68%, with an average of 0.92%. It is in the immature-high maturity stage, and the shale depth is between 1157 and 1157m. There is a linear positive correlation between the maturity and the shale burial depth at -2,761m, and FuYe1 well shale is mainly formed in delta plains, shore- shallow lakes, and deep lake and semi-deep lake environments. Kerogen types of the shale are mainly of type II~III. (2) The TOC contents in the Shahai and Jiufotang formation are between 0.3%-3.44% with an average of 1.59%. Due to the variation of the lake rising levels, the TOC contents of the shale changes regularly. Among them, the organic matter abundance in the lower fourth member of the Shahai Formation in FuYe-1 well is the greatest, followed by the middle member of the Jiufotang formation. The content of organic carbon in the Shale of the lower Jiufotang formation is relatively lower, indicating that the TOC content is controlled by the shale depositional environment. (3) The pore volume, specific surface area, and average pore size of the shale are 0.01740.0307×10-3cm3/g, 4.2367-13.2277 m2/g, and 7.8753-18.269 nm respectively, and the average value is 0.0242×10-3 cm3/g. At 7.0785m2/g and 12.166nm, the specific surface area of the shale is positively correlated with the total pore volume, and negatively correlated with the Average


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pore size. The specific surface area of the shale and total pore volume are positively correlated with TOC contents. (4) The Langmuir’s volume of the shale ranges from 0.17 m3/t to 1.98 m3/t, with an average value of 1.21 m3/t, the Langmuir’s Volume pressure PL ranges from 0.02 to 1.30 MPa, with an average value of 0.74 MPa. The methane adsorption capacity is controlled by TOC contents and pore structure of the shale and is positively correlated with the total clay contents, the specific surface area and the total pore volume ratio of the macropore are negatively correlated with the methane adsorption capacity. The specific surface area and the total pore volume ration of the mesopore and the micropore are positively correlated with the methane adsorption capacity, which indicates that the micropore and mesopore are major contributors to the specific surface area of the shale. Aknowlegment: we appreciated Department of Land and Resources of Liaoning Province, China; 107 exploration team of China Northeast Coalfield Bureau, Fuxin municipal government and Liaoning technical university , which offered a lot help for this article. Note. Corresponding Author *E-mail, [email protected]; Notes The authors declare no competing financial interest.


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Characteristics of Lacustrine Shale Reservoir and Its Effect on

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