**5. Evolution of pore structure in the shale of detachment layers**

It is believed that the shale composition controlled the pore structure character, and the diagenetic processes controlled the shale composition [38, 48, 50]. In the previous analysis, we discussed the pore characteristic data obtained by different analytical methods of MICP and LPG separately [51]. **Figure 11** shows the correlation between porosity and pore volume for different pore diameters, indicating the positive relationship between porosity and micro- and mesopores PV (R2 > 0.85) for undeformed shale, while the porosity of deformed shale was only related to the macropores PV (R2 = 0.77). Such difference between the deformed and undeformed samples indicated that the micropores and mesopores (nanoscale pores) dominate the total porosity and controlled the diagenetic processes and shale compositions in the primary shale reservoir, while the macropores controlled the total porosity in the intense tectonic deformation shale. It is found that there were no significant changes in porosity between deformed and undeformed shale samples. In order to further study the evolution of pore size distribution in deformed shales, analysis method of combined the mercury intrusion and gas adsorption was used to determine the relative content percentage of micro-, meso- and macropores in this article by set the mesopores as a referenced value (**Table 2** and **Figure 12**). The balance of micro-, meso- and macropores were weakened in deformed shale samples, and the percentages of PV in different pore diameters were changed as macropores increased and mesopores decreased.

Organic matter can affect the evolution of pore structure, especially for different organic matter types and organic matter maturity. Jiang et al. reported the pore structure of a lacustrine oil shale in the Ordos Basin and indicated that the mesopores are dominant in samples [52]. Chen and Xiao measured the evolution of pore structure of artificially matured samples during an anhydrous pyrolysis, finding that the microporosity and mesoporosity increase with thermal maturity after the oil window stage [53]. The effect of organic matter on shale pore structure is mainly concentrated on micropore and mesopore [53–55] and achieved by the difference type and maturity. In the present study, all samples have a similar kerogen type and maturity stage, consistent with the similar micropores. Clay minerals can influence pore structure evolution and always have comparable organic contents. As compared to the deformed shale samples, the undeformed shale samples have higher values in micro-, meso-, total pore SA and adsorption quantity, because they have a higher clay content

To fully evaluate the pore size distribution and porosity, samples were crushed (2–5 mm), dried at 110°C and then preformed both mercury porosimetry and low pressure gas adsorption analyses. The mercury injection capillary pressure (MICP) analysis using a Poremaster GT60 and intruded with mercury from 1.5 to 60,000 Psi, the measured pressure range equates to the pore diameter range of 0.003–1000 μm (7–120,000 nm in this study). The pore size distributions of mesopores and macropores were determined using the Washburn equation [47]. The minimal pore diameter limit of 7 nm is within the mesopore range, and mercury porosimetry cannot detect micropores within the pore structure. Porosity is determined by mercury immersion (bulk density) coupled with helium pycnometry (skeletal density). Low pressure gas adsorption analyses have been used to measure the PSD both micropores and mesopores using both nitrogen adsorption at −196°C and carbon dioxide adsorption at 0°C by a Micromeritics ASAP

The relative contents of different pore size distributions with PV were calculated by set the mesopore PV as referenced

**Gas absorption MICP Relative content\***

**Macropore (μL/g)**

U1 6.4 4.8 186.7 231.9 2.0 37.6 27.9 34.6 U2 5.7 8.4 175.5 122.9 1.6 28.5 42.0 29.4 U3 10.4 20.9 2198.4 330.0 7.5 30.3 60.6 9.1 D1 4.5 2.2 73.9 185.7 1.3 36.6 18.1 45.4 D2 5.3 6.7 73.8 143.3 1.6 21.2 26.8 52.0 D3 2.2 0.4 14.2 273.2 3.8 22.6 3.8 73.5 D4 4.5 3.8 148.7 172.1 2.5 28.5 23.8 47.7 D5 5.2 4.4 214.3 387.4 4.5 15.1 12.9 71.9

**Porosity (%)**

**Micropore (%)**

**Mesopore (μL/g)**

by the same calculation models of density function theory (DFT) [48, 49]. The development of DFT models has led to a better understanding of adsorption processes in well-ordered systems compared to the more conventional models, used in the present study to analyzed N2

It is believed that the shale composition controlled the pore structure character, and the diagenetic processes controlled the shale composition [38, 48, 50]. In the previous analysis, we

 gas adsorption data. The pore characteristics including SA, PV and PSD will be different between the two techniques (MICP and gas adsorption analyses) because of sample preparation, analytical models and calculation models; analysis method of combined the mercury intrusion and gas adsorption only used to determine the relative content percentage of micro-, meso- and macropores in this article by set the mesopores as a referenced values (**Table 2**).

and CO2

 **(%)**

**Mesopore (%)**

**Macropore (%)**

gas adsorption

and

2020 HD88 analyzer. The PSD, PV and SA analysis of combined the N2

**5. Evolution of pore structure in the shale of detachment layers**

CO2

**Sample ID**

\*

value.

**Micropore (μL/g)**

58 Tectonics - Problems of Regional Settings

**Mesopore (μL/g)**

**Table 2.** Pore volume characteristics of the samples.

**Figure 11.** Relationship between total porosity and pore volume of micro-, meso- and macropores in shale samples from different analytic methods of LPG and MICP.

detachment folds. Although the deformations between the Jura Mountain and eastern Sichuan look similar at the regional scale, their deformation mechanisms are different. There are many detachment layers occurring in the strata from the Neoproterozoic to the Mesozoic in the study region that controlled the deformation. Due to the multi-tectonic movement, shale reservoirs in China are highly diversified and complicated, which transformed the texture of shale beds and resulted in structure deformed shale with unique pore properties. The structural deformation has a significant effect on the evolution of pore size distribution,

A Discussion on the Detachment Structural Deformation and Its Influence on Pore Structure…

http://dx.doi.org/10.5772/intechopen.72245

61

This research was funded by the Science and Technology Research and Development Program of China Petroleum and Chemical Corporation (No. P06088), Nonprofit Special Research Program (No. 200811015), Land Resource Survey Project of the Ministry of Land and Natural Resources, China (No. 1212010782003), Major State Research Development Program of China

(No. 2016YFC0600202) and the China Geological Survey (CGS, No. DD20160183).

Mingliang Liang, Zongxiu Wang\*, Linyan Zhang, Huijun Li, Wanli Gao and Chunlin Li

Institute of Geomechanics, Key Lab of Shale Oil and Gas Geological Survey, Chinese

[1] Suppe J. Geometrv and kinematics of fault-bend folding. American Journal of Science.

[2] Jamison WR. Geometric analysis of fold development in overthrust terranes. Journal of

[3] Poblet J, McClay K, Storti F, Munoz JA. Geometries of syntectonic sediments associated with single-layer detachment folds. Journal of Structural Geology. 1997;**19**:369-381 [4] Marrett R, Bentham PA. Geometric analysis of hybrid fault-propagation/detachment

[5] Liu C, Zhang YK, Shi B. Geometric and kinematic modeling of detachment folds with growth strata based on Bézier curves. Journal of Structural Geology. 2009;**31**:260-269 [6] McClay KR. Glossary of thrust tectonics terms. In: McClay KR, editor. Thrust Tectonics.

especially for the increase in the proportion of macroporous.

\*Address all correspondence to: wangzongxiu@sohu.com

Academy of Geological Sciences, Beijing, China

Structural Geology. 1987;**9**:207-219

Chapman and Hall; 1992. pp. 419-433

folds. Journal of Structural Geology. 1997;**19**(3-4):243-248

**Acknowledgements**

**Author details**

**References**

1983;**283**:684-721

**Figure 12.** The pore volume percentages for shale samples.

rather than because of their undeformation. Ross and Bustin suggested that shales enriched in both clays and organics have the largest micropore volumes, suggesting a micropore contribution from both the organic and clay fractions. In the present study, the shale sample of D3, which is poor in clay and TOC, has the least micro- and mesopores. Kareem et al. found that the clay minerals are over-represented at the pore surfaces and in pore spaces compared to the other major minerals such as quartz and feldspar [56]. The knowledge of effective mineralogy complicated the influence of clay minerals on the pore structure. The biogenic quartz produced by precipitation during diagenesis with silica is derived from graptolite and radiolarians [57], which may also control the pore volume and structure in shales. This type of quartz affects microporosity significantly and has certain correlation to TOC content. There are no clearly defined relationships between quartz mineralogy and pore structure, because the micropore characteristics of the samples did not change with the quartz content in this study. Furthermore, the parts of extra quartz content in fracture filling of deformed shale may come from the hydrothermal source, after the tectonic deformation and fracture generations. The relationship between shale compositions and pore structure is not well reflected in the change of shale pore structure in deformed shale samples for the present study. There is no significant difference in organic matter content in deformed and undeformed samples, which agrees with a similar microporosity on all of the samples. Allscale pore structure analysis reveals that the deformed shale had notable higher macropores percentages than undeformed shale. At the same time, the total porosity and micropores were constant, suggesting that the evolution of pore structure in structural deformed shale was due to part of mesopores was disappeared due to compression of the tectonic stress, and macropores were generated due to the development of microcracks.

#### **6. Conclusions**

A series of comb-like folds and trough-like folds in eastern Sichuan Basin were the deformation controlled by multilayer detachment, which is different from the typical Jura type detachment folds. Although the deformations between the Jura Mountain and eastern Sichuan look similar at the regional scale, their deformation mechanisms are different. There are many detachment layers occurring in the strata from the Neoproterozoic to the Mesozoic in the study region that controlled the deformation. Due to the multi-tectonic movement, shale reservoirs in China are highly diversified and complicated, which transformed the texture of shale beds and resulted in structure deformed shale with unique pore properties. The structural deformation has a significant effect on the evolution of pore size distribution, especially for the increase in the proportion of macroporous.
