**4. Detachment layers in shale of Longmaxi Formation and its structural deformation characteristics**

**Figure 8.** Block model of the deformation assemblage of multi-layer detachment and thrust fault between the Huayin

B-type and AB-type inverted folds, recumbent folds and small thrust faults character the deformation of the detachment layers, which generally imply the top-to-the-northwest detachment. And, the symmetrical large open folds and large reclined folds character the deformation of the non-detachment layers, and most geomorphic highs are located at the large reclined folds, such as the Huayin Shan, Tongluo Shan, Mingyue Shan, Fangdou Shan and Qiyao Shan.

Most anticlines between the Fangdou Shan and the Huayin Shan (where the narrow-anticline style folds developed) are narrow with steep northwest limbs dipping at 70–85° and shallow dipping southeast limbs dipping at 40–65°. Between the anticlines are the open synclines. And most northeast trending anticlines between the Fangdou Shan to the west and the Xuefeng Shan to the east (where the narrow-syncline style folds formed) are large open asymmetrical folds with steep northwest limbs and shallow southeast limbs except for some folds with steep southeast limbs such as the asymmetrical folds in Wanzhou along the east bank of the Yangtze River. The field observation showed that thrust faults often developed in the cores and steep limbs of these asymmetrical folds, and the thrust faults in the cores did not cut through the folds (**Figure 6**); similar characteristics could be found in the seismic profiles (**Figure 7**). These large reclined folds resulted from the thrusting; however, the thrust faults


, lower Triassic-upper Permian; P1

, lower Cambrian; Z, Sinian; Pt, Precambrian (Banxi Gp);


Mountain and the Xuefeng Mountain. T-J, Triassic-Jurassic; T<sup>1</sup>


Devonian; S, Silurian; Є<sup>2</sup>

DS1, detachment layer and its number.

54 Tectonics - Problems of Regional Settings

To obtain information about the influence of tectonic deformation on shale pore characteristics, the geochemical, mineralogical, structural and textural properties analysis, porosity and pore structure feature investigations are performed using two sets of shale (deformed shale and undeformed shale) collected from the same shale bed of the Longmaxi Formation (Lower Silurian) of southeast of Sichuan Basin, China. Previous studies have shown that the Upper and Middle Yangtze plates have superior marine hydrocarbon geological conditions and immense potential for shale gas. Compared to North America, the geological conditions of gas shale reservoirs in South China are highly diversified and complicated due to the detachment structural deformation, which transformed the structure of shale seams and resulted in structure deformed shale with unique reservoir properties. The detachment layers in the study region show a wide range of deformation styles caused by shearing along the layers: A-type; S-C fabric; sheared puddings; cleavage and thrusts. As the main detachment structure belt [35], the Longmaxi Formations shale layer developed multilayer subdivided slip structural deformation. The primary structure of deformed shale is damaged and the parallel bedding has almost disappeared due to deformation. The plastic deformation of shale is obvious, and the cleavage structure and cleavage surface are smooth with finegrained powder coatings. The detachment fault mirrors (FMs), scratches and micro-fold deformation phenomena were commonly present in Longmaxi shale outcrops. A suite of samples were subset to two sets of deformed shale and undeformed shale for this study due to their variability of texture, fabric and structure properties (**Figures 9** and **10**). To evaluate the influence of detachment deformation on shale reservoir characteristics by comparing the deformed shale and undeformed sample subsets, 14 shale samples (9 undeformed samples from 3 beds) were collected from this Yongshun outcrop (**Figure 9**). However, not all the 14 samples were examined. All samples were analyzed to determine the organic geochemistry and mineralogy, the undeformed samples taken from same bed have similar composition

**Figure 9.** Location of study area is in southeastern Sichuan Basin, Yongshun, China. And the detachment structural deformation control across the Longmaxi Formations shale in this area. (a) The stratigraphic relationships and structural deformation characteristics of the samples, (b) S-C structural deformation shale, (c) Sheath-like structure deformation shale (d) Fault mirrors (FMs) of the deformed shale. : Deformed shale samples, : Undeformed shale samples. The height of the notebook is 20 cm (a), length of the hammer is 38 cm (b) and the length of the pen is 14 cm (c). See **Figure 2** and **Table 1** for texture and structure properties of these deformed samples.

**Figure 10.** Undeformed shale and deformed shale samples; the sample ID was abbreviated as U1, U2, U3 (Undeformed shale) and D1, D2, D3, D4, D5 (Deformed shale). The undeformed shales have original parallel bedding, and the primary structure of shale can be observed. The detachment fault mirrors (FMs) can be observed on the deformed shales. And the deformed samples were subset to two sets of strong deformed shale and weak deformed shale due to their difference on the degree of deformation strength. The D1, D2 and D3 were strong deformed shales that the primary structure was damaged and the parallel bedding has almost disappeared due to deformation, and the fractures and mineral filling development in the strong deformed shales. The D4 and D5 were weak deformed shales that have original parallel

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57

**Carbonate (%)**

U1 3.9 2.57 −30.4 41 nd 44 Undeformed shales. Shale primary structure

D1 2.1 3.01 −30.6 77 nd 14 Strong deformed shales. The primary structure

D4 1.8 2.90 −30.6 46 nd 41 Weak deformed shales. Shale has original

TOC, total organic carbon (%); VRo, vitrinite reflectance values (%); (Number), number of measured samples.

**Clay (%)**

**hand specimens**

**Texture and fabric features of macroscopic** 

can be observed. Shale has original parallel

of shale is damaged and the parallel bedding has almost disappeared due to deformation. Fractures and mineral filling development. The plastic deformation of shale is obvious. Shale shows cleavage structure and cleavage surface is smooth with fine-grained powder coatings

parallel bedding. Shale shows cleavage structure and cleavage surface is smooth with

fine-grained powder coatings

bedding, and FMs developed in the surface of fault. The coin diameter is 20.5 mm.

**Quartz (%)**

bedding U2 2.5 2.78 −30.8 <sup>40</sup> <sup>2</sup> <sup>41</sup>

2.8 2.76 −30.7(3) 42 4.6 38

**Sample ID TOC**

U-mean (9)

D-mean (5)

**(%)**

**VRo (%)**

**δC<sup>13</sup> ‰ (PDB)**

U3 2.0 2.77 −30.9 34 11 33

D2 6.6 2.86 −31.0 64 nd 26 D3 1.0 2.91 −31.3 77 nd 16

D5 3.0 3.00 −31.1 71 nd 20

**Table 1.** The features of the experimental samples.

2.9 2.93 −30.9 67 nd 23

characteristics and eight samples (three undeformed samples and five deformed samples) were selected for mercury injection capillary pressure (MICP) and low-pressure gas (LPG) adsorption to determine the pore structure, including the porosity, pore-size distribution (PSD), surface area (SA) and pore volume (PV).

The sample number was abbreviated as U1, U2, U3 (Undeformed shale) and D1, D2, D3, D4, D5 (Deformed shale), the deformed samples subsets to strong deformed shale (D1, D2, D3) and weak deformed shale (D4, D5). The samples of U3 were collected from the thick sandy shale, and the other samples were collected from the thin siliceous shale. The stratigraphic relationships of the samples are shown in **Figure 9**. The features of the experimental samples are shown in **Table 1**. Before geochemical and mineralogical analyses, samples crushed to 180–200 mesh. The TOC content was collected using a Leco C/S-344 Carbon/Sulfur analyzer. The stable carbon isotope was determined using a Finnigan MAT 252 mass spectrometer. A 3Y–Leica DMR XP microscopy equipped with a microphotometer was used to measure the vitrinite reflectance values (VRo) of samples. Each sample was determined at least by 30 measurements on vitrinite particles. The crushed samples were mixed with ethanol, hand ground in a mortar and pestle and then smear mounted on glass slides for X-ray diffraction (XRD) analysis using a D/Max-III analyzer at 40 kV and 30 mA. The relative mineral contents were estimated and semi-quantified using the area under the curve for the major peaks.

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**Figure 10.** Undeformed shale and deformed shale samples; the sample ID was abbreviated as U1, U2, U3 (Undeformed shale) and D1, D2, D3, D4, D5 (Deformed shale). The undeformed shales have original parallel bedding, and the primary structure of shale can be observed. The detachment fault mirrors (FMs) can be observed on the deformed shales. And the deformed samples were subset to two sets of strong deformed shale and weak deformed shale due to their difference on the degree of deformation strength. The D1, D2 and D3 were strong deformed shales that the primary structure was damaged and the parallel bedding has almost disappeared due to deformation, and the fractures and mineral filling development in the strong deformed shales. The D4 and D5 were weak deformed shales that have original parallel bedding, and FMs developed in the surface of fault. The coin diameter is 20.5 mm.


TOC, total organic carbon (%); VRo, vitrinite reflectance values (%); (Number), number of measured samples.

**Table 1.** The features of the experimental samples.

characteristics and eight samples (three undeformed samples and five deformed samples) were selected for mercury injection capillary pressure (MICP) and low-pressure gas (LPG) adsorption to determine the pore structure, including the porosity, pore-size distribution

**Figure 9.** Location of study area is in southeastern Sichuan Basin, Yongshun, China. And the detachment structural deformation control across the Longmaxi Formations shale in this area. (a) The stratigraphic relationships and structural deformation characteristics of the samples, (b) S-C structural deformation shale, (c) Sheath-like structure deformation shale (d) Fault mirrors (FMs) of the deformed shale. : Deformed shale samples, : Undeformed shale samples. The height of the notebook is 20 cm (a), length of the hammer is 38 cm (b) and the length of the pen is 14 cm (c). See **Figure 2**

The sample number was abbreviated as U1, U2, U3 (Undeformed shale) and D1, D2, D3, D4, D5 (Deformed shale), the deformed samples subsets to strong deformed shale (D1, D2, D3) and weak deformed shale (D4, D5). The samples of U3 were collected from the thick sandy shale, and the other samples were collected from the thin siliceous shale. The stratigraphic relationships of the samples are shown in **Figure 9**. The features of the experimental samples are shown in **Table 1**. Before geochemical and mineralogical analyses, samples crushed to 180–200 mesh. The TOC content was collected using a Leco C/S-344 Carbon/Sulfur analyzer. The stable carbon isotope was determined using a Finnigan MAT 252 mass spectrometer. A 3Y–Leica DMR XP microscopy equipped with a microphotometer was used to measure the vitrinite reflectance values (VRo) of samples. Each sample was determined at least by 30 measurements on vitrinite particles. The crushed samples were mixed with ethanol, hand ground in a mortar and pestle and then smear mounted on glass slides for X-ray diffraction (XRD) analysis using a D/Max-III analyzer at 40 kV and 30 mA. The relative mineral contents were

estimated and semi-quantified using the area under the curve for the major peaks.

(PSD), surface area (SA) and pore volume (PV).

56 Tectonics - Problems of Regional Settings

and **Table 1** for texture and structure properties of these deformed samples.


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

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**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.

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

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

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 2020 HD88 analyzer. The PSD, PV and SA analysis of combined the N2 and CO2 gas adsorption 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 and CO2 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**).
