**2. Deformation of the detachment layers**

There are two main detachment layers of shale in the covering layers in the study region, i.e., the Lower Cambrian Niutitang Formation and Lower Silurian Longmaxi Formation. These two detachment layers controlled the strong deformation of the study region during the late Mesozoic and showed different roles in different zones.

**Figure 3.** Shear folds formed by shear deformation in the lower Cambrian detachment layer. (a) A-type shear folding; (b) pencil structure indicates SE-NW shear (location: Labachong, Luoyixi Town, Yongshun County, Hunan); (c, d) B-type

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

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

51

**Figure 4.** Assemblage of thrust faults and shear folds in the lower Cambrian (location: Fumeng Village, Liangshuijin

Town, Yuanling County, Hunan).

shear folding (location: Mengfu Village, Liangshuijing Town, Yuanling County, Hunan).

#### **2.1. Deformation of the Cambrian system**

The detachment layer is the Niutitang Formation (Є<sup>1</sup> n) in the Lower Cambrian, which is mainly composed of black carbonaceous shale and black shale and about 70–200 m thick. This layer crops out in the region between Shizhu and Anhua-Xupu Fault, but the strong shearing along the layer only occurred in the region between the Cili-Dayong Fault and the Anhua-Xupu Fault.

Different types of small shear folds and thrust faults character the deformation of the detachment layer, including angular folds, congruous inverted folds, sheath folds and recumbent folds. B-type and A-type folds can be found (**Figures 3** and **4**). The fold hinges strike mainly northeastsouthwest. However, the fold hinges of sheath folds trend northwest-southeast, which indicated the direction of shearing. The axial planes of these folds mainly dip to the southeast at the angle of 40–70°. Combined with sheath folds, top-to-the-northwest thrusting occurred along this detachment layer, which is similar to the deformation of the Xuefeng Mountain.

## **2.2. Deformation of the Silurian detachment layer**

The Silurian detachment layer is composed of the Longmaxi Formation (S<sup>1</sup> l) of the Lower Silurian. It is the thickest detachment layer (more than 2000 m) in the study region, which is mainly composed of shale with some silty mudstone and siltstone. This layer mainly distributes in Shizhu and Baojing; it also crops out to the west of Anhua-Xupu Fault. Field observations showed that the deformation of this layer mainly occurred in the region between Shizhu and Baojing (the region where the so-called narrow syncline style folds developed).

The B-type folding characters the deformation of the Silurian detachment layer. The congruent inverted fold is the main type of the folds in this layer (**Figure 5**). Most fold hinges strike A Discussion on the Detachment Structural Deformation and Its Influence on Pore Structure… http://dx.doi.org/10.5772/intechopen.72245 51

for illite-rich shale [37]. Shale gas and coalbed methane (CBM) reservoirs are in formally grouped as unconventional because gas is trapped in part by sorption processes [38]. Several previous workers have focused on the pore characteristics and adsorption gas characteristics of tectonically deformed coals [39, 40], which provide references to the study on deformed shale. Although previous studies have provided useful insight to evolution of pore structure in shale during thermal maturity and in CBM related to deformation structures, the prediction of structural deformation change in pore structure of shale in geological condition still remains quite challenging. In the present study, pore structure feature investigations are performed using two sets of shale samples (deformed shale and undeformed shale) collected from the typical shale bed of the Longmaxi Formations (Lower Silurian) of Western of the Xuefeng Mountain, South China. The influence of structural deformation on pore structure

There are two main detachment layers of shale in the covering layers in the study region, i.e., the Lower Cambrian Niutitang Formation and Lower Silurian Longmaxi Formation. These two detachment layers controlled the strong deformation of the study region during the late

composed of black carbonaceous shale and black shale and about 70–200 m thick. This layer crops out in the region between Shizhu and Anhua-Xupu Fault, but the strong shearing along the layer only occurred in the region between the Cili-Dayong Fault and the Anhua-Xupu Fault. Different types of small shear folds and thrust faults character the deformation of the detachment layer, including angular folds, congruous inverted folds, sheath folds and recumbent folds. B-type and A-type folds can be found (**Figures 3** and **4**). The fold hinges strike mainly northeastsouthwest. However, the fold hinges of sheath folds trend northwest-southeast, which indicated the direction of shearing. The axial planes of these folds mainly dip to the southeast at the angle of 40–70°. Combined with sheath folds, top-to-the-northwest thrusting occurred along

this detachment layer, which is similar to the deformation of the Xuefeng Mountain.

and Baojing (the region where the so-called narrow syncline style folds developed).

Silurian. It is the thickest detachment layer (more than 2000 m) in the study region, which is mainly composed of shale with some silty mudstone and siltstone. This layer mainly distributes in Shizhu and Baojing; it also crops out to the west of Anhua-Xupu Fault. Field observations showed that the deformation of this layer mainly occurred in the region between Shizhu

The B-type folding characters the deformation of the Silurian detachment layer. The congruent inverted fold is the main type of the folds in this layer (**Figure 5**). Most fold hinges strike

The Silurian detachment layer is composed of the Longmaxi Formation (S<sup>1</sup>

n) in the Lower Cambrian, which is mainly

l) of the Lower

was discussed.

50 Tectonics - Problems of Regional Settings

**2. Deformation of the detachment layers**

Mesozoic and showed different roles in different zones.

The detachment layer is the Niutitang Formation (Є<sup>1</sup>

**2.2. Deformation of the Silurian detachment layer**

**2.1. Deformation of the Cambrian system**

**Figure 3.** Shear folds formed by shear deformation in the lower Cambrian detachment layer. (a) A-type shear folding; (b) pencil structure indicates SE-NW shear (location: Labachong, Luoyixi Town, Yongshun County, Hunan); (c, d) B-type shear folding (location: Mengfu Village, Liangshuijing Town, Yuanling County, Hunan).

**Figure 4.** Assemblage of thrust faults and shear folds in the lower Cambrian (location: Fumeng Village, Liangshuijin Town, Yuanling County, Hunan).

**Figure 5.** B-type shear folds with SE dipping axial planes in the Silurian detachment layer (location: (a, b) Shaoha Village, Shaoha Town, Yongshun county, Hunan; (c, d) Heixi Town, Pengshui county, Chongqing).

**Figure 6.** Widespread thrust folding in the eastern Sichuan Basin (location: Wanfo Village, Gaoleshan Town, Xianfeng

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

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

53

**Figure 7.** Multi-layer detaching-thrusting deformation section shown by the seismic reflection. Tt<sup>3</sup>

, upper Permian; Tp1

, middle Cambrian; TЄ<sup>1</sup>

, lower Permian; Ts2

, lower Cambrian.

j, lower Triassic Jialingjiang Fm; Tp<sup>2</sup>

m, lower Ordovician Meitan Fm; TЄ<sup>2</sup>

x, upper Triassic

, lower

, middle Silurian; Ts1

County, Hubei).

Xujiahe Fm; Tt<sup>1</sup>

Silurian; To1

northeast-southwest; some AB-type inverted folds whose hinges strike north-east are also found. The axial planes of these folds mainly dip to the southeast with dip angle ranging from 20 to 60°. And some inverted folds with axial plane dipping to the northwest are also developed. In addition, many minor thrust faults top to the northwest are also developed in the layer. These structures imply that the top-to-the-northwest thrusting parallel to the bedding is the main deformation mechanism, and the inverted folds with axial plane dipping to the northwest resulted from the back thrusting.
