A New Insight into Shale-Gas Accumulation Conditions and Favorable Areas of the Xinkailing Formation in the Wuning Area, North-West Jiangxi, China

: In north-west Jiangxi, China, most shale-gas exploration has been focused on the Lower Cambrian Hetang and Guanyintang formations, whereas the Upper Ordovician Xinkailing formation shale has been ignored for years due to heavy weathering. This study systematically analyzed gas source conditions, reservoir conditions and gas-bearing ability in order to reveal the shale-gas accumulation conditions of the Xinkailing formation. The results show that the Xinkailing formation is characterized by thick deposition of black shale (10–80 m), high organic content (with total organic carbon between 1.18% and 3.11%, on average greater than 2%), relatively moderate thermal evolution (with vitrinite reﬂectance between 2.83% and 3.21%), high brittle-mineral content (greater than 40%), abundant nanopores and micro-fractures, very good adsorption ability (adsorption content between 2.12 m 3 /t and 3.47 m 3 /t, on average about 2.50 m 3 /t), and strong sealing ability in the underlying and overlying layers, all of which favor the generation and accumulation of shale gas. The Wuning-Lixi and Jinkou-Zhelin areas of the Xinkailing formation were selected as the most realistic and favorable targets for shale-gas exploration and exploitation. In conclusion, the Wuning area has great potential and can provide a breakthrough in shale gas with further investigation.


Introduction
Shale-gas exploration and development have seen enormous commercial success in the United States over the past 10 years [1][2][3], altering its status from an oil and gas importing country to an exporting one. Inspired by that, shale gas research has been conducted throughout marine, transitional and continental strata in China recently [4][5][6][7]. Significant progress has been made in shale gas exploration and exploitation in the Upper Yangtze Platform, most notably in the Upper Ordovician Wufeng formation-Lower Silurian Longmaxi formation [8,9].
With the growing demand for national energy production, several studies have been carried out concerning Lower Paleozoic black shale in the middle-upper Yangtze area [10][11][12][13]. Most of these studies have focused on the two high-quality formations [14][15][16][17][18]: the Upper Ordovician Wufeng formation/Lower Silurian Longmaxi formation (collectively known as the Longmaxi formation) and the Lower Cambrian Niutitang formation (including the Shuijingtuo formation or Qiongzhusi formation, collectively known as the Niutitang formation). It is worth noting that both formations For this study, the thickness of Xinkailing shale in the middle of the Xiuwu Basin was investigated using a measured profile. Then, features such as organic chemistry, mineral composition, reservoir properties, and pore distribution and structure were analyzed using laboratory testing of shale samples from the water well. The primary objectives of this paper are to: (1) characterize the hydrocarbon generation conditions of the Xinkailing shale; (2) determine whether the reservoir is suitable for shale gas; and (3) discuss the gas-bearing potential and favorable distribution areas within the Xinkailing formation. For this study, the thickness of Xinkailing shale in the middle of the Xiuwu Basin was investigated using a measured profile. Then, features such as organic chemistry, mineral composition, reservoir properties, and pore distribution and structure were analyzed using laboratory testing of shale samples from the water well. The primary objectives of this paper are to: (1) characterize the hydrocarbon generation conditions of the Xinkailing shale; (2) determine whether the reservoir is suitable for shale gas; and (3) discuss the gas-bearing potential and favorable distribution areas within the Xinkailing formation.

Overview
The study area is situated in the eastern Xiuwu Basin, Lower Yangtze Plate. The administrative regions are mainly comprised of Wuning county, in the north-west area of Jiangxi province (as shown in Figure 2a). The formation and evolution of the area can be broken down into four main stages (as shown in Figure 2b): (1) the Mesoproterozoic-Silurian (Pt 2 -S) basal formation and deformation stage; (2) the Devonian-middle Triassic (D-T 2 ) passive continental margin stage; (3) the Late Triassic-Cretaceous (T 3 -K) intense orogeny stage; and (4) the Jurassic-Quaternary (J-Q) down-faulted red basin stage. Due to tectonic compression during the T 3 -K, folds and faults are well developed in the study area (as shown in Figure 2c). The folds are present in a primarily east-west syncline. The faults in the study area are primarily detachment faults and normal faults, both of which appear in an east-west trend. The Ordovician-silurian shale tectonic deformation was weaker than the Cambrian shale formation [24].

Overview
The study area is situated in the eastern Xiuwu Basin, Lower Yangtze Plate. The administrative regions are mainly comprised of Wuning county, in the north-west area of Jiangxi province (as shown in Figure 2a). The formation and evolution of the area can be broken down into four main stages (as shown in Figure 2b): (1) the Mesoproterozoic-Silurian (Pt2-S) basal formation and deformation stage; (2) the Devonian-middle Triassic (D-T2) passive continental margin stage; (3) the Late Triassic-Cretaceous (T3-K) intense orogeny stage; and (4) the Jurassic-Quaternary (J-Q) down-faulted red basin stage. Due to tectonic compression during the T3-K, folds and faults are well developed in the study area (as shown in Figure 2c). The folds are present in a primarily east-west syncline. The faults in the study area are primarily detachment faults and normal faults, both of which appear in an east-west trend. The Ordovician-silurian shale tectonic deformation was weaker than the Cambrian shale formation [24].

Shale-Distribution Characteristics
The organic-rich shale of the Xinkailing formation was developed in a transgressive system tract environment, which is primarily made up of deep-water sand-mud shelf sediments and a carbon-mud shelf. The strata profile measurements of the Xinkailing formation was conducted in section PM1 (the location of which as shown in Figure 2), which revealed both the top and bottom boundary of the Xinkailing formation (as shown in Figure 3).

Shale-Distribution Characteristics
The organic-rich shale of the Xinkailing formation was developed in a transgressive system tract environment, which is primarily made up of deep-water sand-mud shelf sediments and a carbonmud shelf. The strata profile measurements of the Xinkailing formation was conducted in section PM1 (the location of which as shown in Figure 2), which revealed both the top and bottom boundary of the Xinkailing formation (as shown in Figure 3). Above the top surface, silty mudstone from the Lishuwo formation was found, and beneath the bottom surface was argillaceous shale from the Huangnigang formation, both of which are characterized by a powerful sealing capacity. The lithology of the Xinkailing formation changed vertically. In the lower section, black shale dominated with intermittent bedding of grey-black silty shale, and this Above the top surface, silty mudstone from the Lishuwo formation was found, and beneath the bottom surface was argillaceous shale from the Huangnigang formation, both of which are characterized by a powerful sealing capacity. The lithology of the Xinkailing formation changed vertically. In the lower section, black shale dominated with intermittent bedding of grey-black silty shale, and this horizontal bedding was well developed with few graptolites. In the middle section, black calcareous shale developed with numerous graptolites and the horizontal bedding was clearly visible. In the upper section, highly-weathered grey black shale appeared with a pale-yellow surface, and the presence of graptolites decreased as depth from the surface decreased. The total thickness of these three members was 72.56 m. Fortunately, a water well (drilling began in the lower section) nearby PM1 revealed 18 m of black shale with sustained thickness and developed pyrite, which was a positive signal of un-weathered Xinkailing-formation shale. This suggested that Xinkailing formation shale developed in a deep-water shelf with a strong anoxic environment, which is favorable for the preservation of organic matter [25,26]. The lithology of the Xinkailing formation was mainly comprised of black mud shale and carbonaceous shale. In the whole area, the thickness of organic-rich shale varies from 10 m to 80 m (with an average thickness of 50 m). The sub-surface depth is controlled by the tectonic distribution and in the Wuning-Lixi area is relatively deep (ranging from 500 m to 3000 m), while the depth in the Jinkou-Zhelin area is generally less than 1500 m.

Data and Methodology
Thirty core samples and 33 outcrop samples were taken from the water well 18 m shale and section PM1 75 m shale, respectively. With regard to these samples, relevant experiments were conducted and abundant data gathered. Source-rock data (including organic matter type, total organic carbon, vitrinite reflectance) were obtained from 30 core samples and 33 outcrop samples and reservoir data (including mineral composition, porosity and permeability, scanning electron microscope (SEM) (FEI Quanta200F, FEI, Hillsboro, OR, USA), Brunauer-Emmett-Teller (BET) specific surface area and isothermal adsorption) were obtained from 30 core samples. The types of organic matter were divided using the following criteria [13]: type I kerogen carbon isotopes are less than −29 organic matter, while type II are greater than −29 . The R o values were calculated using Equation (1) [27]: where R o refers to vitrinite reflectance (%) and R b refers to bitumen reflectance (%). In addition, illite crystallinity could also reflect high levels of evolution for marine shale maturity [28]. Source-rock data (organic matter type, total organic carbon, and vitrinite reflectance) and reservoir data (mineral composition, porosity and permeability, BET specific surface area, and isothermal adsorption) were provided by the Keyuan Engineering Testing Center of Sichuan Province. SEM observations were conducted in the China Petroleum University, Beijing.

Hydrocarbon-Generation Conditions
Based on the criteria of Liang et al. [13], samples from the Xinkailing formation in Wuning were analyzed using the kerogen isotope analysis method to determine their carbon isotope percentage (as shown in Figure 4). Results range from −30.67 to 27.34 with an average value of −29.03 . For that reason, the main types of organic matter were classified into type I and type II.
Values for total organic carbon content of shale with commercial potential are generally more than 2% [29]. Statistical data analysis of 30 core samples and 33 outcrop samples showed that the total organic carbon content of the core samples ranged from 1.18% to 3.11%, with an average value 2.11% (as shown in Figure 5). The majority of these samples were greater than 2%, representing resource rock with good potential, while the TOC content for the outcrop samples ranged from 0.1% to 1.21%, with an average value of 0.4%. The majority of these samples were less than 1%, representing resource rock with low potential. Comparison between core samples and outcrop samples demonstrated that the shale outcrops are easily weathered, resulting in lower total organic carbon content. Consequently, the outcrop samples do not accurately represent the actual organic matter abundance in the Wuning area. The R o values for 10 core samples from the Xinkailing formation shale were calculated based on the method of Feng et al. [27], and values ranged from 2.83% to 3.21%, with an average value of 2.99%. Illite crystallinity values ranged from 0.35 to 0.37, with an average of 0.356, which implies that the Xinkailing formation has been beneath the surface for a long time at a very low diagenetic stage. Generally speaking, the analyses of shale samples indicated that the degrees of evolution for the Xinkailing formation in Wuning were relatively high and the thermal evolution of the Xinkailing shale occurred during the early-middle stage of over-maturity [30,31], with dry gas predominating.
Energies 2018, 11, 12 6 of 17 Illite crystallinity values ranged from 0.35 to 0.37, with an average of 0.356, which implies that the Xinkailing formation has been beneath the surface for a long time at a very low diagenetic stage. Generally speaking, the analyses of shale samples indicated that the degrees of evolution for the Xinkailing formation in Wuning were relatively high and the thermal evolution of the Xinkailing shale occurred during the early-middle stage of over-maturity [30,31], with dry gas predominating.  These three geochemical research results, which found that the shale organic matter type was optimum, the organic matter carbon content met good source rock conditions, and maturity remained relatively moderate, imply that the Xinkailing formation possesses excellent conditions for hydrocarbon generation. Comparisons of the organic geochemical test results between Xinkailingformation black shale in Wuning area and lower Paleozoic shale in the middle-upper Yangtze area were conducted (as shown in Table 1), with results showing that Xinkailing formation shale has excellent gas-resource potential with similar characteristics.  Illite crystallinity values ranged from 0.35 to 0.37, with an average of 0.356, which implies that the Xinkailing formation has been beneath the surface for a long time at a very low diagenetic stage. Generally speaking, the analyses of shale samples indicated that the degrees of evolution for the Xinkailing formation in Wuning were relatively high and the thermal evolution of the Xinkailing shale occurred during the early-middle stage of over-maturity [30,31], with dry gas predominating.  These three geochemical research results, which found that the shale organic matter type was optimum, the organic matter carbon content met good source rock conditions, and maturity remained relatively moderate, imply that the Xinkailing formation possesses excellent conditions for hydrocarbon generation. Comparisons of the organic geochemical test results between Xinkailingformation black shale in Wuning area and lower Paleozoic shale in the middle-upper Yangtze area were conducted (as shown in Table 1), with results showing that Xinkailing formation shale has excellent gas-resource potential with similar characteristics.  These three geochemical research results, which found that the shale organic matter type was optimum, the organic matter carbon content met good source rock conditions, and maturity remained relatively moderate, imply that the Xinkailing formation possesses excellent conditions for hydrocarbon generation. Comparisons of the organic geochemical test results between Xinkailing-formation black shale in Wuning area and lower Paleozoic shale in the middle-upper Yangtze area were conducted (as shown in Table 1), with results showing that Xinkailing formation shale has excellent gas-resource potential with similar characteristics.

Permeability and Porosity
Porosity and permeability test results for Xinkailing-formation black shale revealed that the porosity of the samples ranged from 1.10% to 1.22%, with an average value of 1.16%, and the permeability of the samples ranged from 0.000445 md to 0.001297 md, with an average of 0.000872 md. This means that both overall porosity and permeability are extremely low. Comparison of the physical characteristics of the Xinkailing formation black shale in the Wuning area and the lower Paleozoic shale in the middle-upper Yangtze area showed that low porosity and permeability are general characteristics of shale reservoirs (as shown in Figure 6) [35].

Permeability and Porosity
Porosity and permeability test results for Xinkailing-formation black shale revealed that the porosity of the samples ranged from 1.10% to 1.22%, with an average value of 1.16%, and the permeability of the samples ranged from 0.000445 md to 0.001297 md, with an average of 0.000872 md. This means that both overall porosity and permeability are extremely low. Comparison of the physical characteristics of the Xinkailing formation black shale in the Wuning area and the lower Paleozoic shale in the middle-upper Yangtze area showed that low porosity and permeability are general characteristics of shale reservoirs (as shown in Figure 6) [35].

Reservoir Storage-Space Types
Based on the core observations and SEM, the reservoir space types were also studied. At the macro level, the fracture types of the Xinkailing formation in Wuning are dominated by structural fractures and non-structural fractures. Structural fractures are the main fracture type of the Xinkailing formation in Wuning, such as vertical shearing cracks (as shown in Figure 7a) with straight seams and no fill. Non-structural fractures take second place; including interlayer lamellation seams and diagenetic shrinkage joints (as shown in Figure 7b). Diagenetic shrinkage joints are generally found in shale with high silicon content, formed by a shrinkage effect due to chemical changes during diagenesis. Additionally, vertical shearing fractures and interlayer lamellation seams could form abundant reticular fracture systems (as shown in Figure 7c), which could greatly enhance the reservoir space and permeability, forming crucial pathways for shale gas migration from the source to the reservoir [17,36,37].
At the micro level, Xinkailing-formation shale included four types: pores (cracks) between clay minerals, intergranular dissolution pores, pores between minerals, and organic matter pores. Pores (cracks) between clay minerals are about 1μm in size, dominated by micro fractures in the mineral edges (as shown in Figure 7d). Intergranular dissolution pores are relatively large, ranging from 1μm to 20 μm (as shown in Figure 7e). Pores between minerals are mainly composed of pyrite intergranular nanometer pores, whose apertures range from 20 nm to 200 nm (as shown in Figure 7f). The diameters of the organic matter pores range from 2 nm to 100 nm, featuring irregular striping or a honeycomb irregular elliptical shape (as shown in Figure 7g-i). Different pore types and apertures have different contributions to and effects on reservoir capacity and shale-gas production [36,38,39]. Therefore, fractures and micro-pores were greatly developed in the Xinkailing formation of Wuning, indicating that the Xinkailing-formation shale had abundant reservoir-space types and excellent reservoir conditions.

Reservoir Storage-Space Types
Based on the core observations and SEM, the reservoir space types were also studied. At the macro level, the fracture types of the Xinkailing formation in Wuning are dominated by structural fractures and non-structural fractures. Structural fractures are the main fracture type of the Xinkailing formation in Wuning, such as vertical shearing cracks (as shown in Figure 7a) with straight seams and no fill. Non-structural fractures take second place; including interlayer lamellation seams and diagenetic shrinkage joints (as shown in Figure 7b). Diagenetic shrinkage joints are generally found in shale with high silicon content, formed by a shrinkage effect due to chemical changes during diagenesis. Additionally, vertical shearing fractures and interlayer lamellation seams could form abundant reticular fracture systems (as shown in Figure 7c), which could greatly enhance the reservoir space and permeability, forming crucial pathways for shale gas migration from the source to the reservoir [17,36,37].
At the micro level, Xinkailing-formation shale included four types: pores (cracks) between clay minerals, intergranular dissolution pores, pores between minerals, and organic matter pores. Pores (cracks) between clay minerals are about 1µm in size, dominated by micro fractures in the mineral edges (as shown in Figure 7d). Intergranular dissolution pores are relatively large, ranging from 1 µm to 20 µm (as shown in Figure 7e). Pores between minerals are mainly composed of pyrite intergranular nanometer pores, whose apertures range from 20 nm to 200 nm (as shown in Figure 7f). The diameters of the organic matter pores range from 2 nm to 100 nm, featuring irregular striping or a honeycomb irregular elliptical shape (as shown in Figure 7g-i). Different pore types and apertures have different contributions to and effects on reservoir capacity and shale-gas production [36,38,39]. Therefore, fractures and micro-pores were greatly developed in the Xinkailing formation of Wuning, indicating that the Xinkailing-formation shale had abundant reservoir-space types and excellent reservoir conditions.  Figure 8a-d, It can be observed that the N2 adsorption-desorption isotherms of the shale samples belong to the type IV isotherm (isotherm with a hysteresis loop) according to the International Union of Pure and Applied Chemistry (IUPAC) classification [40], indicating that the shale samples contain both mesopores (2-50 nm) [41] and macropores (>50 nm).
Additionally, nanometer-scale spatial ordering occurs in the meso-pore-macro-pore range for all the samples, which may be associated with the organic matter pores and pores (cracks) between clay minerals [42,43].  Figure 8a-d, It can be observed that the N 2 adsorption-desorption isotherms of the shale samples belong to the type IV isotherm (isotherm with a hysteresis loop) according to the International Union of Pure and Applied Chemistry (IUPAC) classification [40], indicating that the shale samples contain both mesopores (2-50 nm) [41] and macropores (>50 nm).
Additionally, nanometer-scale spatial ordering occurs in the meso-pore-macro-pore range for all the samples, which may be associated with the organic matter pores and pores (cracks) between clay minerals [42,43].

Mineral Composition and Lithofacies
The Upper Ordovician Xinkailing shale-formation lithology is mainly comprised of black carbonaceous shale with abundant interlayer lamellation seams and layered pyrite (as shown in Figure 7a-c). X-ray sampling of whole-rock mineral content found that the mineral composition of Xinkailing-formation shale is dominated by brittle minerals and clay minerals (as shown in Figure 9a). In addition, pyrite developed throughout the formation.

Mineral Composition and Lithofacies
The Upper Ordovician Xinkailing shale-formation lithology is mainly comprised of black carbonaceous shale with abundant interlayer lamellation seams and layered pyrite (as shown in Figure 7a-c). X-ray sampling of whole-rock mineral content found that the mineral composition of Xinkailing-formation shale is dominated by brittle minerals and clay minerals (as shown in Figure 9a). In addition, pyrite developed throughout the formation.
The clay mineral content ranges from 32% to 46% (average: 39.66%); the quartz content ranges from 32% to 46% (average: 39.46%); the feldspar content ranges from 10% to 22% (average: 15.76%); the carbonate mineral content ranges from 0% to 9% (average: 0.83%); the siderite content ranges from 1% to 3% (average: 1.33%); and the pyrite content ranges from 1 to 12% (average: 2.83%). The brittle mineral (including quartz, feldspar, and carbonate) content ranges from 51% to 64% (average: 56.06%) (as shown in Figure 9a). Results from the clay minerals' analysis showed that clay minerals in the Xinkailing formation contain few chlorites, kaolinite, and illite-montmorillonite mixed-layers, and are dominated by illite. The illite content ranges from 73% to 97% (average: 85%) (as shown in Figure 9b). The mineral composition is closely related to high thermal evolution and diagenesis. and are dominated by illite. The illite content ranges from 73% to 97% (average: 85%) (as shown in Figure 9b). The mineral composition is closely related to high thermal evolution and diagenesis. Research has classified the shale in the Fuling area into four lithofacies (major classes) and 16 lithofacies (minor classes) [44] based on a 3-end-mumber diagram analysis of the silicon minerals, carbonate minerals, and clay minerals, and has found that mixed siliceous shale and clay-rich siliceous shale were most conducive to formation of lithofacies. Applying the 3-end-mumber diagram analysis to Xinkailing-formation shale, results showed that clay-rich siliceous shale contained most lithofacies (as shown in Figure 10). Above all, Xinkailing-formation shale is characterized by a high brittleness index and can easily be fractured. Research has classified the shale in the Fuling area into four lithofacies (major classes) and 16 lithofacies (minor classes) [44] based on a 3-end-mumber diagram analysis of the silicon minerals, carbonate minerals, and clay minerals, and has found that mixed siliceous shale and clay-rich siliceous shale were most conducive to formation of lithofacies. Applying the 3-end-mumber diagram analysis to Xinkailing-formation shale, results showed that clay-rich siliceous shale contained most lithofacies (as shown in Figure 10). Above all, Xinkailing-formation shale is characterized by a high brittleness index and can easily be fractured.
Energies 2018, 11, 12 10 of 17 and are dominated by illite. The illite content ranges from 73% to 97% (average: 85%) (as shown in Figure 9b). The mineral composition is closely related to high thermal evolution and diagenesis. Research has classified the shale in the Fuling area into four lithofacies (major classes) and 16 lithofacies (minor classes) [44] based on a 3-end-mumber diagram analysis of the silicon minerals, carbonate minerals, and clay minerals, and has found that mixed siliceous shale and clay-rich siliceous shale were most conducive to formation of lithofacies. Applying the 3-end-mumber diagram analysis to Xinkailing-formation shale, results showed that clay-rich siliceous shale contained most lithofacies (as shown in Figure 10). Above all, Xinkailing-formation shale is characterized by a high brittleness index and can easily be fractured.

Isothermal Adsorption Characteristics
Ten core samples of Xinkailing-formation shale were obtained in the Wuning area to perform isothermal adsorption testing under conditions of 30 • C and pure methane equilibrium humidity. The experimental data demonstrated that the saturated adsorption volume V L ranges from 2.12 m 3 /t to 3.47 m 3 /t, with an average value of 2.50 m 3 /t, and the Langmuir pressure P L ranges from 1.55 to 2.35 MPa (average: 1.86 MPa) (as shown in Figure 11). Adsorption volume was found to increase as the pressure increased, and all the adsorption volumes were greater than 2 m 3 /t. Consequently, Xinkailing-formation shale has strong adsorption capacity and can absorb more natural gas in situ with reservoir pressure. In addition, the adsorption capacity of Xinkailing-formation shale has a clear positive correlation with the total carbon content (as shown in Figure 12). The richer the organic matter, the higher the shale-adsorption capacity. Previous studies have concluded that gas adsorption volume first increases and then decreases as organic maturity increases [45]. Tests of organic matter maturity and gas-adsorption volume reveal that the adsorption volume of Xinkailing formation shale has a clear negative correlation with R o and that adsorption volume decreases with increasing organic matter maturity when the R o ranges from 2.8% to 3.2% (as shown in Figure 13).

Isothermal Adsorption Characteristics
Ten core samples of Xinkailing-formation shale were obtained in the Wuning area to perform isothermal adsorption testing under conditions of 30 ˚C and pure methane equilibrium humidity. The experimental data demonstrated that the saturated adsorption volume VL ranges from 2.12 m 3 /t to 3.47 m 3 /t, with an average value of 2.50 m 3 /t, and the Langmuir pressure PL ranges from 1.55 to 2.35 MPa (average: 1.86 MPa) (as shown in Figure 11). Adsorption volume was found to increase as the pressure increased, and all the adsorption volumes were greater than 2 m 3 /t. Consequently, Xinkailing-formation shale has strong adsorption capacity and can absorb more natural gas in situ with reservoir pressure. In addition, the adsorption capacity of Xinkailing-formation shale has a clear positive correlation with the total carbon content (as shown in Figure 12). The richer the organic matter, the higher the shale-adsorption capacity. Previous studies have concluded that gas adsorption volume first increases and then decreases as organic maturity increases [45]. Tests of organic matter maturity and gas-adsorption volume reveal that the adsorption volume of Xinkailing formation shale has a clear negative correlation with Ro and that adsorption volume decreases with increasing organic matter maturity when the Ro ranges from 2.8% to 3.2% (as shown in Figure 13).

Isothermal Adsorption Characteristics
Ten core samples of Xinkailing-formation shale were obtained in the Wuning area to perform isothermal adsorption testing under conditions of 30 ˚C and pure methane equilibrium humidity. The experimental data demonstrated that the saturated adsorption volume VL ranges from 2.12 m 3 /t to 3.47 m 3 /t, with an average value of 2.50 m 3 /t, and the Langmuir pressure PL ranges from 1.55 to 2.35 MPa (average: 1.86 MPa) (as shown in Figure 11). Adsorption volume was found to increase as the pressure increased, and all the adsorption volumes were greater than 2 m 3 /t. Consequently, Xinkailing-formation shale has strong adsorption capacity and can absorb more natural gas in situ with reservoir pressure. In addition, the adsorption capacity of Xinkailing-formation shale has a clear positive correlation with the total carbon content (as shown in Figure 12). The richer the organic matter, the higher the shale-adsorption capacity. Previous studies have concluded that gas adsorption volume first increases and then decreases as organic maturity increases [45]. Tests of organic matter maturity and gas-adsorption volume reveal that the adsorption volume of Xinkailing formation shale has a clear negative correlation with Ro and that adsorption volume decreases with increasing organic matter maturity when the Ro ranges from 2.8% to 3.2% (as shown in Figure 13).    Based on the above analyses, it can be concluded that Xinkailing-formation shale in the Wuning area is characterized by low porosity and permeability, abundant reservoir space type, high brittleminerals content, and excellent adsorption capacity, which are reservoir conditions that have excellent potential for shale-gas accumulation and development. Comparisons between Xinkailingformation black shale in the Wuning area and Lower Silurian Longmaxi-formation shale in the middle-upper Yangtze were also conducted (as shown in Table 2), and results show that Xinkailingformation shale has very good capacity to preserve shale gas with similar characteristics.

The Xinkailing Shale Gas-Bearing Potential
As many scholars have noted [9,17,30,46], high-quality shale thickness, TOC, Ro, quartz content, and absorbed gas content have become the crucial parameters influencing the generation and development of shale gas. Accordingly, these five key parameters were chosen to quantitatively evaluate the Xinkailing shale gas-bearing potential. While the thickness of the black shale found in Wuning is lower than in three other areas (as shown in Figure 14), the TOC average is over 2%, the Ro ranges from 2.5% to 3.0%, the quartz content is approximately 40%, and the absorbed gas content is more than 2 m 3 /t in all four areas. Although only 18 m (the actual thickness could be found to be far more than that by drilling) of black shale were obtained from the water well, the characteristic parameters of Xinkailing shale in the study area are advantageous when compared with the Longmaxi shale in the Zhaotong, Pengshui, and Fuling areas, as well as the Upper Yangtze Platform, which underscores its gas-bearing capacity. If geological research drilling were undertaken, it is likely that more than 18 m of black, organic-rich shale would be discovered, also making a breakthrough for the gas-bearing content.  Based on the above analyses, it can be concluded that Xinkailing-formation shale in the Wuning area is characterized by low porosity and permeability, abundant reservoir space type, high brittle-minerals content, and excellent adsorption capacity, which are reservoir conditions that have excellent potential for shale-gas accumulation and development. Comparisons between Xinkailing-formation black shale in the Wuning area and Lower Silurian Longmaxi-formation shale in the middle-upper Yangtze were also conducted (as shown in Table 2), and results show that Xinkailing-formation shale has very good capacity to preserve shale gas with similar characteristics.

The Xinkailing Shale Gas-Bearing Potential
As many scholars have noted [9,17,30,46], high-quality shale thickness, TOC, R o , quartz content, and absorbed gas content have become the crucial parameters influencing the generation and development of shale gas. Accordingly, these five key parameters were chosen to quantitatively evaluate the Xinkailing shale gas-bearing potential. While the thickness of the black shale found in Wuning is lower than in three other areas (as shown in Figure 14), the TOC average is over 2%, the R o ranges from 2.5% to 3.0%, the quartz content is approximately 40%, and the absorbed gas content is more than 2 m 3 /t in all four areas. Although only 18 m (the actual thickness could be found to be far more than that by drilling) of black shale were obtained from the water well, the characteristic parameters of Xinkailing shale in the study area are advantageous when compared with the Longmaxi shale in the Zhaotong, Pengshui, and Fuling areas, as well as the Upper Yangtze Platform, which underscores its gas-bearing capacity. If geological research drilling were undertaken, it is likely that more than 18 m of black, organic-rich shale would be discovered, also making a breakthrough for the gas-bearing content.

The Optimization of Favorable Areas
A multi-factor superposition method was adopted to identify favorable areas for shale-gas production. Referring to the shale-gas target area screening index for the marine shale in south China [47], seven parameters, including the thickness of organic-rich shale, sub-surface depth, organic carbon content, organic-matter maturity, reservoir conditions, brittle-minerals content and surface geomorphic conditions, were selected to evaluate favorable areas (as shown in Figure 15).

The Optimization of Favorable Areas
A multi-factor superposition method was adopted to identify favorable areas for shale-gas production. Referring to the shale-gas target area screening index for the marine shale in south China [47], seven parameters, including the thickness of organic-rich shale, sub-surface depth, organic carbon content, organic-matter maturity, reservoir conditions, brittle-minerals content and surface geomorphic conditions, were selected to evaluate favorable areas (as shown in Figure 15).

The Optimization of Favorable Areas
A multi-factor superposition method was adopted to identify favorable areas for shale-gas production. Referring to the shale-gas target area screening index for the marine shale in south China [47], seven parameters, including the thickness of organic-rich shale, sub-surface depth, organic carbon content, organic-matter maturity, reservoir conditions, brittle-minerals content and surface geomorphic conditions, were selected to evaluate favorable areas (as shown in Figure 15).   (1) Previous research has clarified that the key factors for the accumulation of shale gas are the thickness and sub-surface depth of organic-rich shale [47]. Organic-rich shale must be thick enough to form effective source rocks and reservoirs. Sub-surface depth not only has a significant effect on the production and accumulation of shale gas, but also directly impacts shale-gas development costs. The proper conditions of sub-surface depth with moderate temperature and pressure can trigger shale to generate hydrocarbons (including biogenic gas and thermogenic gas). It is unfavorable to accumulate free gas at greater sub-surface depths because large depth leads to greater pressure, in turn leading to smaller porosity. In the Wuning area, research results showed that the thickness of organic-rich shale increased from west towards east, ranging from 10 m to 80 m. The sub-surface depth of the syncline wings was shallower than at the core of the syncline. The lower limit of thickness was determined to be 30 m during the process of optimizing favorable areas. The sub-surface depth of the Xinkailing formation in the whole area ranged from 500 m to 3000 m, which was a favorable depth overall. (2) TOC is a significant influencing factor of hydrocarbon-generating intensity, which determines the hydrocarbon-generating quantity [48] and plays an important role in the control of shale-gas accumulation. In addition, TOC also has a significant effect on shale gas by preserving gas due to the solution and adsorption effects ( Figure 12). In Wuning, the overall TOC was greater than 1%. TOC appeared to be higher in the middle of the area, but decreased toward the two sides. The lower limit of TOC was determined to be 2% during the process of optimizing favorable areas. (3) In thermogenic shale-gas reservoirs, hydrocarbons are generated by the interaction of time, temperature and pressure. Kerogen maturity could not only predict hydrocarbon-generating potential, but also influences adsorption gas volume ( Figure 13). In Wuning, the degree of thermal evolution showed a trend of "west high, east low" and "core high, side low". The upper limit of organic matter maturity was determined to be 3% during favorable-area optimization. (4) Brittle-mineral content is a key parameter influencing the effect of hydraulic fracturing treatment [17]. In Wuning, the brittle-mineral content of Xinkailing-formation shale ranges from 51% to 64% (average 56.06%) which can make the shale easily fractured. (5) Preservation conditions also exert significant effects on the accumulation of shale gas [16], which are determined by the intensity of tectonic movement. To determine the shale-gas resources potential and exploitation direction it is important to comprehensively analyze reservoir-preservation conditions. The optimization of structurally favorable areas within the Xinkailing formation in Wuning was conducted under consideration of tectonic location, attitude of stratum (direction and angle of dip), distance from outcrops to the target layer, and the development of fractures. The optimization results showed that syncline cores are favorable areas for shale-gas accumulation due to their greater distance from the target layer outcrop, gentle dip angle, and poor fault development.
Based on the discussion above, the conclusion may reasonably be arrived at that it is reliable to use those key parameters to optimize favorable areas and effectively guide shale-gas reservoir exploration and exploitation that include organic-rich shale thickness, buried depth, TOC, brittle-mineral content and preservation conditions. By comprehensively analyzing the elements, gas accumulation in Xinkailing-formation shale in Wuning had mainly developed in the Wuning-Lixi and Jinkou-Zhelin areas ( Figure 15). The thickness of organic-rich shale ranged from 10 m to 80 m, TOC was relatively high (greater than 2%, overall), and Ro ranged from 2.5% to 3.0% in the over-mature stage, all of which are conditions for excellent gas-bearing potential. Organic-rich shale was well developed and had good preservation conditions, which indicate good potential for shale-gas development and exploitation.

Conclusions
To better understand the characteristics of Xinkailing-formation shale and shale-gas accumulation conditions in the Wuning area, abundant work has been conducted and relevant data has been gathered. The shale of the Xinkailing formation differs greatly between core samples and outcrop samples. The outcrop profile confirms that the shale is very thick with severe weathering, while analysis of the water well revealed high-quality shale deposited under stable, anoxic deep-water sediment. Geochemical analysis (organic matter type, TOC, and R o ) suggests that the Xinkailing shale obtained from the water well acts as a good source rock and has considerable shale-gas resource potential. Pores (cracks) between clay minerals, intergranular dissolution pores, pores between minerals, organic matter pores, and interlaminated fractures and structural fractures can be identified in the Xinkailing shale, among which the organic pores, pores (cracks) between clay minerals, and fractures may provide the main storage space. The pore sizes are mainly predominated by meso-pores (2-50 nm) and macro-pores (>50 nm). Compared with the Longmaxi shale in the middle-upper Yangtze area, it can be concluded that Xinkailing shale possesses advantageous shale-gas accumulation conditions. By selecting such key parameters, including organic-rich shale thickness (more than 30 m), sub-surface depth (more than 500 m and less than 3000 m), TOC content (over 2%), thermal evolution degree (more than 2% and less than 3%), preservation condition (poor fault development nearby), and geomorphologic factors (flat), the Wuning-Lixi and Jinkou-Zhelin favorable areas are optimized, and additional shale-gas breakthroughs could be discovered in the Lower Yangtze area through drilling.