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

energies

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

Shangru Li 1,2, Li Zeng 1,2, Zhongpeng Wang 1,2,*, Jinchuan Zhang 3 and Dong Ma 4,* 1 Jiangxi Geo-Engineering (Group) Corporation, Nanchang 330001, China; [email protected] (S.L.); [email protected] (L.Z.) 2 GanZhongNan Institute of Geology and Mineral Exploration, Nanchang 330001, China 3 School of Energy Resources, China University of Geosciences, Beijing 100089, China; [email protected] 4 School of Petroleum Engineering, Yangtze Univeristy, Wuhan 430100, China * Correspondence: [email protected] (Z.W.); [email protected] (D.M.); Tel.: +86-157-2766-7223 (Z.W.)

Received: 27 November 2017; Accepted: 19 December 2017; Published: 21 December 2017

Abstract: 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 m3/t and 3.47 m3/t, on average about 2.50 m3/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.

Keywords: north-west Jiangxi; Wuning area; Xinkailing formation shale; gas-generating conditions; reservoir conditions; gas-bearing potential; favorable area

1. Introduction Shale-gas exploration and development have seen enormous commercial success in the United States over the past 10 years [1–3], 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 [4–7]. 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 [10–13]. Most of these studies have focused on the two high-quality formations [14–18]: 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

Energies 2018, 11, 12; doi:10.3390/en11010012 www.mdpi.com/journal/energies Energies 2018, 11, 12 2 of 17 Energies 2018, 11, 12 2 of 17 formation, collectively known as the Niutitang formation). It is worth noting that both formations are are made up of high-quality source rock deposits. Due to the influence of its sedimentary age and made up of high-quality source rock deposits. Due to the influence of its sedimentary age and sub-surface depth, the Longmaxi formation was better developed than the Niutitang formation [19], sub-surface depth, the Longmaxi formation was better developed than the Niutitang formation [19], and commercial gas production has been achieved in the areas of Fuling, Weiyuan-Changning, and commercial gas production has been achieved in the areas of Fuling, Weiyuan-Changning, Zhaotong, and Pengshui [20]. The parameters, including thickness, TOC (total organic carbon), Zhaotong, and Pengshui [20]. The parameters, including thickness, TOC (total organic carbon), Ro R (referring to vitrinite reflectance), reservoir conditions and so on, have been testified to be the key (referringo to vitrinite reflectance), reservoir conditions and so on, have been testified to be the key factors of accumulation conditions and favorable areas of shale gas [17]. factors of accumulation conditions and favorable areas of shale gas [17]. Due to their similar tectonic dynamics [21], the lower Yangtze and middle-upper Yangtze areas also Due to their similar tectonic dynamics [21], the lower Yangtze and middle-upper Yangtze areas share similar sedimentary-tectonic evolution and shale-layer development. Previous studies concerning also share similar sedimentary-tectonic evolution and shale-layer development. Previous studies shale formation with rich organic matter in north-west Jiangxi focused on the shale development concerning shale formation with rich organic matter in north-west Jiangxi focused on the shale developmentcharacteristics characteristics of the Lower of Cambrian the Lower Hetang Cambrian formation Hetang and formation the Guanyintang and the formation Guanyintang [22,23 ]. formationHowever, [22,23]. there areHowever, few published there are accounts few publish thated describe accounts the that hydrocarbon describe the source-rockhydrocarbon potential source- of rockthe potential Upper Ordovician of the Upper Xinkailing Ordovician formation. Xinkailing Geochemical formation. Geochemical analysis data analysis of outcrop data samplesof outcrop from samplessection PM1from hassection shown PM1 that has the shown Xinkailing that the shale Xinkai is severelyling shale weathered is severely and weathered not suitable and fornot exploitation suitable for(as exploitation shown in Figure (as shown1A). Accordingly, in Figure 1A). it hasAccordingly, been ignored it has for been years. ignored for years. However,However, the the core core samples, samples, which which were were acquired acquired from from a water a water well well beside beside the the section section PM1, PM1, revealedrevealed excellent excellent shaleshale developmentdevelopment characteristics characteristics (as (as shown shown in Figurein Figure1B)and 1B)an and enormous an enormous potential potentialfor shale for gas shale resources gas resources in the Xinkailing in the Xinkailing formation. formation. Jiangxi Province Jiangxi Province is characterized is characterized by a continuous by a continuousdeficit in oil deficit and gas. in oil If and a breakthrough gas. If a breakthrough can be made can in be the made extraction in the ofextraction Xinkailing of shale,Xinkailing it may shale, change itthe may energy change structure the energy of the structure province. of the province.

Figure 1. (A) The characteristics of an outcrop sample of Xinkailing-formation shale; (B) the characteristics Figure 1. (A) The characteristics of an outcrop sample of Xinkailing-formation shale; (B) the of a core sample of Xinkailing-formation shale. (The location of outcrop sample and core sample can characteristics of a core sample of Xinkailing-formation shale. (The location of outcrop sample and core be seen in Figure 2). sample can be seen in Figure2).

ForFor this this study, study, the the thickness thickness of of Xinkailing Xinkailing shale shale in inthe the middle middle of ofthe the Xiuwu Xiuwu Basin Basin was was investigated investigated usingusing a ameasured measured profile. profile. Then, Then, features features such such as asorganic organic chemistry, chemistry, mineral mineral composition, composition, reservoir reservoir properties,properties, and and pore distributiondistribution and and structure structure were were analyzed analyzed using using laboratory laboratory testing testing of shale of shale samples samplesfrom the from water the well. water The well. primary The primary objectives objectives of this paper of this are paper to: (1) are characterize to: (1) characterize the hydrocarbon the hydrocarbon generation conditions of the Xinkailing shale; (2) determine whether the reservoir is generation conditions of the Xinkailing shale; (2) determine whether the reservoir is suitable for shale gas; suitable for shale gas; and (3) discuss the gas-bearing potential and favorable distribution areas and (3) discuss the gas-bearing potential and favorable distribution areas within the Xinkailing formation. within the Xinkailing formation.

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2. Geological Setting 2. Geological Setting 2.1. Overview 2.1. Overview The study area is situated in the eastern Xiuwu Basin, Lower Yangtze Plate. The administrative regionsThe are study mainly area comprised is situated of in Wuning the eastern county, Xiuwu in the Basin, north-west Lower Yangtzearea of Jiangxi Plate. province The administrative (as shown inregions Figure are 2a). mainly The comprisedformation ofand Wuning evolution county, of inthe the area north-west can be broken area of Jiangxidown into province four (asmain shown stages in (asFigure shown2a). Thein Figure formation 2b): and (1) evolutionthe Mesoproterozoic-Silurian of the area can be broken (Pt2-S) down basal into formation four main and stages deformation (as shown stage;in Figure (2) 2theb): Devonian–middle (1) the Mesoproterozoic-Silurian Triassic (D-T2) passive (Pt 2-S) basalcontinental formation margin and stage; deformation (3) the Late stage; Triassic– (2) the CretaceousDevonian–middle (T3-K) Triassicintense (D-Torogeny2) passive stage; continentaland (4) the margin Jurassic-Quaternary stage; (3) the Late(J-Q) Triassic–Cretaceous down-faulted red (Tbasin3-K) stage. intense Due orogeny to tectonic stage; compression and (4) the during Jurassic-Quaternary the T3-K, folds and (J-Q) faults down-faulted are well developed red basin in stage. the studyDue to area tectonic (as shown compression in Figure during 2c). The the folds T3-K, are folds present and in faults a primarily are well east–west developed syncline. in the study The faults area (asin the shown study in Figurearea are2c). primarily The folds detachment are present in faults a primarily and normal east–west faults, syncline. both of The which faults appear in the studyin an east-westarea are primarily trend. The detachment Ordovician-silurian faults and shale normal tectonic faults, deformation both of which was appear weaker in than an east-west the Cambrian trend. Theshale Ordovician-silurian formation [24]. shale tectonic deformation was weaker than the Cambrian shale formation [24].

Figure 2. (a) The location of Xiuwu Basin, Jiangxi Province, China; (b) the structural characteristics of Figure 2. (a) The location of Xiuwu Basin, Jiangxi Province, China; (b) the structural characteristics of the study area and the site of the water well; (c) schematic diagram showing the stratigraphy and the study area and the site of the water well; (c) schematic diagram showing the stratigraphy and basin basin evolution of Xiuwu Basin. evolution of Xiuwu Basin.

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Energies 2018, 11, 12 4 of 17 2.2. Shale-Distribution Characteristics 2.2. Shale-Distribution Characteristics The organic-rich shale of the Xinkailing formation was developed in a transgressive system tract 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 environment, which is primarily made up of deep-water sand-mud shelf sediments and a carbon- shelf. The strata proﬁle measurements of the Xinkailing formation was conducted in section PM1 (the mud shelf. The strata profile measurements of the Xinkailing formation was conducted in section location of which as shown in Figure2), which revealed both the top and bottom boundary of the PM1 (the location of which as shown in Figure 2), which revealed both the top and bottom boundary Xinkailing formation (as shown in Figure3). of the Xinkailing formation (as shown in Figure 3).

FigureFigure 3. 3.Xinkailing-Lishuwo Xinkailing-Lishuwo formationformation sedimentary and and stratigraphic stratigraphic section section from from PM1. PM1.

AboveAbove the the top top surface,surface, silty mudstone mudstone from from the the Lishuwo Lishuwo formation formation was wasfound, found, and beneath and beneath the thebottom bottom surface surface was wasargillaceous argillaceous shale from shale the from Huangnigang the Huangnigang formation, both formation, of which bothare characterized of which are characterizedby a powerful by sealing a powerful capacity. sealing The lithology capacity. of The the Xinkailing lithology offormation the Xinkailing changed formation vertically. changedIn the lower section, black shale dominated with intermittent bedding of grey-black silty shale, and this

Energies 2018, 11, 12 5 of 17 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.

3. 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 Ro values were calculated using Equationh (1) [27]: h Ro = 0.3195 + 0.6790 × Rb (1) where Ro refers to vitrinite reflectance (%) and Rb 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.

4. Results

4.1. 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 Figure4). Results range from −30.67 to 27.34 with an average value of −29.03 . For that reason, the main types of organic matter wereh classiﬁedh into type I and type II. h 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 Figure5). 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. Energies 2018, 11, 12 6 of 17

Energies 2018, 11, 12 6 of 17 EnergiesThe 2018R, 11o values, 12 for 10 core samples from the Xinkailing formation shale were calculated based6 of 17 on the method of Feng et al. [27], and values ranged from 2.83% to 3.21%, with an average value of Illite crystallinity values ranged from 0.35 to 0.37, with an average of 0.356, which implies that the Illite2.99%. crystallinity Illite crystallinity values ranged values from ranged 0.35 from to 0.37, 0.35 with to 0.37, an average with an of average 0.356, ofwhich 0.356, implies which that implies the Xinkailing formation has been beneath the surface for a long time at a very low diagenetic stage. Xinkailingthat the Xinkailing formation formation has been hasbeneath been beneaththe surface the for surface a long for time a long at timea very at low a very diagenetic low diagenetic stage. Generally speaking, the analyses of shale samples indicated that the degrees of evolution for the Generallystage. Generally speaking, speaking, the analyses the analyses of shale of shale samples samples indicated indicated that that the the degrees degrees of of evolution evolution for for the the Xinkailing formation in Wuning were relatively high and the thermal evolution of the Xinkailing XinkailingXinkailing formation inin WuningWuning were were relatively relatively high high and and the the thermal thermal evolution evolution of the of Xinkailingthe Xinkailing shale shale occurred during the early-middle stage of over-maturity [30,31], with dry gas predominating. shaleoccurred occurred during during the early-middle the early-middle stage stage of over-maturity of over-maturity [30,31 [30,31],], with drywith gas dry predominating. gas predominating.

FigureFigure 4. 4. CarbonCarbon isotope isotope variation variation of of Xinkaili Xinkailing-formationng-formation shale shale samples samples in in Wuning. Wuning. Figure 4. Carbon isotope variation of Xinkailing-formation shale samples in Wuning.

100% 88.57% 100% 88.57% 80% 80% 60.00% 60% 60.00% 60% 36.67% 40% 36.67% Frequency 40% Frequency 20% 11.43% 20% 11.43% 3.33% 0.00% 0.00%3.33% 0.00% 0% 0.00% 0.00% 0.00% 0% ＜1 1-2 2-3 ＞3 ＜1 1-2 2-3 ＞3 TOC distribution scope (%) TOC distribution scope (%) Well Samples Outcrop Samples Well Samples Outcrop Samples Figure 5. The total organic carbon comparison between well samples and outcrop samples in Wuning. Figure 5. The total organic carbon comparison between well samples and outcrop samples in Wuning. Figure 5. The total organic carbon comparison between well samples and outcrop samples in Wuning. These three geochemical research results, which found that the shale organic matter type was These three geochemical research results, which found that the shale organic matter type was optimum,These thethree organic geochemical matter carbon research content results, met which good found source that rock the conditions, shale organic and maturity matter type remained was optimum, the organic matter carbon content met good source rock conditions, and maturity remained optimum,relatively moderate,the organic imply matter that carbon the Xinkailing content met formation good source possesses rock excellentconditions, conditions and maturity for hydrocarbon remained relatively moderate, imply that the Xinkailing formation possesses excellent conditions for relativelygeneration. moderate, Comparisons imply of thethat organic the Xinkailing geochemical formation test results possesses between Xinkailing-formationexcellent conditions black for hydrocarbon generation. Comparisons of the organic geochemical test results between Xinkailing- hydrocarbonshale in Wuning generation. area and Comparisons lower Paleozoic of the shale organic in the geochemical middle-upper test Yangtze results areabetween were Xinkailing- conducted formation black shale in Wuning area and lower Paleozoic shale in the middle-upper Yangtze area formation(as shown black in Table shale1), within Wuning results showingarea and thatlower Xinkailing Paleozoic formation shale in the shale mi hasddle-upper excellent Yangtze gas-resource area were conducted (as shown in Table 1), with results showing that Xinkailing formation shale has werepotential conducted with similar (as shown characteristics. in Table 1), with results showing that Xinkailing formation shale has excellent gas-resource potential with similar characteristics. excellent gas-resource potential with similar characteristics. Table 1. Comparison of the organic geochemical parameters between the Xinkailing-formation shale in Table 1. Comparison of the organic geochemical parameters between the Xinkailing-formation shale TableWuning 1. andComparison the Lower of Paleozoic-formation the organic geochemical shale inpara middle-uppermeters between Yangtze the area.Xinkailing-formation (Data from [17,32 shale–34]). in Wuning and the Lower Paleozoic-formation shale in middle-upper Yangtze area. (Data from [17,32–34]). in Wuning and the Lower Paleozoic-formation shale in middle-upper Yangtze area. (Data from [17,32–34]).

Area Area Formation Formation Organic Organic Matter Matter Type Type TOC TOC (%) (%) Ro (%)Ro (%) Area Formation Organic Matter Type TOC (%) Ro (%) WuningWuning Xinkailing Xinkailing I, I,II II 1.18–3.11 1.18–3.11 2.83–3.21 2.83–3.21 Wuning Xinkailing I, II 1.18–3.11 2.83–3.21 Weiyuan-RongxianWeiyuan-Rongxian I, I,II II1 1 1.09–6.511.09–6.51 2.41–2.642.41–2.64 Weiyuan-Rongxian LongmaxiLongmaxi I, II 1 1.09–6.51 2.41–2.64 Fuling FulingLongmaxi I I 1.04–5.891.04–5.89 2.42–2.82.42–2.8 Fuling I 1.04–5.89 2.42–2.8 Jingyan-QianweiJingyan-Qianwei I I 1.15–3.55/1.871.15–3.55/1.87 2.64–3.02.64–3.0 Jingyan-Qianwei QiongzhushiQiongzhushi I 1.15–3.55/1.87 2.64–3.0 WeiyuanWeiyuan Qiongzhushi I I 2.0–4.22.0–4.2 2.8–3.62.8–3.6 Weiyuan I 2.0–4.2 2.8–3.6

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4.2. Reservoir Conditions

4.2.1. Permeability and Porosity Porosity and permeability test results for Xinkailing-formationXinkailing-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 ofof thethe samplessamples ranged ranged from from 0.000445 0.000445 md md to to 0.001297 0.001297 md, md, with with an averagean average of 0.000872 of 0.000872 md. Thismd. This means means that boththat overallboth overall porosity porosity and permeability and permeability are extremely are extremely low. Comparison low. Comparison of the physical of the characteristicsphysical characteristics of the Xinkailing of the Xinkailing formation blackformation shale inblack the Wuningshale in areathe andWuning the lower area Paleozoicand the lower shale inPaleozoic the middle-upper shale in the Yangtze middle-upper area showed Yangtze that low area porosity showed and that permeability low porosity are generaland permeability characteristics are ofgeneral shale characteristics reservoirs (as shownof shale in reserv Figureoirs6)[ 35(as]. shown in Figure 6) [35].

6 0.5 4.8 5 0.4 4 0.3 3 0.2 2 1.78 1.75 1.07 1.16 0.1 Average Porosity (%) Porosity Average 1 0.74 Average Permeability (md) Permeability Average 0 0 Well JY1 Well Well Well Well Wuning PY1 LD1 XY1 YY1 Average Porosity (%) Average Permeability (md)

Figure 6. Comparison of the porosity and permeability between the Xinkailing-formation shale in Wuning andand thethe Longmaxi-formationLongmaxi-formation shaleshale inin thethe middle-uppermiddle-upper YangtzeYangtze area.area.

4.2.2. Reservoir Storage-Space Types 4.2.2. Reservoir Storage-Space Types Based on the core observations and SEM, the reservoir space types were also studied. At the 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 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 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 formation in Wuning, such as vertical shearing cracks (as shown in Figure7a) with straight seams and no fill. Non-structural fractures take second place; including interlayer lamellation seams and 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 diagenetic shrinkage joints (as shown in Figure7b). Diagenetic shrinkage joints are generally found in shale with high silicon content, formed by a shrinkage effect due to chemical changes during 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 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 abundant reticular fracture systems (as shown in Figure7c), which could greatly enhance the reservoir reservoir space and permeability, forming crucial pathways for shale gas migration from the source space and permeability, forming crucial pathways for shale gas migration from the source to the to the reservoir [17,36,37]. reservoir [17,36,37]. At the micro level, Xinkailing-formation shale included four types: pores (cracks) between clay 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 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 (cracks) between clay minerals are about 1µm in size, dominated by micro fractures in the mineral edges edges (as shown in Figure 7d). Intergranular dissolution pores are relatively large, ranging from 1μm (as shown in Figure7d). Intergranular dissolution pores are relatively large, ranging from 1 µm to 20 µm to 20 μm (as shown in Figure 7e). Pores between minerals are mainly composed of pyrite intergranular (as shown in Figure7e). Pores between minerals are mainly composed of pyrite intergranular nanometer nanometer pores, whose apertures range from 20 nm to 200 nm (as shown in Figure 7f). The diameters pores, whose apertures range from 20 nm to 200 nm (as shown in Figure7f). The diameters of the organic of the organic matter pores range from 2 nm to 100 nm, featuring irregular striping or a honeycomb matter pores range from 2 nm to 100 nm, featuring irregular striping or a honeycomb irregular elliptical irregular elliptical shape (as shown in Figure 7g–i). Different pore types and apertures have different shape (as shown in Figure7g–i). Different pore types and apertures have different contributions to and contributions to and effects on reservoir capacity and shale-gas production [36,38,39]. Therefore, effects on reservoir capacity and shale-gas production [36,38,39]. Therefore, fractures and micro-pores fractures and micro-pores were greatly developed in the Xinkailing formation of Wuning, indicating were greatly developed in the Xinkailing formation of Wuning, indicating that the Xinkailing-formation that the Xinkailing-formation shale had abundant reservoir-space types and excellent reservoir shale had abundant reservoir-space types and excellent reservoir conditions. conditions.

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Figure 7. The reservoir reservoir storage-space types of Xinkailing Xinkailing shale in in Wuning: Wuning: ( (aa)) vertical vertical shearing shearing crack; crack; (b) diageneticdiagenetic shrinkage shrinkage joint joint and and interlayer interlayer lamellation lamellation seam; seam; (c) vertical (c) vertical shearing shearing crack and crack interlayer and interlayerlamellation lamellation seam; (d) poreseam; (crack) (d) pore between (crack) clay between minerals; clay minerals; (e) intergranular (e) intergranular dissolution dissolution pore; (f) pyrite pore; (intergranularf) pyrite intergranular pore; (g) organicpore; (g matter) organic pore; matter (h) organicpore; (h matter) organic pore; matter (i) organic pore; (i matter) organic pore. matter pore.

4.2.3. Pore-Structure Characteristics

Figure 8a–d shows the N2 adsorption–desorption isotherms of four of the Xinkailing shale Figure8a–d shows the N 2 adsorption–desorption isotherms of four of the Xinkailing shale samples. samples. from Figure 8a–d, It can be observed that the N2 adsorption–desorption isotherms of the from Figure8a–d, It can be observed that the N 2 adsorption–desorption isotherms of the shale samples shalebelong samples to the type belong IV isotherm to the type (isotherm IV isotherm with a hysteresis(isotherm loop)with accordinga hysteresis to theloop) International according Unionto the Internationalof Pure and Applied Union of Chemistry Pure and (IUPAC) Applied classiﬁcation Chemistry (IUPAC) [40], indicating classification that the [40], shale indicating samples that contain the shaleboth mesoporessamples contain (2–50 nm)both [mesopores41] and macropores (2–50 nm) (>50 [41] nm). and macropores (>50 nm). Analysis revealedrevealed that that the the BET BET specific specific surfaces surfaces values values range fromrange 15.69 from m 215.69/g to 20.41m2/g mto2 /g20.41 (average: m2/g 17.83(average: m2/g). 17.83 The m2 central/g). The radii central of the radii pores of the range pores from range 9.27 fr nmom to 9.27 11.36 nm nm, to 11.36 with annm, average with an of average 6.8 nm. ofThe 6.8 total nm. poreThe total volume pore of volume the Xinkailing of the Xinkailing shale varies shale from varies 20.8 from× 20.810−3 × cm10−33/g cm³/g to 30.5to 30.5× 10× 10−3−3cm cm³/g3/g (average: 25.9225.92 ×× 1010−−3 3cm³/g).cm3/g). The The micro-pore micro-pore (<2 (<2 nm) nm) [41] [41 volu] volumeme of of the the Xinkailing Xinkailing shale shale ranges ranges from from 0.36 ×× 10−3 3cm³/gcm3/g toto 0.46 0.46 ×× 1010−3− cm³/g3 cm3/g (average: (average: 0.420.42 ×× 1010−3 −cm³/g);3 cm3/g the); the meso-pore meso-pore (2–50 (2–50 nm) nm) volume volume ranges fromfrom 11.6 11.6× ×10 10−3−3cm cm³/g3/g to 16.816.8× × 1010−−33 cm³/gcm3/g (average:13.85 (average: 13.85 × 1010−−3 3cm³/g);cm3/g );and and the the macro-pore macro-pore (>50 nm)nm) volumevolume rangesranges from from 8.6 8.6× ×10 10−3−3cm cm³/g3/g toto13.3 13.3× ×10 10−3−3cm cm³/g3/g (average:(average: 11.6411.64× ×10 10−−33 cmcm³/g)3/g) (as shown in Figure 88e–h).e–h). The percentages of micro-pores,micro-pores, meso-pores, andand macro-pores areare 1.63,1.63, 53.44 and 44.93%, 44.93%, respectively, respectively, implying that meso-pores and macro-pores predominate among the Xinkailing shale.shale. Additionally, nanometer-scalenanometer-scale spatialspatial ordering ordering occurs occurs in in the the meso-pore–macro-pore meso-pore–macro-pore range range for for all allthe the samples, samples, which which may may be associatedbe associated with with the th organice organic matter matter pores pores and and pores pores (cracks) (cracks) between between clay clayminerals minerals [42,43 [42,43].].

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25 B4 25 B13

20 Adsorption isotherm 20 ) )

-1 15 Desorption isotherm -1 15

10 10 V (cm³.g V (cm³.g

5 5

0 0 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 P/Po P/Po a b

20 B22 20 B28 18 18 16 16 14 14 ) )

-1 12 -1 12 10 10 8 8 V (cm³.g 6 V (cm³.g 6 4 4 2 2 0 0 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 P/Po P/Po c d

5 B4 5 B13 4.5 4.5 ） 4 ） 4

cm³/g 3.5 cm³/g 3.5 -3 3 -3 3 10 10

（ 2.5 （ 2.5 2 2 1.5 1.5 1 1

Pore Volume Pore 0.5 Volume Pore 0.5 0 0 1 10 100 1000 1 10 100 1000 Pore diameter（nm） Pore diameter（nm） e f

4 B22 3.5 B28

） 3.5 ） 3 3 2.5 cm³/g cm³/g

-3 2.5 -3

10 10 2 2 （（ 1.5 1.5 1 1 0.5 0.5 Pore Volume Pore Volume Pore 0 0 1 10 100 1000 1 10 100 1000 Pore diameter（nm） Pore diameter（nm） g h

Figure 8. (a–d) Low-pressure N2 adsorption–desorption isotherms of four of the Xinkailing shale Figure 8. (a–d) Low-pressure N adsorption–desorption isotherms of four of the Xinkailing shale samples; (e–h) representative pore-size2 distribution from the studied samples. samples; (e–h) representative pore-size distribution from the studied samples. 4.2.4. Mineral Composition and Lithofacies 4.2.4. Mineral Composition and Lithofacies The Upper Ordovician Xinkailing shale-formation lithology is mainly comprised of black Thecarbonaceous Upper Ordovician shale with abundant Xinkailing interlayer shale-formation lamellation seams lithology and layered is mainly pyrite comprised (as shown ofin black carbonaceousFigure 7a–c). shale X-ray with sampling abundant of whole-rock interlayer mineral lamellation content seamsfound that and the layered mineral pyrite composition (as shown of in Xinkailing-formation shale is dominated by brittle minerals and clay minerals (as shown in Figure 9a). Figure7a–c). X-ray sampling of whole-rock mineral content found that the mineral composition of In addition, pyrite developed throughout the formation. Xinkailing-formationThe clay mineral shale content is dominated ranges from by brittle32% to minerals 46% (average: and clay39.66%); minerals the quartz (as shown content in ranges Figure 9a). In addition,from 32% pyrite to 46% developed (average: throughout39.46%); the feldspar the formation. content ranges from 10% to 22% (average: 15.76%); Thethe claycarbonate mineral mineral content content ranges ranges from from 32% 0% toto 46%9% (average: (average: 0.83%); 39.66%); the siderite the quartz content content ranges ranges from 32%from to 1% 46% to 3% (average: (average: 39.46%); 1.33%); and the the feldspar pyrite content content ranges ranges from from 1 to 10% 12% to(average: 22% (average: 2.83%). The 15.76%); the carbonatebrittle mineral mineral (including content quartz, ranges feldspar, from 0% and to carb 9%onate) (average: content 0.83%); ranges the from siderite 51% to content 64% (average: ranges from 1% to 3%56.06%) (average: (as shown 1.33%); in Figure and 9a). the Results pyrite contentfrom the rangesclay minerals’ from 1analysis to 12% showed (average: that 2.83%).clay minerals The brittle mineralin (includingthe Xinkailing quartz, formation feldspar, contain and few carbonate) chlorites, kaolinite, content and ranges illite–montmorillonite from 51% to 64% mixed-layers, (average: 56.06%) (as shown in Figure9a). 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 Figure9b). The mineral composition is closely related to high thermal evolution and diagenesis. Energies 2018, 11, 12 10 of 17

EnergiesEnergiesand2018 are 2018, 11 dominated,, 1211, 12 by illite. The illite content ranges from 73% to 97% (average: 85%) (as shown10 of 17in10 of 17 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. 29 29 27 27

2925 2925 2723 2723 2521 2521 2319 2319 2117 2117 1915 1915 1713 1713 Sample Number Sample Number 1511 1511 139 139 Sample Number Sample Number 117 117 95 95 73 73 51 51 0 20406080100 3 3 0 20406080100 Whole rock-mineral content (%) Clay mineral content (%) (a)1 (b)1 0Clay Quartz 20406080100Feldspar Carbonate Siderite Pyrite 0illite 20406080100Kaolinite Chlorite Illite Smectite Whole rock-mineral content (%) Clay mineral content (%) (a) (b) FigureClay 9. (Quartza) The Feldspartotal rockCarbonate mineralSiderite compositionPyrite of the Xinkailingillite shaleKaolinite in Wuning;Chlorite (b)Illite the Smectite clay mineral Figure 9. (a) The total rock mineral composition of the Xinkailing shale in Wuning; (b) the clay mineral composition of the Xinkailing shale. compositionFigure 9. ( ofa) theThe Xinkailingtotal rock mineral shale. composition of the Xinkailing shale in Wuning; (b) the clay mineral compositionResearch has of theclassified Xinkailing the shale. shale in the Fuling area into four lithofacies (major classes) and 16 Researchlithofacies (minor has classiﬁed classes) [44] the based shale on in a the 3-end-mumber Fuling area diagram into four analysis lithofacies of the silicon (major minerals, classes) and 16 lithofaciescarbonateResearch(minor minerals, has classes)classified and clay [ 44the] minerals, basedshale in on theand a 3-end-mumberFuling has found area into that diagramfour mixed lithofacies siliceous analysis (major shale of theclasses) and silicon clay-rich and minerals, 16 carbonatelithofaciessiliceous minerals, shale (minor were and classes) most clay conducive minerals,[44] based to andon formation a has 3-end-mumber found of lithofacies. that mixeddiagram Applying siliceous analysis the shale 3-end-mumberof the and silicon clay-rich minerals, diagram siliceous shalecarbonateanalysis were most to minerals, Xinkailing-formation conducive and toclay formation minerals, shale, ofresults and lithofacies. has showed found Applyingthat that clay-rich mixed the siliceous 3-end-mumbersiliceous shale shale contained and diagram clay-rich most analysis to Xinkailing-formationsiliceouslithofacies shale (as wereshown most shale,in Figure conducive results 10). Aboveto showed formation all, that Xi ofnkailing-formation clay-richlithofacies. siliceous Applying shale shale the is 3-end-mumber characterized contained most by diagram a lithofacies high analysis to Xinkailing-formation shale, results showed that clay-rich siliceous shale contained most (as shownbrittleness in Figure index and10). can Above easily all, be Xinkailing-formationfractured. shale is characterized by a high brittleness lithofacies (as shown in Figure 10). Above all, Xinkailing-formation shale is characterized by a high indexbrittleness and can index easily and be fractured.can easily be fractured.

Figure 10. The ternary diagram of mineral composition for dividing lithofacies. S: siliceous shale lithofacies; S-1: calcarenite-rich siliceous shale lithofacies; S-2: mixed siliceous shale lithofacies; FigureFigureS-3: 10. clay-rich 10.The The ternary siliceousternary diagram diagram shale of lithofacies;of mineral mineral compositionCM:composition argillaceous for for dividing dividingshale lithofacies;lithofacies. lithofacies. S:CM-1: siliceous S: siliceoussilica-rich shale shale lithofacies;lithofacies;argillaceous S-1: S-1: calcarenite-richshale calcarenite-rich lithofacies; siliceousCM-2: siliceous mixed shale shale argillace lithofacies; lithofacies;ous shale S-2:S-2: mixedmixedlithofacies; siliceoussiliceous CM-3: shale calcarenite-rich lithofacies; S-3: clay-richS-3:argillaceous clay-rich siliceous shalesiliceous shale lithofacies, lithofacies; shale lithofacies;C: CM:calcareous argillaceous CM: shale argillaceous shalelithofacies; lithofacies; shale C-1: lithofacies; silica-rich CM-1: silica-rich CM-1:calcareous silica-rich argillaceous shale shaleargillaceouslithofacies; lithofacies; C-2:shale CM-2: mixed lithofacies; mixed calcareous argillaceous CM-2: shale mixed lithofacies; shale argillace lithofacies; C-3:ous clay-richshale CM-3: lithofacies; calcareous calcarenite-rich CM-3: shale calcarenite-rich lithofacies; argillaceous M: shale argillaceous shale lithofacies, C: calcareous shale lithofacies; C-1: silica-rich calcareous shale lithofacies, C: calcareous shale lithofacies; C-1: silica-rich calcareous shale lithofacies; C-2: mixed lithofacies; C-2: mixed calcareous shale lithofacies; C-3: clay-rich calcareous shale lithofacies; M: calcareous shale lithofacies; C-3: clay-rich calcareous shale lithofacies; M: mixed shale lithofacies; M-1: calcareous/siliceous mixed-shale lithofacies; M-2: argillaceous/siliceous mixed-shale lithofacies: M-3: argillaceous/calcareous mixed-shale lithofacies. Energies 2018, 11, 12 11 of 17 Energies 2018, 11, 12 11 of 17

Energiesmixed2018 ,shale11, 12 lithofacies; M-1: calcareous/siliceous mixed-shale lithofacies; M-2: argillaceous/siliceous11 of 17 mixed shale lithofacies; M-1: calcareous/siliceous mixed-shale lithofacies; M-2: argillaceous/siliceous mixed-shale lithofacies: M-3: argillaceous/calcareous mixed-shale lithofacies. mixed-shale lithofacies: M-3: argillaceous/calcareous mixed-shale lithofacies. 4.2.5. Isothermal Adsorption Characteristics 4.2.5. Isothermal Adsorption Characteristics Ten corecore samplessamples ofof Xinkailing-formationXinkailing-formation shale were obtained in the Wuning area to perform Ten core samples of Xinkailing-formation shale were◦ obtained in the Wuning area to perform isothermal adsorption testing testing under under conditions conditions of of 30 30 ˚CC and and pure pure methane methane eq equilibriumuilibrium humidity. humidity. isothermal adsorption testing under conditions of 30 ˚C and pure methane equilibrium humidity.3 The experimental datadata demonstrateddemonstrated thatthat thethe saturatedsaturated adsorptionadsorption volume volume V VLLranges rangesfrom from 2.12 2.12 m m3/t/t The experimental3 data demonstrated that the 3saturated adsorption volume VL ranges from 2.12 m3/t to 3.473.47 mm3/t, with with an an average average value of 2.50 m3/t, and and the the Langmuir Langmuir pressure pressure P L rangesranges from from 1.55 to to 3.47 m3/t, with an average value of 2.50 m3/t, and the Langmuir pressure PL ranges from 1.55 to 2.35 MPa (average: (average: 1.86 1.86 MPa) MPa) (as (as shown shown in in Figure Figure 11). 11 ).Adsorption Adsorption volume volume was was found found to toincrease increase as 2.35 MPa (average: 1.86 MPa) (as shown in Figure 11). Adsorption volume was found3 to increase as asthe the pressure pressure increased, increased, and and all all the the adsorption adsorption volumes volumes were were greater greater than 22 mm3/t./t. Consequently, Consequently, the pressure increased, and all the adsorption volumes were greater than 2 m3/t. Consequently, Xinkailing-formation shale has strong adsorption ca capacitypacity and can absorb more natural gas in situ 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 with reservoir pressure. In addition, the adsorption capacity of Xinkailing-formation shale has a clear positive correlationcorrelation with with the the total total carbon carbon content content (as shown(as shown in Figure in Figure 12). The 12). richer The richer the organic the organic matter, positive correlation with the total carbon content (as shown in Figure 12). The richer the organic thematter, higher the thehigher shale-adsorption the shale-adsorption capacity. capacity. Previous Previous studies havestudies concluded have concluded that gas that adsorption gas adsorption volume matter, the higher the shale-adsorption capacity. Previous studies have concluded that gas adsorption ﬁrstvolume increases first increases and then and decreases then decreases as organic as maturityorganic maturity increases increases [45]. Tests [45]. of organic Tests of matter organic maturity matter volume first increases and then decreases as organic maturity increases [45]. Tests of organic matter andmaturity gas-adsorption and gas-adsorption volume reveal volume that reveal the adsorption that the adsorption volume of volume Xinkailing of Xinkailing formation shaleformation has a shale clear maturity and gas-adsorption volume reveal that the adsorption volume of Xinkailing formation shale negativehas a clear correlation negative correlation with Ro and with that Ro adsorptionand that adsorption volume decreasesvolume decreases with increasing with increasing organic organic matter has a clear negative correlation with Ro and that adsorption volume decreases with increasing organic maturitymatter maturity when the whenRo ranges the Ro ranges from 2.8% from to 2.8% 3.2% to (as 3.2% shown (as shown in Figure in 13Figure). 13). matter maturity when the Ro ranges from 2.8% to 3.2% (as shown in Figure 13). 3.5 3.5 3 3 2.5 2.5 2 2 1.5 1.5 1 1 0.5 0.5

Absorbed gas content (m³/t) gas content Absorbed 0 Absorbed gas content (m³/t) gas content Absorbed 0 024681012 024681012 Pressure (MPa) Pressure (MPa) B1 B4 B7 B10 B13 B16 B19 B22 B25 B28 B1 B4 B7 B10 B13 B16 B19 B22 B25 B28

Figure 11. The isothermal adsorption characteristics of the Xinkailing formation in Wuning, related Figure 11. The isothermal adsorption characteristics of the Xinkailing formation in Wuning, related to pressure. to pressure. 2.6 2.6 y = 0.5174 x + 1.2694 y = 0.5174 x + 1.2694 2.5 R² = 0.8533 2.5 R² = 0.8533 2.4 2.4 2.3 2.3 2.2 2.2 2.1

Absorbed gas content (m³/t) gas Absorbed content 2.1

Absorbed gas content (m³/t) gas Absorbed content 2 21.70 1.90 2.10 2.30 2.50 1.70 1.90 2.10 2.30 2.50 TOC (%) TOC (%)

Figure 12. Positive relationship between total organic carbon (TOC) content and adsorbed gas content Figure 12. Positive relationship between total organic carbon (TOC) content and adsorbed gas content Figureof Xinkailing 12. Positive shale. relationship between total organic carbon (TOC) content and adsorbed gas content of Xinkailing shale.

Energies 2018, 11, 12 12 of 17 Energies 2018, 11, 12 12 of 17

2.6 y = -0.7846 x + 4.7419 R² = 0.8204 2.5

2.4

2.3

2.2

2.1 Absorbed gas content (m³/t) 2 2.80 2.85 2.90 2.95 3.00 3.05 3.10 3.15 3.20 3.25 Ro (%)

Figure 13. Negative relationship between vitrinite reflectance (Ro) and adsorbed gas content of Figure 13. Negative relationship between vitrinite reﬂectance (Ro) and adsorbed gas content of Xinkailing shale. Xinkailing shale. Based on the above analyses, it can be concluded that Xinkailing-formation shale in the WuningBased area on isthe characterized above analyses, by it low can porositybe concluded and permeability,that Xinkailing-formation abundant reservoir shale in the space Wuning type, higharea is brittle-minerals characterized by content, low porosity and excellent and permeab adsorptionility, abundant capacity, reservoir which are space reservoir type, high conditions brittle- thatminerals have content, excellent and potential excellent for shale-gasadsorption accumulation capacity, which and development. are reservoir Comparisonsconditions that between have Xinkailing-formationexcellent potential for blackshale-gas shale accumulation in the Wuning and area development. and Lower Silurian Comparisons Longmaxi-formation between Xinkailing- shale information the middle-upper black shale Yangtze in the Wuning were also area conducted and Lower (as shownSilurian in Longmaxi-formation Table2), and results shale show in that the Xinkailing-formationmiddle-upper Yangtze shale were has also very conducted good capacity (as show ton preserve in Table shale 2), and gas results with similar show that characteristics. Xinkailing- formation shale has very good capacity to preserve shale gas with similar characteristics. Table 2. Comparison of the reservoir parameters between the Xinkailing-formation shale in Wuning andTable the 2.Lower Comparison Paleozoic-formation of the reservoir shale parameters in the middle-upper between the YangtzeXinkailing-formation area. shale in Wuning and the Lower Paleozoic-formation shale in the middle-upper Yangtze area. Isothermal Brittle Porosity Permeability Reservoir AdsorptionIsothermal Area Formation Pore Size BrittleMineral Lithofacies Porosity(%) Permeability(md) SpaceReservoir Type AdsorptionVolume Area Formation Pore Size Mineral(%) Lithofacies (%) (md) Space Type (mVolume3/t) (%) (m³/t) Organic pores Meso pores 1.10~1.22/ 0.000445~ Clay-rich Wuning Xinkailing Net-shapedOrganic andMeso Macro 51.6~56.06 2.12~3.47/2.5 1.10~1.22/1.16 0.0012970.000445~ pores pores and siliceousClay-rich shale Wuning Xinkailing fractures pores 51.6~56.06 2.12~3.47/2.5 1.16 0.001297 Net-shaped Macro siliceousMixed shale Organic pores 1.17~7.98/ 0.0038~ fractures pores siliceous shale Fuling Longmaxi Net-shaped Meso pores 62.42~64.76 0.98~6.89/3 4.8 0.9813 Clay-rich fracturesOrganic Mixed 1.17~7.98/ 0.0038~ pores Meso siliceoussiliceous shale shale Fuling Longmaxi 62.42~64.76 0.98~6.89/3 4.8 0.9813 Net-shaped pores Clay-rich fractures siliceous shale 5. Discussion

5.1.5. Discussion The Xinkailing Shale Gas-Bearing Potential

5.1. TheAs manyXinkailing scholars Shal havee Gas-Bearing noted [9, 17Potential,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 As many scholars have noted [9,17,30,46], high-quality shale thickness, TOC, Ro, quartz content, shale gas. Accordingly, these five key parameters were chosen to quantitatively evaluate the Xinkailing and absorbed gas content have become the crucial parameters influencing the generation and shale gas-bearing potential. While the thickness of the black shale found in Wuning is lower than in development of shale gas. Accordingly, these five key parameters were chosen to quantitatively three other areas (as shown in Figure 14), the TOC average is over 2%, the Ro ranges from 2.5% to 3.0%, evaluate the Xinkailing shale gas-bearing potential. While the thickness of the black shale found in the quartz content is approximately 40%, and the absorbed gas content is more than 2 m3/t in all four Wuning is lower than in three other areas (as shown in Figure 14), the TOC average is over 2%, the areas. Although only 18 m (the actual thickness could be found to be far more than that by drilling) Ro ranges from 2.5% to 3.0%, the quartz content is approximately 40%, and the absorbed gas content of black shale were obtained from the water well, the characteristic parameters of Xinkailing shale in is more than 2 m3/t in all four areas. Although only 18 m (the actual thickness could be found to be the study area are advantageous when compared with the Longmaxi shale in the Zhaotong, Pengshui, far more than that by drilling) of black shale were obtained from the water well, the characteristic and Fuling areas, as well as the Upper Yangtze Platform, which underscores its gas-bearing capacity. parameters of Xinkailing shale in the study area are advantageous when compared with the If geological research drilling were undertaken, it is likely that more than 18 m of black, organic-rich Longmaxi shale in the Zhaotong, Pengshui, and Fuling areas, as well as the Upper Yangtze Platform, shale would be discovered, also making a breakthrough for the gas-bearing content. 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.

Energies 2018, 11, 12 13 of 17 Energies 2018, 11, 12 13 of 17 Energies 2018, 11, 12 13 of 17

Figure 14. Comparison of the key shale-gas parameters between the Xinkailing-formation shale in Figure 14. Comparison of the key shale-gas parameters between the Xinkailing-formation shale in Figure 14. Comparison of the key shale-gas parameters between the Xinkailing-formation shale in Wuning and the Longmaxi shale in the middle-upper YangtzeYangtze area.area. Wuning and the Longmaxi shale in the middle-upper Yangtze area. 5.2. The Optimization of Favorable Areas 5.2. The Optimization of Favorable Areas A multi-factor superposition method was adopted to identify favorable areas for shale-gas A multi-factor superposition method was adopted to identify favorable areas for shale-gas production.A multi-factor Referring superposition to the shale-ga sme targetthod area was screening adopted index to identify for the marine favorable shale areas in south for China shale-gas [47], production. Referring to the shale-gas target area screening index for the marine shale in south sevenproduction. parameters, Referring including to the shale-ga the thicknesss target area of organi screeningc-rich index shale, for sub-surfacthe marine eshale depth, in south organic China carbon [47], China [47], seven parameters, including the thickness of organic-rich shale, sub-surface depth, organic content,seven parameters, organic-matter including maturity, the reservoirthickness conditions, of organic-rich brittle-minerals shale, sub-surfac contente and depth, surface organic geomorphic carbon carbon content, organic-matter maturity, reservoir conditions, brittle-minerals content and surface conditions,content, organic-matter were selected maturity, to evaluate reservoir favorable conditions, areas (asbrittle-minerals shown in Figure content 15). and surface geomorphic geomorphicconditions, were conditions, selected were to evaluate selected fa tovorable evaluate areas favorable (as shown areas in (as Figure shown 15). in Figure 15).

Figure 15. The optimization of favorable shale-gas areas for the Xinkailing formation in Wuning district. Figure 15. The optimization of favorable shale-gas areas for the Xinkailing formation in Wuning district. Figure 15. The optimization of favorable shale-gas areas for the Xinkailing formation in Wuning district.

Energies 2018, 11, 12 14 of 17

(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.

6. 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 Energies 2018, 11, 12 15 of 17 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 Ro) 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.

Acknowledgments: The work presented in this paper was supported by the National Shale Gas Resource Potential and Favorable Areas Project (Grant No. 2009GYXQ15-06-05) and Jiangxi Province Science and Technology Major Project (Grant No. 20152ACE50014). We are grateful to Tang Zhaoyou, Cao Haifeng, Xiongkuan and others for their help with the fieldwork. Author Contributions: These authors contributed equally to this work. Conflicts of Interest: The authors declare no conflict of interest.

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