International Journal of Coal Geology 137 (2015) 111–123

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International Journal of Coal Geology

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The effect of sedimentary redbeds on coalbed methane occurrence in theXutuan and Zhaoji Coal Mines, Huaibei Coalfield, China

Kan Jin, Yuanping Cheng ⁎, Liang Wang, Jun Dong, Pinkun Guo, Fenghua An, Limin JiangFaculty of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, ChinaNational Engineering Research Center for Coal and Gas Control, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China

⁎ Corresponding author at: National Engineering ResControl, China University of Mining and Technology, XTel.: +86 516 83885948; fax: +86 516 83995097.

E-mail addresses: ypc620924@gmail.com, jinkan@out

http://dx.doi.org/10.1016/j.coal.2014.11.0090166-5162/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 August 2014Received in revised form 25 November 2014Accepted 25 November 2014Available online 3 December 2014

Keywords:RedbedsMineralogyPore structureSealing capabilityGas occurrence

The lithologies and permeabilities of caprocks have significant effects on coalbed methane (CBM) preservation.To study the controlling effects of redbeds deposits on CBM occurrence in the Xutuan and Zhaoji Coal Mines ofthe Huaibei Coalfield in China, the variations in mineralogical composition, microstructure, pore structure,permeability and diffusion among the redbeds caprock samples, Neogene clay rock and ordinary coal measurestrata (sandstone, mudstone and siltstone) have been investigated. The results indicate that redbeds are porousrocks with weaker sealing capabilities than coal strata caprocks, allowing CBM to escape from the coal seam. Theformation of the lithological properties of the redbeds is closely related to their mineralogical compositionand diagenesis. Redbeds are mainly composed of illite (57–64%), calcite (25–33%), quartz (6–12%), muscovite(3–8%) and pyrite (b1%). The high content of fragile minerals and the later formation age of this rock lead to un-apparent compaction and cementation, resulting in a loose structure, low degrees of consolidation and porestructure development. Mercury intrusion experiments show that the porosity of redbeds is 3.8–5.2 times higherthan that of sandstone and 8–13 times higher than those of mudstone and siltstone; the pore size distribution ofthis rock is primarily centralized in transition pores (57.82%–74.72%) and mesopores (9.08%–22.51%), whichfavors gas diffusion. The permeability and diffusion coefficient tests demonstrate that the sealing capability ofredbeds ismuchweaker than that of coal strata caprocks. Although the origins of the CBM gas pool in the HuaibeiCoalfield remain controversial, the findings of this study along with the mining practices indicate that the depo-sition of thick redbeds can be treated as indicative of low gas contents in the coal mines, which has a significantmeaning for the engineering of coal mine gas control.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Coal is both the source and the reservoir for coalbedmethane (CBM),which is mainly stored within the coal matrix by adsorption (Pan andConnell, 2012). CBM has always been recognized as an explosive andoutburst hazard in safe mining, but it is also a potential energy resourceas amajor type of unconventional gas (Bustin and Clarkson, 1998). Gen-erally accepted theory considers that because CBM primarily occurs inan adsorbed form rather than in a free state, a conventional-type trap-pingmechanism is not entirely required (Zhang et al., 2005a). However,recent studies from CBM explorations and exploitations around theworld argue against this traditional viewpoint. In the coal basins of Po-land (Kędzior, 2009; Kędzior et al., 2013), Czech (Hemza et al., 2009),Australia (Saghafi and Pinetown, 2011), Indonesia (Sosrowidjojo andSaghafi, 2009) and China (Fu et al., 2009; Hong et al., 2005; Jiang et al.,2011; Li et al., 2014; Meng et al., 2014; Su et al., 2005; Wang et al.,

earch Center for Coal and Gasuzhou, Jiangsu 221116, China.

look.com (Y. Cheng).

2013; Wu et al., 2012; Xu et al., 2012; Zhang et al., 2005b), higher gascontent seem to concentrated in areas where coal seams are overlainby impermeable overburden, indicating that the CBM accumulationsare strongly inhomogeneous due to the features of caprocks. Conclu-sions drawn from the above studies suggest that although CBM reser-voirs are accumulated dominantly by adsorption mechanism, and therequirements for caprocks are weaker than conventional oil/gas reser-voirs, however the lithologies and permeabilities of caprocks still havesignificant effects on CBM accumulation (Kędzior, 2009; Meng et al.,2014; Saghafi, 2010; Wei et al., 2010).

The accumulation of CBM is actually the process of CBMpreservation(Song et al., 2012b). Studies byHong et al (2005) and Song et al (2012a)indicate that the dissipation of CBM has three paths: (1) free gas dissi-pates by overcoming capillary pressure of sealing rocks; (2) dissolvedgas diffuses driven by concentration difference; (3) hydrodynamic lossby water flushing. The former two paths are both significantly influ-enced by the properties of caprocks. The mechanism of caprocks affect-ing the gas retention in sorption-type CBM reservoirs is that thecaprocks with favorable sealing capabilities can effectively prevent thegas dissipation through the overburden of the coal seam (Meng et al.,2014). Despite the presence in a state of adsorption gas (accounting

Fig. 1. Map showing the distribution of redbeds in the study area and data locations for stratigraphic characteristics.

112 K. Jin et al. / International Journal of Coal Geology 137 (2015) 111–123

for 80–90%), there are still significant amount of CBM (accounting for10–20%) mainly stored in a free gas form through fractures andmacropores and with a further very small amount of gas dissolved inwater (Fu et al., 2009; Moffat and Weale, 1955). Since these threeforms of gases keep in a dynamic balancing state in the CBM reservoir,and the quantity of gas stored in adsorbed phase depends on the pres-sure exerted by free gas in the pore void volume (Saghafi et al., 2010).Any dissipation of free or dissolved gas due to theweaker sealing capac-ities of caprockswill result in decrease of reservoir pressure and desorp-tion of adsorbed CBM, and finally lower the gas contents of thereservoirs. Therefore, the properties of caprocks play a key role in CBMconservation.

Accordingly, recent AHP (analytic hierarchy process) modelsestablished for evaluating CBM potential begin to regard the caprockproperties as important evaluation parameters (Cai et al., 2011; Menget al., 2014). In addition, the studies on CO2 sequestration in coal

Fig. 2. Generalized stratigraphy of the Xutuan and Zhaoji Coal Mines (elevation of theground is +25 m).

seams also pay much attention to the integrity and sealing capabilityof caprocks (Leung et al., 2014; Shukla et al., 2010).

The CBM sealing capabilities of the caprocks are related to theirphysical properties, which formed in different sedimentary environ-ments and were acted on by geological movements. However, re-searches on the sealing properties of caprocks to CBM are stillseriously lacking. Saghafi et al (2010) only measure the differences ofpermeabilities and diffusivities between claystone and sandstone.Zhang et al (2011) compare the difference in sealing propertiesamong mudstone, siltstone, sandstone and limestone but without pro-viding any data. However, since the sealing mechanism is similar toCBM reservoir, the findings from conventional oil/gas reservoir can becited as a reference to some extent. Buttinelli et al (2011) suggest thatthe sealing capabilities of caprocks with the same thickness decreasein the following order as: clays, chalky clays, evaporitic rocks N clayishmarls, sandy clays, schistose clays N marls and flyschoid sediments(clayish sandstones) N sandstones, limestones, intrusive rocks, meta-morphic rocks N conglomerates, gravels and alluvial deposits.

Redbeds are red clastic sedimentary deposits, which typically consistof sandstone, siltstone and shale that are predominantly red due to thepresence of ferric oxides (Sheldon, 2009), and are widely distributedthroughout China (Chan, 1938; Huang and Opdyke, 1996; Rong et al.,2012; Tan et al., 2003). Currently studies on redbeds primarily focus ontheir sedimentological, geomorphological and engineering geologicalproperties (Besly, 1988; Perri et al., 2013; Ting et al., 2003; Wang,1993). Studies about the petroleum geology and reservoir characteristicsof redbeds have recently indicated that with effectively impermeablesealing barriers the redbeds can act as reservoir rocks and trap the oil–gas that migrates into it (Besly et al., 1993; Liu et al., 2012b; Wang et al.,2012b). Moreover, Akintunde et al (2013) evaluated the supercriticalCO2 storage in redbeds capped by basalts and/or diabase sills with ex-tremely low porosities and permeabilities. However, the effects ofredbeds deposits on the occurrences of CBM have rarely been reported.

The redbeds of the Huaibei Coalfield are primarily distributed in theXutuan and Zhaoji Coal Mines. The mining practices below the redbedsdeposits indicate a significant difference in the pattern of gas occurrencewith that of the coal seam in the non-redbeds covered area. This article

Fig. 3. Schematic diagram showing the burial history and stratigraphic evolution of the coal measure strata in the Xutuan and Zhaoji Coal Mines.

113K. Jin et al. / International Journal of Coal Geology 137 (2015) 111–123

uses the mineralogy, microstructure, pore structure, permeability anddiffusion characteristics of redbeds to compare the physical differencesbetween redbeds and ordinary coal measure strata rocks (sandstone,mudstone and siltstone), and investigates the effects of sedimentaryredbeds on CBM occurrence and its significance for coal mine gascontrol.

2. Geological background

2.1. Regional geology and formation of redbeds

The Huaibei Coalfield is in the northern Anhui Province of China,which is one of the major coalfields in this country, with 23 activeunderground coal mines (Zheng et al., 2008). The main coal-bearinglayer of the coalfield belongs to the Carboniferous–Permian system.Most of the coal measure strata in the coalfield are covered by

Fig. 4. Location, tectonic divisions and stratig

Neogene–Quaternary strata; only part of the coalfield containsMesozoicstrata (Jiang et al., 2010).

The Xutuan and Zhaoji CoalMines are located in the southern regionof theHuaibei Coalfield, and cover 52.59 km2 and50.79 km2, respective-ly (Fig. 1). The Paleogene redbeds are primarily distributed in the III3District of the Xutuan Coal Mine and in most regions of the Zhaoji CoalMine. These redbeds are unconformable to the Permian coal measureswith sedimentary thicknesses of 0–492.59 m and 23.25–922.76 m, re-spectively. The sedimentary characteristics of redbeds and the correla-tion between redbeds and coal seams are shown in Fig. 2. The absenceof Triassic–Cretaceous strata is a result of stratigraphic evolution,which is discussed in the following section. Geological data from thetwomines indicate that the interval between the bottom of the redbedsand the No. 32 coal seam ranges from 30 m to 380 m, and thatthe absence of the No. 24 coal seam in the redbeds deposition area iscommon.

raphic framework of the Guzhen Basin.

Fig. 5. Correlation between gas content and the elevation of the No. 32 coal seam.

Fig. 6. Correlation between CH4 concentration and the elevation of the No. 32 coal seam.

114 K. Jin et al. / International Journal of Coal Geology 137 (2015) 111–123

The formation of redbeds is closely related to paleoclimate andpaleogeography. Studies indicate that this type of sedimentary rockwas deposited in ancient basins or lakes under hot arid climates (Pinti,2011).

Fig. 7. Graph showing the distribution of redbeds

During the Paleogene, the central region of China was controlled byan inland arid subtropical climate (Sun and Wang, 2005) and the in-tense weathering of rocks (sandstone, limestone, shale and mudstone)provided a wealth of weathering products such as quartz sands, musco-vite scraps and clay minerals for the formation of redbeds in a deposi-tional environment of lacustrine facies. High-temperature indicatorminerals such as gypsum and glauberite (Fig. 9), which are found inredbeds that are intersected through drilling, establish the paleoclimateof when the redbeds were deposited.

The paleogeographic condition of redbeds deposition is closelyrelated to the stratigraphic evolution of the study area. According tothe geological data of the Huaibei Coalfield, the burial history and thestratigraphic evolution of the coal measure strata in the Xutuan andZhaoji Coal Mines are interpreted and shown in Fig. 3.

The sedimentary process of the Huaibei Coalfield began in the lateCarboniferous, when widespread transgression formed a 1700 m thickdeposit of coal-bearing clastic sedimentary rocks and carbonate, provid-ing the basematerial and reservoir capping conditions for the generationof CBM.

In the late Indo-Chinese epoch (approximately 210 Ma), themaximum burial depth of coal-bearing strata reached 3000 m andexperienced amaximumearth temperature of 140–180 °C, and the plu-tonic metamorphism caused the degree of coal metamorphism to reachRo=0.8–1.0%. During the Yanshanian period (200Ma–134Ma) the coal-bearing strata began to uplift due to the impact of strong tectonic activity,and the total erosion thickness of strata reached 2000–2500 m in lateCretaceous (70 Ma). The Triassic formation of the Huaibei Coalfield waseroded and exhausted during this period, and even parts of the Permianstrata were denuded in some places. In the late Yanshanian (approxi-mately 150 Ma), six sedimentary basins began to form in the northernAnhui Province due to the effects of geological movements and theMesozoic/Cenozoic sedimentary deposits in these basins gradually accu-mulated. The location of Xutuan andZhaoji CoalMineswere formed closeto the Guzhen Basin (Fig. 4). In the Himalayan period (66 Ma), the strataof the coalfields were under a tensile stress field, representing uplift orsettlement in different blocks; finally, a structural framework with upliftand depression segmented by faults was formed. During this period, theGuzhen basin continually developed westward, and the northwesternedge of the basin reached the Xutuan and Zhaoji Coal Mines, providinga favorable paleogeography for the formation of redbeds. The redbedsdeposits in the two mines belong to the Paleogene sedimentary stratain the Guzhen Basin. In the Neogene–Quaternary periods, the HuaibeiCoalfield was in a subsiding region, and the strata eventually formed.

in the III3 District of the Xutuan Coal Mine.

Fig. 8. Correlation between distance from the redbed boundary and gas emission quantity.

115K. Jin et al. / International Journal of Coal Geology 137 (2015) 111–123

2.2. Differences in gas accumulation

A statistical analysis of the gas content in the III3 District of theXutuan Coal Mine and in the Zhaoji Coal Mine is presented in Fig. 5.

The data indicate a high gas content in the non-redbeds coveredarea. However, in the redbeds deposition area, although the depth ofthe coal seam is deeper than in the shallow region, the gas content ismuch lower, with an average of only 2.52 m3/t.

Table 2Hydrogeological characteristics of the aquifers in the Xutuan and Zhaoji Coal Mines.

Aquifer Horizon Avgthickness(m)

Specific yi(L/s · m)

Aquifer #1 of Cenozoic Floor elevation: -7.50 to-16.55 23 0.563−1.8

Aquifer #2 of Cenozoic Floor elevation: -57.80 to -80.50 16 0.119–0.21Aquifer #3 of Cenozoic Floor elevation: -118.40 to-187.70 40 0.217–0.25Aquifer #4 of Cenozoic Floor elevation: -250.70 to -369.74 15 0.105–0.28Sandstone aquifer ofNo. 3 coal seam

60–90 m below the floorof No. 32 coal seam

8 0.004–0.00

Sandstone aquifer ofNo. 5–8 coal seams

Roof and floor of Nos. 7 and 8coal seams

17 0.00292–0

Sandstone aquifer ofNo. 10 coal seam

Roof and floor of No. 10 coal seam 8 0.0015

Limestone aquifer ofTaiyuan Formation

117.38–144.1 m below the floorof No. 10 coal seam

64 0.028–0.11

Limestone aquifer ofOrdovician

Ordovician of Lower Paleozoic, 133–230 mbelow the bottom of Permian strata

135 2.712–11.2

Table 1Proximate, petrographic analyses and adsorption constants of the No. 32 coal seam samples fro

Sample Elevation(m)

Proximate analysis (wt.%) Petrograph

Macerals

Mois Ash VM FC V

XT-1 -548 to -753 1.21 15.91 23.16 63.84 88.65XT-2 -425 to -840 1.24 17.33 28.06 58.72 78.77ZJ-1 -828 to -1115 1.01 24.42 36.85 47.25 73.18ZJ-2 -715 to -858 1.73 22.89 32.29 51.30 74.72

Mois=Moisture; Ash is on a dry basis; VM=Volatile matter, on dry ash free (daf) basis; FC=CA= CarbonateXT-1 and ZJ-1 are average experimental results of 9/52 samples obtained below redbeds from Xobtained in non-redbeds covered area from Xutuan/Zhaoji Coal Mine.

Based on the percentage of CH4 in CBM, coal seam gas vertical zoningcan be divided into the gasweathering zone (CH4 percentage≤ 80%) andthemethane zone (CH4 percentageN 80%) (Wang et al., 2012a). The anal-ysis of CBM composition (Fig. 6) indicates that in the shallow part, theCH4 concentration rapidly increases to 80% with increasing depth, indi-cating that the coal seam gas vertical zoning changes from the gasweathering zone to the methane zone. However, in the redbeds deposi-tion area, although the elevation increases from -400 m to -1200 m,the CH4 concentration is less than 80%, with an average of only 45.51%.TheCBMoccurrence shows characteristics of the gasweathering zone, in-dicating that the coal seam below the redbeds suffered a much strongerdiffusion effect than the coal in the non-redbeds covered area.

Mining in the Xutuan Coal Mine indicates the differences in gasoccurrence between the redbeds deposition area and the non-redbedscovered area. At present, four working faces in the III3 District of theXutuan Coal Mine have been mined. Two of these working faces arenear or below the redbeds, while the others are far away from the depo-sition boundary (Fig. 7). A calculation of the gas emission quantities ofthe four mined working faces indicates that the average gas emissionquantities of the working faces near/below redbeds are 6.22 m3/min(3233working face) and 8.09m3/min (3235working face). These valuesare smaller than those of the working faces far from the depositionboundary of redbeds (12.93 m3/min in the 3234 working face and14.61 m3/min in the 3236 working face).

The statistics of the gas emissionquantities of the 3234 and3235work-ing faces, which have similar mining elevation (-530 m ~ -570 m) indi-cate that the gas emission quantity increases obviously as the miningareamoves from the redbeds deposition area to the non-redbeds coveredarea (Fig. 8). The average gas emission quantity in the non-redbedscovered area (12.41 m3/min) is 1.87 times higher than the quantity inredbeds deposition area (6.65 m3/min). Furthermore, the gas emissionquantity decreases with increasing redbeds deposits thickness.

eld Hydraulicconductivity(m/day)

Salinity(g/L)

Water yieldproperty

Water quality type

37 2.40–5.80 0.348–0.863 Medium–rich HCO3·SO4–Na·Mg·GaHCO3·Cl·SO4–Na·Mg·Ga

6 0.877–3.80 0.926–1.022 Medium HCO3·Cl·SO4–Na·Mg·Ga2 1.194–2.18 0.893–1.157 Medium–rich HCO3·Cl·SO4–Na·Mg·Ga2 0.317–0.987 0.893–1.214 Weak–medium HCO3·Cl·SO4–Na·Mg·Ca894 0.0247–0.068 0.855–0.955 Weak–medium HCO3·Cl·SO4–K·Na

.294 0.0527–0.392 0.458–0.993 Weak–medium HCO3·Cl·SO4–Na·Mg·Ga

0.004 0.51 Weak HCO3·Cl–Na·Mg·Ga

6 0.099–0.461 0.896–0.953 Weak–medium Cl·HCO3–K·Na·Ca

9 6.22–17.92 2.362–1.303 Rich–extremely rich Cl·SO4–K·Na·CaSO4·HCO3·Cl–Na·Ca

m the Xutuan and Zhaoji coal mines.

ic composition (vol.%) Ro (%) Adsorption constant

Minerals

I L CL P CA VL

(m3/t)PL(MPa)

9.19 1.07 1.57 0.60 0.44 1.062 22.3216 1.216916.23 1.22 3.96 0.65 0.20 0.893 22.9612 1.236811.07 7.75 8.38 0.29 0.44 0.972 20.9535 1.175411.13 6.97 8.54 0.23 0.37 0.954 19.7445 1.1263

fixed carbon (daf basis); V=Vitrinite; I = Inertinite; L= Liptinite; CL= Clay; P= Pyrite;

utuan/Zhaoji Coal Mine, respectively; XT-2 and ZJ-2 are average results of 23/19 samples

Fig. 9.Graph showing the sampling sites of redbeds and the prepared φ50mm× 100mmcaprock samples.

116 K. Jin et al. / International Journal of Coal Geology 137 (2015) 111–123

3. Sampling and methods

3.1. Sample collection

From the viewpoint of gas geology, the gas content of coal is a geo-logical residual content of tectonic evolution that is determined notonly by the amount of generation but also by the diffusion/migrationsituation of the gas and the storage abilities of the coal seam (Creedy,1988). Thus, the present gas content is primarily decided by the capac-ities of the coals (adsorption properties and porosities) and by the pres-ervation capability of the reservoir. The physical parameters of the No.32 coal seam (Table 1) in the redbeds deposition area and the non-redbeds covered area are almost the same, indicating that the currentdistinction in gas occurrence may be due to the differences in preserva-tion conditions.

CBM preservation is mainly controlled by tectonic evolution, hydro-dynamics and sealing conditions (Song et al., 2012b); however the tec-tonic variations in a district or even in a coalmine are unremarkable andplay a secondary role in the control of gas accumulation. Thehydrogeological characteristics of the Xutuan and Zhaoji Coal Mines(Table 2) show that, unlike in many commercial CBM reservoirswhere the coal beds commonly act as regional aquifers (Scott et al.,1994), the caprock (including the redbeds and overlying Permian coalmeasure strata) and the direct floor of the No. 32 coal seam in thesetwo mines are both aquifuges. The thicknesses of the aquifuges rangefrom 36.06 m to 922.76 m (300.97 m on average) in the caprock and60m to 90m in thefloor. In this case, the effect of hydrodynamic sealing

Table 3Mineral composition of caprock samples.

Sample Mineral composition (%)

Quartz Muscovite Calcite

Redbeds 6–12 3–8 25–33Neogene clay rock b1 b2 7–12Sandstonea 42–53 3–6 1–2Mudstonea 3–6 b5 –

Siltstonea 12–21 b2 6–12

a Sampled from the caprocks of No. 32 coal seam which belong to Permian sedimentary roc

on CBM accumulation is weakened. Therefore, we can conclude that thedistinction in gas occurrence is mainly due to the differences in the cap-rocks. Moreover, Diamond and Schatzel (1998) indicate that thecoalbed gas reservoirswhich are notwater-saturatedwill contain largervolumes of free gas, thus for the No. 32 coal seam in the study area theeffects of caprocks on the preservation of CBM will be more significant.

The caprocks of the No. 32 coal seam, including redbeds and otherrocks suchasNeogene clay rock andordinary coalmeasure strata (sand-stone, mudstone and siltstone) were sampled by surface drilling in thesoutheast region of the Xutuan Coal Mine's industry square (Fig. 7).The total drilling depth 1076 m, and the thickness of redbeds intervalwas approximately 400 m (from -336 m to -736 m). After sample col-lection, roof rock samples with different lithologies were selected fortesting the physical parameters, and some intact cores were selectedto make φ50 mm × 100 mm standard rock samples (Fig. 9).

3.2. Experimental methods

To study the differences in physical parameters among the redbedsand other rocks, mineralogical composition, cementation, diagenesis,microstructure, pore structure, permeability and diffusion characteris-tics of the rock samples were analyzed.

The analysis of mineralogical composition, cementation and diagen-esis were conducted in the Rock Identification Laboratory, ChinaUniversity of Mining and Technology. The microstructures of thecaprock samples were observed by a scanning electron microscope(SEM, Quanta 250, FEI, USA). The pore structure differences were deter-mined using the mercury intrusion method with an AutoPore IV 9510(Micromeritics, USA), and the data were modeled to the WashburnEquation (Gürdal and Yalçın, 2001; Washburn, 1921):

pc ¼2σ cosθ

rð1Þ

where pc is the capillary pressure (MPa); σ is the surface tension of Hg(dyn/cm2); θ is the contact angle between Hg and the coal surface (°);and r is the pore/throat radius (nm).

Rock permeability was determined bymeasuring the stress and per-meability of coal/rock using a coupling instrument (Chen et al., 2013).The testing data were modeled by a pressure transient method (Braceet al., 1968; Evans and Wong, 1992), and the basic governing equationfor the pressure pulse through a specimen can be written as (Wanget al., 2011):

Pup tð Þ−Pdown tð Þ ¼ Pup t0ð Þ−Pdown t0ð Þ� �

e−αt ð2Þ

α ¼ kAμβL

� 1Vup

þ 1Vdown

!ð3Þ

where Pup(t)− Pdown(t) is thepressure difference between the upstreamand downstream reservoirs at time t (MPa); (Pup(t0)− Pdown(t0)) is theinitial pressure difference between the upstream and downstreamreservoirs at time t0 (MPa); α is the slope of the line when plotting thepressure decay Pup(t) − Pdown(t) on semi-log axis against time; k is the

Illite Kaolinite Feldspar Chlorite Pyrite

57–64 – – – b184–89 – – – b17–10 19–35 28–46 0.5–1 –

10–26 37–62 10–14 7–11 –

19–45 – 1.5–7 22–67 –

ks, the same below.

Table 4Texture, cementation and diagenesis of caprock samples.

Sample Texture Type of cementation Diagenesis

Redbeds Argillaceous- Aleurite Pelitic, calcareous Compaction, cementation, eluviationNeogene clay rock Argillaceous Pelitic, calcareous Compaction, cementation, eluviationSandstone Coarse-medium sand Carbonate, siliceous Compaction, cementation, recrystallization, dissolutionMudstone Argillaceous Pelitic Compaction, cementation, recrystallizationSiltstone Aleurite Pelitic, calcareous Compaction, cementation, recrystallization

117K. Jin et al. / International Journal of Coal Geology 137 (2015) 111–123

permeability (mD); A is the cross-sectional area (m2); L is the length ofthe specimen (m); μ is the dynamic viscosity of the gas (MPa·s); β is thecompression factor of the gas; and Vup and Vdown are the volumes of theupstream reservoir and downstream reservoir (mL), respectively.

The CH4 diffusion coefficients of rocks were measured by the sameinstrument with an additional module based on the free gas concentra-tion method (Liu et al., 2012a).

4. Results and analysis

4.1. Mineralogy of redbeds

4.1.1. Mineralogical composition, cementation and diagenesisThe mineral composition, texture, cementation and diagenesis of

the caprock samples are listed in Tables 3 and 4, and more detailedidentification characteristics of redbeds are shown in Table 5.

Paleogene redbeds are one type of lacustrine facies of clasticsedimentary rocks and are deposited under arid climates. Clay mineral(illite) is the main ingredient of redbeds (accounting for 57–64%),while calcite and quartz are secondary components (25–33% and6–12%, respectively). Small quantities of muscovite (3–8%) and pyrite(b1%) are also present. Redbeds display argillaceous to aleurite textureswith even and finemassive structures. Their color is due to iron impreg-nation (hematite) of the illite. Overall, the lithology of redbeds liesbetween siltstone and mudstone. Due to the later formation age ofredbeds, the effects of compaction and cementation are unapparent,leading a loose structure, a low degree of consolidation, and easierdisintegration when dried or immersed in water.

4.1.2. Microstructural analysesThe microstructures of caprock samples were analyzed by scanning

the selected smooth surfaces of natural fresh rock sections. The SEMimages are shown in Fig. 10.

The microstructural images of redbeds, Neogene clay rock andsandstone show a large number of micron-size pores, resulting in highconnectivities and developed pore structures. Moreover, fractures insandstone also enhance the permeability. Thus, the CBM sealingcapabilities of these rocks are weak.

Table 5Rock identification characteristics of redbeds.

Mineralcompositions

Percentage(%)

Identification characteristics

Quartz 6–12 Granular shape, particle size 0.02–0.04 mmMuscovite 3–8 Long strip shape, particle size 0.03–0.3 mm, aleuritic

texture overall and fine sand texture in some positionCalcite 25–33 Two types of calcite can be observed in petrographic

thin section: 1. particle size b 0.01 mm, xenomorphicgranular texture, scattered among the clay minerals;2. particle size 0.02–0.06 mm, aleuritic texture,subangular–subrounded psephicity

Illite 57–64 Thin scaly shape, particle size b 0.01 mm, evendistribution. Because of iron impregnation(hematite), the illite renders a maroon color.

Pyrite b1 Subhedral–euhedral, black, opaque, particlesize 0.03 mm

Compared with redbeds and sandstone, the structures of mudstoneand siltstone are much denser. The matrixes of these two caprocks arecomposed of flaggy and schistose grains, with limited pore structuresof sizes b5 μm. These types of caprocks are conducive to the sealing ofCBM.

4.2. Pore structures, permeabilities and diffusion coefficients of rocksamples

4.2.1. Mercury curves and pore structureThe pore structures of samples were obtained from the widths of

hysteresis loops and the volume differences between mercury injec-tion/ejection curves (Cai et al., 2012; Chen et al., 2012). The mercurycurves of different rock samples (Fig. 11) show similar curve sharpsfor redbeds sampled from different depths. All four redbeds samplesexhibit broad hysteresis loops in the mercury volume versus pressurecurves and large differences in volume between mercury injection andejection. This indicates that transition pores constitute a large ratio ofthe total pore volume within the tested range and that the pores arewell connected. The mercury ejection curve is slightly convex or hori-zontal at first and then concave, indicating the presence of open poresand some semi-closed pores (Chen et al., 2012). The formation of theredbeds pore structure is closely related to the diagenesis of the rock.Redbeds are young strata whose burial depth is shallower than1000 m in the study area; therefore, the compaction and cementationof redbeds are less remarkable, resulting in looser structure with morepores, and the eluviation to the redbeds would lead to the developmentof induced porosity. Moreover, the high content of fragile minerals(quartz, muscovite and calcite) of redbeds also benefits the conserva-tion of pore space (Chen et al., 2011). In summary, this type of porestructure is in favor of gas diffusion and penetration.

For Neogene clay rock, the mercury curve shape and total porevolume are similar to that of redbeds, indicating that transition poresconstitute a large ratio of the total pore volume and that the pores arewell connected. The diagenesis of this rock results in a looser structurethat also favors gas diffusion and penetration.

For sandstone, themercury curve shape is similar to that of redbeds,while the total pore volume is smaller as a result of the irreversiblereduction in pores caused by the maximum burial depth of nearly3000 m (compaction) and the carbonate–siliceous cementation type.The recrystallization of secondary quartz may also contribute to thesmaller pore volume. At low injection pressure, there is an obviousincrease in pore volume that indicates a large ratio of macropores andsuper macropores exist. The mercury ejection curve is slightly convex,the hysteresis loop is broad and the volume difference between injec-tion and ejection is great; this indicates that transition pores andmesopores constitute a large ratio of the total pore volume and thatthe pores are well connected (Chen et al., 2012). This type of porestructure is also favorable for gas diffusion and penetration.

Thefineparticle size of clastic grains, high degree of compaction, anddense microstructure result in poor pore structures for mudstone andsiltstone. The hysteresis loops in the mercury curves of mudstone andsiltstone are narrow, and the volume differences between injectionand ejection are low; this indicates that open pores are rare and thatthe pores have bad connections. This type of pore structure is notconducive to gas diffusion and penetration.

Fig. 10. SEM images of different rock samples.

118 K. Jin et al. / International Journal of Coal Geology 137 (2015) 111–123

To quantitatively analyze the experimental mercury data of caprocksamples, the Hodot classification (Wu et al., 2011a) for coal/rockpore size was used as follow: micropores (3–10 nm), transition pores(10–102 nm), mesopores (102–103 nm) and macropores (N103 nm).The pore size distribution of the rock samples is shown in Fig. 12 and

Fig. 11. Mercury curves of di

the data calculated from mercury intrusion experiments are listed inTable 6.

A comparison between redbeds and other rocks indicates that theporosity of redbeds reaches 17.0283–12.4175%, which is 3.8–5.2 timeshigher than sandstone and 8–13 times higher than mudstone and

fferent caprock samples.

Fig. 12. Correlation between incremental pore volume and pore diameter.

119K. Jin et al. / International Journal of Coal Geology 137 (2015) 111–123

siltstone. In addition, as the depth increases, the reduction in redbedsporosity is unremarkable. The total pore volume of redbeds reaches0.0654–0.0969 mL/g, which is 4–6 times higher than that of sandstoneand 10–15 times higher than those of mudstone and siltstone. Thepore size distribution of redbeds is mainly concentrated in transitionpores (approximately 57.82%–74.72%) and mesopores (9.08%–22.51%).In contrast, sandstone exhibits an even distribution within the testedrange (29.25% in transition pores andmacropores, 37.41% inmesopores),while the distributions of mudstone and siltstone are dominated bymicropores (54.17% and 43.55%, respectively).

Table 6Porosity and pore volume distributions of rock samples.

Sample Elevation(m)

Porosity(%)

Pore volume distribution (%) Vt

(mL/g)V1/Vt V2/Vt V3/Vt V4/Vt

Redbeds #1 -348 13.8752 12.94 57.82 22.51 6.74 0.0742Redbeds #2 -404 17.0283 15.07 74.72 9.08 1.14 0.0969Redbeds #3 -560 12.469 17.13 64.07 15.75 3.06 0.0654Redbeds #4 -670 12.4175 12.59 63.88 20.79 2.73 0.0659Neogene clay rock -256 15.1245 6.48 36.19 50.37 6.97 0.0818Sandstone -742 3.2615 4.08 29.25 37.41 29.25 0.0147Mudstone -755 1.5145 54.17 29.17 4.17 12.50 0.0072Siltstone -770 1.2897 43.55 29.03 6.45 20.97 0.0062

V1=pore volume ofmicropores; V2=pore volumeof transition pores; V3=pore volumeof mesopores; V4 = pore volume of macropores; Vt = total pore volume.

Fig. 13. Permeability of different caprock samples.

Generally, the transition pores in coals and rocks mainly constitutethe space for capillary condensation and gas diffusion, while themesopores constitute the slow penetration intervals. The mercurycurves and pore size distribution of redbeds indicate that this type ofrock has characteristics of high pore volume and high porosity; thepores in redbeds are well connected and open pores are abundant.Due to the large ratio of transition pores/mesopores and the insignifi-cant reduction in porosity reduction as depth increases, this type ofpore structure is favorable for gas diffusion and penetration in caprocks.

4.2.2. PermeabilityAsmentioned, thedissipation of CBMmainly has three paths as: cap-

illary loss, diffusion loss and hydrodynamic loss (Hong et al., 2005; Songet al., 2012a). Thus, the sealing mechanisms of CBM can be divided intothree corresponding types as: capillary sealing, hydrocarbon concentra-tion sealing and hydrodynamic sealing. Considering the caprock and

Table 7Diffusion coefficients of CH4 in different caprocks.

Sample Diffusion coefficient (m2/s)

Redbeds 1.8509 × 10-9

Neogene clay rock 1.6592 × 10-9

Sandstone 8.4695 × 10-10

Mudstone 2.0902 × 10-11

Siltstone 2.1859 × 10-11

Fig. 14. δ13C1 value of CBM in the Huaibei Coalfield.

Table8

Hyd

roge

olog

ical

analyses

ofthegrou

ndwater

intheXutua

nan

dZh

aojiCo

alMines.

Sample

Elev

ation

(m)

Tempe

rature

(°C)

Salin

ity

(g/L)

pHCa

tion

(mmol/L)

Anion

(mmol/L)

K++

Na+

Ca2+

Mg2

+Fe

3+

NH4+

Cl-

SO42-

HCO

3-

CO32-

NO2-

Xutua

n-5

2to

-721

17.5–36

.10.34

8–1.21

47.65

–8.25

2.68

4–25

.636

0.08

4–2.54

30.01

2–3.02

70–

0.00

70–

0.03

80.43

4–9.81

20.01

3–3.63

13.39

1–17

.198

0–1.73

80–

0.00

8Zh

aoji

-705

to-103

535

.2–43

.80.45

8–2.36

27.4–

8.6

6.12

7–23

.802

0.33

5–0.77

10.60

3–1.69

70–

0.00

80–

0.03

10.46

7–13

.186

0.36

9–2.47

86.95

5–9.85

80–

0.63

40–

0.22

3

120 K. Jin et al. / International Journal of Coal Geology 137 (2015) 111–123

floor of the No. 32 coal seam are both aquifuges, thus the hydrodynamicsealing effect in the study area is unremarkable.

The capillary sealing mechanism of caprocks is essentially the crea-tion of a permeability barrier due to the presence of free gas in layeredsedimentswith varied grain sizes (Revil et al., 1998). Thus, permeabilitycontrols the transport of free gas through the caprock and can beused toillustrate the capillary sealing ability of the caprock. Lower permeabil-ities are associated with stronger capillary sealing abilities.

The permeability experiments were conducted at 30 °C with a porepressure of 0.5 MPa. The testing medium was CH4 and the confiningpressure ranged from 2 to 15 MPa. The permeability testing results(Fig. 13) indicate that, under the same confining pressure, the perme-ability of redbeds is similar to that of Neogene clay rock and approxi-mately 68% that of sandstone; however, it is 23 times higher than thepermeability of siltstone and 35 times higher than that of mudstone.These different magnitudes of permeability indicate that siltstone andmudstone are much more effective at preventing the escape of gasfrom reservoirs. The sharp reduction in the permeability of clay rockunder high pressure may be related to its loose structure, while thehigh permeability of sandstone may be attributed to the existence offractures and the large ratio of mesopores and macropores.

4.2.3. Diffusion coefficientCapillary sealing can only prevent the free gas from dissipating; the

main mechanism of gas migration in a geological system is dominatedby diffusion in transition pores. Krooss et al (1988) indicated that signif-icant amounts of light hydrocarbons can diffuse over distances of hun-dreds of meters from the reservoirs into caprock units during geologictime. Saghafi (2010) indicates that the differential diffusive flow in cap-rocks can create significant differences in the level of saturation in coal.Gas diffusion, which follows Fick's first law, is controlled by the differ-ence in hydrocarbon concentration between reservoir/caprock and dif-fusion coefficient of medium itself (Nelson and Simmons, 1995). Thus,the hydrocarbon concentration sealing capabilities of caprocks can beevaluated by the diffusion coefficients of the rocks.

Diffusion coefficientmeasurementswere conducted at 30 °Cwith anequilibrium pressure of 0.5 MPa, and the testing medium was CH4. Theexperimental results are listed in Table 7.

Transition pores and mesopores constitute a large ratio of the totalpore volume in redbeds and Neogene clay rock, which is favorable forgas diffusion in caprocks. The experiment results indicate that the diffu-sion coefficient of CH4 in redbeds is similar to that in Neogene clay rock,2.2 times higher than the coefficient in sandstone, 88.6 times higherthan that of mudstone and 84.7 times higher than that of siltstone.Therefore redbeds have a much weaker sealing capability for CBMthan coal strata caprocks. In addition, the methane that diffuses or mi-grates into the redbeds will quickly escape into the atmosphere due tothe weak sealing characteristic of this rock; thus, the redbed depositionarea is characterized by low gas pressure and low gas content.

5. Discussion on the origins of the CBM gas pool and the effects ofredbeds

The stable isotope analysis of the methane is a principal meansof identifying the gas in question as being thermally generated ormicrobially generated (Rightmire et al., 1984). Rice (1993) suggestedthe δ13C1 value of the biogenic gas is -90 to -55‰while that of thermo-genic gas is greater than -55‰. Recent studies regarding the origins ofCBM in the Huaibei Coalfield indicate that the CBM has obvious charac-teristics of a secondary biogenic gas (Tan et al., 2010; Wu et al., 2011b),with most of the δ13C1 values smaller than -55‰ (Fig. 14). In addition,evidence has also been found in the Huainan Coalfieldwhich is adjacentto the Huaibei Coalfield, the percentage of secondary biogenic methanein the Huainan Coalfield ranges from 43% to 79% (Tao et al., 2007; Wuet al., 2011b; Zhang et al., 2005a).

Fig. 15. Schematic diagram of the CBM accumulation pattern resulting primarily from the effects of erosion.

121K. Jin et al. / International Journal of Coal Geology 137 (2015) 111–123

The hydrogeological analyses of the study area also indicate suitableconditions for the generation of secondary biological gas (Table 8).With suitable groundwater salinity (b4 g/L), ion type and concentration(low SO4

2-, low Cl- and high Na+), temperature (30 °C–40 °C) and pH(5.9–8.8), methanogens introduced by meteoric water can decomposethe organic matter in coal and generate secondary biogenic gas(McIntosh et al., 2010; Scott et al., 1994). From this perspective, theCBM in the Huaibei Coalfield is composed of secondary biogenic gasand some thermogenic gas.

However, the gas generationmechanism of CBM is complex, and or-ganic geochemistry studies point to uncertainties in the interpretationsof the stable carbon isotope data for gases associated with coals that areconnected to different types of macerals/kerogens of humic organicmatter and the fractionation process of CBM during migration and/ormixing (Kotarba, 2001). Therefore the processes in this area still havelarge uncertainties, and the present natural gas classification schememay not be able to be directly used to explain the origin of the CBM.

The Huaibei Coalfield underwent several tectonic movements afterits formation. The coalfield began a long period of denudation in theearly Mesozoic. In the Cenozoic, along with the entire stretch functionin eastern China, the extensional normal faults were widely developed,and the coal measure strata were eroded and exposed to the atmo-sphere due to uplift; this led to the emission of a large quantity of the

Fig. 16. Schematic diagram of the CBM accumulation pat

thermogenic methane (Wang et al., 2014; Wu et al., 2011b), whichwas generated in early times.

Themultiple tectonic movements alongwith the disputes regardingthe origin of the CBMmake the accumulation andmigration patterns ofCBM much more controversial. If the existing CBM in the HuaibeiCoalfield mainly belongs to thermogenic methane, the low gas contentunder the redbeds can then be attributed to coalbed degassing causedby strata denudation (Bustin and Bustin, 2008) during the Mesozoic.In redbeds deposition areas, much more serious erosion results in athinner residue of Permian strata and the emission of a larger quantityof gas than in the non-redbeds covered area. Because the redbedsformed much later, gas diffusion along the erosional contact betweenthe Permian rocks and Paleocene redbedswas limited, and the presenceof redbeds could only act as secondary cause, this results in the differ-ence between the two areas being maintained or increased (Fig. 15).

If the existing CBM belongs primarily to secondary biogenic gas, theroot cause of the different gas occurrences can then be ascribed to theredbeds since the generation stage of secondary biogenic gas began atthe later period of upliftwhen themethanogens introduced bymeteoricwater and continued until now. Thehigh porosity andhigh permeabilitycharacteristics of redbeds lead to a very weak sealing capability formethane; thus the secondary biogenic gas generated in the coal seambelow thick redbeds would easily escape through this type of caprock

tern resulting primarily from the effects of redbeds.

122 K. Jin et al. / International Journal of Coal Geology 137 (2015) 111–123

or diffuse along the unconformity interface (outcrop) between thePermian coalmeasures and Paleocene redbeds. However, the secondarybiogenic gas that is generated in the non-redbeds covered area wouldbe preserved because the overlying caprocks are much denser (Fig. 16).

Whether the CBMescaped from theNo. 32 coal seam is primarily dueto the effects of erosion, the weak sealing capability of redbeds, or acombination of both remains controversial and requires further study.However, the mining practice indicates that the deposition of thickredbeds can be treated as indicative of low gas content areas in coalmines, which is significant for the engineering of coal mine gas control.

6. Conclusions

1) The redbeds deposited in the Xutuan and Zhaoji Coal Mines are la-custrine facies clastic sedimentary rocks containing illite (57–64%),calcite (25–33%), quartz (6–12%), muscovite (3–8%) and pyrite(b1%). Iron impregnation (hematite) in the illite makes the color ofthe rock predominantly red. The formation age of this rock isyoung, resulting in unapparent compaction and cementation,which leads to a loose structure and a low degree of consolidation.SEM images showing themicrostructures of redbeds contain numer-ous micron-sized pores that are highly connected.

2) The mercury intrusion experiments demonstrate that redbeds havehigh porosities (12.469%–17.0283%), high pore volumes (0.0654mL/g–0.0969 mL/g) and unremarkable reduction in porositywith increasing depth. The pore size distributions are mainly con-centrated on transition pores (57.82%–74.72%) and mesopores(9.08%–22.51%) which is favorable for gas diffusion and penetrationin caprocks. The permeability and diffusion coefficient tests showthat the sealing capability of redbeds for gas diffusion is similar tothat of friable Neogene clay rock but much weaker than those ofmudstone, siltstone, and even sandstone. Therefore, the methanediffused (through caprocks) or migrated (through outcrop) intothe redbeds will quickly escape into the atmosphere, leading tolow gas pressure and low gas content in the redbeds deposition area.

3) The complex tectonic movements in the Huaibei Coalfield and thecontroversy in the origins of CBM make the accumulation patternof CBM gas pool much more controversial, and further study is stillrequired. However, from the aspect of engineering, the presence ofthick redbeds deposits can be treated as indicative of low coalmine gas content, as confirmed by mining operations. This is signif-icant for coal mine gas control.

Acknowledgments

The authors are grateful to the financial support from the NationalNatural Science Foundation of China (No. 51374204, No. 51204173, No.51304188), the China Postdoctoral Science Foundation (No.2014T70561), Youth Technology Funds of China University of Miningand Technology (No. 2010QNB02) and Doctoral Funds of Ministry of Ed-ucation (No. 20120095120001).

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