Coal petrology and genesis of Jurassic coal in the Ordos Basin, China

GEOSCIENCE FRONTIERS 3(1)(2012)85-95availableatwww.sciencedirect.comGEOSCIENCEChina University of Geosciences(Beijing)FRONTIERSGEOSCIENCE FRONTIERSGEOSCIENCEFRONTIERSjournalhomepagewww.elsevier.com/locate/gsfORIGINAL ARTICLECoal petrology and genesis of Jurassic coal in the OrdosBasin ChinaWeihua Ao, Wenhui Huang,s, Chengmin Weng, Xiuling Xiao, Dameng Liu,Xiuyi Tang ng Chen b Zhigen Zhao b Huan Wan Robert B Finkelmana School of Energy Resources, China University of Geosciences, Beijing 100083, Chinab Anhui University of Science and Technology, Huainan, Anhui 232001,ChinaThe University of Texas at Dallas, Richardson, TX 75083-0688, USAReceived 16 September 2010: accepted 20 July 2011Available online 7 October 2011KEYWORDSAbstract Multiple sets of thick coal beds characterized by simple structure and shallow burial depth wereLower-Middle jurassic.developed in the Early and Middle Jurassic strata of the Ordos Basin, northwestern China. The huge reCoal-formingof this high quality coal have a high commercial value. We studied the coal's petrologic characteristicsenvironment:maceral distribution to determine the macerals contribution to generation of oil and gas. The results show thatCoal maceralthe Jurassic coals in the Ordos Basin have special petrological features because of the Basins unique depo-sitional environment which was mainly a series of high-stand swamps in the upper fluvial system. These petro-graphic features are a result of the development of typical inland lakes where some sand bodies were formedby migrating rivers. After burial, the peat continued to undergo oxidizing conditions, this process generatedextensive higher inertinite contents in the coals and the vitrinite components were altered to semi-vitrinite. Themacroscopic petrographic types of these Jurassic coals are mainly semi-dull coal, dull coal, semilustrous andlustrous coal. The proportions of semi-dull coal and dull coal are higher in the basin margins, especially in thearea near the northern margin. The numbers of semilustrous and lustrous coals increase southwards andtowards the central basin This situation indicates that different coal-forming swamp environments have majorcontrolling effects on the coal components. Another observation is that in the Ordos'coal sequences, espe-cially in the lower part, some sandstone beds are thick, up to 20 m with a coarse grain size. The higher fusinite1674-9871@ 2011, China University of Geosciences(Beijing)and PekingUniversity. Production and hosting by Elsevier B.V. All rights reserved.Peer-review under responsibility of China University of GeosciencesBeijing)doi:10.1016/1sf201109004Production and hosting by ElsevierELSFVIER中国煤化工CNMHGw. Ao et al. Geoscience Frontiers 3(1)(2012)85-95content in the macerals accompanies a higher semi-vitrinite content with more complete and regular plant cellstructure. The fusinite structure is clear and well preserved. After burial, the lithology of the roof and floorrocks can continue to affect the evolution of coal petrology. The sand bodies in the roof and floor exhibit goodphysical conditions so that pore water can maintain a long-term state of oxidation, circulation and connectionto the coal So coal components remain in an oxidation environment for a long time. Conversely, in the basincenter, lacustrine facies developed and peat was rapidly covered by mudstone after burial and subsequent coalbeds rapidly entered a reducing environment. As a result, abundant gelatification occurred and the vitrinitecontent increased. Exinite often accumulated in a specific position in the coal bed. Although the averageexinite content is not high on the whole, it does significantly contribute to the total hydrocarbon generation.The exinite content has been underestimated, especially the amorphous bituminous fluid and its importance isemphasized here. The reason is that the fluid flows easily into fusinite which has strong rigidity, or flows intosome fissures, where it is commonly neglected.@2011, China University of Geosciences( Beijing) and Peking University. Production and hosting by ElsevierB.V. All rights1. Introductiondeveloped on a stable craton with a stratigraphic thickness generallover 4000 m, and up to 6000 m in the central basin. During itsHuge reserves approaching 1500 Gt of Jurassic coal are present in tectonic evolution, the basin underwent a Paleozoic platform phase.Nw China. The coal rank is low, but because of its specialforeland basin flexural phase in the Middle and Late Triassicproperties, the coal can be used in several ways. No matter what similar, and a relatively simple subsidence phase during theapplication is considered, a basic understanding of the maceral and Jurassic. The long-term development of this superimposed basin ledmineral content of the coals is necessary. For example, coal prop- to the occurrence of Permo-Carboniferous sequences, theerties are important for efficient direct coal liquefaction; theseyaobao coal sequences in the Triassic and the Yan'an Formationproperties include high hydrogen content with the h/C atomic ratio coal sequences in the Jurassic. The coal sequences coexist in somegreater than 0.8, and low inertinite content, generally lower than area, but also have different areal development. From the standpoint20%. a direct coal liquefaction technique has been developed by of coal resources, the Jurassic coals with burial depths of less thanthe Shenhua Company, the biggest coal mining company in China 2000 m are estimated to be 14,878 Gt, accounting for 75% of the(Zhang, 2006). It has been proven successful because it directlytotal resources(Zhang et al, 2005)exploits transitional semi-vitrinite and active semi-inertiniteThe major coal-bearing seams in the Ordos Basin are locatedcomponents in coal ( the macerals are described in detail in in Early and Middle Jurassic strata and are widely distributedsection 4.1).hroughout the basin. The petrography of the Jurassic coals hasSome of the Jurassic coals in Nw China have oil-generating been well studied(Qian et al, 1987; Huang et al., 1995; Dai et alpotential. For example, coal-generated oil occurs in the Tuha Basin. 2007, 2008), and indicates that Ordos's coals are very different inwhere coals from the Middle Jurassic Xishanyao Formation provideeir properties and genetic types in different parts of the basinone of the major oil source rocks. The main concern is to determineCoal formation in Ordos Basin can be divided into four temporalwhich macerals are the major oil-generation materials in the Tuha phases, namely, Early Jurassic, Middle Jurassic, Late Jurassic,andBasin. Suberinite was once considered the primary oil-generation Early Cretaceous. The Early Jurassic is an important coal-formingcomponent. Degradinite, containing ultramicro-lipid components, period with a total thickness of coal-bearing beds of about 300 mwas also regarded as another major oil-generation contributor. Zhao (Wang and Zhang, 1999). The lower part of the sequence is namedet al.(1994)have conducted many studies in the Tuha, Junggar, the Fu County Formation and the upper part is the Yan'anTarim and other basins. They consider that the primary Formation (Fig. D). Recent surveys have found that the MiddleJurassic Zhiluo Formation also has recoverable coal beds(Zhangalgainite and bitumenite)and fluorescent degradinite. However, Liu and Jian, 2007), but the Yan'an Formation, with the most exten-et al. 2000) considered most of northwestern China's Jurassic coal sive distribution and simplest structure of recoverable coal beds, isto be poor in exinite. As with other areas in Nw China, the Jurassicstill the major coal strata in the Ordos Basin.coals in the Ordos Basin possess high inertinite content. However,Because of the influences of tectonism and sedimentationthere are large differences in the proportion of each maceral and configuration and centers of basinal subsidence are different insome coal seams are rich in exinite. Coal-forming environments every phase(Figs. 2, 3). As a result, coal-accumulation zonescontrol changes in the maceral content, therefore, research on the display differences in the number of coal seams, their thicknessespaleogeographic framework of the Ordos Basin and the sedimen- and their lateral extents in different areas of the Ordos Basirtary environments of each of its major coal beds will bring benefits The Jurassic coal-rich belt in the Ordos Basin has a uniforto the prediction of maceral contents within them and the potential distribution, the center of the coal-accumulation zone and depoof the coathe thick coal seam is a broad gentle syncline and its axis islocated at N35%E in the line from the Wushenqi to Huan County2. Regional geologic settingThis trend parallels the distribution direction of the coal-rich beltThe coal-accumulation center in the Ordos basin is located nearThe Ordos Basin is a huge basin in China rich in coal, oil and gas Wushenqi(Fig 4). The maximum thickness is over 40 m and getsresources. It is almost 400,000 km" in area and is located on the thinner towards the periphery. Local coal accumulations occur inwestern margin of the North China Platform(Fig. 1). The basinHuating in the中国煤化工CNMHGW. Ao et al./ Geoscience Frontiers 3(1)(2012)85-95StrataGeneticLithological column SequeneSedimental environmentstratig-RaFaciesSubfaciesFlood plainNatural levee3 Distributary channel3Crevasse splayCrevasse delta1201-7Interdistributary bayFlood plaing Crevasse splayDistributary channelg Crevasse splayCrevasse deltaInterdistributary bay70Distal-bar sandCrevasse deltaInterdistributary bay116fLood plaing Crevasse delta8Flood plainHorizontal stratificationBioturbation structureA wavy cross bedding//Carbon-shaleFigure 1 Diagram of stratigraphy and coal seams in the Yan'an Formation of Suishi mine, Lingwu coalfield, Ordos Basin中国煤化工CNMHG88W. Ao et al. Geoscience Frontiers 3(1)(2012)85-95environmentdepositional microfacies are important in0120km8Yinshan Mountains3. Characteristics of coal petrologyx sHangjinDongshen3.1. Macroscopic types of coalsThe coals of the Jurassic Yan'an Formation form in two generaEtuokeqidepositional environments. One is high-stand swamp oxidation,which mainly develops in swamp environments in upper Aluvialsystems near the sediment provenance. Coarse sand bodiesdevelop in the lower coal beds and a thin mudstone layer alwaysoccurred in the bottom of the sand bodies. This type of environment mainly develops semi-dull coals, dull coals with lesserlarite, and bright coals. It is mainly distributed in the northeasternDongsheng area. The proportion of dull coal is higher from thebasin,s south edge to the north. Commonly, the combinedpercentage of semi-dull coal and dull coal can account for 60%tomore than 75% of high-stand swamp coals with a graduallydeclining trend from the basin north edge to the south. It should bePingliangManchengpointed out that in previous studies(Yang et al., 2005).muchbright coal and clarite, approximately 90% or more, was observedTongchuanin the Ruqigou area. These coals were considered to be a specialpe. Actually, the metamorphic degree of these coals is so highWei RiveO Xi'anthat they are anthracitic and even their liptinite and inertinitecannot be distinguished by common petrographic methods. Asa City or County River channela result, the typc of swamp environment cannot be determined byErosion boundary Open lake area z Alluvial plaincoal petrology or the types of macerals in such highly mcta-morphosed coals Semi-dull coal and dull coal are dominant in theFigure 2 Distribution of sedimentary environments of the Early- mining areas with a moderate degree officationMiddle Jurassic Yanan Formation(after Wang and Zhang, 1999)(Rmax=0.5%-08%). Judging from our observabright and bright coals always occurred in the bottom of thecoal seams. The other type of Ordos's coal environment is calleda low-stand swamp. It mainly develops in swamp environments insouthwestern Ordos Basin and in northeastern areas(Zhang et al., the upper fluvial system or in lakes(lacustrine facies). In this1995). Coal sequences in the Ordos Basin show a thinning trend cnvironment, fine sandstone bodies occurred in the lower, poorlyfrom north to south; coal seams in the northern basin are always developed, coal seams. The depositional layers are verymore than 20 m-thick but in the southern basin they decreasefine-grained muddy shale and coaly mudstone, which are well10 m. Moreover, the coal seams also become non-continuousdeveloped in the floor and the roof. The precursor peat quicklyThe distribution of the Jurassic coal-rich belt has a specific becomes anoxic aftcr burial and undergoes major gelatificationzoning property. From the basins western margin to its eastem Thereforc, the vitrinite content is high. The macroscopic types ofarea,coal sequences show a gradual thinning trend. Although at coal arc mainly bright coal and semi-bright coal, such that thethe western margin, coal sequences are thick, individual coal combined proportion of dull coal and semi-dull coal is less thanseams are thinner and the phenomena of branching off and 50%. This type of coal occurs in Shenfu and Yulin-Hengshan inmerging are common. The development of the coal-forming Shaanxi Province in the central and southcrn basin and in Fuswamp was near the basins western sedimentary provenance, County in the southeastern basin. Although semi-bright coal anda depositional setting not favorable for the formation of thick bright coal are dominant in this environment, some semi-dull coalindividual coal beds. For this reason the economic coal-rich belt is and dull coal is well developed locally, indicating that theonly developed somewhat east of the depositional centersedimentary microfacies still controls the distribution of theAccording to the coal seams' thickness and the numbers, the coal typesOrdos Basin has two coal-rich districts. One is in the northcrn areaIn a single coal sequence, oxidation generally characterizes thelocated along the line of Xinzhao, Wushenqi and Dongsheng. lower coal beds, in contrast to reducing conditions in the upperThe Lingwu district, in western margin of northern basin, has sequence. This phenomenon results from microfacies changes.Ina great number of coal seams with large thickness, more than 22 m general, oxidizing swamp waters were in circulation in early peric(Figs. 1, 5). The number of coal scams gradually declines and they later backwater cnvironmental conditions were reductive. The wholethin southwards to Huan County and Qingyang City. Some coal- sedimentary cycle presents a normal graded bed fining-upwardrich belts occur in the southern margin with coal seams more sequence. If overlying water conditions are stagnant and reductivehan 20 m-thick appearing locally in Pingliang, Huating, Toenough, no matter whether in peat swamps, upper fluvial or upperchuan and Huangling. There are no coal deposits in the Dali River lacustrine systems, and the gelatification process is adcquate. theLo the south, Hulu River to the north and Wuqi area to the cast of vitrinite content will be high. This phenomenon is common in thethe basin(Fig. 4). Based on our observation, the sedimentary Ordos Basin,butTH中国煤化工the same lawCNMHGW Ao et al. Geoscience Frontiers 3(1)(2012)85-95NHangjinhouqi. BaotouTuoketuo CountHangjingongshengChinaWushengOrdos basilYaDaning排 River faciesQingyangXiangningLake faciesAlluvial faciesJiaopingHualinRiver-delta faciesTongguanRiver-lake faciesoXi'anFigure 3 Sedimentary facies distribution of Jurassic rocks in the Ordos Basin(after Yang et al., 2005).The macroscopic types of coal display specific differences microscope, the sapropelic coal can be distinguished from humic(Table 1). However, it should be pointed out that the reason whcoal because it has no internal structure or layers. Other propertiesthe types of coals in Ruqigou are mainly bright coal is that the include a unique texture and high strength(Zhang et al., 1996)coal rank is anthracite. This metamorphic degree makes it difficult The Huangling area had been a sag in the Se basin in the depo-to distinguish exinite and vitrinite and this is the main reason for sitional period of the coal-bearing sequences. At that time, thestatistical compositional errorslake was deep, which was not favorable for the development ofThe low-stand type of coal environment cannot develop coals coal-forming swamps, but sapropelic coal can be formed in deepbundantly for two reasons. One is that some areas do not possess and semi-deep lacustrine environments. The areal extent of thethe occurrence of this kind of environment is commonly too short. in the Early-Middle jurassic sedimentary bsi of the deep lakesufficient water to cover the swamp. The other is that the timing of sapropelic coal corresponds exactly to the extentIn many coal sequences, individual sequences display the domi-nant dull coal and semi-dull coal in their lower part, more and more 3. 2. Characteristics of maceralssemi-bright coal and bright coal in their middle part and thenncreasing semi-dull coal and dull coal contents upwards. The reason Many studies have been done on the characteristics of macerals infor this is that the upper swamp environment with sufficient water to mining areas in the Ordos Basin(Wu et al., 1995; Shu et al. 1996cover the swamp was rapidly interrupted and then replaced by an Zeng, 2000; Bai and Li, 2001; Bai et al., 2002; Duan et al., 2004oxidizing swamp environment. In short, in many incomplete coal- Chen et al., 2006; Wang and Tang, 2007). In the comprehensivebcaring sequences, the coal beds display changes from dull coal to research of the whole basin, vitrinite was absolutely the predombright coal, then to dull coal, according to the cycle development. inant maceral group in the Permo-Carboniferous coals, inertiniteIt has also bcen discovered in this study that in the huangling was next in abundance and exinite was minor. Thesc abundancesmining arca in the se part of the basin, the No. 4 coal bed in the are basically similar with those in other areas in North China. Inlower Yan'an Formation contains sapropelic and cannel coals. the Jurassic, large-scale inland lacustrine and fuvial systemsThese oily coals are black with black streak and a dark silky luster. became the major cnvironment with accompanying changes inDepending on the vitrinite content, the luster appears glassy or climate and plant types. These changes resulted in an increase ofearthy. The cannel coal density is low; it is flexible and has a flal, inertinite in the Jurassic coals of the Ordos, with lesser vitriniteconchoidal or conchoidal-like fractured surface. Under the and still lesser exiniteTH中国煤化工CNMHGW. Ao et al. Geoscience Frontiers 3(1)(2012)85-95⊙ HangingWe2617We|2833⊙ BaotouTuoketuo CountyCoal seam 9Coal seam 9Coal seam 10Coal seam 10L Coal seam 1Ea DaningCoal seam 11CounBHu River⊙BaopHorizontal stratificationo⊙ctyX>30m05-5mr Bioturbation structureSandstoneountyNN20-30mCross beddingSiltstoneL*Fault structureZ1-20mavy cross bedding.. Jurassic pinch-out lineQ 60kmFi4 Distribution of jurassic coals thickness in the OrdosFigure 5 Diagrammatic sections of the rock sequence with coaligureseams in the lingwu coalfield area.4. Maceral distribution featuresThe maceral types in Jurassic coals are mainly vitrinite andinertinite, with inertinite most abundant. The most common The macerals in Yan'an Formation coals in different mining areasitrinite components are anthraxylon, homocollinite andof the Ordos Basin(Fig. 3)are mainly vitrinite and liptite: thedinite. The most common inertinite components arevitrinite content is lower than that of liptite (Table 2); and there isfusovitrite and macrinite, and the most common exiniterare cxinite and little mineral matter. The vitrinite contentnents are microsporinite, cutinite and amorphous bitumen. Some generally ranges from 32% to 60%, except in the Rujigou miningresinite, cortical tissue can be found, but no macrosporinite. We area. The distribution trend of the macerals is such that thenoted that exinite can be accumulated locally in the middle part of vitrinite content increases from the basin s periphery to the center;thick coal beds, which is very important for strong hydrocarbon the fusinite content has an opposite trend. The vitrinite content isgenerating and reserve capacity. Fusinite often acts as the matrix lowest in the Dongsheng area on the northern periphery ofmaterial. In most situations, the fusinite and vitrinite coexist in the basin, ranging from 27.5% to 40%, This is because the waterbent, irregular, or lenticular shapes and is distributed in blocky cover in the coal-forming swamp became well-oxygenated nearforms or in a dispersed state. Some fusinite is distributed as the northern part. The vitrinite content is higher in thecomplicated blocky forms or in a dispersed state. Fusinite has Yulin-Hengshan mining area, ranging from 60% to 70%a high rigidity so that cavities and pores are developed, which is of the high coal rank in Rujigou, Shitanjing, Huating and otherfavorable for the migration and the storage of hydrocarbons. So it mining areas near the western margins of the basin; in other areasis apparent that fusinite acts to enhance the reserve capacity of the the vitrinite value is about 40%. From old strata to young strata,coal beds. Vitrinite and exinite have strong hydrocarbon genera- the vitrinite content in the Yan'an Formation coals changes fromtion capacities without many vugs and pores. Therefore, further low to high, then from high to low. The changes of liptiniteresearch on the distribution and allocation of macerals in the thick content are exactly oppositecoal seams of the Ordos Basin will be useful to evaluate theMicroscopic observations of the macerals show that the vitrihydrocarbon generation capacity of the coal sequences and to nite is mainly anthraxylon, homocollinite or degradinite. Liptitemake an assessment of the reasonable utility of various compo- contains mainly fusinite, with a little micrinite and sclerotinitenents in the coal bedsThe main comTH中国煤化工and cutinite withCNMHGW. Ao et al. Geoscience Frontiers 3(1)(2012)85-95Table 1 Macro-petrologic types as percentages(%)of Jurassicin transmitted light; it is distributed uniformly as uneven bands andoal in the Ordos basilenses with locally orange-yellow color. Vertical innate fissures canbe observed occasionally(Plate Ia), with cutinite fringes locally orMining areaBright Semi-bright Semi-dull Dullscattered orange-yellow cuticularized aggregatescoalLingxin64412TianshuibaoDegradinite is one of the primary macerals in this area presenting31amounts up to 30% in most coal samples. In transmitted light,18degradinite appears as a sheet or"cement"(Plate lb). Its colorRujigousometimes reddish orange to orange-yellow without a stable shapeWayaobao10or cell structure. In addition, some degradinites can cementMajiatan139598606499695485macrinite and clay minerals(Plate lc).SuishijingShenfu4.1. 4. CorpocolliniteYulin-Hengshan 123326Corpocollinite occurs singly or as cell-filling material; it is mainlyderived from phlobaphinite and is seldom found in this area. ItsHuangling33color is brownish red-maroon and is distributed as oval. circularBinchangand lenticularThe vitrinite reflectance of Jurassic coals generally ranges from0.5% to 0.8%, which is in the range of low metamorphic biturare resinite. Overall, fusinite (iptite) content is high and the minous coal. Bai et al. (2005)considered that the volatile yield ofcontent of mineral materials is lowsuch low metamorphic degree coals does not correlate well with4.1. Vitrinite: telinite, collinite, degradinite, corpocollinite caused by different degrees of reductio othe maceral composition and vitrinite reectance; it is probatThe cell walls of telinite are reddish-brown to brownish-red in sclerotinie fusinite, semi-fusinitre,macrinite, micrinite,4. Telinitetransmitted light. The cell structure is well preserved witha regular arrangement that can be observed clearly. Some cell 4.2.1. Fusinite, semi-fusinite, macrinitewalls expand and then the cell cavities (lumen) shrink gradually; Fusinite has clear cell structure. It appears black and brownish-ome of them are empty(Plate la, b). Others are filled in with black in transmitted light(Plate ld). It is distributed in bands,collinite, resinite, microsome or clay. Extrusion and deformation lenses, strips and irregular shapes. The cell wall is white in oilof the structure of the cell cavities are common and occasionally immersion and reflected light (Plate Ik, I). Semi- fusinite oftenthe telinite cell cavities are filled in by cutinite fragmentsoccurs in the Ordos Basin and generally appears in a transformedstate with anthraxylon. The color ranges from reddish-black to4. 2. Collinitebrownish-black in reflected light(Plate le). The reflectance ofBased on its shape and genesis, collinite sometimes can be semi-fusinite is lower than that of fusinite, but higher than vitrisubdivided. In the Ordos Basin. its homocollinite and nite. Its cell structure is not preserved as well as that of fusinitedegradinite with a little corpocollinite. Collinite can evolve from Macrinites are observed as different-size circular and oval grainsgelatinous substance, which could have been wood fibers or other without original cell structure. It also appears as a mixture withcomponents in plants. Under the microscope, the homocollinite degradinite in the matrix(Plate If)appears uniform but shows different degrees of gray color in oilimmersion and reflected light. The color is reddish orange to brown 4.2.2. Micrinite, sclerotiniteMicrinite occurs as white aggregates observed in oil immersionTable 2 Maceral statistic data(in %)and mean reflectance ofand reflected light. It often appears in bands or cell-cavity fillingssome Jurassic Yan'an Formation coal samples from the OrdosMinor sclerotinite is also present in some Ordos coals(Plate 1j); itlooks like a circular white vein in oil immersion and reflectedBasinlight. It often occurs in degradinite with black sclerotiumMining area Vitrinite Liptite Exinite MineralAfter their study on low grade Jurassic coals in Nw China, BaiDongsheng 56et al.(2005)considered that the high content of inertinite andRejigtransmitted light components has some connection with a weak0.55reducing environment. The Jurassic coal-forming swamp wasSuishijing46ShenfuYulin- Hengshan 60Jiaoping98747340.70mainly of the high-stand type. Because of drought and forest fires,0.63burnt fusinite occasionally appears as intercalated layers in thecoal. The Jurassic coals in Nw China are generally non-caking0.64coals and weakly caking coals( Han et al., 1996). Because ofweak reduction the coal has low volatile yield, low hydrogenHuangling520.79 content, but high oxygen content.55After his study on inertinite-rich coal in the Shendong(Shenfu-Huating 84 6 7Dongsheng) mining area, a famous coal mine near Yulin City, LiNote: R mm mean vitrinite reflectance(2005)considered that the main conditions for formation of thisd coal were dry weatherTYH中国煤化工CNMHGW. Ao et al./Geoscience Frontiers 3(1)(2012)85-95车的d5ie医f9中国煤化工CNMHGW Ao et al. Geoscience Frontiers 3(1)(2012)85-95级队不Plate 1 Petrographical micrographs of Jurassic coal samples from the Ordos Basin. a: anthraxylon, orange-red, clear and regular arrangementof cell texture, void cell cavity, transforming into semi-vitrinite and fusinite and surrounded by cutinite. Micro-fissures are developed in thenetwork shape Dongsheng. J1-2y, transmitted light, 50x, [orange-yellow components shown in Plate la are homocollinites]; b: mainly cutinitemacrosporinite and microsporinite, in amorphous degradinite, the latter dark brown in reflected blue light. The macrosporinite(banded cement inthe middle)is yellow and shows strong fluorescent light; the microsporinite and cutinite are yellow and show weak or dark brown fluorescent light.Yuhebao, Yulin, J1-2, blue reflected light, 50X; c: degradinite, medium-degree degradation, amorphous matrix, cemented macrinite. ShenmuJ1-2y, transmitted light, 130x;d: fusinite, brownish-black with clear cell texture between degradinite bands with sharp boundaries and withoutany transformational phenomenon. Yuhebao, Yulin, J1-2y, transmitted light, 50x; e: transition from anthraxylon, semi-fusinite to fusinite(varyingfrom left to right); the rate of change is slower on the left and quicker on the right. There are intercalated vitrinite bands; several horizontalpartings in the bands are the result of shrinkage of vitrinite gelatination Dongsheng, J1-2, transmitted light, 50x: f: macrinite, black, distributedirregularly in amorphous degradinite, the latter orange-red in mixed distribution, but still can be connected. The macrinite may result from theresidue of separation of vitrinites during hydrocarbon evolution. Yuhebao, Yulin, J1-2y, transmitted light, 125x: g: cutinite, thin cell wall, fine andbent as a wire, distributed in degradinite. Shenmu, J1-2, transmitted light, 130x; h: phlobaphinite, yellow fluorescent light, the arrangement ofthe cell texture of suberitoid is irregular with the separation of bitumen. Dongsheng, J1-2y, blue reflected light, 600x; i: seepage bitumen, strongvellow fluorescent light, infilling bedding fissures, showing flowing phenomenon. Well Yi-23, Etuokeqi, J1-2y, blue reflected light, 780x:j: sclerotinite(emphasized by white rings), white, void rings, sclerotium, in degradinite. Well Yi-23, Etuokeqi, J1-2, oil-immersed reflected light310x; k: fusinite, lined, very regular and complete arrangement of cell texture, the product of early-stage fusainization, void cell cavityDongsheng, J1-2y oil-immersed reflected light, 390x; 1: burnt fusinite, easily broken. The cell wall is compressed and broken, extending in onedirection, showing that well-textured burnt fusinite was broken in-situ by compression creating better permeability of the coal bed. Well Yi-23Etuokeqi, J1-2y, oil-immersed reflected light, 490xsurface. In addition, the plant remains underwent the influence of types. Adjacent to the northern basin the inertinite content is thefungal activity, which has been indicated as an important mech- highest, exceeding 80%; the content often reaches more than 60%anism for forming inertinite (Stach et al., 1982). This study(see towards Jiaoping, Huangling and Binchang mining area in thePlate 1j) has not shown that thecoexists with fusinite southern part of the basin. Fire-formed fusinite is minor, but thewhich has a complete cell structure. However, the fungus always content of semi-fusinite is high. The features of clear cell structureexists with semi-vitrinite, fine grain cutinite and microsporinite in and fracturing of fire formed fusinite are quite different from thedull coal or semi-dull coal beltstructure of semi- fusinite or semi-vitrinite(Plate 11). It is notedWe have also found that in these coals the inertinitethat in previous studies ( Chandra, 1965; Stach et al 1982:varies depending on the coal-forming environment andDai et al., 2003: Tang中国煤化工2005),theCNMHGW. Ao et al. Geoscience Frontiers 3(1)(2012)85-95semi-fusinite or semi-vitrinite, is always categorized as inertinite, 4.3.6. Mineralsand basically still remains active. Thus semi-fusinite and The mineral content of the Jurassic coal in the study area is mainlysemi-vitrinite should not be classified as inertinite but should be kaolinitic clay distributed in disseminated or small lenticularreassessed according to their percentage in the whole maceral shape. The sulfur content is low, which indicates that an acidicgroups or its hydrocarbon ratio. The high content of these tran- terrigenous environment was predominant. Pyrite crystals occur insitional components in the middle part of Ordos Basin should be some coal beds, but framboidal pyrite is rare. In addition, a kind ofgiven considerationmatrix mineral bitumen that doesn' t belong to any maceral can bedistinguished in plain light. This component is very common in43. Exinitebulk rock samples and has not been identified.4.3.1. Sporophyte5 ConclusionsSporophyte is sclerine of pollen or sporinite. Microsporinitegenerally exhibits strong yellow fluorescence under ultraviolet In this study, we found that the coal petrological features oflight. It has compressed elongated shapes and occurs in degra- Jurassic coal in the Ordos basin in Nw China are its transitionalbut in low concentrations. Macrosporinite is rare in this area and components, semi-vitrinite and semi-fusinite. The coal'swhen present, has weak fluorescence(Plate 1b).semi-fusinite content is significantly high. Therefore, the chemicalcharacteristics of these coals and their technical properties areThis study discovered that exinite often accumulated locally indifferent from other coals with the same metamorphic degree inthe middle part of coal beds. Its average content is not high in thewhole coal bed, but the presence increases the capacity of liquidof ioa: other areas. In-depth research on the genesis of these transitionalcomponents will be helpful for better predictions on the varyinghydrocarbon generation. In previous research(Stach et al., 1982), trend of the technical properties of these Jurassic coals.the exinite content is often underestimated because exinite has fineCoal petrology research on the Jurassic coal sequences in thegrain size, occurs in other components, and it is always difficult to Ordos Basin shows that macroscopically the coals are mainlyanalyze accuratelysemi-dark and dark with a little semi-bright and bright coalThe coals are mainomponent petrographic system4.3.2. CutiniteCutinite is from plants leaves, branch or skin secretion, and it isminite and fusinite. The coals maceral composition has closedepositional environment evolution. nograyish black in reflected light and oil immersion and is often matter whether in horizontal or stratigraphic distribution it mainlyfilled with vitrinite(Plate Ig). It occurs in single bodies or layers. shows the features of vitrinite content of Jurassic coal. On planeIn fluorescent light, it appears yellowish green to brownish yellow distribution, vitrinite content gradually increases from the basinperiphery to the center, but the fusinite content exhibits a reverseis yellow or orange-yellow. In transmitted light, it is brightend. It indicates that the oxidation and fusinization are strongeryellowish white and distributed in banding, limbate and fake on the basin periphery than in the center. When the covering watershape. Cutinite is widely distributed, but present in low concen-tration in the study areadeepens in the basin and forms a stable and closed water body, thehigher vitrinite content in basin centers. In stratigraphic distils breduction strengthens, which lead to stronger gelatification an433. ResiniteResinite was the plants resin or oil secretion, in the coal beds itbeds it tion, the lower vitrinite content in Jurassic coal at the two edgesgenerally occurs in oblong, oval, kidney shape particles, with thinbut higher in the middle, indicates the evolution of the depotional environment. In other words, the basin underwent a processand thick banding or dispersed and locally as fillings in cellcavities. Occasionally the resinite appears compressed and or advance-retreat- advance with the lake in a state of expandeformed In transmitted light, it appears homogeneous and brightsion-shrinkage-expansion, and the paleoclimatic environmentyellow and in reflected light it is dark grayish black. In thein a damp-drought-dampExinite is often observed in middle part of thick coal beds inexcitation of blue reflected light, the color of fluorescence vanes the Ordos Basin. Although its average content is not high, itfrom brownish yellow to yellowish greencontributes a lot on the total hydrocarbon generation capacity ofcoal beds. The hydrocarbon generated from coal seam can iow4.3.4. Phlobaphiniteinto and storage in layers, which are rich in fusinite. In OrdosPhlobaphinite(cortical tissue) is one of the more important Basin, the exinite content increases in the south. Its content ismacerals in the Ordos Basin coals. Its cell structure is well lower than other components in the coal beds, but if it rises a littlepreserved. The regular degree of the cell structure arrangement isthe whole hydrocarbon generation capacity strengthens. Andeven, mostly imbricated, and elongated along the beds. Clearfurther research should put more attention to the observation ofcortical structure can be observed. The color is dark grayish light thin sections, the precise description of the distribution state ofgray in reflected light and oil immersion(Plate 1h). The fluoreexinite in coal beds, and an objective estimation of its content,cence is yellow, brownish yellow and yellowish green in blueeflected light excitationespecially fluid bitumen. The reason is that it can flow easily intohard fusinite or fissures and then is always neglected.4.3.5. Seepage amorphous bitumenSeepage amorphous bitumen is very common. It has features ofAcknowledgmentslow reflectance and mostly weak light brown fluorescent light.In blue reflected light, it shows dark-gray or yellow fluorescent This paper is supported by Chinese Ministry of Science and Tech-light. The bitumen fills in fissures and pores; it sometimes appears nology Project (No. 2006CB202202), and the National Naturalto have a flowing structure( Plate 11)ndNo.40772095)中国煤化工CNMHGW. Ao et al. Geoscience Frontiers 3(1)(2012)85-9595The authors are very grateful for these financial supports. We would Qian, LJ, Bai, QZ, Xiong, C.W., 1987. The Accumulating Features oflike to thank Prof. Gregory A. 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