Significance of Coal Seam in Recognition of Sequence Boundaries and Systems Tract of the Jurassic in

2006Petroleum ScienceVol 3 No. 2Significance of Coal Seam in Recognition of Sequence Boundaries and SystemsTract of the Jurassic in East Fukang Slope, Junggar Basin, Chinahang Qin 2, Zhu Xiaomin" and Song Gan(1. Geosicences Department, China University of Petroleum, Beijing 102249, China)(2. Key Laboratory for Hydrocarbon Accumulation under Ministry of Education,China University of Petroleum, Beijing 102249, China)Abstract: The Jurassic in the East Fukang Slope can be divided into six sequences based on sequence stratigraphy bycombining logging, core and seismic data. The indicators of sequence boundaries include unconformity, coal seams,change of spore and pollen abundance, scour surfaces and base conglomerate, change of logging curve and sedimentaryfacies. How to determine the location of the first flooding surfaces and the maximum flooding surface is the key step todivide the systems tract. There occurred a topographic slope break in the East Fukang Slope when the Jurassic wasdeposited, and therefore we can recognize the location of the first flooding surface and establish the sequencestratigraphic framework with the slope break in the study area. Coal seams regionally distributed are correlatable andisochronic, and record the termination of a depositional event or episode. So, the regional coal seam(more than 60percent coverage)can be used as the genetic stratigraphic sequence boundary, while locally distributed coal seam(lessthan 60 percent coverage)can be used as the systems tract boundary. The thick coal seams distributed regionally in themiddle of the Badaowan Formation and the lower part of the Xishanyao Formation in the study area act as the sequenceboundaries, while the thin and locally distributed coal seam acts as the systems tract boundary, which results in thecorrelation of the division of sequence stratigraphy of the Jurassic to the whole basin where coal seams are developedextensively.Key words: Jurassic, sequence boundary, systems tract, coal seamanalysis of logging, cores and seismic data. Each1. Introductionsequence corresponded with the reflected wave group ofThe east Fukang Slope is located in the east of the J, J, J, J,J", J,K, etc(Fig. 2)(Zhang, et alChangji Sag and on the junction of the middle-south of 2001; Wang, 1995). Furthermore by comparing thesequence divisions, based on the seismic, well loggingin the northeast of the Junggar Basin, Northwest China, and core data, the consistency of the sequencewith an area of about 5,000 square kilometres(Fig. 1). stratigraphic division of the Jurassic, was achieved inWhen the Jurassic was deposited, a huge slope with athe study areanorth-south dip was formed, which consisted of theWucaiwan Sag, front of the Kelameili Mountain2. Identification of sequence boundarypiedmont, the Baijiahai Salient and the east FukangThe key to defining a sequence stratigraphic modelSlope in the narrow sense. Because of the slight in a sedimentary basin is how to recognize the sequenceinfluence of the Yanshan Movement, the traps formed boundaries of the different grades. Based on the wellby tectogenesis were not developed. So it is importanlogging, core and seismic data, the sequence boundariesto conduct a study of the sequence stratigraphy about of different grades are identified by the indication ofthe Jurassic in the East Fukang Slope, so that subtle tectonic movement interface, coal seam, change oftraps might be found(Gu, 1995). The jurassiclithology and lithofacies, unconformity etcomposed of lower series Badaowan Formation J1b) division of the sequence stratigraphy of the East Fukangand Sangonghe Formation ( JIs), middle serieYH中国煤化工Xishanyao Formation (J2x)and Toutunhe Formation J2t,table sequenceand upper series Qigu Formation (J3a) and Kalazha boundaCNMHGFormation J3)(Pu, et al., 1994)from bottom to top inA sequence is defined as the relatively conformablethe East Fukang Slopeelated in genesis betweenThe jurassic in the study area can be divided into six unconformities or conformities which can be correlatedsequences based on sequence stratigraphy through an with the unconformity. The retlection termination typesPetroleum Science2006indicating the sequence boundary include onlap, Yanshan movement, spread widely and cause extensivedownlap, truncation, toplap, etc. Three unconformities truncations of the underlying formations, whileindicating respectively the tectonic movement of the unconformities that developed locally exist betweenIndosinian(J), I episode(J")and II episode(K)of the other sequences in the Jurassic(Fig 3)o Cai311. Lunan Uplift3. Baijiahai Salient4. Wucaiwan Salient5.Zhangbei Fault andDrape Zonengo7. Santai SalientTai22 o Bei34818.Mountain FrontFault-fold zoneU Study areaO Cai3 1-LocationFig 1 Locationof the East Fukang Slope41942943b2o0keAsszwdudkGKaSSSSiOFig. 2 The sequence stratigraphic division and sequence boundary of the Jurassic in the East Fukang Slope(Section w9241)2)Coal seams distributed regionally are markers for characteristics of reflected wave termination show thatthe construction of a sequence stratigraphic framework. onlap or downlap lies on the top of a reflected waveThe coal seams occurring respectively in the middle group中国煤化工 seismic section.Forpart of the Badaowan Formation(corresponding to the examplreflected wave group of J 2)and the lower part of on theYHtCNMHGoWnlaps can be founde: .,. Broup(z)of coal seamXishanyao Formation(corresponding to the reflected on the JH8715 seismic section( Fig 3), and a series ofwave group of J ) in the Jurassic are distributed onlaps transferred gradually downwards the basin onextensively and stably, and they were easy to trace and the JH8725 seismic section could be identified on thecorrel乃数据the whole Junggar Basiwave group J. Therefore, the main coal seamsVol 3 No. 2Significance of Coal Seam in Recognition of Sequence Boundaries and Systems tract9occurring respectively in the middle part of the reflected wave group respectively) act as the sequenceBadaowan Formation and the lower part of the boundary in the process of sequence stratigraphicXishanyao Formation(corresponding to J2 and Jt3 divisionerosionFig 3 Sequence stratigraphic division and sequence boundary of the Jurassic (erosion, onlap and downlap3)There are decreased differentiation and fossils conglomerate of the channel of Toutunhe Formationabundance near the sequence boundarydirectly overlies the Sangonghe Formation, while theBased on the analysis of sporopollen fossil in the Xishanyao Formation disappeared, and the sangongheJurassic in Well Caican-2, there is notable decreased Formation was eroded and partly disappeareddifferentiation and abundance of sporopollen fossil near6) There is a change of sedimentary facies near thethe bottom of sequence II and sequence Ill, such as sequence boundarypores of cyatheaceae, dicksoniaceae, osmundaceaeThe sedimentary facies spreads discontinuously orand lotus seed, which was caused by the fluctuation of changes suddenly near the sequence boundary. Shore andthe lake water surface and the formation of the sequence shallow lake mudstones were developed on the coalboundary. The differentiation and abundance of spores seams(the swamp sediment of delta plain, belonging toand pollens increase obviously in a transgressive system the highstand systems tract of Sequence III) in the lowertract and in the early stage of a highstand system tract. part of the Xishanyao Formation, and occurred4) There is lithology change of the sedimentary correspondingly in a new period of transgression(Fig 4)formation and the development of basal conglomeratechannels infilling and infilling sediment in river channel 3. Identification of systems tract boundaryand on scour surfaces on the bottom of the sequencesWhen the Jurassic was deposited, a topographiThere developed basal conglomerates and scourslope-break zone came into being on the baijiahaisurfaces near the bottom of sequence III in Wells Fu4 and Salient in the Fukang Slope, where the topography isCailol, where conglomerate JIs) lies on the scouredsteep from the Baijiahai salient to the Changji Sag andsurface with mudstone (J1b)existing below it. The basalgentle from Cainan to the Kelameili Mountain, so thatconglomerate, such as pancake boulder clay, quartzitic the first flooding surface and three types of systemsgravel and carbonaceous fragments, are parallel to the tract, including lowstand, transgressive and highstandscour surface or in imbricate arrangementsystems tract, could be determined.5)The shape of log curves changes suddenly near the1)A stably spreading seismic event on the channelequence boundedil中国煤化工 g surfaceThe channel sediments which developed on theCNMHGact was formed,thesequence boundary, with the box-shaped, bell-shaped or separatlciclilauui ui topography are obviousmultiple bell-shaped logging curves, are greatly where some isolated small scale lakes and depressionsdifferent from the underlying bed. For example, this were developed discontinuously, and a large number ofkind of difference exists on bottom boundatndy mised valley sediment was developed in the lowstandof incised valley and alluvial infilling were developed. TheSequenc方数搪 Well Be-34, where the sarPetroleum Science2006systems tract of Sequence I in JH8715 seismic section strong amplitude is regarded as the first lacustrineand its seismic reflection was shown as local onlap, on flooding surface.which the stable and the farther-spreading event with aPotential kolor Lithological Resistivity Strati- Sequ-bystemsSedimenSequencegraphy ence Tractacle12HSTMaximumLakeflooding surfaceⅣ|TsT|LakeRetrogradationEtangJ2··。PlainHSTDeltaProgradationFrontMaximumflooding surface●三TST EtangRetrogradationFirst flooding surfaceBraidLSTAggradationFig. 4 Sequence boundary between sequences 3 and 4 of the Jurassic in Well Cai310-Grayish white, &-Grayish green, 12-Black, grayish black, 13-Dark grey, 14- Light grey2) A coal seam or carbargilite distributed parasequence between different systems tractssporadically indicates the first flooding surfaceLowstand systems tracts are infilled with alluvialThe coal seam or carbargilite shows the rise of free sediment and have a typical bivariate structure, shownwater surface that relates to the first lacustrine flooding. as the succession of vertical accretion and retrogressionBut, with the rapid rise of the lake water surface, the in the study area, while transgressive systems tract isconditions for formation of carbargilite were no longer mainly sediments of deep water lake or delta and itsavailable. For example, a lowstand system tract of lower part is usually the parasequence sets withequence I in Well Cai3 l consists of coarse pebblestone, increasing thickness of mudstone and a decreasing ratiosandy conglomerate and fine pebblestone with 3 sets ofof sandstone and mudstone, and its upper part ispositive rhythms, which is shown as box shape in the generally mudstone of lake facies with accretivelog and is classified as the braid sediment Sediments of parasequence sets. The maximum flooding surface(thedelta plain(containing 2 sets of thin coal seams) and bounda中国煤化工 only characterized asdelta front were developed in the lower part of the surftransgressive systems tract. The coal seams could actprogrHCNMHcressive or accretive toor from mudstone andthe markers of the first flooding surface. The coal seam silty mudstone of lake facies or prodelta with a positivwith less spreading was thinner than that of a highstand or faintly positive rhythm to argillaceous siltstone,systems tractsiltstone and fine medium sandstone of delta front with3)1号极辖 nange in the superimposition mode of a reverse rhythmVol 3 No. 2Significance of Coal Seam in Recognition of Sequence Boundaries and Systems tract4)The coal seam or carbargilite distributed locally combined with the relation of the sequence strata, thects sometimes as the marker of the maximum flooding distribution of the main coal seams and the locations ofsurfacethe first flooding surface and maximum floodingWhen the maximum flooding happened, the climate surface, the sequence stratigraphic analysis waswas humid and the accommodation space increased performed in the Jurassic in the East Fukang slopegradually with no supply of terrigenous clastics, which The tripartite version, namely lowstand, transgressivewas in favor of the development of plants and and highstand systems tracts was adopted in the divisionaccumulation of carbargilite, and the coal seam was of systems tract of Jurassic sequence. Because theformed under the free water surface and abobvious topographic slope-break existed in the studycarbargilite sediment. The coal seam or carbargilite area when the Jurassic was deposited, namelysent on the fine clastic sediment of deep-water for topographic slope-break came into being on thefirst time might act as the marker of the maximum Baijiahai salient, which resulted from the fact that theflooding surface. it is thicker and more continuous than topography was steep from Baijiahai salient to Changjithat near the first flooding surface. For example, in the sag and gentle from Cainan to Kelameili Mountain. As atransgressive systems tract in the Sequence IV in Well result, the location of the first flooding surface could beCai31 grey mudstone of shore-shallow lake was verified and the sequence stratigraphic framework coulddeveloped, and highstand systems tract consists of be built with topographic slope-break(Fig. 5). Thiscarbargilite, siltstone and silty mudstone of shore-lake model shows that the sequence stratigraphic frameworkwamp. Then the carbargilite could act as the marker of of the Jurassic is affected obviously by the relativee maximum flooding surface(Fig 4)change of lake surface in the East Fukang Slope, and4. Sequence stratigraphic frameworkegularly with the fluctuations of lake surfaceAccording to the change of ancient landformChangji sagBaijiahai salientKelameili mountainand Cainan areaVI ASTMaximum flooding surfaceFirst flooding surfaceSlope break zone of landforml--VI: SequeAST: Alluviation systems tractHST: Highstand systems tractLST: Lowstand systems tractTST: Transgressive systems tractFig. 5 The sequence stratigraphic model of the Jurassic in East Fukang Slopesequence boundary and provided two pieces of5. Significance of coal seams as the marker ofevidences:(a) Coal seam distributed regionally issequence boundary or systems tract boundary correlatable. In the majority of cases, the accumulationquence boundary is theof coal seam is distributed regionally. The distributionboundary consisting primarily of muddy rock according o coaL'e when the supply ofto W. E. Galloway's definition(Gallal.1997:clastic中国煤化工s. And coal seam can1995).The research by Douglas S. Hamilton in the be cutCN MH Gnt when it begins toUniversity of Texas shows that the coal seam can act as come from the fluvial channel, but coal seam is stillthe sequence boundary(Hamilton, et al., 1994)with the correlatable. (b )The accumulation of coal seam has thGunnedah Basin in Australia as an example. Wu(1996) characteristics of isochronism. In theory, coal seamdid some ssez ch about the coal seam which acts as the could be regarded as the considerable depositionalPetroleum Science2006break surface studied by Frazier(1974)and this surface coverage) with poor stability, complex structure, andmarks the end of a depositional event or depositional lens-shape on the seismic section occurred near the firstepisode. Coal seam distributed locally could separate a flooding surface and was mainly deposited in thedepositional event, while coal seam distributed carbargillite swamp facies of prograding delta plairegionally could separate a depositional episode. In fact, During the maximum flooding, the climate was humidthe supply of clastic sediments will break the and the accommodation space increased gradually withcontinuation of the accumulation of coal. so. a the absence of terrigenous clastics which was in favordepositional event occurring in the swamp environment of development of plant and accumulation ofis corresponding to that caused by transgression and carbargilite, and coal seam was formed under the freeregression proposed by Frazier, which is isochronous water surface and on the carbargilite sediment. The thinand is able to act as a parasequence boundary. In the coal seam or carbargilite occurred on the fine clasticsame way, coal seams distributed regionally could be sediment of deep-water for the first time and distributedcompared to the muddy bed formed during the period of locally (less than 20 percent coverage) is the marker ofmaximum transgression of Frazier's depositional the maximum flooding surface. The coal seam or thepisode, and is similarly isochronous and is able to act carbargilite is thicker and more continuous than thatas the genetic sequence boundary(SB),toonear the first flooding surface and is classified as shoreTherefore, in the division of sequence and systems and shallow lake facies. So, the thin coal seam whichtract of the Jurassic in the east Fukang slope, the main occurred for the first time in the lower part ofcoal seams distributed regionally in the middle part of Badaowan Formation( Sequence I)is regarded as theBadaowan Formation and the lower part of Xishanyao marker of the first flooding surface and the boundary ofFormation (more than 60 percent coverage) are transgressive and highstand systems tracts, whileregarded as the sequence boundariethee coaI carbargilite or coal seam occurring on the fine clasticseam with a new onlap on it toplaps and truncates sediment in the upper part of the Xishanyao Formationlocally the underlying formation on the seismic section is regarded as the marker of the maximum flooding(Fig. 6). The depositional environment is carbargillite surface. In addition, the depositional environment andwamp of retrograding delta plain in the study area. organic petrology characteristics of theWhile the thin coal seam distributed locally acts as thethe boundaries of sequence and systems tract areboundary of systems tract: the coal seam or carbargilite different, and need to be studied further. Therefore, coalwhich first occurred on the coarse clastic sediment seams play an important role in the division of sequencereflects the uprise of the free water surface related to the and systems tract in the study area, which make thefirst flooding surface. But with the rapid uprise of the sequence stratigraphic framework of the Jurassiclake water surface, the formation condition of correlatable regionally in the basin with extensive coalcarbargillite no longer existed. Therefore, the thin coal seamsseam distributed sporadically (less than 5 percentRegionally extended swampSequence bounM业三业以以业↓业Locally extended swampMaximum flooding surfaceSprinkling extended swampFirst lacustrine flooding surfaceSlope break zone of landform中国煤化工CNMHGFig. 6 The paraphrase of coal seam as the sequence boundary, the first flooding surface and the maximum flooding surfaceVol 3 No. 2Significance of Coal Seam in Recognition of Sequence Boundaries and Systems tractCoal seam is believed to be the boundary of Gu J. Y(1995) Framework concepts and models of sequencesequence or systems tract in well sections, but itstratigraphy in nonmarine petroliferous Basin. Petroleumdifficult to identify on the seismic section, for it isExploration and Development, 22(4), 6-10(in Chinese)difficult to recognize thin coal seams, and especially Hamilton D. S and Tadros N. Z. (1994) Utility of coal seamwhen multilayered thin coal seams and thin clasticgenetic stratigraphic sequence boundaries in nonmarinedeposits are interbedded with each other, the single coalbasins: An example from the Gunnedah Basin, australiaseam and thin clastic deposit interlayer cannot beAAPG,78(2)distinguished because of the shielding effect by the coal Pu R. H, Mei, Z. C and Tang Z H.(1994)A preliminaryseam. As a result, it is rather easy to recognize the coaldiscussion of Jurassic non-marine sequence stratigraphy,seam related to the sequence boundary on the seismicEastern Junggar Basin. Xinjiang Petroleum Geology, 15(4section, but the recognition of the coal seam related to335-341(in Chinesethe systems tract boundary is limitedWang L. Z.(1995)A study on Mesozoic-Cenozoic continentalaphy and its applicaBAcknowledgementsXinjiang Petroleum Geology, 16(4), 324-330(in Chinese)We thank Messieurs Kuang Jun and Zhang Yijie of Wu Y. Y.(1996)Coal seam--Genetic stratigraphic sequenceXinjiang Oil Field for their help and support in thisboundary in nonmarine basins. Acta Petrolei Sinica, 17(4), 8study35 (in Chinese)Zhang Q, Wang G w, Zhu x. M. and Zhang M. L(2001) WellReferenceslogging sedimentary facies of the Jurassic in East FukangSlope of Junggar Basin. Journal of Paleogeography, 3(3), 41Frazier D. E.(1974)Depositional episodes: their relation to the47(in Chinese)Quatenary stratigraphic framework in the northwesternportion of the Gulf Basin. University of Texas at Austinabout the first authorBureau of Economic Geology Geological Circular, 74(1), 28Galloway W.E., Yancey M.S. and Whipple A P(1977)SeismicZhang Qin was born in 1973stratigraphy--Applications to hydrocarbon exploration. AAPGPh. D She is engaged in research onMemoir26,439-449sequence stratigraphy and reservoirGalloway W. E(1995)Depositional positional processes, regimesedimentologyvariables, and development of siliciclastic stratigraphicE-mail address: zdkzq cup. edu.cnzqlw(asina.comNorwegian Petrol SOC CONF(Stavanger: Norway, 9/6-8/1995)Received January 5, 2006)PROC. NPF SPEC Publication 8: 117-140(Edited by Yang Lei)中国煤化工CNMHG