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This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institutionand sharing with colleagues.Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:http://www.elsevier.com/copyright Author's personal copy The structure and formation of diapirs in the Yinggehai e Song Hong Basin,South China Sea Chao Lei a , * , Jianye Ren a , Peter D. Clift b , Zhenfeng Wang c , Xusheng Li c , Chuanxin Tong c a Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan 430074, China b School of Geosciences, University of Aberdeen, Meston Building, Aberdeen AB24 3UE, United Kingdom c China National Offshore Oil Corporation Ltd., Zhanjiang 524057, China a r t i c l e i n f o Article history: Received 6 July 2010Received in revised form7 December 2010Accepted 1 January 2011Available online 8 January 2011 Keywords: Gas chimneyPockmarksPalaeo-craterConcentric faultsLayer-bounded faultsInversion structureRed River FaultYinggehai e Song Hong BasinSouth China Sea a b s t r a c t The occurrence of shale diapirs in the Yinggehai e Song Hong (YGH e SH) Basin is well documented, as istheir association with big petroleum fi elds. In order to better understand how and why the diapirs formwe performed a detailed geophysical analysis using a new regional compilation of high-resolution two-and three-dimensional seismic re fl ection data, as well as drilling data that cover the diapirs in YGH e SHBasin. As many as 18 diapirs were identi fi ed and are arranged in six N e S-striking vertical en échelonzones. On seismic re fl ection sections gas chimney structures, diapiric faults and palaeo-craters aregenetically linked with the process of diapirism. Here we use geophysical and geological observations topropose a three-stage model for diapirism: initiation, emplacement, and collapse. During these threestages, different diapiric structure styles are formed, which we describe in detail. These include burieddiapirs, piercing diapirs and collapsed diapirs. We link the diapirism to activity on the offshore contin-uation of the Red River Fault, as shown on our high-resolution seismic re fl ection data, which is alsorelated to a high paleogeothermal gradient caused by crustal thinning. We also recognize the role of loading by the very large volume of sediment eroded from the edges of the Tibetan Plateau and deliveredby the Red River to the basin. 2011 Elsevier Ltd. All rights reserved. 1. Introduction Diapirs have been observed in a number of sedimentary basinsworldwide( Jackson,1995;VanRensbergenetal.,1999).Accordingtothe style of remobilization and the injection materials, diapirs aredivided into several kinds, e.g., shale, salt and igneous diapirs(Stewart, 1999). Over the past few decades, as a result of intenseexploration for hydrocarbons, shale diapirism has been explored indetail and structures associated with shale diapirs have beenrecognizedasbeingofgreatsigni fi canceinhydrocarbonexplorationandproduction.Thisisbecauseoftheirimpactonsource,migration,reservoir,trapandsealaspectsofthehydrocarbonsystem(Dimitrov,2002; Woodbury et al., 1980). However, uncertainties in the shalediapiric structure also contribute to operational risks in drilling(Ebrom, 2004; Fertl et al., 1976). Therefore, shale diapirism is of considerable interest to both academic and industrial geoscientists.TheYinggehai e SongHong(YGH e SH)Basinisanidealregionforstudying shale diapiric structures. In the southernpartof the basin,therearegasseepagesandpockmarksonthesea fl oorcausedbythediapirism (Huang et al., 2009; Xie et al., 2003a,b; Zhang and Hu, 1992), especially close to the offshore extension of the Red RiverFault, which forms the eastern margin of the basin. Evidence of migration pathways for shale diapirs, such as gas chimneys in theCentral Yinggehai Depression, was fi rst revealed by conventionalseismic re fl ection pro fi ling in the 1980s (Gong and Li,1997). It hasbeen speculated that fl uids fl owed from great depth upward alongthesezones,resultingingeothermalandoverpressureanomaliesinthe shallow sediments adjacent to the diapiric structures (Haoet al., 2000; Wang and Huang, 2008). Recent exploration hasshown that the commercial gas fi elds surround the diapiric struc-tures, especially in shallow reservoirs of Upper Pliocene andQuaternary age (Wang and Huang, 2008).Most published works on the shale diapirism in the YGH e SHBasinhavefocusedonthegeochemistryofsedimentsandgaswithintheshale(Haoetal.,1996,1995;Huangetal.,2003,2004,2009;Luoet al., 2003; Xie et al., 2001, 1998, 2008) or the characteristics of individual shale diapirs (Clift and Sun, 2006; Wang and Huang,2008; Xie et al., 2003a,b; Zhu et al., 2009). However, there isa general poor understanding of how mud is remobilized in the * Corresponding author. E-mail address:
[email protected] (C. Lei). Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo 0264-8172/$ e see front matter 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.marpetgeo.2011.01.001 Marine and Petroleum Geology 28 (2011) 980 e 991 Author's personal copy diapirs in the Central Yinggehai Basin or of what controls the stylesof the shale diapirs. Furthermore, there has been little research onthe relationship between diapirism and regional tectonic events. Inthis paper, we present the results of analyzing both 3D and high-resolution2Dseismicdata,togetherwithdrillingdatathatcoverthediapirs and their associated structures. This paper presents a de-tailedstructuralanalysisofthediapirsintheYGH e SHBasinandusesthis to improve our understanding of the emplacement processes.We further explore the link between the diapirism and regionaltectonic processes in Southeast Asia. 2. Regional geology The YGH e SH Basin is a Cenozoic pull-apart basin in the north-western corner of the South China Sea with its long axis orientedNNW e SSE (Fig.1). The YGH e SH Basin is an important petroleum-bearingprovinceandisdevelopedoverlyinganMesozoicbasement(Gong and Li,1997; Xia et al.,1998). The basin differs in its tectonicdevelopment and sedimentary evolution from other Cenozoicbasins on the northern continental margin of South China Sea (Cliftand Sun, 2006; Gong and Li,1997; Ren et al., 2002; Sun et al., 2003; Figure 1. Maps showing the region around the YGH e SH Basin (a), and the solid lines and the solid rectangle in (b) indicates 2D and 3D multichannel seismic data respectively. Notelocation of Fig. 11 is shown in (a). C. Lei et al. / Marine and Petroleum Geology 28 (2011) 980 e 991 981 Author's personal copy Wang and Li, 2009). The basin consists of two major depressions,the northern Hanoi Depression and the southern YinggehaiDepression, betweenwhich there is a structural high, known as theLingao Uplift (Fig. 1). Diapirs are concentrated in the CentralYinggehai Depression, with Cenozoic sedimentary fi ll reachinga maximum thickness of up to 17 km (Gong and Li, 1997).The Cenozoic tectonic evolution of the YGH e SH Basin can bedivided into three periods (Fig. 2): the syn-rifted period (Eocene e Oligocene),thepost-riftedstage(MidtoLateMiocene)andaperiodofrenewedacceleratedsubsidence(Pliocene e Holocene)(GongandLi, 1997; Ru and Pigott, 1986; Xie, 2009; Xie et al., 2004). Eocenestrata have not been penetrated in the YGH e SH Basin because of their deep burial depth, but are characterized by low continuity,subparallel and chaotic re fl ections on seismic sections. These areinterpreted to represent lacustrine, fl uvial and alluvial deposits(Gong and Li, 1997). In addition, the coeval sediments in the adja-cent Hanoi Depression, Beibuwan Basin and Pearl River MouthBasins are dominated by alluvial, lacustrine and coal-bearing allu-vialplainsedimentaryrocks(GongandLi,1997;Ranginetal.,1995).The Oligocene syn e rifted sequence comprises littoral and neriticdeposits,asrevealedbydrillingdataontheEasternSlope(GongandLi,1997; Xie, 2009). During the Miocene, basement tectonic subsi-denceratesgreatlydecreased,because theregionalsubsidencewasthen mainly driven by lithospheric thermal cooling effects (Gongand Li, 1997; Lin et al., 1997; Xie, 2009). Sedimentation in theMiocene was characterized by neritic e bathyal calcareousmudstones. The overlying sediments of the Pliocene YinggehaiFormation and other Quaternary strata that accumulated during Figure 2. Stratigraphic column of the Yinggehai Basin. Modi fi ed after Gong and Li (1997), Xie (2009). C. Lei et al. / Marine and Petroleum Geology 28 (2011) 980 e 991 982 Author's personal copy a phase of accelerated subsidence, include neritic shelf sandstones,bathyal turbidite clastic sedimentary rocks and marine mudstones,which form favorable hydrocarbon reservoir-seal assemblages(Gong and Li,1997; Xie et al., 2001; Xie, 2009). 3. Data sources Thisstudyisbasedonaseriesofmultichannelseismicre fl ectionpro fi les that cross and parallel the strike of the YGH e SH Basin,including high-resolution 2D and 3D seismic re fl ection data, andtogetherwithlogsandcoresprovidedbyChinaNationalOffshoreOilCorporation (CNOOC). The Eastern Slope and the Central YinggehaiDepressionarecoveredwitharegionalgridofseismiclinesspacedat2 2 km to 4 4 km. Two 3D seismic re fl ection datasets cover thestructures and overlying strata of Diapirs LD221 and DF11 (Fig.1).Age controls are present on regional mapped seismic horizonsfor32Ma (T70),23.8Ma (T60),15.5Ma (T50),10.5 Ma(T40), 5.5 Ma(T30), 2.4 Ma (T27) and 1.64 Ma (T20). These are mainly based onnannofossil biostratigraphy provided by CNOOC from drilling sitesin the YGH e SH Basin, Qiongdongnan Basin and Pearl River MouthBasin. We supplement these with new ages for the strata at OceanDrilling Program (ODP) Site 1148 (Wang and Li, 2009) in northernSouth China Sea. There are larger uncertainties in the ages of sedimentary rocks older than 32 Ma because no core data areavailable and dates are constrained only from limited drillingcuttings. Nonetheless, the ages available here do provide someframework and are suf fi cient for the objectives of this study.Correlation of faults and horizons between seismic pro fi les insedimentary rocks older than Neogene in the Central YinggehaiDepression involves a large amount of uncertainty, because theresolution of structures deeper than w 5 km is poor. Our interpre-tation also relies on published seismic interpretations from thewestern and northern edges of the Yinggehai Depression (Ranginet al., 1995; Roques et al., 1997; Zhu et al., 2009), because wehave no direct access to the srcinal data in those areas. 4. Seismic re fl ection image description 4.1. Diapirs arrangement in the plan view Good-quality seismic images in the Central Yinggehai Depres-sion allows at least 18 diapirs to be delineated, which are alignedalongsixzoneswithanN e Senéchelonstrikingorientation(Fig.1).These are numbered1 to6 fromwest to east. Forexample the No.2diapiric zone comprises four diapirs, including DF11 and DF291. 4.2. Gas chimneys Shale diapirs are typically imaged as re fl ection-free zones onseismic data, or as chaotic re fl ection patterns that disrupt there fl ections in the surrounding strata. These characteristics likelyre fl ect the ascent of the mud, which leaves the diapirs internallyhomogeneous and hence unre fl ective (Graue, 2000; Hanyi et al.,2002; Pieter et al., 2003). Surrounding the diapiric body, a pull-down re fl ection pattern is usually visible and interpreted as beinga product of gas escape. Gas within the sediments signi fi cantlydecreases the seismic velocities of P-waves, which leads to pull-downofthere fl ectors(LudmannandWong,2003;Yunetal.,1999).High amplitude anomalies, i.e., bright spots, are often observedabove or at the fl anks of the mud diapirs and these are usuallyinterpretedasindicatinggaschargingofthepermeablebedsaroundthediapirs(Fowleretal.,2000;Hustoftetal.,2007;Sethetal.,2009),In ourstudya numberofseismically-de fi ned gas chimneys havebeen identi fi ed from the newly processed seismic pro fi les (Fig. 1),which allow us to map the structural relationships between theshalediapirandthesedimentaryrockssurroundingthestructureindetail(Figs.3 e 9).Forexample,theverticaldistributionofstructureswithintheDiapirDF11isshowninFig.3.ThecrestofDiapircontainsseismically resolved folds dipping outward at high angles (Fig. 3).Turtleback-like strata are observed at the diapir head. Diapir DF11cuts through strata from a deep srcin and strataon the diapircrestare seen to be deformed and arched. We interpret Diapir DF11 ashavingformerlyactivelyrisenandintrudedintotheshallowerlevelsof the basin sedimentary fi ll without reaching the surface.In contrast, Diapir LD81 located to the southeast (Fig. 4) haspropagated upwards to slightly shallower levels compared withDiapir DF11. Diapir LD81 is a pipe-like intrusion that actively pier-ces the sediments as far as the current sea fl oor. Currently it isobserved to discharge gas and to form pockmarks on the seabed, asdemonstrated byside-scan sonar imageryand shallow-penetratingsparker pro fi les (Zhang and Hu,1992).InFigure 5two diapirtypesare shown.Inthemiddle-lowerpartof the core zone of Diapir LD141 we observe pronounced deformedseismicre fl ectors.Meanwhilestratasurroundingthe upper, middleand lower parts of the strata surrounding Diapir LD141 vary in dipand are domed above the diapir. Diapir LD131 has characteristicssimilartothoseseenwithinandaroundDiapirLD141,exceptfortheshallow-penetrating seismic re fl ectors. High amplitude anomaliesin the cap rocks of Diapir LD131 are recognized close to the seabed.The seismic re fl ections here display brightening and increasedcontinuity. Shallow seismic anomalies can be interpreted asexpressions of gas- fi lled unconsolidated sediments. Both Diapirs Figure 3. Seismic section showing key structural features of Diapir DF11. Core photographs show the micro-fractures observed from the mudstone cores of the well DF112. C. Lei et al. / Marine and Petroleum Geology 28 (2011) 980 e 991 983
