Indirect Indicators of Gas HydrateOccurrence within Submarine Sediments

JurnuofChiaUniversyorGeosciences,vo,l,No.1,p.422-425,Dhcemnber200ISsNl02-075Printed in ChinuIndirect Indicators of Gas HydrateOccurrence within Submarine SedimentsZhao Xingmin Wu bihaoInstitute o/ Mineral De posits, Chinese Academy of Geological Sciences, Beijing 100037Wang YapingExperimental Institute. Chinese Academy of Geological Sciences, Beijing 100037L u ZhenquanInstitute of Mineral De posits, Chinese Academy of Geological Sciences. Beijing 100037ABSTRACTGas hydrate, a new kind energy resources discoverd over the past ten years, has aroused much attention from many countries around the globe, because the gas hydrate has played a very important rolein energy resources exploration. submarine geohazards precaution. and changes in global climate. Sofar. gas hydrates have been found in about sixty places on continental margins and polar regions in theworld, but the majority of them is recognized by all kinds of indireet indicators owing to their limmitedexistence conditions, only minority of them is collected. Thereby, the indirect marks of gas hydratesplay an important role in exploration of the clathrate compounds, and the authors discuss themKEY WORDS gas hydrate, recognition indicator. marine geolegyMade up of water and methane, gas hydrates are ice-like such as India and Pakistan have also made greatly invested inclathrate compounds that formed in the conditions of low tem this arca, since the gas hydrates show the fallowing three feperature(0-10 Y) and high pressure(>10 MPa)(Max and tures: their great potcntial of fossil fuel, whose totaf amount isDillon, 1998). Generally speaking, they are also called meth twice as mush as the total conventional energy resources suchanc hydrates because methane is a dominant composition. Be. as coal. petroleum and gas around the world: their role in sub-ides methane, the gas constituting gas hydrates includes little marine geohazard and their effects on global climate changeof ethane. propane, nitrogen and hydrogen sulfide. Owing to Anyway, research into gas hydrates is fairly important in reheir special formation condition, the compounds usually occur sources exploration, precaution against submarine geohazardin the sediments, at the water depth of 300-3000 m, of the and protection of existing human environments.continental slopes and deep sea basins and within theUp to now, a few gas hydrate samples were collected durfrost. But gas hydrates associated with continental slopes and ing the period of seafloor drilling and submarine sediment samdeep sea basins occupy more than 90 of the total ones pling because of their special conditions of formation and occurthroughout the world.rence. Usually, the occurrence of gas hydrates worldwide wasAt present, great importance is attached to the research conformed by means of geology geophysics and geochemistrynd development of gas hydrates world wide. The developed ete.. In this case, indirect recognition methods related to gascountries such as USA. Japan and Canada have drafted their hydrates are given as follows.commercial plans about research into and development of gashydrates and begin to do experiment in exploitation of the GEOLOGICAL INDICATIONSclathrate compounds. Meanwhile, the developing countriesnd cators of gas hydra中国煤化工 TThis paper is supported by National High Tech Research and develCNMH Gydrates are geologicallycaracteristics of submaopment(863 Prugramrine sediments and sedimentology etc.Manuseript rcceived January 14. 2000Seafloor topography Seafloor associated with gas hydratesManuscript accepted September 10, 2000.is often related to pockmarks. mud volcanoes and mudIndireet Indicutors uf Guy Hydrate Occurrence within Submarine Sedimentsmounds. Within the sediments available for gas hydrate production, directional mig rution of lydrucurlon gus-luaded fluidis requisite for the clathrate compound formation (ITovland andblank zoGallagher, 1997: Hyndman and Davis, 1992). Intense activityof animating hydrocarbon gug-rith fluid within the sedimentwhere gas hydrate is distributed leads to the distinctive gak-disBSR, reversal polricharging topography on seafloor.ative to seafloorSuhmarine sediments The core containing gas byureteyhas a soupy or mousse-like appearance, close to the anomalous-ly dry sediment layera, Moreover. mud diapirs often occur nearthe areu ol gus hydrate occurrence. During the course of gas Fignre 1. Responses of gas hydrate te seismic reflectionhydrate formatinn, crystallization of water and hydrocarbonmodifled from Lee et al. (1994))gas, which draws the water within the sediments adjacent togus hydrate layers towards the crystallization or freezing front, limit of blanking zones is a gradational transition, and the lowcauses the interstitial water within stratu nearby to deplete, re- er boundary is sharply in touch with the underlying free gassulting in anomalously dry sediment layers (Hovland and Gal- zone( Andreassen et al., 1997)(Fig. 1)lagher, 1907). At the same time, intense activity of hydrocarReversal polarity Reversal polarity, relative to the stuhon gasrich fluid near gas hyu rute layers initiates the mud diu- floor reflection Fig. 1), coincides with two seismic rellectionptrs widespread in the strata nearbymarks described above, and is the most prominent features thatSediment composition Besides the two features described BSR's hEs. Reversal polarity, depended on the reflection coeffiabove, there widely occur authigenic rarhnnate nodules, car. cients uf relle tion boundary thut relutedl to acoustic impedancebonate buildups and carbonate reefs within the horizons under- contrast (p,.v p.n)of two-side sediments of the hounds-Bin by gas hydrate bearing intervals, or it is true that enhanced ry, indicates that the acoustic impedance contrast is less thanauthigenic carbonate minerals und lowered sulfale minerals ure zero(A,.v-A2.w< 0). Reversal polarity is the key to dedistributed in tle sediments above ltydrule-uhurged sTrut. For- termination of real BSR's (that is to say, some BSR's that apmation of the authigenic carbonate minerals is explained by oxi- pear to be parallel to seafloor reflection free of reversal polaridization of methane in ascending hydrocarbon gas rich fluid by ty, ure not rcal unes), espceinlly fur the level sedimentssulfate ions ( IL, I so:-microD4 Process-TIS-1IC(XVAMPs (velocity amplitnde structure) The type af re-Hg0)while the sulfate minerals are almost depleted( Claytonflection characteristics is firstly discovered m the Aliutian basinetaL,1997)located in the north Pacific and sometimes reappears in theIn addition, there usually exist hinherms, bacterial matsOmun Gulf lutterly. The structure shows that the arched shapechemosynthetic pipes and clams on the seafloor associated with results from velocity pullup above BSk and the funnel appeargus hydrate.ance,caused by velocity pulldown(Fig. 2). The two shapesoften occur in doubles, however, sometines only in velocityGEOPHYSICAL IDENTIFICATIONSpulldow n structure. It is generally thonght that velocity pullupCp tu now, geophysical way is still the most important structure is generated from massive gas hydrate assemblagesmeans for gas hydrate exploration, Reognition nf geophysical and velocity pulldown is caused by the underlying free gasfeatures is very significant in gas hydrate survey.(Scholl and Hart. 1999)Avo(amplitnde versus offset) AV(), A type nt distineSeismic Reflection Featnrestive reflection structure, is represented by genomic reflectionBSRs( bottom-simulating reflectors) BSR's often inclined amplitude variation with offset (incidence angle). The strue-to depositional bedded surfaces (Kvcnvuldcn, 1993) ure tbe ture feature is ubtuincd from Poisson's ratio (a) contrast hedistinctive rellection approximately parallel to seafloor (Fig. tween gas hiydrate-bearing sediments ahove HSR and underlying1). It is generally believed that BSR's represent the acoustic free gas-filled strata( Hyndman and Davis. 1992). It, being animpedance contrast (p. v)between gas hydrate-beuring luyer effective evidence of oil and ges reservoir exisling in strati, has(gus bydrule stuble zome) und underlying low-velncity free gas widely been used in oil and gas exploration industry. At presznne.(f the geophysical indicator. BSR's are the most com ent, the met hod has heen used to determine the amount of gasmon, most reliable and easiest identification method. To dute hydrate and free gas associated with the BSR.most of the gas hydrates occurring throughout the world are中国煤化工Frequently defined by nSRs,Well- LwK ChCNMHGBlanking zones Blanking zones resulted from even Iu additon uIm a m as kk, geophysical downacoustic respunse to gus hydrute-cemnentution sediments usually hole surveying is also a very important method for explorationconcur with lSRs, they are called blanking zones for their even of gas hydrate. The logging methods successfully used for gAsreflection on seismic reflection profiles. Vertically, the upper bydrule study are as Follows: caliper, spontaneous potentialnatural gamma-ray, lateral resistivity, sonic velocity and neu-124Zhao xingmin. Wu Bihso, Wang Yaping and Lu ZhenquanCALI CR/API SPimv g-m (ns.m)A%313601501000492164050BSR-GIdrateintervalHSR-DDFigure 3. Response of gas hydrate to loggIng(modified frnmKvenvolden (1993)). CAL, caliper.Sonic logglng Sonic moveout (At) from gus hydrate bear-00ing intervala decrcases (lig. 3). Deduted from the furmulaArel/v, it is seen that sonic moveout of medin is inverselyFigure 2. VAMPs structnre of Ras hydrate on seismie see- proportional to transmission velocity of sunir wave. Further-more, transmission velocity of Swave or Iwave through thetlon(adpted from Scholl and Hart(1997)).media closely relates to mechanical properness f them, As verilied in corresponding experiments, formution of gas hyruletron porosity.und its cementation of sediments can lead to rising of the Youn'sCaliper logging Borehole diameter value of logs for gas modulus(E) of the sedimentaE-(F.L)(S.AL.), Fishydrate layer relative to tIe nther neghboring horizons is elr force exerted on substance, L is length of subsume, s is sublarger(Fig 3 ). The phenomenon is evoked hy gas hydrute de-stance urea of cross scetion, AL ig deformation per lengthomposition resulting from the heating that occur during the unit). reducing in density (p) of them and changes in othercourse of drillingmechanical properties. If a change in loisson's rutin o) is nottaneous potential There is a decreuse in spontaneous taken into consideration, inFerred from the velocity model=palentialvalue(SP)for RAs hydrate-beuring interval eompared [E(1-a)/(P(1+o)(1-2a))"(Chen et aL.,1994),formawith those of the other nearby sediment strata(Fig 3). Apart tion of gas hydrate initiates enhancements of w, resulting in thefrom expansion of well diameter for gaM-hydrnte mones, gas hy- deercases in sonic moveout value (Ar)drate decomposition resulting fmm drilling lauds to decreases in Neutron log porosity There is un increase in neutron pthe ion concentration and activity of mud solution so that lighr oxity (N,) at gas hydrate layers(Fig. 3).In general. neutronuctivity strata waler in adjacent intervals diffuses into ile well logging valve reflects the content of hydrogen elements withinsection for gus hydrate layer(diffusive velocity of cI ions strata, and further reflects the porosity filled with the nuid forgreater than Nu'ions (Chen et al.. 1994))and negative elec sandy sediments. Formation of gas hydrate always draws atric charge numbers of the slush in the well seetion rise No as to great deal of water out of nearby atrata into the hydrate-formmproduce negative potentinl abnormality.ing interval, so thut und makes solid methane replace 20%wa-Natural gammaray Natural gammu-ray value(GR) for ter of the erystalline compounds per volume unit so as to causegas hydrate on well logs shows valley-shaped fall (Fig 3).As the content of hydrogen elements within sediments per vuleverybody knows, natural gammaray energy intensity of sedi- unit to raise a lot. Even if that sume reduced density resultingments is related to the content of clay within them. In the from gas-hydrate formation provoke the content of hydrogen elcourse ol gas hydrate formation, absorption of both u great ements within sediments to fall a little is taken into accountdeul nf water and u lot of hydrocarbon gas gives rise to drop but there is an ullimate outcome thml the content of hydrogenping in the clay content within sediments per volume unit. elements within sediments per volume unit is added. As a re-Consequently, gamma ray energy intensity decrease in gas hy- sult, neutron porosity of gas hydrate bcaring sediments risesr lowersIn addit中国煤化工 d dipiars and other.ateral resistivity(LR) The well log of lateral resistivity gaR-dischargeK sonar maps andat gas hydrate-bearing interval displays sharply-enhanced box seismie reileCNMHm are also indirectshape(Fig. a). Owing to its formation, gas hydrate changes marks of gas hydrate existence.into solid compounds, resulting in prevention of the nuid insediments from migrating, so thut the resistance of gas hydrate GEOCHEMICAL MARKS發搪Aside from geological and geophysical indications, thereIndirect Indicators of Gas Hydrate Occurrence within Submarine sedimentsare also a great number of geochemical signs associated with sediments is characterized by geology, geophysics and geogas hydratechemistry. Supposing that sampling and drilling on seafloor isnot made, occurrence of them is also inferred by all kinds ofOrganie Geochemistry Symbolsmarks described above. However, for accurately recognizingAbnormal methane concentration of sea bottom water In them, it would be best if we analyze the signs comprehensivegeneral, there is a prominent abnormality of methane concen- ly, so as to improve the surveying precision of gas hydrate.tration in the water over the seafloor where gas hydrate occurs.Between gas hydrate layer and underlying free gas zone exists a REFERENCES CITEDdynamic equilibrium. While faults cut through gas hydrate sta. Andreassen K, Hart PE, MacKay M. 1997. Amplitude Versus(Hfserility zone (GHSZ) and further join the gas hydrate intervalModeling of the Bottom Simulating Reflection Associated withand free gas zone to the seafloor, the methane gas comes out ofSubmarine Gas Hydrates. Marine Geology, 137 25-40submarine sediments into the bottom water and vields gas Chen Y M. Zhu H D, Ren K, et al, 1994. 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Journal of Geophysical Research. 97Suddenly-reduced chlorinity Usually, chlorinity at gas(B5):7025-704hydrate zone decreases sharply. When gas hydrate decomposes Kastner M. Kvenvolden K A, Lorenson T D, 1998. Chemistry. Isoowing to drilling, the fresh water drawn into gas hydrate intertope Composition, and Origin of a Met hane Hydrogen Sulfideval during its formation is set free so that gas hydrate-bearingHydrate at the (acadia Subduction Zone. Earth and PlanetaScience ietters, 156: 173-18sediments has a lower chlorinity and is freshened. In theory, Kvenvolden K A. 1993. A Primer on Gas Hydrate. US Geological Surconcentrations of all the ions within gas-hydrate zone should bevey Professional Paper 1570,297-291dropped, but degree of chlorinity decrease turns greater( Kast- Lee M w. Hutchinson D R. Agena W F, et al. 1994. Seismc Characner et al., 1998)er of Gas Hydrate on the South U.S. Continental Margin,MaDecreased concentration of sulfateofrine Geophysical Researchessulfate ion for gas hydrate layer tends to decrease as chlorinity Max M D, Dillon W P, 1998. (ceanic Methane Hydrate: The Characdoes, In addition to the fresh pore water resulting from gas hyCbility Zone, and the Potentialdrate formation, in the course of hydrocarbon rich fluid moving中国煤化工 r Petroleum Geol,1(3)up to seafloor, consumption of sulfate ions caused by methaneCNMHGreduction of sulfate ions in submarine sediments indicates a deand Amplitude Structures onSeismic-Reflection Profiles-Possible Massive Gas Hydrate Decrease in sulfate ions from the seafloor to gas hydrate zoneposits and Underlying Gas Accumulations in the Bering Sea Hain. US Gcolgocal Survey Professional Paper 1570. 331-351CONCLUSIONAs described above, gas-hydrate existence in submarine万方数