A Preliminary Study of the Gas Hydrate Stability Zone in the South China Sea

VoL. 76 No.4 ACTA GEOLOGICA SINICA Dcc. 2002A Preliminary Study of the Gas Hydrate Stability Zonen the South China SeaJIN Chunshuang"and WANG Jiyang1 Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 1000292 Strategic Research Center for Oil and Gas Resources,Ministry of Land and Resources, Beijing 100034Abstract Based on the analysis of sea-bottom temperature and geothermal gradient, and by means of the phaseoundary curve of gas hydrate and the sea-bottom temperature versus water depth curve in the South China Sea, thispaper studies the temperature and pressure conditions for gas hydrate to keep stable. In a marine environment methanehydrate keeps stable at water depths greater than 550 m in the South China Sea. Further, the thickness of the gas hydratestability zone in the South China Sea was calculated by using the phase boundary curve and temperature-depth equationsThe result shows that gas hydrate have a better perspective in the southeast of the Dongsha Islands, the northeast of theXisha Islands and the north of the Nansha Islands for thicker stability zones.Key words: South China Sea, gas hydrate, stability zone, geothermal gradient, sea-bottom temperature1 Introduction1992)In the south China sea. the bsrs were identified inThe gas hydrate is an ice -like crystalline compound many crests of anticlines, mud volcanoes and mudcontaining methane or other low-molecular weight diapirs in the offshore collisional zone of Taiwan( Chigases in lattices of water molecules. As the main et al., 1998), and in some seismic reflection profiles ofomposition is methane, it is also called methane the Dongsha and Xisha Islands(Yao, 1998).Thesehydrate Gas hydrate are stable under temperature and BSRs confirmed the prospect of gas hydrate in theconditions generally found in permafrost South China Sea, but it is difficult to know theregions and within marine sediments at water depths distribution and thickness of the gas hydrate stabilitygreater than 500 m, and are commonly seen beneath the zone (GHSZ) throughout the areas because of thecontinental slope of both active and passive continental quantity and quality limitation of seismic reflectionmargins (Trehu et al., 1999). They have recently profiles. However, we can determine the distribution ofbecome a major focus of international research because the GHSZ and calculate its thickness through the phasethey represent a valuable global reservoir of boundary curve of gas hydrate and the pressurehydrocarbons(Kvenvolden, 1998), a potential source of temperature equations based on sea-bottom temperaturemethane, an important greenhouse gas(Paull et al., and geothermal gradient. This is of great importance for1991, Dickens et al., 1997), and a possible cause for the the evaluation of gas hydrate resources.instability of continent-marginal sediments( CarpenterAs for the formation pressure-temperature condition1981, Paull et al., 1996)of gas hydrate, Roozeboom(1884, 1885)made the firstIn the past years, the main approach used to evaluate pressure-temperature plot for SO2. When determininge presence of gas hydrate in sediments was based on a hydrate in pipelines, Hammerschmidt( 1934)publishedgeophysical proxy known as the bottom-simulating a correlation summary of over one hundred hydratereflector (BSR). The BSR is a seismic reflector that formation data points. Shortly afterwards, Professor Dmimics the sea floor, cuts original stratigraphic L. Katz and his colleagues began their experimentalreflections in general, and has a negative polarity, study中国煤化工 s that the energyindicating that it results from a decrease in acousticimpedance(Shipley et al., 1979; Hyndman and Spence, ThenCNMHmore attention toial distribution ofPreliminary Study of gas Hydrate Stability zoneJin and wamethane hydrate beneath the European continental 60@C/km and the sea-bottom temperatures are 2C andmargins. Rao(1999)calculated the thickness of gas 4C respectively. The GHSZ is about 500 m thick withhydrate stability zone beneath the India continental a sea-bottom temperature of 2 C and a geothermalmargins. In this paper we use the gas hydrate phase gradient of 40C/km, about 440 m thick with aoundary curve and pressure-temperature equationssea-bottom temperature of 4C and a geothermaldetermine the distribution of the GHSZ and calculate its gradient of 40oC/km, and about 330 m thick withthickness in the South China Sea. The parameters used sea-bottom temperature of 2C and a geothermale water depth, gradient of 60C/km. Thus, we can conclude that theea-botlom temperature, latitude and geothermal greater the geothermal gradient is, the smaller thegradient of sedimentsthickness of the GHSZ will be, and that the higher thethe thinner the ghsz will2 Gas Hydrate Stability Zone and Its be. Similarly, with the same sea-bottom temperaturethe deeper the GHSZ will beThe thickness and distribution area of the ghSz areindispensable for the evaluation of gas hydrate 3 Sea-bottom Temperature and Geothermalresources by means of the cubature. The amount of Gradient in the South China Seamethane in gas hydrate is determined based on theng eqtIn order to determine the thickness and distribution ofV=A×AxHEthe Ghsz, we have analyzed more than 200where V is the amount of methane in gas hydrate; A: theution area of gas hydrate; Az: the mean thicknessof the ghsz; p: the mean porosity of sediments; Hthe compaction percentage of gas hydrate; and E: thecubage-multiplying factor of gas hydrate.The gas hydrate phase diagram in Fig. I reveals thephase equilibrium between three phases, i.e. gashydrate, gas and water in the marine environment. Onthe left of the phase boundary, the pressure andPhase boundary curvemperature conditions are favorable for hydrateBSR and phase boundary curve is the gas hydrate aformation. The area enclosed by the ocean bottom, theGeothermal gradientstability zone. From Figsea-bottom temperature, geothermal gradient, waterSea bottomdepth, and the phase boundary curve determine theGHSZ thickness. The phase boundary curve isBSRcontrolled by pressure and temperature, gascomposition and pore-water salinity(Holder et al140℃/km1987). Because we do not have the data of gasGas+watercomposition and pore-water salinity in the South ChinaSea, here we suppose that methane is thecomponent of natural gas and the salinity of pore waTemperature(C)is the standard salinity of seawater. Provided pressure Fig. 1. A diagram showing the phase of methane hydrate in(water depth), pore water salinity and gas composition a marine environment and the effects of sea-bottare the same, the geothermal gradient and sea-bottom temperature and geothermal gradient on the gas hydratetemperature determine the GHSZ thickness. Fig. I stability zone(modified from Miles, 1995)shows the thicknesses of the ghsz as the water depthBSRGHSZ中国煤化工 ure of the base of theis 3000 m, geothermal gradient is 40oC/km andCNMHGACTA GEOLOGICA SINICAmeasurements of sea-bottom temperature and isobath of 550 m corresponds to the boundary of theeothermal gradient in the South China Sea. Statistics methane hydrate stability zone. In the marginal seas ofshow that the current sea-bottom temperature is about the Indian Ocean, however, the water depth boundary60-14C on the continental shelf, about 2-6C on the for the stability zone is greater than 750 m(Rao, 1999).continental slope, and about 2-3 C in the central basin. The reason is that the sea-bottom temperature of theThe sea-bottom temperature decreases with the increase South China Sea is lower than that of the margins of theof water depth( Fig. 2). When the water depth is more Indian Ocean. Taking a water depth of 1000 m as anthan 2800 m, the sea-bottom temperature tends to be example, the sea-bouton temperature is about 8.C forstable at 2.2C: while the water depth is less than 2800 the latter case, but about 5C for the formerm, the water depth and sea-bottom temperature show a The geothermal gradient data used in this paperlinear relationship in the logarithmic coordinate, and obtained by seafloor heat flow measurements, and thethe fitting equation is as followsmeasurement depth in sediments is about 5 m From theln(D=-1.52434×n(T)+2.35192geothermal gradient versus water depth diagram( Fig. 3)where D(km)is the water depth and T(oC) is the we can see that the geothermal gradients have no directsea-bottom temperaturerelationship with water depth, and most of the valuesThe intersection of the sea-bottom temperature curve are greater than 50.C/km. Because of the compactionand the phase boundary curve shows a water depth of of sediments, the thermal conductivity of sediments550 m(Fig. 2). Thus, the methane hydrate in the South usually increases with depth, which will result in theChina Sea can keep stable in sediments only at water decease of geothermal gradient. However, we useddepths greater than 550 m, and in areas at water depths geothermal data obtained by means of the 5mless than 550 m, the possibility of stability of methane measuring depth and treated as constants in calculatinghydrate is minimal. We therefore conclude that the the thickness of the GHSZ, which may yield smallerlues due to the negative correlationbetween the geothermal gradient and the thickness ofthe GHSZ4 Thickness of Gas hydrate Stability ZoneFree gasin the South China SeaPrevious studies(e. g, Hyndman et al., 1992; Trehuet al., 1995; Brown et al. 1996)have confirmed that thein the South China Sea Phase boundary curveBSR occurs approximately at the base of the GHSz, sowe can also predict the depth of the BsR through thebase of the ghSz where we have no seismic profilesGas hydrate, if exists, will remain stable from seafloor to certain depths which can be determined by thegeothermal gradient(Fig. 1). From the methane hydratephase diagram, if we have the data of sea-bottomtenon, we can determine the temperature(T,)at the base ofthe gHSZ from the intersection point of the phaseboundary curve in the sediments. The depth of the baseof the GHSZ can be determined with this T, value. Thecalculation methods we used, similar to Rao's (1999)are introduced belowFig. 2. Sea-bofiom temperature versus water depth in theThe equations develoPed by Miles(1995)on theSouth China seabasis of中国煤化工 am are usedCNMHGA Preliminary Study of Gas HydratJin and where tz (c) is thof the base of theGHSZ, i.e. at the depth of d(meters below sea level)200D=zo+z, Zo is the water depth(meters below sea level),and Z is the thickness of sediments up to the base of theGHSZ from the seafloor(meters below sea floorConversion from pressure P(MPa) to depth D(m)6000P=(1+C)D+C2D2]×4000Water depth (m)hereC:=(5.92+5.25sin(Lat)x10; Lat is the latitudein degrees, and C2=2.21x10. Transforming cFig 3. Geothermal gradient data in the South China Sea. (2) into a function of Z and then substituting it intoequation (3), we haveP=[(l+C1)(z0+(7-T△z△+gzhouSouth ChinaC2+(7-T△Z/△n3)×10-2(4)which is the hydrostatic P-Trelationship within the sediments in theas in equation (1). Thesimultaneous solutions for equation(1)and (4)can be analytically obtainedwith a single algorithm. The left hand1616 these two equations can yield Ti at the/Zhongsha Islandsintersection of the phase-boundaof gas hydrate and the geothermalgradient curve(see Fig. 1). Altogethefour solutions can be obtained since we1212 have biquadratic simultaneousequations and the positive real root ofis the temperature (T,) at thlower limit of the GHSZSubstituting T, into equation (2)yields the value of Z, the thickness ofthe GHSZ. Then we plotted theisopaches of the GHSZ shown in Fig 4,hich also represent the depth of thBSR. Figure 4 shows that the GHSz isZenmu basin4 about 100-300 m thick and that thereorcoare thicker GHSZs in the southeasternnortheastern part of the Xisha IslandsFig. 4. Isopach map of the methane hydrate stability zone(in meter)and the northern part of the NanshIslandswhere a=1.559474×10-,b=4.8275×102,c=-2.78083x10-d=1.5922x10, P (MPa)is the 5 Discussion and Conclusionspressure and T(C)the temperature.The temperature-depth function from the sea-bottom Asahnve. sea-hottom temperaturetemperature(To)and the geothermal gradient (AT/AZ) is g中国煤化工 gas composition andTz=T+(∧T△Z)z(2)YHCN Kness of the GHSZ76No.4ACTA GEOLOGICA SINICADec.2002In order to know the effects of gas composition and composition in a standard marine environment, and thepore water salinity on the GHSZ, we propose three geothermal gradient data applied in this paper may beas in terms of their compositions: A smaller than those in actual situation. Therefore, theindicating 100% methane: B, 96% methane, 3% ethane distribution and thickness of the GHSZ provided in thisand I% propane; C, 90% methane, 7% ethane and 3% paper represent the smallest range and the thinnestpropane; and two types of pore water in terms of their GHSZ in the South China Sea,salinity: one is pure water and the other is standardeawater.We applied the water depth, sea-bottom Acknowledgmentstemperature and geothermal gradient of site 1164, ODPeg 184(Wang et al., 2000)to the above equations to This work was financially supported by the Marine 863calculate the thickness of the GHSZ. The results are Project( Grant 820-Exploration-5). We thank Professorshown in Table 1Wu Bihao, Dr. Zhu Youhai and Lu Zhenquan in theThe results show that the minimum water depthChinese Academy of Geological Sciences for theirbecome smaller and the GHSZ will become thicker great help in the researchwith increasing contents of ethane and propane in gamixture no matter in a pure-water or sea-waterChinese manuscript received Oct. 23, 2001environment. And, the minimum water depth becomesaccepted Mar 8, 2002larger and the gHSz becomes thinner with the increaseedited by Zhang Yuxuof salinity for a given gas compositionEnglish manuscript edited by Liu Xinzhubottom temperature, geothermal gradient, waterdepth. gas component and pore water salinity determine Referencesthe thickness of the GHSZ. The greater the geothermal Brown, KM. Bangs, NL, Froelich. P.N., et al., 1996.Thegradient is, the thinner the thickness of the GHSZ will nature, distribution, and origin of gas hydrate in the Chilebe:the higher the sea-bottom temperaturetriple junction region. Earth and Planetary Science Letters139:471-483the thinner the GHSZ will be; and the greater the water Carpenter, G.B., 1981. Coincident sediment slump/clathratedepth is, the thicker the GHSZ will be. With theincrease of ethane and propane contents in gas mixture 29-32complexes on the U.S. Atlantic slope. Geo-Marine Letters, (D)and the decrease of salinity, gas hydrate are easy to Chi, wC.Reed, D L, Liu, C.S., et al. 1998. Distribution of theorm and to keep stable in sediments beneath sea floorbottom-simulating reflector in the offshore Taiwan collisionand the ghSz will become thickerzone. Terrestrial, Atmosphere, Ocean, 9(4):779-794or a marine environment, methane hydrate can keepDickens, G R. M., Castillo, M und Walker, J C, 1997. a blast ofstable at water depth greater than 550 m, and the GHSZgas in the latest Paleocene: Simulating first-order effects ofis about 100-300 m in the South China Sea. Gasmassivc dissociation of oceanic methane hydrate. Geology, 25259-262.hydrate have a better perspective in the southeast of the Holder, G.D., Malone, R D. 1987. Effects of gas compositionDongsha Islands, the northeast of the Xisha Islands and and geothermal properties on the thickness and depth ofthe north of the Nansha Islandsnatural gas-hydrate zones, Journal of Petroleum Technology,In this paper, the minimum water depth and thickness Sept: 1147-1152of the GHSz are derived from pure methaneHyndman, R.D., and Spence, G D, 1992. A seismic study ofmethane hydrate marine bottom simulating reflectors. JTable 1 The minimun water depth for stable gasGeophys Res, 97: 6683-6698hydrate at sea bottom and the thickness of the ghszHyndman, R D, Foucher, J P, Yamano, M. et al., 1992. DeepPore water salinityPure waterStandard seawaterea bottom-simulating- reflectors: Calibration of the hydratestability ficld as used for heat flowGas compositionPlanetary Science Letters, 109: 289-301Kvenvolden, K, 1988. Methane hydrate-A major reserve ofdepth(m)550400310carbon in the shallow geosphere? Chemical Geology. 171:GHSZ thickness30833336926830g344abergK 1006 Gaindica中国煤化工Note: The gas hydrate phase equilibrium used for the calculation isfrom Laberg and Andreassen(1996).3(8)CNMHetroleum geologyability Zonein andMiles. P.R. 1995. Potential distribution of methane hydrate Oregon continental margin. Geology. 27(10): 939-942beneath the Europe continental margins. Geophysical Research Wang. P, Prell, L. Blum, P, 2000. Proceedings of the OceanLetters,22(23):3179-3182.Drilling Program, Initial Reports V 184. College StationPaull, C.K., Buelow, w. J, et aL., 1996. Increased Ocean drilling Program, 1-103continental-margin slumping frequency during sea-level Yao Bochu, 1999. Preliminary exploration of gas hydrate in thelow-stands above gas hydrate-bcaring sediments. Geology,northern margin of the South China Sea. Marine Geology24(2):143-146.Quatenary Geology, 18(4): 11-18(in Chinese with EnglishPaull. C K, Ussler, w,Ill, and Dillon. w.P.. 1991. Is the extent abstract)of glaciation limited by marine gas-hydrate? GeophysicalResearch Letters. 18: 432-434About the first authorRao YH. 1999. C-program for the calculation of gas hydrate Jin Chunshuang Born in October 1974: graduatedtability zone thickncss. Computers Geosciences, 25705-707from the Department of Petroleum Exploration,Shipley, T.H., Houston, M.H., Buffler R T, et al. 1979. Seismic University of Petroleum( East China) in 1996; got aevidence for widespread possible gas hydrate horizonsM.s. degree at the Department of Geosciencecontinental slopes and rises. AAPG Bull, 63: 2204-2213University of Petroleun(Beijing)in 1999, a Ph.Dloan.ED,1998.Clathrate Hydrate of Natural Gases(second degree at the Institute of Geology and Geophysics,dition). New York: Marcel Dekker, Inc,7-11Trehu, A M. Lin, G, Maxwell. E, et al., 1995. A seismiChinese Academy of Sciences in 2002reflection profile across the Cascadia subduction zonc offshore researcher of Strategic Research Center for Oil and Gascentral Oregon: New constrains on the deep crustal structureMinistry of Land and Resourcesand on the distribution of methane in the accretionary prism. J engaged in researches on gas hydrate and petroleum101-15,116geologyTrehu, A.M., Torres, M. ct al., 1999. Temporal and spatialevolution of a gas hydrate-bearing accretionary ridge on中国煤化工CNMHG