南海北部神狐海域天然气水合物形成聚集的数值模拟研究
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摘要
利用2D数值方法对南海北部陆坡神狐海域水合物形成聚集过程进行了模拟,对气烟囱、泥底辟与水合物成藏间的关系进行研究.模拟结果表明,来自深部的甲烷热解气在向上运移过程中以垂向运动为主,且局限在某一狭窄的范围内,故在地震剖面上显示为气烟囱及顶部BSR.只有当其越过水合物稳定带底界,才能形成水合物,此时BSR等于水合物稳定带底界.而一旦水合物形成,该地层即成为封堵层,从而阻止甲烷继续向上运移.因此,水合物仅仅在水合物稳定带底界上方很薄的一层,饱和度却可以很高.此后如果深部气源停止供气,那么气烟囱会逐渐消失,只留下水合物存在.因此,气烟囱的存在并不一定对应着水合物存在.另外,模拟结果还显示,断裂流体活动(泥底辟)对水合物形成并不十分有利.来自深部的甲烷流量与流体流量之间的匹配很重要,在甲烷流量一定的情况下,流体流量越大,越不容易形成水合物.
Shenhu is located in the middle of northern South China Sea.The bottom simulating reflector(BSR)exists widely in the area,and the strong BSR appears mostly in the diapiric structure or at the top of gas chimney.Drilling showed that gas hydrate was only distributed in the upper 10～25 m range above the bottom of hydrate stability zone(BHSZ),but with the maximum saturation of up to 48%.We designed a 2D numerical model to simulate the formation and accumulation in the marine sediments,aiming to understand this unique feature of gas hydrate,as well as to study the relationship between gas chimneys,mud diapirism and gas hydrate reservoir.Modeling results indicate that the methane from deep moves upward vertically and is confined to a rather narrow range,which is shown as gas chimney in the seismic section and BSR at its top.During the process of slow movement of gas,the BSR(top of gas chimney)is always lower than the BHSZ controlled by temperature and pressure.Unless it crosses the BHSZ hydrate cannot form.In case the hydrate forms,the stratum becomes a sealing layer,thereby preventing the continued migration of methane.Consequently,the hydrate only forms within a very thin layer above the BHSZ,but the saturation can be very high.After that,if the gas supply from deep stops,the gas chimney will gradually disappear,leaving only the hydrate.So, the existence of gas chimney does not necessarily correspond to hydrate.In addition,the modeling results also show that the fracture fluid activity(diapir)is not always favorable to the hydrate formation.If the fluid flow from deep is very large,all the methane may dissolve in fluid once produced,and is taken away.Thus,no free gas and hydrate is able to form.Only when the methane flow matches the fluid flow can the hydrate form.Therefore,the larger the fluid flow, the more difficult to form hydrate.
Shenhu is located in the middle of northern South China Sea.The bottom simulating reflector(BSR)exists widely in the area,and the strong BSR appears mostly in the diapiric structure or at the top of gas chimney.Drilling showed that gas hydrate was only distributed in the upper 10～25 m range above the bottom of hydrate stability zone(BHSZ),but with the maximum saturation of up to 48%.We designed a 2D numerical model to simulate the formation and accumulation in the marine sediments,aiming to understand this unique feature of gas hydrate,as well as to study the relationship between gas chimneys,mud diapirism and gas hydrate reservoir.Modeling results indicate that the methane from deep moves upward vertically and is confined to a rather narrow range,which is shown as gas chimney in the seismic section and BSR at its top.During the process of slow movement of gas,the BSR(top of gas chimney)is always lower than the BHSZ controlled by temperature and pressure.Unless it crosses the BHSZ hydrate cannot form.In case the hydrate forms,the stratum becomes a sealing layer,thereby preventing the continued migration of methane.Consequently,the hydrate only forms within a very thin layer above the BHSZ,but the saturation can be very high.After that,if the gas supply from deep stops,the gas chimney will gradually disappear,leaving only the hydrate.So, the existence of gas chimney does not necessarily correspond to hydrate.In addition,the modeling results also show that the fracture fluid activity(diapir)is not always favorable to the hydrate formation.If the fluid flow from deep is very large,all the methane may dissolve in fluid once produced,and is taken away.Thus,no free gas and hydrate is able to form.Only when the methane flow matches the fluid flow can the hydrate form.Therefore,the larger the fluid flow, the more difficult to form hydrate.
引文
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[5] 陈多福,黄永样,冯正等.南海北部冷泉碳酸盐岩和石化微生物细菌及地质意义.矿物岩石地球化学通报,2005,24:185～189 Chen D F,Huang Y Y,Feng Z,et al.Seep carbonate and preserved bacteria fossils in the northern of the South China Sea and their geological implications.Bulletin of Mineralogy,Petrology and Geochemistry (in Chinese),2005,24 :185～189
[6] 蒋少涌,杨涛,薛紫晨等.南海北部海区海底沉积物中孔隙水的Cl和SO42～浓度异常特征及其对天然气水合物的指示意义.现代地质,2005,19:45～54 Yang S Y,Yang T,Xue Z C,et al.Chlorine and sulfate concentrations in pore waters from marine sediments in the north margin of the South China Sea and their implications for gas hydrate exploration.Geoscience (in Chinese),2005,19:45～54
[7] 祝有海,饶竹,刘坚等.南海西沙海槽S14站位的地球化学异常特征及其意义.现代地质,2005,19:39～44 Zhu Y H,Rao Z,Liu J,et al.Geochemical anomalies and their implication from site 14,the Xisha Trough,the South China Sea.Geoscience (in Chinese),2005,19:39～44
[8] 张光学,黄永样,祝有海等.南海天然气水合物的成矿远景.海洋地质与第四纪地质,2002,22(1) :75～81 Zhang G X,Huang Y Y,Zhu Y H,et al.Prospect of gas hydrate resources in the South China Sea.Marine Geology and Quaternary Geology (in Chinese),22(1) :75～81
[9] 吴能友,梁金强,王宏斌等.海洋天然气水合物成藏系统研究进展.现代地质,2008,22:356～362 Wu N Y,Liang J Q,Wang H B,et al.Marine gas hydrate system..State of the Art.Geoscience (in Chinese),2008,22:356～362
[10] 吴能友,杨胜雄,王宏斌等.南海北部陆坡神狐海域天然气水合物成藏的流体运移体系.地球物理学报,2009,52(6) :1641～1650 WuN Y,Yang S X,Wang H B,et al.Gas bearing fluid influx sub system for gas hydrate geological system in Shenhu Area.ChineseJ.Geophys.(in Chinese),2009,52(6) :1641～1650
[11] 吴时国,龚跃华,米立军等.南海北部深水盆地油气渗漏系统及天然气水合物成藏机制研究.现代地质,2010,24:433～440 Wu S G,Gong Y H,Mi L J,et al.Study on hydrocarbon leakage system and associated gas hydrate reservoirs in the deep water basin of northern South China Sea.Geoscience (in Chinese),2010,24:433～440
[12] Rempel A W,Buffett B A.Formation and accumulation of gas hydrate in porous media.J Geophys Res,1997,102;10151～110164
[13] Rempel A W,Buffett B A.Mathematical models of gas hydrate accumulation.In:Henriet J P,Mienert J,eds.Gas Hydrates:Relevance to World Margin Stability and Climate Change.London:Geological Society,1998,137:63～74
[14] Xu W,Ruppel C.Predicting the occurrence,distribution,and evolution of methane gas hydrate in porous marine sediments.J Geophys Res,1999,104:5081～5095
[15] Davie M K,Buffett B A.A numerical model for the formation of gas hydrate below the seafloor.J/ Geophys Res ,2001,106:497～514
[16] Davie M K,Buffett B A.A steady state model for marine hydrate formation;Constraints on methane supply from pore water sulfate profile.J Geophys Res ,2003,108(B10) :2495,doi:2410． 1029/2002JB002300
[17] Gering K,Cherry R S,Weunberg D M.Mechanisms for methane gas accumulation under hydrate deposits in sediments.Ann NY Acad Sci,2000,912:623～632
[18] Gering K.Simulations of methane hydrate phenomena over geologic timescales.Part I-Effects of sediment compaction rates on methane hydrate and free gas accumulations.Earth Planet Sci Lett,2003,206:65-81
[19] He L,Matsubayashi O,Lei X.Methane hydrate accumulation model for the Central Nankai accretionary prism.Marine Geology ,2006,227:201～214
[20] Garg S K,Pritchett J W,Katoh A,et al.A mathematical model for the formation and dissociation of methane hydrates in the marine environment.J Geophys Res,2008,113(B0120l):doi:10． 1029/2006JB004768
[21] Moridis G J,Kowalsky M B,Pruess K,eds.Tough+Hydrate vl.0 User s Manual;A Code for the Simulation of System Behavior in Hydrate-Bearing Geological Media.Berkeley:Lawrence Berkeley National Laboratory,2008
[22] 张功成,米立军,吴时国等.深水区南海海洋油气勘探的新领域.石油学报,2 007,28:15～21 Zhang G C,Mi L J,Wu S G,et al.Deepwater area-the new prospecting targets of northern continental margin of South China Sea.Acta Petrol Ei Sinica (in Chinese),2007,28:15～21
[23] 徐行,施小斌,罗贤虎等.南海西沙海槽地区的海底热流测量.海洋地质与第四纪地质,2006,26(4) :51～58 Xu X,Shi X B,Luo X H,et al.Heat flow measurements in the Xisha trough of the South China Sea.Marine Geology and Quaternary Geology (in Chinese),2006,26(4) :51～58
[24] 徐行,陆敬安,罗贤虎等.南海北部海底热流测量及分析.地球物理学进展,2005,20(2) :562～565 Xu X,Lu J A,Luo X H,et al.The marine heat flow survey and the result discussion in the northern part of South China Sea.Progress in Geophys.(in Chinese) ,20(2) :562～565
[25] 张毅.南海北部陆坡天然气水合物成矿的地温场及成藏动力学初步研究[博士论文].北京:中国科学院地质与地球物理研究所,2010 Zhang Y.Preliminary studies on the temperature field and formation mechanism of gas hydrate in the norther margin of the South China Sea [Ph.D.thesis].Beijing:Institute of Geology and Geophysics,Chinese Academy of Sciences,2010
[26] 张毅,何丽娟,徐行等.南海北部神狐海域甲烷水合物BHSZ与BSR的比较研究.地球物理学进展,2009,24(1) :183～194 Zhang Y,He I J,Xu X,et al.The disagreement between BSRs and the base of methane hydrate stability zones in the Shenhu Area north of the South China Sea.Progress in Geopohys.(in Chinese) ,2009 ,24(1) :183～194
[27] 吴能友,张海啟,杨胜雄等.南海神狐海域天然气水合物成藏系统初探.天然气工业,2007,27(9) :1～6 Wu N Y,Zhang H Q,Yang S X,et al.Preliminary discussion on natural gas hydrate reservoir system of Shenhu Area,north slope of South China Sea.Natural Gas Industry (in Chinese),2007,27(9) :1～6
[28] GuanJ,Liang D,Wu N Y,et al.The methane hydrate formation and the resource estimate resulting from free gas migration in seeping seafloor hydrate stability zone.Journal of Asian Earth Sciences,2009,36:277-288
[29] 杨涛,蒋少涌,葛璐等.南海北部神狐海域浅表层沉积物中孔隙水的地球化学特征及其对天然气水合物的指示意义.科学通报,2009,54:3231～3240
[2] 姚伯初.南海北部陆缘天然气水合物初探.海洋地质与第四纪地质,1998,18(4) :11～18 Yao B C.Preliminary exploration of gas hydrate in the northern margin of the South China Sea.Marine Geology and Quaternary Geology (in Chinese),1998,18(4) :11～18
[3] 宋海斌,耿建华,Fong King等.南海北部东沙海域天然气水合物的初步研究.地球物理学报,2001,44:687～695 Song H B,GengJ H,Hong King,et al.A preliminary study of gas hydrates in Dongsha region north of South China Sea.ChineseJ.Geophys.(in Chinese),2001,44(5) :687～695
[4] 吴时国,董冬冬,杨胜雄等.南海北部陆坡细粒沉积物天然气水合物系统的形成模式初探.地球物理学报,2009,52(7) :1849～1857 Wu S G,Dong D D,Yang S X,et al.Genetic model of the hydrate system in the fine grain sediments in the northern continental slope of South China Sea.Chinese J.Geophys.(in Chinese),2009,52(7) :1849～1857
[5] 陈多福,黄永样,冯正等.南海北部冷泉碳酸盐岩和石化微生物细菌及地质意义.矿物岩石地球化学通报,2005,24:185～189 Chen D F,Huang Y Y,Feng Z,et al.Seep carbonate and preserved bacteria fossils in the northern of the South China Sea and their geological implications.Bulletin of Mineralogy,Petrology and Geochemistry (in Chinese),2005,24 :185～189
[6] 蒋少涌,杨涛,薛紫晨等.南海北部海区海底沉积物中孔隙水的Cl和SO42～浓度异常特征及其对天然气水合物的指示意义.现代地质,2005,19:45～54 Yang S Y,Yang T,Xue Z C,et al.Chlorine and sulfate concentrations in pore waters from marine sediments in the north margin of the South China Sea and their implications for gas hydrate exploration.Geoscience (in Chinese),2005,19:45～54
[7] 祝有海,饶竹,刘坚等.南海西沙海槽S14站位的地球化学异常特征及其意义.现代地质,2005,19:39～44 Zhu Y H,Rao Z,Liu J,et al.Geochemical anomalies and their implication from site 14,the Xisha Trough,the South China Sea.Geoscience (in Chinese),2005,19:39～44
[8] 张光学,黄永样,祝有海等.南海天然气水合物的成矿远景.海洋地质与第四纪地质,2002,22(1) :75～81 Zhang G X,Huang Y Y,Zhu Y H,et al.Prospect of gas hydrate resources in the South China Sea.Marine Geology and Quaternary Geology (in Chinese),22(1) :75～81
[9] 吴能友,梁金强,王宏斌等.海洋天然气水合物成藏系统研究进展.现代地质,2008,22:356～362 Wu N Y,Liang J Q,Wang H B,et al.Marine gas hydrate system..State of the Art.Geoscience (in Chinese),2008,22:356～362
[10] 吴能友,杨胜雄,王宏斌等.南海北部陆坡神狐海域天然气水合物成藏的流体运移体系.地球物理学报,2009,52(6) :1641～1650 WuN Y,Yang S X,Wang H B,et al.Gas bearing fluid influx sub system for gas hydrate geological system in Shenhu Area.ChineseJ.Geophys.(in Chinese),2009,52(6) :1641～1650
[11] 吴时国,龚跃华,米立军等.南海北部深水盆地油气渗漏系统及天然气水合物成藏机制研究.现代地质,2010,24:433～440 Wu S G,Gong Y H,Mi L J,et al.Study on hydrocarbon leakage system and associated gas hydrate reservoirs in the deep water basin of northern South China Sea.Geoscience (in Chinese),2010,24:433～440
[12] Rempel A W,Buffett B A.Formation and accumulation of gas hydrate in porous media.J Geophys Res,1997,102;10151～110164
[13] Rempel A W,Buffett B A.Mathematical models of gas hydrate accumulation.In:Henriet J P,Mienert J,eds.Gas Hydrates:Relevance to World Margin Stability and Climate Change.London:Geological Society,1998,137:63～74
[14] Xu W,Ruppel C.Predicting the occurrence,distribution,and evolution of methane gas hydrate in porous marine sediments.J Geophys Res,1999,104:5081～5095
[15] Davie M K,Buffett B A.A numerical model for the formation of gas hydrate below the seafloor.J/ Geophys Res ,2001,106:497～514
[16] Davie M K,Buffett B A.A steady state model for marine hydrate formation;Constraints on methane supply from pore water sulfate profile.J Geophys Res ,2003,108(B10) :2495,doi:2410． 1029/2002JB002300
[17] Gering K,Cherry R S,Weunberg D M.Mechanisms for methane gas accumulation under hydrate deposits in sediments.Ann NY Acad Sci,2000,912:623～632
[18] Gering K.Simulations of methane hydrate phenomena over geologic timescales.Part I-Effects of sediment compaction rates on methane hydrate and free gas accumulations.Earth Planet Sci Lett,2003,206:65-81
[19] He L,Matsubayashi O,Lei X.Methane hydrate accumulation model for the Central Nankai accretionary prism.Marine Geology ,2006,227:201～214
[20] Garg S K,Pritchett J W,Katoh A,et al.A mathematical model for the formation and dissociation of methane hydrates in the marine environment.J Geophys Res,2008,113(B0120l):doi:10． 1029/2006JB004768
[21] Moridis G J,Kowalsky M B,Pruess K,eds.Tough+Hydrate vl.0 User s Manual;A Code for the Simulation of System Behavior in Hydrate-Bearing Geological Media.Berkeley:Lawrence Berkeley National Laboratory,2008
[22] 张功成,米立军,吴时国等.深水区南海海洋油气勘探的新领域.石油学报,2 007,28:15～21 Zhang G C,Mi L J,Wu S G,et al.Deepwater area-the new prospecting targets of northern continental margin of South China Sea.Acta Petrol Ei Sinica (in Chinese),2007,28:15～21
[23] 徐行,施小斌,罗贤虎等.南海西沙海槽地区的海底热流测量.海洋地质与第四纪地质,2006,26(4) :51～58 Xu X,Shi X B,Luo X H,et al.Heat flow measurements in the Xisha trough of the South China Sea.Marine Geology and Quaternary Geology (in Chinese),2006,26(4) :51～58
[24] 徐行,陆敬安,罗贤虎等.南海北部海底热流测量及分析.地球物理学进展,2005,20(2) :562～565 Xu X,Lu J A,Luo X H,et al.The marine heat flow survey and the result discussion in the northern part of South China Sea.Progress in Geophys.(in Chinese) ,20(2) :562～565
[25] 张毅.南海北部陆坡天然气水合物成矿的地温场及成藏动力学初步研究[博士论文].北京:中国科学院地质与地球物理研究所,2010 Zhang Y.Preliminary studies on the temperature field and formation mechanism of gas hydrate in the norther margin of the South China Sea [Ph.D.thesis].Beijing:Institute of Geology and Geophysics,Chinese Academy of Sciences,2010
[26] 张毅,何丽娟,徐行等.南海北部神狐海域甲烷水合物BHSZ与BSR的比较研究.地球物理学进展,2009,24(1) :183～194 Zhang Y,He I J,Xu X,et al.The disagreement between BSRs and the base of methane hydrate stability zones in the Shenhu Area north of the South China Sea.Progress in Geopohys.(in Chinese) ,2009 ,24(1) :183～194
[27] 吴能友,张海啟,杨胜雄等.南海神狐海域天然气水合物成藏系统初探.天然气工业,2007,27(9) :1～6 Wu N Y,Zhang H Q,Yang S X,et al.Preliminary discussion on natural gas hydrate reservoir system of Shenhu Area,north slope of South China Sea.Natural Gas Industry (in Chinese),2007,27(9) :1～6
[28] GuanJ,Liang D,Wu N Y,et al.The methane hydrate formation and the resource estimate resulting from free gas migration in seeping seafloor hydrate stability zone.Journal of Asian Earth Sciences,2009,36:277-288
[29] 杨涛,蒋少涌,葛璐等.南海北部神狐海域浅表层沉积物中孔隙水的地球化学特征及其对天然气水合物的指示意义.科学通报,2009,54:3231～3240
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