Methane seepage is the signal of the deep hydrocarbon reservoir. The determination of seepage is significant to the exploration of petroleum, gas and gas hydrate. The seepage habits microbial and macrofaunal life whic...Methane seepage is the signal of the deep hydrocarbon reservoir. The determination of seepage is significant to the exploration of petroleum, gas and gas hydrate. The seepage habits microbial and macrofaunal life which is fueled by the hydrocarbons, the metabolic byproducts facilitate the precipitation of authigenic minerals. The study of methane seepage is also important to understand the oceanographic condition and local ecosystem. The seepage could be active or quiescent at different times. The geophysical surveys and the geochemical determinations reveal the existence of seepage. Among these methods, only geochemical determination could expose message of the dormant seepages. The active seepage demonstrates high porewater methane concentration with rapid SO42- depleted, low HaS and dissolved inorganic carbon （DIC）, higher rates of sulfate reduction （SR） and anaerobic oxidation of methane （AOM）. The quiescent seepage typically develops authigenic carbonates with specific biomarkers, with extremely depleted 13C in gas, DIC and carbonates and with enriched 34S sulfate and depleted 34S pyrite. The origin of methane, minerals precipitation, the scenario of seepage and the possible method of immigration could be determined by the integration of solutes concentration, mineral composition and isotopic fractionation of carbon, sulfur. Numerical models with the integrated results provide useful insight into the nature and intensity of methane seepage occurring in the sediment and paleo- oceanographic conditions. Unfortunately, the intensive investigation of a specific area with dormant seep is still limit. Most seepage and modeling studies are site-specific and little attempt has been made to extrapolate the results to larger scales. Further research is thus needed to foster our understanding of the methane seepage.展开更多
The Shenhu gas hydrate drilling area is located in the central Baiyun sag, Zhu I! depression, Pearl River Mouth basin, northern South China Sea. The gas compositions contained in the hydrate-bearing zones is dominated...The Shenhu gas hydrate drilling area is located in the central Baiyun sag, Zhu I! depression, Pearl River Mouth basin, northern South China Sea. The gas compositions contained in the hydrate-bearing zones is dominated by methane with content up to 99.89% and 99.91%. The carbon isotope of the methane （δ^13C1） are -56.7%0. and -60.9%0, and its hydrogen isotope （δD） are -199%0 and -180%0, respectively, indicating the methane from the microbial reduction of CO2. Based on the data of measured seafloor temperature and geothermal gradient, the gas formed hydrate reservoirs are from depths 24-1699 m below the seafloor, and main gas-generation zone is present at the depth interval of 416-1165 m. Gas-bearing zones include the Hanjiang Formation, Yuehai Formation, Wanshan Formation and Quaternary sediments. We infer that the microbial gas migrated laterally or vertically along faults （especially interlayer faults）, slump structures, small-scale diapiric structures, regional sand beds and sedimentary boundaries to the hydrate stability zone, and formed natural gas hydrates in the upper Yuehai Formation and lower Wanshan Formation, probably with contribution of a little thermogenic gas from the deep sedments during this process.展开更多
Elemental sulfur(ES) is one of the intermediates in the inorganic sulfur cycle and thus plays a key role in the fractionation of stable sulfur isotopes in different reservoirs and the marine environment. In this study...Elemental sulfur(ES) is one of the intermediates in the inorganic sulfur cycle and thus plays a key role in the fractionation of stable sulfur isotopes in different reservoirs and the marine environment. In this study, solid ES is discovered in sediments near the Jiulong Methane Reef in the northern South China Sea by scanning electron microscopy and Raman spectroscopy. Combining the morphology and distribution of ES, pyrite concentrations, and sulfur isotopes, we conclude that:(1) solid ES coexists with pyrite microcrystals and sulfide(oxyhydr)oxides as well as clay minerals, and they are mainly distributed on the surface of mineral aggregates;(2) ES mainly occurs within and near the sulfate-methane transition zone(SMTZ) despite little morphological diversity;(3) ES formation might be related to hydrogen sulfide oxidation and is therefore linked with fluctuations in the SMTZ. Within the SMTZ, hydrogen sulfide is produced and pyrite precipitates because of enhanced anaerobic oxidation of methane coupled with dissimilatory sulfate reduction. This enhances the efficiency of the inorganic sulfur cycle and provides favorable conditions for ES formation. The discovery of solid ES in sediments near the Jiulong Methane Reef suggests an important relationship with SMTZ fluctuations that could have implications for the evolution of methane hydrate in the South China Sea.展开更多
Shenhu Area is one of the most promising areas for gas hydrate exploration in the northern South China Sea （SCS）. Pore water sulfate gradient, sulfate-methane interface （SMI） depth, and sulfate flux were analyzed ...Shenhu Area is one of the most promising areas for gas hydrate exploration in the northern South China Sea （SCS）. Pore water sulfate gradient, sulfate-methane interface （SMI） depth, and sulfate flux were analyzed at 53 sites in this area. SO42- gradient ranges between 0.33 and 4.43 mmol L-L m-1. SMI depths are from 7.7 to 87.9 mbsf. Sulfate flux varies between 2.0 and 26.9 mmol m-2 yr L, with a mean of 11.7 mmol m-2 yr1. Correlation coefficient between SMI depth and methane flux for the 53 sites is -0.80, implying that methane flux regulates the rate of anaerobic methane oxidation （AMO）, SMI depth, and sulfate flux. Twelve anomalous fields with high methane flux and steep sulfate gradients were recognized. Bottom simulating reflector （BSR） is distributed mainly in areas where SMI depth is less than 50 mbsf or places with sulfate flux larger than 3.5 mmol m-2 yr-1. It is suggested that the Baiyun Sag and the Southern Uplift are potential areas for gas hydrate exploration.展开更多
基金The National Natural Science Foundation of China under contract No.41376076the Natural Science Foundation of Guangdong Province under contract No.2015A030313718+1 种基金the Scientific Cooperative Project by China National Petroleum Corporation and Chinese Academy of Sciences under contract No.2015A-4813the National Marine Geological Project,China Geological Survey under contract No.GZH2012006003
文摘Methane seepage is the signal of the deep hydrocarbon reservoir. The determination of seepage is significant to the exploration of petroleum, gas and gas hydrate. The seepage habits microbial and macrofaunal life which is fueled by the hydrocarbons, the metabolic byproducts facilitate the precipitation of authigenic minerals. The study of methane seepage is also important to understand the oceanographic condition and local ecosystem. The seepage could be active or quiescent at different times. The geophysical surveys and the geochemical determinations reveal the existence of seepage. Among these methods, only geochemical determination could expose message of the dormant seepages. The active seepage demonstrates high porewater methane concentration with rapid SO42- depleted, low HaS and dissolved inorganic carbon （DIC）, higher rates of sulfate reduction （SR） and anaerobic oxidation of methane （AOM）. The quiescent seepage typically develops authigenic carbonates with specific biomarkers, with extremely depleted 13C in gas, DIC and carbonates and with enriched 34S sulfate and depleted 34S pyrite. The origin of methane, minerals precipitation, the scenario of seepage and the possible method of immigration could be determined by the integration of solutes concentration, mineral composition and isotopic fractionation of carbon, sulfur. Numerical models with the integrated results provide useful insight into the nature and intensity of methane seepage occurring in the sediment and paleo- oceanographic conditions. Unfortunately, the intensive investigation of a specific area with dormant seep is still limit. Most seepage and modeling studies are site-specific and little attempt has been made to extrapolate the results to larger scales. Further research is thus needed to foster our understanding of the methane seepage.
基金supported by the National Major Fundamental and Development Project of China (No. 2009CB219501)the National Natural Science Foundation of China (No. 41202099)
文摘The Shenhu gas hydrate drilling area is located in the central Baiyun sag, Zhu I! depression, Pearl River Mouth basin, northern South China Sea. The gas compositions contained in the hydrate-bearing zones is dominated by methane with content up to 99.89% and 99.91%. The carbon isotope of the methane （δ^13C1） are -56.7%0. and -60.9%0, and its hydrogen isotope （δD） are -199%0 and -180%0, respectively, indicating the methane from the microbial reduction of CO2. Based on the data of measured seafloor temperature and geothermal gradient, the gas formed hydrate reservoirs are from depths 24-1699 m below the seafloor, and main gas-generation zone is present at the depth interval of 416-1165 m. Gas-bearing zones include the Hanjiang Formation, Yuehai Formation, Wanshan Formation and Quaternary sediments. We infer that the microbial gas migrated laterally or vertically along faults （especially interlayer faults）, slump structures, small-scale diapiric structures, regional sand beds and sedimentary boundaries to the hydrate stability zone, and formed natural gas hydrates in the upper Yuehai Formation and lower Wanshan Formation, probably with contribution of a little thermogenic gas from the deep sedments during this process.
基金supported by the National Natural Science Foundation of China(Grants Nos.41472085 and 41172102)the National Basic Research Program of China(Grants Nos.2011CB808805 and 2009CB21950605)the National Project of Exploration and Test Production for Gas Hydrate(Grants Nos.GZH20110030-50603 and GZH20110030-6WX02)
文摘Elemental sulfur(ES) is one of the intermediates in the inorganic sulfur cycle and thus plays a key role in the fractionation of stable sulfur isotopes in different reservoirs and the marine environment. In this study, solid ES is discovered in sediments near the Jiulong Methane Reef in the northern South China Sea by scanning electron microscopy and Raman spectroscopy. Combining the morphology and distribution of ES, pyrite concentrations, and sulfur isotopes, we conclude that:(1) solid ES coexists with pyrite microcrystals and sulfide(oxyhydr)oxides as well as clay minerals, and they are mainly distributed on the surface of mineral aggregates;(2) ES mainly occurs within and near the sulfate-methane transition zone(SMTZ) despite little morphological diversity;(3) ES formation might be related to hydrogen sulfide oxidation and is therefore linked with fluctuations in the SMTZ. Within the SMTZ, hydrogen sulfide is produced and pyrite precipitates because of enhanced anaerobic oxidation of methane coupled with dissimilatory sulfate reduction. This enhances the efficiency of the inorganic sulfur cycle and provides favorable conditions for ES formation. The discovery of solid ES in sediments near the Jiulong Methane Reef suggests an important relationship with SMTZ fluctuations that could have implications for the evolution of methane hydrate in the South China Sea.
基金supported by the National Basic Research Program of China (Grant Nos. 2009CB219508 and 2009CB219502)Research Program for Non-profit Industries of the Ministry of Land and Resources of the People’s Republic of China (Grant No. 200811014)
文摘Shenhu Area is one of the most promising areas for gas hydrate exploration in the northern South China Sea （SCS）. Pore water sulfate gradient, sulfate-methane interface （SMI） depth, and sulfate flux were analyzed at 53 sites in this area. SO42- gradient ranges between 0.33 and 4.43 mmol L-L m-1. SMI depths are from 7.7 to 87.9 mbsf. Sulfate flux varies between 2.0 and 26.9 mmol m-2 yr L, with a mean of 11.7 mmol m-2 yr1. Correlation coefficient between SMI depth and methane flux for the 53 sites is -0.80, implying that methane flux regulates the rate of anaerobic methane oxidation （AMO）, SMI depth, and sulfate flux. Twelve anomalous fields with high methane flux and steep sulfate gradients were recognized. Bottom simulating reflector （BSR） is distributed mainly in areas where SMI depth is less than 50 mbsf or places with sulfate flux larger than 3.5 mmol m-2 yr-1. It is suggested that the Baiyun Sag and the Southern Uplift are potential areas for gas hydrate exploration.
