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Wang X.,CAS Qingdao Institute of Oceanology | Lee M.,U.S. Geological Survey | Wu S.,CAS Qingdao Institute of Oceanology | Yang S.,Guangzhou Marine Geological Survey
Geophysics | Year: 2012

Wireline logs were acquired in eight wells during China's first gas hydrate drilling expedition (GMGS-1) in April-June of 2007. Well logs obtained from site SH3 indicated gas hydrate was present in the depth range of 195-206 m below seafloor with a maximum pore-space gas hydrate saturation, calculated from pore water freshening, of about 26%. Assuming gas hydrate is uniformly distributed in the sediments, resistivity calculations using Archie's equation yielded hydrate-saturation trends similar to those from chloride concentrations. However, the measured compressional (P-wave) velocities decreased sharply at the depth between 194 and 199 mbsf, dropping as low as 1.3 km/s, indicating the presence of free gas in the pore space, possibly caused by the dissociation of gas hydrate during drilling. Because surface seismic data acquired prior to drilling were not influenced by the in situ gas hydrate dissociation, surface seismic data could be used to identify the cause of the low P-wave velocity observed in the well log. To determine whether the low well-log P-wave velocity was caused by in situ free gas or by gas hydrate dissociation, synthetic seismograms were generated using the measured well-log P-wave velocity along with velocities calculated assuming both gas hydrate and free gas in the pore space. Comparing the surface seismic data with various synthetic seismograms suggested that low P-wave velocities were likely caused by the dissociation of in situ gas hydrate during drilling. © 2012 Society of Exploration Geophysicists.

Chen J.-L.,CAS Guangzhou Institute of Geochemistry | Xu J.-F.,CAS Guangzhou Institute of Geochemistry | Ren J.-B.,Guangzhou Marine Geological Survey | Huang X.-X.,CAS Guangzhou Institute of Geochemistry | And 2 more authors.
Gondwana Research | Year: 2014

Ore-bearing porphyritic rocks are widely distributed in the Zhongdian arc in the southern part of the Yidun arc, eastern Tibet. New U-Pb zircon dates, and previous results, show that the porphyritic rocks formed mainly between 221 and 211Ma, with a peak at 217-215Ma. These Late Triassic porphyritic rocks and associated volcanic rocks are primarily calc-alkaline igneous rocks, some of which have geochemical affinities with adakite, such as high SiO2 (≥56wt.%), Al2O3 (≥14wt.%), and Sr, and low Y and heavy rare earth element contents. However, moderate Sr/Y and La/Yb ratios of these rocks compared with typical adakites characterize them as being transitional between adakites and normal arc rocks. Those rocks that do not have adakitic affinities are typical normal arc volcanic rocks. The porphyritic and associated volcanic rocks occur in the eastern and western parts of the Zhongdian arc, and both have the same geochemical characteristics and ages. The new dates, geochemical data, and Sr-Nd isotopic ratios, combined with previous data on the Zhongdian arc (particularly the Xiaxiaoliu basalt that has enriched mid-ocean ridge basalt characteristics), suggest that these rocks are probably related to slab break-off or slab-tearing of the westward subducting Garze-Litang oceanic crust in the Late Triassic. The enriched mantle wedge metasomatized by subducted fluids and sediments was heated by ascending asthenosphere and underwent partial melting. These magmas then probably interacted with underplated mafic material and experienced a melting-assimilation-storage-homogenization process (MASH) in the lower crust and/or with slab-derived melts, resulting in formation of the porphyritic rocks and associated porphyry deposits in the Late Triassic Zhongdian arc. © 2013 International Association for Gondwana Research.

Wang Y.,China University of Geosciences | Sun G.,Guangzhou Marine Geological Survey | Li J.,Chinese Academy of Geological Sciences
Bulletin of the Geological Society of America | Year: 2010

The development of structures and their age along the segment of the Altyn Tagh fault system, and the eastward extension of the Tianshan orogenic belt, remain speculative. Recent investigations on the structural framework, granitic intrusions, and metamorphic rocks in the eastern Tianshan and adjacent areas show that the NE-striking Xingxingxia sinistral ductile shear zone, NW China, is subparallel to the Altyn Tagh fault zone and is superposed on the eastern Tianshan orogenic belt. U-Pb zircon sensitive high-resolution ion microprobe (SHRIMP) dating, and muscovite, biotite, and K-feldspar 40Ar/39Ar thermochronology indicate that sinistral shear along the Xingxingxia shear zone initiated at ~240-235 Ma, broadly at the same time as initial formation of the Altyn Tagh fault zone, but later than initiation of dextral strike-slip motion along ~EW-trending eastern Tianshan orogenic belt at ~270-245 Ma. Formation of the Xing-xingxia ductile shear zone was associated with Gondwanaland convergence along the southern margin of the Eurasian continent during the Late Permian-Early Triassic. © 2009 Geological Society of America.

Wang X.,CAS Qingdao Institute of Oceanology | Hutchinson D.R.,U.S. Geological Survey | Wu S.,CAS Qingdao Institute of Oceanology | Yang S.,Guangzhou Marine Geological Survey | Guo Y.,Guangzhou Marine Geological Survey
Journal of Geophysical Research: Solid Earth | Year: 2011

Gas hydrate saturations were estimated using five different methods in silt and silty clay foraminiferous sediments from drill hole SH2 in the South China Sea. Gas hydrate saturations derived from observed pore water chloride values in core samples range from 10 to 45% of the pore space at 190-221 m below seafloor (mbsf). Gas hydrate saturations estimated from resistivity (Rt) using wireline logging results are similar and range from 10 to 40.5% in the pore space. Gas hydrate saturations were also estimated by P wave velocity obtained during wireline logging by using a simplified three-phase equation (STPE) and effective medium theory (EMT) models. Gas hydrate saturations obtained from the STPE velocity model (41.0% maximum) are slightly higher than those calculated with the EMT velocity model (38.5% maximum). Methane analysis from a 69 cm long depressurized core from the hydrate-bearing sediment zone indicates that gas hydrate saturation is about 27.08% of the pore space at 197.5 mbsf. Results from the five methods show similar values and nearly identical trends in gas hydrate saturations above the base of the gas hydrate stability zone at depths of 190 to 221 mbsf. Gas hydrate occurs within units of clayey slit and silt containing abundant calcareous nannofossils and foraminifer, which increase the porosities of the fine-grained sediments and provide space for enhanced gas hydrate formation. In addition, gas chimneys, faults, and fractures identified from three-dimensional (3-D) and high-resolution two-dimensional (2-D) seismic data provide pathways for fluids migrating into the gas hydrate stability zone which transport methane for the formation of gas hydrate. Sedimentation and local canyon migration may contribute to higher gas hydrate saturations near the base of the stability zone. Copyright 2011 by the American Geophysical Union.

Gong C.,China University of Petroleum - Beijing | Wang Y.,China University of Petroleum - Beijing | Peng X.,Guangzhou Marine Geological Survey | Li W.,British Petroleum | And 2 more authors.
Marine and Petroleum Geology | Year: 2012

Using an integrated multi-beam bathymetry, high-resolution seismic profile, piston core, and AMS 14C dating data set, the current study identified two sediment wave fields, fields 1 and 2, on the South China Sea Slope off southwestern Taiwan. Field 1 is located in the lower slope, and sediment waves within it are overall oriented perpendicular to the direction of down-slope gravity flows and canyon axis. Geometries, morphology, and internal seismic reflection configurations suggest that the sediment waves in field 1 underwent significant up-slope migration. Field 2, in contrast, is located more basinward, on the continental rise. Instead of having asymmetrical morphology and discontinuous reflections as observed in field 1, the sediment waves in field 2 show more symmetrical morphology and continuous reflections that can be traced from one wave to another, suggesting that vertical aggradation is more active and predominant than up-slope migration.Three sediment wave evolution stages, stage 1 through stage 3, are identified in both field 1 and field 2. During stage 1, the sediment waves are built upon a regional unconformity that separates the underlying mass-transport complexes from the overlying sediment waves. In both of these two fields, there is progressive development of the sediment waves and increase in wave dimensions from the oldest stage 1 to the youngest stage 3, even though up-slope migration is dominant in field 1 whereas vertical aggradation is predominant in field 2 throughout these three stages.The integrated data and the depositional model show that the upper slope of the study area is strongly dissected and eroded by down-slope gravity flows. The net result of strong erosion is that significant amounts of sediment are transported further basinward into the lower slope by gravity flows and/or turbidity currents. The interactions of these currents with bottom (contour) currents induced by the intrusion of the Northern Pacific Deep Water into the South China Sea and preexisting wavy topography in the lower slope result in the up-slope migrating sediment waves in field 1 and the contourites as observed from cores TS01 and TS02. Further basinward on the continental rise, turbidity currents are waned and diluted, whereas along-slope bottom (contour) currents are vigorous and most likely dominate over the diluted turbidity currents, resulting in the vertically aggraded sediment waves in field 2.The results from this study also provide the further evidence for the intrusion of the Northern Pacific Deep Water into the South China Sea and suggest that this intrusion has probably existed and been capable of affecting sedimentation in South China Sea at least since Quaternary. © 2011 Elsevier Ltd.

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