International Conference on Gas Hydrates (ICGH) (6th : 2008)

GEOCHEMICAL ANOMALY OF PORE WATERS AND IMPLICATIONS FOR GAS HYDRATE OCCURENCE IN THE SOUTH CHINA SEA Jiang, Shao-Yong; Yang, Tao; Ge, Lu; Yang, Jing-Hong; Wu, Neng-You; Liu, Jian; Zhang, Guang-Xue; Chen, Dao-Hua Jul 31, 2008

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
59278-5759.pdf [ 98.45kB ]
Metadata
JSON: 59278-1.0041007.json
JSON-LD: 59278-1.0041007-ld.json
RDF/XML (Pretty): 59278-1.0041007-rdf.xml
RDF/JSON: 59278-1.0041007-rdf.json
Turtle: 59278-1.0041007-turtle.txt
N-Triples: 59278-1.0041007-rdf-ntriples.txt
Original Record: 59278-1.0041007-source.json
Full Text
59278-1.0041007-fulltext.txt
Citation
59278-1.0041007.ris

Full Text

Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  GEOCHEMICAL ANOMALY OF PORE WATERS AND IMPLICATIONS FOR GAS HYDRATE OCCURRENCE IN THE SOUTH CHINA SEA Shao-Yong Jiang ∗ , Tao Yang, Lu Ge, Jing-Hong Yang State Key Laboratory for Mineral Deposits Research and Center for Marine Geochemistry Research, Department of Earth Sciences, Nanjing University 22 Hankou Road, Nanjing 210093 CHINA Neng-You Wu, Jian Liu, Guang-Xue Zhang, Dao-Hua Chen Guangzhou Marine Geological Survey, China Geological Survey Guangzhou 510760 CHINA ABSTRACT Except for direct drilling and sampling of marine gas hydrates, the occurrence of gas hydrates has been identified generally by inference from indirect evidence, derived from geological, geophysical, and geochemical data. In this paper, we intend to discuss the geochemical anomalies of pore waters and their implications for gas hydrate occurrence in the northern continental slope of the South China Sea. The molecular concentration and isotopic composition of methane in sediments can provide clues to gas sources, whereas ionic and isotopic compositions of pore waters, such as steep SO42- gradients, shallow SMI (sulfate-methane interface) depths; decreasing pore water chlorinity, and heavy oxygen isotopic compositions, are used to identify gas hydrate occurrence and the distribution and thickness of sediment layers containing gas hydrates. Other good geochemical indicators include anions and cations concentrations such as Br-, I-, PO43-, NH4+, Ca2+, Mg2+, Sr2+, B3+, Li+, and Ba2+ in pore waters. We also found that the very negative carbon isotopic compositions of dissolve inorganic carbon (DIC) in pore waters can serve as good indicators for gas hydrate occurrence. In the South China Sea, three most promising target areas for gas hydrates include the Dongsha, Shenhu, and Xisha Trough. Keywords: gas hydrates; isotopic; geochemical indicators; South China Sea INTRODUCTION Gas hydrate is an ice-like solid substance predominantly composed of water and methane, which commonly occurs in deep-water (7002500m) marine sediments under appropriate pressure, temperature, and salinity conditions. In the world oceans, >220 gas hydrate deposits have been found, >100 wells drilled and kilometers of hydrated cores studies [1]. In the northern continental slope of the South China Sea, we have started to explore and study gas hydrates using ∗  combined geological, geophysical, and geochemical methods since 1999. In 2004, a joint Sino-Germany cruise SO-177 of R/V SONNE found one of the largest areas of cold seep carbonates in the world oceans at the Shenhu area [2]. In 2007, gas hydrate cores were drilled out and sampled in the Shenhu area of the South China Sea during a cruise organized by China Geological Survey. In the exploration and investigation of marine gas hydrates, geochemical methods have played an important role, and this paper discusses  Corresponding author: Phone: +86 25 83596832 Fax +86 25 83592393 E-mail: shyjiang@nju.edu.cn  the geochemical anomalies of pore waters in marine sediments and their implications for gas hydrates in the South China Sea. THE NORTHERN CONTINENTAL SLOPE OF THE SOUTH CHINA SEA The northern continental slope of the South China Sea contains many sedimentary basins with large source of oil and natural gas (Fig. 1). In recent years, increasing evidence has shown that this region is a favorable place for the formation of gas hydrates [2-9]. Three prospecting target areas for occurrence of gas hydrates have been indicated in the South China Sea, namely, the Xisha Trough, the Shenhu and Dongsha areas (Fig.1).  Figure 1: Three prospecting target areas for gas hydrate occurrence in the northern continental slope of the South China Sea Yao [9] found BSRs for the first time in the northern margin of the South China Sea after a 100,000 Km seismic profile survey, and suggested that gas hydrates might occur both within the Xisha Trough and in the south of the Dongsha area according to the BSRs and the tempeture- pressure regimes there. In the following years, other researchers also reported BSRs and other geophysical indicators such as blanking and wipeout below the BSR in the South China Sea [6, 1011]. They suggested a link of these BSRs to the potential occurrence of gas hydrates in the region. Gas hydrates occur either in the accretionary prism of active continental margin or in the crest and trough in passive continental margin. In South  China Sea occur these two tectonic settings. The northwestern of the South China Sea is a passive tectonic margin, whereas the eastern margin is an actively collisional margin. Typical tectonic and sediment structures related to gas hydrates are found in South China Sea, such as accretionary prism, mud diapirs, slumps, cold seep venting and fluid-induced structures [6, 12]. In the South China Sea, the sediment thickness can reach up to several thousand meters and the sedimentation rates range from 33 to 1400 m/Ma, which are similar to the sedimentation rates at Blake Ridge [13]. GAS COMPOSITIONS Elevated downward hydrocarbon gas concentrations in headspace gas samples and sediments are good indicators for gas hydrates. In the South China Sea, the ODP drilling and the pistol cores both show an increase in methane concentration. In the ODP 184 Hole 1146 in the Dongsha area, head space methane concentration as high as 85,000 ppmv has been determined at 563.2 mbsf. High head space ethane concentration of 155 ppmv has also determined at 572.8 mbsf in this drill core [14-15]. Wang JQ et al [16] reported methane concentrations in sediments from the ODP 184 Hole 1146 in the range of 15.7 to 394.1 μl/kg, which show a similar increasing trend with depth to those headspace values reported by Zhu et al [15]. These authors have suggested that the high methane concentrations at the deep sections of the ODP 184 holes likely indicate decomposition of gas hydrates nearby, which migrated laterally to site 1146 through faults or bedded planes. In the gas hydrate occurrences, methane venting or high methane anomaly may be observed in the water column (e.g., Blake Ridge, Gulf of Mexico, and Arabian Sea etc.). Active methane venting has not been found in the South China Sea, but the discovery of large area of cold seep carbonates suggests to us that gas seeping may have occurred in this region, and high methane content anomaly in bottom seawater has also been determined in several areas in the South China Sea. SULFATE GRADIENTS AND SMI DEPTHS Steep, linear sulfate gradients and shallow SMI depths are excellent geochemical indicators for gas hydrate occurrence [17]. In the Xisha Trough, the SO42-concentrations of pore waters vary from 19.9  to 36.8 mM, with a downhole decreasing trend for most of the cores, and the calculated SMI (sulfatemethane interfaces) depths and sulfate gradients using least-square linear regression are between 21 and 47 mbsf, and between -0.7 and -1.7 mM/m, respectively [13]. In the Dongsha area, the SMI depths at ODP 184 Sites 1144 and 1146 are 11 and 65 mbsf, respectively [14-15]. The piston cores in the Dongsha area show very shallow SMI values of 7.5 to 14.2 mbsf [18]. These values are quite similar to those at gas hydrate locations in the world oceans [17].  values. For example, in ODP 164 site 997, below ~400 mbsf Br- contents are about three times of seawater and I- contents are three orders of magnitude of seawater values [20]. In this study, we found high Br- concentrations (up to 278 ppm) in pore water samples in several pistol cores in the Dongsha area. High I- concentrations are both found in the Dongsha and Shenhu areas. In particular, pore water samples from the BSR fields in the Shenhu area show significantly higher Icontents (4997-5878 ppm) than those from the non-BSR fields (147-1103 ppm).  CHLORINITY Significant decrease of chlorinity in pore waters in gas hydrate bearing zone sediments was first found in DSDP Leg 67 [19]. Now, pore water chlorinity has become one of the most important evidence for gas hydrate existence. Two trends for chlorinity may relate to gas hydrate. One is decreasing chlorinity due to the dissociation of gas hydrate during core recovery. Another is roof chlorinity increase due to salt exclusion effect during the formation of gas hydrate [20].  PO42- AND NH4+ In the gas hydrate occurrence area, the pore water show significantly higher ammonia and phosphate concentrations in shallow marine sediments [21]. An increasing trend of pore water phosphate concentrations with depths is also observed, which match the SMI curve of SO42- gradients in pore water. We suggest that the anomalies of ammonia and phosphate concentrations in pore water may be used as new geochemical tracers to prospect marine gas hydrate [21].  In the South China Sea, ODP 184 Hole 1146 shows a decrease in Cl- concentrations with depth [15]. Below 540.4 mbsf, Cl- shows a significant decrease, with values of 547 mM at 559.7 and 579.0 mbsf and a minimum value of 536 mM at 598.2 mbsf, which were probably produced by decomposition of gas hydrate nearby and migrated to site 1146 along faults or bedded planes [15]. In the Shenhu area, the gas hydrate drill cores recovered in 2007 also show low chlorinity (333.2 mM in core SH-2, 418.1 mM in core SH-3, and 337.4 mM in core SH-7) in the gas hydrate bearing zones.  CATIONS Cations, such as K+, Na+, Ca2+, Mg2+, Li+, B3+, Sr2+, and Ba2+, in pore waters may be useful indicators for gas hydrate existence. In ODP 164 sites 994, 995, and 997, pore water Ca2+ and Mg2+ concentrations decrease dramatically and Mg/Ca and Sr/Ca ratios increase significantly in the shallow depths of <40 mbsf, whereas Sr2+ concentrations and Sr/Cl ratios fall below seawater values between 100 and 450 mbsf corresponding to the gas hydrate bearing zones at these sites [22]. Mazurenko et al. [23] used pore water Mg/Cl ratios to identify mixing of various fluids in the gas hydrate deposits in the Ginsburg and Yuma mud volcano sediments, Moroccan Margin.  Pore water samples with Cl- concentrations significantly higher than normal seawater are also found in shallow marine sediments in the South China Sea [13]. But these samples may not relate to the roof effect of chlorinity as reported at sites with shallow gas hydrate occurrence elsewhere since a general linear correlation between Cl- and Br- (and Na+, K+, Mg2+) are observed and a mixing between normal seawater and a saline brine is the simplest explanation [13]. BR- AND IIn the gas hydrate locations, pore water Br- and Iconcentrations show a large increase over seawater  In the Xisha Trough, pore-water Mg/Ca and Sr/Ca ratios increase with depths in pistol cores HX-17 and HX-21 [13]. These trends are probably caused by precipitation of carbonates due to AMO (anaerobic methane oxidation) or SOM (oxidation of sedimentary organic matter through sulfate reduction) processes, which produce CO32- [17, 20, 24]. In the Dongsha area, pore waters from ODP 184 show a sharp increase in Li+ concentration at depth up to 1337 μM at the base of the hole, whereas Ca concentrations decrease quickly from 16.7 mM at surface to a minimum value of 2.6  mM at 65.35 mbsf, corresponding to sulfate reduction zone [15]. OXYGEN AND HYDROGEN ISOTOPES A sharp increase in pore water δ18O values has been used as good indicator for the gas hydrate bearing zones, because heavy isotope 18O is preferentially incorporated in the solid phases of gas hydrate. When gas hydrate dissociated, isotopically heavy water is released to cause the 18 O enrichment in the pore waters [19-20]. Hydrogen isotopes behave essentially the same as that for oxygen. In the Dongsha area, pore waters from ODP 184 cores did not show any systematically significant shift in both δ18O and δ2H from typical seawater stored in sediments, except for several samples with anomalously lower values of oxygen (-3.7‰) and hydrogen (-14‰) [15]. In the Xisha Trough, pore water samples also show similar δ18O (0.0 to 5.0‰) and δ2H values (-5 to -39‰) to seawater, but in core A-14 a slightly increasing trend with depth in both δ18O and δ2H is observed, and Yang et al. [25] suggest a possible indication for gas hydrate occurrence in this site, although further work is needed to test it. SEEP CARBONATES AND THEIR CARBON AND OXYGEN ISOTOPES Due to high carbonate activity in pore waters related to methane oxidation reactions, authigenic carbonates may precipitate at gas hydrate deposits. These carbonates usually preserve a very negative carbon isotopic signature, indicating a gas hydrate related origin. In the northern continental slope of the South China Sea, cold seep authigenic carbonates have been found in all the three prospecting target areas (Xisha Trough, Shenhu, Dongsha). For example, authigenic carbonate crust and chimneys are found in the southeast Dongsha, offshore southwest Taiwan, with very negative δ13C values of -56.9 to -32.8‰, and δ18O values of 2.2 to 5.0‰ PDB [26]. In the southwest Dongsha area, dolomite and siderite nodules are found with δ13C values of -18.2 to -36.1‰, and δ18O values of 0.4 to 2.8‰ PDB [27]. Chimney-like carbonates dredged from the Dongsha area show the most negative δ13C values of -51.25 to -51.76‰ and highest δ18O values of 4.76 to 5.11‰ PDB [3]. In the Xisha Trough, seep carbonate crust composed mainly of aragonite show δ13C values of -13.3 to -  29.1‰, and δ18O values of 2.3 to 3.7‰ PDB [28]. These data suggest that the carbon in seep carbonates was derived from oxidization of methane of thermogenic or microbial origin that may relate to gas hydrates. DIC AND CARBON ISOTOPES Dissolved inorganic carbon (DIC) is a major carbon component in pore waters and its contents mainly depend on the fluid pH and are related to biogeochemical processes. The concentration and carbon isotopic composition of DIC have been used as good tracers for gas hydrate study in recent years [5,20,24,29]. For example, at Blake Ridge, very negative δ13C-DIC of -39‰ has been suggested to be related to strong sulfate reduction by anaerobic methane oxidation related to gas hydrate formation [20]. In the Shenhu area, carbon isotopic compositions of DIC are reported for two pistol cores with δ13C-DIC values of -9.1 to 20.0‰, a general downhole decrease of δ13C-DIC and increase of DIC concentrations are observed in the deeper sediment column of the cores. It is suggested that an anaerobic methane oxidation (AMO) process occurred in the sediments with large methane flux from depth in this area [5]. In the Xisha Trough, pore-waters from a pistol core (X-01) show rather constant δ13C-DIC value around -8.0‰ [29]. In contrast, pore-waters from a pistol core (D-01) at Dongsha area show very large variations in δ13C-DIC values from -3.7 to 28.8‰ and display good linear correlations between sulfate gradients and δ13C-DIC values, suggesting a genetic link to strong AMO process in this site [29]. CONCLUSIONS Geochemical methods are useful tools to study gas hydrate formation. In recent years, we conduct a detailed geochemical investigation of pore waters in the northern continent slope of the South China Sea, including anions (Cl-, Br-. I- SO42-), cations (K+, Na+, Ca2+, Mg2+, Sr2+, B3+, Li+, Ba2+, and NH4+), and isotopes (δ18O- and δ2H- pore water, δ13C-DIC, δ13C- and δ18O- Carbonate). The results show that geochemical anomalies occurred in three prospecting target areas including the Xisha Trough, Shenhu and Dongsha areas. In 2007, first deep drilling program for gas hydrate organized by China Geological Survey already found gas hydrates in the Shenhu area. The geochemical evidence indicates that the Dongsha area is also  one of the most promising target areas for gas hydrate occurrence in the South China Sea. ACKNOWLEDGEMENTS This study was financially supported by grants from National Science Foundation of China (40773029) and a Key Scientific Project of Investigation and Evaluation of Marine Gas Hydrate in China Seas (GZHZ200203-04-02) organized by China Geological Survey.  REFERENCES [1] Makogon YF, Holditch SA, Makogon TY. Natural gas-hydrates — A potential energy source for the 21st Century. Journal of Petroleum Science and Engineering 2007;56(1-3):14-31 [2] Wu N, Dong H, Suess E, Jiang H, Ye Y, Zhang C, Rowe H, Huang Y. Carbonate build-up found in the northeastern South China Sea. 2006 Western Pacific Geophysics Meeting, EOS Trans. AGU Abstract 2006;87:OS41F-01. [3] Chen DF, Huang YY, Yuan XL, Cathles III LM. Seep carbonates and preserved methane oxidizing archaea and sulfate reducing bacteria fossils suggest recent gas venting on the seafloor in the Northeastern South China Sea. Marine and Petroleum Geology 2005;22:613-621. [4] Wu S, Zhang G, Huang Y, Liang J, Wong HK. Gas hydrate occurrence on the continental slope of the northern South China Sea. Marine Petroleum Geology 2005;22:403-412. [5] Yang T, Jiang SY, Yang JH, Ge L, Wu NY, Liu J, Chen DH. Dissolved inorganic carbon (DIC) and its carbon isotopic composition in sediment pore waters from the Shenhu area, northern South China Sea. Journal of Oceanography 2008;64:303310. [6] McDonnell SL, Max MD, Cherkis NZ Czarnecki MF. Tectono-sedimentary controls on the likelihood of gas hydrate occurrence near Taiwan. Marine Petroleum Geology 2000;17:929936. [7] Jiang SY, Ling HF, Yang JH, Lu ZL, Chen DH, Ni P. Geochemical anomaly of marine gas hydrates from shallow sediments and pore waters. Marine Geology & Quaternary Geology 2003;23(1):87-94. [8] Lu Z, Wu B, Zhu Y. Preliminary discussion on origin and formation of potential gas hydrates in South China Sea. Mineral Deposits 2002;21(3), 232-239.  [9] Yao BC. Preliminary exploration of gas hydrate in the northern margin of the South China Sea. Marine Geology & Quaternary Geology 1998;18(4):11-18. [10] Ma ZT, Geng JH, Dong LG, Song HB. Seismic recognition studies on marine gas hydrates. Marine Geology & Quaternary Geology 2002;22: 1-8. [11] Zhang GX, Huang YY, Zhu YH, Wu BH. Prospect of gas hydrate resources in the South China Sea. Marine Geology & Quaternary Geology 2002;22(1): 75-81. [12] Wang HB, Zhang GX, Yang MZ, Liang JQ, Liang J, Zhong GJ. Structural circumstance of gas hydrate deposition in the continent margin, the South China Sea. Marine Geology & Quaternary Geology 2003;23(1): 81-86. [13] Jiang SY, Yang T, Ge L, Yang JH, Wu NY, Liu J, Chen DH. Geochemistry of pore waters in sediments of the Xisha Trough, northern South China Sea and their implications for gas hydrates. Journal of Oceanography 2008;64: 459-470. [14] Wang PX, Prell WL, Blum P. Proceeding of the Ocean Drilling Program, Initial Reports, South China Sea, College Station TX (Ocean Drilling Program), 1999;184:18-20. [15] Zhu Y, Huang Y, Matsumoto R, Wu B. 2003, Geochemical and stable isotopic compositions of pore fluids and authigenic siderite concretions from Site 1146, ODP Leg 184: Implications for gas hydrate. In: Prell WL, Wang P, Blum P, Rea DK, Clemens SC, editor. Proceeding Ocean Drilling Program Scientific Results 2003;184:1-15. [16] Wang JQ, Zhu YH, Wu BH, Fang NQ. Geochemistry of hydrocarbon gases from site 1146, ODP Leg 184, the South China Sea and their implications. Marine Geology & Quaternary Geology 2005;25(3): 53-60. [17] Borowski WS, Paull CK, Ussler W. Global and local variations of interstitial sulfate gradients in deep-water, continental margin sediments: sensitivity to underlying methane and gas hydrates. Marine Geology 1999;159:131–154. [18] Jiang SY, Yang T, Xue ZC, Yang JH, Ling HF, Wu NY, Huang YY, Liu J, Chen DH. Chlorine and sulfate concentrations in pore waters from marine sediments in the North Margin of South China Sea and their implications for gas hydrate exploration. Modern Geology 2005; 19(1):45-54. [19] Hesse R, Harrison WE. Gas hydrates (clathrates) causing pore-water freshening and oxygen-isotope fractionation in deep-water  sedimentary sections of terrigenous contienetal margins. Earth and Planetary Sciences Letters 1981; 55:453-462. [20] Hesse R. Pore water anomalies of submarine gas-hydrate zones as tool to assess hydrate abundance and distribution in the subsurface: What have we learned in the past decade? Earth Science Reviews 2003; 61:149-179. [21] Yang T, Jiang SY, Yang JH, Ge L, Ling HF, Wu NY, Chen DH. Anomaly of ammonia and phosphate concentration in pore waters: A potential geochemical indicator for prospecting marine gas hydrate. Modern Geology 2005; 19(1): 55-60. [22] Rodriguez NM, Paull CK, Borowski WS. Zonation of authigenic carbonates within gashydrate bearing sedimentary sections on the Blake Ridge: offshore southeastern North America. In: Paull CK, Matsumoto R, Wallace PJ, Dillon WP, editor. Proceeding Ocean Drilling Program Scientific Results 2000;164:301-312. [23] Mazurenko LL, Soloviev VA, Gardner JM, Ivanov MK. Gas hydrates in the Ginsburg and Yuma mud volcano sediments (Moroccan Margin): results of chemical and isotopic studies of pore water. Marine Geology 2003;195:201-210. [24] Borowski WS, Hoehler TM, Alperin MJ, Rodriguez NM, Paull CK. Significance of anaerobic methane oxidation in methane-rich sediments overlying the Blake Ridge gas hydrates. In: Paull CK, Matsumoto R, Wallace PJ, Dillon WP, editor. Proceeding Ocean Drilling Program, Scientific Results 2000; 164:87-99. [25] Yang T, Xue Z, Yang JH, Jiang SY. Oxygen and Hydrogen Isotopic Compositions of Pore Water from Marine Sediments in the Northern South China Sea. Acta Geoscientica Sinica 2003; 24(6):511-514. [26] Lu HF, Liu J, Chen F. Mineralogy and stable isotopic composition of authigenic carbonates in bottom sediments in the off shore area of southwest Taiwan, South China Sea: Evidence for gas hydrates occurrence. Earth Science Frontiers 2005;12( 3): 268-276. [27] Chen Z, Yan W, Chen M. Discovery of cold seep carbonates in the northern continental slope of the South China Sea: new evidence for gas seeping activity. Chinese Science Bulletin 2006;51 (9):1065-1072. [28] Chen Z, Huang C, Yan W, Chen M, Lu J, Wang S. Authigenic carbonates as evidence for seeping fluids in Xisha Trough of South China Sea.  Journal of Tropical Oceanography 2007;26(2): 2633. [29] Yang T, Jiang SY, Yang JH, Ge L, Wu NY, Zhang GX, Liu J. Comparison of carbon isotopic compositions of dissolved inorganic carbon (DIC) in pore waters in two sites of the South China Sea and significances for gas hydrate occurrence. Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.59278.1-0041007/manifest

Comment

Related Items