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

PRELIMINARY DISCUSSION ON GAS HYDRATE RESERVOIR SYSTEM OF SHENHU AREA, NORTH SLOPE OF SOUTH CHINA SEA Wu, Nengyou; Yang, Shengxiong; Zhang, Haiqi; Liang, Jinqiang; Wang, Hongbin; Su, Xin; Fu, Shaoying Jul 31, 2008

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  PRELIMINARY DISCUSSION ON GAS HYDRATE RESERVOIR SYSTEM OF SHENHU AREA, NORTH SLOPE OF SOUTH CHINA SEA   Nengyou WU 1 , Shengxiong YANG 1 , Haiqi ZHANG 2 , Jinqiang LIANG 1 , Hongbin WANG 1 , Xin SU 4 , and Shaoying FU 1  1. Guangzhou Marine Geological Survey,Guangzhou 510075, P. R. CHINA 2. China Geological Survey,Beijing 100022, P. R. CHINA 3. China University of Geosciences (Beijing), Beijing 100083, P. R. CHINA    ABSTRACT Gas hydrate is a very complicated reservoir system characterized of temperature, pressure, gas composition, pore-water geochemical features, and gas sources, gas hydrate distribution within the gas hydrate stability zone. Temperature, pressure and the gas composition of the sediments were suitable for gas hydrate formation in the gas hydrate reservoir system of Shenhu Area, north slope of South China Sea. The high-resolution seismic data and the gas hydrate drilling getting high concentrations of hydrate (>40%) in a disseminated form in foram-rich clay sediment showed that gas hydrate is distributed heterogeneously at all spatial scales in all drill holes, and the hydrate-bearing sediments ranged several ten meters in thickness are located in the lower part of gas hydrate stability zone (GHSZ), just above the bottom of gas hydrate stability zone (BGHSZ). It is likely seem that the methane to crystallize gas hydrate is from in-situ microbial methane.  Keywords: gas hydrate reservoir system, gas hydrate drilling, Shenhu Area, South China Sea    Corresponding author: Phone: +86 20 87623546 Fax +86 20 87765102 E-mail: INTRODUCTION Gas hydrate is a kind of ice-like solid substance formed by the combination of certain low-molecular-weight gases such as methane, ethane, and carbon dioxide with water. Gas hydrate mainly occurs naturally in sediments beneath the permafrost and the sediments of the continental slope with the water depth greater than 300 m. The reason why marine gas hydrate geological systems are important to the economic society and environments, is not only that gas hydrate in some places may be concentrated enough to be an economically viable fossil fuel resources, but also gas hydrate can cause the geo- hazards through the large-scale slope destabilization [1] and can release methane, a potential greenhouse gas, to impact the global change [2-7]. Gas hydrate and its associated sediments have also become an important focus for biogeochemical study of the deep biosphere [8]. The stability of gas hydrate fundamentally depends on four factors of temperature, pressure, gas composition, and pore water composition. The nucleation and growth of gas hydrate also depend on sediment grain size, shape, and minerals [9]. These factors, which control gas hydrate formation and stability, are affected by a series of physical and chemical processes in the marine sediments, resulted in a variation of gas hydrate dynamics on different timescale [2, 5, 10-13]. Therefore, the gas hydrate is not continuously distributed all over the Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008. world in the vertical and horizontal scales [14-16]. Because the geological setting and the parameters controlling the gas hydrate formation are different in various locations, scientists established the different geological models based on the formation mechanisms, gas sources and dynamics. Gas hydrate deposits can be classified into two end- member regimes of the high-flux gas hydrate deposit, and low-flux gas hydrate deposit, based on the mechanisms that control gas transport into the gas hydrate stability zone, although both regimes operate simultaneously in many regions [13]. Chen et al. (2006) grouped the hydrates in marine environment into two categories, diffusion gas hydrates and vent gas hydrate [17]. The diffusion gas hydrates occur widely in an area with bottom simulation reflector (BSR) recorded in seismic profiles, and is a thermodynamic equilibrium system of hydrates and water with dissolved methane within gas hydrate stability zone (GHSZ). The hydrates are buried in a distance apart from the seafloor and are characterized by low concentrations. The vent gas hydrates occur in an area where gas vents out of the seafloor. It is a thermodynamic disequilibrium system of hydrate, water and free gas, occurs in a zone that extends from the base of GHSZ to the seafloor, and is characterized by high concentration [18-19]. However, these concept models should be verified by the actual evidences of all over the world. Scientists have begun to integrated research gas hydrate reservoir system, which is the intrinsic relationship among gas hydrate source, gas transport and gas hydrate formation [15, 20].  But its definition was not given yet, and it just mainly means the balance system with free gas and water, organic ecosystem with methane oxidation and sulfate reduction in deep ocean conditions [21], or the restriction of geological factor to the gas hydrate formation in suitable temperature and pressure condition [15]. Since 2001, the geological, geophysical and geochemical investigations for gas hydrate in the north slope of South China Sea have been carried out and the evidences for gas hydrate’s occurrence have preliminarily suggested the great promise for gas hydrates. Shenhu Area is near southeast of Shenhu Underwater Sandy Bench in the middle of north slope of South China Sea, between Xisha Trough and Dongsha Islands.  The research area is located in the Zhu Ⅱ Depression, Pearl River Mouth Basin (Figure 1), where is in the process of the tectonic subsidence since the middle Miocene, to form good geological conditions for gas hydrate reservoir. Recent years, we have been carried out the geological, geophysical investigation of gas hydrate in this area. In order to determine the nature and distribution of gas hydrate, a gas hydrate drilling expedition bas been initiated by using M/V Bavenit along with specialized Fugro& Geotek equipments in Shenhu Area in 18 April – 11 June of 2007. Mainly from hydrate drilling results, we preliminary discussed the gas hydrate reservoir system of Shenhu Area, north slope of South China Sea, based on the basic formation conditions, the pore-water geochemical features of shallow sediments and their inflected gas sources, gas hydrate distribution and seismic characteristics in this paper. Figure 1  Location of research area, Shenhu Area, north slope of South China Sea   CONDITIONS FOR GAS HYDRATE FOR- MATION Temperature and pressure According to in-situ measurement of the geothermal gradient and the thermal conductivity of 19 sites sediment samples, the heat flow in Shenhu Area ranges from 74.0 mW·m -2 to 78.0 mW·m -2 , with an average of 76.2 mW·m -2 . The geothermal gradient ranges from 45 ℃/km to 67.7℃/km in the research area by in-situ temperature measurement of 5 drilling sites.  In general, the heat flow and geothermal gradient in research area are in middle-low range in the South China Sea. These indicated that the research area is rather optimistic for gas hydrate formation. The depth and thickness of gas hydrate distribution is controlled by the temperature and pressure.  We calculated the temperature and pressure of 76 BSR (Bottom Simulating Reflector) points in 5 BSR seismic sections. The results showed that the temperature commonly ranges from 15℃to 25℃, and the pressure ranges from 12 MPa to 18MPa of the BSR in researched area, so the area is in suitable temperature and pressure conditions for gas hydrate formation. Gas composition Table1 shows the gas composition of core sediments in SH2 drilling site by using the GC (gas chromatogram) analysis. The content of methane is 96.10-99.82%, and the content of ethane and propane is very low, suggesting that gas hydrate in sediments is the type I gas hydrate.  Table 1  Gas composition in SH2 site sediments of Shenhu Area No. Depth (mbsf) Gas composition C1/C2 CH4 (wt%) C2H6 (ppm) C3H8 (ppm) 1 70.73 96.10 440 LD 2185 2 73.31 96.72 477 LD 2027 3 146.46 98.49 646 97 1524 4 146.89 98.02 632 88 1551 5 150.00 99.53 552 67 1804 7 181.65 99.36 675 65 1473 8 182.07 97.75 533 LD 1833 9 185.39 99.21 581 61 1708 10 185.54 99.32 824 53 1205 11 195.40 99.32 1611 LD 616 12 197.50 99.82 1737 LD 575 13 221.71 99.68 1559 LD 639 14 228.00 99.70 1653 LD 603 15 238.58 99.06 1346 LD 736 Note: mbsf means meter below the seafloor; LD means lower than detection limit; C1/C2 is the molecule ratio of methane to ethane.  PORE-WATER GEOCHEMICAL FEA- TURES AND GAS SOURCES When gas hydrate is distributed in the marine sediments, the gas hydrate related gas diffusion or transport, geochemical processes and biogeochemical processes in sediments would result in some features above the gas hydrate- bearing sediments: (1) The concentration of Cl -  is quickly decreased with the depth increasing, and accompanied with heavier 18O and D value in the pore-water of sediments [22-23];  (2) The concentration of sulfate is sharply decreased in pore-water, and the depth of SMI (sulfate-methane interface) is shallow [24-25];  (3) The concentrations of Ca 2+、Mg2+、Sr2+ are obviously decreased, the ratios of Mg 2+ /Ca 2+、Sr2+/Ca2+ are sharply increased, and the concentration of Ba 2+ is increased with the depth increasing [25]. The concentrations of sediments pore-water ions in the core sediments of 337、338、373、 381、389、500 (large gravity piston sampling site) are listed in the Table 2.  As the depth increasing, the concentration of Cl - is quickly decreased, the concentration of SO4 2- is sharply decreased in pore-water, the concentration of CH4 has the tendency of slow increasing, the depth of SMI is shallow, the concentrations  of Ca 2+、Mg2+ 、 Sr2+ are obviously decreased, the ratios of Mg 2+ /Ca 2+ , Sr 2+ /Ca 2+  are sharply increased, and the concentration of Ba 2+ is obviously increased (Figure 2).  All of the characteristics strongly indicate that gas hydrate occurs in the deep sediments.                 Figure 2 Characteristics of pore-water ions concentrations of 381 site’s sediments  Depth of SMI (Table 2) in research area was figured out depending on the variation trend of SO4 2-  and CH4concentrations in pore-water of sediments collecting from SH1、SH2、SH3、 SH5、SH7 drilling cores and large gravity piston cores in the same area.  It is obviously that the depth of SMI is different at site to site from 10 to 27mbsf, and the sediments’ SMI depth of drilling 2 4 6 8 10 12 C a 2+(m M ) 900 800 700 600 500 400 300 200 100 0 D e p th (c m ) 40 44 48 52 56 M g 2+(m M ) 900 800 700 600 500 400 300 200 100 0 60 80 100 Sr2+(M ) 900 800 700 600 500 400 300 200 100 0 0 1 2 3 4 5 6 Ba 2+(M ) 900 800 700 600 500 400 300 200 100 0 480 560 640 720 B 3+(m M ) 900 800 700 600 500 400 300 200 100 0 520 560 C l -(m M ) 900 800 700 600 500 400 300 200 100 0 D e p th (c m ) 0 10 20 30 SO 4 2-(m M ) 900 800 700 600 500 400 300 200 100 0 4 6 8 10 12 14 M g 2+/C a 2+ 900 800 700 600 500 400 300 200 100 0 8 12 16 20 24 Sr2+(M )/C a 2+(m M ) 900 800 700 600 500 400 300 200 100 0 5 10 15 20 25 30 C H 4 ( l/kg ) 900 800 700 600 500 400 300 200 100 0 site is deeper than the SMI depth of large gravity piston sampling site (Table 2).  Table 2 Depth of SMI figured out based on the variation trend of SO4 2-  and CH4 concentrations in sediments pore-water Site Geographical position WD( m) SMI( mbsf) Latitude (°N) Longitude (°E) 337 19°53.77’  115°08.49’ 1252 11.6 338 19°54.40’ 115°08.78’ 1121 14.2 373 19°51.28’  115°12.09’  1402 10.6 381 19°50.45’ 115°12.60’ 1442 10.3 389 19°51.48’ 115°13.31’ 1380 13.9 500 19°52.92’ 115°09.65’  1230 22.3 SH1 19°52.55’ 115°09.82’ 1262 27.0 SH2 19°52.82’ 115°09.20’ 1230 26.0 SH3 19°52.94’ 115°09.81 1245 27.0 SH5 19°51.80’ 115°13.13’ 1423 21.0 SH7 19°54.40’ 115°09.08’ 1105   17.0 Note: SMI represents sulfate-methane interface; mbsf means meter below the seafloor; WD means water depth.  There are two gas sources to form gas hydrate, which is biogenic gas and thermogenic gas. Biogenic methane gas is from anywhere organic matter and micropopulation occurs [26]. Heterogeneously methanogenesis is microbial action by consuming methane [27-30]. AOM (Anaerobic Methane Oxidation) of sulfate is the main reason of methane consuming zone (non- methane) occurrence from the seafloor to certain depth below the seafloor in the marine sediments. And the depth can be hundreds meter in special conditions. The thickness of methane consuming zone, which is the same as the bottom of sulfate consuming zone, is a significant indication for methane flux in diffusion pattern gas hydrate [24- 25].  However, gas hydrate was also found on the seafloor or near the seafloor [24-25, 31-33].  The formation and stability of gas hydrate on seafloor surface needs fast methane fluid and geochemical property that thermogenic gas from the deeper position below the seafloor occurs in many areas [13]. Therefore, the big depth of SMI in Shenhu Area, north slope of South China Sea, indicated that the vertical flux of methane gas forming gas hydrate is minor, and the methane gas sources is biogenic methane gas, but not thermogenic gas in deep position.  GAS HYDRATE DISTRIBUTION BSR is the main response of gas hydrate on seismic section, and is also one of important parameters for researching gas hydrate distribution.  Most of BSRs are oblique to the sediment layers in the research area.  And the amplitude and the continuity shows that the amplitude of BSR does not change too much, strong and moderate amplitude is dominated, and moderate-strong amplitude reflects the big difference of the physical property (velocity and density) between the above and below sediment layers of BSR, difference of wave impedance is much big, and it is presumed that the main reason is there is abundant of free gas below the BSR. Except few BSRs show weak continuity, and the others all show strong or moderate-strong continuity.  That reflects the physical property (velocity and density) between the above and below sediment layers of BSR is comparative stability in horizontal distribution.  Meanwhile, continuous distributed BSR with moderate-strong amplitude features indicates that gas hydrate (or free gas) is high abundance in research area, and horizontal distribution is steady (Figure 3).  We found that most polarity reversal BSR wave patterns have a pair of peaks, just like amplitude trough-crest of wave, and just few weak continuous BSR wave patterns have indistinct polarity, depending on wave pattern composition, polarity feature, and seismic waveform section.  In addition, the gas hydrate distribution is reflected by the seismic attribution anomalies, including seismic velocity anomaly (Figure 3), wave impedance, AVO, instantaneous amplitude, instantaneous phase, instantaneous frequency and so on, in the BSR and the stratify above the BSR of the research area.  Figure 3  The comparison between stack velocity anomaly of Trace900 in Shenhu Area Line130 and the BSR position of seismic section  From horizontal analysis, that can be known BSR distributes as different size of block in research area, and the extended length of BSR is different in different blocks, the normal extended length ranges from 1.5 to 2.5 km. BSR is always below the seafloor 150~350 m for the vertical distribution depth of BSR. It was discovered that almost all the upper part of BSR developed the weak amplitude or amplitude black zone by analyzing the details of the seismic section of research area. Further more, there are characteristics as fellow: ①The development of the black zone is heterogeneously, and developping disparity of different BSR black zone is large in the same block and the same survey line. The black zone displaies as lumpy or intrabed black (Figure 3).  That reflects the occurence characteristics of gas hydrate;  ②The appearance of visible black zone has corresponding relationship with stong amplitude and ultra- continued BSR. During April to June of 2007, China Geological Survey first organized and implemented the gas hydrate drilling expedition, and collected SH2, SH3 and SH7 three sites gas hydrate bearing sediment samples in research area [34].  The drilling, wireline well logging, coring, in-situ temperature measuring, pore-water collecting, in-situ test analysis and so on all indicated that gas hydrate-bearing zone in research area was situated below the seafloor 153- 225m, the thickness is about 18-34m, and the gas hydrate saturation can be as high as 48%. The depth and thickness of gas hydrate-bearing sediments is different in different sites, but there is one common feature of gas hydrate distribution: (1) In vertical, the depth of gas hydrate occurrence is  deeper, and gas hydrate-bearing sediments zone is just right on certain depth above the BSR.  (2)In horizontal, not all the BSR blocks have gas hydrate, but the gas hydrate was drilled and discovered in all the blocks with high BSR reflection feature.  (3) The gas hydrate-bearing sediments are composed with fine grained foraminiferal clay or sand clay.  Meanwhile, ultra-saturation gas hydrate even and decentralized distributes in the sediments.  DISCUSSION AND CONCLUSION Gas hydrate is a very complicated reservoir system.  Tréhu et al. (2006) summed up the main experiences how to get gas hydrate by scientific ocean drilling which is recently implemented by the ODP, IODP, DOE and JIP [13].  Gas hydrate can be classified into two categories, FHF (Focused, High-Flux Gas Hydrate) and DLF (Distributed, Low-Flux Gas Hydrate), according to the mechanism how to control the gas enter the gas hydrate stability zone [13].  FHF system always forms massive gas hydrate and locates near the seafloor, and DLF system forms decentralized gas hydrate and locates in deep position of sediments. Deeper SMI depth of sediments represents that the vertical flux of methane fluid is lower in research area.  It can be known that methane gas forming gas hydrate is probably from in-situ micro-biogenic methane, and the highest content of gas hydrate is around the bottom of gas hydrate stability zone (BGHSZ, near BSR), but the content is always relatively lower (pore space ranges from several percent to twenties percent), according to the gas hydrate distribution features.  This is similar to the gas hydrate distribution features of the Blake Ridge and Hydrate Ridge [14, 23]. However, fluid convection is still key factor for migrating the methane-bearing fluid to gas hydrate stability zone, only when the in-situ micro- biogenic methane gas production is more than our general thought.  This fluid diffusion is very low, and it’s not fast enough to form thick gas hydrate in reasonable geologic time scale.        The research area has the features of stable tectonic setting, relatively even fine grained sediments, fine grained foraminiferal clay or sand clay preference, and lower penetration rate. Therefore, the gas hydrate probably belongs to DLF (Distributed, Low-Flux Gas Hydrate) reservoir system in Shenhu Area, north slop of South China Sea. 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