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Marine Geology 366 (2015) 69–78 Contents lists available at ScienceDirect Marine Geology journal homepage: www.elsevier.com/locate/margeo A seepage gas hydrate system in northern South China Sea: Seismic and well log interpretations Zhibin Sha a,b,⁎, Jinqiang Liang a, Guangxue Zhang a, Shengxiong Yang a, Jingan Lu a, Zijian Zhang c, Daniel R McConnell c, Gary Humphrey c a b c MLR Key Laboratory of Marine Mineral Resources, Guangzhou Marine Geological Survey, Guangzhou 510075, China Faculty of Resources, China University of Geosciences, Wuhan 430074, China Fugro GeoConsulting, Inc., Houston 77081, USA a r t i c l e i n f o Article history: Received 12 September 2014 Received in revised form 30 March 2015 Accepted 25 April 2015 Available online 27 April 2015 Keywords: Gas hydrate Fluid flow Seismic interpretation Well log interpretation South China Sea a b s t r a c t The second gas hydrate expedition of the Guangzhou Marine Geological Survey drilled 13 sites and recovered a large amount of gas hydrate in the east of the Pearl River Mouth basin of the South China Sea In this study, we examine three of these sites (sites 05, 08, and 16) using multichannel seismic data, LWD logging data, and multi-beam echo sounder data The geophysical data suggest and core samples confirm that gas hydrates occur near the BSR at all three sites, whereas near-surface gas hydrates exist at sites 08 and 16 with high fluid flux We present a conceptual model for the formation and accumulation of gas hydrates in our study area, controlled by the activity of dissolved and free gas-rich fluid Interpreted abundant gas sources below the BSR contribute to the generation of overpressure, resulting in the movement of gas-rich fluid Dissolved gas-rich fluid forms pore filling hydrates near the BSR, and the relatively deep hydrate layer thickens over time by making its way through the growing hydrate layer and adding to the top, if the fluid is supplied sufficiently Free gas-rich fluid is more favorable to generate near-surface nodules or massive hydrates, which could be stable in natural conditions As sediments deposit, the relatively shallow hydrates are buried deeper and keep their morphologies Furthermore, gas hydrate formation and stability imply that gas hydrates may be increasingly considered as a potential energy resource © 2015 Elsevier B.V All rights reserved Introduction A gas hydrate is an ice-like solid composed of water molecules encasing methane and other gases Because the quantity of gas within hydrate-bearing sediments is substantial, naturally occurring gas hydrates represent a potential energy resource (Kvenvolden, 1993; Boswell, 2009) Gas hydrates are stable under the temperature and pressure conditions typical of water depths greater than 500 m in oceanic sediments along the continental margins In the South China Sea, gas hydrates were found by the Guangzhou Marine Geologic Survey (GMGS) in 2007 (Zhang et al., 2007) In 2013, GMGS once again conducted a drilling program that targeted the gas hydrate deposits in the northern South China Sea A total of 23 holes were drilled at 13 sites (G Zhang et al., 2014a; Fig 1) At all of these sites, a pilot logging while drilling (LWD) or wireline logging was conducted At sites with good logging indicators of potential gas hydrate occurrences, coring was performed The logging data and core analysis indicate the occurrences of gas hydrate from ⁎ Corresponding author at: MLR Key Laboratory of Marine Mineral Resources, Guangzhou Marine Geological Survey, Guangzhou 510075, China E-mail address: shazb2008@163.com (Z Sha) http://dx.doi.org/10.1016/j.margeo.2015.04.006 0025-3227/© 2015 Elsevier B.V All rights reserved approximately m to 220 m below mudline (BML) Gas hydrate occurs as solid nodules, disseminated within pore spaces of sediments and fracture filling in veins (G Zhang et al., 2014a,b; Yang et al., 2014) Bottom simulating reflector (BSR) on seismic profiles has been used to interpret the occurrence of gas hydrates for approximately 30 years, although more recently, it is common to use BSRs primarily to confirm the presence and extent of gas hydrate stability (Shipley et al., 1979) A continuous BSR is a seismic event that is parallel to the seafloor with opposite polarity and cut lithological reflectors A dis-continuous BSR consists of several high-amplitude bright-spots lining up to the seafloor (McConnell and Kendall, 2002; Shedd et al., 2012) Many studies have explained that the presence of the BSR is associated with hydrates and free gas (Singh et al., 1993; Mackay et al., 1994; Lu and McMechan, 2002; Zhang and McMechan, 2006) However, without logging and sampling data to validate interpretations and estimates based on the seismic profiles, it is difficult and uncertain to determine the amount of gas hydrate, investigate the formation of gas hydrate, and assess its geotechnical impact to seafloor stability and near-surface sediments In this paper, we interpret seismic and LWD data to analyze the distribution and formation of gas hydrates with calibration of core samples The three sites used in this investigation are sites 05, 08, and 16 (Fig 1) Drilling results allow us to better understand the gas hydrate formation, 70 Z Sha et al / Marine Geology 366 (2015) 69–78 Fig Bathymetric map showing the seafloor rendering of the study area, marked as a red block in the inset local map Red indicates shallow water, whereas blue indicates deep water Notice that sites 05 and 08 lie on a smoother ridge than does site 16 (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) dissolved and free gas migration, and fluid expulsion in marine We document a larger amount of gas hydrate with variant morphology in the South China Sea than previously studied Regional setting This study area focuses on the east of the Pearl River Mouth basin of the South China Sea, where the passive continental margin is typically characterized by Neogene extensional tectonics that form ENE–WSW trending normal faults (Pautot et al., 1986; Taylor and Hayes, 1980) Several NW–SE trending submarine canyons have developed in the shelf edge down to the lower slope since Pleistocene The canyons transported large amounts of sediments from the continental shelf into the slope and rise, where submarine landslides occur The predominant sedimentary facies are fine-grained marine clays with various sand-rich turbidite sediments that were carried by gravity flows along submarine canyon and channel systems Authigenic carbonate accumulations and cold seeps have been observed at many places, while BSRs have been identified in the study area (Suess, 2005) More detailed descriptions of structure and sediments with geological feature maps and profiles in the study area were presented by Yi et al (2007), Huang et al (2008), and G Zhang et al (2014c) GMGS II gas hydrate expedition 3.1 Project overview The GMGS II gas hydrate expedition took place on the M/V REM Etive between May and September 2013 and included LWD, wireline logging, and sampling The wireline logging or LWD pilot holes were first drilled in 13 sites Resistivity and velocity logs were used to identify gas hydrate (G Zhang et al., 2014b) The 13 sites can be separated into two groups: sites containing slightly high resistivity and/or velocity anomalies, and sites containing significantly high anomalies From these sites, of them were sampled by using Fugro coring tools and all contain gas hydrate (G Zhang et al., 2014a,b; Yang et al., 2014; Figs and 3) Wireline coring tools consisted of rotary and no-rotary corers for different lithologies, which include three non-pressurized cores (Fugro Hydraulic Piston Corer (FHPC), Fugro Corer (FC), and Fugro Marine Core Barrel (FMCB)) and two pressure corers (Fugro pressure corer (FPC) and Aumann pressure core tool with ball valve (PCTB)) (Yang et al., 2014) The pressure coring tools were designed to recover gas hydrate samples at in-situ pressures The objectives of the coring were to sample target layers or representative lithologies with and without gas hydrate to determine the concentration, nature, and distribution of gas hydrate, to recover cores containing gas hydrate for description and sample storage, and to study the lithology of study area All non-pressure cores were logged using Multi Sensor Core Loggers (MSCLs), which include infrared scanning to look for early signs of gas hydrate dissociation Other core loggings on whole cores are comprised of a variety of geophysical parameters and X-ray imaging (Yang et al., 2014) Pressure cores were processed using the Pressure Core Analysis and Transfer System (PCATS) This device enables the pressure cores to be transferred from the pressure core autoclaves into a core logging chamber and cut into subsamples at full pressure (Yang et al., 2014) Thus X-ray imaging, density and P-wave velocity can be measured at Z Sha et al / Marine Geology 366 (2015) 69–78 full pressure as well During the course of this expedition subsamples were taken for controlled depressurization to accurately determine gas hydrate concentrations 71 Geophysical identifications and interpretations of gas hydrate and gas layers 4.1 Site 05 3.2 Gas hydrate distribution and morphology Gas hydrate-bearing sediments identified from the coring include two main morphologies: invisible pore-filling gas hydrates and visible fracture filling, nodules, and massive gas hydrates (Figs and 3) Disseminated, or called pore-filling, gas hydrate in deep fine-grained sediments was identified in sites 05, 09 and 16, while disseminated gas hydrate within coarse-grained sediment was found in site 16 (Fig 3) This gas hydrate morphology is invisible to the naked eye but was found to occur in very high concentrations Gas hydrate saturations above the BSR in site 05 were calculated from porewater freshening at 23–31% of pore volume The highest gas hydrate saturation occurs at site 16 with up to 50% of pore space (Yang et al., 2014) In contract to ODP 164 (Paull et al., 2000), IODP 204 (Tréhu et al., 2006a, 2006b), IODP 311 (Riedel et al., 2006) and others, highly concentrated finegrained gas hydrate is relatively unusual However, it was found during the 2007 GMGS I expedition in the Shenhu area of South China Sea (Zhang et al., 2007) Fractured filling gas hydrate was found in fine-grained sediments at sites 08 and 16 (Fig 2) These fracture filling structures can be clearly observed from X-rays imaging measured in pressure cores Gas hydrate concentrations are highly variable in this morphology, but were measured at up to 40% of core volume Massive and nodule gas hydrate was found at sites 07, 08 and 09 (Figs and 3) In site 08, the massive gas hydrates were well preserved in pressure cores, whereas gas hydrates were recovered as remnant pieces of massive hydrate in non-pressure cores sites 07 and 09 These forms of gas hydrate are essentially considered as 100% pure hydrate Within the gas hydrate stability zone (GHSZ), the disseminated gas hydrate shows in deeper sections greater than 90 m near the BSR in sites 05 and 16, whereas massive and nodule gas hydrate occurs in shallower sections less than 90 m in sites 08 and 16 (Fig 3) These three sites were selected as the main focus of our study because sites 05 and 08 represent two typical types of gas hydrate, and site 16 is a reference site which contains the two types and was cored continuously to obtain an almost complete sedimentary record Site 05 is located at a water depth of 1127 m in the flank of a seafloor ridge in the center of the study area (Fig 1) Seismic characters are shown in Fig A BSR delineates the base of gas hydrate stability Above the BSR, seismic facies are characterized by a low-to-moderate amplitude, high continuity reflection configuration indicating overlap/ onlap canyon fill sequences The canyon fill is interpreted to be predominantly clays Below the BSR, a chaotic reflection configuration is inferred to gassy sediments Faults and factures usually allow for rapid migration of fluid along with the gas from the depths (Heggland, 1997; Roberts et al., 2006) However, there are no obvious faults near the site High seismic amplitude can be generated by a change of acoustic impedance of sediments when fluid is replaced with solid gas hydrate With gas hydrate accumulations, an increase of the acoustic impedance is expected at the top of the hydrate filling layer, while a decrease would occur at the bottom If fluid comes with the gas, especially if the gas dissolves or gas bubbles dissipate within the fluid, some portion of the acoustic energies scatter and are absorbed (Judd and Hovland, 2007; Z Zhang et al., 2014) Therefore, low amplitude, chaotic reflections are present at the gas-bearing interval below the hydrate accumulations The downhole recorded gamma ray log data from the seafloor to 250 m BML is characterized by generally consistent values of approximately 80–90 API, which suggest clay rich sediments The most striking feature in the log data recorded in site 05 is a 13.5-m-thick interval with a high resistivity and velocity between 193 m and 206.5 m BML, which was interpreted as a significant occurrence of gas hydrate (Fig 5) The logging record is consistent with the high amplitude seismic reflection in the interval Gas hydrate occurrence is also inferred from analyses of no-pressurized core samples (Yang et al., 2014) Gas hydrate saturations calculated from porewater freshening of core samples, based on chlorinity as a conservative species, estimate approximately 25–30% of hydrate filling within the pore space in the interval Another interesting feature is a slightly low velocity and higher resistivity interval below the BSR between 213 m and 247 m BML, suggesting possible gas below the BSR (Fig 5) Sonic data not show dramatic velocity decreases; thus, it is inferred that only small amounts of free gas, if any, could be present at the site Vertical seismic profile is an effective tool to detect free gas Fig Examples of gas hydrate illustrate pore filling gas hydrate (A), fracture filling gas hydrate (B, C and D), nodules gas hydrate (E and F), and massive gas hydrate (H to G) The first sample at A was recovered from site 16 and the other samples from B to G were recovered from site 72 Z Sha et al / Marine Geology 366 (2015) 69–78 Fig Profile containing sites indicates zones of hydrate occurrence or well log anomalies, different morphologies of gas hydrate, and different average saturations estimated from relative freshening of the pore water below the BSR (Tréhu et al., 2006a, 2006b) However, it was not conducted in our program 4.2 Site 16 Site 16 was selected to test hydrate occurrences in a topographic high on the eastern ridge (Fig 1) The top of the high is approximately 100 m wide and 250 m long at a water depth of 871 m The distinctive seismic feature near site 16 is a continuous BSR (Fig 6) Just above the BSR, reflections show high amplitudes Seismic pull-downs may affect the reflections below the BSR Relatively low-interval-velocity free gas layers are inferred At site 16, gas-charged sediments are inferred to extend up to 320 m below the BSR At site 05, this depth was much less (120 m) Above the BSR, the reflection pattern consists of a low amplitude mound in the upper section and relatively higher amplitude continuous reflectors in the lower section (Fig 6) The downhole recorded gamma ray log data from the seafloor to 230 m BML suggests clay-dominated sediments The resistivity log indicates two zones with high resistivity anomalies (Fig 7) The upper zone is located at 10 to 23 m BML, with high resistivities of 14–24 Ω ∗ m compared to the background resistivity of 1.2 Ω ∗ m The velocity increases from 1505 to 1640 m/s in the zone Core samples show nodular or fracture-filling gas hydrates in this interval (Fig 3, Yang et al., 2014; G Zhang et al., 2014c) The lower zone is located at 191 to 230 m BML with high resistivities of 2.5–3 Ω ∗ m The velocity increases from 1608 to 1937 m/s between 193 and 203 m BML, whereas it decreases to 1500 m/s from 203 to 211 m BML In contrast to site 05, the higher velocity and resistivity suggest higher hydrate saturations in site 16, which is supported by the porewater analysis of core samples The hydrate saturations from porewater freshening are estimated to be up to 50% of pore space near 200 m BML This is consistent with the thicker gas layer interpreted from seismic data, which indicates sufficient gas supply The velocity decreases from 1650 to 1500 m/s, also indicating that free gas could occur within the gas hydrate layer 4.3 Site 08 Site 08 characterizes an interpreted buried relict venting system Fig 8a shows the seismic section crossing the site The BSR at the site is not evident; however, it can be recognized to both sides of the site by high-amplitude events Above the high-amplitude events, there is a convex-upward shape on the seismic section The sediments above the buried mound have characteristic moderate-amplitude, continuous reflections, indicating the presence of clay-prone sediments near the surface Recovered samples indicate that the buried mud mound is covered by carbonate on the top In the well logs of site 08, the resistivity, sonic, and porosity responses indicate significant hydrate occurrence in three intervals: (1) 9–22 m, in which hydrate occupies fine-grained sediments with Z Sha et al / Marine Geology 366 (2015) 69–78 73 Fig Seismic section through site 05 (Fig 1), showing high continuity reflectors above the BSR, high-amplitude reflectors at the BSR, and chaotic reflectors below the BSR The highamplitude event corresponds to high resistivity and velocity in LWD logs, which is indicative of gas hydrates high resistivity up to 17 Ω-m, (2) 58–62 m, in which hydrate is inferred to occur within carbonate with high velocity, high resistivity and low porosity, (3) 65–98 m, in which extreme high resistivity exceeding 1000 Ω ∗ m suggests the occurrence of thick gas hydrates (Fig 8b) Occurrence of gas hydrate as a fracture fill was observed in cores taken from this interval (Fig 3) Discussion 5.1 Possible driving mechanism of fluid flow: shallow overpressure Overpressure is fluid pressures in excess of hydrostatic pressure The lateral flow transfers fluid pressure from thick overlying sediments Fig Crossplot of velocity and resistivity at site 05, showing the deep hydrate zone and dissolved gas High resistivity and velocity between 195 m and 201 m BML are caused by the presence of gas hydrate Slightly high resistivity and low velocity between 224 and 237 m are interpreted as dissolved gassy clays 74 Z Sha et al / Marine Geology 366 (2015) 69–78 Fig Seismic section through site 16 (Fig 1), illustrating that gas is sufficiently supplied below the BSR and possible fluid migration paths Seismic characteristics, such as chaotic reflectors, high amplitude, and pull down, indicate thick gas accumulations between 1.4 and 1.82 s TWT (approximately 350 m) Fluid could migrate upwards along slumps or discontinuous updip reflectors A landslide deposit is interpreted near the seafloor where overburden pressure is high to thin overlying sediments where overburden pressure is low; therefore, overpressure often occurs at the continental slope (Dugan and Flemings, 2000) On a small scale, the overpressure is generated in the vertical direction, if gas is present in the fluid, due to gas buoyancy and gas bubble expansion (Hunt, 1979; Sahagian and Proussevitch, 1992) The overpressure generated Fig Crossplot of velocity and resistivity shows shallow and deep hydrate zones at site 16 Compared to water clays between 26 and 80 m BML, the shallow hydrate zone between 10 and 23 m BML has higher resistivity and velocity The deep hydrate zone between 193 and 203 m BML has water clays above it and gassy clays below, which are distinctly differentiated from each other Z Sha et al / Marine Geology 366 (2015) 69–78 75 Fig a: Site 08 (Fig 1) is interpreted to penetrate through a buried mound in the seismic section Notice that the mound appears as a dome shape, and similar reflections are observed on both sides of the mound Thick dashed lines indicate possible faults or fractures b: Summary of the LWD logs of site 08; shallow and deep hydrate zones are shown in the log data Note that a significant high anomaly of neutron porosity shown between 65 and 98 m BML indicates highly concentrated gas hydrate or massive hydrate Because gas hydrate and pure water have similar hydrogen index values, neutron porosity consists of water saturation and hydrate concentration (Collett and Lee, 2011) Thus, the higher neutron porosity than the normal compaction trend, the higher hydrate concentration from fluid movements is applicable to our study area, where gassy fluid has widely taken place as discovered by analyzing seismic and LWD data Submarine landslide deposit is found along the southern flank in the western ridge and others (Fig 1) The most common conditions generating submarine landsides are a significantly inclined seafloor, a rapid sedimentary rate and faulting (Schwab et al., 1991; Hampton et al., 1996; Locat, 2001) For example, Fig shows a prominent erosional escarpment trending from northwest to southeast with vertical relief up to roughly 100 m and seafloor gradients locally exceeding 20° in the eastern ridge The area below the escarpment shows a distinctly more rugged appearance interpreted as landslide deposits, whereas the topography above the escarpment is the top of the ridge (Figs and 6) In addition to these factors, shallow overpressure was also suggested as a mechanism of slope failure (Orange and Breen, 1992; Orange et al., 1997) It drives fluids along permeability pathways and lowers the effective stress of the sediment, thus resulting in sediment failure High-amplitude, continuous reflections cut by the escarpment are inferred to be the permeability pathways (Fig 6) The seafloor image shows that the top of the western ridge is smoother and wider than the eastern ridge (Fig 1) This geomorphic expression suggests that the western ridge is less active and has not recently failed The lack of recent failure in the western ridge may have resulted from an increase in sediment strength due to relatively compacted and cemented sediments exposed at the seafloor and a decrease in the overpressure due to a decrease in the fluid flux The seafloor morphology appears not to be affected by the presence of hydrates In contrast, gas hydrates would intensify the sediment strength This is constituent with the findings of Trehu et al (1999) in the Hydrate Ridge However, many studies indicate that gas hydrate disassociation could result in numerous slope failures worldwide (Kayen and Lee, 1991; Bouriak et al., 2000; Weaver et al., 2000; Sultan et al., 2004) 5.2 Gas hydrates and fluid flow Gas hydrates were recovered in sites 05, 08, and 16, along with others, by conventional and Fugro pressure core samplers (Figs and 3) They show different habits in the core samples, including nodules, laminar veins, lenses, and fractures filled within fine-grained sediments, disseminated within fine-grained and coarse-grained sediments Our geophysical results suggest that fluid flux largely controls gas hydrate growth and the depth of gas hydrate occurrence The deepwater subsurface sediments are saturated with fluids Fluid movement from deep to shallow sediments may be efficient, predominantly depending on permeability In less permeable clay layers, fluids move through the pore space between the solid grains of clay The formation of gas hydrates blocks fluid transports into shallower sections; therefore, the gas hydrate accumulates at the base of the GHSZ, e.g., there is not significant near-surface gas hydrate accumulation in site 05 Fractured media has a much higher permeability than porous media Fluid migrates upwards through the base of the GHSZ by the fractures and forms gas hydrate in shallower sections, such as site 08 Sager et al (2003) generally describe three types of seep mounds that are related to gassy fluid flux: the authigenic carbonate mound, the hydrate mound, and the mud volcano The carbonate mound usually characterizes low flux hydrocarbon seeps, while the hydrate mound indicates higher fluxes The mud volcano is often associated with the highest fluid and sediment flux Carbonates in site 08 are interpreted to be authigenic carbonates formed by precipitation of carbonate on 76 Z Sha et al / Marine Geology 366 (2015) 69–78 the ancient seafloor and near the seafloor in association with the release of gas from deeper sediment sections A large amount of massive gas hydrate found near the ancient seafloor below the carbonates suggests that fluids, including deep source gases, move up along faults or fractures to supply sufficient gas for hydrate formation The formation of the authigenic carbonates requires a slower gas flux and the same migration conduits Although it is unclear if the hydrate forms before the authigenic carbonate, it appears that the fluid flux has changed Moreover, no gas hydrate is found between the shallower hydrates near the current seafloor and the deeper hydrates near the ancient seafloor One interpretation of this finding is that the shallower hydrate forms in another period of rapid fluid expulsion Thus, buried carbonate material interlayered by two intervals of gas hydrate suggests that fluid expulsion was episodic The following section discusses the relation between hydrate formation and fluid movements 5.3 The interpretations of hydrate formation and accumulation The rate of hydration within GHSZ is primarily controlled by the availability of gas in the deepwater environment along with others (Collett et al., 2009) The microbial generation of gas mainly depends on the amount of organic matter in sediments and the temperature of sediments Biogenic and thermogenic diagenesis of organic carbon results in the gas production (Tissot and Welte, 1978; Collett et al., 2009) The mechanisms of gas migration and hydrate accumulation can be distinguished into advection of dissolved gas, advection of free gas, and diffusion (Ginsburg and Soloviev, 1999) The processes of diffusion and advection are responsible for the movement of fluid The diffusion is defined as the movement of gas molecules without being carried in another fluid The advection is fluid flow through lithological strata by physical forces, such as high-pressure layer under normal compression layers The following evolution of hydrate accumulation can be sketched based on the interpretation of seismic and well logs in sites 05, 08, and 16 In site 05, slow hydrate accumulation mainly results from changes in the solubility of the dissolved gas until hydrate equilibrium is reached during upward fluid flow (Fig 9I) Diagenesis of organic carbon results in methane production in the deep sections Due to the diffusion and advection of dissolved gas, methane is transported through sediments into the base of the GHSZ Numerical models of this system have been studied by Rempel and Buffett (1997), Xu and Ruppel (1999), Davie and Buffett (2001), and others The amount of precipitated hydrate is determined by the excess of dissolved gas along with other factors On the one hand, gas hydrates being precipitated from filtering over-saturated water partially fill the pore spaces of sediments Consequently, gas hydrate-bearing sediments grow and form a thin hydrate layer On the other hand, the pore-filling hydrate forms a low permeable barrier to hamper the movements of water flow Free gas begins to form when solution gas is oversaturated and does not form a gas hydrate This leads to relatively higher gas saturation than was previously observed below the base of the GHSZ The higher the gas concentration is, the higher the diffusion is As a result, more hydrate will form and the hydrate layer thickens near the base of the GHSZ (Fig 9II) Pore filling gas hydrate usually form in this scenario In site 16, gas supplies are inferred to be more sufficient than in site Dissolved and free gases are interpreted below the base of the GHSZ However, there are no obvious faults and fractures within the GHSZ Filtration of dissolved gases appears to govern the occurrence of gas hydrates within the GHSZ The reduction of gas solubility in water as the temperature decreases can result in hydrate crystallization Nearseafloor sediments tend to become preferential locations for the initiation of gas hydrates due to relatively lower gas solubility Once crystallization is underway, the hydrates are further controlled by capillarity-related stresses in small-diameter pores In very shallow sediments, pressure generated by hydrate formation may push away these pores Thus, in shallow fine-grained sediments, gas hydrates increase in volume by preferentially displacing the sediment rather than by filling the pore space (Fig 9III) Typical forms of gas hydrate morphology in fine-grained sediments include small crystals, small nodules, lenses and partings, and veins A more sophisticated analysis is provided by Torres et al based on observations of gas hydrate in IODP 204 (2004) Their model predicts effective overburden stress inhibits gas hydrate growth where water depth deeper than 20 m BML Fig Sketch of a geological interpretation of the hydrate formation in the study area (I) A thin hydrate accumulation near the base of the GHSZ (II) The hydrate accumulation thickens with progressive supplies of dissolved gas fluid (III) Hydrate accumulation occurs in the shallow section near the seafloor (IV) Upward shift of the base of the GHSZ, and shallow hydrate accumulation is buried more deeply with deposition (V) Dissolved and free gas accumulation due to dissociation of hydrate and supplies of gas fluid (VI) Hydrate formation in the shallow section into which free gas breaks through the base of the GHSZ and migrates along the mound and fractures Z Sha et al / Marine Geology 366 (2015) 69–78 As new sediments deposit and the older layers get buried beneath, the base of the GHSZ migrates stratigraphically upwards Consequently, shallow gas hydrates are buried in a deeper section (Fig 9IV) If gas supplies not significantly decrease, new shallow hydrates would form in a similar manner in site 16 Gas accumulations in deep sections focus an upward flow of fluids, often leading to gas venting and hydrate formation on the seafloor Hydrate mounds on the seafloor have been observed in the Gulf of Mexico (Sassen et al., 2001; Sager et al., 2003) Petroleum seepage is common in the deepwater of the Gulf of Mexico; vent sites with oil seeps and gas hydrate are found at the edges of salt mini-basins (Sassen et al., 2001) These hydrate mounds can extend several meters to tens of meters above the seafloor; site 08 is interpreted as an exposed hydrate mound Seismic profiles showing a mound-like shape indicate that the mound was stable for a long period Carbonate covering the mound suggests that fluid flux was reduced before it was buried Although faults or fractures cannot be mapped within chaotic zones below the GHSZ, it is inferred that overpressurized fluid releasing and flowing along them results in the high flux of site 08 The base of the GHSZ migrates upwards, while the hydrate mound is completely buried with sediments during deposition (Fig 9V) Consequently, dissociation of gas hydrates leads to free gas accumulations below the base of the GHSZ Continuous flux of gas-rich fluids promotes the precipitation of gas hydrates above the GHSZ and free gas below it Near the top of the buried mound, fractures could form and develop because of local high slopes Along the buried mound and fractures, dissolved gas fluids flow upwards, together with free gas, to form gas hydrates at a shallower buried depth (Fig 9VI) Conclusion Gas hydrates have been well investigated in the east of the Pearl River Mouth basin of the South China Sea by a seismic survey, LWD logging, and coring program The analysis of seismic reflection indicates that BSR occurs widely in the area Site 05 was drilled through the BSR Analyses of samples show that the gas hydrate fills the pore space of fine-grained sediments Site 16 was drilled on an erosional ridge The hydrate samples of the site were found near the seafloor and just above the BSR The borehole of site 08 penetrates a buried seafloor mound Gas hydrates occur near seafloor fine-grained sediments and extend vertically to the mound and fine-grained and coarsegrained sediments inside the mound Gas supply and migration are important factors determining the occurrence of gas hydrates The three sites represent different fluid-gas expulsion rates in different geologic times Site 05 has a slow flux rate in comparison to sites 16 and 08 Dissolved gas from deep subsurfaces accumulates at the base of GHSZ to form pore filling gas hydrates As the flux rate increases and pathways formed, dissolved gas migrates upwards and accumulates in the shallow section Near-seafloor nodule gas hydrate deposits are present in site 16 under these conditions Site 08, which is characterized by a fast flux rate with free gas over a long period, has the largest accumulation of gas hydrates among the three sites Fracture acts as conduits for gassy fluid migration and reservoirs for gas hydrate accumulations If fluid is amply supplied with time, nodule hydrates at the near-surface would grow and form massive hydrates The results provide additional insight into fluid-gas expulsion behavior within the gas hydrate stability zone, especially fluid-gas migration and gas hydrate accumulation in fine-grained sediments near the seafloor in the South China Sea The results also suggest a link between overpressure, the hydrate stability zone, and slope instability However, more research and expeditions are needed to identify the gas hydrate volume, the rate of gas hydrate growth and dissociation as well as assess related, potential impacts to the seafloor and subsurface sediments as geohazards 77 Acknowledgments We thank GMGS for providing the data and allowing publication of the study We acknowledge the crew and scientists of GMGS's vessels “Fengdou 4” and “Haiyang 6” for seismic data acquisition We also acknowledge the crew and scientists of the vessel “Rem Etive,” operated by Fugro, for collecting log data and taking core samples We would also like to thank Ray Boswell, Nengyu Wu, editor Michele Rebesco, and another anonymous reviewer for reviewing this paper and providing constructive comments This research is supported by the National High Technology Research and Development Program of China (863 Program, Grant No 2013AA092501) and the China Geological Survey Program (Grant No GZH201100303 & No GZH201100305) References Boswell, R., 2009 Is gas hydrate energy within reach? Science 325, 957–958 Bouriak, S., Vanneste, M., Saoutkine, A., 2000 Inferred gas hydrates and clay diapirs near the Storegga slide on the southern edge of the Voring Plateau, offshore Norway Mar Geol 163 (1–4), 125–148 Collett, T.S., Lee, M.W., 2011 Downhole well log characterization of gas hydrates in nature — a review Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom, July 17–21, 2011 Collett, T.S., Johnson, A.H., Knapp, C.C., Boswell, R., 2009 Natural gas hydrates: a review In: Collett, T., Johnson, A., Knapp, C., Boswell, R (Eds.), Natural Gas Hydrates—Energy Resource Potential and Associated Geologic Hazards AAPG Memoir 89, pp 146–219 Davie, M.K., Buffett, B.A., 2001 A numerical model for the formation of gas hydrate below the seafloor J Geophys Res 106, 497–514 Dugan, B., Flemings, P.B., 2000 Overpressure and fluid flow in the New Jersey Continental Slope: implications for slope failure and cold seeps Science 289, 288–291 Ginsburg, G.D., Soloviev, V.A., 1999 Methane migration within the submarine gashydrate stability zone under deep-water conditions Mar Geol 137, 49–57 Hampton, M.A., Lee, H.J., Locat, J., 1996 Submarine landslides Rev Geophys 34, 33–59 Heggland, R., 1997 Detection of gas migration from a deep source by the use of exploration 3-D seismic data Mar Geol 137, 41–47 Huang, Y., Suess, E., Wu, N., et al., 2008 Methane and gas hydrate geology of the northern South China Sea: Sino-German Cooperative SO-177 Cruise Report Geological Publishing House, Beijing Hunt, J.M., 1979 Petroleum Geochemistry and Geology Freeman, San Francisco (617 pp.) Judd, A.G., Hovland, M., 2007 Seabed Fluid Flow: The Impact on Geology, Biology and the Marine Environment Cambridge University press (475 pp.) Kayen, R.E., Lee, H.J., 1991 Pleistocene slope instability of gas hydrate-laden sediment on the Beaufort Sea margin Mar Geotechnol 10, 125–141 Kvenvolden, K.A., 1993 Gas hydrates as a potential energy resource — a review of their methane content In: Howell, D.G (Ed.), The Future of Energy Gases U.S Geological Survey Professional Paper 1570 United States Government Printing Office, Washington, pp 555–561 Locat, J., 2001 Instabilities along ocean margins: a geomorphological and geotechnical perspective Mar Pet Geol 18 (4), 503–512 Lu, S., McMechan, G.A., 2002 Estimation of gas hydrate and free gas saturation, concentration, and distribution from seismic data Geophysics 67, 582–593 MacKay, M.E., Jarrard, R.D., Westbrook, G.K., Hyndman, R.D., 1994 Origin of bottomsimulating reflectors: geophysical evidence from the Cascadian accretionary prism Geology 22, 459–462 McConnell, D.R., Kendall, B.A., 2002 Images of the base of gas hydrate stability, northwest Walker Ridge, Gulf of Mexico Proceedings, Offshore Technology Conference, Houston, 6–9 May 2002 OTC, p 14103 Orange, D.L., Breen, N.A., 1992 The effects of fluid escape on accretionary wedges II: seepage force, slope failure, headless submarine canyons and vents J Geophys Res 97, 9277–9295 Orange, D.L., McAdoo, B.G., Moore, J.C., Tobin, H., Screaton, E., Chezar, H., Lee, H.J., Reid, M., Vail, R., 1997 Headless submarine canyons and fluid flow on the toe of the Cascadia accretionary complex Basin Res 9, 303–312 Paull, C.K., Matsumoto, R., Wallace, P.J., Dillon, W.P (Eds.), 2000 Proc ODP Sci Results 164 Ocean Drilling Program, College Station, TX Pautot, G., Rangin, C., Briais, A., Tapponier, P., Beuzart, P., Lericolais, G., Mathieux, X., Wu, Y., Han, S., Li, Y., Zhao, J., 1986 Spreading direction in the central South China Sea Nature 321, 150–154 Rempel, A., Buffett, B.A., 1997 Formation and accumulation of gas hydrate in porous media J Geophys Res 102, 10,151–10,164 Riedel, M., Collett, T.S., Malone, M.J., and the Expedition 311 Scientists, 2006 Proc IODP 311 Integrated Ocean Drilling Program Management International, Inc., Washington, DC Roberts, H.H., Hardage, B.A., Shedd, W.W., Hunt Jr., J., 2006 Seafloor reflectivity—an important seismic property for interpreting fluid/gas expulsion geology and the presence of gas hydrates Lead Edge 25, 620–628 Sager, W.W., MacDonald, R.I., Hou, R., 2003 Geophysical signatures of mud mounds at hydrocarbon seeps on the Louisiana continental slope, northern Gulf of Mexico Mar Geol 198, 97–132 Sahagian, D.L., Proussevitch, A.A., 1992 Bubbles in volcanic systems Nature 359, 485 Sassen, R., Sweet, S.T., Milkov, A.V., DeFreitas, D.A., Kennicutt, M.C.K.I.I., 2001 Thermogenic vent gas and gas hydrate in the Gulf of Mexico slope: is gas hydrate decomposition significant? Geology 29, 107–110 78 Z Sha et al / Marine Geology 366 (2015) 69–78 Schwab, W.C., Danforth, W.W., Scanlon, K.M., Masson, D.G., 1991 A giant submarine slope failure on the northern insular slope of Puerto Rico Mar Geol 96, 237–246 Shedd, W., Boswell, R., Frye, M., Godfriaux, P., Kramer, K., 2012 Occurrence and nature of “bottom simulating reflectors” in the northern Gulf of Mexico Mar Pet Geol 34 (1), 34–40 Shipley, T.H., Houston, M.H., Buffler, R.T., Shaub, F.J., McMillen, K.J., Ladd, J.W., Worzel, J.L., 1979 Seismic reflection evidence for the widespread occurrence of possible gas hydrate horizons on continental slopes and rises AAPG Bull 63, 2204–2213 Singh, S., Minshull, T.A., Spence, G., 1993 Velocity structure of a gas hydrate reflector Science 260, 204–207 Suess, E., 2005 RV SONNE cruise report SO 177, Sino-German cooperative project, South China Sea Continental Margin: geological methane budget and environmental effects of methane emissions and gas hydrates IFM-GEOMAR Reports (http://store.pangaea de/documentation/Reports/SO177.pdf) Sultan, N., Cochonat, P., Foucher, J.-P., Mienert, J., 2004 Effect of gas hydrates melting on seafloor slope instability Mar Geol 213, 379–401 Taylor, B., Hayes, D.E., 1980 The tectonic evolution of the South China Basin In: Hayes, D.E (Ed.), Tectonic and Geologic Evolution of Southeast Asian Seas and Islands Geophys Monogr vol 23 American Geophysical Union, pp 89–104 Tissot, B.P., Welte, D.H., 1978 Petroleum Formation and Occurrence Springer-Verlag, New York (538 pp.) Torres, M.E., Wallmann, K., Trehu, A.M., Bohrmann, G., Borowski, W.S., Tomaru, H., 2004 Gas hydrate growth, methane transport, and chloride enrichment at the southern summit of Hydrate Ridge, Cascadia margin off Oregon Earth Planet Sci Lett 226, 225–241 Trehu, A.M., Torres, M.E., Moore, G.F., Suess, E., Bohrmann, G., 1999 Temporal and spatial evolution of a gas-hydrate-bearing accretionary ridge on the Oregon Continental margin Geology 27, 939–942 Tréhu, A.M., Torres, M.E., Bohrmann, G., Colwell, F.S., 2006a Leg 204 synthesis: gas hydrate distribution and dynamics in the central Cascadia accretionary complex In: Tréhu, A.M., Bohrmann, G., Torres, M.E., Colwell, F.S (Eds.), Proc ODP Sci Results 204 Ocean Drilling Program, College Station, TX, pp 1–41 Tréhu, A.M., Bangs, N.L., Guerin, G., 2006b Near-offset vertical seismic experiments during Leg 204 In: Tréhu, A.M., Bohrmann, G., Torres, M.E., Colwell, F.S (Eds.), Proc ODP Sci Results 204, pp 1–23 ([Online] Available from World Wide Web: http://wwwodp.tamu.edu/publications/204_SR/VOLUME/CHAPTERS/120.PDF) Weaver, P.P.E., Wynn, R.B., Kenyon, N.H., Evans, J., 2000 Continental margin sedimentation with special reference to the Northeast Atlantic margin Sedimentology 47 (Suppl 1), 239–256 Xu, W., Ruppel, C., 1999 Predicting the occurrence, distribution, and evolution of methane gas hydrate in porous marine sediments J Geophys Res 104, 5081–5095 Yang, S., Zhang, G., Zhang, M., Liang, J., Lu, J., Schultheiss, P., Holland, M., 2014 A complex gas hydrate system in the Dongsha area, South China Sea: results from drilling expedition GMGS2 Proceedings of the 8th international conference on gas hydrates (ICGH8-2014), Beijing, China, 28 July-1 August, 2014 Yi, H., Zhong, G., Ma, J., 2007 Fracture characteristics and basin evolution of the Taixinan Basin in Cenozoic Pet Geol Exp 29 (6), 560–564 Zhang, Z., McMechan, G.A., 2006 Elastic inversion for distribution of gas hydrate, with emphasis on structural controls J Seism Explor 14, 349–370 Zhang, H., Yang, S., Wu, N., Schultheiss, P., GMGS-1 Science Team, 2007 China's first gas hydrate expedition successful: fire in the Ice Methane Hydrate Newsletter—Spring/ Summerp Zhang, Z., Mcconnell, D., Yao, Q., 2014a Velocities and attenuations of gas hydrate-bearing sediments in fractured and sand reservoirs Proceedings of the Eighth International Conference on Gas Hydrates, July 28–August 1, 2014, Beijing, China Zhang, G., Yang, S., Zhang, M., Liang, J., Lu, J., Melanie, H., Peter, S., and the GMGS2 Science Team, 2014b Gmgs2 Investigates a Rich and Complex Gas Hydrate Environment in the South China Sea: Fire in the Ice Methane Hydrate Newsl 14 (1), 1–5 Zhang, G., Yang, S., Zhang, M., Liang, J., Lu, J., Schultheiss, P., Holland, M., Su, X., Sha, Z., Fun, S., Gong, Y., 2014c Introduction to GMGS2 gas hydrate drilling expedition: overview Proceedings of the 8th International Conference on Gas Hydrates (ICGH8-2014), Beijing, China, 28 July–1 August, 2014 Zhang, G., Liang, J., Lu, J., Yang, S., Zhang, M., Su, X., Xu, H., Fu, S., Kuang, Z., 2014d Characteristics of natural gas hydrate reservoirs on the northeastern slope of the South China Sea Nat Gas Ind 34 (11), 1–10 ... hydrates IFM-GEOMAR Reports (http://store.pangaea de/documentation/Reports/SO177.pdf) Sultan, N., Cochonat, P., Foucher, J.-P., Mienert, J., 2004 Effect of gas hydrates melting on seafloor slope instability