Gauging formation dynamics of structural-seepage methane hydrate reservoirs in Shenhu area of northern South China Sea: Impact of seafloor sedimentation and assessment of controlling factors

Marine and Petroleum Geology 107 (2019) 185–197

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Research paper

Gauging formation dynamics of structural-seepage methane hydrate reservoirs in Shenhu area of northern South China Sea: Impact of seafloor sedimentation and assessment of controlling factors

T

Jinan Guana,b,c,d, Lihua Wana,b,c,d, Deqing Lianga,b,c,d,∗ a

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China Key Laboratory of Gas Hydrate, Chinese Academy of Sciences, Guangzhou 510640, China c Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China d Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, China b

ARTICLE INFO

ABSTRACT

Keywords: Shenhu area Methane hydrate Structural-seepage type Formation dynamics Seafloor sedimentation

As massive methane hydrate reservoirs in Shenhu slope area of northern South China, the structural-seepage hydrate-bearing sediments, which mainly include thick-bedded and disseminated types of hydrates, show favorable advantages of high content, great thickness, and excellent exploitation value. They are generated by thermogenic methane vertically migrated from deep strata through faults, and finally accumulate in shallow sand layers. In order to investigate the formation dynamics of massive methane hydrate reservoirs in Shenhu area, a vertical fluid flow-methane hydrate formation reaction-seafloor sedimentation model is designed to gauge the accumulation mechanism. From the analysis of Peclet number which weighs the relative importance between deposition and formation reaction, local methane hydrate looks more likely to actively gather together in the sediments primarily, then gradually evolve into massive reservoirs accompanying passive seafloor sedimentation. Through choosing three typical formation stages (50 ka, 3 Ma, and 5 Ma) to exhibit the evolution process of these hydrate reservoirs, the change of local pressures, temperatures, dissolved methane and salt, phase saturations, stratum permeability, and pore capillary pressure deduces how the structural-seepage hydrate reservoir operates. The investigation also shows that after 5 Ma theses hydrate-bearing layers can proceed to take on similar appearance with current occurrence when average seafloor sedimentation rate and the initial seafloor are 5 cm/ka and 988 m, respectively. Finally, the performance of five controlling factors, including methane flux, kinetic coefficient, initial fluid position, permeability and seafloor sedimentation rate, has been quantitatively assessed in this formation model. Our work verifies this methane hydrate formation reaction-seafloor sedimentation mechanism is adequate for studying Shenhu structural-seepage hydrate reservoirs. The findings further suggest the combination of small methane flux and small reaction coefficient should be preferentially recommended to breed this type of methane hydrate-bearing layers.

1. Introduction Natural gas hydrate is widely distributed in global onshore permafrost areas and offshore active/passive continental margins (Boswell, 2016; Koh et al., 2012). Because methane is dominant among natural gas molecules (> 90%), it is also called methane hydrate (MH). Collett et al. (2015) summarized the current progress of international MH field expeditions, and emphasized the importance of understanding these MH systems and assessing their future potential as an energy resource for mankind. Although still facing technical and environmental problems, such as sand plug in wellbore, and gas escape from hydrate

∗

reservoirs, results from recent experimental and numerical studies indicate a huge possibility for humans to extract this type of chemical bonding methane, which is stored in natural hydrate reservoirs, through improved technologies in the field of conventional oil and gas recovery (Heeschen et al., 2016; Nagao, 2012; Yamamoto, 2015). For the last decade, oil and gas companies, universities and research institutes in China have conducted field investigations and laboratory tests on the aggregation and occurrence of natural MH layers. Largescale MH-bearing sediments in Shenhu slope area of northern South China Sea (SCS) have been found and reconfirmed during three scientific gas hydrate drilling expeditions GMGS1, GMGS3, and GMGS4 (Wu

Corresponding author. Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China. E-mail address: [email protected] (D. Liang).

https://doi.org/10.1016/j.marpetgeo.2019.05.024 Received 8 October 2018; Received in revised form 17 May 2019; Accepted 17 May 2019 Available online 20 May 2019 0264-8172/ © 2019 Elsevier Ltd. All rights reserved.

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et al., 2011; Yang et al., 2015, 2017a). The first trial production of MH in this zone was conducted from May 10 to July 9 in 2017, and it smoothly yielded a total of approximately 309,000 m3 (under atmospheric pressure) of gas (Su et al., 2018). Undoubtedly, these results further demonstrated the great potential of Chinese marine MH resources. Five types of gas hydrates have been identified in Shenhu area now, including thick-bedded, disseminated, plaque, near fault and thinbedded (Yang et al., 2017b). They bond with solid porous skeletons and are irregularly distributed within unconsolidated clayey silt fine grained layers which located hundreds of meters away from seafloor. The origin of MH reservoirs in this area is a matter of interest. Discerning local sequence stratigraphic framework and understanding the evolution process of upper crust in this area are crucial to reasonably speculate the accumulation periods of these MH-bearing layers. By means of elaborate interpretation on local seismic reflection data, three structural layers have been identified to mark out different tectonic stages from the lower crust to shallow seafloor: the late Miocene (11.6 Ma), the Pliocene (5.5 Ma), and the Quaternary (2.0 Ma) (Su et al., 2014a). The regional bottom simulating reflections (BSRs) are mostly present near the interface between the Quaternary and Pliocene layer, indicating that the formation of MH-bearing sediments probably begins at 5.5 Ma, and also their ages cannot be more than 11.6 Ma. Since the late Miocene, the accelerated tectonic subsidence and rapid uploading of clay silt/silt deposits should provide preferential accumulation conditions for MHs (Cong et al., 2013; Yu et al., 2014). Within this period, diverse and strong geological activities, such as sea-level fluctuation, submarine landslide, etc., have been detected to prevail in northern SCS (Chen et al., 2014; Sun et al., 2018). These activities should alter the occurrence and distribution of MH reservoirs. Because of the tremendous advantages of high content, great thickness, and focused distribution area, the two types of thick-bedded and disseminated MHs, which are also named as structural-seepage hydrates, constitute the massive MH reservoirs for primary economic exploitation. Although biogenetic methane in local hydrate stable zone (HSZ) is not enough to produce such high MH content layers (maximum 48%), deep-seated biogenetic and thermogenetic methane can flow upward into the HSZ through favorable migration pathways (Wu et al., 2017). The structural-seepage MH reservoirs are speculated to be generated by thermogenic methane vertically migrated from deep strata through faults. Their vertical distribution is nearly controlled by local faults and upward gas-rich fluids. Plentiful and powerful proofs from field seismic and logging data analysis and laboratory drilling core sample tests support the viewpoint that free gas in local sediments contributes to MH generation (Hui et al., 2016; Liu et al., 2017a). The occurrence and accumulation of these structural-seepage MH sediments in Shenhu area is rather similar to parts of gas hydrate sites in the Gulf of Mexico. Smith et al. (2014) presented a one-dimensional (1D) model coupling the thermodynamics of MH solidification with multiphase flow to illustrate how free gas pierces the HSZ in the Ursa vent of the Gulf of Mexico. The similar behaviors may have occurred in Shenhu submarine hydrate systems. Along this idea, Zhang et al. (2017b) proposed a migration and accumulation model to illustrate how massive MH-bearing layers originate and propagate. It reveals the importance of gas-liquid two-phase fluids migrating upward from below in the formation process of local massive MH-bearing reservoirs. From the perspective of geochronology, how this structural-seepage MH reservoir in Shenhu area operates and evolves within its corresponding geologic time is still unclear and controversial. Su et al. (2014b) first combined the downward sedimentary compaction and upward fluid flow in their numerical model to discuss the hypothetical dual accumulation pattern of Shenhu MH layers during the late Pliocene Epoch-early Pleistocene Epoch, but they did not consider local seafloor alternation resulting from various environmental and geologic movements. Generally, structural-seepage MH reservoirs associate with free gas zones firmly adjacent to HSZ bottoms. Based on amplitude

analysis of local seismic reflection profiles, the existence of free gas zones below BSRs has been identified, and the sign of gas movement within dipping strata has been confirmed (Wang et al., 2016; Yang et al., 2017c). Thus the importance of free gas migration process and MH formation reaction kinetics shall not be ignored because of the extensive participation of free gas in MH production. Furthermore, the unsaturated fluid flow in sedimentary pores below the HSZ determines the supply of methane which will turn into MH as the reactive material. However, the indication on the top occurrence of MH and the scale of pore methane flux inferred from pore water geochemistry of local shallow sediments are not very consistent with the results from the investigations via well logging (Feng et al., 2017). Perhaps associated characteristic and conditional parameters in this area should be unified to enhance the reliability on the study of the massive MH accumulation mechanism. In theory, a certain amount of thermogenic methane carried by pore fluids migrates along sedimentary faults upward across the HSZ bottom to react under favorable conditions of MH formation. During the initial stage, gas phase pressure is sufficient enough so that thick-bedded types of MHs are easily generated. With the good cementation between hydrates and grains, the strata permeability decreases sharply to block the pass through of subsequent fluids. The late-formed MH has to gather under the early thick-bedded MH layers. After such a long period of evolution, usually the gas pressure is gradually reduced to adapt to the new environment in pores. As a result, the disseminated and plaque MHs appear and disperse in porous strata. They are generally detected below thick-bedded layers in accumulation sequence in Shenhu slope area. In order to probe the formation dynamics of the structural-seepage MH reservoirs, the environmental and geologic influence on Shehu hydrate system has been considered in this study. A methane transportation-MH formation reaction model coupled with seafloor sedimentation has been adopted to gauge the evolution pattern. During this formation process, different influence factors, including methane flux, permeability, fluid flow distance, MH formation kinetic reaction coefficient (or abbreviated as kinetic coefficient in the following contents), and seafloor sedimentary rate, have been matched together to assess their controlling roles. 2. Study area and background The northern slope area of SCS is an east-northeast-trending rifted continental margin. It suffers long-term geological interactions from the Eurasian plate, the Pacific plate, and the Indo-Australian plate. As the study zone, the Shenhu hydrate drilling field is located in the lower part of this slope area with the Zhu III Depression to the northwest, the Kaiping-Baiyun Depressions to the northeast, and the Xisha Basin to the south (Fig. 1). This zone has experienced three evolutionary phases including rifting during late Cretaceous-early Oligocene, rifting depression during late Oligocene-early Miocene, and depression from the middle Miocene to the present, resulting in complex geological conditions in the region (Yang et al., 2017c). Widely developed faults, fractures, and gas chimneys provide good pathways for pore fluid migration into potential host rocks of bathyal-abyssal layers in the shallow crust. The great potential for biogenetic and thermogenic methane buried in local deeper sediments makes this area suitable for MH generation and accumulation (Liu et al., 2017b; Wu et al., 2017). Whole MH reservoir with a thickness of 10–80 m occurs in finegrained layers, and stretches just above the base of HSZ which is close to local BSRs. The results from field investigations and subsequent indoor tests, including logging while drilling borehole image, resistivity spectrum scan, and hydrate-bearing sand counting analysis, show that the valuable thick-bedded and disseminated hydrate reservoirs associate with the vertical thermogenic gas-bearing fluids. The tectonic structures, especially high-angle faults, are presumed to play an important role in influencing the development of MH layers (Yang et al., 186

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Fig. 1. The location of Shenu slope area in northern SCS and its seafloor rendering. During the GMGS1 scientific expedition hydrate core samples have been collected in three red drilling sites. In the following expeditions (GMGS3, GMGS4) the hydrate-bearing sediments in this zone have been verified (revised from Wu et al., 2017). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 2. The phase districts of the Shenhu area. On the left side the HSZ is enclosed by the hydrostatic line (black solid line BD) and the dissociation lines (AC, AD, AE). On the right side, the hydrate-liquid (dissolution methane) two-phase lines (FH, FI, FJ) and the hydrate-liquid-gas three-phase lines (GH, GI, GJ) corresponding to different geothermal gradients have been listed (red line-0.047 °C/m, blue line-0.037 °C/m, green line-0.057 °C/m). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) Fig. 3. The supposed alteration of the seafloor location and its subsequent influence on the HSZ bottom boundary in Shenhu area. Their present positions are 1238 mbsl and 240 mbsf, respectively. If vs is set to be 20 cm/ka (Chen et al., 2016; Cong et al., 2013), after 1 Ma they will change to be 1438 mbsl and 262 mbsf, accordingly.

2017c). The rich gas-bearing fluid pathways and adaptive sediment bodies indicate this hydrate field is a kind of typical gas hydrate migration and accumulation system (Wu et al., 2017).

seawater level often happens in Shenhu area, the current seawater depth is speculated to become deeper than before (Han et al., 2016). Within this period, the deposition process is very obvious. The maximum deposition rate can reach about 280 m/Ma here, and this situation has lasted for a rather long period (Huang et al., 2013). On the one hand, the seafloor sediments will be continuously compressed downward to the bedrock. On the other hand, tender cohesive forces in seafloor shallow unconsolidated sediments can easily cause slope instability and subsequent slumps when local layers are subjected to lateral extrusion from continental plate, possible hydrate decomposition and free gas accumulation in pores (Sun et al., 2017; Wu et al., 2018). The violent and abrupt submarine slope slump will rapidly deepen the seafloor location. It is so hard to quantify the impact of these factors on seabed position in corresponding geological periods in Shenhu slope area. In this work, we used a simplified average sedimentation rate to comprehensively reflect the seafloor change with time. It has the linear relationship as follows:

3. Data and 1D numerical methods The data from representative site SH2 in GMGS1 are used for the following discussions. The seafloor depth is about 1238 m below sea level (mbsl) now, and the present seafloor temperature is 3.4 °C (Wang et al., 2014; Su et al., 2012). The current thermodynamic phase districts have been shown in Fig. 2. The pressure-temperature-salt (3.4 wt% in pore) thermodynamic environment and the methane supply conditions determine the scope and boundary of the HSZ. It also represents the influence of ambient environment change on the HSZ. For example, if the local geothermal gradient becomes warmer from the present 4.7 °C/ 100 m to 5.7 °C/100 m, the HSZ bottom moves from the current 240 m below seafloor (mbsf) to 205 mbsf, accordingly. If it changes to 3.7 °C/ 100 m, the HSZ bottom will accordingly change to 325 mbsf. The research area in this model mainly focuses on upper sediments (0–300 mbsf), and the temperature shift in this zone is probably slight. Thus the linear geothermal gradient is always used in the simulation. The seafloor position mainly follows the geologic long-term change of seawater depth and the compaction effect of sedimentary seabed. Since late Miocene, although oscillatory variation of low and high

Pb = P0 + vs × t × Gd,

(1)

where Pb and P0 (MPa) are the changing seafloor pressure and initial seafloor pressure, respectively; vs (cm/ka) is local average sedimentation rate; t (a) is time; and Gd (MPa/m) is the hydrostatic pressure gradient (Fig. 3). On account of the presence of free gas in the pore, the analysis of how the venting gas at cold seep sites in Hydrate Ridge formed gas 188

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Fig. 4. The comparison of formation reaction kinetics between our model (c) and other's. a): Sun and Mohanty (2006); b): Cao et al., 2013.

hydrate by Cao et al. (2013) can be applied in Shenhu area. An improved quasi-equilibrium kinetics which describes the MH formation reaction can be expressed by

rh =

0

exp

Ea 1 R T

1 Teq −3

0 (1

Sh )(f

feq ),

t

(2)

t

−1

krg gk µ g

m l Sl Xl )] +

( Pg

krl lk µ ( l

=

m rm

Pl

g

g

lg

z) z ) Xlm

m l Dl

Xlm

+

lk

krl ( Pl µl

lg

z ) Xlw =

w rw

+ qw

(4)

The component salt (s) is confined to only dissolve in the pore aqueous liquids:

where rh (mol·m ·a ) is the MH reaction rate; ζ0 (mol·m−3 MPa−1·a−1) is the kinetic coefficient; f and feq (MPa) represent the fugacity under the local thermodynamic and equilibrium conditions, respectively; ΔEa/R is 9752.73 K; T and Teq (K) are the local and equilibrium temperatures, respectively; φ0 is the initial layer porosity; Sh, Sg, and Sl is the phase saturation of MH, gas and liquid. The change of reaction rate is shown in Fig. 4. According to above descriptions of the occurrence features of structural-seepage MH reservoirs, a vertical 1D dynamic fluid flow-MH generation-seafloor sedimentation process is competent to survey the accumulation dynamics of the massive MH deposits here. The component methane (m) disperses in both the gaseous (g) and aqueous phases (l), and the methane molecular diffusion in the gaseous phase is neglected:

[ ( g Sg + t

w l Sl Xl )

(

s l Sl Xl )

(

+

lk

krl ( Pl µl

lg

z ) Xls

s l Dl

Xls = qs

(5)

The component hydrate (h) is:

t

( Sh

h)

=

h rh

(6)

And the energy (e) balance equations can be expressed by above phases and porous skeleton (r):

[(1

) r Cr +

=

z

T z

e

( h Ch Sh +

l Cl Sl

+

g Cg Sg )]

T + [ g ug Cg + t

+ Qh + qe

l ul Cl]

T z (7)

3

In expressions (3)–(7), ρg, ρl, ρh, and ρr (kg/m ) is phase density of gas, liquid, hydrate and rock respectively; Xlm, Xlw, and Xls is mass fraction of methane, water, and salt in pore aqueous liquid (also be written as Xm, Xw, Xs); ug and ul (m/s) is Darcy velocity of gas and liquid, and they are written as

.

ug =

+ qm

k

(3)

and ul =

The water component (w) disperses only in aqueous phase (l). The water molecular diffusion in aqueous phase and pore vapor has been neglected:

krg µg

( Pg

k

krl ( Pl µl

g

g

z ),

lg

(8)

z ).

(9)

Pg and Pl (kPa) is phase pressure of gas and liquid in pore, and they 189

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×(3.949 14.41Sl + 23.67Sl2 have the relationship: Pg Pl = Pc0 × 12.61Sl3) , here the pore entry pressure Pc0 is 2.6 kPa (Guan et al., 2009); μg and μl (Pa·s) is gas and liquid viscosity; Cg and Cl (J·kg−1·k−1) is specific heat of gas and liquid; k (m2) is sediment instinct permeability; krg and krl is the phase relative permeability of gas and liquid; Dlm and Dls (m2/s) is diffusion coefficient of methane and salt in liquid; g (kg/m3) is gravitational acceleration; z (m) is distance along the gravity direction; qm, qw, and qs (kg·m−2·a−1) is component flux of methane, water and salt, and qe (J·m−2·a−1) is heat flux; ηm, ηw and ηh is mole mass of methane, water and hydrate; rw (mol·m−3·a−1) is water consumption (=5.86rh), and rm (mol·m−3·a−1) is methane consumption (=rh). The corresponding hydrostatic and thermal properties, such as permeability k, effective heat conductivity e (W·m−1 K−1), hydrate latent heat Qh (J·m−3·a−1), etc., have been explained in another works (Guan et al., 2009; Guan and Liang, 2018). One of the differences is that in this work the initial dissolved methane concentration is set to be 0 because in this model the in-situ methane is ignored. Another is that the seafloor compaction causes the reduction of local pore space. An exponential function fitted with the measured porosity data (φ) here describes this kind of change when there is no MH presence in sediments: 0, z

=

0,

+(

0,0

0,

)e

z,

Fig. 5. The Peclet number calculated from different sedimentation rates and SMT thicknesses in the Shenhu area. On the right side of the vertical pink dashed line, where Pe is over 1, MH production in the pore inclines to follow the seabed sedimentation, but in the left zone the MH will be mostly generated prior to the burial. This area is estimated to locate in the delta-shaped region (ABC). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

(10)

where φ0,0 is the porosity at the seafloor (=0.65); φ0,∞ is the porosity at infinite depth (=0.35); and β is an attenuation coefficient (=0.012) (Guan et al., 2014). After MH emerges in the pore, the new porosity, φh, will be defined by h, z

=

0, z (1

Sh, z ).

(11)

two conditions, Pe is calculated to be about 0.61. Thus, it can be roughly inferred that the MH will tend to preferentially form in the pore and abundantly gather together for the most part of the evolution period in the Shenhu slope area. Subsequently, the MH-bearing skeletons will be passively compacted and buried by the sedimentation to distribute in the submarine layers. Although these two activities are occurring at the same time, the kinetic formation reaction shall play a more significant and active role in comparison with the deposition.

4. Modeling results 4.1. The formation pattern The MH formation kinetic reaction and the sedimentation conjunctly constitute the basis for understanding the massive accumulation mechanism of Shenhu MH area. The effect of burial will accelerate the advection of pore fluids. Accordingly, the MH formation rate becomes faster in this situation than when influenced only by the molecular diffusion in static pore solutions. The dimensionless Peclet number, Pe, has often been applied to judge the relative importance of the effects between molecule diffusion and fluid advection in the research of marine gas hydrate systems (Bhatnagar et al., 2007; Chatterjee et al., 2014). Malinverno and Pohlman (2011) explained how to use the Pe to compare the effects between deposition and MH formation reaction. Following this principle, we can roughly trace the possible evolutionary pattern of structural-seepage MH-bearing layers in the area. The expression is:

Pe =

vs z SMT , Dlm

4.2. The formation process evolution Considering the change of local seafloor, we set the initial seafloor location at 988 mbsl. The start position of upward migrating gasbearing pore fluids is treated to follow the seafloor alternation although the bottom of HSZ is changing in this reaction-sedimentation process. Therefore it is fixed at 300 mbsf. During this process, the upward methane flux at the bottom boundary is 0.5 kg m−2·a−1, and the kinetic coefficient is 0.2 mol m−3 MPa−1·a−1 (Guan and Liang, 2018). In order to better exhibit the correlation among various attributes of MH layers during this process, three representative stages were chosen to show the state of local sediments in different evolution stages (Fig. 6). When time goes forward to reach 50 ka (initial), 3 Ma (growing) and then 5 Ma (current), the seafloor location moves down to 990.5, 1138 and 1238 mbsl, respectively (Fig. 6a), and the HSZ bottom accordingly changes to 195, 175, and 165 mbsf (or 1185.5, 1313, and 1403 mbsl) because of the alteration of thermodynamic conditions. With the pore dissolved methane gradually reaching its saturation which is supplied by methane flux (qm) below, free gas accordingly appears in the HSZ (Fig. 6d) and the dissolved salt shows obviously abnormality (Fig. 6c) with massive MH aggregates there (Fig. 6d). Within the whole process, sediment capillary pressure (Fig. 6f) shows a positive change tendency with the MH content, while the layer permeability (Fig. 6e) displays negative change. It indicates that the accumulation of MH may hold back the further formation itself. Notably, in this model the local layer always keeps within the MH existence environment according to local thermal field (Fig. 6b). The MH content from Shenhu drilling investigation has been added

(12)

where zSMT (m) is the thickness of the sulfate-methane transition (SMT), the sedimentation is approximately equal to the local average sedimentation rate vs (cm/ka) in Eq. (1), and the diffusion coefficient of methane in bulk sediment is also regarded as the coefficient Dlm (m2/s) in Eq. (3). From Fig. 5 it can be inferred that if the strata burial dominates the accumulation of seafloor MH-bearing sediments, it needs a large sedimentation rate and SMT thickness. Because of the influence of seawater and MH, the base line of SMT in south part of Shenhu area appears deeper than in north part. In General, the scope of SMT in this area ranges within 7.7–50 m (Feng et al., 2017). The current base of the SMT is about 27 mbsf at site SH2 (Su et al., 2014b). Considering the complex submarine geological activities in northern SCS, if vs is so rapid to be 100 cm/ka, the corresponding Peclet numbers are 0.1878, 0.6585 and 1.2195 (Fig. 5). On the whole, the vs and zSMT in this area have been estimated to be less than 50 cm/ka and 50 m, respectively. Using these 190

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Fig. 6. The strata state during these three evolution stages. The subscript 0 here represents the initial state. a) the local hydrostatic pressure and phase pressure; b) the strata temperature (Tem) and phase temperature (Teq); c) the dissolved methane (Xm) and dissolved salt (Xs) in local pore; d) three phase contents (gas-Sg, hydrate-Sh, and liquid-Sl), and the drilling results of hydrate distribution (pink dots); e) the ratio of layer permeability (k) to initial value (k0); f) the difference change between capillary pressure (Pc) and initial state (Pc0). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

191

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Fig. 6. (continued)

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Fig. 7. The MH formation reaction rate at three stages (a), and the MH saturation contour (b) in Shenhu MH area.

into Fig. 6d in order to compare with the simulation profile at 5 Ma. Although these two results do not coincide with each other completely, the rationality of this transportation-reaction model is been proved for practicably explaining local MH accumulation and capably describing MH occurrence from morphology. The complex geological conditions, uncertain geothermal field history, and possible alternations between MH formation and decomposition increase the difficulty for exploring this structural-leakage hydrate reservoir (Zhang et al., 2017a). It urges us to gauge this dynamic system using important controlling factors as much as possible. There are several typical and meaningful characteristics in this dynamic process. The theoretical MH reaction rate without driving force, rh/(f - feq), at these three stages is demonstrated in Fig. 7a. The results verify that this rate is negatively related to the MH formation and the time. Furthermore, the three-dimensional continuous colorful contour which illustrates the MH saturation change with the local seafloor sedimentation and the MH kinetic reaction simultaneously is also shown, as in Fig. 7b. From this contour, it can be seen that the change of the seafloor position forces the MH layer to form a subulate shape with a high content peak. Another issue is the geothermal field and its influence on the occurrence boundary of MH. In this model, two types of thermal sources have been involved into the research area. One is the external upward geothermal flow from local deep strata. It is constant (=qe) and does not change with time during the formation reaction-seafloor sedimentation process (Fig. 8a). The other is the inner MH reaction heat. In this model, it is whole released by MH formation because there is no obvious MH dissociation. A simple arithmetic sum, Qht, can be used to describe the inner heat evolution with time (Fig. 8b). And one of the

effects of these heats is that they will lead to the rise of the HSZ bottom. The difference, between the current bottom that the layer has already been heated and the initial bottom when there is no heat, has been compared in Fig. 8c. The maximum shift in this case is ～0.8 m. 4.3. The assessment on controlling factors In this type of MH formation model, five parameters, including methane supply, layer permeability, start position of the fluids, kinetic coefficient, and seafloor sedimentary rate, are usually deemed as key controlling factors to restrain the evolution process of structural-seepage MH-bearing layers. What on earth their performances are, are our eager concerns. 4.3.1. No sedimentation In order to conveniently probe the restriction of four factors, including methane flux, kinetic coefficient, sedimentary permeability, and the start position, on the massive MH-bearing sediment formation and distribution, the sedimentation influence in this part was not considered during the discuss. Five methane fluxes qm (5, 0.5, 0.05, 0.005, and 0.0005 kg m−2·a−1) which range from 100 to 10−4 kg m−2·a−1, and five kinetic coefficients ζ0 (200, 20, 2, 0.2 and 0.02 mol MPa−1·m−3·a−1) which range from 102 to 10−2 mol MPa−1·m−3·a−1 are compared. Firstly, when qm is set to be 5 kg m−2·a−1, the results of MH and free gas distributions from all these five coefficients are distorted and cannot be right in this model. Secondly, when ζ0 is 0.02 mol MPa−1·m−3·a−1, the scope of qm within 10−110−4 kg m−2·a−1 cannot form effective massive subuliform MH-bearing 193

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Fig. 8. The local geothermal field and its influence. (a) the input heat flux from below; (b) the MH reaction heat; (c) the shift of HSZ bottom.

sediments in the same way as the Shenhu area. Finally, if we set the combination of a small methane flux (0.0005 kg m−2·a−1) and a large kinetic coefficient (200 mol MPa−1·m−3·a−1) to run this model, there is no hydrate production. They indicate the qm should not be larger than 100 kg m−2·a−1 and the ζ0 should not be less than 10−2 mol MPa−1·m−3·a−1 in Shenhu area. The combination of small methane fluxes (< 10−3 kg m−2·a−1) and large kinetic coefficients ζ0 (> 102 mol MPa−1·m−3·a−1) is also inappropriate if the sedimentation is not considered here. Therefore, five combinations of qm and ζ0 which express different reaction scales have been designed to probe these effects (Fig. 9). The smaller methane flux needs more time to build this kind of submarine subuliform MH reservoir. Although all these combinations can be useful, large methane fluxes and rapid formation reaction rates are more eligible for Shenhu hydrate area in this mechanism.

The flow distance of methane-bearing fluids to the HSZ bottom, L (m), has already been noticed in several studies on marine hydrate systems (He et al., 2011). Obviously, the greater this distance, the more time the fluid needs to reach the HSZ bottom. The greater this time, the greater the free gas content in the departure position when MH initially appears in the HSZ pore. For example, using the basic setting, two fluxes are set to be 0.5 and 0.0005 kg m−2·a−1. When L is 800 m, the free gas contents are 4.32% and 1.37% accordingly. When L is 300 m, the free gas contents then become 3.38% and 0.41%, respectively. In our model, the layer intrinsic permeability displays a very weak effect on the final distribution and occurrence of massive MH-bearing sediments, even though it is likely to have some impacts on the evolution process. We have tried a low permeability (k = 3 × 10−16 m2), a high permeability (k = 3 × 10−9 m2), and a linear permeability (k = 194

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Fig. 9. The effects of five combinations of qm and ζ0 on MH (a) and gas (b) content.

(10−18–10−12) × z + 3 × 10−10 m2), but the ultimate results of MH saturation and free gas are almost the same.

geological timescales, Chatterjee et al. (2014) concluded that localized, focused, vertical, advective flux within high-permeability conduits determines the content of MH and free gas, relative to molecule diffusion in pore. These similar geologic phenomena have been displayed in some related BSR studies (Liu et al., 2017b). This situation shows the diversity of MH sediments in Shenhu area. The lateral sedimentary structures and the body inhomogeneity of upper strata shall be noticed to influence the occurrence of MH-bearing layers here, including the impact on structural-seepage types. The quantitative assessment on local thermal field has not been technically executed because their field in-situ data are explicit, and there is no obvious dispute on the possible variation range of them. The seafloor temperature and geothermal gradient both have great influence on HSZ bottom boundary. If we use the approximate history matching relation between seawater depth Lw (m) and seafloor temperature Tsb (°C), Tsb = exp {[2.0339 ln (L w /1000)]/1.3361, instead of the current seafloor temperature, the temperature variation is less than 0.72 °C when the seafloor location changes from 988 mbsl to 1238 mbsl. The sedimentation effect will compact the porous skeleton, and make the supposed linear geothermal gradient become nonlinear. Some impacts of this disturbance have been exhibited in Fig. 2. When the gradient changes from 3.7 °C/100 m to 5.7 °C/100 m, the HSZ bottom will move upward 120 m. The impacts of upward heat flux ranging from 27 °C/1000 m to 87 °C/1000 m have been discussed in the work by Guan and Liang (2018), so there is no need to make more statements here.

4.3.2. The effect of sedimentation In order to facilitate a unified discussion of the effects of seafloor sedimentation and compare it with the discussion in the previous section, we employed a small qm (=0.0005 kg m−2·a−1) and a general ζ0 (= 2 mol m−3 MPa−1·a−1) in this case. The two initial seafloor positions are set at 1000 and 1100 mbsl. The comparisons of the two representative average sedimentation rates (0.2 and 2 cm/ka) are shown in Fig. 10. After 5 Ma the MH-bearing sediments will form subulate shape, and the peaks of MH contents are very close. When the seafloor starts at 1000 mbsl, this peak will be 41.89% and 42.18% if vs is 0.2 and 2 cm/ka, respectively (Fig. 10a). When the seafloor starts at 1100 mbsl, this peak will be 43.67% and 42.55% if vs is 0.2 and 2 cm/ka, respectively (Fig. 10d). The corresponding trajectories of the seafloor and HSZ bottom with time under different sedimentation rates have also been presented in order to overall elaborate on the sedimentation-reaction style (Fig. 10b, c, 10e and 10f). The evidence shows the rationality and feasibility of this type of evolution process and mechanism when studying the Shenhu MH system. It seems that the small methane flux and relatively small kinetic coefficient are more applicable here. 5. Discussion This study numerically surveyed the dynamic formation of structural-seepage MH reservoirs in Shenhu slope area, referring to the theoretical summary on gas hydrate migration and accumulation system of this area proposed by Wu et al. (2017). The external seafloor sedimentation and inner MH formation kinetics were considered as two crucial aspects which controlled the distribution and accumulation of local massive MH layers. Furthermore, through characterizing Pe, the MH reaction process was speculated to be more prominent and active than the seafloor sedimentation process within this accumulation dynamics of these MH reservoirs. Because of the rapid MH formation mechanism (Guan and Liang, 2018), it also indicated another probability: much of MH was produced in local HSZ pore within a rather short term; and, the role of long-term seabed compaction contributed to this evolution into massive MH reservoirs. When counting the effect of the fluid advection and methane diffusion in the analysis of Pe, merely the upward fluid flux below the bottom of research zone has been considered in this fluid flow-MH formation-seafloor sedimentation process. Although the influence of fluid flux related to seabed sedimentation and compaction was ignored, it does not alter the assessments on the general formation pattern of massive MH reservoirs according to the current cognition of local SMT. Based on a two-dimensional model accounting for mass transfer over

6. Conclusion After the successful trial of recovering MH in Shenhu area, it is urgent to comprehensively grasp the aggregation state of structuralseepage MH sediments because these massive reservoirs provide the preferential industrial production. In order to gauge their formation dynamics, the seafloor sedimentation and several important factors controlling the evolution process have been clearly assessed by means of a 1D dynamic fluid flow-MH generation-seafloor sedimentation model. Our evaluation shows the formation pattern of structural-seepage MH sediments, demonstrates how the MH-bearing sediments evolve to the similar appearance with current occurrence through three geologic episodes (50 ka, 3 Ma, and 5 Ma), and presents possible variation scopes of these five controlling factors. The investigation reveals that this seafloor sedimentation-MH formation reaction style is suitable and feasible for studying the formation mechanism of Shenhu massive MH layers. Several geological phenomena remain unresolved in this study. The pathway of gas movement along sedimentary faults below HSZ, which is often driven by both overpressures and cooling effects, has not been

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Fig. 10. After 5 Ma the effect of two sedimentation rates on MH distribution when the initial seafloor is 1000 (a) and 1100 (d) mbsl, and the corresponding seafloor and HSZ bottom change over time (b, c, e, and f).

dealt with in this work. Additionally, except for thick-bedded and disseminated MHs, this model cannot be applied to explain the heterogeneous presence of stratigraphic-diffusive types of MH layers which have also been found in the GMGSs. These problems offer new challenges for understanding the interesting Shenhu hydrate system.

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Acknowledgements This work was jointly supported by the National Natural Science Foundation of China [Grant No.: 41344149, 51576197], the Science and Technology Program of Guangzhou, China [Grant No.: 201707010252], and the Chinese National Marine Research Program [Grant No.: GHZ2012006003]. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.marpetgeo.2019.05.024. 196

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