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Patterns of gas hydrate accumulation in mass transport deposits related to canyon activity: Example from Shenhu drilling area in the South China Sea
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Chao FU1, Shengli LI1, *, Xinghe YU1, Jinqiang LIANG2, Zenggui KUANG2, Yulin HE2, Lina JIN1
Acta Oceanologica Sinica | 2019, 38(5) : 118 - 128
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Acta Oceanologica Sinica | 2019, 38(5): 118-128
• Marine Geology •
Patterns of gas hydrate accumulation in mass transport deposits related to canyon activity: Example from Shenhu drilling area in the South China Sea
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Chao FU1, Shengli LI1, *, Xinghe YU1, Jinqiang LIANG2, Zenggui KUANG2, Yulin HE2, Lina JIN1
Affiliations
  • 1 School of Energy Resources, China University of Geosciences, Beijing 100083, China
  • 2 Guangzhou Marine Geological Survey, China Geological Survey, Guangzhou 510760, China
Published: 2019-05-25 doi: 10.1007/s13131-019-1443-1
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Since 2017, a plenty of gas hydrates have been drilled in a new area of Shenhu, and good heterogeneity has been found throughout the spatial distribution of the reservoir. After distinguishing different sedimentary sequence types and matching their formation with slope deposition settings, this study proposes three mass transport deposit (MTD) patterns related to canyon activity that occurred contemporaneously or epigenetically with it: well preserved MTDs, MTDs eroded by canyon migration, and MTDs dislocated by contemporaneous faults. Based on seismic reflection characteristics, this study proposed methods of quantitatively analyzing sedimentary factors, such as measuring the turbidities flow rate in the canyon, and results are interpreted with respect to canyon activity. Combining the above parameters and their relationship with gas hydrate accumulation, fine-grained seals overlapping coarse MTDs reservoirs are found to be indispensable to gas hydrate accumulation, as they prevent the release of free gas. Based on grain size data of hydrate samples from drilling wells, multi-layered gas hydrate reservoirs capped by fine-grained sediments and overlapping mud show favorable hydrate-bearing prospects. The release of gas hydrates, however, is mostly caused by the lack of mud sealing in relation to canyon activity, such as turbidities flow erosion and contemporaneous fault breaking. Canyon migration with respect to MTDs may be the actual cause of erosion of overlapping syn-sedimentary layers, and high bottom flows may contribute to an increase in the release of free gas. It is believed that contemporaneous faults caused by unstable canyon walls may break the muddy over layers and decrease the accumulation pressure of gas hydrate bearing. Thus, according to the sedimentary characteristics of MTDs and the hydrate accumulation process, three responding accumulation or releasing patterns are proposed, which respond to the different types of MTDs distinguished above: a well-preserved MTD accumulation pattern; a canyon migration eroded MTD release pattern; and a micro-contemporaneous fault dislocated MTD release pattern.

South China Sea  /  gas hydrate  /  sedimentary characteristics  /  MTDs  /  hydrate responding pattern
Chao FU, Shengli LI, Xinghe YU, Jinqiang LIANG, Zenggui KUANG, Yulin HE, Lina JIN. Patterns of gas hydrate accumulation in mass transport deposits related to canyon activity: Example from Shenhu drilling area in the South China Sea[J]. Acta Oceanologica Sinica, 2019 , 38 (5) : 118 -128 . DOI: 10.1007/s13131-019-1443-1
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1 Introduction
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Natural gas hydrates are quasi-ice crystalline compounds composed of water and natural gas that are formed in a low temperature and high-pressure environment (Holbrook et al., 1996). Former studies have shown that mass transport deposits (MTDs) are favorable reservoirs for gas hydrate accumulation (Wu et al., 2011; Yu et al., 2014). Complex hydrodynamic settings in slope regions and canyon activity migration science the Miocene have led to the development of many MTD types, which makes investigations into MTD preservation and corresponding gas hydrate accumulation patterns challenging (Yu et al., 2014; Riedel et al., 2013b).
If pore types are effective and suitable accumulation conditions are provided, such as suitable temperatures and pressures, it is believed that MTDs could enable gas accumulation and serve as potential reservoirs (Wu et al., 2011; Yu et al., 2014). In addition, many different deposition processes intervene with MTDs, and successive reworking lead to valuable gas hydrate accumulation patterns therein. For example, Scholz et al. (2012) and Riedel et al. (2013a) analyzed seismic data and core samples from the Ulleung Basin and found hydrate-bearing heterogeneity in different MTD reservoir types (Scholz et al., 2012; Riedel et al., 2013b). However, they also found extremely low gas hydrate saturation in a single MTD, but as it did not have a fine-grained seal bed they determined that gas hydrates mainly occur in multiple MTDs (multi-MTDs). Multi-MTDs create thick reservoirs and have good sealing conditions that are suitable for gas hydrate accumulation. Bangs et al. (2011) found variable bottom simulating reflections (BSR) between well-preserved MTDs where there was no later canyon activity reworking, and displaced one during study of the hydrate ridge in the Northwest Pacific in Western North America (Bangs et al., 2011). Nevertheless, the relationship between MTDs and hydrate accumulation has not been adequately defined and currently remains unclear.
The GMGS-3 drilling area (Fig. 1a) is located at the bottom of the northern slope of the South China Sea (SCS). Earlier research has studied the complex sedimentary setting and its depositional characteristics in the area (He et al., 2013, Chen et al., 2016). For example, per studied sedimentary characteristics in the earlier drilling area of Shenhu (GMGS-1: location shown in Fig. 1) and found that MTDs could sever as an ideal gas hydrate reservoir and that their classification can be based on topographical changes (Yu et al., 2014). From the perspective of bottom flow interaction and reworking, Gong et al. (2016) showed that the contourites or bottom currents rebuild canyons lead to canyon migration, and that the intensity of the bottom flow may also act as an external mechanism for MTD formation. He classified MTDs into two types: normal MTDs and MTDs eroded by external turbidities flow. However, due to the lack of deep-water core samples from Shenhu, the characteristics of MTD reservoirs and their hydrate response patterns therein could not be clarified.
This study uses measurements of MTDs, such as thickness, distribution length, scale covered, sedimentary characteristics, and core-log-seismic integration analyses to categorize MTDs into three types in relation to canyon activity. Unidirectional canyon migration (UCM) and erosion of the canyon wall enabled the number of MTDs in the study area to be identified using 3D seismic data interpretation and attribute analyses. In addition, seismic facies were distinguished and their profiles interpreted, which enabled classification of MTDs into three types: (1) well-preserved MTDs, (2) MTDs eroded by canyon migration, and (3) MTDs dislocated by contemporaneous faults. An analysis of hydrate accumulation theory has led to the suggestion of three corresponding patterns, namely: (1) a well-preserved MTD accumulation pattern (such as the sandwich pattern, which has two layers of fine sediment beneath and above a coarse-grained reservoir and was introduced by Malinverno (2010) in the Cascadia margin); (2) a canyon-migration eroded MTD release pattern (such as the canyon pattern introduced by Boswell et al. (2012), in the Gulf of Mexico and Vadakkepuliyambatta et al. (2015), in the Barents Sea, in which the decrease in heat flow and increase in the erosion rate in the canyon area was defined); and (3) a micro- contemporaneous faults dislocated MTD release pattern (which is determined in this study). Turbidites flow may lead to canyon wall erosion, which then causes sediments to become unstable; therefore, the above patterns may also form micro-contemporaneous faults. In each of these three patterns, a fine reservoir seal is found to be indispensable to hydrate accumulation and release. It is considered that the response patterns presented in this paper will enhance understanding of the relationship between MTD development and hydrate accumulation, and enable improved hydrate prediction in the high dip, angular, slope of passive continental margins (an area that facilitates MTD development).
This paper is organized as follows: the geological settings, methodologies, and data preparation are firstly presented to provide a background to this study. The different types of MTDs are then identified through seismic-log-core integration analysis, and MTD hydrate-bearing patterns are constructed in relation to the hydrate distribution area. Finally, factors impacting accumulation patterns are discussed.
2 Geological setting
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The new drilling area in Shenhu is located in the central section of the northern slope of the SCS and constitutes a deep-sea sedimentary area of the Baiyun Sag in the southern Zhujiang (Pearl) River Mouth Basin (ZRMB) (115°10′ to 115°20′E) (Fig. 1a). The ZRMB is a Cenozoic sedimentary basin that has experienced three tectonic stages since the Paleogene, namely a rift stage during the Paleocene to the early Oligocene, a transition in the late Oligocene, and a depression during the early Miocene to Quaternary (Han et al., 2016). During the late Oligocene to early Miocene, “Baiyun” movement (belonging to the latest tectonic stage) caused the Zhujiang (Pearl) River Estuary shelf slope to break and drift northwards, thereby becoming the depositional center of the Baiyun Sag; it then evolved into a deep-water slope environment (Clift et al., 2002; Han et al., 2016).
Han et al. (2016) proposed the following: sea levels in SCS have been rising continuously since the late Miocene, there has been an increase in the extent of SCS transgression, the seaward progress of the Zhujiang River Estuary shelf delta is limited, and there has been a simultaneous and significant northward migration of the sedimentation process. The Shenhu area lies in the southern deep-water area of the ZRMB (Fig. 1b), which is far from the provenance form the Zhujiang River; therefore, transported sediment is relatively small and fine-grained (Li et al., 2015).
The main well sites studied here are distributed in the canyon area (well sites shown in Fig. 1b). The canyon is located in the central part of the Baiyun Sag and lies mostly in a NNW direction (Fig. 1b). The drilled hydrate reservoirs are mainly composed of hemipelagic foraminifera, and fine silt and shale in the Yuehai and Wanshan Formations (Fig. 1c) (Wang et al., 2014; Sha et al., 2015). Based on multi-beam submarine topography data (Fig. 1b), the upper part of the slope is found to mainly have an asymmetric V-shaped or symmetric U-shaped structure, whereas the lower part of the slope is relatively flat. The study area is located at depths of 800–2 000 m, and there is a large gradient throughout the entire study area, with a maximum gradient of 3°.
The modern ocean circulation in the SCS is mainly controlled by semi-enclosed basin physiography and can be divided into seasonal surface-water circulation (<350 m), intermediate water circulation resulting from North Pacific intermediate water (NPIW; 350–1 350 m), and deep-water circulation associated with North Pacific deep water (>1 350 m). Along-slope bottom (contour) currents generally involve significant water masses at certain bathymetric depth ranges that persist for very long time intervals. It is therefore considered that the bottom currents associated with intermediate water circulation resulting from the NPIW (NPIW-BCs) are most likely to have had a profound influence on the development of the unidirectional migrating deep-water channels that are studied here, which are located primarily in water depths ranging from 500–1 300 m.
3 Data and methodology
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Pseudo-three-dimensional (3D) survey data were acquired by GMGS between 2008 and 2009, using a frequency of 65 Hz and a sampling interval of 1 ms over an area measuring approximately 15 km by 25 km. To accurately describe the hydrate-bearing layers, time windows were set at 3 ms per slice. Log data were acquired by Fugro and processed by Schlumberger in 2015. Data acquired by the Guangzhou Marine Geological Survey (GMGS)-3 include the use of natural gamma-rays, gamma density, resistivity, and sonic velocity. In situ core samples in the target layers were taken using two kinds of pressure-core-barre tools (FPC, PCTB) and three non-pressure-core-barre tools (FHPC, FMCB, FXMCB). Due to the salt extraction effect of hydrate, the value of the resistivity log was abnormally high in the target layers (Riedel et al., 2013a). The grain size was measured using Mastersizer® 2000 with a laser particle size analyzer (SYE209) in Well11 (W11), Well17 (W17), Well18 (W18), and Well 19 (W19).
Studies of variable MTD development patterns are based on the integration of seismic data, log data, and core analyzes. Mitchum (1977) first proposed the seismic facies analysis methodology to study sedimentary characteristics; this is based on the morphology of seismic reflectors, reflection of the seismic boundary, and the contact relation between different seismic facies (see the seismic facies listed in Fig. 2).
In this study, the turbidities flow rate in the canyon was calculated using the following four steps: (1) dividing the seismic data into three slices (Figs 3a–c: 1 500 ms, 2 000 ms and 2 500 ms); (2) decompacting the surface of each sequence using 2DMove software and reconstructing the paleo-geomorphology of each slice (Figs 3a–c); (3) calibrating the depositional rate of each well (W02, W05, W08, W16, W17, W18, W19) using the equation: vs=4.2Φ–0.2, where Φ is the corresponding grain size (see the Rose graph in Fig. 3); and (4) calibrating each well, subtracting the thickness of adjacent slices, then calculating the turbidities flow rate with well-integrated data.
The methodology used to distinguish hydrates is based on seismic reflection and corresponding log calibration. Using core samples from Wells W11, W17, W18 and W19 (Fig. 4) and their corresponding resistance curves (Rt) at depths of 40–100 m and >120 m (Fig. 4), the hydrate saturation of the bearing layers was calculated (Table 1) using Archie’s formula (the empirical parameter is determined as ‘a’ = 1.0 and ‘m’ = 1.8 (unconsolidated), through in situ pore turbidities flow salinity data).
4 Seismic geomorphology and recognition of MTDs
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4.1 Seismic facies analysis
With respect to studies of 3-D seismic volume work conducted in the study area, and according to seismic reflection results and strengths and weaknesses, the seismic facies can be divided into two types: concave facies and chaotic facies (Fig. 2). Concave facies are characterized by down lap reflector packages (Fig. 2: concave) and chaotic facies are characterized by discontinuous and disorderly reflector packages (Fig. 2: radom); these are further described as follows.
Concave facies: Based on the external geometric morphology of a seismic reflection, concave facies can be divided into lateral-concave facies (Sf. 1) and horizontal-concave facies (Sf. 2). Lateral-concave facies (Sf. 1) are characterized by discontinuous reflector packages, high-amplitude reflections, and unidirectional migration of the truncated surface (reflector packages undergo upward migration following aggrading), which can be explained as being the directional migration of the canyon (Wang et al., 2014). Horizontal-concave facies (Sf. 2) have continuous reflector packages, low-amplitudes, and show horizontal migration of the truncated surface.
Chaotic facies: chaotic facies present as discontinuous internal seismic reflectors and can be divided into mound-chaotic (Sf. 3) and chimney-chaotic (Sf. 4) facies. Mound-chaotic facies (Sf. 3) have a chaotic internal reflector package with a high amplitude and a mound reflection boundary, representing MTDs; but chimney-chaotic facies (Sf. 4) also have a chaotic internal reflector package. The chimney-shaped reflection boundary is thought to be an abnormal reflection caused by the upward movement of free gas from the bottom.
4.2 Seismic profile analysis
This paper refers to two NNW-direction profiles through well sections (inline 300 and inline 500) that were selected to study the vertical MTD distribution of sedimentary facies. Positions of the profiles are shown in Fig. 1b, where the section indicates 20 km of inline direction and 15 km of crossline direction.
Profile A (Fig. 5: inline 300) is located in the western part of the study area (profile position is shown in Fig. 1b) and passes through Well W05. Use of 1 250–1 750 ms distinguishes mound-chaotic facies (Sf. 4), and two types can be identified in the profile: (1) mound-chaotic facies (Fig. 5c) have a continuous seismic facies boundary, cluttered internal seismic event, and high attributes, while the seismic event of the upper overburden is not significantly broken; (2) mound-chaotic facies (Fig. 5d) have an indistinct facies boundary, and the seismic event of the upper overburden has an obvious dislocation.
Profile B (Fig. 6: inline 500) is located in the east of the study area (profile position is shown in Fig. 1), passes through wells W02, W08, W19, and W18, and mainly through Canyon A and Ridge B. Use of 1 250–1 500 ms enables identification of three types of mound-chaotic facies in the profile: (1) MTDs (Fig. 6c) are influenced by the effect of erosion and have a serious seismic erosion surface with a high amplitude; (2) chaotic facies (Fig. 6d) and (3) fault lines of mound-chaotic facies (Fig. 6e) are also development with a maximum length of more than 100 m and a small overall thickness.
5 Corresponding MTD classification and sedimentary characteristics
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5.1 Type I MTDs
Well-preserved MTDs developed along the unstable slope setting, and most developed along slope-breaks, such as within the southern Cretan Sea, Peri-Adriatic Sea, and SCS (Strozyk et al., 2009; Di Celma, 2011; Zhou et al., 2015). Profile B, shown in Fig. 6, shows that multi-layer developed over the well-persevered MTDs, meaning the. Combined with the corresponding hydrate-bearing core sample in Fig. 4, W11, W17, W18 and W19 show rhythmic variations in MTD grain sizes, which can be referred to as “sandwich layers”.
5.2 Type II MTDs
Based on interpretation of the seismic profile shown in Fig. 6c above, it appears that most Type II MTDs developed upstream of Type I MTDs and therefore underwent more intensive and complex reworking by the flow. Alsop et al. (2016) showed that sedimentation and turbidities flow rates are important factors in MTD formation. Meanwhile, the fine-grained layers over Type II MTDs are mostly reworked during later turbidities flow erosion.
5.3 Type III MTDs
Based on the seismic profiles shown in Figs 5b, 6c and the interpretation above, Type III MTDs are developed over a larger region. However, most are developed in the downstream slope-bottom of Type I MTDs (Fig. 7a: Type III). Micro-contemporaneous faults may have developed in relation to the following factors: free gas flow and a decrease in cementation between particles, differential settlement, and tectonic movement. For example, due to tectonic movement, the feet of Ridges A and B are formed in areas that have undergone rapid geomorphic basement changes (the dip angle changes from 5° to 2°–4° per km). Contemporaneous faults are formed when the shear stress is larger than the shearing strength of the MTD; when the sheer force of MTDs increases, the slump in progress restarts. Thus, contemporaneous faults may dislocate the above fine-grained over-layers (as in profiles Figs 5d, 6e).
6 Corresponding MTD hydrate-bearing patterns
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6.1 Hydrate reflection characteristics
Hydrate saturation in each well was determined by the pressure release in the core through in-situ observations of the four core wells (W11, W18, W17 and W19) (Table 1). However, due to the restricted number of wells in the GMGS-3 area and other parameters, and therefore the lack of data for the drilling area, it was necessary to study and evaluate the seismic reflection characteristics of the hydrate-bearing layers (Table 2) to determine the hydrate distribution. From a comparison between original sections and synthetic seismograms from high-resistivity regions (Wang et al., 2011), the bottom of the hydrate-bearing layer (BGHSZ) in GMGS-3 was found to be developed with an amplitude anomaly due to free gas, and this was found to decrease the P-velocity. Nevertheless, the BGHSZ shows a blank reflection band in the seismic profiles (Horozal et al., 2015), and the hydrate response in the BSR indicates skewed reflection characteristics and exhibits an amplitude anomaly.
6.2 Response pattern of hydrate-bearing MTDs
The progress of gas hydrate accumulation is controlled by many sediment-related factors, such as grain size, sand content, porosity, and turbidities flow rate (Wood et al., 2008; Behseresht and Bryant, 2012). The combination of a hydrate-bearing core sample and hydrate accumulation system, where the above MTDs types could be projected into three patterns, to respond to hydrate accumulation/ released (Wu et al., 2011; Sha et al., 2015; Vadakkepuliyambatta et al., 2015) (AM.1, RM.2 and RM.2) (Fig. 8), and these are explained as follows.
6.2.1 Well-preserved MTDs accumulation pattern
Type I MTDs shown in seismic profiles (Figs 5a, 6b) are well-preserved MTDs that have developed with a complete sedimentary structure, namely crown cracks, basal shear surface, and toe thrust (Fig. 7a2). These kinds of MTDs develop with moderate thicknesses (40–80 m per MTDs) and have moderate spans (100–200 m2 per MTDs). The continuous seismic events occurring above MTDs can be interpreted by the fine layer, serving as a seal and cap for free gas. Based on hydrate calculations and the volume of MTD development (as shown in Fig. 7a3) Type I MTDs have good gas hydrate reflections.
The well-preserved MTD accumulation pattern can be simplified and referred to as a“sandwich accumulation pattern” (Fig. 8a3). As shown by the porosity curve in Fig. 8a3, hydrate-bearing sand and fine mud mutually overlaps in Type I MTDs. During gas hydrate resource appraisal, the following are considered: the thermal genetic gas supply, reaction time, effective migration, reservoir litho-facies, effective porosity, and gas hydrate stability zone. This type of sandwich pattern (AM.1) not only provides sufficient reaction time for the formation of clathrate hydrates but also provides sufficient porosity for hydrates. Therefore, AM.1 types serve as reservoirs for fine-grained filling hydrate under suitable temperatures and pressures.
6.2.2 Canyon migration eroded MTD release pattern
In Type II MTDs (shown in the seismic profile of Fig. 6a), canyon migration eroded MTDs developed with an incomplete depositional structure (Fig. 7a2), small thicknesses (20–40 m per MTD) and on a moderate scale (100–200 m2 per MTD). Despite not procuring a drilling sample, the type of BRS reflection and low amplitude (Profile B and Well 2 shown in Fig. 6) prove that these MTDs have low saturation levels (Fig. 7b3). When UCM erosion has occurred and the MTD structure has been broken, a release pattern (RM. 1) is likely to form. The fine layer overlapping the coarse hydrate-bearing siltstone is also eroded. In addition, with respect to the high turbidities flow rate caused by turbidities passing (the turbidities flow rate is shown in Fig. 3), the release of free gas leads to a decrease in gas saturation in these reservoirs. Thus, as shown by RM.2 in Fig. 8b3, free gas migrates from the gas chimney and is easily released into the sea water. In addition, there is insufficient reaction time, and excess turbidities flowing in the canyon may change the heat flux distribution and geochemistry, thereby influencing hydrate accumulation.
6.2.3 Micro- contemporaneous faults dislocated MTD release pattern
Type III MTDs shown in the seismic profile (Figs 5c and 6c) are related to micro-contemporaneous faults that have dislocated MTDs (Fig. 7c2). These kinds of MTDs develop with moderate thicknesses (20–40 m per MTD) and on a small scale (<100 m2 per MTD). The seismic amplitude is less intense than that of Type I, which implies that Type III MTDs have a lower hydrate saturation than Type I MTDs (Fig. 7c3).
A previous study showed that normal faults can not only break the primary structure but can also serve as a pathway for free gas migration (Lee et al., 2012; Jeong et al., 2014). Meanwhile, normal faults provide available space for hydrates and can also serve as reservoirs for fracture-filling hydrates.
Therefore, as shown in Figs 8a3 and c3, fault slip may be a key factor for gas accumulation. If the slip distance is less than half the thickness of the MTD, it is classified as AM.1, and if not it is classified as RM. 2.
The three different MTD types shown above have different hydrate responses, and all patterns can be summarized into litho-facies assemblages. If fine layers act as a seal for the hydrate reservoir, free gas is not easily released and hydrate accumulation progresses easily. Thus, in a fine canyon system, UCM and uneven subsidence of the canyon floor may be the main causes of broken MTD reservoirs in the Shenhu new drilling area.
7 Factors impacting gas hydrate bearing-MTDs
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The hydrate accumulation pattern in Fig. 8 considers both porosity and the fine-grain seal above the MTD. In addition, according to the hydrate accumulation system, the temperature, pressure, and gas source also have an impact on accumulation.
7.1 Effect of pressure-temperature
Hydrate accumulation pressure is comprised of hydrostatic pressure and litho-static pressure. In the drilling area, the water depth ranges from 820 m to 1 445 m. On the northern slope of the SCS, values of heat flow range from 6 to 191 mW/m2 with an average value of 76 mW/m2 (Shi et al., 2003; Zhang et al., 2011).
However, differences between the preservation settings of MTDs may lead to considerable heterogeneity in the heat flow. According to the study of Vadakkepuliyambatta et al. (2015) in the Barents Sea, canyon migration cannot not only break the original MTD structure, but it can also impact the local heat distribution, which may lead to an increase in heat flow on the sides of the canyon walls. Therefore, due to the minimal impact from later canyon activity, the pressure-temperature balance of well-preserved MTDs fluctuate and has an impact on hydrate accumulation. However, when later canyon activity reworks MTDs, the original litho-static pressure and its corresponding heat flow distribution may be changed, and the gas hydrate is more easily decomposed and released.
7.2 Effect of available water and gas resources
Available water provides a clathrate molecule skeleton; therefore, an adequate amount of free water is required within pores to effectively form hydrates. However, excess available water in pores may occupy spaces between free gases and have a negative impact on hydrate accumulation. In addition, Handa (1990) found that a suitable matching relation between available water and the gas source is crucial for enabling a hydrate synthesis reaction. In pattern AM.1, abundant gas sources are provided from the bottom gas chimney, and the well-preserved MTDs provide a suitable storage space and available water. Furthermore, the fine-grain seal above the MTD acts as a seal that stops free gas diffusion, and thereby provides enough refection time between the methane gas and available water.
In patterns RM.1 and RM.2, the accumulation factors, such as gas sources, suitable storage space, and available water, are similar to those for AM.1. However, canyon-migration erosion and normal-fault dislocation are capable of breaking the original fine-grain layers above the MTD, thereby suggesting that it is difficult for gas hydrate refection over a longer time period.
7.3 Effect of later canyon migration
The three types of MTDs have relationships with canyon development and migration. Hence, the three corresponding pattern types have mutual transformations. For example, the flow action of later turbidities may lead to an unstable and unconsolidated ridge and the development of MTDs, and later geological movements may cause conversion between these three patterns. Later canyon migration leads to basement erosion and may increase the occurrence of normal MTD development. However, if the canyon migrates to a region where MTDs are developing, the AM.1 pattern may be converted into a RM.1 pattern. Furthermore, if canyon migration is limited and tractive current develops, fine sediment (sedimentary wave or levee) may cover the reservoir and serve as a seal for free gas, which may then lead to the conversion of RM. 1 and RM. 2 in to AM. 1.
8 Conclusions
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Based on the analysis of the deep-water canyon system in GMGS-3, this paper describes three types of MTDs. Three patterns are summarized based on results of a core-log-seismic integration study and with respect to hydrate accumulation.
(1) Two hydrate-bearing layers are distinguished based on an analysis of electronic log and a core sample. The upper layer is fracture-filling hydrate and the lower layer is fine-grain-filling hydrate. A saturation count of the hydrate was conducted and reflective characteristics of 3D seismic data were evaluated. The regions overwhelmed by BSR distribution fit well with the MTDs along the ridge.
(2) Through seismic data interpretation, three kinds of MTDs are classified in the new drilling area at Shenhu: well preserved MTDs, MTDs eroded by canyon migration, and MTDs dislocated by contemporaneous faults.
(3) Three types of patterns are proposed based on a comparison of the three types of MTDs and their relationships with hydrate accumulation. Results show that a fine-grained seal is the key to gas accumulation. With respect to canyon migration and normal faults, the primary structure of the MTD often breaks and layers are sealed. Sand and fine mud mutually overlap in a sandwich pattern, which is the ideal type of MTD for accumulating hydrates on the northern slope of the SCS.
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doi: 10.1007/s13131-019-1443-1
  • Receive Date：2017-12-06
  • Online Date：2026-03-31
  • Published：2019-05-25
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  • Received：2017-12-06
  • Accepted：2018-03-26
Affiliations
    1 School of Energy Resources, China University of Geosciences, Beijing 100083, China
    2 Guangzhou Marine Geological Survey, China Geological Survey, Guangzhou 510760, China

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表12种不同金属材料的力学参数
科
Family		 属数
Number of
genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
Number of
species		 占总种数比例
Percentage of total
species (%)
鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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