1 INTRODUCTION

Stabilized under specific low temperatures and high pressure, gas hydrate has been recovered throughout the global oceans. In nature, the gas hydrate stability zones were mainly formed in a limited range of water depth, and they were widely present in submarine and permafrost sediments. The accumulation of gas hydrate is commonly controlled by temperature, pressure, gas compositions, and methane flux (Lüdmann et al., 2004; Collett et al., 2009; Rajan et al., 2013; Portnov et al., 2016). In terms of the reservoir conditions of gas hydrate in deep sea, many published studies have proven that gas hydrate was preferentially accumulated and preserved in the turbidite and channel-levee systems that commonly consist of sand-rich units with relatively high porosity and permeability (Collett et al., 2009; Noguchi et al., 2011; Waite et al., 2019). In some cases, gas hydrate reservoirs occurred in submarine canyon systems, such as the Keathley Canyon in the Gulf of Mexico (Winters et al., 2008), the Dnieper Canyon of the Black Sea (Lüdmann et al., 2004), and the Penghu Canyon of South China Sea (SCS) (Sha et al., 2015). Submarine canyons are important and complex geomorphologic components that incise the continental margin, acting as conduits for delivering sediments from continental shelves to slopes, as well as deep-water basins (Bouma, 2001; Popescu et al., 2004; Antobreh and Krastel, 2006; Harris and Whiteway, 2011; Li et al., 2012). Meanwhile, it can also provide accommodation for deposition of turbidites, as well as formed high-quality hydrocarbon or gas-hydrate reservoirs (Posamentier and Kolla, 2003; Boswell et al., 2012; Lin et al., 2014). Recently, some submarine canyon systems served as important target regions for gas hydrate exploration and production. For instance, the Gulf of Mexico Gas Hydrate Joint Industry Project set several drilled wells at the Green Canyon, Keathley Canyon, and surrounding areas (Ruppel et al., 2008; Boswell et al., 2012). Additionally, the MH21 national program and several academic cooperative research projects investigated the area with the development of pockmarks and submarine canyons on the eastern margin of the Japan Sea (Matsumoto et al., 2011; Nakajima et al., 2014).

The northern continental slope of SCS has a long history of oil and gas exploration and production on basin scale, including the Qiongdongnan Basin, the Zhujiang (Pearl) River Mouth Basin (ZRMB), and the Taixinan Basin (Pang et al., 2007; Yang et al., 2015, 2017; Zhang et al., 2017; Wang et al., 2020). The Shenhu Area, situated in the middle part of the northern continental margin, has well-developed seventeen slope-confined canyons, named the Shenhu Submarine Canyon Group (SSCG) (Su et al., 2020), and it serves as a test field of gas hydrate (Fig. 1a). The occurrence of gas hydrate in the Shenhu Area has been identified and investigated through several drilling expedition by Guangzhou Marine Geological Survey (GMGS) (Yang et al., 2015, 2017; Su et al., 2016; Zhang et al., 2020a, b).

According to the previous grain-size analysis, the C-M patterns reveal that hydrate-bearing sediments at coring sites in the Shenhu Area are products of turbidity currents (Su et al., 2016, 2021). Jin et al. (2020) and Zhang et al. (2020a) documented that gas hydrate stable zones identified at the drilling sites located in the inter-canyon area were developed in the turbidite channel-levee systems. Moreover, the gas hydrate-bearing sediments distributed in the intercanyon areas (C9–C10) might be transported along turbidite channels in the north of the continental slope (Su et al., 2016). However, Sites W18 and W19 are located at the transitional zone between C11–C12 inter-canyon and C12 intra-canyon areas, where the sedimentary system is complex due to the interaction among multiple depositional processes from inter- and intra-canyon areas (Fig. 1b & c) (Ding et al., 2013; Su et al., 2020). In addition, previous relevant literature show no details about the depositional process of hydrate-bearing turbidites in the Shenhu Area, which limited our understanding on the formative mechanism of gas hydrate-bearing sediments, as well as the prediction of future exploration zones.

With abundant drilled gas hydrate sites, the intercanyon (C11–C12) and intra-canyon (C12) regions were selected as the studied area for investigating gas hydrate-bearing turbidites of the Shenhu Area. This study aims to delineate internal architecture in the C11–C12 inter-canyon area based on highresolution 3D seismic interpretation. In this study, we used detailed analysis of paleogeomorphologic variation to speculate the depositional architecture of the Quaternary section within the C11–C12 intercanyon area with a better understanding of the interaction between the formation mechanism of gas hydrate-bearing sediments and the sediment transport pattern of the submarine canyon system.

2 GEOLOGICAL SETTING

The ZRMB is an offshore sedimentary basin with a well-developed petroleum system covering an area of around 175 000 km², and this basin is bounded by the Qiongdongnan Basin to the west and the Taixinan Basin to the east (Fig. 1a). The ZRMB is the result of continental rifting and seafloor spreading during the SCS opening, and developed along the northern continental margin of SCS (Pang et al., 2007; Zhou et al., 2009; Ma et al., 2015). The lithology of basement in the ZRMB consists of Cretaceous and Jurassic granites, Mesozoic sedimentary rocks, Paleozoic quartzite, and metamorphic rocks (Zhou et al., 2009; Ding et al., 2013), and it contains up to 14 km of Cenozoic sedimentary infillings (Xie et al., 2014) (Fig. 2). The Cenozoic sedimentary evolution in the ZRMB is generally divided into syn-rift and post-rift phases, separated by the breakup unconformity dating as ca. 32 Ma (correlating to the T7 boundary interpreted in the seismic profile) (Fig. 2). During the rifting stage from the Late Cretaceous to the Early Oligocene, the depositional environment of the ZRMB was continental to shallow marine systems and deposited shallow lacustrine and fluvial to deltaic sediments, forming the Paleocene Shenhu Formation, Eocene Wenchang Formation, and Lower Oligocene Enping Formation. The stage of postrifting subsidence, from Late Oligocene to the present, has experienced a transition from neritic to bathyal-abyssal facies by the shift of shelf break from north to south (Ding et al., 2013).

Tectonically, the Shenhu Area is situated in the continental slope region of the Baiyun sag, the largest sag in the southern part of ZRMB between the Southern Uplift in the west and Dongsha Uplift in the east. The modern seafloor in the Shenhu Area is dissected by the SSCG, consisting of several slope-confined submarine canyons with uniformspaced distribution. These canyons are entirely restricted to the slope area and do not incise onto the shelf region (Zhu et al., 2010; Gong et al., 2013; He et al., 2014; Li et al., 2016; Wang et al., 2017). Some studies have documented that the canyon group in the Shenhu Area contain 18 or more canyons, because the parent and canyon tributaries were inclusive (e.g., Li et al., 2019; Zhou et al., 2021). This study only defined that the SSCG is comprised of 17 large-scale submarine canyons with length more than 15 km. These 17 canyons were named from west to east: C1 to C17 canyons, extending in a general NNW-SSE trend in water depths approximately ranging from 300 m to 1 600 m (Fig. 1a). The SSCG is the major sedimentary routing system within the ZRMB, to transport large volumes of terrestrial materials sourced from the Zhujiang River Delta and settling into the abyssal plains, in the Zhujiang River Canyon and the Southwest Subbasin to the south of the SSCG (Ding et al., 2013; Su et al., 2020).

The expeditions of GMGS01 in 2007, GMGS03 in 2015, and GMGS04 in 2016 were conducted in the Shenhu Area to obtain core samples, well log, and seismic datasets to be used to analyze the formation and accumulation mechanisms of gas hydrate stable zones (Wang et al., 2014b; Su et al., 2015; Zhang et al., 2020b). During GMGS01, all core samples containing gas hydrate were located in the inter-canyon areas between the C9 and C10 canyons. In 2015, gas hydrates stable zone are present in the Quaternary deposits at 19 drilling sites of GMGS03 expeditions, and most of these sites are distributed in inter- and intra-canyon regions of the SSCG system (Yang et al., 2015; Su et al., 2016). At the sites W11, W17, W18, and W19 of GMGS03, the core samples and logging-while-drilling (LWD) data identified significant reservoirs of gas hydrate (Zhang et al., 2017, 2020b; Jin et al., 2020). The drilling site of W07 from GMGS04 in 2016 was also situated in the C11–C12 inter-canyon area (Zhang et al., 2020a). The reservoir units at these coring sites acquired during the GMGS01, GMGS03, and GMGS04 consisted of mostly silt and silty clay, with 20% to 48% average saturation of gas hydrate (Wang et al., 2014a; Yang et al., 2015).

3 DATA AND METHOD

The primary data used in this study is highresolution 3D and conventional 2D seismic data. The 3D seismic survey was conducted in 2018 by GMGS in the middle part of the Shenhu Area. The total area of the 3D seismic survey is about 850 km², and it has incline interval of 6.25 m and crossline interval of 12.5 m, with vertical range from 0 to 8 000 ms of two-way travel time (TWT). This survey fully covers C9, C10, C11, C12, and partial area of C8 and C13 canyons, ranging from 400 to 1 600 m water depth. The sedimentary and geomorphologic features indicated by the 3D seismic data were interpreted by using the software GeoframeTM. These horizons were manually tracked every 20 to 10 crossline and inline in average. The seawater column velocity used to calculate the water depth of presentday seafloor is 1 500 m/s.

To delineate gas hydrate-bearing turbidites in the 3D seismic volume, the base and top surfaces of gas hydrate stable zones shown in the seismic profile were determined by analysis of well-log data at Sites W07, W18, and W19 that obtained from the publication released by previous literature (Yang et al., 2015, 2017, 2020; Zhang et al., 2017, 2020a, b; Jin et al., 2020). Classification of seismic facies was based on seismic reflector features, including amplitude, textures, external configuration, and reflection terminations. To characterize the internal architecture of gas hydratebearing turbidites, selected seismic key horizons and unconformities within the C11–C12 inter-canyon and C12 intra-canyon regions above the base of Quaternary (T1) have been interpreted and mapped throughout the studied area. The seismic stratigraphic framework was calibrated by drilling data from the Ocean Drilling Program (ODP) Sites 1146 and 1148 in the southeast of ZRMB (Su et al., 2019). The slice extracted from variance cube and root-meansquare (RMS) amplitude was computed from the interpretation of selected seismic horizons using restricted time windows of 10 ms, and it was used to observe and analyze plan-view paleogeomorphology of gas hydrate-bearing sediments distributed in the C11–C12 inter-canyon area of the SSCG.

4 RESULT 4.1 Seismic facies analysis of Quaternary succession within the inter-canyon area

Seismic facies, occurred within the inter-canyon area and adjacent transition zone between the C11 and C12 canyons, are classified into five types from Facies A to Facies E, differentiated by their external geometry, internal reflector configuration, continuity, amplitude, and strata termination. Those seismic facies were converted to sedimentary facies by considering different seismic characteristics and depositional environments.

Facies 1 is characterized by subparallel reflections of low amplitude and moderate continuity. The overall external shape is lenticular, and the internal reflectors onlapped against the erosive bases. The bottom surfaces of these facies exhibit U-shaped incised channels with variable scales (Fig. 3a). Through analysis of seismic facies, this type of facies was considered as mud-rich turbidites with low net-to-gross interbedded sandstones (Mayall et al., 2006; Su et al., 2016).

Facies 2 is shown as variable amplitude, discontinuous, chaotic seismic reflections with irregular external forms. Most of the features is widely distributed within the transition zones between inter- and intra-canyon areas, and commonly thin away from topographic high (Fig. 3b). Facies 2 can be interpreted as slumping (debris-flow) deposits that glided from the intercanyon ridge down toward the valley.

Facies 3 is a fill-type facies characterized by a concave-up basal surface, displayed as moderateamplitude, highly-continuous, and subparallel seismic package with lenticular geometry (Fig. 3c). These seismic facies are interbedded with some layers of low-amplitude to transparent reflections. The basal surface was recognized as high-amplitude reflection truncating surrounding reflections, and it is well developed within the inter-canyon area. This type of seismic facies could be interpreted as the high net to gross channel fills due to gravity flows.

Facies 4 features low-amplitude to transparent, discontinuous, and chaotic internal reflector structure and exhibits external lenticular configuration. The basal surfaces of Facies 4 show distinct U-shaped incised channels. The facies are usually interbedded with the Facies 3 and filled with the valleys. Commonly, this facies type is usually interpreted to be mass transport deposits (MTDs).

Facies 5 consists of low- and moderate-amplitude, high-continuity, and parallel to subparallel wavy seismic reflections with thick layers. The overall geometry of these facies is sheet-like, draping conformably onto underlying topography (Fig. 3e). They commonly capped Facies 3 and 4, and fully covered the inter-canyon areas (Fig. 3c–d). Within the canyon transitional zones, these seismic facies display highly wavy features resulting from internal deformation. Facies 5 could be the product of pelagic deposition or dilute unconfined turbidity currents, and the wavy reflectors were probably affected by slope failure and were reworked by along-slope contour currents (Rebesco and Stow, 2001; Qiao et al., 2015).

4.2 Sedimentary and geomorphologic variation of the C11–C12 inter-canyon area

The C11–C12 inter-canyon area extends in nearly N-S orientation along the continental slope, with 550–1 400-m water depth, and the average width is about 2.5 km. In the slope profile, the C11–C12 inter-canyon area shows a linear shape, and it could be subdivided into three main sections, containing upper, middle, and lower segments from north to south (Fig. 1b). The upper segment ranges from 745–950 ms (TWT), converting to 550–700 m of water depth, with a slight change in slope gradient averaging 1.11°. The gradient profiles within the middle segment from 950–1 440 ms (TWT) (700–1 100 m of water depth) has fluctuation, and the average gradient is about 2.10° is higher than the upper segments. The slope gradient of the lower segment from 1 440–1 850 ms (TWT) (1 100–1 400 m of water depth) is highly variable, with 3.67° on average.

4.2.1 The upper segment

The Quaternary succession in the 3D seismic volume reaches 300–380 ms (TWT) thick within the upper segment. The base of the Quaternary (H1) often shows a distinct unconformity with highamplitude reflection in the seismic section. C9 canyon with higher maturity deeply incised into the Quaternary deposits, but the heads of C11 and C12 canyons were not developed in the upper segment. Internally, numerous truncations and onlapped terminations are shown in the lower part of the Quaternary succession and are resulted from the development of channel system. A dense of Ushaped channel features, about 450–1 000 m wide and 50–85 ms (TWT) deep, were clearly imaged above the H1, and they are interpreted as smallscale turbidity channels through the upper segment from north to south. The primary fill features in those small-scale channels are Facies 1, characterized by flat to inclined semi-continuous, low-amplitude seismic reflections (Fig. 4) that are considered as fined-grained deposits from low-density gravity flows along paleo-axial channels. Minor channel-fill deposits with Facies 3 and 4 are also observed and spatially distributed in the lower and middle part of the Quaternary succession (Fig. 4). Those channels developed in the upper segment are sporadically distributed, appearing predominantly disorganized stacking pattern. Those channel-fill units are overlain by a thick sedimentary layer, up to 200 ms (TWT), mainly consisting of Facies 5 characterized by continuous, subparallel wavy reflections that are considered as the shelf-edge deltaic deposits extending successively during the Late Quaternary (Su et al., 2016, 2019). Facies 5 often change to irregularly wavy layered reflections on the canyon flanks, indicating sedimentary slide with slight deformation by sediment failures (Fig. 4).

4.2.2 The middle-lower segment 4.2.2.1 Characteristics of gas hydrate-bearing sediments at W07 site

Zhang et al. (2020a) used integrated analysis of seismic data and LWD to characterize the gas hydrate stable zone of Site W07 located at the middle segment, and they demonstrated that the dynamic process of hydrate formation and dissociation is associated with the channel-levee system erosion and sedimentation. The gas hydrate-bearing sediments at Site W07 are identified and ranged from 122 to 153 mbsf, corresponding to relatively high electrical resistivity and compressional wave travel time (DTCO) of P-wave (sonic) in the log dataset (Fig. 5a). The mean grain size of Site W07 has no significant vertical variation within the interval between 42 and 156 mbsf, with 6–7 φ. The lithology of gas hydratebearing sediments is characterized with silt and clayey silt, consisting of sand (0.4%–10%), silt (67%–82%), and clay (14%–32%) (Fig. 5b). The C-M diagram of grain-size analysis exhibits that the major population trend of plots in the gas hydratebearing sediments almost mimic the baseline of C=M, which indicated that the reservoir units at the Site W07 are often considered as fine-grained turbidites (Fig. 5c). By the observation of SSE-trending seismic profile, multiple small-scale turbidite channels were spatially distributed in the upper part of middle segment mainly filled with Facies 1, and are absent in the location of Site W07 and surrounding area (Fig. 6). In the seismic profiles, the gas hydratebearing sediments at Site W07 are correlated to highamplitude seismic anomaly over bottom simulating reflector (BSR) mimicking the seafloor topography, showing different seismic characteristics from the facies of small-scale turbidity channels occurring in the middle segment (Fig. 6). The external geometries of reservoir unit are featured with mounded or undulation shapes extending about 4 km along slope, which is similar to its overlaying layers F5 (Fig. 6). Moreover, some vertically aligned features with low-amplitude, transparent, and chaotic texture were observed, commonly penetrated the unconformity of H1, and they were terminated at the shallow layers mainly characterize with highamplitude seismic anomalies (Fig. 6). The occurrence of vertically aligned features indicated welldeveloped fluid migration system within the C11–C12 inter-canyon area transporting hydrocarbons from deep to shallow successions by gas chimneys, faults, and diapirs, the important paths of gas hydrate formation process.

4.2.2.2 Analysis of seismic geomorphologic in the middle-lower segment

In the NE-SW trending seismic profile, the gas hydrate-bearing sediments of Site W07 were located at topographic high in the eastern part of the inter-canyon area. A distinct U-shaped incised valley is identified as deep-cutting the H1 erosive surface, interpreted by onlap and truncated terminations. This incised feature was filled with sediments exhibiting partial highamplitude seismic anomaly between the H1 and H2 surfaces, which is similar to the seismic characteristics of gas hydrate-bearing sediments at Site W07 and surrounding sediments of BSRs (Fig. 7a). In the middle segment, U-shaped valley was developed in the center of inter-canyon area with depth of 110 ms (TWT), and seismic facies analysis revealed that the channel fills were Facies 3 and 4, interpreted as MTDs and turbidites deposited by gravity-driven flows along the slope (Fig. 7b). This channel complex consisted of multiple channel elements with predominantly vertical stacking pattern, recording the multiple phases of cuts and fills by debris or turbidity flow events. The sedimentary strata below the channel complexes are commonly characterized by seismic anomalies of high-amplitude reflections and low-amplitude chaotic textures, regarded as an indicator of fluid migrations in the deep marine (Fig. 7b).

The plan-view morphology of this incision valley within the C11–C12 inter-canyon area is also well imaged by the variance cube slice created by interpreted H2 surface (Fig. 8a). The incised valley was channelized from the middle to lower segment along NNW-SSE orientation, and it was about 8 km long, with average width of 1 km. The flanks of channel are constituted by multiple steep scours with lunate shapes in plain view, ranging from 380 to 760 m (Fig. 8b). Seismic profiles show most steep scours commonly overlaying the stratified unit with subparallel and wavy reflections and chaotic unit (Facies 4), which were interpreted as sediment slumps, gliding from topographic highs (Fig. 7b). A set of linear features observed in the plan-view map were distributed in the eastern part of the inter-canyon area. They extended along the nearly W-E trend and extended to the intra-canyon areas (Fig. 8b). In the high-relief topography, these linear features are generally associated with the effects of mass wasting events. The RMS amplitude was extracted from time windows 0–5 ms (TWT) below the H2 surface. The high RMS amplitude attribute shows that the high-amplitude seismic reflections are mainly distributed within the inter-canyon and transitional regions, showing a zonal distribution pattern trending SSE direction (Fig. 7d). The location of incised channel is consistent with the occurrence of high RMS amplitude in the western part of C11–C12 inter-canyon area, and it reveals that channel-fill units within the inter-canyon area might be served as reservoirs of gas hydrate and free gas.

4.2.3 Canyon transitional zones

The canyon transition zone is the canyon flank between intra-canyon and inter-canyon areas. The drilling sites of W18 and W19 are located at the canyon transitional zone between the inter- and intra-canyon regions in the lower segment of the Shenhu Area (Fig. 1c). Based on the analysis of grain-size datasets, the lithology of gas hydratebearing sediments at Sites W18 and W19 is mainly composed by silty and clayey silt, and they were generated by turbidity currents (Su et al., 2021). Through integrated analysis of core samples and log data, gas hydrate stable zones at Sites W18 and W19 were present at the intervals of 25 and 28 m thick, respectively, which corresponds to a thin layer featured with high-amplitude seismic anomalies in seismic data (Jin et al., 2020; Zhang et al., 2020b). The geomorphologic features of Site W18 and W19 in cross-section views display a channel-levee system with channel depth up to 45 ms (TWT) above the H2 surface. This channel-levee system is disconnected from channel complexes within the C11–C12 intercanyon area and totally capped by canyon-fill deposits (Fig. 9a). The internal seismic reflections are very poorly imaged in the channel-levee system, and the facies of channel-fill units are difficult to interpret, due to the seismic anomalies caused by gas hydrate or free gas accumulation (Fig. 9a & b). It indicates that the channel-fill and levee deposits might be reservoir units hosting gas hydrate and free gas.

The variance cube slice shows that Site W19 is situated on the erosional surface in the transitional zone, interconnected with the inter- and intra-canyon regions. The geomorphologic characteristics in the plain view trend SE direction with about 1.7 km long and 270–750 m wide, and appear semicircular heads with amphitheatre-like rims. The internal structure exhibits a train of crevasse-shaped bedforms along its axial channel, and it suggests that a volume of sediments was funneled downslope by repeated mass wasting via this active channel from inter- to intra-canyon (Fig. 8c). Additionally, Site W18 is located at the west of erosional surface (Fig. 8c), where the depositional setting might be strongly associated with suspended transport by overflows from mass wasting events.

5 DISCUSSION

The canyon infilling characteristics within the intra-canyon areas of SSCG responded to different sedimentary transported processes from the canyon head area to the lower reach (He et al., 2014; Qiao et al., 2015; Su et al., 2020). Depending on the variation in geomorphologies and internal architectures with time, we can infer the formational mechanisms of gas hydrate-bearing sediments that are impacted by various depositional conditions within different parts of the inter-canyon area.

5.1 Gas hydrate-bearing sediments form by slide processes

The seismic profile shows the lack of channelization and Facies 1 at Site W07 located in the middle segment of the C11–C12 inter-canyon area, indicating that the gas hydrate-bearing units mainly consisting of fine-grained turbidites might not be associated with channelized incision by sea-level lowstand. In addition, the geometry of gas hydrate-bearing sediments exhibits mounded or undulation shapes, which is similar to overlaying Facies 5 units with wavy internal reflections interpreted as internal deformation caused by slope failure (Fig. 6). Owing to homogeneous lithology consisting of silts or muddy silts, the fine-grained turbidite transported from the upper slope remained at Site W07 providing reservoir units of gas hydrate. Therefore, the gas hydrate-bearing sediments at Site W07 might be deposited by gravity-driven destabilization, depositing as slide features within the middle segment.

In terms of the sediment source, the gas hydratebearing sediments at the W07 site are more likely to be supplied from the upper slope region in the north. Based on geomorphology and internal architecture, the channel system extended along NEE-SWW orientation, and it implies that those small-scale channels developed in the upper segment are more likely to be fed by sediment fluxes from the continental shelf in the north. The distribution pattern of the small-scale channels with cut-and-fill features (Facies 1) suggested that eroded materials and sediments from the shelf were downslope delivered southward and were mainly restricted within the upper segment, where slope gradient might be relatively low at the initial stage during early Quaternary (Fig. 10). The gravity flows eroded onto pre-existing topography producing minor angular unconformity (H1) and a set of small-scale incised valleys. The age of H1 unconformity is dated as approximately 2.5 Ma (Fig. 2), corresponding to the contact between the Pliocene and Quaternary strata. Curves of oxygen (δ¹⁸O) and carbon (δ¹³C) isotopes of benthic foraminifer reflect the amplitude of sea level fluctuation. The base of Quaternary (H1) in the Baiyun Sag is highlighted in low value of δ¹⁸O or high value of δ¹³C, recording a regression event in the study area associated with a change in global climate or glaciation during this period (Wang et al., 2014a; Lin et al., 2018). The widespread channels with shallow incision suggest sea-level lowstand of very short duration occurred during the early Quaternary.

5.2 Gas hydrate-bearing sediments form by secondary turbidite currents

In the middle to lower segment, there exists an erosive channelized feature developed downslope along the C11–C12 inter-canyon area. The flanks of channel appear multiple and individual lunateshaped slide scars that are the products of sediment instability. It indicates that the development of channelized feature across the inter-canyon area has changed in the gradient of surrounding sediments, leading to partial sediments movement from topographic highs to lows (Dugan and Fleming, 2000; Ridente et al., 2007; Ercilla et al., 2008). In the plan-view attribute map, the locations of Sites W07, W18, W19 and the incised channel show high RMS amplitude characteristics, representing higher concentration of free gas and/or gas hydrate compared with the surroundings (Fig. 8d). Thereby, the gas hydrate-bearing sediments of Site W07 at topographic high tended to be failed away by mass wasting events to the channelized feature, and further transport to lower segment.

Additionally, this transition zone, generally showing steeper slope gradients, could be controlled by both sedimentary processes from inter- and intra-canyon effects. During the evolution of submarine canyons, canyon heads migrated with repeated retrogressive erosion and penetrated to the upper part of the middle segment, and lateral canyon flanks continuously widened (Pratson and Coakley, 1996; Puga-Bernabéu et al., 2013; He et al., 2014; Tournadour et al., 2017). The submarine canyon produced the geomorphologic change within the continental slope system, increasing the slope gradient within the transitional zone, where multiple failure scars were recognized. By observation of the variance cube slice, the direction of failure scars is almost perpendicular to the canyons, and they are more visible in the eastern part of the C11–C12 inter-canyon area, where slope gradients are gentler than in the western part (Fig. 10). Near Sites W18 and W19, gas hydratebearing sediments dispersed in topographic lows generated by the erosive surface of sedimentary slump/slide on the western flank of C12 canyon. This negative topographic feature offered accumulation space for sediment deposition generating turbidite reservoir in this part. The occurrence of multiple scars suggests that partial sediments within the lower segment were migrated by lateral migration of turbidity flows to the canyon floor, and it indicates those erosive surfaces created by sediment failures might be small-scale conduits to interconnecting the inter-canyon and intra-canyon areas.

5.3 Reconstructing model for the formation of gas hydrate-bearing sediments

By analyzing variation in sedimentary conditions, we reconstructed a model of the depositional patterns of gas hydrate-bearing sediments from the upper to lower segment of the inter-canyon area (Fig. 10). During the early Quaternary, multiple small-scale channels were formed by the erosion dominant environment in the sea-level stand with very short-term period, triggering a large volume of sediments entering and settling in the upper slope. Abundant sediments supplied from the shelf tended to be delivered through the small-scale channels, and they increased sedimentary thickness and slope gradient within the upper slope. When exceeding the maximum limit of sediment strength or stability, sediment failures would commonly occur in the inter-canyon area and they were further transported by gravity destabilization and were settled at the middle segment, showing generally slight deformation of internal structures. For the gas hydrate-bearing sediments in the lower segment, it is mainly determined by the evolution of channelized features that serve as a conduit system interconnecting with the middle and lower segment to downward deliver failed sediments by turbidity flows. The turbidity currents also flowed across the canyon transition zone by lateral migration, remobilizing sediments in the lower segment from the inter-canyon area to the canyon floor, and sediments were furtherly delivered along the axial incision of the intra-canyon regions.

In the marine system, gas hydrate with sufficient methane supply can be accumulated in different types of deep-water sediments, such as turbidites, MTDs, and contourites (Lüdmann et al., 2004; Collett et al., 2009; Rajan et al., 2013; Portnov et al., 2016). Gas hydrate was preferentially preserved in the fine-grained turbidites in the inter-canyon areas of the Shenhu Area (Su et al., 2016, 2021; Zhang et al., 2017). The sedimentary processes of the intercanyon areas primarily controlled the distribution of gas hydrate reservoirs, and they might directly or indirectly influence the gas hydrate saturations and reservoir porosity at different drilled wells. However, due to the lack of well logging data and coring samples collected from drilled sites, we only obtained information about gas hydrate saturation and sediment porosities from previous studies that contain limited data of these parameters. They mentioned that the maximum gas hydrate saturation of the W07 site is about up to 65%, with an average saturation of 43% (Yang et al., 2020). W18 and W19 have relatively 25% and 45% of average gas hydrate saturation (Jin et al., 2020; Zhang et al., 2020b). The average porosities of the gas hydrate reservoirs are relatively high in fine-grain sediments at W07 (56%–62%), W18 (56.7%), and W19 (48.3%) (Zhang et al., 2017, 2020b; Yang et al., 2020). The average saturation and reservoir porosity show a subtle variation among these three drilled sites, and the sample size is insufficient. In addition, based on the results of lithologic analysis, there is no significant difference in the sedimentary features of the gas hydrate-bearing sediments at W07, W18, and W19 that are predominantly composed of fine-grained turbidites with silt and clayey silt (Zhang et al., 2020a; Su et al., 2021). It is difficult to prove and explain the variation in gas hydrate saturation and reservoir porosities by comparing sedimentary features at different drilled sites.

6 CONCLUSION

High-resolution 3D seismic data, integrated with LWD and grain-size data derived from Sites W07, W18, and W19, were used to characterize sedimentary architectures in the C11–C12 inter-canyon area of Shenhu Area. This work demonstrated that free gas and gas hydrate are recognized as high-amplitude seismic anomalies and partially distributed in the C11–C12 inter-canyon area, and the distribution of this seismic facies is consistent with positions of the channelized feature in the middle-lower segment and slide erosive surface in the canyon transition zone. By analysis of seismic facies and geomorphologic features, the gas hydrate-bearing sediments in the middle and lower segment of inter-canyon area were deposited by different depositional patterns. In the middle segment, the turbidite features might be initially deposited in the upper segment that is featured by multiple small-scale channel system, and they were further delivered by slope failure generating sliding features. The gas hydratebearing sediments in the lower segment might be formed by downslope gravity flows via the channelized features. Furthermore, with evolution of submarine canyons, the oversteepened topography of inter-canyon area was easy to trigger sediment failure, producing multiple or individual failure scars or erosive surfaces, which tend to serve as small-scale conduit and also provided accommodation space for turbidity deposition. The work in this study demonstrates that distributions of free gas and gas hydrate reservoir units within the Early Quaternary succession are mainly controlled by a complex sedimentary transport model of the intercanyon area, from the upper to lower segment.

7 DATA AVAILABILITY STATEMENT

Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.

8 ACKNOWLEDGMENT

We are grateful to Guangzhou Marine Geological Survey (GMGS) for providing seismic and well data as well as permitting to publish. We also thank the anonymous reviewers for their comments and suggestions, which improved the early version of this manuscript.