This study focused on the gas hydrate reservoir and adjacent layers of site W19 (114.4–171 mbsf) and 39 samples were obtained for SEM observations, CT scans, and specific surface area (SSA) analyses (Table 1; Figure 3). The gas hydrate in the core samples decomposed under changes in temperature and pressure conditions (Figure 2A). However, gas hydrate decomposition has a limited influence on the mineral and biological components and SSA in sediments (Zhang et al., 2016; Li et al., 2019; Waite et al., 2019). The microstructure of the core samples is not in the in-situ state with gas hydrates. However, the samples can represent the microstructural features of the sediment before gas hydrate formation, which is significant for gas hydrate accumulation analysis. The samples used for SEM observation and CT scans in this study were obtained using plastic square cubes (2 × 2 × 2 cm), which helped preserve the original microstructural features as much as possible (Figure 2B). The samples were stored on dry ice until the tests were conducted in the laboratory.

As the square cubes were made of plastic, which has no effect on the CT scan, the samples and square cubes were tested directly to obtain CT scan images and data. A total 12 samples were used for CT scan. The CT scan was performed by Nanjing Hongchuang Geological Exploration Technology Service Co., Ltd.

The samples used for the SEM observation were prepared as follows:

(1) First, the epoxy resin and curing agent proportions were adjusted. We used more epoxy resin and less curing agent, which prolongs the curing time of epoxy resin and facilitates infiltration.

The samples for SEM observation were prepared at the Institute of Geology and Geophysics, Chinese Academy of Sciences.

The analysis of gas hydrate saturation of site W19 was mainly based on the voyage report of GMGS3. In this study, resistivity log data, Cl⁻ anomalies of the pore water in sediment, and quantitative degassing of the pressure core were used to analyze the gas hydrate variation trend (Dvorkin et al., 1999; Qian et al., 2018; Ye et al., 2019).

Biological data were obtained from the voyage report of GMGS3 and were tested at the Test Center of the Guangzhou Marine Geological Survey.

SSA analysis was conducted using Quantachrome Autosorb-1 and Quantachrome Nova Station A instruments at the Institute of Analysis and Testing, Beijing Academy of Science and Technology. Before the analysis, the samples were degassed at approximately 383.15 K under vacuum for approximately 12 h to remove adsorbed moisture and residual volatiles. The N₂ sorption isotherms were determined at 77.35 K within a relative pressure (P/Po = absolute/saturation pressure) range of 0.009–0.995. The specific surface area was calculated using the Brunauer-Emmet-Teller equation (Brunauer et al., 1938).

The lithological data of site W19 were collected from the voyage report of the GMGS3. In the study interval at site W19, the sediments were dominated by clayey silt and silt. The sand content of the sediments ranged from 0.12 to 10.85%, with an average of 3.34%. The silt content of the sediments ranged from 55.64 to 80.07%, with an average of 70.00%. The clay content of the sediments ranged from 17.21 to 44.19%, with an average of 26.66%. In the upper part of the study interval (114.4–145 mbsf), the sediments were dominated by clayey silt with only a few silt layers in the bottom part (Figure 3). In the lower part of the study interval (145–171 mbsf), the sediments were dominated by silt with several interbedded clayey silt layers (Figure 3). In Site W19, the sediments mainly consist of quartz, feldspar, calcite, and clay minerals, and a few dolomite, siderite, and anhydrite appear in some layers. The clay minerals were dominated by illite/smectite, illite, kaolinite, and chlorite.

Based on the SEM observations, the sediments at site W19 mainly consisted of quartz, feldspar, carbonates, and clay minerals (Figure 4A). The quartz mainly exhibited a granular shape with a diameter of 8–30 μm. The edge of the quartz was smooth and exhibited some abrasion (Figure 4B). The feldspar in the sediment could be classified into two categories, K-feldspar and plagioclase, which both have similar morphology. The diameter of the feldspar ranged from 10 to 30 μm, while its edge was smooth but indicated some traces of corrosion (Figures 4A, C). The carbonates in the sediment were mostly calcite, which appeared as fossils. Some fossils were calcareous ultramicrofossils with diameters of less than 2 μm (Figure 4D). Other fossils were foraminiferal shells, which consisted of calcite. The calcite particles in the foraminiferal shells were fibrous with unclear edges. The diameter of the foraminiferal shells was >20 μm (Figure 4E). Many clay minerals in the sediment included filamentous or flaky illite/smectite (I/S) (Figure 4F), illite (Figures 4G, H), sheet-like kaolinite, and flaky chlorite (Figure 4I). The diameter of the clay mineral particles was less than 1 μm.

In this study, gas hydrate saturation refers to resistivity log data, Cl⁻ anomalies of the pore water in the sediment, and quantitative degassing of the pressure core. The gas hydrate saturation ranged from 10 to 54% based on the resistivity log data (Figure 3). Based on the Cl⁻ anomalies of the pore water in the sediments, the gas hydrate saturation ranged from 0.21 to 70.90% (Figure 3). According to the voyage of GMGS3, only three pressure cores (12, 13, and 21A) were successfully obtained, from which we collected 234 L, 228 L, and 190 L, respectively, of methane gas. The gas hydrate saturation values calculated for the three pressure cores were 60, 58, and 49%, respectively (Figure 3).

The biological component mainly consisted of foraminifera and calcareous ultramicrofossils based on SEM observations. The abundance of the foraminifera in the study interval ranged from 238.8 ind/g to 39,014.4 ind/g, with an average of 3,214.5 ind/g. The abundance of calcareous ultramicrofossils ranged from 758 ind/g to 2,895 ind/g, with an average of 1,872.1 ind/g.

The SSA data are presented in Table 2; Figure 3. The SSA of the sediments in the study interval ranged from 11.49 m²/g to 24.86 m²/g, with an average of 15.6 m²/g.

The quantitative degassing of the pressure core provides the most accurate gas hydrate saturation. However, owing to the high cost of obtaining a pressure core, data on gas hydrate saturation with quantitative degassing of the pressure core are scarce. It is difficult to indicate the variation in gas hydrate saturation. The gas hydrate saturation estimated by the resistivity logs was continuous and independent of the sediment samples. However, resistivity logs have multiple solutions (Chand et al., 2004; Collett et al., 2012; Wang et al., 2014). The gas hydrate saturation estimated by the Cl⁻ anomalies of pore water in sediment is relatively accurate and relatively continuous (Lu and McMechan, 2002; Su et al., 2021). Meanwhile, the study of microstructures depends on the sediment sample, and it can be well matched with the gas hydrate saturation estimated by the Cl⁻ anomalies of the pore water in the sediment.

The gas hydrate saturation was very low in the upper part of the study interval, highest in the middle part, decreased with fluctuation, and was lower in the bottom part (Figure 3). Combined with the methane gas migration direction, the study interval was divided into upper and lower sections with 134 mbsf as the boundary. The upper section was the non-gas hydrate layer (114.4–130.39 mbsf). Furthermore, the lower section was the gas hydrate reservoir layer, which was subdivided into three layers according to the gas hydrate saturation: 1) I-gas hydrate reservoir (I-GHR) layer, where the gas hydrate saturation ranged from 58.48 to 70.90%, with an average of 64.84% (Table 3); 2) II-gas hydrate reservoir (II-GHR) layer, where the gas hydrate saturation ranged from 17.27 to 49.27%, with an average of 33.95% (Table 3); 3) III- gas hydrate reservoir (III-GHR) layer, where the gas hydrate saturation ranged from 0.21 to 6.44%, with an average of 1.88% (Table 3).

The study of the sediment microstructure relied on SEM observations and CT scans. CT scans can directly display the microstructural features of sediment, particularly the size and distribution of pores (Jones et al., 2007; Bian et al., 2020; Wu et al., 2020). SEM observations can reveal more details of microstructural features, such as more accurate size, morphology, and sediment composition of pores (Bohrmann et al., 2007; Klapp et al., 2010; Oshima et al., 2019; Cai et al., 2020).

The SEM observation and CT scan results indicate that there are 1) intergranular pores between coarse grains (CG-intergranular pores), 2) intergranular pores between fine grains (FG-intergranular pores), and 3) biologic grain pores (BG-pores) in the study interval sediments (Figure 5A).

CG-intergranular pores occur between coarse grains such as quartz, feldspar, or foraminiferal fossils and have irregular pore shapes. They have pore sizes between 10 and 100 μm (Figure 5B). FG-intergranular pores occur between coarse grains, such as clay minerals. Their pore shape varies with the morphology of the clay mineral, while their pore size is typically less than 1 μm (Figure 5C). BG-pores consist of coeloms of foraminifera. Their pore shape is either spherical or ellipsoidal, while the pore size is commonly greater than 50 μm (Figure 5D).

The microstructural features of the sediments among the non-GH, I-, II-, and III-GHR layers were significantly different. In the non-GH layer, the CT scan showed that the sediment had massive structures with many FG-intergranular pores and a few CG-intergranular pores (Figures 6A–C). The BG-pores in the non-GH layer were specific, and many FG-intergranular pore aggregates showed spherical BG-pore shapes visible in CT scan images (Figures 6A–C). The SEM observations confirmed that the BG-pores were filled with other components (Figure 6D).

In the I-GHR layer, the sediment microstructure was loose compared to the non-GH layer and had many CG-intergranular pores, BG-pores, and some FG-intergranular pores (Figures 7A, B). The SEM observations showed features similar to those observed in the CT scan (Figures 7C, D).

In the III-GHR layer, CG-intergranular pores and BG-pores were rare, while there were few FG-intergranular pores (Figures 8A–C). However, the clay fraction content in the III-GHR layer was not significantly different from those in the non-GH and I-GHR layers (Figure 3). This indicates that the structure of the clay minerals in the III-GHR layer was different from that in the non-GH and I-GHR layers.

In the II-GHR layer, some samples exhibited similar features to samples of the I-GHR layer (Figures 9A, C), while others exhibited similar features to those of the III-GHR layer (Figures 9B, D). Thus, the II-GHR layer acted as a transitional layer between the I-GHR and III-GHR layers, which is consistent with the wide range of gas hydrate saturation in the II-GHR layer (Figure 3).

The effect of the microstructure on gas hydrate accumulation is generally reflected in porosity and permeability (Jones et al., 2007; Klapp et al., 2010; Cai et al., 2020; Wang et al., 2020; Li et al., 2021). Previous studies have shown that large pores formed by coarser sediments are conducive to gas hydrate accumulation (Colwell et al., 2004; Boswell et al., 2009; Ito et al., 2015; Heeschen et al., 2016; Bian et al., 2020). The grain size of sediments does not affect the total porosity but may significantly affect the effective reservoir space and permeability (Wang et al., 2020). It may be difficult for extremely small pore spaces to form and accumulate gas hydrates owing to the migration ability of gas or pore water and capillary pressure (Brown and Ransom, 1996; Gamage et al., 2011; Li et al., 2021). Meanwhile, at the same porosity, more small pores lead to a significant decrease in sediment permeability (Brown and Ransom, 1996; Gamage et al., 2011; Oshima et al., 2019; Wu et al., 2020). Thus, a sediment layer with more CG-intergranular pores has more effective reservoir spaces and high permeability, which is beneficial for gas hydrate accumulation.

Many previous studies have shown that BG-pores consisting of foraminiferal coeloms can improve the quality of gas hydrate reservoirs, as foraminiferal coeloms are large and can thus provide additional effective reservoir space (Wang et al., 2011; Chen et al., 2013; Wang et al., 2014; Zhang et al., 2015; Li et al., 2019). In the I-, II-, and III-GHR layers, BG-pores gradually decreased, which is consistent with previous research results. However, according to Figure 3, the abundance of foraminifera was the highest in the non-GH layer, in contrast with previous research results. Meanwhile, BG-pore filling was verified both by SEM observations and CT scans (Figure 6 and Figure 10). The filling components were clay minerals and many calcareous ultramicrofossils (Figure 10). Calcareous ultramicrofossils have very small grain sizes (typically less than 2 μm) and clay fraction content. Thus, calcareous ultramicrofossils will reduce the effective reservoir space of the BG-pores and reduce the permeability of the sediment together with clay minerals.

The adsorption of methane by quartz and carbonates is significantly lower than that of clay minerals. The strong adsorption of methane by clay minerals is usually related to the complex structure and large SAA of clay minerals (Venaruzzo et al., 2002; Volzone et al., 2002; Ji et al., 2012). The SEM observations confirmed that the FG-intergranular pores were mainly clay minerals (Figure 5C). The microstructure of clay minerals controls the FG-intergranular pores. SSA is an effective parameter to characterize the microstructure of clay minerals (Dogan et al., 2006; Macht et al., 2011; Zhang Y. et al., 2019). Some studies have reported that the SSA is negatively correlated with gas hydrate saturation (Zhang et al., 2016; Zhang et al., 2017). The negative correlation between SSA and gas hydrate saturation may be determined by the clay mineral content and microstructure. The more complex microstructure of clay minerals leads to the formation of more FG-intergranular pores, affecting the permeability of sediment and the adsorption of free gas and pore water on sediments (Dogan et al., 2006; Gamage et al., 2011; Macht et al., 2011). The adsorption of free gas and pore water on sediments has potential significance for controlling gas hydrate formation. The weak adsorption capacity of sediments may make it difficult for free gas or pore water to remain in sediment and form gas hydrates. Conversely, the strong adsorption capacity of sediment may make it difficult for free gas or pore water to flow, thereby potentially acting as a sealing bed.

The in-situ porosity (calculated by LWD data) from the voyage report showed a sharp porosity drop in the I-GHR layer (Figure 3). The decreased porosity in the I-GHR layer is probably caused by gas hydrate enrichment. The porosity of the non-GH layer is not significantly higher than that of the underlying I-GHR layer. However, the abundance of the foraminifera in the non-GH layer is significantly higher than in the others. The high abundance of the foraminifera did not result in increased porosity. The saturation of gas hydrate in the non-GH layer is low (Figure 3) due to the BG-pores filling with calcareous ultramicrofossils and clay minerals. In the II- and III-GHR layers, the trend of porosity is consistent with that of gas hydrate saturation (Figure 3).

The CG-intergranular, FG-intergranular, and BG-pores were smaller in the III-GHR layer. The sediment in the III-GHR layer had low SSA. The reservoir space and adsorption capacity of the sediment were low and weak, respectively. After gas and water form and migrate to the III-GHR layer, it may be difficult to stay and no sufficient effective space to form the gas hydrate. The CT scan of samples shows FG-intergranular pores dominate the microstructure of the III-GHR layers (Table 4). The gas hydrate saturation of the CT scan samples in this layer is also very low (0.63–1.08%), indicating that the gas hydrate is not enriched. Hence, this layer may act as a gas migration channel (Figure 11). In the I-GHR layer, the sediments had many CG-intergranular pores and BG-pores (Table 4), indicating a more effective reservoir space and higher permeability (Figure 11). The CT scan samples’’ gas hydrate saturation in this layer is very high (58.48–62.89%). Thus, the gas hydrate had sufficient space and conditions to form and accumulate in the I-GHR layer. The II-GHR layer is a transition layer between the III- and I-GHR layers. The gas hydrate saturation of the CT scan samples in this layer (18.35–42.47%) is lower than in the I-GHR layer (Figure 3 and Table 4). Sediments with microstructural features similar to the I-GHR layer are favorable for gas hydrate accumulation, and those similar to the III-GHR layer are unfavorable for gas hydrate accumulation (Figure 11). Although many BG-pores in the non-GH layer were filled, CG-intergranular pores were rare, making it difficult to form an effective reservoir space (Figures 6, 10). Based on CT scan samples, the microstructure of sediments in the III-GHR layer is dominated by filled BG- and FG-intergranular pores (Table 4). The gas hydrate saturation of the CT scan samples in this layer is very low (1.45–3.41%). Meanwhile, the sediments in the non-GH layer had high SSA, which indicates that there were many FG-intergranular pores in the sediment, thereby leading to low permeability and strong adsorption of the sediments and making free gas or pore water migration difficult (Figure 11). The sediment in the I- and III-GHR layers had the same low SSA, indicating a weak adsorption capacity for free gas or pore water. However, the non-GH layer overlying the I-GHR layer had strong adsorption capacity and lacked an effective reservoir space. Thus, free gas and pore water may accumulate in the I–GHR layer and form gas hydrates (Figure 11).