Synthetic seismic records of gas hydrate-bearing layers were obtained using the reflection characteristics of seismic wave groups (i.e., external geometrical morphology, internal reflection structure, continuity, amplitude, frequency, and layer velocity), as well as regional geological data. Gas hydrate-bearing strata generally show strong amplitude anomalies in the synthetic seismic records (Riedel et al., 2010). Seismic impedance can well reflect the characteristics of gas hydrates (such as BSRs), and the applications of wave impedance in the identification of gas hydrate and the study of the concentration assessments have achieved good results (Riedel and Shankar, 2012; Wan et al., 2016; Xue et al., 2016; Li et al., 2019). In this study on the characteristics of hydrate-bearing reservoirs, we performed well-free acoustic impedance inversion using seismic data because there is no logging data in the study area. The low-frequency impedance model was obtained by seismic velocity data.

Most of the strata at the target location have large dip angle, which imposes higher requirements for the low-frequency model. It is necessary to improve the accuracy of seismic interpretation in the early stage to ensure the accuracy of horizon information. In order to ensure the rationality of the low-frequency impedance model, we extracted wavelets at multiple locations and obtained the optimal wavelet. Then, we used the inversion results to correct the initial model iteratively and finally obtained the final impedance inversion results. The high saturation hydrate reservoirs on the impedance inversion profile can be characterized by high impedance anomalies.

A comparative analysis was conducted on the characteristics (e.g., lithology, thickness, and sedimentary facies) of high saturation hydrate layers using the seismic and logging data of high saturation hydrate concentrated areas confirmed by drilling in Nankai Trough and Shenhu area in South China Sea. In this way, the characteristics of the seismic reflections and inversed impedance anomalies of high saturation hydrate concentrated areas were determined. Based on this information, the authors can predict high saturation hydrate concentrated zones in the Makran Accretionary Prism.

According to the interpretation of newly acquired high-resolution seismic data, the BSR in the study area shows noticeable reflection characteristics and is continuously distributed, and fluids are transported by massive highly permeable strata (indicated with blue arrows in Figure 3) as well as deep thrust faults and shallow normal faults. Most of the highly permeable strata are located in the piggyback basin under the BSR. They correspond to the negative topography of the seafloor and are present as inclined amplitude-enhanced reflectors in the seismic profile (Figure 3). As revealed by previous studies, the amplitude-enhanced reflectors above the BSR are generally the high saturation hydrate concentrated zones (Guo et al., 2017), whereas those below the BSR are usually free gas concentrated zones (Riedel et al., 2010). Analysis shows that the free gas layers under the BSR in the Makran Accretionary Prism are 200–300 m thick (Grevemeyer et al., 2000; Sain et al., 2000; Ojha and Sain, 2008), indicating that the Makran Accretionary Prism has abundant gas sources. However, there are no apparent amplitude-enhanced reflectors in the shallow depth strata near seafloor of the Makran Accretionary Prism. This may be related to the small particle size of surface sediments (mainly including silty clay) and the low saturation hydrates with sporadic distribution (Figure 4) (Bohrmann and Ohling, 2008; Gong et al., 2017).

Seismic data interpretation has revealed that the gas hydrates in the Makran Accretionary Prism are mainly concentrated in the anticlinal ridge and have a two-story structure (Gong et al., 2016; Gong et al., 2018a). To date, surface hydrate samples have been obtained from gravity cores (Figure 4), but the deep hydrate layers close to the BSR have not been drilled. Shallow hydrate-bearing sediments show weak amplitude or blank reflections in the seismic profile (Figure 3). By contrast, the unconfirmed deep hydrate layers above the BSR are present as amplitude-enhanced reflectors in the seismic profile (Figures 3, 5) and show high impedance anomalies in the impedance inversion profile (Figure 5). Seismic data interpretation and the analysis of sedimentary conditions indicate that deep hydrate reservoirs may consist of coarse-grained turbidites. Comprehensive analysis suggests that the strong amplitude and high impedance above the BSR in the study area probably indicate high saturation hydrate concentrated zones as well as coarse-grained sediments (Guo et al., 2017). By contrast, the strong amplitude and low impedance under the BSR reflect free gas concentrated zones (Figure 5). It should be noted that the piggyback basin under the BSR show the interbeds of slightly higher and low impedance, reflecting both the low impedance of free gas and the slightly higher impedance of turbidite sands.

Two hydrate production tests have been conducted in 2012 in the Nankai Trough offshore Japan, which is a typical example of the exploration and production tests of gas hydrates in active continental margins. According to Zhao, 2019, the high saturation hydrate reservoirs in the Nankai Trough were identified mainly based on the clear and continuous BSR, the existence of amplitude-enhanced reflectors above the BSR, high P-wave velocity, and sand-rich turbidites. The logging interpretation results show that the hydrates discovered in the first well (AT1-C) for coring and hydrate production tests in Japan are mainly concentrated in the wing of anticlinal ridges and show two layers of strong reflections above the BSR on the seismic profile (Figure 6) (Fujii et al., 2015). The upper layer of strong reflections is mainly composed of sheeted turbidite sands with high lateral continuity. By contrast, the lower layer of strong reflections consists of thick channel sands with poor lateral continuity, both sand layers have almost the same hydrate saturation of 50–80%. In addition, Komatsu et al. (2015) divided the 60 m thick hydrate concentrated zone of well AT1-C into four sedimentary facies zones based on logging curves (Figure 6). Analysis of sedimentary facies associations shows that the hydrate concentrated zone can be divided vertically into three portions, namely bottom turbidity channels, middle sheeted turbidity sands, and top basin-bottom sediments. Among them, the bottom and middle portions have the gas hydrates at the highest concentrated level (Ito et al., 2015).

Therefore, the relatively continuous strong-amplitude reflections above the BSR indicate high-saturation hydrate deposits and highly permeable turbidites are high-saturation hydrate concentrated zones in the Nankai Trough.

Three hydrate production tests have been conducted in the Shenhu Area in the South China Sea. Therefore, the Shenhu Area is also a model for the exploration and production tests of gas hydrates of passive continental margins. According to comprehensive analyses (Zhang et al., 2014; Li et al., 2018; Su et al., 2020; Ning et al., 2020), the conditions for the concentration of high saturation hydrate deposits in the Shenhu Area are similar to those in the Nankai Trough, Japan (Figure 7), despite they are in different tectonic backgrounds. The logging curves show that the average P-wave velocity of hydrate concentrated zones in the Shenhu Area is about 2.03 km/s, while that of non-hydrate layers under the BSR is only 1.1–1.7 km/s (Ye et al., 2020).

In 2015, logging-while-drilling was conducted at 23 sites in the Shenhu Area carried out by the Guangzhou Marine Geological Survey. The logging data obtained during this expedition showed a hydrate saturation of up to 64%. Several sets of hydrate layers are vertically developed along Well W11, with a total thickness of over 70 m and a maximum hydrate saturation of up to 53%. By analyzing the seismic profile and synthetic seismic records of this well, Guo et al. (2017) found that the hydrate horizons are present as three strong reflections in the seismic profile and show strong impedance in the profile of synthetic seismic records. Li et al. (2019) obtained a detailed description of hydrate deposits passing through Well W11 using the well-free broadband seismic inversion technology (Figure 8). The inversion results clearly showed the top boundary, bottom boundary, and internal characteristics of the hydrate deposits in the Shenhu Area. Hydrate deposits in the Shenhu Area are located above the BSR and are manifested as three strong reflection axes (left, Figure 8), while the free gas under the BSR show weak impedance anomalies.

According to the above comparison, although there are different tectonic geological backgrounds, the corresponding relationship between strong reflection and gas hydrate is the same. We believe that the strong amplitude reflectors above BSR represents the existence of high saturation hydrate in both active and passive continental margins.

The seismic reflection characteristics of Makran Accretionary Prism are similar to those of Nankai Trough, and also can be compared with ODP204 drilling results (Tréhu et al., 2004). The drilling results of ODP204 show that gas hydrate is mainly distributed in three zones (Figure 9): 1. It is a high saturation accumulation area related to cold seeps, and its saturation can usually reach 30–40% (Figure 9B). The cold seep organisms found in the anticline ridge of Makran and the clear vertical channel on the seismic profile indicate that this type of hydrate distribution exists in the Makran area, However, its distribution range and thickness are relatively small, and the amount of resources is relatively small; 2. The weak reflections between seafloor and BSR, which is thicker than the cold seeps zones, and the saturation is generally 2–8% (Figure 9A), but its distribution area is relatively large and its thickness is also large, so the amount of hydrate resources should be relatively large; 3. Strong reflection above BSR, with saturation of about 15–20% (Figure 9C), distribution zone between 1 and 2, thickness of about 100m, and contains large hydrate resources. In addition, it is strongly reflected under the BSR, which can be determined as free gas can be considered as a part of the gas hydrate system, with a thickness of up to 1000m, which contains a great amount of resources (Figure 9C).