1 INTRODUCTION

The seafloor substrate plays an important role in the application of marine engineering, seafloor resource exploration and development, seafloor environmental monitoring and military defense construction (Jackson and Richardson, 2007). The seafloor substrate can be divided into clay, granular sand and gravel, and carbonates. Carbonate sediment is an important sediment type, especially carbonates in shallow-water areas, as shallow water is the primary realm of interest in high-frequency acoustics (Jackson and Richardson, 2007). The Zengmu Basin lies in the shallow waters of the southern margin of the South China Sea. It is surrounded by the Borneo Island and the BruneiSabah Basin to the east, and the Beikang Basin and the Wan'an Basin to the north (Fig. 1). At present, a small number of studies have been done on the seafloor substrate of the Zengmu Basin, and concluded that the substrate is mainly silty sand and find sand (Yang et al., 2015). Usually, submarine oil and gas leaking can form submarine pockmarks and mud volcanoes, where there is hard substrate on the seafloor due to the development of carbonate nodules (Johnson et al., 2003; Judd and Hovland, 2007; Vaular et al., 2010; Yan et al., 2017). In recent years, a large number of oil and gas fields have been discovered in the Zengmu Basin (Fig. 1) (Xu et al., 2019). The fractures are developed in this area (Xu et al., 2019), but it is not clear whether the oil and gas transported to the shallow layer and formed submarine pockmarks. Therefore, it is still unclear whether patches of carbonate nodules appeared on the seafloor of the Zengmu Basin, and then leading to changes in the seafloor substrate.

In this study, we analyzed the stratigraphic structure, the seismic properties, the seafloor topography, and the spatial variation characteristics of its backscatter intensity through multi-channel seismic (MCS) data, the latest acquired sub-bottom profiles and multibeam data to explain whether oil and gas leaking have occurred on the seafloor and whether there is abundant carbonate sediment in the Zengmu Basin. This study provided valuable basic information for the submarine substrate, submarine stability, and marine engineering construction in the southern boundary line area of the South China Sea.

2 GEOLOGICAL SETTING

The Zengmu Basin is a large Cenozoic sedimentary basin in the southern margin of the South China Sea. During the Late Cretaceous-Early Oligocene, the southern margin of the South China Sea was a foreland area for plate subduction, and the PaleoSouth China Sea subducted southward under the Borneo landmass, triggering the collision between the Nansha block and Northwest Borneo (Taylor and Hayes, 1983; Hall, 1996), which led to the continuous uplift and erosion of Sabah in northern Borneo (Hutchison, 1996) and prompted a large amount of sediment in the Sabah area to move northward, forming a giant delta sediment in the Zengmu Basin (Xu et al., 2019) with a thickness of nearly 20 km (Huang and Wang, 2006).

The Zengmu Basin can be divided into eight secondary tectonic units, such as the Kangxi Depression, the Nankang Platform, the East Balingian Depression, the West Balingian Uplift, the Tatao HorstGraben, the La'nai Uplift, the Suokang Depression, and the West Slope. This basin is characterized by a high sedimentation rate (0.3–0.45 cm/a) (Mahmood et al., 2011) and great thickness (nearly 20 km) (Huang and Wang, 2006). Its major source rocks are the Oligocene and Lower Miocene delta plain coalbearing and marine source rocks, which are in a high-maturity and over-maturity gas generation stage, the secondary source rock is Miocene marine source rock (Zhang et al., 2017). Zengmu Basin has favorable oil-gas geological conditions and better oil-gas resource potential (Xie et al., 2015). By 2012, 514 exploratory wells have been drilled, 134 oil-gas fields and geological reserves with 5 billion tons of oil equivalent have been discovered (Lei et al., 2019), making this area one of giant oil and gas areas in the world (Zhang et al., 2017). East Balingian Depression located in the northeastern of the Zengmu Basin is an oil-prone one, while the other areas in Zengmu Basin are rich in gas mainly (Zhang et al., 2015).

Due to the limitation of geological sampling, there is a lack of in-depth research on the substrate types in the Zengmu Basin. Recently, some researchers believe that the seafloor substrate of the Zengmu Basin is mainly silty sand and find sand, while the marginal part of the basin is calcium-bearing sand, for example, calcareous sand, calcareous clay silt, calcareous silt clay (Fig. 2) (Yang et al., 2015).

3 DATA ACQUISITION AND PROCESSING 3.1 Sub-bottom profile data acquisition and processing

Sub-bottom profile SUB2021 crosses the central segment of the Zengmu Basin in NNE-SSW direction (Fig. 1). This line, 180 km long, was acquired aboard the R/V Shiyan 2 of South China Sea Institute of Oceanology, Chinese Academy of Sciences in 2021. A StrataBoxTM Sub-Bottom Profiling System, suitable for shallow water, was used to record subbottom profile data. The main performance and acquisition parameters of the instrument are as follows (Table 1). Conventional sub-bottom profile data processing sequence, e.g., Automatic Gain Control and filter were applied.

3.2 Multibeam data acquisition and processing

The R/V Fugro Equator conducted a large number of multibeam surveys in the Nansha area in 2019 (Fig. 1). Multibeam lines cross the hinterland of the Zengmu Basin in NWW-SEE (EQT190010) and NEE-SWW (EQT190007 and EQT190011) direction, with a length of about 2 000 km. A Kongsberg EM302 0.5°×1.0° System was used to record multibeam data. Conventional multibeam data processing sequence, e.g., import data, load zero tide data, merging data, data edit, and create water depth surface, was applied.

3.3 MCS data acquisition and data processing

MCS lines 87n-6a and 99n-1 cross the central segment of the Zengmu Basin in NNE-SSW and NWW-SEE direction, respectively (Fig. 1). These seismic lines were acquired aboard the R/V Shiyan 2 of South China Sea Institute of Oceanology in 1987 and 1999. A Digital Field System V seismometer and a 48-receivers streamer were used to record seismic data in 50-m space for both shot and receiver intervals. Three Bolt guns with total capacity of 2 200-in3 shot as source. The seismic data were processed in the Computer Processing Center, China National Offshore Oil Corporation. Conventional seismic processing sequence, e.g., frequency filtering, normal moveout correction, multiple suppressing, stacking and migration, was applied. We use the frequency attenuation gradient method to process the spectral information on seismic profiles and predict their hydrocarbonbearing properties.

4 RESULT 4.1 MCS profiles and sub-bottom profiles

Seismic profiles show that there are some gasbearing blank zones, mud-diapir structures, and fracture structures in this area (Figs. 3–4 & 5a). Geophysical bright spots exist in many places on the seafloor surface (Fig. 3: CDP700–1100; Fig. 5a: CDP2900–3200, CDP1500–1800), where are obvious seismic negative polarity and reflection blank zones below them. The internal reflections of the mud-diapir structures are chaotic (Figs. 3 & 5a) and extend upward to the seafloor surface (~1 s) (Fig. 5a). Fractures are also developed and extend upward to the seafloor surface (Figs. 3–4). The seismic attribute profiles show characteristics of high frequency attenuation and low frequency increase (Fig. 5b–c).

The sub-bottom profiles (Figs. 6–8) show that most of the seismic reflections are clear, but there are a lot of blank zones and mud-diapir structures in the seafloor surface (Figs. 6–7). In some areas, the blank zone is directly under the seafloor (Fig. 8), and the distribution range is wide, up to ~4 km.

4.2 Seafloor topography

The seafloor in the Zengmu Basin is relatively flat, with a water depth of 50–200 m and an average depth of 100 m (Fig. 1). High-resolution multibeam topographic maps (Figs. 9–11) show that a large number of depressions exist on seafloor, which are circular, elliptical, or elongated, in general width of several tens of meters, and a depth of 2–4 m, and some depressions are large in width of more than 400 m (Fig. 11). The distribution of depressions is dense, and their width range along the survey line is 1.8–8 km, up to ~50 km with an area of more than 20 km². The locations of depressions are shown in Fig. 2.

4.3 Backscatter intensity in the depressions area

Since there are many depressions in the study area (Fig. 2), we selected a typical depressions area (Fig. 11) for backscatter intensity imaging. The result shows that the values of the backscatter intensity range from -42 to -22 dB (Fig. 12). This area can be roughly divided into high and low value areas. The high value area has values ranging from -28 to -24 dB, with an average value of -26 dB, and their locations correspond roughly to the locations of the depressions (Figs. 11–12). The low value ranges from -40 to -34 dB, with an average value of -38 dB, and their locations correspond to the outer side of the depressions (Figs. 11–12).

5 DISCUSSION 5.1 Shallow stratigraphic characteristics in the Zengmu Basin

It has been shown that mud-diapir structures are developed in the western of the Zengmu Basin (Kangxi depression), where a large number of oil and gas fields have been discovered (Xu et al., 2019). Figures 3 & 5a show the mud-diapir structures, which extends from deep (~5 s) to shallow (~1 s) (Fig. 5a). The high-resolution sub-bottom profiles even show uplifted structures on the seafloor, with a synchronous uplift of the basement and the seafloor (Figs. 6–7). The interior of the uplifted structures are blank reflections, without lateral accretion bedding, and the seafloor on both sides of the uplift is relatively low (Figs. 6–7), indicating that the diapir structures extend upward to the seafloor.

Bright spot is an effective seismological indicator of oil and gas (Hammond, 1974), which are also present on the seafloor surface above the mud-diapir structures (Figs. 3 & 5a), and their seismic polarity is reversed. The phenomenon of "low frequency increase and high frequency attenuation" is also a seismological indicator of oil and gas (Biot, 1962). There are also abnormally-high values around the bright spot and the mud-diapir structures in the seismic property profiles (Fig. 5b–c), indicating that the lowfrequency component increases significantly and the high-frequency component attenuates strongly when the seismic waves pass through the bright spot and the mud-diapir structures, which is an obvious oil and gas response (Castagna et al., 2003; Liu et al., 2019). The sub-bottom profiles show a large number of gasbearing blank zones and mud-diapir structures near the surface of the seafloor (Figs. 6–8), and the blank zones are directly beneath the seafloor in some areas (Fig. 8) which are widely distributed (up to ~4 km long). In addition, the fractures are development and extend upward to the submarine surface (Figs. 4, 6, & 7). Based on the above features, it is hypothesized that the fractures caused the upward transport of deep gas to the seafloor. The strong reflections above the mud-diapir structures (Figs. 6–7) even suggests the existence of a subsurface micro-reservoir, which further indicates the upward gas transport to the seafloor. In addition, there is a near-vertical columnar structure near the blank zone, which is 1.5 m high and up to 10 m wide, with wider roots and narrower tops and strong reflections (Fig. 7b), and its features are similar to those of carbonate chimneys, which also suggest upward gas transport to the seafloor.

5.2 Geomorphological features of the Zengmu Basin and geological origin of depressions

Pockmarks are usually submarine depressions formed when deep fluids are strongly ejected or slowly seeped to the seafloor through transport channels, such as faults or weak zones, and then eroded loose sediments on the seafloor (Hovland et al., 2002; Judd and Hovland, 2007; Webb et al., 2009; Cathles et al., 2010). It is easy to form pockmarks when the surface sediment of the seafloor is loose (Cathles et al., 2010). The main feature of the pockmarks is the submarine depressions (Hovland et al., 2002, 2010), hard seafloor that can be detected and mapped by multibeam (Hovland and Svensen, 2006; Sen et al., 2016) and the gas chimney structure beneath it (Cathles et al., 2010). The diameter of pockmarks ranges from several meters to several kilometers (Cole et al., 2000; Hovland et al., 2010), and is usually small in shallow waters, such as the Yinggehai Basin in the South China Sea and the Oslofjorden in the Norway, water depth less than 100 m, where pockmarks are less than 50 m in diameter and less than 10 m in depth (Zhang et al., 2019). The pockmarks are usually circular, elliptical, crescent, elongated, chain-like or complex (Hovland et al., 2002; Zhang et al., 2020). In addition, the formation of pockmarks is closely related to oil and gas leakage or hydrate decomposition (Judd and Hovland, 2007; Vaular et al., 2010).

The submarine depressions present in the Zengmu Basin are circular, elliptical, or elongated, generally 2–4 m deep and tens of meters wide (Figs. 9–11). The backscatter intensity derived from the multibeam data is higher (-26 dB) in the depressions, much higher than the surrounding intensity (-38 dB), indicating that the seafloor at the depressions is harder (Fig. 12). These characteristics are similar to those of typical pockmarks (Hovland et al., 2002; Hovland and Svensen, 2006; Sen et al., 2016; Zhang et al., 2020). The Zengmu Basin is rich in gas (Xie et al., 2015), and a large number of oil and gas wells have been drilled in this area (Fig. 1). The depressions area shown in Fig. 9 is 6 km from the location of a gas show well (Fig. 1) (Yang et al., 2015). Multi-channel seismic data and sub-bottom profiles show that a large amount of gas has leaked into the surface layer of the seafloor in this area (Figs. 3 & 8). Combined with the submarine topographic features and the backscatter intensity in the depressions, we hypothesize that these submarine depressions are pockmarks. In addition, the sediment type of the Zengmu Basin is mainly silty sand and find sand (Yang et al., 2015), and the surface sediments on the seafloor are loose, it is easy for fluid eruption out of the seafloor and formation of pockmarks (Cathles et al., 2010).

Unit-pockmarks are very small seabed depressions, typically less than 5 m wide and up to 0.5 m deep, usually found inside and around normal pockmarks in groups, and their most distinctive feature is the dense mono-sized circular depressions (Hovland et al., 2002, 2010). Inside a giant semi-elliptical (230 m long, 66 m wide, and 5 m deep) pockmark in the Zengmu Basin, more than 110 mono-sized small circular pockmarks, with a depth of less than 1 m and a width of 5 m, also appear in an area of less than 0.03 km² (Fig. 13). Although the sedimentation rate in the Zengmu Basin is high, 0.3–0.45 cm/a (Mahmood et al., 2011), these mono-sized small pockmarks are not obliterated by sediment infilling. Therefore, these small pockmarks are most likely unit-pockmarks. Since unit-pockmarks can represent the most recent and most active local seep locations (Hovland et al., 2010), the unit-pockmarks in the Zengmu Basin are or recently have been active.

In addition, sub-bottom profiles also show some depressions near the gas-bearing blank zones and mud-diapir structures (Figs. 6–7), which are 20–40 m wide and up to 0.2 m deep. These small depressions are not obliterated by sediment infilling with the high sedimentation rate (0.3–0.45 cm/a) (Mahmood et al., 2011). Therefore, these small depressions are also submarine pockmarks.

The submarine pockmarks identified in the South China Sea are mainly located in northern, western, and southern margins of the South China Sea, such as the Yinggehai Basin, the Qiongdongnan Basin, the Xisha and Zhongsha Islands, the Zhongjiannan Basin (Sun et al., 2011, 2013; Di et al., 2012; Bai et al., 2014; Chen et al., 2015), the Liyue Basin (Zhang et al., 2019), the Beikang Basin, the Zhujiang (Pearl) River Basin, and Andu Basin (Zhu et al., 2020). Zengmu Basin is rich in hydrocarbon resources and fractures are developed, where it is easy to form pockmarks, but past studies have not found pockmarks in this area. This is the first report on the discovery of pockmarks in the Zengmu Basin.

5.3 Effect of pockmarks on seafloor stability in the Zengmu Basin

Submarine pockmarks activity can increase the likelihood of submarine landslides and poses a safety hazard to submarine engineering construction (Hovland et al., 2002). The depth of the pockmarks found in the Zengmu Basin is generally shallow, 2–4 m deep revealed by multibeam data, 0.2 m deep revealed by sub-bottom profile data. A large number of pockmarks even are present with a depth less than 1 m (Fig. 13). These depressions are not obliterated by sediment infilling with the high sedimentation rate (0.3–0.45 cm/a) (Mahmood et al., 2011). Therefore, these pockmarks are active or recently active pockmarks and their activities have a large impact on the stability of the seafloor and marine engineering construction.

5.4 Indication of substrate types in the Zengmu Basin

The seafloor substrate of the Zengmu Basin is less studied, and existing studies suggest that small areas of calcareous sand, calcareous clay silt, and calcareous silt clay exist in the northern and southeastern parts of the Zengmu Basin (Fig. 2) (Yang et al., 2015). In contrast, the seafloor substrate in a large area of the Zengmu Basin is silty sand and fine sand (Fig. 2) (Yang et al., 2015).

Multibeam data and sub-bottom profile data show a large area of dense pockmarks, with a usual distribution range of 1.8–8 km along the survey line, up to 46 km with an area of more than 23 km² (see Fig. 2 for location). The main feature of active or recently active pockmarks is large number of carbonate nodules on the seafloor and the hard seafloor (Johnson et al., 2003; Judd and Hovland, 2007; Vaular et al., 2010), but with the increase in the time of cessation of pockmark activity, the carbonate nodules around the pockmarks decrease and the seafloor becomes soft in seafloor substrate. The depth of the pockmarks found in the Zengmu Basin is generally shallow, 2–4 m deep, some less than 1 m deep, and not obliterated by sediment infilling with the high sedimentation rate (0.3–0.45 cm/a) (Mahmood et al., 2011), and the backscatter intensity derived from the multibeam data in the pockmarks (-26 dB) is much greater than that in the surrounding non-pockmarks areas (-38 dB) (Fig. 12). These facts indicate the presence of a large number of carbonate nodules on the seafloor around the pockmark, thus extensive carbonate sediments are embedded in silty sand and fine sand (Fig. 2). Therefore, there is extensive hard seabed in the Zengmu Basin, which led to a complex and variable substrate type in the Zengmu Basin.

6 CONCLUSION

Multibeam data across the Zengmu Basin reveals a large number of depressions, with depths of 2–4 m, widths of several tens of meters, large distribution range of 1.8–8 km along the survey line, and their backscatter intensity (-26 dB) is much greater than that of the surrounding area (-38 dB). Combined with the developed mud-diapir structures and fracture structures, and abundant oil and gas resources in the Zengmu Basin, these dense depressions are presumed to be pockmarks. This is the first report on the discovery of pockmarks in the Zengmu Basin. Furthermore, a large number of mono-sized small circular unit-pockmarks are observed, implying active or recently active gas leakage, which have a large impact on the stability of the seabed and marine engineering construction in China's maritime boundaries.

Seismic profiles across the Zengmu Basin show characterization of upward migration of hydrocarbons, expressed as mud-diapir structures, bright spots in the shallow formation with characteristics of "low frequency increase and high frequency attenuation". High-resolution sub-bottom profiles even show the mud-diapir structures nearly exposed at the seafloor, as well as the gas-bearing blank zones beneath the seafloor. These features suggest large gas leaking from the seafloor. According to the fact that the Zengmu Basin has abundant gas fields, combined with a large number of pockmarks discovered from multibeam data and sub-bottom profile data, we speculate that a large amount of gas is transported upward to the surface of the seafloor, resulting in the occurrence of large amounts of carbonate nodules on the seafloor.

7 DATA AVAILABILITY STATEMENT

The seismic data and sub-bottom profile data that support the conclusions of this study are available from the corresponding author upon reasonable request. The multibeam data were obtained from https://www.ngdc.noaa.gov/mgg/bathymetry/multibeam.html.

8 ACKNOWLEDGMENT

We would like to thank the crews of R/Vs Shiyan 2 and Fugro Equator for their collection of the geophysical data. We would also like to thank Jinfeng MA (Guangzhou Marine Geological Survey), Shengxuan LIU (Guangzhou Marine Geological Survey), Xiuya LÜ (South China Sea Bureau of Ministry of Natural Resources), Yongzheng QUAN (Ocean University of China), and Fugang LIU (First Institute of Oceanography of the Ministry of Natural Resources) for their help in multibeam data processing. Some of the figures in this study are generated with Generic Mapping Tools (GMT) (Wessel and Luis, 2019).