Focused Fluid Flow, Shallow Gas Hydrate, and Cold Seep in the Qiongdongnan Basin, Northwestern South China Sea

The 3D seismic data acquired in the central Qiongdongnan Basin, northwestern South China Sea, reveal the presence of shallow gas hydrate, free gas, and focused fluid flow in the study area, which are indicated by multiple seismic anomalies, including bottom simulating reflectors, polarity reverses, pulldowns, minor faults, and gas chimneys intensively emplaced within the shallow strata. A new cold seep is also discovered at approximately 1520 m water depths with an ~40 m wide crater in the west part of the study area. Water column imaging, seafloor observation, and sampling using the remotely operated vehicle “Haima” demonstrate ongoing gas seepages and shallow gas hydrates at this site. Thermogenic gas in the study area migrates from the deep reservoir through the gas hydrate stability zone along deep faults and gas chimneys, forms shallow gas hydrate and free gas, and sustains localized gas seepage within this cold seep. The results provide insight into the relationship between shallow gas hydrate accumulation and deep hydrocarbon generation and migration and simultaneously have important implications for hydrocarbon explorations in the Qiongdongnan Basin, northwestern South China Sea.


Introduction
According to hydrocarbon supplies and metallogenic characteristics, gas hydrates can be characterized into two categories: diffusive and leakage types. The diffusive gas hydrates markedly indicated by bottom simulating reflectors (BSRs) dominantly develop in the Shenhu area, northern South China Sea (SCS), and have received extensive and comprehensive research studies [1][2][3]. Moreover, both the Shenhu area and the deepwater slope area of the Qiongdongnan Basin (QDNB) provide favorable conditions for leakage gas hydrate formation. The leakage gas hydrates are commonly related to focused fluid flows [4][5][6] or/and cold seeps [7,8] at many active and passive continental margins around the world. The leakage gas hydrate deposit zone is between the seafloor and the base of the gas hydrate stability zone (BGHSZ) [9][10][11], commonly has high methane flux and coexists with free gas [12]. Gas hydrate destabilization may occur and release methane which migrates along faults, fractures, pipes, mud diapirs, gas chimneys, etc., into oceans and will nourish cold seeps on the seafloor.
The shallow gas hydrates in the QDNB belong to the leakage type, and their accumulation within the shallow marine sediments is attributed to focused fluid flows in the basin [8,13]. Active cold seeps in the SCS have been reported as northwestern "Haima" cold seeps [14] and northeastern site F [15,16]. Now, a new and active cold seep is found, approximately 50 km northeast of the "Haima" cold seeps. In this paper, we aim to study the distributions and subsurface features of the shallow gas hydrates, focused fluid flows, and cold seep in the QDNB, northwestern SCS, by use of 3D seismic data acquired in 2018. Based on the new results, the relationship between shallow gas hydrate accumulation and deep hydrocarbon generation and migration is discussed. Furthermore, the newly discovered cold seep is also briefly presented.
The QDNB structural framework is constituted by three depressions and two uplifts: the northern depression, the northern (central) uplift, the central depression, the southern uplift, and the southern depression, which can be further divided into different sags and uplifts of third-order structural regions [29,32]. The study area covers an area of 800 km 2 and lies in the central depression, with a relatively flat topography at water depths ranging from 1450 m to 1700 m ( Figure 1). Most of the study area lies in the Lingnan Low Uplift, and the rest includes part of the transitional zones between the Lingnan Low Uplift and the Beijiao Sag. Since Late Miocene, the whole study area is in the subsidence stage with a thick sequence of marine sediments and generates large amounts of thermogenic hydrocarbons. Seafloor spreading in the SCS results in extensive normal faults ( Figure 2) [33], which can provide pathways for the upward migration of deep gas and fluids. Tectonic activities in the QDNB have become weak since Pliocene [33], and hence, almost all normal faults do not extend to the seafloor [28].
We have no access to the borehole data in the study area and cannot directly estimate the seismic sequence ages. Hence, we collect and analyze previous seismic interpretations [22,25] and well data from adjacent regions [8,34]. According to the unconformities and the seismic reflection characteristics of conformable interfaces, we have identified seven reflecting horizons (T 0 -T g ), which divide the basin strata into 6 sequences (

Data and Method
Here, we mainly utilize the 3D seismic data to analyze the distributions, the characteristics, and the subsurface structures    , which explicitly image the subsurface structures of the shallow gas hydrates and focused fluid flows in the study area. In addition, the water column imaging, seafloor observations, and samplings using the remotely operated vehicle (ROV) "Haima" are used to describe the newly discovered cold seep. A multibeam echo sounder provides an efficient method for hydroacoustic detection, which is a first-order remote proof of active gas seepage at the cold seep. The impedance contrasts between the water and gas bubbles released from cold seeps bring about strong backscattering of the transmitted acoustic wave, which will create gas flares in echograms [7].

Focused Fluid Flows, Shallow Gas
Hydrates, and Free Gas in the Study Area. A variety of seismic anomalies are observed in the 3D seismic sections (Figures 3 and 4), including polarity reverses, pulldowns, minor faults, gas chimneys, and BSRs, which indicate the presence of focused fluid flows, free gas, and shallow gas hydrates in the central QDNB, northwestern SCS. The first type of seismic anomaly is polarity reverse. It marks a localized decrease in acoustic impedance (the product of velocity times density) [36,37], which indicates the presence of free gas in the strata [38]. The sediment interface in the shallow strata is normally displayed in the seismic section as a positive polarity reflection; however, polarity reverses break the continuous reflection event and exhibit a negative seismic phase (Figures 3 and 4). Large-scale polarity reverse zones in the study area mainly include the polarity reverse_A (PR_A) and the polarity reverse_B (PR_B) and else may occur at the top of gas chimneys (Figure 3(b)). The PR_A and PR_B are all located in the southwestern part of the study area, with an area of 1.516 km 2  5 Geofluids underlying strata through minor faults or shallow gas chimneys (Figures 5(a) and 5(c)), which may serve as pathways for gas migration.
Minor faults are the most widely distributed features in the study area and cover more than half of the study area ( Figure 4). They are proved to play important roles in gas accumulations and gas hydrate deposits, as they provide focused fluid flow pathways through the shallower sediments [5,39,40]. In most cases, these minor faults in the study area are highlighted by obvious broken reflections. Sometimes, fluid flows charged in the minor faults will reduce acoustic velocity and result in apparent downward deflection of otherwise horizontal reflectors, called pulldowns. Both minor faults and pulldowns in the study area occur within the shal-low strata, and their vertical extents are no more than 150 ms TWT ( Figure 6). They occur in different environments, such as those related to the polarity reverse zones (Figure 5(a)) or within the paleochannel (Figure 6(a)). Most of them do not extend to the seafloor, but some minor faults, such as the pulldown above the PR_B (Figure 6(c)), may pierce the seafloor and provide the pathway for fluid flows into the ocean.
Gas chimneys in the study area are characterized by columnar disrupted zones in the seismic sections (Figures 7-9). Gas hydrates filled in gas chimneys will increase acoustic velocity and result in unusual seismic anomaly features. The reflection events of their flanks are nearly unfaulted at the edge, and their internal events show a regular convex upward geometry. They are different from other ordinary  6 Geofluids gas chimneys, which commonly show chaotic and/or blank reflection patterns caused by gas-charged fluid flows [5]. Shallow gas chimneys mainly develop in the shallow Quaternary strata and are commonly hundreds of meters in diameter, and most of them are observed at 2080~2250 ms TWT, between BSRs and seafloors. Several shallow gas chimneys may develop side by side and form a cluster (Figure 7(e)). Except for these shallow gas chimneys, some chimneys will extend down into deep sediment layers. As shown in Figures 7(d) and 8, the gas chimney_A (GC_A) extends straightly down to the basement, while the gas chimney_B, gas chimney_C, and gas chimney_D (GC_B, GC_C, and GC_D) are connected with the basement by faults instead of directly reaching it. Most of the deep gas chimneys upwardly terminate below the BSRs, and only the GC_A pierces the seafloor and generates a cold seep in the study area.

Newly Discovered Cold Seep.
On the lower continental slope of the northwestern SCS, approximately 50 km northeast of the "Haima" cold seeps, a newly discovered cold seep is situated at~1520 m water depths (Figures 1(a) and 9(c)), which is connected with the GC_A. The GC_A shows columnar vertical convex aligned reflectors in the seismic sections (Figures 7(e), 8, and 9(b)), originates from a basal uplift with an approximate width of 3.5 km, narrows up gradually to less than 1 km, and finally forms a 40 m diameter crater on the 8 Geofluids seabed. In addition, a flare-shaped backscatter feature rising about 800 m away from the seafloor, with a width of 400 m, is delineated at this site on the echogram (Figure 9(a)). Observations made with the ROV "Haima" confirm the existence of gas seepages, shallow gas hydrates, and cold seep ecosystems at this site ( Figure 10). All these prove the ongoing gas seepage activities at this newly discovered cold seep.

Discussion
The analysis from the "Haima" cold seeps and the sites GMGS5-W08 and GMGS5-W09, which are located northeast of the study area and drilled during the fifth "China National Gas Hydrate Drilling Expedition" (GMGS5), demonstrates that the hydrate-methanes in the QDNB are mixed gases, and the thermogenic hydrocarbons generated by Eocene and Oligocene Yacheng formations serve as the main sources [8,13,34,[41][42][43]. The geochemical analysis conducted elsewhere in the QDNB also confirms the presence of Eocene and Oligocene source rocks [44]. Certainly, biogenic gases are also involved in this process, as the research studies of Liang et al., Lai et al., and Zhang et al. [8,34,45] verify that the self-generation and self-accumulation biogenic gas supply systems develop in the basin. As shown in Figure 8, we propose that the thermogenic gases originated from deep E-O strata migrate upward along deep faults and/or deep gas chimneys, such as GC_B~D, into the gas hydrate stability zone, which form diffusive gas hydrates indicated by BSRs or a mix of leakage gas hydrate and free gas filled in the minor faults, polarity reverse zones, and gas chimneys. In addition, the thermogenic gases may migrate along the deep gas chimney, such as GC_A, directly to the seafloor, sustain localized gas seepages, and nourish an active cold seep system. During the hydrocarbon migration, the leakage gas hydrates will be generated under suitable conditions filling in the gas chimneys and result in unusual convex upward reflections within the gas chimney and unfaulted flanks at the edges of the gas chimney. The thermogenic hydrocarbons with the characteristic of lower generation and upper accumulation result in the related seismic anomalies dominantly distributed in the Lingnan Low Uplift (Figure 4). This result provides insight into the relationship between shallow gas hydrate accumulation and deep hydrocarbon generation and migration and simultaneously has important implications for hydrocarbon explorations in the QDNB, northwestern SCS.

Conclusions
Multiple seismic anomalies, including polarity reverses, pulldowns, minor faults, gas chimneys, and BSRs, are recognized in the central QDNB, northwestern SCS, which indicate intensively distributed shallow gas hydrates, free gas, and focused fluid flows within the shallow strata in the study area. In addition, the newly discovered cold seep related to a deep gas chimney with an internal convex upward geometry also demonstrates ongoing focused fluid flows and shallow gas hydrates at this site. The thermogenic gases originated from deep E-O strata are the main hydrocarbon sources, and they migrate along deep faults and gas chimneys to the shallower Lingnan Low Uplift and contribute to the genesis of the shallow gas hydrates, cold seeps, and focused fluid flows in the study area. The characteristics and distributions of the seismic evidence indicative of shallow gas hydrates and focused fluid flows provide insights into the understanding of the coupling relationship between seafloor cold seeps, shallow gas hydrates, and focused fluid flows in depth and have important implications for hydrocarbon explorations in the QDNB, northwestern SCS.

Data Availability
The seismic and other data used to support the findings of this study have not been made available because the data are confidential.

Conflicts of Interest
The authors declare that they have no conflicts of interest.