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Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom, July 17-21, 2011.

POLYGONAL FAULT, POCKMARK AND GAS HYDRATE OCCURRENCE

IN THE NORTHERN SLOPE OF THE NORTHERN SOUTH CHINA SEA

Wu Shiguo∗

Key Laboratory of Marine Geology & Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China

Chen Duanxin, Qin Zhiliang, Sun Qiliang

Key Laboratory of Marine Geology & Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China

and Wang Xiujuan

Graduate University of Chinese Academy of Sciences, Beijing 100049, China

ABSTRACT Based on 2D/3D seismic data sets and multi-beam bathymetry data, occurrence of polygonal fault systems and pockmarks has been mapped in the northern slope of the South China Sea (SCS). 3D seismic coherence slices show clear pipe characteristics of the triple junctions along the polygonal faults. The fluid flow along the polygonal fault could be demonstrated by the pipe attribute. And chaotic reflectors, submarine slides occurred over the polygonal fault. In the western Qiongdongnan basin the fluid flow can arrive to the seafloor and pockmarks are formed over the polygonal faults. We mapped the distribution of the bottom simulating reflectors (BSRs) indicating the existence of gas hydrate and discovered that BSRs usually locate above the polygonal faults. Therefore, it is inferred that the polygonal faults serve as the conduits for fluid flow and provide the pathways for the fluid flow with methane gas. Furthermore, it contributes formation of the gas hydrate in the deepwater of northern South China Sea margin.

Keywords：polygonal fault, gas hydrate, chaotic reflection, bottom simulating reflector, South China Sea

∗ Corresponding author: Tel & Fax: +86-532-82898544; Email: swu@qdio.ac.cn

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INTRODUCTION Polygonal fault is a network of layer-bound, meso-scale (throws from 10-100 m) extensional faults arranged in a polygonal structure Cartwright [1]. Henriet [2] firstly discovered these structures in sedimentary layers. The term ‘polygonal fault’ was named by Cartwright 1994 [3] after he analyzed the shale sedimentary layers in North Sea basin throughout the 3D seismic data. Up to now, polygonal faults exist in more than 50 basins. However, formation mechanism of polygonal fault is still a disputed issue [4-7]. The polygonal faults in the Qiongdongnan basin, South China Sea, were firstly reported according to 3D seismic data [8]. The pipe attribute of the polygonal faults linking the hydrocarbon source rock with the gas hydrate reservoir have been demonstrated in the Lower

Congo Basin [9], the Norway continental margin [10], the Scotian slope of the eastern Canada continental margin [11]. It is known that Neogene strata with few tectonic activities limit the thermal gas generated in the syn-rift sequence to migrate upward into the gas hydrate stability zone (GHSZ). However, a lot of gas chimneys occur close to seafloor [12]. Therefore, we infer if the polygonal faults can serve as the pathway for fluid flow in the post-rift strata to enhance gas concentration in the GHSZ (Figure 1). The study focuses in deepwater of Qiongdongnan basin, Pearl River Mouth basin and Zhongjiannan basins, northern SCS and discusses the seismic characteristics of polygonal faults and its implication on formation of gas hydrate occurrence.

Figure 1. a The schematic map of the northern South China Sea, including Qiongdongnan basin, Zhongjiannan basin and Pear River Mouth basin. The map shows the distributions of the polygonal faults pockmarks and the potential gas hydrates zone. b The multibeam bathymetry map of the deepwater basins in southern Pearl River Mouth basin. The figure displays the distribution of polygonal faults, pockmarks on the seafloor, gas hydrate well. c The 3D seismic survey in deep water of Qiongdongnan basin. The basemap is the time depth of the base of upper Miocene.

GEOLOGICAL SETTING The geological evolution of the northern SCS

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margin can be divided into two main stages: syn-rift stage and post-rift stage. Large volume of source rocks and syn-rift faults are well developed in the Paleogene syn-rift stage, whereas fewer faults occur in the Neogene post-rift stage. The separation for syn-rifting and post-rifting stage is a breakup unconformity T60 between Oligocene and Miocene. And the giant submarine erosion surface at the base of the Upper Miocene produced to massive submarine canyons which could be induced by sea level change. The Paeogene sedimentary facies varied from alluvial, delta and lacustrine facies to shallow marine facies, whereas the Noegene facies were characterized by shallow, bathyal and abyssal facies [13] (Figure 2).

Figure 2. The sequence stratigraphy in Qiongdongnan basin and Pear River Mouth basin

SEISMIC IMAGING OF THE POLYGONAL FAULTS The polygonal faults discovered in deepwater of northern SCS margin have the similar attributes as those in North Sea basin and other basins of the world Cartwright [1]. With the coherent slice and flattening technique, some analyses were implemented on the 3D and partial 2D seismic data in the deep-water of the Qiongdongnan basin and the Zhongjiannan basin. The polygonal faults concentrated at the lower Huangliu Formation and

the upper Meishan Formation and were limited by the layer-bound strata of the base of the Pliocene and the base of the Meishan Formation. The research flattened the stratigraphic boundary of the base of the Huangliu Formation T40, which was set to be the 0 ms and the strata below the boundary was positive. It could be viewed that the polygonal faults were tensional fault system with the length from 150 to 1500 meters, the average space from 40 to 800 meters, and the throw from 10 to 40 meters (Figure 3). The seismic section also displayed two tiers visible. The upper tier lay in 2.5-2.7s and the lower one was in 2.7-3.0 s, manifesting the stratigraphically delimited subunits within the deformed interval. The meso-faults with large throw could connect the two tiers, and pierce the base of Pliocene and Meishan Formation. Hence, the faults with large throw were the significant pathway for the fluid flow upward the polygonal fault system with two or more tiers. Furthermore, the chaotic reflections with discontinuous and sinuous seismic events were also obviously visible overlying the polygonal faults interval in the profile. The view of the coherence slice from the 3D seismic data (Fig 3), showed the irregular polygonal map geometry. In the plane view, the distinct meso-faults were easily identified, and with much inspection, the smaller scale and faults could be found dim among the distinct meso-faults. The dim faults might indicate that smaller meso-faults existed, and could be observed among the larger scale faults with the higher resolution seismic data set.

Polygonal faults in Pearl River Mouth basin were fault systems with the length 100-1400 m, the space 200-800 m, and the throw 10-30 m. And locate at lower Miocene aged 18.5 Ma (Figure 1b).

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Figure 3. The 8 ms coherence slice after flattening below T40 and seismic profile of line AA’

The occurrence areas of the polygonal faults in the study area arrive to 6000 km2 according to our seismic data, distributing in the deep-water area of the southern depression of the Qiongdongnan basin, northeastern Zhongjiannan basin, and southern Pearl River Mouth basin (Figure 1a). Many articles have reported an abundance of the pockmarks, gas chimneys, mud diapirs, slides and channels, which are considered to have the linkage with the fluid migrations in northern South China Sea, and the polygonal fault can provide the additional pathway for the fluid flow [14-16]. BSRS OCCURRENCE IN THE DEEPWATER BASIN, SCS The distribution of BSRs in the deepwater of northern SCS margin has been delineated based on recent high resolution seismic data and conventional 2D/3D seismic data (Fig. 1a) [12, 14, 17-18]. Most of BSRs in the Qiongdongnan basin is discontinuous because the Neogene and Quaternary strata are parallel to the seafloor and middle to high amplitude. Great majority of the

BSRs locate at the upper slope with the water depth of 350-1500 m. A few BSRs occur in the middle and lower slope with water depth of 1400-2000m where tectonically it belongs to the central depression, as well as the southern faulted horst in the basin. The continuity of BSRs is cut by the faults which are good pathway for the methane fluid migration. Gas chimneys are also well developed in the Qiongdongan basin and Pearl River Mouth basin [12] (Figure 4). Besides geophysical data, some geochemical characteristics for the gas hydrates in the Qiongdongnan basin are discovered, such as higher hydrocarbon gas concentrations and higher thermal flux value, and the ascent of the concentration of SO4

2- and the descent of the salinity [19]. The prospect areas of the gas hydrates are described in Figure 1a. The leakages of the gas were frequently viewed through the seismic data and the line PP’ displayed the phenomenon of gas leakage. In the view of the profile, it shows that the seal of the gas led to the generation of the strong reflectors, which were common appearance in this area. Additionally,

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gas chimneys underlying the slide were considered to be the primary factor for the formation of the slide. Lots of pockmarks lay on the seafloor and

provided vents for deep gas migrating via the chimney upwards into the sea.

Figure 4. The seismic profile of the line PP’. The strong reflectors of sealed gas layers, the gas chimneys and pockmarks are present.

DISCUSSION Focused fluid flow associated along polygonal fault One accepted model was the model of the episodic hydrofracturing proposed by Cartwright [3]. It was interesting that, when the natural hydraulic fractured and pressure bled off, the fluid in the strata underlying the compartment could migrate into the reservoir overlying the layer-bound strata before the resealing of the pressure compartment. What’s more, the tectonic activities could destroy the resealing. As a result, it became possible that the polygonal fault served the pathway for the fluid migration.

Processes controlling the propagation of fluids that expelled during compaction strongly depend on sedimentation rate, lithology and stratigraphy [20]. Sandy sediments generally have high permeability and hydraulic conductivity, which allows pore fluids to dissipate quickly even if loading and associated fluid expulsion from minerals or advection rates from deeper strata are high. Sedimentary consolidation and fluid expulsion are generally slow processes that do not cause large deviations from hydrostatic pressure. However, some contracting and water-expelling rocks are under high excess pore

fluids from dissipating at the same rate as the load increases during sediment compaction. It can also be caused by the presence of impermeable rocks above. Such a situation can arise as a result of deposition, i.e. a stratigraphy in water-rich sediments is overlain by impermeable sediments. In some cases, the overpressure changes the fluid flow pattern from homogeneous dissipation to focused flow by self-enhances permeability.

Polygonal faults are formed by sediment compaction. Seismic data from Qiongdongnan basin show polygonal faults are confined to the Meisha Formation and Huangliu Formation, which consists of very fine-grained hemiplegic sediments. As these faults are layer-bound, they cannot be the results of tectonic activity. Frequently, polygonal faults terminate in pipe structures, which conduct the expelled pore fluids towards the surface [21]. Therefore, it is difficult to assess flux rates and the importance of these pipes.

The triple junction of three neighboring hexagonal cells of polygonal faults represented a preferential pathway for upward fluid migration from deeper levels [9]. The seismic section and coherence slice after flattening displayed the dim reflections of the

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triple junctions supported the existence of fluid migration via the triple junctions of hexagonal cells. The plane view shows the vents of the magma at the position of the triple truncation of neighboring

hexagonal cell in the study. The pipe attribute of the polygonal faults can also be lead to the occurrence of the chaotic reflection depositions and the gas hydrates.

Figure 5. a The seismic profile of line SS’. b The schematic interpretation of line SS’. The figure shows the fluid migrates into the GHSZ through the polygonal faults. And the BSRs and acoustic blanking zones are distinctly visible overlying the polygonal faults.

Gas hydrate occurrence associated with polygonal faults BSRs which represent the occurrence of gas hydrate overlie the polygonal faults, which show direct relationship between the polygonal fault and the distribution of BSRs [9-11]. The seismic data confirmed that the polygonal faults could serve as conduits for the fluid to migrate into the GHSZ. The models developed in the world can provide guidance for the research on the connection between the pipe attribution of polygonal faults and the concentration of gas

hydrates in deepwater basins of northern SCS margin. In this study, we map the distributions of BSRs which are regarded as the possible existence of the gas hydrates (Figure 1a), especially in the area south of the 3D survey in Qiongdongnan basin. The discontinuous BSRs occur over the polygonal faults (Fig. 5a) and the acoustic blanking zones overlie the BSRs. Furthermore, the negative relieves are discovered below the BSRs, which may be caused by the seismic events pushing down as a result of the gas concentration below the GHSZ.

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Figure 6. a the distribution of gas chimney, gas hydrate, mud diaper and polygonal faults in Baiyun sag, Pearl River Mouth basin. b coherence slice of polygonal faults. c seismic profile displaying polygonal faults and BSR. d seismic profile displaying gas chimney, BSRs, mud diaper and polygonal faults

The main hydrocarbon source rocks are the littoral facies in Early Oligocene Yacheng Formation, neritic facies in Late Oligocene Lingshui Formation and the marine facies in Miocene Sanya Formation. The focus fluid flows with sufficient methane gas increase concentration of the gas hydrate in GHSZ [22]. The fluid flows need a pathway to migrate sufficient gas to GHSZ, and in this study area, large volume of thermogenic methane gas from the source rocks could migrate upward along the polygonal fault system and formed gas hydrates in the strata of appropriate temperature and pressure. Hence, the polygonal faults served as the pathway linking the source rocks and the reservoir of the gas hydrates. Figure 6 shows the distribution of BSR, gas hydrate, gas chimney and polygonal faults in Pearl River Mouth basin. It is inferred that thermogenic gas ever escaped from the primary reservoirs through polygonal faults. CONCLUSIONS

The polygonal faults discovered in deepwater basin of northern South China Sea are tensional normal faults with the length from 150 to 1000 meters. The throws shift from 10 to 40 meters and the average spaces change between 40 and 800 meters. The layer-bound stratigraphic surfaces are easily identified from the seismic profile. Frequently, two or more tiers can be recognized. And the polygonal geometry view fault patterns are the apparent characteristics for the recognizing the polygonal fault system in plane view. Furthermore, the study suggests that the pipe attribute of polygonal faults can make a contribution to the concentration of the gas hydrates in northern South China Sea. The polygonal faults can serve as the conduit to migrate the fluid flow upwards to form the gas hydrates. ACKNOWLEDGEMENTS We thank the Guangzhou Marine Geological Survey and China National Offshore Oil Corporation for their permission to release the data used in this paper. The research work reported here

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was supported by the National Natural Science Foundation of China (40930845)， International Science & Technology Cooperation program of China (2010DFA21740) and the National Basic Research Program (2009CB219505). REFERENCES [1] Cartwright JA, Dewhurst DN. Layer-bound compaction faults in fine-grained sediments. Geological Society of America Bulletin 1998;110(10):1242-1257. [2] Henriet JP, de Batist M, van Vaerenbergh W. Seismic facies and clay tectonic features of the Ypresian clay in the Southern North Sea. Bull. Belg. Ver. Geol 1989;97:457-472. [3] Cartwright JA. Episodic basin-wide hydrofracturing of overpressured Early Cenozoic mudrock sequences in the North Sea Basin. Marine and Petroleum Geology 1994;11(5):587-607. [4] Henriet JP, Batist D, Verschuren M. Early fracturing of Palaeogene clays, southernmost North Sea: relevance to mechanisms of primary hydrocarbon migration. In: Generation, Accumulation and Production of Europe's Hydrocarbons (Ed. A. M. Spencer). Spec. Pub., Oxford University Press, Oxford, 1991. [5] Davies R, Cartwright JA, Rana J. Giant hummocks in deep-water marine sediments: Evidence for large-scale differential compaction and density inversion during early burial. Geology 1999;27(10):907-910. [6] Higgs WG, McClay KR. Analogue sandbox modeling of Miocene extensional faulting in the Outer Moray Firth. In: Williams GD, Dobb A (eds) Tectonics and seismic sequence stratigraphy. Special Publications, London, 1993:141-162. [7] Cartwright JA, Lonergan L. Volumetric contraction during the compaction of mudrocks: a mechanism for the development of regional-scale polygonal fault systems. Basin Research 1996;8(2):183-193. [8] Wu SG, Sun QL, Wu TY Yan SQ, Ma YB, Yao GS. Polygonal fault and oil-gas accumulation in

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