Characteristics and genesis of the polygonal fault system in southern slope of the Qiongdongnan Basin, South China Sea

Characteristics and genesis of the polygonal fault system in southern slope of the Qiongdongnan Basin, South China Sea

Accepted Manuscript Characteristics and genesis of the polygonal fault system in southern slope of the Qiongdongnan Basin, South China Sea Jianhui Han...

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Accepted Manuscript Characteristics and genesis of the polygonal fault system in southern slope of the Qiongdongnan Basin, South China Sea Jianhui Han, Leng Jigao, Yingmin Wang PII:

S0264-8172(15)30144-6

DOI:

10.1016/j.marpetgeo.2015.11.022

Reference:

JMPG 2404

To appear in:

Marine and Petroleum Geology

Received Date: 22 April 2015 Revised Date:

30 October 2015

Accepted Date: 25 November 2015

Please cite this article as: Han, J., Jigao, L., Wang, Y., Characteristics and genesis of the polygonal fault system in southern slope of the Qiongdongnan Basin, South China Sea, Marine and Petroleum Geology (2015), doi: 10.1016/j.marpetgeo.2015.11.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Characteristics and genesis of the polygonal fault system in southern slope of the Qiongdongnan Basin, South China Sea1 Han Jianhui a,b,*, Leng Jigao c, Wang Yingmind,e

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Corresponding author: Jianhui Han

Affiliation of correspongding author: Chengdu University of Technology Email address of the corresponding author: [email protected]

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College of Energy Resources, Chengdu University of Technology, Chengdu, Sichuan 610059, People’s Republic of China b State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, Sichuan 610059, People’s Republic of China c China Energy Reserve Corporation, Beijing 100107, People’s Republic of China d Ocean College, Zhejiang University, Hangzhou 310058, China e State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing), Beijing 102249, People’s Republic of China

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Abstract: Based on 3D seismic data, the polygonal fault system(PFS) discovered in the southern slope of the Qiongdongnan(QDN) Basin was studied through fine seismic interpretation and coherent cube analysis. The results show that: the PFS is mostly composed of small plate-type faults, with fault length of 1-3 km, maximum fault throw of 20-40 m and dip angle of 25°-45°; the PFS is mainly observed in the Huangliu Formation, deposited in the early stage of fast slope depression; the PFS is layer-bound and can be separated into two tiers by T31’ interface with obvious channel incision features; the PF of tier2 exhibit ”y” structural style, including two parallel faults, master fault and associated fault, dipping in opposite directions. Their intersection relationship can be divided into three classes, and 14 sub-classes based on the fault intersection relationship of master fault and its associated fault; the PFS is dominated by two strikes: NE60° and NW150°, indicating that it was controlled by tensile stress produced by differential settlement between the Xisha uplift and QDN basin; and its genesis was jointly controlled by the syneresis of clay minerals and overpressure cyclical dehydration. Key words: South China Sea, the Qiongdongnan Basin, polygonal fault 1. This paper is jointly sponsored by the Doctoral Program of Higher Specialized Research Fund (20125122120022) and the National Natural Science Foundation of China (No. 41372115). About the first author: Han Jianhui, Ph.D, male, born in 1976, works in Chengdu University of Technology as a lecturer. He is mainly engaged in Sequence stratigraphy and petroleum geoscience. Tel: 13568865352, E-mail: [email protected] 1

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1. Introduction

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Polygonal fault(PF) is a typical fault with non-structural genesis (Cartwright et al., 1998). Cartwright et al. (2003) defined it as a layer-bound extension fault system mainly in fine-grained strata, with variable and intersecting fault strikes, which form polygons. This type of fault has been discovered in more than 200 basins in the world, predominately in passive continental margin basins and craton basins, covering an area of 10×109km2(Cartwright et al., 1998).

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A PF generally extends a hundred meters to several kilometers. It grows in the manner of radiation propagation (Barnett et al., 1987). Its fault tendency is non-selective and its strike is random. Such faults are straight with a dip angle of 25°-65°, or occasionally listric when a fluidized bed exists at the bottom of a deformed layer (Stuevold et al., 2003). The maximum displacement values for polygonal faults range globally from a few metres or less to over 120 m(Cartwright, 2011). The maximum displacement is often in the central fault surface, and gradually decreases towards fault endpoint. The faults across two layers generally have their two biggest displacement points, located in the central part of their layers (Stuevold et al., 2003). Most PFs are distributed in the slope of passive continental margin (Cartwright, 2011), but some faults are discovered in the lake basin, such as the Sanzhao Sag in the Songliao Basin(Ding et al., 2013; Fu and Song, 2008).

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Wu et al. (2009) and Sun et al. (2010) indicated that the PFS occur in three tiers of the upper Meishan Formation and the Huangliu Formation. And the PFs are almost oriented equidirectionally . Different from them, Han (2009) insists that the PFS have dominant fault strikes.

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Today, the study on PF in the QDN Basin is still at its preliminary stage in two aspects. Firstly, fault features are not analyzed adequately. Previously, scholars diagnosed fault features by upward extending and downward extending different time thicknesses into time slices after flatting T40 interface (namely T31 herein). In this case, the fault features were not controlled by isochronous interfaces, resulting in inaccurate layer-bound fault features and fault phases. Secondly, the genetic mechanism of PFs in this area needs to be further studied. Inaccurate analysis on fault features makes it difficult to draw an accurate conclusion on the fault formation mechanism. In this paper, using high resolution seismic data, a coherent cube is obtained, and then systematic study is conducted through extracting coherent data along the horizon based on the fine interpretation of isochronous interfaces. Finally, some new conclusions are achieved.

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Fig.1. Location map. Study area located within area of 3D seismic data and several 2D seismic sectioins.

2. Regional geology

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The QDN Basin locates in the northwest of the South China Sea, and neighbors Hainan Island to the north, Red River – Yuedong Fault to the west, Xisha Uplift to the south and Xisha Trough Basin to the east. The basin formation and evolution were jointly controlled by large-scale Red River strike slip fault and South China Sea extension. The study area is located in the southern slope of the QDN Basin, at the position where the QDN Basin, Xisha Uplift and the northern slope of Guangle Uplift are conjunct(Fig.1).

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Gong et al. (2011) have divided the Cenozoic in the QDN Basin into rifting and post-rifting sequence. Han (2009) divided the Cenozoic into three tectonic sequences by taking break-up unconformity (T60) and continental slope transform unconformity (T40) as boundaries(Fig.2): rift tectonic sequence (Tg-T60); depression tectonic sequence (T60-T40); and intense depression tectonic sequence (T40-seabed). The depression tectonic sequence includes the Sanya Formation and Meishan Formation, representing the stage from initial lithosphere fracturing and formation of the oceanic crust of south-west sea basin extension during the extensional process of the the southwestern sea basin. This stage is characterized by low subsidence rate and depositional rate, when deltaic sediments and carbonate deposits occurred. The intense depression tectonic sequence includes Huangliu Formation, Yinggehai Formation and Ledong Formation, representing the continental slope subsidence stage after extension, when the tectonic subsidence rate was high, but depositional rate was 3

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Three tectonic sequences represent the sedimentary strata of three tectonic evolution stages including rifting, depression and intense depression. The sedimentary strata of an intense depression stage is mainly discussed in this study.

Fig.2 Stratigraphic framework showing the ages of key Formation in study area along with key seismic stratigraphic marks. A representative section of seismic reflection data from 3D survey is shown to illustrate the typical seismic stratigraphy.

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Fig 3. Major unconformities and seismic reflector packages in the QDN basin. Seismic interface and its age are determined by Well YC35-1-2, and its seimic calibration after (Gong et al., 2011).

3. Database and methodology

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A high resolution three-dimensional (3D) seismic reflection cube and several 2D seismic sections, located on the QDN basin, covering an area of 1100 km2, was made available for this study(Fig 1). The seismic data are near zero phase with SEG normal polarity. The 3D seismic data sampling interval of 4 ms. Line spacing of this survey is 25 m (inlines) and 12.5 m (crosslines). The dominate frequency of the 3D seismic data is between 25 and 40HZ. The vertical resolution for this interval is estimated to be respectively 18m, using an average velocity value of 2000ms-1.

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This study was carried out through fine interpretation and coherent cube analysis. Firstly, five seismic horizons are interpreted finely; then, coherent cube was calculated; finally, the a coherent data was extracted along these horizons and mapping for PF analysis. This method can more effectively reflect the relationship between seismic interface and PF, so as to effectively identify the geological significance of PF.

4. Results 4.1. Seismostratigraphy Fig. 2 shows the main unconformities identified by seismic reflection discordance. T60 is characterized by erosional truncation, T40 by onlap in the north part of the slope and T30 by numerous channels. T40 was the beginning of intense 5

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PFs are widely developed throughout the study area. The PFs predominantly occur in T40-T30 in the study area (Figs.4 and 5), namely, they are located in the Huangliu Formation, rather than in the Meishan and Huangliu Formations proposed by previous scholars (Sun et al., 2010)(Fig.2). This contradiction is mainly attributed to the different recognitions on regional tectonic evolution and seismic reflection interface. Due to limited drilling data, in this paper, Well YC35-1-2 in the northern part of the QDN Basin is selected to seek for drilling interface, and the geological significance of different seismic reflection interfaces is determined based on regional tectonic evolution, in order to constrain the interpretation of seismic interface based on well data.

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The continuous seismic reflection interfaces with strong-amplitude between two unconformity interfaces (T40 & T30) act as regional marker horizon, named T31. This interface is consistent with previous T40 (Sun et al., 2010). Seismic profile shows that this interface presents the features of parallel reflection configuration, weak amplitude and high continuity, which are consistent with that of abyssal sea argillaceous deposits. The seismic reflective wave group represents a trend from weak to strong, and then back to weak from bottom to top; its cyclicity represents a completed process from sea level rise to decline, while T31 in the middle part represents the condensed section of the high sea level phase. According to the analysis above, it is considered reasonable that the PFs are mainly developed in the condensate section with argillaceous deposits, and on the other hand, the bathyal environment after T40 is more suitable for the development of PFs. Therefore, it is more reasonable that the PFs in the study area predominantly occur in the Huangliu Formation.

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4.2. Fault architecture

Sun et al. (2010) have made T40-centred(namely T31 inhere) time coherence slices, and subdivided the PFS into three tiers based on fault density and throw. We found that these three tiers can’t divide each other neatly. It should be a complex tier. We named them tier1. We also found there is a new tier above these “three tiers”. This tier is simpler than tier1, and named tier2(Figs.4 and 5). Two tiers are bounded by seismic interface closely related to the incised channel developed in the deep water region. Tier1 occurs between T31′ and T30, and tier2 occurs between T31′ and T40. The two layers are consistent in seismic facies – both parallel seismic facies in deep sea quiet water, with moderate-weak amplitude and high continuity. The lower faults are mostly cut at T31′ interface, revealing that the incised water channel was formed after formation of PF (Figs.4, 5 and 6). The upper faults are also terminated at the T31′ 6

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interface, indicating that the layer-bound nature of PFs is influenced by sedimentary types. This type of fault only occurs in the deep water region dominated by shale deposits, but not in water channel, since it represents continuous parallel seismic facies. The PFs seldom occur above T30 at the top of the Huangliu Formation represented by the incised channels, thus making the interface with the incised channels (T31′, T30) as the stratigraphic mark of PFs. In this way, the “layer-bound” nature of PFs is highlighted.

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Fig.4. Uninterpreted (top) and interpreted(bottom) seismic profile I-I′. The figure shows the vertical distribution of PFs. X-X′ representing the intersection point with profile II-II′ in Fig.5. See Fig.1 for location.

Fig.5. Uninterpreted (top) and interpreted(bottom) seismic profile II-II′.The figure shows the vertical distribution of PFs. X-X′ represents the intersection point with profile II-II′ in Fig.4. See Fig.1 for location.

4.2.1. Tier 1 The characteristics of the PFs are consistent with those obtained from previous 7

ACCEPTED MANUSCRIPT studies. The PFs show that there exist two primary fault trends, 30–50N and 115–130N. Most PF lengths are between 200m and 1500m(Fig.6).

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Three faults were selected at random for analyzing their fault throw in Fig 7, showing the maximum fault throw at the middle part of the fault surface, and gradually decreasing downwards and upwards, until missing (Fig.7). Some PFs cut across T31’, which may indicate that tier1 and tier2 were not divided absolutely. The max fault throw occupying different depths may imply that tier1 is also a complex of PF formed at a different time.

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Sun et al. (2010) adopts to calculate the mudstone thickness at 1800 m/s after the result of the Y13-1 block(Hao et al., 2000). Considering that our study area is much deeper than the Y13-1 block, we assume that the average velocity is 2000 m/s. Based on this value, the maximum fault throw is about 10-15m. Based on the analysis above, the fault dip angle can be determined (Eq.1): 180 ߨ

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A = ATAN

∆‫ݐ‬ ܸ× 2 ∆‫ܮ‬

×

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Where, A is the fault dip angle; V is formation average velocity; ∆t is the two-way travel time of vertical fault section; ∆L is the horizontal distance of fault section. The results show that the dip angle of faults in the study area are generally about 30°-45º, much smaller than 50°-90° in previous studies (Sun et al., 2010). Compared with the reported PFs in other basins around the world, the PFs in the study area have smaller dip angles.

Fig. 6. Distribution of tier1. Rose diagrams of fault strickes for the PF networks show that the NE and SE are the main directions. See Fig.1 for location.

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Fig. 7. Seismic cross-section of tier1 and vertical throw variation on three fault of tier1.

Fault distribution in different time slices show that the faults in T31’’(See Fig 2 &7 for the interface location) have more density and shorter fault length, and T31’’ is near the center of tier 1. The lateral distribution is controlled by the incised channels, such as in T30. 4.2.2. Tier 2

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4.2.2.1. Fault geometry of tier2

We have interpreted two new reflected surfaces besides the T40, T30,T31, and have named them as T31-60ms and T31-140ms. The data extracted from the coherent cube along these horizons can be used to study the PF cut through each surface.

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The statistics indicate that the tier2 have two dominate directions, i.e. NE60° and NW150°, and most extend about 1-3km (Fig. 8), with fault throw range from 20m to 40m, gradually increasing from top to bottom, reaching the maximum of T31 and missing at bottom(Fig. 9). The dip angle of faults are generally about 25°-43º. (1) “y” style PFs

Insets showing a small area of time coherence along T31 show that most PF are associated by a small fault. Both of them are parallel to each other(Fig.10, a & c) and have opposite dip directions(Fig. 10,b & c). The seismic section of PF in Fig.10,e shows that the “y” style is the basic style. This structural style includes a master fault with higher throw and an associated fault with low displacement. Fig.10,b shows the attribute fusion of coherence and dip azimuth. Most faults show two parallel lines with opposite color in the chromosphere, such as green lines and purple lines, meaning that the master faults and the associated fault have the same mode of origin. We speculate that this style is associated with fluid escape along the PF.

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Fig 8. Insets showing a sample area of tier2 for different surfaces and rose diagrams of fault strikes and lenghts for each surface. The color maps were drawn according the coherence data along four seismic reflect interfaces. See Fig.9 for the location of these four surfaces in the seismic section. 10

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Fig.9. Fault throw distribution of PF in tier2. Gradually increasing from top to bottom, reaching the maximum in T31 and missing at the bottom. Black solid line

(2) Main fault intersection types for PFS

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Recognizing the parallel associated fault, we can refine the fault intersection types for the PF presented by Lonergan et al. (1998). Three main class were recognized. And they can be subdivided into 14 sub-classes based on the fault intersection relationship of master fault and its associated faults.

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Cass A. The class A intersection is orthogonal. Most sub-classes are controlled by two master faults with cross strikes. And the different conditions of the associated faults, existing or not, cut through or limited, control the section type. This class, especially A-2 and A-5, are the most common intersection type in the study area.

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Class B. This class exhibits one fault set limited by another. In this area, B-1(one master fault was limited by another) and B-4 (one set was limited by the master fault of another set) are more common. Class C. This class is rare in the study area. The angle between the branch is always bigger than 90. One or two branches with an associated fault and the other branch only having only a master fault(C-2) is the common type in this class.

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Fig 10. Typical combination of PF. A: insets showing a small area of time coherence along T31.B:

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Attribute fusion of coherence and dip azimuth.C, D: A zoomed in portion of A, showing the details of PFS in T31. See A for location. E: Main structural style of PFS. The big faults with more displacement are associated with small faults which have opposate dip drections. F: Schematic diagram of main fault intersection types for PFS. Typical examples of each type are shown in D.

4.2.2.2. PFs classes

This fault class is subdivided into 3 different groups: 1st order PF, 2nd order PF and associated fault(Fig.11). The 1st order faults usually cut across T40 and T31, having a higher displacement than others’. The max throw is centered on T31(Fig.9). Most of them are master faults in y” style with PFs in the same way. The 2nd order faults usually cut across the strata above T31. The bottom often 12

ACCEPTED MANUSCRIPT passes through T31. They have lower displacement and often cut through the 1st fault, composing “X” conjugate faults or are limited by the 1st faults(Fig.10, Fig.9). The max throw of a 2nd fault is usually 10-13m, and is higher than the 1st’ in vertical ). These suggest that the 2nd faults may occur later than the 1st direction(Fig. 9, order faults. The associated faults are seen with the associated faults in equally

y” style PFs.

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Fig.11 shows that the same fault which cut through the surfaces at a different depth may exhibit quite different horizontal characteristic. So, the method dividing tiers by fault density and throw should not been recommended in a complex PF tier.

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Fig.11. Insets showing a small area of PF for different surfaces and main sets of PFs in the sections. Five 1st fault segments in different seismic sections and their intersection line in each surface have been shown in this Figure. See Fig. 5 and Fig. 1 for location. 4.2.2.3. PF distribution Statistics about a sample area covered 280km2(Fig.8) indicate that the number of PFs increased upward from T40 to T31-60ms, and then reduced sharply from T31-60ms to -140ms. T31-60ms, at center of tier2, is a densely packed, highly mature system. The top and bottom, such as T40, has low fault density. Rose diagram of PF strikes show that each surface has two orthogonal strikes, and they have turned anticlockwise from bottom to center(Fig.8 and 12). Towards the top, PF strikes turn back. PFs in T40 exhibit flat segments with almost free tips, while there is a more bent segment with almost no free tips in T31&T31-60ms. 13

ACCEPTED MANUSCRIPT Refering to Fig. 11, faults in T40 are mainly of the 1st order PF, and faults in T31&T31-60ms are overlapping 1st order, 2nd order and associated faults.

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The Fig.8 and Fig.4 & 5 suggest that lateral variations of PFS were influnced deeply by chanell. T31-140ms has been erased by T30, which was formed by channels.

Fig.12. Overlapping diagram of PF strikes and length in different surface. 4.3 Relationship between PF and underlying fault

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Distribution of faults on 0 ms, 36 ms and -48 ms time slices was identified by upward extending and downward extending T31 interface. Then, the strikes of PFs were calculated and rose diagrams were prepared for analyzing features of such strikes. The results show that the fault strikes in the study area are variable, without regularity ((Sun et al., 2010),Fig.6). In this paper, based on T40 and T31 isochronous interfaces, a coherent cube along horizon is extracted and mapped. The results indicate that, compared with T40, T31 reveals more intensive PFs and more arc-shaped faults. The statistics of strike and length of the PFs in two isochronous interfaces suggest that the strike is very uniform - mostly NE60º and NW150° (Fig.8). As can be seen from Fig.4 and Fig.5, there are no obvious faults cutting between T40 & T31 where PFs are developed, and the underlying T50. However, the statistics of the faults in T50 in the same area show that the dominant fault strike is also about 150°, the same as in the case of one of the predominant PFs’ strike direction(Fig.7). The PFs with a certain predominant direction were not rare. Gay and Berndt (2007) 14

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used the predominant direction of PFs in different horizons to distinguish the development phases of such faults. In addition, Hansen (2004) reported the phenomenon that structural faults were perpendicular to PFs in the same horizon and pointed out that regional stress around structural faults could influence propagation of PFs, resulting in the generation of PFs perpendicular to structural faults. However, the PFs in the study area have obviously different features: they are not in the same horizon as the underlying (T50) structural faults; moreover, the underlying structural faults don’t cut into the Huangliu Formation, where PFs are located. PFs have two predominant strikes that are perpendicular. Generally, the PFs with strike consistent with that of underlying structural faults constrain the PFs with strike perpendicular to underlying structural fault. The uniformity of underlying structural fault and PFS might implicate that there was there was a uniform stress field perative throughout this time period, from T50 through to T30. This period is the depression to intense depression of north-west slope of the South China Sea. Our study area is located in the slope between the Xisha uplift and subsiding center of the QDN basin. The process of differential settlement can put the study area under a tensile stress, nearing the SN direction, and lead to NE and SE failure.

5. Discussion

5.1 Genesis and influencing factors of the PF system in the northwestern South China Sea

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As for formation mechanism, Lonergan et al. (1998), Jeffrey and Scherer (1990) and Cartwright (1996) considered that PFs were products of the syneresis of mud-rich sediments. Cartwright et al. (1998) pointed out that the structure and geometry of fault systems were controlled by the colloidal properties of sediments, and the volume shrinkage measured in seismic profiles could be interpreted by the syneresis of montmorillonite gel during early compaction period. Cartwright (1994) also suggested that the genesis of PFs was related to the periodic dehydration of an overpressure layer, which was consistent with the fact that PFs had a layer-bound nature and their planner distribution scope was consistent with facies belt boundary and lay thickness. Higgs and McClay (1993) and Clausen et al. (1999) analyzed the relationship between the formation of PFs and the gravity collapse of fine-grained sediments on slopes. Goulty (2002) considered that the formation of PFs was related to the repeated slip resulting from the low frictional strength of compacted sediments. Henriet et al. (1988) and Watterson et al. (2000) stated that PFs were formed due to the deformation of soft sediments and their genesis was related to the density inversion and collapse produced by the mud-rich sediments with abnormally high pore fluid pressure. Ding et al. (2013) pointed that the genesis of PFs in the Sanzhao Sag was related to the shear fracturing driven by dissolution. These mechanisms are all reasonable. However, further study is required as to whether the PFs in the study area originate from any or none of these mechanisms. For the genesis of the PFs in the study area, previous scholars (Sun et al., 2010; 15

ACCEPTED MANUSCRIPT Wu et al., 2009) more supported the syneresis of clay minerals. However, the author believes that another factor can’t be ignored, namely, overpressure. The PFs in the study area are likely to have been formed under the joint actions of the syneresis of clay minerals and overpressure cyclical dehydration. The former was discussed systematically. This paper will only focus on overpressure.

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The PFs in the study area were formed during a rapid depression period, when the entire area subsided as a whole. Containing argillaceous sediments, they usually presented in the southern slope of the QDN Basin, which was far away from the continent. During this period, the northwestern continental slope subsided rapidly(Yao et al., 1994) and the Red River fan deposited rapidly with sediments severely uncompacted, forming favorable conditions to generate overpressure. Widespread overpressure systems have been discovered in the QDN Basin (Hao, 2005; Xie et al., 2003).

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The T31 interface, where PFs are frequently present, displays strong reflection with good continuity in seismic profile. According to the analysis of sequence stratigraphy, T40 represents the beginning of the northwestern continental slope transiting from shallow water to deep water, while 5.5Ma (T30) represents a period when sea level was very low during continental slope subsiding and correspondingly, where a large quantity of undersea channel deposits were extensively distributed (Jiang, 2005; Xie et al., 2008). T31 interface, which represents the maximum flooding surface, displays continuous and strong reflection and is an argillaceous sediment concentrated interval. Previous scholars had realized the influence of overpressure fluid. For example, Hao et al. (2004) found that the episodic expulsion of overpressure fluids resulted in obvious constraint on the Ya21-1 structure in the QDN Basin during organic matter thermal evolution. Wang et al. (2013) reported that the existence of underlying overpressure was the premise for the formation of the massive transport complex pervasive in the QDN Basin. The massive transport complex in the study area mainly occurs in the strata above T30, just at the top of Huangliu Formation (T40-T30), where PFs are located, indicating the existence of overpressure. The episodic expulsion could be the dynamic mechanism for the generation of PFs.

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Some studies indicate that overpressure would result in the decrease of a fracturing limit (Hao et al., 2004). It is usually considered that hydraulic fracturing of formation would take place when pore fluid pressure reaches or exceeds 85% of net rock pressure (Holm, 1998; Roberts and Nunn, 1995). Under the circumstance of over-pressure, the Mohr circle shifts left, and the displacement is equal to the pore fluid pressure, thus the Mohr circle is easier to tangent to a rupture envelope, and the strata is more likely to fracture(Fig.13) (Hao, 2005), and the hydraulically fractured faults have random strikes. The study on Central Graben in the North Sea showed that, in shallow burial stage, overpressure would cause extensional fracturing in formation, inducing conical fissures, while during deep burial stage, cumulative overpressure would result in shear fracturing (Conybeare and Shaw, 2000). Generally, the hydraulic fractures produced by overpressure are difficult to preserve, unless they are filled by liquid sandstone and various mineral deposits (Cosgrove, 2001). But, the faults would 16

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be easily preserved due to repeatedly opening and closing in case of overpressure episodic expulsion within formation.

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Fig.13. Left shift of More circle caused by overpressure and result fracturing. A: The differential stress is smaller than 4T and extensional failure will be generated; B: The different stress is greater than 4T and shear failure will be generated(Hao, 2005). τ, shear stress; T, tensile strength; Pf, pore fluid pressure.

Fig.14. an example of over-pressured rupture of the underlying strata and the liquid’s

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6 Conclusion

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Formation and development of the PFs play an important role in oil and gas formation, migration and accumulation. Many scholars have focused on the role of PFs in hydrocarbon migration and considered them as an important part of oil and gas seepage systems (Geng et al., 2014; Xu et al., 2014). Many strata in the QDN Basin have already reached the threshold of hydrocarbon generation. Thus, the episodic expulsion of PFs resulted from overpressure is essential for hydrocarbon accumulation in the following aspects: (1) intense magmatic activities can increase heat effect, accelerate the maturity of oil and gas and relieve or offset the impact generated by shallow burial depth; and (2) overpressure episodic expulsion may induce a series of faults, which can act as expulsion channels for organic matters within mudstone. Therefore, it is speculated that a new oil and gas accumulation model with migration through PFs exists in the study area. Under this model, the oil and gas was sourced from the marine mudstones of Meishan Formation and Huangliu Formation; as such hydrocarbon-rich mudstones became gradually mature along with their subsidence and thermal evolution. Their volume expanded and then episodic hydrocarbon expulsion occurred as oil and gas migrated along PF system and finally accumulated in overlying deepwater turbidite sandstone. The further seepage of these oil and gas resulted in typical undersea terrains, such as undersea mud volcanos and pockmarks.

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The PFs in the study area, mainly in the Huangliu Formation, are controlled by the seismic reflection interface marked by incised water channels. They represent an obvious “layer-bound” nature, generally being in two tiers (T40-T31 'and T31'-T30). Most PF are associated by a small fault, being parallel to the master fault and dipping in opposite directions. Their intersection types can been divided based on the fault intersection relationship of master faults to their associated fault. This PF class is subdivided into 3 different groups: 1st order PF, 2nd order PF and associated fault. Most 1st order faults exist with associated faults. 2nd and 1st order faults can form “X” conjugate faults. The fine interpretation and statistics of the PFs in the QDN Basin indicate that the fault strikes are mostly in 60° and 150°, indicating that they were controlled by tensile stress produced by differential settlement between the Xisha uplift and the QDN basin; 18

ACCEPTED MANUSCRIPT The PFs in the study area originated in two mechanisms: clay mineral synersis and overpressure. The Huangliu Formation, where PFs are frequently observed, has the basic conditions for overpressure. It is speculated that there is also a new oil and gas evolution model in the study area, under which marine source rocks episodically expelled hydrocarbon under overpressure after an evolution and maturity process. In this model, PFs act as hydrocarbon migration pathways.

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ACCEPTED MANUSCRIPT The polygonal faults(PF) are controlled by the interface marked by incised channels. Most big PF are associated by a small parallel fault, dipping in opposite directions. The PF strikes may controlled by tensile stress produced by differential settlement. The PF mechanisms include clay mineral synersis and overpressure.

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PF may act as hydrocarbon migration pathways in hydrocarbon expulsion.