CHINA PETROLEUM EXPLORATION

Volume 21, Issue 2, March 2016

Received date: 23 Dec. 2015; Revised date: 29 Jan. 2016. Corresponding author. E-mail: liushx2@cnooc.com.cn Foundation item: State Natural Science Fund Project “Control of Tectonic Cycle in South China Sea over Hydrocarbon Accumulation in Zengmu Basin” (91525303); Special and Significant Project of National Science and Technology “Oil & Gas Exploration in and Key Technologies for Offshore Deepwater Area” (2016ZX05026). Copyright © 2016, Petroleum Industry Press, PetroChina. All rights reserved.

Control of tectonic cycle in the South China Sea over hydrocarbon accumula-tion in the Zengmu Basin

Liu Shixiang, Zhang Gongcheng, Zhao Zhigang, Xie Xiaojun, Wang Long, Song Shuang, Guo Jia, Wang Shenglan, Bi Yankun, Wang Yibo

CNOOC Research Center

Abstract: The Zengmu Basin was gradually formed with the evolution of tectonic cycle in the South China Sea. Since the Ceno-

zoic it has undergone three major structural evolution stages, i.e., foreland fault depression in the Eocene to the Early Miocene,

strike-slip transformation in the Middle Miocene and regional subsidence in the Late Miocene to present. The tectonic cycle in the

South China Sea has controlled the structural evolution of the Zengmu Basin and has played an important role in the hydrocarbon

generation and accumulation conditions. Foreland fault depression is the major controlling factor in the development of principal

source rocks in this basin, and the formation of sandstone reservoirs and structural traps at its southern flank. During the foreland

fault depression stage, delta coal source rocks and terrigenous marine source rocks were generated. Carbonate reservoirs and car-

bonate formation occurred in the central-northern area of this basin as part of the strike-slip transformation stage. The formation

and distribution of regional cap rocks in this basin have been dominated by the regional subsidence since the Late Miocene.

Key words: tectonic cycle in South China Sea, Zengmu Basin, structural evolution, hydrocarbon accumulation

The South China Sea, one of the most important marginal seas in the West Pacific Ocean, is located at the southeastern margin of the Eurasian plate. Affected jointly by the Eura-sian, Indo-Australian and Pacific plates, it has a very com-plex regional tectonic setting and dynamic condition[1–3]. Since the Cenozoic, the South China Sea has undergone mul-tiple stages of spreading. Its structural evolution is charac-terized by multi-cycle and multi-stage, when two major tec-tonic cycles of marginal sea arose, namely, the Paleo-South China Sea and the Neo-South China Sea[4]. The South China Sea region exhibits different geological features at different evolution stages. The tectonic cycle of this marginal sea is the major factor controlling the formation of the tectonic framework in the South China Sea and has played an im-portant role in formation, structural evolution, sedimentary filling, hydrocarbon accumulation and oil & gas resource potential of basins in the South China Sea area. Study on the Zengmu Basin suggests that the basin was formed gradually with the evolution of tectonic cycle in the South China Sea. The South China Sea has controlled the structural evolution and hydrocarbon accumulation condition of the Zengmu Basin and, therefore, can be considered as a major factor controlling the formation and evolution of the basin.

1. Geological overview of the Zengmu Basin

The Zengmu Basin is a Cenozoic foreland basin located

in the southern part of the South China Sea. The basin cov-ers a total area of 17×104 km2; topography is complicated, reef-flat and shoal are broadly developed, and the water depth is generally less than 300 m. According to the geotec-tonic division on a regional scale, the Zengmu Basin mainly lies within the Sunda Shelf area and partially extends into the Borneo Shelf area. The basin is separated from the Wan’an and West Natuna Basins to the west by the SN-trending Xiya and Natuna uplifts, and the Brunei-Sabah Basin to the east and the Beikang Basin to the north by a NW-trending Tingjia fault zone. To the south, the basin is situated onto a suture zone between the Zengmu block and the Borneo accretion system (Fig.1).

Utilizing basement properties, structural features, sedi-mentary responses and geophysical data, the Zengmu Basin can be divided into eight second-order tectonic units, i.e. Western slope, Kangxi depression, Nankang platform, East Balinjian depression, Suokang depression, Lanai uplift, Ta-tao horst-graben and West Balinjian uplift (Fig.1). The Kangxi depression comprises the majority of the Zengmu Basin and is also the primary depocenter in the basin. This is a sedimentary depression, where over ten thousand meters of sediments were deposited, exceeding a maximum deposi-tional thickness of 15000 m[5].

The Zengmu Basin is one of the most important basins for the oil & gas strategic region in the South China Sea. Since the discovery of the first oil/gas field in 1953, the

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Fig. 1 Location map of regional structures in Zengmu Basin

basin has experienced 4 large-scale exploration stages. As of 2013, a total of 846 wells had been drilled, and 26×104 km 2D seismic lines and 3.6×104 km2 3D seismic surveys had been acquired by foreign oil companies in the Zengmu Ba-sin. Within the basin, 134 oil & gas fields were discovered, which are distributed in 4 tectonic units (Western slope, Nankang platform, East Balinjian depression and West Balinjian uplift). At present, 103 oil & gas fields are located within the traditional territory of China, totaling 4.89×108 t of recoverable oil in place, 4.43×1012 m3 of recoverable gas in place and 1.60×108 m3 of recoverable condensate in place (the equivalent of 50.79×108 tons of oil and gast). This study of structural evolution and hydrocarbon accumulation will provide valuable guidance to upcoming oil & gas ex-ploration operations.

2. Structural evolution features

2.1. Tectonic cycles in South China Sea

Tectonic cycles in the South China Sea during the Ceno-zoic were manifested by two tectonic cycles of the marginal sea, i.e. the Paleo-South China Sea and the Neo-South China Sea. They evolved in two stages. 2.1.1. Stage of Paleo-South China Sea’s dying out and Neo-South China Sea’s development

The Paleo-South China Sea was developed on the basis of the residual Paleo-Tethys Ocean[6]. It began to spread during the Early Jurassic and was jointly affected by the

Meso-Tethys and the Paleo-Pacific. Its northern margin has witnessed a transition from the compressional setting to the extensional setting as the rate and direction of movement of the Pacific plate changed during the Late Cretaceous. At that time, the collision between the Indosinian and Eurasian plates resulted in the southeastward flow of a deep astheno-sphere situated between them and under the effects of NS stress. This asthenosphere was then obstructed by the Pa-cific plate to the southeast, forming the ascending mantle plume[8]. The interaction of these plates caused the Pa-leo-South China Sea to begin to subduct southward and die out. The Neo-South China Sea, opened up during the Oli-gocene, pushed the Nansha block to drift southward and also accelerated the dying out of the Paleo-South China Sea, forming the Lupar Line ophiolite belt and a series of sub-duction-accretion belts. The drifting of the Nanhai block terminated when it collided with the Borneo accretion sys-tem in the Middle Miocene, the Paleo-South China Sea to-tally died out, and the Neo-South China Sea completely opened up. 2.1.2. Stage of Neo-South China Sea's rapid subsidence and shrinkage

From the Middle Miocene to present, the NS seafloor spreading of the Neo-South China Sea has remained static. Main tectonic movements include the subduction of the Philippine plate from southeast to northwest. The northern and western margins of the Neo-South China Sea are in rapid subsidence. The subsidence effect decreases succes-

Liu Shixiang et al., Control of tectonic cycle in the South China Sea over hydrocarbon accumulation in the Zengmu Basin 3

sively from the central oceanic basin to shelf basin, leaving thick sediments at the influx of a river into the sea. In the eastern part of the Neo-South China Sea, the obducted Phil-ippine Island Arc caused the oceanic crust lithosphere in the Neo-South China Sea to subduct eastward, forming a sub-duction edge[9]. As a result, the eastern part of the Neo-South China Sea remains closed.

2.2. Tectonic evolution of Zengmu Basin

The formation and evolution of the Zengmu Basin are controlled primarily by tectonic evolution of the Paleo- and Neo-South China Sea. During the Eocene-Early Miocene, the Zengmu block collided to the south with the Borneo ac-cretion system to form a foreland basin due to the impacts of the Paleo-South China Sea’s gradual dying out and the Neo-South China Sea’s spreading. In the Middle Miocene, the spreading of the Neo-South China Sea ceased, the strike-slip transformation of the basin occurred as a result of the counterclockwise rotation of the Borneo accretion sys-tem, and under this kind of tectonic stress a torsional fault depression basin was formed. Since the Late Miocene, the basin as a whole has entered the regional subsidence stage. In general, since the Cenozoic, the Zengmu Basin has un-dergone three major tectonic evolution stages, i.e. foreland fault depression, strike-slip transformation and regional subsidence. 2.2.1. Foreland fault depression stage (Eocene to Early Miocene)

The Lucania block on which the Zengmu Basin rests was located in the northern part of the Borneo by the end of Late Cretaceous[10], and was separated by the Paleo-South China Sea. At that time, the Zengmu Basin was situated on the passive continental margin (Fig.2a). In response to the southward subduction of the Paleo-South China Sea, the Lucania block drifted southward[11], and in the Late Oligo-cene it collided to the south with the Borneo accretion sys-tem (Fig.2b). As a result, thrust nappe structure was devel-oped in the southern part of the Zimu Basin, forming a foreland basin. This effect lasted to the end of the Early Miocene (Fig.2c). The Mesozoic to Eocene strata, under the effects of this tectonic stress, was gradually uplifted to ex-perience erosion or metamorphism, forming the basement of the basin. Because of collision and compression, the south-ern part of the basin was highly uplifted to form a foreland basin and an orogenic belt, which provided the basin with abundant sediments at later stage (Fig.2d). The northern part of the Zengmu Basin is predominately in an extensional set-ting, with extensional normal faults and locally developed mud diapirs and volcanoes. A considerable thickness of strata was deposited during the foreland fault depression

stage, and at that time the Kangxi depression, the Nankang platform and the East Balinjian depression were a unified depression rested on foredeep of the foreland basin. There-fore, during the foreland fault depression stage, the southern part of the basin had collided and was compressed to form a foreland basin and an orogenic belt. Since the effect of this compression was stronger in the south than that in the north, the structural pattern of the basin can be divided into two distinct belts – the compressional structure in the south and the extensional structure in the north. Because the depocen-ter and subsidence center were located in the northern part of the basin, the northern part has accommodated thicker sediments than the southern part. 2.2.2. Strike-slip transformation stage (Middle Mio-cene)

The Middle Miocene witnessed a considerable change in regional tectonic settings across the Zengmu Basin and ad-joining areas. Firstly, the spreading of the Neo-South China Sea ceased and the southern part of the basin was no longer affected by collision and compression. Secondly, the open-ing up of the Celebes Sea and the Sulu Sea to the east of the Borneo, and northward progression of the Australian plate to the south,[12] caused the Borneo, as well as the Zengmu Basin, to rotate counterclockwise[13]. As a result of this rota-tion, two strike-slip fault zones were formed in the northern part of the basin. One is named the western South China Sea fault zone[14], and the other is the Lizhun-Tinjia fault zone[15]. The northern part of the basin was therefore transformed by a large number of strike-slip faults. This transformation was, however, relatively weak in the southern part. Therefore, it can be concluded that the strike-slip transformation was strong in north and weak in south. A number of reversal an-ticlines were formed in the proximity of strike-slip fault zones under the effect of strike-slip extrusion. Angular un-conformities can be formed on the top of these anticlines where truncation and erosion commonly occurred, and car-bonate rocks could form at high positions of anticlines. Many carbonate formations are present across the basin during the strike-slip transformation stage, forming a set of high-quality reef limestone reservoir. Specific examples include reservoirs of the L gas field on the West slope and some oil & gas fields in the Nankang platform, all of which were formed in this setting. 2.2.3. Regional subsidence stage (Late Miocene to pre-sent)

Since the Late Miocene, the Zengmu Basin and adjoining areas as a whole have entered a regional subsidence stage, during which the basin was predominately subsided on a re-gional scale under a relatively stable tectonic setting (Fig.2d). During this stage, the Zengmu Basin underwent a

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Fig. 2 Tectonic evolution section of Zengmu Basin

slow subsidence in the Late Miocene and then a rapid sub-sidence since the Pliocene. However, the tectonic setting generally remained stable and was neritic to bathyal-do-minated.

3. Petroleum geological conditions

3.1. Source rock

Exploration practices have confirmed the presence of two sets of source rocks across the Zengmu Basin, i.e., the Oli-gocene delta coal source rock (including mudstone, coal bed and carbonaceous mudstone) and the Lower Miocene terri-genous marine source rock[16–17]. The Oligocene coal source rock was developed in the southern part of the basin. It con-tains Types II2-III kerogen with TOC of 1%–2%, and can be 4000–5000 m thick as revealed by seismic data. Organic matter in the Lower Miocene source rock is predominately

Type III and partially Type II2 kerogen with TOC of 0.3%–5.5% or 1.4% on average. This can be therefore clas-sified as moderate to good source rock. Geochemical analy-sis of these source rocks suggests that the Oligocene-Lower Miocene source rocks are dominant in the basin. It means that the principal source rock in the Zengmu Basin was de-veloped during the foreland fault depression stage in the Eocene-Early Miocene.

3.2. Reservoir

The statistics of discovered oil & gas fields across the Zengmu Basin indicate the presence of two types of reser-voirs across the basin, i.e. sandstone reservoirs in the Oli-gocene-Middle Miocene strata and carbonate reservoirs in the Middle to Upper Miocene strata.

Sandstone reservoirs are distributed in the East Balinjian depression and acts as the primary reservoir of oil fields

Liu Shixiang et al., Control of tectonic cycle in the South China Sea over hydrocarbon accumulation in the Zengmu Basin 5

discovered in that depression. River mouth bar, distributary channel and delta plain are main sedimentary facies. Lithology is dominated by fine- to moderate-grained sand-stone. The porosity ranges from 10% to 38%, averaging 14%, and tends to increase from the lower part to the upper part. The permeability generally ranges from 100 mD to 1500 mD. They are therefore interpreted to have good physical properties. A series of oil & gas fields with sand-stone reservoirs have been discovered by far in the East Balinjian depression, the Zengmu Basin. Recoverable re-serves of these sandstone reservoirs total about 2.6×108 tons of oil equivalent, accounting for about 9% of the basin’s re-coverable reserves.

Carbonate reservoirs, dominated by organic reef-shoal, grainstone and marlstone[16], were developed in the West slope and the Nankang platform. By far over 200 carbonate formations, either platform- or tower-shaped, have been dis-covered. The porosity is 10%–40%, or 26% on average. The permeability is 20–1000 mD. They are therefore interpreted to have very good physical properties. Over 30 gas fields holding considerable gas reserves have been discovered by far in West slope and Nankang platform, the Zengmu Basin. Particular examples are L, E11, F6 and M1 gas fields. Recoverable reserves of carbonate reservoirs in the Zengmu Basin total 26.5×108 tons of oil equivalent, accounting for about 91% of the basin’s recoverable reserves. Therefore, carbonate reservoirs are considered the main producing lay-ers in the Zengmu Basin.

3.3. Cap rock

The Cenozoic strata in the Zengmu Basin were deposited in a setting in which the sea level rose continually and the sea area spread gradually, which allowed for the formation of favorable cap rock. The Late Miocene-Quaternary is a in which the Zengmu Basin subsided on a regional scale. Un-der this setting a section of neritic-bathyal sediments were deposited, which could act as a good regional cap rock across the basin, since it extends continuously and is thickly bedded and free of tectonic movement[17]. In addition, the Oligocene-Lower Miocene mudstone interlayer is consid-ered a local cap rock for structural traps.

3.4. Source-reservoir-cap-rock assemblage

The Zengmu Basin has undergone three stages of tectonic evolution, with two sets of source rocks (Oligocene and Lower Miocene), two sets of reservoirs (Oligocene-Middle Miocene clastic rock and Middle to Upper Miocene carbon-ate rock), a local cap rock (Oligocene-Lower Miocene) and a regional cap rock (Upper Miocene-Quaternary). It is de-positionally characterized by presence of multi-cycle and

rhythmicity. Vertically, the source rocks, reservoirs and cap rocks are combined to four favorable assemblages.

Assemblage I: the Oligocene mudstone, carbonaceous mudstone and coal bed as the source rocks, the Oligocene sandstone as the reservoir, and the Oligocene mudstone in-terlayer as the cap rock, which together form a self-genera-tion & self-preservation assemblage.

Assemblage II: the Lower Miocene mudstone and car-bonaceous mudstone as the source rocks, the Lower Mio-cene delta sandstone as the reservoir, and the Lower Mio-cene mudstone interlayer as the cap rock, which together form a self-generation & self-preservation assemblage.

Assemblage III: the Oligocene mudstone, carbonaceous mudstone and coal bed, and the Lower Miocene mudstone and carbonaceous mudstone as the source rocks, the Middle Miocene sandstone and carbonate rock as the reservoirs, and the Upper Miocene-Quaternary mudstone as the cap rock, which together form a lower-generation & upper-preserva-tion assemblage.

Assemblage IV: the Lower Miocene shale and carbona-ceous mudstone as the source rocks, the Upper Miocene carbonate rock/organic reef as the reservoir, and the Upper Miocene-Quaternary mudstone as the caprock, which to-gether form a lower-generation & upper-preservation as-semblage.

For the Zengmu Basin, the hydrocarbon-generating po-tential of source rocks and the stratigraphic position of oil & gas discoveries indicate that, Assemblages III and IV are deemed to be most prospective, and Assemblages I and II can be classified as good assemblages.

3.5. Trap type

The Zengmu Basin holds well-developed traps[18], in-cluding structural traps (e.g., fault blocks, fault anticlines, draping anticlines and rollover anticlines) and organic reef lithologic traps. Structural traps, such as fault blocks, drap-ing anticlines and rollover anticlines, were formed in the southern part of the basin during the foreland fault depres-sion stage in the Oligocene-Early Miocene and the strike- slip transformation stage in the Middle Miocene (Fig.3). The Organic reef lithologic traps, which have a large area and significant amplitude, were developed mainly in the West slope and the Nankang platform (Fig.3) and are con-sidered to be the main trap for the Zengmu Basin.

4. Control of tectonic evolution over hydro-carbon accumulation

Oil & gas reservoirs are products of the formation and evolution of a basin. The formation of reservoirs is con-

6 CHINA PETROLEUM EXPLORATION Vol. 21, No. 2, 2016

Fig. 3 Distribution of major traps in the Zengmu Basin

trolled strictly by the tectonic evolution of a basin. The tec-tonic cycle in the South China Sea has controlled the struc-tural evolution of the Zengmu Basin as well as the hydro-carbon accumulation in the basin. This control is evident in the development features of source rocks, reservoirs and caprocks, formation of traps, and geological conditions for hydrocarbon migration.

4.1. Control on formation of source rocks

Two sets of source rocks are developed in the Zengmu Basin, i.e. the Oligocene delta coal source rock and the Miocene terrigenous marine source rock. In the evolution process of the basin, both source rocks were formed during

the foreland fault depression stage in the Eocene-Early Miocene. Because of the continual southward subduction of the Paleo-South China Sea, the Lucania block on which the Zengmu Basin rests collided with the Borneo accretion sys-tem. As a result, the southern part of the basin was com-pressed to form uplifts and then the provenance zone for the basin (Fig.4). The substantial presence of fluvial and delta sediments may provide sufficient materials for formation of source rocks. At that time, a subsidence area was formed in the northern part of the basin under an extensional regime, providing the accommodation space for source rocks. Within the East Balinjian depression, which was uplifted later to a shallow buried depth, a coal source rock was

Fig. 4 Palaeogeographic map of the Zengmu Basin at the foreland fault depression stage

Liu Shixiang et al., Control of tectonic cycle in the South China Sea over hydrocarbon accumulation in the Zengmu Basin 7

formed and, given the relatively low geothermal gradient, has predominately generated oil. Within the Kangxi depres-sion and the Nankang platform, where marine strata were continually deposited, a terrigenous marine source rock was formed and, given the relatively high geothermal gradient (due to a deep burial depth) and connection to deep heat source (through strike-slip faults formed in a later), has predominately generated gas[19].

4.2. Control on formation of reservoirs

During the foreland fault depression stage, the Zengmu Basin formed the depositional setting of a large-scale slope that dips northeastward, with uplifts formed in the southern part (in response to the compression effect) and depressions formed in the northern part. As a result, a number of sand bodies of alluvial fan and delta depositional systems were developed on a gentle slope in the southern part of the basin. These sand bodies, together with coastal plain mudstone and

neritic mudstone, comprise a self-generation & self-preser-vation assemblage. Reservoir bodies of this assemblage ex-hibit a complex distribution law and are distributed unstead-ily. This kind of oil/gas field, such as D18 (Fig.5), often has multiple producing layers and unsteady productivity.

During the strike-slip transformation stage, a number of uplifted structures were developed on the West slope and the Nankang platform in the Zengmu Basin, due to the effect of strike-slip tectonic deformation in the northern part. Car-bonate formations were formed onto these uplifted struc-tures, forming a section of carbonate reservoir. Carbonate reservoir bodies, which are steadily distributed, exhibit sig-nificant thickness and have good physical properties; they comprise a favorable assemblage in combination with the overlying bathyal mudstone, a continuously distributed, high-quality cap rock. This kind of oil/gas field, such as L (Fig.6), is commonly block-shaped and has a thick produc-ing layer, significant production scale and stable productivity.

Fig. 5 Hydrocarbon accumulation section of D18 oil and gas field

Fig. 6 Hydrocarbon accumulation section of L gas field

8 CHINA PETROLEUM EXPLORATION Vol. 21, No. 2, 2016

4.3. Control on distribution of cap rock

Since the regional subsidence stage of the Zengmu Basin, a set of thickly bedded and broadly distributed neritic- bathyal mudstone has been developed in the basin. This mudstone, with a thickness of 2–6 km and an excellent seal-ing capability, is considered a favorable regional cap rock for the basin, and has enabled a good preservation of oil & gas reservoirs in this area. The drilling results indicate that, all oil & gas reservoirs in this area were discovered at depths below the Upper Miocene. This clearly illustrates the control effect of this cap rock on vertical distribution of hydrocarbon.

4.4. Control on formation of trap

During the foreland fault depression in the Eocene-Early Miocene, the intense tectonic movement in the southern part of the Zengmu Basin allowed for formation of a number of fault blocks and faulted anticline traps. Since the effect of strike-slip transformation was strong in north and weak in south, a series of structural traps (e.g., rollover anticlines and draping anticlines) were formed in some low-lying ar-eas and a series of carbonate lithologic traps were formed in high-lying areas across the central and northern parts of the basin. Therefore, the structural traps have been shaped in the central and northern parts of the basin by the end of Middle Miocene and in the southern part by the end of Early Mio-cene.

4.5. Control on hydrocarbon migration.

Based on an analysis of major oil & gas fields discovered in the Zengmu Basin, it is acknowledged that faults formed during the foreland fault depression and strike-slip trans-formation stage are primary channels for hydrocarbon mi-gration. Since the tectonic movements in the strike-slip transformation stage was strong in north and weak in south, some deep faults developed in the central and northern parts of the basin during the Middle Miocene are deemed to be favorable for connecting the Oligocene-Lower Miocene source rocks in the lower part with the Middle to Upper Miocene carbonate reservoirs in the upper part, which to-gether comprise the lower-generation & upper-preservation reservoirs. In the southern part of the basin, faults were mostly terminated in the Lower Miocene, since the tectonic movement in the Middle Miocene was relatively weak, and the Oligocene-Lower Miocene self-generation & self-pre-servation reservoirs were therefore commonly formed.

5. Conclusions

(1) The Zengmu Basin, controlled by the tectonic cycle in

the South China Sea, has undergone three major evolution stages, i.e., foreland fault depression in the Eocene to the Early Miocene, strike-slip transformation stage in the Mid-dle Miocene, and regional subsidence stage in the Late Miocene to present.

(2) The Oligocene-Lower Miocene strata hold the most principal source rock of the Zengmu Basin. During the foreland fault depression and strike-slip transformation stages, a variety of depositional systems including the flu-vial-delta, coastal plain-neritic, and carbonate platform were developed and various reservoir-cap-rock assemblages were formed. During the regional subsidence stage, good cap-rocks were formed under the effect of a steady regional subsidence.

(3) The tectonic cycle in the South China Sea controlled the structural evolution of the Zengmu Basin as well as the hydrocarbon accumulation in the basin. This control is evident in the development features of source rocks, reser-voirs and cap-rocks, formation of traps, and geological con-ditions for hydrocarbon migration.

(4) Since the Zengmu Basin is a key basin in the South China Sea, a study on main factors that control the hydro-carbon accumulation of this basin can provide reference and guidance for the future exploration of this basin.

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