Petroleum geology controlled by extensive detachment thinning of continental margin crust: A case study of Baiyun sag in the deep-water area of northern South China Sea

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 45, Issue 1, February 2018 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2018, 45(1): 29–42.

RESEARCH PAPER

Petroleum geology controlled by extensive detachment thinning of continental margin crust: A case study of Baiyun sag in the deep-water area of northern South China Sea PANG Xiong1, 2, *, REN Jianye3, ZHENG Jinyun1, 2, LIU Jun1, 2, YU Peng4, LIU Baojun1, 2 1. CNOOC Ltd.-Shenzhen, Shenzhen 518054, China; 2. CNOOC Ltd.-Deepwater, Shenzhen 518054, China; 3. Faculty of Marine Science and Technology, China University of Geosciences (Wuhan), Wuhan 430074, China; 4. Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, China

Abstract: The relationships between crustal stretching and thinning, basin structure and petroleum geology in Baiyun deep-water area were analyzed using large area 3D seismic, gravity, magnetic, ocean bottom seismic (OBS), deep-water exploration wells and integrated ocean drilling program (IODP). During the early syn-rifting period, deep-water area was a half-graben controlled by high angle faults influenced by the brittle extension of upper crust. In the mid syn-rifting period, this area was a broad-deep fault depression controlled by detachment faults undergone brittle-ductile deformation and differentiated extension in the crust. In the late syn-rifting period, this area experienced fault-sag transition due to saucer-shaped rheology change dominated by crustal ductile deformation. A broad-deep fault depression controlled by the large detachment faults penetrating through the crust is an important feature of deep-water basin. The study suggests that the broad-deep Baiyun sag provides great accommodation space for the development of massive deltaic-lacustrine deposition system and hydrocarbon source rocks. The differentiated lithospheric thinning also resulted in the different thermal subsidence during post-rifting period, and then controlled the development of continental shelf break and deep-water reservoir sedimentary environment. The high heat flow background caused by the strong thinning of lithosphere and the rise of mantle source resulted in particularities in the reservoir diagenesis, hydrocarbon generation process and accumulation of deep-water area in northern South China Sea. Key words: northern South China Sea; Zhujiangkou Basin; Baiyun sag; deep-water area; continental margin crust; detachment fault; broad-deep fault depression; continental shelf break; petroleum geology

Introduction Petroleum exploration has experienced the development process from continent to ocean and from shallow water to deep water. This is dictated by not only engineering techniques but also knowledge of general geology and petroleum geology. For some time, our cognitions on basin structures in deep-water regions of continental margin is based on shallow-water continental shelf basins where data can be easily obtained[1]. According to the classic extension model, basins in deep-water regions of continental margin have the similar structural pattern to intra-continents and shallow-water basins: featuring rift basin series controlled by high-angle normal faults[2]. In addition, petroleum geologic features of deepwater basins are similar to that of intra-continent and shallow-water basins, and exploration model in shallow-water

areas can be applied in deep-water areas. However, as a typical deep-water basin in the northern South China Sea, ten years of petroleum exploration and research of the Baiyun sag allow the discovery of the oil and gas reservoir; more important is that they revealed a series of geologic phenomena that are different from shallow-water basins: (1) Since no large high-angle boundary faults have been found in the Baiyun sag, how is the wide-deep rift style formed? And whether it is a depression or rift has long been controversial[36]. (2) According to the convention extension model, the formation and force source mechanism of rift basins should be multiphase and migratory[67]. Hence, it was thought that the deposits of the Cenozoic rift basins in the northern South China Sea had the trend of migrating and becoming newer from continent to ocean; moreover, it was de-

Received date: 25 Aug. 2017; Revised date: 15 Nov. 2017. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the Science and Technology Project of CNOOC Ltd. (YXKY-2012-SHENHAI-01); China National Science and Technology Major Project (2011ZX05025-003; 2016ZX05026-003); and the National Natural Science Foundation of China (91128207). Copyright © 2018, Research Institute of Petroleum Exploration & Development, PetroChina. Publishing Services provided by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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duced that the deposits of intra-continental basins began in early Paleogene, and gradually became newer toward the ocean direction[6,89]; and the deposition in the Baiyun sag started later at the late Eocene[7]. On the basis of the above stratigraphic cognition, though the deposits in the Baiyun sag are very thick, the Lower-Middle Eocene Wenchang Formation, a major source rock formation in most rift basins should be absent in the Baiyun sag. If this was true, the potential of the source rock in the sag should be much smaller. However, deep-water exploration in the past few years has revealed the Wenchang Formation[10], which indicates that the traditional model is controversial. (3) By seismic interpretation and combination inversion of gravity, magnetic and seismic data, it has been found that the depression structure is controlled by a large low-angle detachment fault system that almost cut through the earth’s crust[11]. The crystallized crust in the Baiyun sag is very thin (only several kilometers thick), the thinning crust has a mirror-image relationship with the Moho upwelling, and the extension degree of the lower crust is bigger than that of the upper crust[1214], which is very different from intra-continent and shallow-water basins. The thinning degree of the crust and the sag structure feature different from the typical shallow-water half-grabens mean that petroleum geology in the deep-water areas has its particularities. (4) The Baiyun sag had the most intense crust thinning during rift period, and stronger thermal subsidence than normal rift basins in shallow-water area of the continental shelf during post-rifting period[8], not only receiving up to 6 000 m thick sedimentary formations, but evolving into a continuous deep-water basin, which shows that it has been controlled by special subsidence mechanisms. (5) Wells drilled in deep water show that the Baiyun sag has very high geothermal gradient[12,1516], which is embodied by special sandstone reservoir diagenesis and hydrocarbon generation and accumulation under high geothermal gradient. During long-term petroleum exploration of the deep-water area of the Baiyun sag, we have tried to understand the petroleum geologic features of the deep-water basins[12], but our past understanding was apparently constrained by the conventional extension mode[2]. Based on the new model of detachment thinning of extending continental margin crust[17], we attempt to analyze the petroleum geology of the Baiyun deep-water area on the background of the extending detachment thinning of continental margin crust. We hope to promote the quickly advancing petroleum exploration in the deep-water area of the South China Sea.

1. Hyper-extended detachment thinning of continental margin crust and basin structure 1.1.

Detachment faults in extending background

The detachment faults in this study refer to normal faults with listric or flat-ramp pattern fault planes, dip angles between 1030, and horizontal displacements more than 2 times vertical displacements in extending background. Pierce

(1963)[18] firstly proposed the concept of detachment fault as the bottom fault of imbricate thrust faults in compressional stress background. Till the 1980s, the cognition on the development mechanism of detachment faults in extending background has been gradually established[19], and become the important basis of extending, thinning and break-up theory of lithosphere[20]. After studying the extending detachment faults in the Basin and Range Province in the Western North America, Wernicke (1981)[21] presented the simple shear deformation model of lithosphere extending deformation, constituting two end models of extending deformation theory of lithosphere together with the pure shear extending deformation model proposed by McKenzie (1978)[22]. Miller (1983) propounded that detachment fault is the boundary between brittle deformation at hanging walls and ductile deformation at the footwalls[23], and considered that extending detachment faults were initially formed and active with low angles (<30). In 1984, Spencer found that the isostatic rebounding caused by unloading of extending faults could lead to rotating of the footwalls[24], which suggests that detachment faults is the results of the tilting and domino rotating of the fault blocks at hanging walls during the extending process of high-angle normal faults. Since then, researchers further analyzed the variations of the brittle-ductile rheology of lithosphere such as depth, temperature, pressure and component etc[25], and proposed the rolling hinge model of low-angle normal fault development[26]. This theory suggests that once detachment faults are formed, the extending strain of the hanging walls will be concentrated on the detachment planes, and generate extensive long-distance displacement, while the strain below the fault planes conducts as ductile extending deformation[27]. 1.2. Crust detachment thinning caused by different rheological property of sphere According to classic plate tectonic model, the extending and breaking of passive continental margin is a type of uniform in depth and instantaneous process that can be divided into two stages: syn-rifting extension and post-rifting thermal subsidence[20,28]. It is believed that the rift basins in deep-water areas at continental margin have similar rifting configuration and structure to intra-continental rift or continental shelf rift in shallow-water areas, and the analogy between rift basins is an effective method to guide oil and gas exploration in deepwater areas. However, exploration practices in the Baiyun deep-water area of the passive continental margin of the northern South China Sea demonstrate that it has very different crust thickness, basin structure, deposit filling, heat flow evolution, hydrocarbon generation and accumulation from the rift basins in shallow-water continental shelf areas (Fig. 1)[10,12]. Fig. 2a is a regional large seismic section through the Zhu-I Depression, the Baiyun sag and the Liwan sag at the continental margin of the northern South China Sea. Based on the

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Fig. 1.

Location map and stratigraphic column of the Baiyun sag in the study area (After reference [12]).

structural-stratigraphic framework on the main section of this region in Fig. 2b, by analyzing velocity structure, inversion model of rock properties (velocity and density) is established and joint inversion profile of gravity-magnetic-seismic data is obtained (Fig. 2c). With the depths of crust bottom interface (Moho surface), the bottom interfaces of middle and upper crusts, and basin basal surface worked out from the inversion results, the crustal structure profile is established (Fig. 2c and 2d). Fig. 2 demonstrates that Moho depth is 29 km in northern continental shelf, and becomes 19 km to the continental slope where the Baiyun sag is situated, 15 km at Liwan sag to the south, and only 11.5 km at the abyssal plain with juvenile oceanic crust. The Moho surface uplifts in stepped style from the continental shelf, continental slope to the abyssal plain. If the Cenozoic sedimentary formations are deducted, the earth crust is apparently thinner at continental slope. The crystalline crust in the depocenter of the Baiyun sag is only about 57 km thick, and the Moho surface relief shows a mirror image relationship with the shape of the depositional base[12]. Strong crust thinning and apparent uplifting of the Moho surface in the deep-water area are characteristically different structural features from the continental shelf zone in the northern South China Sea (Fig. 2c). On the basis of the structural-stratigraphic framework and crust structural profile of the above regional main section, by using flexure backstripping, decompaction and post-rifting thermal subsidence inversion modeling[29], the variation profile of extensional ratios (β) of the upper crust, whole crust and lithosphere is obtained. The study shows that the extension and thinning in upper crust are very small, with extensional ratio (β) between 1.1-1.3; while from shallow-water

area at continental shelf to deep-water area at continental slope, the extensional ratios (β) of crust and lithosphere gradually become bigger, and reach the biggest in the Liwan sag (with β value of crust of about 3.5, and that of lithosphere of more than 4), which are much higher than that of upper crust. This indicates that in the direction from current continental shelf to the deep-water area at the continental slope, the extentsional and thinning scales of crust and lithosphere gradually become higher with various extensional ratios; depth-dependence extension or rheomorphism of differential sphere extension occurred in lithosphere. Researchers have made in-depth studies on the differential extension of various spheres in the Baiyun sag. Zhao Zhongxian et al. (2010) suggested that the extensional ratio of the crust at continental shelf in the northern South China Sea is around 1.2[30], while the numerical simulation by Zhang Yunfan et al. (2007) indicated that the maximum extensional ratio of the crust in the Baiyun sag is up to 4, and the stretching factor of lower crust (4 at maximum) is much larger than that of the crust (1.9 at maximum), making it the area with the maximum crustal thinning in the Pearl River Mouth Basin[12]. This means that the lower crust has stronger differential deformation or depth-dependence extension than the upper crust[12,28,31]. Actually, apparent thinning of lithosphere in many passive continental margins (including the Baiyun deep-water area) is hard to be balanced by the extensional amount of the brittle faults in the upper crust. Simulation and calculation results all indicate that the extensional degree of lower crust is much larger than that of upper crust[12,13,3233]. The basin subsidence curve from backstripping history calculation (Fig. 2f) shows that the extensional deformation

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Fig. 2. Composite interpretation profiles of the crust thinning and sedimentary basin structures of continental margin crust in the northern South China Sea (their locations are shown in Fig. 1). (a) Seismic section; (b) Sequence stratigraphic interpretation profile of the Pearl River Mouth Basin (modified from reference [12]); (c) Joint inversion section of gravity-magnetic-seismic data; (d) Composite interpretation section of crust and basin; (e) Stretching factor profiles of upper crust, crust and lithosphere; (f) Subsidence rate profiles during various periods.

and thinning degree of lithosphere have a significant controlling effect on subsidence of the basin above. In shallow-water area of continental shelf, the stretching factors of upper crust,

the whole crust and the whole lithosphere are similar. This means that there is no depth-dependence extension, the deformation mechanism of lithosphere is mainly pure shear de-

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formation; and the subsidence rate and quantity in this part during post-rifting period were relatively smaller. In the deep-water area of continental slope, depth-dependence extension occurred and the extensional deformation of lithosphere features simple shear or joint shear, and the basin in this section experienced faster and larger syn-rifting subsidence and post-rifting subsidence controlled by detachment faults. Intensive ductile extension of lithosphere in deep-water area led to significant subsidence of the surficial basin, forming huge accommodation space. Since the 1980s, IODP at the Iberia-Newfoundland margin in the ocean (Leg103/173/210)[3436] encountered serpentinized peridotite with velocity <8 km/s. By combining with extensive geophysical prospecting and comprehensive studies[37], it has been found that the extensional continental margin crust with poor magma gradually thins and pinches out from the continental shelf, continental slope to deep-water basins, appearing in an overall wedge shape. Ocean-continent transition zone composed of mantle rocks outcrops between the thinned and pinched out the continental crust and the normal oceanic crust. The study results indicate that the detachment faulting due to the differences in extension and rheology of the lithosphere is the main reason that the crust thins and pinches out[3839]; the detachment fault planes with huge displacements in crust mean quick thinning of crust lithosphere, and the final mantle exposure with crust pinching out. This is different from the conventional model in which the crust extension and instantaneous break-up give rise to ocean-con-

Fig. 3.

tinent transition interface (Fig. 3 and Fig. 4)[2,17]. The final breakup of the lithosphere is a tectonic process that is associated with extensional deformation migrating oceanwards, and the process is involved with subsidence of sedimentary basins and thinning of deep crust. Thinning of the lithosphere is accompanied by the development of the crust-scale detachment faults. These faults were initialed with high angles and rotated to the low angle normal faults that root in the detachment surface in the mid-lower crust (Fig. 5). According to our research result, the Baiyun sag is a rift controlled by a group of detachment fault systems cutting into deep crust in the south. These faults dip northwards, with dip angle steeper in its shallower segments and gentler in deeper segments, and extend near Moho surface (Figs. 2a, 2d, and 6). This supra-detachment faults gradually converge near the Moho surface at depth of 8-9 s TWT. The major detachment plane in deep section is generally dipping between 10°-20°, appearing as complex fault geometry with alternating ramp-flat style on some typical profiles. Multiple secondary synthetic listric faults usually develop on the major fault plane, extend downwards in a fan-shape or a “Y” shape, and finally converge on the main detachment plane. The detachment fault belt constituted the southern boundary of the Baiyun sag during its rifting period. This fault belt strikes near EW on the plane, and extends traceably more than 200 km on the seismic sections. Associated with the detachment fault belt, the flexural deformation at its hanging wall produced large roll-over anticlines that involve early faults. The maximum horizontal

Extensional continental margin model based on pure shear extension (after reference [2]).

Fig. 4. Tectonic unit division, basin structure styles and depositional filling of the continental margin lithosphere in middle Southern Atlantic (after reference [17]).

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component of detachment fault displacements is more than 40 km, and its ratio to fault throw can be up to 5:1. In the ultra deep-water areas (such as the Heshan sag, the Liwan sag, and the Xingning-Jinghai sag) in the northern South China Sea (Fig. 1), as well as the deep-water area in the Qiongdongnan Basin, such detachment faults are common. In conclusion, the huge horizontal displacement component of the Baiyun sag is the manifestation of the intensive ductile extension of middle-lower crust in sag structure, while the detachment faults act as differential deformation interface between ductile rheology, intense extension and thinning of lower crust and brittle upper crust. 1.3. Extensional detachment thinning of continental margin crust and wide-deep rift structure Passive continental margin underwent a complete extensional evolution process. Generally, with the lithosphere thinning, deformation was gradually migrated from the continent shelf to the outer continental slope, and concentrated in the area where the lithosphere finally was broken-up and formed oceanic crust. At this time, tectonic stress was completely concentrated in the middle oceanic ridge and finally release the extensional stress (Fig. 4). The progressive deformation process associated with continental lithosphere stretching, thinning, mantle exhumed and breakup shows episodic extension, concentrated strain, and deformation migration and conversion[11,4041]. The continental margin extension firstly began from brittle upper crust breakup caused by pure shear extension, forming the evenly distributed rift group in narrow strip shape controlled by high-angle faults. After that, the highangle faults in brittle layers incised continuously and entered the brittle-ductile belts between upper and lower crusts. The strain difference of extension in upper and lower crusts led to detachment plane. The subsequent extension strain concentrated in lower crust, which caused apparent ductile rheology in ductile layers of lower crust. Because of ductile extension of the lower crust, the hanging walls in upper crust were pull apart along the detachment plane, resulting in huge displacement of slipping and the early rifts in narrow shape evolved into wide-deep rifts (Figs. 5 and 6). The sustained deep extension could lead to further thinning and even absence of the lower crust. Finally, the crust lithosphere was completely broken, and the mantle lithosphere was exhumed[42]. The above evolution model indicates that most rifts controlled by steep normal faults in deep-water area of continental margin were chiefly formed during the early extensional period, with even the extension of the upper crust, small horizontal fault displacement, smaller throws and high angle boundary faults with dips of 60°-68°. These basins are usually smaller, and shorter in the development periods of deep rifts (Fig. 5). In contrast, the rifts controlled by large detachment faults generally have larger fault displacements and throws than the rifts controlled by steep normal faults, and higher

ratio between horizontal fault displacements and throws, so they are wide and deep (Fig. 5). With larger the stretching factor of the ductile lower crust, and stronger thinning degree, the detached horizontal displacement of the deep rifts formed in upper crust will be wider. A wide-deep rift is the most important geomorphic feature of intense thinning belt of continental margin crust, which is apparently different from the narrow rift controlled by high-angle faults with limited extension in intra-continental crust. The Baiyun sag is a massive wide-deep rift controlled by intensive detachment thinning of the crust, about 200 km in length (95 km width), 1.26×104 km2 in area, and with 13 km thick Cenozoic strata and current water depths of 5002 000 m (Fig. 2). According to time-depth conversion of seismic section, the maximum strata thickness was more than 7 km in the Baiyun sag during rifting period; The total vertical fault throw of the sag-controlling fault system is 4.4 km, and the maximum total horizontal displacement is up to 40 km. In Fig. 6, the total horizontal fault displacement on Tg horizon is 21 km, the dip angles of branch fault planes near Tg are between 11°-46°, and the dip angles of fault planes in deep lower crust are sun-horizontal. In response to this, the crust lithosphere stretches intensively and becomes apparently thin (Fig. 2d); Gravity-magnetic-seismic inversion (Fig. 2c) indicates that

Fig. 5. Relationship between the process of the extensional crust thinning and the basin structure.

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Fig. 6.

Structure interpretation of 3D seismic section across the Baiyun sag (Its location is shown in Fig. 1).

the upper crust is broken, and lower crust thins to only several kilometers; the lithosphere stretching factor can be up to 4, and the stretching factor of lower crust is apparently bigger than that of upper crust (Fig. 2e). These features fully demonstrate that it is intensive crust stretching and differential sphere detachment thinning that lead to the massive wide-deep rift in the Baiyun sag.

2. Wide-deep rift and massive lacustrine source rock Because of rheology differences of sphere layers, the extension, detachment thinning of continental margin crust is an orderly evolving process. The corresponding development and evolution of surface rifts experience the following three stages (Fig. 5): (1) Early extension stage (the depositional period of lower Wenchang Formation during early Eocene): the crust extension was limited, the sag witnessed the formation of steep normal faults caused by uniform extension of upper crust which had not broken the upper crust, and weak extension and indistinct thinning of lower crust; Half-graben rifts in narrow strip shape controlled by evenly distributed steep high-angle normal faults developed (Fig. 5). During this stage, the surface of the sag had big topographic relief, alternating uplifts and depressions, and the small basins are isolated. Compared with peripheral provenances, the depositional accommodation space provided by these rifts were limited, thus compensation-over compensation deposits were likely to occur. In the half-grabens, the sediment supply was mainly from the long axis direction of the rifts extending along sag controlling faults and the apparent provenance systems were around the sags. Therefore, the periphery zones of the sag were mainly filled with coarse clastic deposits in fluvial, delta, fan delta and alluvial fan facies; The middle of sags mainly developed under-compensation deposits with smaller scope in lacustrine to semi-deep lacustrine facies, and the area and volume of lacustrine facies account for smaller percentages of the whole sags. Because of limited crust extension, the Zhu-I Depression of the Pearl River Mouth Basin in the continental shelf of the northern South China Sea was characterized by narrow rifts controlled by high-angle faults (Figs. 2 and 7).

Fig. 7. Isopach map of eastern Pearl River Mouth Basin during rifting period, South China Sea Basin.

(2) Middle extension stage (approximately the depositional period of the upper Wenchang Formation during middle Eocene): with increasing extension, strain shifted and concentrated to fault belts, making these faults incise downwards into the ductile rheological layers inside crust, and evolve into large extending detachment planes (Fig. 5), Controlled by these detachment faults, early narrow half-grabens developed into wide-deep rifts. Deep faults, large slippage of hanging walls, and rotating and titling basin structure of the wide-deep rifts, led to a special depositional system. Continuous incision of faults resulted in increasing of fault throws and basin depth. The size of detachment faults reflects the rheological thinning intensity of the ductile layers in lower crust, and the growth of detachment faults strengthened subsidence of the rift. Large horizontal displacements of detachment faults means significant widening of the basin, which resulted in deeper, wider, and larger accommodation space than that in the rifted basin stage formed by early extension. The development of detachment faults led to rotating and

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titling of the hanging walls. The tilted ends of the fault block uplifted strongly and was erode, becoming the most important provenance of the rifts. The pitching end of the fault block turned into deep subsiding areas, where extensive undercompensation deposits of large deep lacustrine or semi-deep lacustrine facies developed. The gentle slope zone of the middle sections of hanging block was the pathway for the deposition from the provenance in the tilted end, massive fluvial and delta deposits developed (Figs. 5c, 8a, 8b). The listric shape of the detachment faults usually led to the rolling of hanging walls into flexural zones, controlling the basin geomorphic form. The flexural anticline axes became the slope break zones separating the delta deposition environment on gentle slope area and the lacustrine – semi-deep lacustrine deposition environment in quick subsidence areas at the pitching end[43] (Figs. 5c and 8b). Because of the rotating and titling process of the detachment faults, the dip direction of the basement rock at the footwall of the steep side is usually opposite to the rifted basins, and there were little eroded deposits entering the rifts. Hence, there were fewer fan deltas at the steep slopes. The wide-deep rifts had a larger proportion of undercompensation deposition; therefore, there was depositional accommodation space for forming large-scale lacustrine source rock. (3) Late extension stage (the depositional period of Enping Formation during late Eocene): The brittle upper crust had been completely broken, fault activity was not apparent, while the ductile extension and rheomorphic deformation in the lower crust still kept on, leading to continuous subsidence and deep depression of crust surface; Consequently, the wide-deep rifts controlled by detachment faults changed into large faulted and depressed basins in dish shape (Fig. 5). The sedimentary formations suffered no or less fault control, featuring onlap filling structure of formations at this time. It should be emphasized that during late extension stage, if the ductile lower crust experienced continuous intense stretching and thinning, there would be apparent surface subsidence, giving rise to the semi-lacustrine depositional environment. During the full extension stage, the deep-water area was in the rifting process with intensive extension all the time. Compared with the shallow-water area (the area with weak crustal extension), the deep-water area showed higher subsidence rate, bigger depositional accommodation space, and the longest rifting period. For instance, during middle extension stage, shifted and concentrated strain led to intense detachment thinning of lower crust in deep-water area, and deep subsidence of the surface, thus wide-deep rifts were developed. During this stage, the shallow-water area had smaller extension degree, weak faulting activity, and thus a limited increase of depositional accommodation space in rifts. It should be addressed that the width of wide-deep rifts was mainly related to extending and thinning degree of the ductile layers of lower crust, while the subsidence amount of rifts was related to the thickness of the brittle layers in the upper crust and isostatic subsidence caused by lower crust thinning.

Moreover, with the thinning of crust and even the lithosphere, gravitational equilibrium might make the surface at continental margin drop below sea level at a certain time of the middle-later extension stage. If there was sea water entering at this time, the marine depositional environment might develop. In summary, the most important features of the deep-water area at the continental margin are wide-deep rifts, and large deltas and lacustrine source rock development during detachment period. The wide-deep rifts of the Baiyun sag have huge accommodation space. The total deposit volume can be calculated by subtracting Enping Formation top (T70) from the base (Tg). The total volume of Eocene Wenchang Formation and the Enping Formation during rifting period amounts is up to 28 828 km3, more than two times the total volume (12 662 km3) of Wenchang Formation and Enping Formation in the 22 half-grabens controlled by steep normal faults in the Zhu-I Depression (Fig. 7). According to comprehensive analysis of the sag structure, stratigraphic sequences and deposition systems, affected by intense extension of the detachment fault system in the south, the Panyu low-uplift at the north rose intensively due to the strong rotating and titling process at hanging walls, turning into the primary provenance. The northern gentle slope zone of this sag developed multi-cyclic superimposed foreset seismic reflections in “S” shape (Fig. 8c, 8d). The progradational seismic reflection facies spanning 40 km long from north to south, which reflects the large deltaic deposits with multiple sequences from the northern provenance (Fig. 8c, 8d), and the deposition from the northern provenance covers the majority of this sag. A flexural slope break zone striking near EW can be identified on the seismic sections in the middle sag (Fig. 5c). The region near the slope break zone has typical “S”-shaped foreset seismic facies (Fig. 8d). It can be calculated after reconstruction by decompaction that the relief of the “S”-shaped reflection between the topset layer to the bottomset layer is more than 150 m, which basically reflects the water depth of the lake basin then[12]. The maximum area of the lacustrine – semi-deep lacustrine facies of the Baiyun sag is up to 3 300 km2. Some exploration wells in the Baiyun deep-water area have penetrated Eocene Wenchang Formation and Enping Formation. Drilling data show that large delta-lacustrine depositional systems with sediments mainly from the northern provenance were developed during rifting. P33 exploration well in northwestern Baiyun sag (Fig. 1) penetrated more than 1 000 meters of Enping Formation of fluvial-delta facies, in which the source rock has mainly type II2 kerogen and an average TOC of 1.76%[12]. W4 exploration well in southeastern Baiyun sag (Fig. 1) proves the existence of the semi-deep lacustrine facies of Wenchang Formation[10]. This well encountered nearly 50 meters of mudstone with rich freshwater planktonic algae and sporopollen fossils in the Wenchang Formation. This mudstone layer has mainly type II 1 kerogen, TOC between 1.36%1.72%, hydrogen indexes of 408565 (mg/g, TOC),

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Fig. 8. Sedimentary facies map and sections of Baiyun sag during rifting period. (a) A 3rd-order sequence depositional system map of the detachment rift episode; (b) Deposition interpretation section of Baiyun sag during rifting period (its location is in Fig. 8a); (c) North-south striking seismic section through the center of Baiyun sag (flatten along 5 s horizon. Its location is in Fig. 8a). This section shows the seismic reflection feature of deposition during detachment rift episode. Flatten horizon restored the original strata during the deposition period; (d) The seismic foreset reflection phase in Fig. 8c shows the delta facies dominated by northern gentle slope provenance, the semi-deep lacustrine facies is represented by continuous and parallel reflection phase, and interpretation of their mutual relationship.

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freshwater planktonic algae contents between 60%90%. The primary algae are pediastrum, and the secondary ones are granodiscus granulatus and leiosphaeridia, with sporadic botryococcus and muricate granulatus. The enrichment of some types of planktonic algae (botryococcus and pediastrum) means that the lacustrine environment was productive with eutrophic water body. Apatococcus lobatus flourish in the lake with higher salinity and deeper water. Therefore, the Wenchang Formation in this well was deposited in freshwater lacustrine environment with certain salinity, and the lake water had high productivity and flouring planktonic algae. Massive coarse-grained terrestrial detrital deposits in northern Baiyun sag indicate that the sediment source was mainly from the north; the controlling detachment faults in this sag led to rotating and titling of the hanging wall, and the uplifting of the Panyu low-uplift, which became the major provenance under erosion. The southern part of the Baiyun sag was the pitching end and quick subsidence led to continuous development of lacustrine basin with large deep water body, which was favorable for developing thick and extensive lacustrine—semi-deep lacustrine source rock (Fig. 8b).

3. Post-rift differential subsidence controlling deep-water reservoirs There have been many studies on the sequence stratigraphic framework, shelf break zone, sedimentary environment, sedimentary system, deep-water sandstone reservoir distribution etc. in the deep-water area of the northern South China Sea[12, 4446]. These study results concluded that the sandstones in deep-water gravity flow and continental shelf margin delta facies are the most important reservoir in the Baiyun deepwater area. The continental shelf break zone and the 3rd-order lowstand systems tract controlled the distribution of these reservoirs. Previous studies also showed that since early Miocene, the Baiyun sag and its south area have been in the deep-water environment of continental slope same as today. The continental shelf break zone has been situated at the north of the Baiyun sag stably for a long period, controlling the depositional environment since Miocene (Fig. 2f). The north area of the continental shelf break zone was in shallow-water continental shelf environment, and affected by eustatic sea level changes, the frequent migrated deltaic sediments from paleo Pearl River deposited on the wide shelf region. In the south area of the shelf break zone, rapid subsidence has maintained a deep-water environment in slope region, receiving massive deep-water gravity flow deposits. Hereinto, the deep-water gravity flow sandstone is an important reservoir in this region[4446]. The post-rifting deposits at the continental slope zone where the Baiyun sag is located are up to 6 000 m thick, more than two times the synchronous stratigraphic thickness in the northern shelf region. The current water depth is between 5002 000 m. For a long time, the special phenomenon of high sedimentation rate in deep-water environment and continental shelf break zone remained generally

stable, has been considered as results of abnormal tectonic subsidence, but the causes are still controversial[4648]. Recent studies suggested that the intensive detachment thinning of the lithosphere at continental margin directly impacted the thermal subsidence and depositional environment during post rifting period. The decompaction calculation of formations has proved that the subsidence rates of the Baiyun sag and Liwan sag were much higher than those in northern depression zone and the Panyu low-uplift (Fig. 2f), which means that there was a positive relationship between thermal subsidence degree during post-rifting period of this basin and the lithosphere stretching factor. With the current continental shelf break zone as the boundary (Fig. 2b, 2f), the south and north parts have apparently differential subsidence during the post-rifting period. During rifting period, the regions with smaller crust stretching and thinning degree also had relatively smaller thermal subsidence degree during post rifting period, and chiefly formed shallow-water continental shelf depositional environment. The regions with intensive crust stretching and detachment thinning (such as the Baiyun sag – Liwan sag), had larger thermal subsidence degree during post rifting period, and evolved into continental slope deep-water to ultra deep-water depositional environment. The differential subsidence resulted in the continental shelf break zone. Even with massive deposit supply, the continental shelf break zone still maintained in the northern slope of the Baiyun sag for quite a long period[46]. The stable location of continental shelf break zone on the thin crust of the continental margin of the northern South China Sea during post rifting period suggests that, the upwelling degree of the mantle asthenosphere caused by lithosphere thinning during rifting period directly affected the thermal subsidence degree during post rifting period, then controlled the depositional environment of shelf and slope during post rifting period, and the distribution of deposition systems and deep-water reservoirs.

4. Origin of high heat flow and its influence on hydrocarbon accumulation The deep-water area of the northern South China Sea shows high surface heat flow feature consistent with the detachment thinning trend of crust lithosphere. The intensive detachment faulting led to drastic crust thinning in the deep-water area and quick uplifting of Moho surface (Fig. 2d, 2e), correspondingly, the mantle lithosphere thinned, and mantle asthenosphere uplifted apparently, leading to ascending of surface heat flow. The Xijiang sag, located in northern continental shelf, has weak extension degree, with about 20 km of crystalline crustal thickness, where the geothermal gradient revealed by drilled wells is 2.6 C/100 m. Geothermal gradient gradually increases to 3.5 C/100 m toward the south at continental shelf margin. The Baiyun sag located in northern continental slope, has only about 7 km of crust due to intensive detachment thinning, where the geothermal gradient in Well W3-1 is 5.2 C/100 m. In the Liwan sag, the geothermal

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gradient increases to 6.5 C/100 m (Well W21-1). For the continental crust margin at the bottom of the continental slope, the geothermal gradient reaches 8.4 C/100 m (Well ODP1148). From continental shelf to slope, heat flow and current geothermal gradient generally show a gradual increasing trend, which is consistent with the overall trend of thinning degree of crust lithosphere, and of the lithosphere (1 300 C adiabatic interface) to gradually rises toward the ocean. Tang Xiaoyin et al. (2014) concluded a good exponential correlation between geothermal heat flow and lithosphere thickness at continental margin of the northern South China Sea[16]. Hence, the detachment thinning degree of lithosphere controlled the total variation trend of geothermal heat flow in the deep-water area; Detachment thinning led by lithosphere extension is an important reason behind the typical “hot basin” feature in deepwater area of the northern South China Sea[15] (Fig. 2). The high heat flow background caused by crust thinning provided stronger diagenesis of sedimentary rock and thermal evolution for hydrocarbon generation in the deep-water area, and led to particularities of hydrocarbon migration and accumulation. The Cenozoic strata in the Baiyun sag are up to 13 km thick. During rifting period, the Wenchang Formation and Enping Formation developed large-scale delta and lacustrine deposits (currently at a burial depth of 313 km), which are potential source rock of the Baiyun sag. Because of long-term deep subsidence, multiple source rock series and thick over-

lain formations, this sag had long consecutive hydrocarbon generation duration. According to theoretical modeling result of hydrocarbon generation history (Fig. 9), when geothermal gradient increases from 3 C/100 m to 5 C/100 m, the burial depth of the source rock entering major oil generation window can be reduced from 4 2005 700 m to 2 6003 600 m, and the burial depth for major gas generation window can decrease from 5 7006 800 m to 3 6004 200 m. Moreover, liquid hydrocarbons are more likely to be generated in high geothermal setting[49]. In the high geothermal gradient (> 5 C/100 m) setting, the source rock shows some special hydrocarbon generation features and evolution process, such as earlier generation, quick maturity, shallower and narrower generation window, more generation of liquid hydrocarbons, and more likely producing hydrocarbon overpressure. The high geothermal gradient can also significantly affect diagenesis of clastic deposits. The abnormal high geothermal gradient can accelerate sandstone compaction rate, advance water-rock reaction, and strengthen various cementation and micro pore generation of clay minerals. Drilled wells in the Baiyun sag show that in high geothermal gradient areas, sandstone diagenetic sequence can occur earlier and the poor reservoir is much shallower. When the geothermal gradient is >5 C/100 m and 4.5 C/100 m, and <4 C /100 m, the corresponding buried depths at which sandstone reservoir porosity drops to 12% and permeability to 1×103 μm2 are 2 300 m,

Fig. 9. Relationship between top of hydrocarbon expulsion and lower boundary of sandstone permeability. The relationship between the source rock top of massive hydrocarbon expulsion and sand reservoir permeability of more than 1×103 μm2 reflects whether the major hydrocarbon expulsion has condition of transporting hydrocarbons by buoyancy.

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Fig. 10. Conceptual patterns of hydrocarbon migration and accumulation in the Pearl River Mouth Basin. With the variation of geothermal gradient from north to south in this basin, there may be two kinds of hydrocarbon migration and accumulation patterns, one by buoyancy transportation in shallow-water area and the other by non-buoyancy transportation in deep-water area. The profile location is in Fig. 1.

2 700 m and 3 500 m, respectively[50]. The increase of geothermal gradient can lead to shallower low limit of favorable reservoirs and narrower hydrocarbon generation window (Fig. 10). When the geothermal gradient is 3.04.0 C/100 m, during the early stage of massive hydrocarbon generation and expulsion, the permeability of peripheral sandstones was generally still good, then hydrocarbons could be expelled and transported by buoyancy, and conventional oil and gas pools were more likely to form. When geothermal gradient was more than 5.0 C/100 m, the peripheral sandstone has been tight when source rocks began to expel considerable hydrocarbons, with large capillary resistance, then it is hard for hydrocarbons to be transported by buoyancy. Hence, overpressure by hydrocarbon generation, phase-potential driving and episodic hydrocarbon expulsion[51] may be the major hydrocarbon accumulation pattern.

5.

Conclusions

Controlled by sequential crustal thinning, the structural evolution of the basins at continental margin shows regular changing. In the regions where crust stretched relatively weaker, narrow rifts controlled by steep normal faults developed. They are featured with weak thermal subsidence during post-rift period, and finally evolved into continental shelf zone. In the hyper-extended regions, wide-deep rifts controlled by detachment faults of different sphere extension developed, recording a more complete extension and rifting process, and more intense thermal subsidence during post rifting period resulted into deep-water continental slope depositional environment. The shallow-water shelf zone and the deep-water slope zone have apparent different structural evolution processes and stratigraphic frameworks. During rifting period, the deep-water area underwent the following evolution process: a narrow rifting stage controlled by high-angle faults due to brittle extension of upper crust during early period, a wide-deep rift stage dominated by lowangle detachment faults due to crustal brittle-ductile differential extension during middle period, and the faulted depression of dish-shaped caused by ductile crust strain during late pe-

riod. During middle-late rifting period, the lithosphere extension shifted and concentrated to the deep-water area at continental margin, the lower crust had apparent ductile thinning. The wide-deep rift controlled by large detachment faults cross-cutting the crust is an important structure feature of deep-water basins. The wide-deep rift of the Baiyun sag formed by intensive detachment thinning of the crust at continental margin provided accommodation space for large-scale delta-lacustrine depositional systems and source rocks. The differential thinning of the lithosphere at continental margin also led to different post-rifting thermal subsidence, and controlled the development of shelf break zone and the depositional setting of the deep-water reservoirs during post rifting period. The high thermal flow background caused by the intense lithospheric thinning and mantle uplifting resulted in the narrower and shallower hydrocarbon generation window, stronger hydrocarbon generation intensity and thermal diagenesis of sandstone reservoirs, thus unique in hydrocarbon accumulation conditions. The extensional detachment controlled by differential sphere rheology is the major mechanism of the intense crust thinning in the deep-water area of the continental margin of the northern South China Sea. The basal lithosphere of the Baiyun sag in the necked zone experienced intense thinning. Its special petroleum geology features need to be studied further and should be paid more attention to in exploring and developing deep-water oil and gas.

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