Tectonic movement refers to tectonic deformation and displacement caused by geodynamic geological process within the earth and shows different development characteristics in regions and localities. The evolution of a rift basin always leads to different levels of tectonic movements^[1], which usually results in transformation of rifting structures, variation and migration of faulting, uplift and subsidence, transition from deposition to denudation, and magmatism. The transformation of these tectonic processes is essentially due to the variation of tectonic dynamic environment of the basin.

In a rift basin, tectonic transition, which often occurs during multi-episode rifting process, is an important indicator for dividing various rifting episodes. For example, from rifting episode I to episode II in the East African Rift System, rifting migrated from the rift margin to the rift center^[2], and the local variation in the stress field and the transition of rifting structures occurred in different rifting episodes in the North Atlantic rifted margin ^[3,4]. Tectonic transition is an important attribute in the multi-episode rifting evolution of a rift basin.

The tectonic transition between different rifting episodes is characterized by extension direction, episodic faults and their distribution, tectonic subsidence, rifting structures, unconformity, volcanism, sedimentary cycle, depocenter migration, and sedimentary system (facies) change, which are factors considered in dividing rifting episodes. However, due to different perspectives and key factors, the rift episodes of the same rift basin are always divided in different schemes (e.g. in the Bohai Bay Basin^[5,6], and the North Sea Basin^[7,8]). These schemes have differences in time limit, span and dynamic properties, which are possibly caused by the differences in rift cycles of different depressions, and even understanding of the rifting episodes in the rift basin. If the rift basin is considered in terms of the time scale, the tectonism varies throughout evolution of the rift basin. According to the tectonic variation of the North Sea Basin, Ravnas et al.^[9] defined the first level as the large-scale (the rift period, spanning dozens to millions of years), the second level as the meso-scale (the rift episode, spanning 4-6 Ma), and the third level as the small scale (the stage, spanning 1-2 Ma), and the fourth level as the smallest scale (fault activity, spanning 1000 to tens of thousands of years). The time span, mode, and expression of tectonism at different scales are different. Henstra et al.^[4] considered that the reasons for the differences in understanding of the rift episode are due to the lack of understanding on the characteristics, evolution, and dynamics of the rift episode, as well as the differences in the division schemes, scopes, and scales.

The tectonic movements in the Pearl River Mouth Basin include the Shenhu Movement, the Zhuqiong Movement during the rifting period, the Nanhai Movement, the Baiyun Movement, and the Dongsha Movement during the depression period^{[10,11,12,13]}. In the east part of the basin, the Wenchang period is defined as episode I of the Zhuqiong Movement, and the Enping period as episode II of the Zhuqiong Movement. Between them, the rifting structure, fault evolution, sedimentation/subsidence, and regional stress field vary significantly^[14]. The Wenchang rifting episode experienced initial rifting, intense rifting and shrinking rifting, forming a complete rifting cycle. The area of the lake, accommodation space, faulting intensity, and subsidence rate are all characterized by cyclic variation. However, recent studies show that from the early to late Wenchang period, the rifting process experienced a certain SN transition and the migration along the fault strike, accompanied by basement uplift, magma diapirism, fault block rotation, and strata denudation, indicating a certain degree of tectonic variation during the early and late Wenchang periods. Further research shows that the tectonic transition during this period had significant effect on the development of hydrocarbon-rich sags, deposition of source rocks, sedimentary filling, development of high-quality reservoirs, hydrocarbon migration and accumulation in the deep Paleogene strata. This study focuses on the tectonic transition between the early and late Wenchang periods, corresponding to the key variation period of global plate tectonic movement at 43 Ma and the interaction between the northern continental margin of the South China Sea and the surrounding Indian-Australian and Pacific plates^[15,16]. It is of great significance to understand the rifting process of the northern continental margin of the South China Sea and surrounding plate movements, and the kinematics and dynamics of the lithosphere evolution from initial rifting to rapid thinning.

The northern continental margin of the South China Sea developed after an complex interaction between the Mesozoic active and passive continental margins, and it was affected by the joint extrusion of the Indian-Australian plate, the Eurasian plate, and the Pacific plate, leading to complex geodynamic background^[17,18]. The Pearl River Mouth Basin is located on the northern continental margin of the South China Sea and it is wholly characterized by zoning in SN direction and blocking in EW direction. From north to south, it is divided into 5 primary tectonic units, including the North Uplift Zone, the North Depression Zone, the Central Uplift Zone, the South Depression Zone, and the South Uplift Zone. The study area is in the Zhu-1 depression on the east side of the North Depression Zone. The rifted zone has an NE-striking axis, and is divided into several secondary negative tectonic units, including the Enping sag, the Xijiang sag, the Huizhou sag and the Lufeng sag, from the west to the east (Fig. 1).

The evolution of the Pearl River Mouth Basin is wholly divided into syn-rift and post-rift stages. According to the cyclic variation of the faulting intensity, the subsidence rate, the lake basin area, and the accommodation space, the syn-rift period is further divided into two rifting episodes, i.e., the Wenchang period (I) and the Enping period (II) (Fig. 2). The upper and lower Wenchang Formation contact with a certain range of unconformity, i.e., the seismic reflection horizon T83, corresponding to the sequence interface WC-SB4. Considering the tectonic characteristics of basement uplift, magmatic diapirism, and rifting migration during the early and late Wenchang periods, the episode I is subdivided into sub-episode I_a and sub-episode I_b. The sub-episode I_a experienced the initial rifting period, extending period and intense rifting period, and the rifting intensity was gradually enhanced, accompanied by extension of the lake basin and increase of the fault activity intensity. The sub-episode I_b experienced the rifting transition, contraction, and shrinkage periods, and the rifting intensity was gradually weakened, accompanied by decrease of the lake basin and reduction of fault activity intensity (Fig. 3). After the syn-rift period, the oceanic crust formed, and the South China Sea began to spread, and the northern continental margin of the South China Sea entered the post-rift period. The post-rift period is further divided into the seafloor-spreading period (late Oligocene-middle Miocene, South China Sea period) and subduction period (middle Miocene-present, Dongsha period) of the South China Sea.

The Wenchang Formation in the study area is divided into 6 tertiary sequences, and from bottom to top, WC-SQ1 to WC-SQ6 correspond to the sixth to first members of the Wenchang Formation (abbreviated as Wen 6 Member to Wen 1 Member, among which Wen 1, Wen 2, Wen 3 Members constitute the upper member of the Wenchang Formation, and Wen 4, Wen 5, Wen 6 Members constitute the lower member of the Wenchang Formation). A complete rifting cycle of initial, intense to shrinking periods occurred. The lower member (WC-SQ1 to WC-SQ3) and the upper member (WC-SQ4 to WC-SQ6) of the Wenchang Formation contact with a local unconformity, i.e. WC-SB4 interface between WC-SQ3 and WC-SQ4, and the lake basin space experienced the significant variation from the intense period to transition period of rift development (Fig. 3). During transition from the early to late Wenchang periods, the rift basin showed SN transition, migration of sag-controlling (sub-sag) faults, basement uplift, magmatic diapirism and denudation.

Through identifying and tracking seismic reflection characteristics, contact relationship, seismic facies differences, and seismic reflection structures, a quasi-secondary sequence interface WC-SB4 with the significance of correlation across the region was identified inside the secondary sequences of the Wenchang Formation. The interface is defined as a tectonic- sedimentation transition surface and divides the Wenchang Formation into the upper and lower members. Through tracking and analyzing the attributive characteristics (strong amplitude interface, onlap above the interface, local truncation below the interface, sedimentary slope break) of the typical characteristic interface, and the stratigraphic characteristics (stratum wedge symmetry, seismic facies difference, and larger inclination angle below the interface than that above the interface) below and above the interface, it was confirmed that the upper and lower members of the Wenchang Formation in the Zhu-1 depression contact with unconformity (Fig. 4). Well PY5-A in the Panyu-4N sub-sag encountered the WC-SB4 interface, and this indicates that the lower member of the Wenchang Formation has obvious truncation, and the upper member of the Wenchang Formation overlaps the lower member (Fig. 5).

According to the denudation range of the lower member of the Wenchang Formation tracked across the whole study area (Fig. 6), the unconformity contact between the upper and lower members of the Wenchang Formation are mainly distributed in the southeast part of the Lufeng sag, the most part of the southern Huizhou sag, the south and north parts of the Xijiang sag, and sparse areas of the Enping depression. The formation of the unconformity is related to the local basement uplifting, magmatic diapirism, fault block rotation and tilting at the end of the early Wenchang period.

From the early to late Wenchang periods, the sag-controlling fault activity and sag subsidence of the Zhu-1 depression varied from south to north. During the early Wenchang period, a series of subsidence centers occurred on north and south sides of the Huizhou sag (Fig. 3). Hereinto, the Huizhou-26 sub-sag in the southern part is the largest subsidence area, and the Xijiang-23, Huizhou-08, Huizhou-09, and Huizhou-10 sub-sags in the northern part are smaller. The sag showed duplex fault structure in the south and north, and the south one was dominant. During the late Wenchang period, some sub-sags at the southern margin of the sag (Huizhou-25, Huizhou-26, Huizhou-26E, Huizhou-22, Huizhou-24 sub-sags) stopped development, while the sub-sags in the northern margin continued to subside, and the Xijiang-23W sub-sag had the most significant subsidence. The Xijiang-24 sub-sag in the north- south transition zone was controlled by the north and south opposite faults. The subsidence centers during the early Wenchang period were close to the south faults, and that during the late Wenchang period migrated to the north faults. The profile across Huizhou-25 sub-sag - Xijiang-30 sub-sag - Xijiang-23 sub-sag shows that the lower member of the Wenchang Formation is the thickest in the Huizhou-25 sub-sag, followed by Xijiang-30 sub-sag, and Xijiang-23 sub-sag as the smallest. On the contrary, the upper member of the Wenchang Formation is the thickest in the Xijiang-23 sub-sag, followed by the Xijiang-30 sub-sag, and not developed in the Huizhou-25 sub-sag (Figs. 6 and 7a, 7b). The profile across Huizhou-22 sub-sag - Huizhou-10 sub-sag also shows the rifting transition in SN direction (Figs. 6 and 7c, 7d). During the early Wenchang period, the south-faulting and north-falling half graben on the south side of the sag was dominant, while during the late Wenchang period, the north-faulting and south- falling half graben on the north side of the sag was dominant.

The SN differences of fault activity intensity and transition characteristics in the Zhu-1 depression during the early and late Wenchang periods can be directly observed in the activity of major north and south sag (sub-sag)-controlling boundary faults (Fig. 8). The activity of the south sag (sub-sag)-controlling faults was stronger during the early Wenchang period, and that of the north sag (sub-sag)-controlling faults was stronger during the late Wenchang period. The variation of geodynamic environment is directly reflected in the transition of structural space and activity transformation during different rifting stages^[2,3,4].

From the early to late Wenchang periods, migration of the sag (sub-sag) controlling faults occurred in addition to SN transition of the rifting center. During the early Wenchang period, the development of the Xijiang-33E sub-sag of the Xijiang sag was strong, and during the late Wenchang period, that of the Xijiang-33W sub-sag was strong, and the sub-sag migrated westward along the sag-controlling faults (Fig. 9). In addition, some boundary faults with different strikes also show the sectional differences of activities. The segments with strong fault activity were in NE-striking during the early Wenchang period and were transformed to the NW-striking during the late Wenchang period. The controlled sub-sags show beaded or oblique distribution in different directions and migrated from the NE sub-sags to the NW sub-sags, e.g. Huibei region^[19], Huizhou-26 sub-sag, Xijiang-23 sub-sag and Lufeng-14 sub-sag.

According to development and thickness of single layers in the lower member of the Wenchang Formation in the Xijiang-23 sub-sag (Fig. 10), the thickness and development number of sub-layers in the lower member of the Wenchang Formation in the Xijiang-23E sub-sag in NE direction are more than that in the Xijiang-23W sub-sag in NW direction, indicating the NE rifting process started earlier and was dominant during the early Wenchang period. The thickness of the upper member of the Wenchang Formation varies along the strike, i.e. the Xijiang-23W sub-sag is thicker, while the Xijiang-23E sub-sag gradually becomes thinner. During the early and late Wenchang periods, the development intensity of the sub-sags varied along the fault strike from NE to NW, indicating variation in the activity intensity during these periods.

Basement uplift and magmatic diapir are caused by vertical tectonism, and basement uplift is often related to deep magmatism. Basement uplift and magmatic diapir widely occurred in the Zhu-1 depression at the end of the early Wenchang period. Comparison of the sedimentary thicknesses of the early and late Wenchang periods, especially that of the tertiary sequences WC-SQ3 and WC-SQ4 (Fig. 3), shows that during the early Wenchang period, strong faulting process occurred on both sides of the rifted zone in the Zhu-1 depression, forming a large-scale sedimentation and subsidence center. The Huizhou-Lufeng area has the most significant subsidence at the southern margin of the rift, and Huizhou-26 sub-sag with a maximum sedimentary thickness up to 2500 m is the largest subsidence within the area. Sedimentation and subsidence occurred in Huizhou-22 sub-sag, Huizhou-24 sub-sag, and the transition position of Huizhou-Lufeng area. At the end of the early Wenchang period, strong uplifting process occurred in southern Huizhou, and the transitional position of Huizhou-Lufeng (the dominant part is in the Huilu low uplift), which was accompanied by massive denudation in the lower member of the Wenchang Formation with the denudation thickness of 100-250 m (Fig. 6). During the late Wenchang period, the above regional sedimentation stopped, and development of Huizhou-25 sub-sag, Huizhou-26 sub-sag, Huizhou- 26E sub-sag, Huizhou-22 sub-sag, and Huizhou-24 sub-sag stopped, and only the northern region continued to rift and received sediments.

The magmatic diapir and the resulting strata uplifting and denudation were important events in tectonic transition during the early and late Wenchang periods. Spatially, diapiric magma has the scattered and dotted distribution and occurs in all sags in the Zhu-1 depression, with representatives in Lufeng 14-4 area, Huizhou-21 area, Xijiang-23 sub-sag, Panyu-4N sub-sag, and Enping-Yangjiang area, which often occur in the upthrown sides of the subsag-controlling boundary faults or the uplifted part of tilting basement. The magmatic diapir in the tilting basement and the sub-sag-controlling boundary faults together enhanced strong rotation of the lower member of the Wenchang Formation, and further caused exposure and denudation of the tilting formations. The strata denudation contours of Panyu-4N sub-sag and Huizhou-21 area are approximately concentric circles, and seismic section shows the magmatic diapir reflection characteristics. During the early Wenchang period, the strata were "domonically" upwarped (Figs. 5 and. 6). Well PY5-A penetrated the diabase in the interval of the 3526-3674 m (lower member of the Wenchang Formation) (Fig. 5). The diabase and the lower member of the Wenchang Formation are both upwarped, and obvious stratigraphic truncation occurs below the WC-SB4 interface, where the upper member of the Wenchang Formation is overlapped on the diabase and the lower member. The relatively concentrated large-scale basement uplift and disperse, and dotted magma diapir at the end of the early Wenchang period are important reflection of the tectonic transition during the early and late Wenchang periods, responding to the variation of the deep dynamic environment and regional structure in the upper basin.

A series of structural events, such as the SN transition of rifting process, migration of sag (sub-sag)-controlling faults along strike direction, basement uplifting, magmatic diapirism and denudation from early Wenchang period to late period, reflect tectonic transition during the early and late Wenchang periods.

The tectonic transition during the early and late Wenchang periods was firstly discovered in the Huizhou area of the Zhu-1 depression, thus it was named as the Huizhou Movement. Its movement level was defined as the sub-episode of the Zhuqiong Movement. The time limit of the movement is restricted in three ways.

(1) The chronological statistics of magmatism in tectonic transition during the early and late Wenchang periods shows that the entire basin was in a magmatism peak from 43.3 to 41.1 Ma. Guo Xiaowen et al.^[20] and Li Shubo et al.^[21] respectively confirmed that the temperature rise of fission track and the significant increase of heat flow value at about 44 Ma according to analysis of the thermal history evolution and fission track of source rocks. In this study, it is thought that regional tectonic-magmatism during the early and late Wenchang periods occurred at about 43 Ma.

(2) The estimation is based on the characteristics of being far away from the sub-sag boundary, stable mudstone sedimentation rate and weak affection by the provenance under the conditions of still water in the lake basin center. Based on the thickness statistics and compaction recovery of pure mudstone in sequences of 22 typical sub-sags in the whole region, especially in deep lakes with 5 completely developed tertiary sequence sub-sags, the thicknesses of the upper and lower members of the Wenchang Formation and their relative ratios in the original sedimentary state were calculated, based on which the proportions of the early and late Wenchang periods in the entire Wenchang period were calculated. It is known that the Wenchang Formation experienced 10 Ma from 49 Ma to 39 Ma, and the approximate time limit of the early and late Wenchang periods can be calculated. The average ratios of pure mudstone thickness of the upper and lower members of the Wenchang Formation in the total pure mudstone thickness of the Wenchang Formation are 40.4% and 59.6% respectively (Table 1). Assuming a positive correlation between the time proportion and the thickness ratio, the time limit of the early and late Wenchang periods were estimated to be about 43 Ma. This estimation result is comparable to the theoretical time limit of the tertiary sequence as 1-2 Ma proposed by Mitchum et al.^[22]

(3) The time of tectonic transition is compared with the tectonic activities of surrounding plates and regional tectonic events. The northern continental margin of the South China Sea is in the jointly squeezed area of the Indian-Australian plate, the Eurasian plate, and the Pacific plate. Its evolution is affected by east and west tectonic domains, i.e., the Tethys tectonic domain on the west side and the Pacific tectonic domain on the east side^{[16, 23-26]}. There is a natural response of regional tectonic effect between the Pearl River Mouth Basin and the two major tectonic domains (Fig. 11). The plate movement varied significantly since Miocene, and hard collision between the Indian plate and the Eurasian plate started around 43 Ma, and the convergence rate decreased significantly, and the Indochina and South China land masses were squeezed out. Meanwhile, the subduction direction of the Pacific plate changed from NNW to NWW. The Huizhou Movement, which occurred at the late stage of early Wenchang period, the early stage of late Wenchang period, about 43 Ma ago, is in good agreement with the this time point. The detrital zircon at the age peak of 43-48 Ma (early Wenchang period) found by Wang et al.^[27] in the Wenchang Formation in the Zhu-1 depression is possibly a record of magmatism during this period, and its age range is close to that based on the mudstone deposition rate and the regional tectonic events under the condition of still water in the lake basin center. Based on agreement of key time points between the Huizhou Movement and the major tectonic variation of two tectonic domains, this study concludes that there is close dynamic genesis relationship among the tectonic transition and the start of the hard collision between the Indian plate and the Eurasian plate, and the subduction direction change of the Pacific plate from NNW to NWW direction during this period (around 43 Ma).

Spatially, the tectonic transition of the Zhu-1 depression during the early and late Wenchang periods was mainly concentrated in the Huizhou sag, Lufeng sag, Xijiang sag, and the low uplift among them, and was most significant in the south part of the southwest Huizhou, the south part of the east Huizhou, and the Huizhou-Lufeng transition area (mainly Huilu low bulge) (Fig. 6). Generally, the tectonic transition in this period was centralized on basement uplift of the Huixi low uplift and the Huilu low uplift Meanwhile, rifting process migrated from south to north, and faulting process migrated along strike direction. The distribution of diapiric magma was scattered and dotted in the Zhu-1 depression. The SN migration of rifting process is reflected in the faulting intensity that was stronger in the south and weaker in the north during the early Wenchang period, but became stronger in the north and weaker in the south during the late Wenchang period. Accordingly, the rift structure changed from a half-graben that was faulted in the south and onlapped in the north to a half-graben that was faulted in the north and onlapped in the south. The distribution of the sub-sag subsidence centers and the variation in subsidence rate of the sub-sag centers are also characterized by SN transition. The migration of faulting process along the strike mainly shows that the section with strong fault activity changed from the NE-trending sub-sags to the NW-trending sub-sags. For instance, the Xijiang-23 sub-sag and Huizhou-08-09-10 sub-sag are closely related to the pre-existing basement structure and the clockwise rotation of the regional stretch direction during the Wenchang period (Fig. 6).

The basement of the Zhu-1 depression experienced multi- stage tectonic actions during Mesozoic period, forming the NE-NEE and the NWW-EW reverse fault systems. The development of the faults in the study area during the rifting period was significantly affected by the pre-existing basement faults, and the controlling-sag (sub-sag) faults on the north and south sides of the depression were almost inherited from the pre-existing basement faults^[28,29]. The transition of rifting process during the early and late Wenchang periods was closely related to the selective activation of pre-existing basement faults, which was also closely related to the scale and occurrence of faults, most importantly the angles between the directions of regional extension stress and the pre-existing faults^[30,31]. At about 43 Ma, the subduction direction of the Pacific plate changed from NNW to NWW, leading to the transition of the extension direction of the Zhu-1 depression from NWW-NW to NW-NNW^[23]. Thus, the development of the NE-NEE pre-existing faults was not dominant any longer, while the NWW-EW pre-existing faults were preferentially and selectively activated, leading to the NE to NW migration of faults along the strike during the early and late Wenchang periods.

The research suggests that the SN transition of basement uplift, magmatic diapirism, and rifting during the early and late Wenchang periods was also possibly related to the initially rapid thinning of the lithosphere in the Pearl River Mouth Basin. In the study on the genesis of the lower crust high-velocity body in the northeast part of the South China Sea, Liu An et al.^[32] suggested that in case of crustal extension and thinning, the decrease in upper mantle pressure promoted partial melting, causing underplating of the basic magma into the bottom of the lower crust to form lava pad, constituting the lower crust high-speed body, and this process lifted the crust^[33]. Based on the study on deepwater areas such as the Baiyun sag in the Pearl River Mouth Basin, this study suggests that the early and late Wenchang periods were the transition stage of the crust from initial rifting to rapid thinning, which provided conditions for partial melting of the lower mantle. Large-scale development of magmatic diapirs in the Pearl River Mouth Basin during the transition period in turn proves the active magmatism at the bottom of the crust. Therefore, it is predicted that the regional basement uplifting and magmatic diapirism during the early and late Wenchang periods was possibly the tectonic-magmatic response when the lithosphere at the northern margin of the South China Sea began rapid thinning.

Based on analysis of the time sequence of the faulting unconformity surface, the starting and ending ages of rifting, the position of the largest rift, the maximum thermal subsidence, and the final opening position of the ocean, it is thought that the continental marginal rifting process in the northern part of the South China Sea migrated from north to south as a whole^{[23, 34-35]}. Studies show that the transition of rifting process at the continental margin is clearly reflected in the transition of the faults from "trending toward the land" to "trending toward the ocean/sea"^[36]. This phenomenon in the Zhu-1 depression during the early and late Wenchang periods was possibly related to the transition of the lithosphere from initial rifting to ductile thinning. Ranero et al.^[36] proposed a “sequence fault model” based on the Anderson model to resolve the contradiction between the asymmetry of the Iberian-Newfoundland conjugate passive continental margin in the northern Atlantic Ocean, the crustal extending and thinning. This model relatively completely explains the evolution process of the lithosphere from extending, thinning to final rifting. During the early period of the rift, the crustal deformation showed a uniform pure shear model, and the faults were sparsely distributed, without dominant trend. As the crust was further extended, the thinning process was enhanced, and the strain concentration turned to the faults "trending toward the ocean". Whether the migration and transition of continental marginal rifting in the South China Sea conforms to this model needs further study.

3.2.1. Restriction of Huizhou Movement on development of hydrocarbon-rich sags and source rocks

Due to variations of tectonic attributes and dynamic mechanism, the tectonic movement during the rifting period had significant restriction on the formation and evolution of the hydrocarbon-rich sags, which in turn affected development and quality of source rocks^[37,38]. In the Pearl River Mouth Basin, the Huizhou Movement caused significant SN transition of the hydrocarbon generation centers of the key source rock (Wenchang Formation) and migration along fault strike directions, resulting in dislocated superimposition or lateral superimposition of continuous high-quality source rocks, vertical superimposition and extended horizontal distribution of multiple hydrocarbon generation centers.

Studies show that the rifting process during the Wenchang period in the Zhu-1 depression resulted in the deepwater lake basin and the under-compensation environment, with rich nutrient substance in the source water system. Volcanism provided rich nutrients for the lake basin, which was conducive to the growth of surface plankton and was the condition of developing high-quality source rocks^[13]. The Huizhou Movement caused migration and transition of sedimentary centers for the upper and lower members of the Wenchang Formation, and the high-quality source rocks mainly occurred above or below the Huizhou tectonic movement plane and in two sets of tertiary sequences with the strongest rifting process, i.e. WC-SQ4 and WC- SQ3 (Table 2).

Spatially, the migration of hydrocarbon-generation centers with high-quality source rocks in two sets of tertiary sequences was controlled by SN transition of rifting process and migration along fault strike/dip directions, developing the source rock superimposed models of transition, self-migration, and ex-migration. The transition model derived from SN transition of rifting process, with the representative of the Huizhou sag. The high-quality source rocks in the lower member of the Wenchang Formation occurred in the south of the rift, and that in the upper member migrated to the north of the rift. The SN transition of the sedimentation and subsidence center led to migration of the hydrocarbon generation centers from south to north, and dislocated superimposition of source rocks (Fig. 12a). The self-migration model derived from the inherited development of rifting process controlled by single boundary fault, with the representative of the Enping sag, where the sedimentation and subsidence center continuously migrated to one side of fault, and the source rocks were overlapped laterally (Fig. 12b). The ex-migration model derived from the fault migration along strike direction, with the representative of the Xijiang sag whose sag (sub-sag) controlling boundary faults were stronger in the east and weaker in the west during the early Wenchang period, and stronger in the west and weaker in the east during the late Wenchang period. The upper and lower members of the Wenchang Formation have “seesaw shaped” unequal-thickness sediments, and the hydrocarbon generation centers migrated along fault, and the source rocks were laterally superimposed (Fig. 12c).

3.2.2. Control of Huizhou Movement on sedimentary system and high-quality reservoirs in deep Paleogene

The Huizhou Movement caused the regional differential uplifting (including magmatic diapirism) / subsidence in different regions, which changed the source-canal-sink system and type in the deposition process, and accordingly the distribution of sedimentary facies. This is beneficial to interactive distribution of multiple-type sediments, forming multiple sets of reservoir-cap combinations and traps of various sedimentary origins. Differential uplifting and fault block rotation led to the early uplifting and shallow burial of Paleogene target reservoirs, and reservoir transformation promoted forming of deep high-quality reservoirs.

Taking Lufeng area as an example, differential uplifting of the Huizhou Movement caused variation in the provenance system to form the migrated source-sink system. During the sedimentary period of lower member of the Wenchang Formation, the Huilu low uplift was denuded in a large range, resulting in sufficient provenance, which provided a large amount of debris transport materials to Lufeng-14 area, etc. During the initial rifting stage, a large braided river delta sedimentary system formed. Affected by tectonic transition during the early and late Wenchang periods, the strong denudation provenance area transformed to Lufeng east low uplift during the late Wenchang period, which provided large-scale sediments for Lufeng-8 sub-sag etc. (Fig. 13). Multiple sets of favorable reservoirs were formed due to migration of main sedimentary area, resulting in a migration- type source-sink system, and development of multi-layer braided river delta sedimentary system. In addition, the Huizhou Movement caused relatively high fall or migration of uplift areas, and the thick massive medium coarse-grained sandstone migrated accordingly, forming multiple sets of dislocated, superimposed and continuous thick massive medium coarse-grained sandstone that was potential Paleogene sweet spot areas with high-quality reservoir (e.g. the high-quality reservoirs with thick massive medium coarse-grained sandstone in the lower member of the Wenchang Formation in structural belts of Lufeng-14, Lufeng-8, etc.).

The differential uplifting process led to early uplift and shallow burial of the Paleogene target reservoirs, and reservoir transformation formed deep high-quality reservoirs. Drilling in Well Lufeng 14-4 confirmed that the declines of porosity and permeability during re-burial of the reservoirs uplifted early was slower or delayed than that during normal burial of reservoirs, and the delay of reservoir diagenesis under the same conditions is conducive to development of deep high-quality reservoirs. Afifi et al.^[39] also provided the same case in the South Suez Rift.

3.2.3. Influences of Huizhou Movement on hydrocarbon accumulation

Many studies show that tectonic movement has important influence on hydrocarbon migration and accumulation^[40,41]. The tectonic transition, sedimentary migration, and hydrocarbon generation center and provenance migration caused by the Huizhou Movement promoted lateral superimposition and intercalation of source rocks and sand bodies, which is conducive to effective configuration of the source-reservoir-cap combinations; multi-stage superimposition and coupling in space and time formed Paleogene multiple layers and multiple types of hydrocarbon accumulation combinations, which is conducive to Paleogene near-source hydrocarbon enrichment and multi-zone composite hydrocarbon accumulation^[13]. The Huizhou Movement controlled migration and variation of source rocks and reservoirs, forming reservoir-controlling models of source rock migration and provenance migration in the Zhu-1 depression.

The reservoir-controlling model of source migration is guided by the hydrocarbon accumulation model in "source controlling theory", which focuses on the core role of source rocks in hydrocarbon accumulation^[42]. The migration and transition of the hydrocarbon generation centers and source rocks caused by the Huizhou Movement fundamentally controlled variation of the "source" and further affected hydrocarbon accumulation, forming the reservoir-controlling model of "source rock migration". Taking the superimposition model of self-migration source rocks in the Enping sag as an example, the source rocks in the lower member of the Wenchang Formation was lifted to the south, and the source rocks in the upper member lifted to the north. Hydrocarbons migrated both southward and northward, forming south and north large commercial oilfield groups in the Enping sag (Fig. 14).

The reservoir-controlling model of provenance migration focuses on migration of source-sink system caused by the Huizhou Movement and the control effect of development of high-quality deep reservoirs on hydrocarbon accumulation. Taking the migration source-sink system in the Lufeng-13E sub-sag as an example, the Huilu low uplift on the southwest side was the main provenance area during the early Wenchang period, and the delta sedimentary system occurred from southwest to northeast. During the late Wenchang period, the provenance was from the southern part of the Lufeng east low uplift, and a delta sedimentary system occurred from east to west (Fig. 14), forming superimposition of different strata of multi-stage delta. Under the background of stratigraphic uplifting and fault communication with hydrocarbon sources, Lufeng oilfield group with continuous superimposition and hydrocarbon accumulation was formed in the superimposed delta sand bodies filled with hydrocarbons (Fig. 15).

The Huizhou Movement refers to the tectonic movement in the Pearl River Mouth Basin during early and late Wenchang periods in middle Eocene of Paleogene, and its age was around 43 Ma. The Huizhou Movement is characterized by SN transition of rifting process, migration along fault strike, basement uplift, magmatic diapirism, and strata denudation, etc. Spatially, the tectonism of the Huizhou Movement was mainly concentrated in Huizhou sag, Lufeng sag, and Xijiang sag of the Zhu-1 depression and the low uplift among them, and was most significant in the south part of the southwest Huizhou, the south part of the east Huizhou, and the Huizhou- Lufeng transition area (mainly Huilu low uplift). The Huizhou Movement is universal in the Pearl River Mouth Basin, and its significance throughout the basin needs further study.

The Huizhou Movement during the early and late Wenchang periods was a comprehensive response of plate interaction and lithosphere thinning process. The transition of lithosphereinitial rifting to rapid thinning possibly caused lava pad at the bottom of the lower crust, thus resulting in basement uplift, magmatic diapirism, and accompanying layer denudation in some areas. Meanwhile, rapid thinning of the crust started, and strain began to migrate to the dominant faults trending toward the sea, which led to SN transition in rifting. The fault migration along strike was closely related to the pre-existing basement structures and clockwise rotation of the regional extension direction during the early and late Wenchang periods. The variation of regional extension direction was possibly related to dynamic genesis relationship between the start of the hard collision of the Indian plate and the Eurasian plate, and the change of subduction direction of the Pacific plate from NNW to NWW direction during this period.

The Huizhou Movement is of great significance to hydrocarbon accumulation in the Pearl River Mouth Basin, especially hydrocarbon accumulation in deep Paleogene. It has an important restriction on the development of Paleogene hydrocarbon-rich sags and source rocks. Spatially, several source rock superimposed models (transition, self-migration, and ex-migration) occurred. The Huizhou Movement has a significant control effect on the sedimentary system and the high-quality reservoirs in deep Paleogene. It changed the source-canal-sink system or its type during sedimentation. The early uplift, shallow burial and reservoir transformation has obviously positive effects on the high-quality reservoirs in deep Paleogene. It has important influence on hydrocarbon migration and accumulation, forming reservoir-controlling models of source migration and provenance migration.