**1. Introduction**

The continental rift basin is one of the most significant types of petroliferous basins. Approximately 31% of oil and gas fields in the world are distributed in continental rift basins with proven reserves of over 0.68 × <sup>10</sup><sup>8</sup> of oil equivalents [1]. Major petroleum systems sourced by Cenozoic sediments occur in a large number of sedimentary basins such as the Kachchh Basin (India), Congo Basin (Africa), Madrid Basin (Spain), Songliao Basin (China), and the basins in the South China Sea [1,2]. Investigation of the deposition evolution process in continental rift basins is critical to locating hydrocarbon reservoirs and predicting potential source rocks [2–6].

In sedimentary basins, stratigraphic and depositional architectures are fundamentally controlled by basin and tectonic evolution [7,8]. The dynamic process of basin filling is

**Citation:** Liu, E.; Deng, Y.; Lin, X.; Yan, D.; Chen, S.; Shi, X. Cenozoic Depositional Evolution and Stratal Patterns in the Western Pearl River Mouth Basin, South China Sea: Implications for Hydrocarbon Exploration. *Energies* **2022**, *15*, 8050. https://doi.org/10.3390/en15218050

Academic Editors: Mohammad Sarmadivaleh and Shu Tao

Received: 18 September 2022 Accepted: 27 October 2022 Published: 29 October 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

mainly controlled by the interaction between sea/lake-level changes, tectonic subsidence, sediment supply, and geomorphic conditions, but the main controlling factor varies in different types of sedimentary basins [9–13]. In continental rift basins, tectonics is the most significant factor controlling depositional processes, including episodic rifting, differential subsidence, and the evolution and linkage of normal faults [4,14–19]. It exerts significant control on sedimentary fills, sequence architecture, and sand dispersal patterns [8,20–24]. By contrast, in marine basins, sequence stratal patterns and depositional architecture are mainly controlled by eustatic sea-level changes [9,10,25–27]. Even in a single basin, different evolutionary stages are characterized by different stratigraphic and depositional architectures [28,29]. For example, in continental rift basins, episodic tectonic subsidence with short stages of accommodation creation in response to pulses of fault reactivation usually occurs in the early stage, while little accommodation is generated due to tectonic quiescence, and sediment supply consumes and fills the available space in the later stage [2,5,8,30,31].

The tectono-stratigraphic archive provides unique insights into the evolution of stratigraphic and depositional architectures in continental rift basins [5,31–35]. Various palaeogeographic features formed by syn-depositional faults are getting more concern since they play a significant effect in the transportation and deposition of sediments [36–38]. Structural slope breaks are important types of palaeogeographic features, controlling the distribution of deposition systems through changes in the accommodation [33,36,39,40]. Overall, the previous studies related to stratigraphic and depositional architecture mainly focused on marine basins. For the continental rift basins, attention has been mainly paid to the rifting stage of continental rift basins [32,33,41,42]. However, studies on the evolutionary features during the long-time process are rare, especially for the evolutionary process from lacustrine to marine environments [22].

The Pearl River Mouth Basin (PRMB) is a significant hydrocarbon-bearing basin located in the central region of the northern margin of the South China Sea continental shelf [43–45]. It is a focus point for intensive hydrocarbon resources exploration because of the high petroleum potential in the Eocene–Miocene interval revealed by recent drilling findings [46,47]. The previous studies show that PRMB has undergone complex structural evolution from the lacustrine environment in the Eocene to the marine environment in the Miocene [48,49]. During the evolution process, multiple types of depositional systems (e.g., lacustrine system, neritic system, fan delta) were developed with great variability in time and space [49,50]. Although previous studies [46,47,51] have investigated the tectonic evolution, depositional systems, and source rock potential of the PRMB, rare study has focused on the depositional evolution and sequence filling processes during the basin evolution from lacustrine to marine environments, and the controlling factors for the sequence architecture and depositional system distribution remain debatable [42]. Moreover, the characteristics, spatial and temporal distribution, and evolutionary process of stratal stacking patterns in different stages of basin evolution are not clear. Given the above disputes over the depositional evolution of the PRMB, this study was conducted with the following aims: (1) to describe the evolution of depositional systems from the Eocene to the Miocene within a sequence stratigraphic framework; (2) to identify and characterize stratal stacking patterns in different basin evolutionary stages; (3) to investigate the characteristics of stratigraphic and depositional architecture throughout the entire history of basin filling; (4) to discuss the controlling factors for basin evolution and sediment filling process. Our study can not only enhance the understanding of sequence filling and depositional evolution processes in the continental rift basins from lacustrine to marine environments, but also serve as a new example for further exploration in the PRMB and the other basins in the South China Sea.

### **2. Geological Setting**

The Pearl River Mouth Basin (PRMB) is a passive margin Cenozoic basin located in the center area of the northern continental margin of the South China Sea [45,52,53]. It covers an area of approximately 26.7 × 104 km2 and consists of four depressions from west to east, including the Zhu I Depression, Zhu II Depression, Zhu III Depression, and Chaoshan Depression (Figure 1A). The target study area, the Zhu III Depression, is located in the western region of the PRMB, covering a region of 12,180 km2 with a water depth of 0–400 m. The Zhu III Depression is bounded by the South China Block to the north, Xisha Island to the south, Hainan Island to the west, and Taiwan Island to the east (Figure 1A). It consists of several subsidence centers (i.e., Wenchang A, B, C, D, and E Sags, Qionghai Sag, Yangjiang Sag) and structural highs (i.e., Qionghai Uplift, Shenhu Uplift, Yangjiang Uplift) (Figure 1B).

**Figure 1.** (**A**) Geological map of South China showing the regional setting and tectonic elements of the PRMB with major modern river systems that discharge into the northern South China Sea (modified after Liu et al. [50]). (**B**) Map showing tectonic elements of the Zhu III Depression and geographic locations of borehole samples. (**C**) Interpreted NE-trending profile in the Zhu III Depression and the location of the profile is shown in (**B**).

The Zhu III Depression has undergone a complex tectonic evolution associated with the continental rifting and evolution process of the South China Sea [52,54]. Its evolution history can be divided into four stages, including a pre-rifting stage (Shenhu Formation, 65–49 Ma), a rifting stage (Wenchang and Enping Formations, 39–30 Ma), a transition stage (Zhuhai Formation, 30–23.3 Ma), and a post-rifting stage (Zhujiang Formation, 23.3 Ma to present) (Figure 2). The Shenhu Formation (65–49 Ma) deposited during the pre-rifting stage and consisted of coarse-grained alluvial and volcanic deposits [49,52].

During the rifting stage (39–30 Ma), Wenchang (49–39 Ma) and Enping (39–30 Ma) Formations formed, and the depression is characterized by intense faulting and the development of several half-grabens [49,52,55]. The Wenchang and Enping Formations are represented by widespread terrestrial facies, including fan deltas and braided river deltas [55]. The boundary between the rifting stage and the transition stage was defined by the significant unconformity T70, which is related to the regional Nanhai Event. The Oligocene Zhuhai Formation (30–23.3 Ma) that deposited during the transition stage with half-grabens

coalesced is mainly composed of tidal flats, semi-enclosed bays, and deltas [49,50,52]. The Miocene Zhujiang Formation (23.3–16.3 Ma) formed during the post-rifting stage when the PRMB began to receive marine sediments from the Miocene [50]. The Zhujiang Formation consists of fan delta and neritic deposits [49,52,56]. The source rocks are deposited in the Wenchang and Enping Formations, and the main reservoirs are located in the stratum of the Zhuhai and Zhujiang Formations. Several commercial oil pools were discovered in the central and marginal areas of the Wenchang A Sag [57].


**Figure 2.** Generalized stratigraphic framework, sedimentary facies, tectonic evolution, and sourcereservoir-cap assemblages of the PRMB. The sediment rate curve is from Xie et al. [49].

### **3. Material and Methods**

*3.1. Sequence Stratigraphic Framework*

The combination of seismic data, drilling cores, logging data, and lithology data provided by CNOOC was used to conduct sedimentological and sequence stratigraphical studies. The seismic club comprises a region of approximately 2000 km2, and its peak frequency is ~40 Hz. Seismic data were calibrated with well data using synthetic seismogram calibration. The sequence stratigraphic technique [9,58–60] was applied to establish a

sequence stratigraphic framework across the depression and to identify different stratal stacking patterns in different structural units.

### *3.2. Analysis of Sedimentary System*

The types and ranges of depositional systems were determined using geological and geophysical methods [61,62]. Over 600 m of drilling cores from 20 wells were described to determine sedimentary facies and depositional characteristics. As drilling cores in the Zhu III Depression are limited, the logging and lithological association data from more than 50 wells, including natural gamma, true resistivity, and acoustic logging data, were also employed to identify depositional facies and to constrain their distribution ranges [40,63]. In no-well regions, the ranges of depositional systems were determined by 3D seismic data using seismic reflection, horizontal slice, and attributive analysis. The analysis of structural slope breaks was conducted based on seismic interpretation of 3D seismic data and analysis of sedimentary systems.

### **4. Results**

### *4.1. Characteristics of Depositional Systems*

Based on the comprehensive sedimentological analysis, five types of depositional systems were identified in the Zhu III Depression, western PRMB, including the fan delta, meandering river delta, tidal flat, neritic shelf system, and lacustrine system (Figures 3–5).

**Figure 3.** Drilling cores, logging, lithologic data, and interpreted depositional facies in Well A3 showing the depositional evolution from the Eocene to the Miocene. The location of Well A3 is shown in Figure 1.

**Figure 4.** Typical sedimentary characteristics of the deposits in the Enping (**a**–**f**) and Wenchang (**g**–**l**) Formations in the Zhu III Depression: (**a**) 4694.4 m in Well W1, gravelly sandstone with normally graded bedding; (**b**) 4698.1 m in Well W1, gravelly sandstone; (**c**) 4695.4 m in Well W1, gravelly sandstone with normally graded bedding; (**d**) 4693.2m in Well W1, gravelly sandstone with normally graded bedding; (**e**) 4694.6 m in Well W1, black mudstone; (**f**) 2.9 m (4696.8–4699.7 m) long core of the underwater distributary channels of the fan deltas; (**g**) 4042.9 m in Well W13, oil-bearing gravelly sandstone; (**h**) 3830.0 m in Well B3, gravelly sandstone with normally graded bedding; (**i**) 3041 m in Well W12, oil-bearing gravelly sandstone with normally graded bedding; (**j**) 3043 m in Well 13, oil-bearing gravelly sandstone; (**k**) 3581 m in Well W9, gravelly sandstone; (**l**) 2.4 m (3833.5–3835.9 m) long core of the underwater distributary channels of the fan deltas.

**Figure 5.** Typical sedimentary characteristics of the deposits in the Zhujiang (**a**–**f**) and Zhuhai (**g**–**l**) Formations in the Zhu III Depression: (**a**) 2092.5 m in Well W4, siltstone; (**b**) 2092.8 m in Well W4, siltstone; (**c**) 2090.2 m in Well W4, fine-grained sandstone; (**d**) 2091.3 m in Well W4, fine-grained sandstone and siltstone with parallel bedding; (**e**) 1220.3 m in Well W32, fine-grained sandstone with normally graded bedding; (**f**) 2.0 m (2091.9–2092.9 m and 2093.9–2094.9 m) long core of the shallowmarine detrital deposits from the Zhujiang Formation; (**g**) 3210.7 m in Well W3, siltstone interbedded with mudstone; (**h**) 3210.9 m in Well W3, siltstone interbedded with mudstone; (**i**) 1861.2 m in Well A3, tidal rhythmites; (**j**) 1816.4 m in Well A3, tidal rhythmites; (**k**) 3211.1 m in Well W3, fine-grained sandstone interbedded with mudstone; (**l**) 3.0 m (3207.3–3210.3 m) long core of the tidal flat deposits in the Zhuhai Formation.

### 4.1.1. Fan Delta

The fan delta system consists of three sub-facies, including the fan delta plain, fan delta front, and pro-fan delta with the fan delta front being the most extensively distributed (Figure 6). Three lithofacies are recognized in the fan delta deposits (Figures 3 and 4): (a) gravelly sandstones with erosional bases, normally graded bedding, and widespread sandstone gravels (Figure 4a–d,g,j,k); (b) normally graded bedded sandstones with mud laminae at the top (Figure 4i,f,l); (c) thin fine-grained sandstones, siltstones with coarsening upward successions (Figure 4h,f,l).

Lithofacies (a) and (b) are interpreted as submerged distributary channels, which are funnel-shaped on the GR log, while lithofacies (c) are interpreted as distal bar deposits, which have a funnel-shaped geometry on the GR log (Figure 3). In the seismic profiles,

they are characterized by oblique progradational reflection configurations, low to medium continuous reflections, and medium to high amplitudes (Figures 7–9).

**Figure 6.** Deposition facies diagrams of the Zhujiang Formation (**a**), Zhuhai Formation (**b**), and Wenchang Formation (**c**) of the Zhu III Depression, PRMB, showing the depositional evolution from the Eocene Wenchang Formation to the Miocene Zhujiang Formation.

**Figure 7.** (**A**) interpreted seismic section bb' showing seismic reflection characteristics and the distribution regions of different stratal stacking patterns; (**B**) the types of depositional facies and their distribution ranges in the section bb' showing the depositional evolution from the Eocene to the Miocene. The location of section bb' is shown in Figure 1.

**Figure 8.** (**A**) interpreted seismic section cc' showing seismic reflection characteristics and the distribution regions of different stratal stacking patterns; (**B**) the types of depositional facies and their distribution ranges in the section cc' showing the depositional evolution from the Eocene to the Miocene. The location of section bb' is shown in Figure 1.

**Figure 9.** (**A**) interpreted seismic section dd' showing seismic reflection characteristics and the distribution regions of different stratal stacking patterns; (**B**) the types of depositional facies and their distribution ranges in the section dd' showing the depositional evolution from the Eocene to the Miocene. The location of section bb' is shown in Figure 1.

Fan deltas are widely distributed in the Wenchang and Enping Formations, and are mainly located in the footwall blocks of the Zhusannan Fault as well as the adjacent region of the Qionghai Uplift in the Wenchang–Enping Formations (Figures 6 and 7). They exhibit fan-shaped plan view geometries and are distributed along the boundary faults.

### 4.1.2. Meandering River Delta

In contrast to fan delta deposits, meandering river delta deposits are significantly finergrained with better sorting and roundness in detrital sediments (Figures 3 and 5a,b). The deposits are mainly composed of two lithofacies, including (a) normally graded bedded sandstones with mud laminae or mudstones at the top and (b) thin fine-grained sandstones, siltstones with coarsening upward successions (Figure 5b,c). Lithofacies (a) are interpreted as submerged distributary channels, whereas lithofacies (b) are interpreted as distal bars deposits.

Meandering river delta deposits are developed in the Zhu III Depression with their distribution ranges increasing from the Eocene to the Miocene (Figure 6). In the Wenchang– Enping Formations, meandering river delta deposits were confined to the northeast of the Zhu III Depression with their sediments from the Yangjiang Uplift (Figures 6, 8 and 9). In the later Oligocene Zhuhai Formation, meandering river delta deposits could be divided into two parts, including the northwestern delta system and the northeastern delta system (Figure 6). In the Miocene Zhujiang Formation, the distribution range of meandering river delta deposits further expanded (Figure 6).

### 4.1.3. Tidal Flat

The lithologies of tidal flat deposits are composed of siltstone, pelitic siltstone, finegrained sandstone, and mudstone (Figures 3 and 5). Two types of lithofacies are developed including (a) siltstones interbedded with thin layers of mudstones (Figure 5g,h,k,l) and (b) tidal rhythmites that are characterized by packages of vertically accreted laminated to thinly bedded fine-grained sandstone and mudstone (Figure 5i,j,l).

Tidal flat deposits are widely distributed in the Oligocene Zhuhai Formation (Figures 6–9). Two types of sub-facies are identified in the Zhu III Depression, including the intertidal zone and the subtidal zone. The intertidal zone was developed in the northern depression ranging from Yangjiang Sag to Wenchang B Sag, while the subtidal zone was distributed in the southern depression (i.e., Wenchang C, D, and E Sags) (Figure 6).

### 4.1.4. Lacustrine System

Lacustrine deposits are widely distributed in the Wenchang and Enping Formations and are composed of two sub-facies (i.e., the shallow-lacustrine and the deep-lacustrine deposits) (Figure 6). The deep-lacustrine deposits developed primarily in the centers of halfgrabens, and the lithofacies consists of thick layers of gray-black mudstones. On the seismic profiles, the deep-lacustrine deposits display continuous, high-amplitude, high-frequency, and unparallel reflections (Figures 7–9). By contrast, the shallow-lacustrine deposits consist of thick gray mudstones interbedded with thin layers of fine-grained sandstones with small-scale cross-bedding and waving bedding (Figure 4e). The shallow-lacustrine deposits are characterized by divergent reflector configurations on seismic sections (Figures 7–9).

### 4.1.5. Neritic Shelf System

Neritic shelf deposits are developed only in the Miocene Zhujiang Formation (Figure 6). They are composed of two types of lithofacies: (a) thick black marine mudstones that contain bivalve, brachiopod shells; (b) massive fine-grained sandstones that are well-sorted and well-rounded (Figures 3 and 4f). Lithofacies (a) is interpreted as offshore black marine mudstone, and Lithofacies (b) is interpreted as the deposits of neritic shelf sand bodies. On the seismic profiles, the neritic shelf deposits display continuous, high-amplitude, high-frequency, and parallel reflections (Figures 7–9).

### *4.2. Sequence Architecture and Stacking Patterns*

### 4.2.1. Sequence Stratigraphic Framework

Sedimentological and sequence stratigraphy analysis show that the sequence architecture in the Zhu III Depression varies in time and space (Figures 7–9). The Eocene Wenchang Formation is composed of three sequences, and the Oligocene Enping Formation is composed of two sequences [42]. These sequences consist of lowstand, transgressive, and highstand systems tracts (Figures 7–9). The lowstand systems tract is located in the centers of the deep lake, which is characterized by thick wedge-shaped progradation seismic reflection. The transgressive systems tract is characterized by medium, continuous parallel reflections, whereas the highstand systems tract features low-angle oblique progradation reflections (Figures 7–9). By contrast, the Oligocene Zhuhai Formation and the Miocene Zhujiang Formation consist of two and three sequences, respectively, composed of only transgressive and highstand systems tracts [42]. In the Oligocene Zhuhai Formation, the highstand systems tract displays strong amplitude and continuous reflection characteristics, while the transgressive systems tract shows moderate–weak amplitude on the seismic reflection (Figures 7–9). Different from the overlying strata, the third sequence (SQZJ3) in the Miocene Zhujiang Formation is composed of shallow-marine detrital deposits, and the continental shelf sand bodies are developed only within the highstand systems tract (Figure 3).

### 4.2.2. Depositional Stacking Patterns

Six types of slope break belts are identified in the Zhu III Depression (Figure 10), and their features are described as follows.

(1) Steep slope fault belt

The steep slope fault belt is defined as a steep slope that is controlled by a synsedimentary boundary fault (Figure 10). Such a boundary fault usually has a large fault displacement and a straight plan pattern (Figure 10). A deep fault terrace is developed at the footwall of the boundary fault where clastic sediments are accumulated (Figure 10). In the Zhu III Depression, the steep slope fault belt is mainly located at the footwalls of the Zhusannan Fault, which constitute the southwest margins of the Wenchang A, B, and C Sags (Figures 7–9 and 11).

(2) Steep-fault bending belt

The steep-fault bending belt features a steep slope that is controlled by both a synsedimentary boundary fault and a pre-existing uplift (Figure 10). Compared with the steep slope fault belt, the slope of the steep-fault bending belt is much gentler, and fault displacement is smaller, which is influenced by the pre-existing uplift (Figure 10). The steep-fault bending belt in the Zhu III Depression developed mainly along the footwall of the No.2 Fault, the north margin of the Qionghai Uplift (Figures 7–9 and 11).

(3) Multi-level step-fault belt

The multi-level step-fault belt is characterized by multiple fault terraces jointly controlled by several syn-sedimentary faults (Figure 10). These faults usually have a laddershaped profile and are parallel to each other (Figure 10). Each fault terrace is controlled by a single syn-sedimentary fault, creating an accommodation zone for sediment deposition (Figure 10). The multi-level step-fault belts are mainly located at the footwalls of the Zhusannan Fault and No.5 Fault during the sedimentary period of the Zhuhai Formation (Figures 7–9 and 11).

### (4) Antithetic-fault slope belt

The antithetic-fault slope belt is featured by multiple fault terraces formed by the development of antithetic faults (Figure 10). The dip direction of antithetic faults is opposite to the direction of sediment transport, which is significantly different from the multi-level step-fault belt (Figure 10). In the Zhu III Depression, the antithetic-fault slope belt is only located on the east margin of the Qionghai Uplift in the stratum of the Wenchang and Enping Formations (Figures 9 and 11).

**Figure 10.** The types of slope break belts and their stratal stacking patterns in the Zhu III Depression, PRMB.

### (5) Gentle slope belt

The gentle slope belt is characterized by limited change of slope gradient with a lack of intensive syn-sedimentary normal faults (Figure 10). The sediments are distributed and generally disappear along the slope (Figure 10). In the Zhu III Depression, the gentle slope belt is mainly located on the margins of the Qionghai and Yangjiang Uplifts during the depositional period from the Wenchang Formation to the Zhuhai Formation (Figures 7–9 and 11).

### (6) Flat continental shelf Belt

The flat continental shelf belt is featured by a nearly horizontal stratum with a low angle of <2◦ (Figure 10). There is neither an intensive normal fault nor gentle slope (Figure 10). Long-distance transported sediments are deposited along the continental shelf as large-scale meandering river deltas (Figure 10). Neritic shelf sandstones are formed due to the modification of storms and tides (Figure 10). In the Zhu III Depression, the flat continental shelf belt is located almost in the whole depression during the depositional period of the Zhujiang Formation (Figures 7–9 and 11).

**Figure 11.** The spatial distributions of slope break belts of the Zhujiang Formation (**a**), Zhuhai Formation (**b**), and Wenchang-Enping Formations (**c**) in the Zhu III Depression, PRMB.

### **5. Discussion**

### *5.1. Tectonic Controls on the Evolution of Depositional Systems*

There are four stages in the tectonic evolution history of the Zhu III Depression, including the pre-rifting stage, rifting stage, transition stage, and post-rifting stage (Figure 2). Each evolutionary stage shows distinctive characteristics in basin configuration and stratigraphic architecture (Figures 6–9). During this evolution process, the representative depositional systems changed from proximal fan delta and lacustrine deposits in the Eocene Wenchang and Oligocene Enping Formations, to tidal flat and fan delta deposits in the Oligocene Zhuhai Formation, and then distal meandering river delta and neritic deposits in the Miocene Zhujiang Formation (Figure 6).

During the rifting stage, the basin is characterized by wedge-shaped geometries with the development of several NE trending half-grabens because of intense rifting (Figures 6–9). The structural highs developed along the basin margin and, among the half-grabens, acted as significant source areas for proximal fan delta deposits, which were accumulated mainly in the footwall scarps of boundary faults (Figures 7–9). During the transition stage, basin rifting was weakened, and the increase in subsidence of the PRMB led to large-scale transgression [56]. Tidal flats were widely developed during this stage, and the fan deltas were deposited at the footwalls of boundary faults. During the postrifting stage, rifting was almost negligible, and the subsidence rate significantly weakened and gradually stabilized (Figures 6–9). As a result, the isolated sub-sages were gradually interconnected to form a unified depression, and the whole depression was covered by the shallow sea shelf (Figure 6). This consequently resulted in the formation of a structural foundation for the development of neritic shelf sandstones and long-distance transported deltas in the continental shelf (Figure 6). Therefore, it can be concluded that tectonic evolution controlled the basin configuration and evolution process of depositional systems.

Normal faults significantly control sequence architecture and syn-rift successions in extensional systems through the generation of accommodation space [33,41,64,65]. In the Zhu III Depression, the Zhusannan Fault was critical in controlling accommodation and the distribution of deposition systems (Figures 7–9). In different basin evolutionary stages, the Zhusannan Fault had different characteristics of fault activity and fault geometry, affecting the development position of sand bodies. During the rifting stage (Wenchang–Enping Formations), the Zhusannan Fault was marked by large displacement and a high activity rate (as high as 250 m/Ma) [42]. Because of the intensive faulting, the high steep slope topography was formed with the fault scarp directly adjoined to the deep lake (Figure 10). The root of the Zhusannan Fault was the largest accommodation space in the depression, providing a depositional site for the nearshore fan delta (Figures 7–9). In this stage, fan delta systems of different sizes were mainly distributed along the Zhusannan Fault with prograding stacking patterns (Figures 7–9).

By contrast, during the transition stage (Zhuhai Formation), the fault activity of the Zhusannan Fault significantly decreased to <100 m/Ma [42], and the subsidence centers gradually moved away from the footwalls of the Zhusannan Fault (Figures 7–9). In this stage, secondary syn-sedimentary faults that were parallel to the Zhusannan Fault were formed and maintained a tectonic low landform (Figures 7 and 8). These faults, together with the Zhusannan Fault, controlled the distribution ranges of fan delta systems (Figures 7 and 8). Compared with the fan deltas in the rifting stage, the distribution ranges of fan deltas are much smaller (Figures 6–9). During the post-rifting stage, the Zhusannan Fault exerted little effect on the distribution of deposition systems. The Shenhu Uplift was gradually submerged and unable to provide sediments to form fan deltas, while the large-scale meandering river delta sourced from the Pearl River in the north became the dominant deposition system [50]. In this stage, the distribution pattern of depositional systems was mainly controlled by sea-level change and sediment supply, which have been documented in previous studies [66].

In summary, the filling patterns and distribution of depositional systems were controlled by several factors (i.e., tectonic activities, sea-level change, and sediment supply), but the main controlling factor is not uniform during the basin evolution process from rifting stage to the post-rifting stage. In the rifting stage, the intense rifting of boundary faults significantly controlled the distribution of depositional systems, whereas sea-level change and sediment supply became significant controlling factors for stratigraphic architecture.

### *5.2. Sequence Architecture Evolution*

Structural and sedimentary slope breaks have significant control over the distribution pattern of depositional systems [12,21,33,67]. Six types of slope break belts have been identified in the Zhu III Depression, exerting significant functions on stratal stacking patterns (Figure 10). The sequence architecture differs greatly in space and time (Figures 10 and 11). For example, in the northern depression, the sequence architecture was mainly controlled by faulting, and two different faulting patterns (i.e., steep slope fault belt, multi-level stepfault belt) were developed during the evolving growth of the Zhusannan Fault (Figure 11). By contrast, the sequence architecture in the regions adjacent to the Qionghai Uplift in the western depression was mainly controlled by the gentle slope belt (Figure 11). Even in the regions along the footwall of the Zhusannan Fault, two different patterns, including the steep slope fault belt in the west and the multi-level step-fault belt in the east, were revealed in the depositional period of the Zhuhai Formation, indicating that the fault geometry (low-angle or high-angle) is also an important factor controlling the depositional patterns (Figure 11). Therefore, our results highlight the diversity and complexity of sequence architecture in the continental rift basin, which resulted from the comprehensive effect of sea/lake-level changes, faulting, palaeogeomorphology, and sediment supply.

The diversity and complexity of sequence architecture are closely associated with the tectonic evolution process [7,21,66]. The dynamic evolution process of sequence architecture can be divided into three stages: (1) During the rifting stage, the steep slope fault belt and steep-fault bending belt were developed in the footwalls of the Zhusannan and No.2 Fault, respectively (Figure 11). The antithetic-fault slope belt and gentle slope belt were developed around the uplifts (Figure 11). The depression was filled with widespread fan delta deposits on the gentle slopes and steep slopes (Figure 6). The distribution of fan deltas in the fault zones was mainly controlled by faulting, while that in the gentle slope was dominantly controlled by palaeogeomorphology (Figure 10). (2) During the transition stage, the steep slope fault belt developed in the footwall of the Zhusannan Fault converted into the multi-level step-fault belt, which controlled the distribution of sedimentary facies from the shallow to the deep lacustrine environment (Figures 7 and 8). By contrast, in the regions of the intrabasin uplifts (e.g., Qionghai Uplift), the previously developed filling patterns have changed to the gentle slope belt (Figure 11). (3) During the post-rifting stage, the end of tectonism caused the cessation of differential subsidence, and the previously developed sequence patterns were replaced by the flat continental shelf belt (Figures 6–9). The depositional systems and their stratal stacking patterns were dominantly controlled by sea-level change and sediment supply [66], and tectonic activities had little effect on the sequence architecture.

In summary, the spatial and temporal features of stratal stacking patterns varied with a single structural unit and between different structural units (Figure 11). From the rifting stage to the post-rifting stage, the dominant stacking patterns have changed from tectonic slope breaks (e.g., steep slope fault belt, steep-fault bending belt) to sedimentary slope breaks (e.g., flat continental shelf belt) (Figure 11). The diversity of sequence architecture resulted from the comprehensive effect of tectonic activities, sediment supply, sea/lakelevel changes, and geomorphic conditions. The sequence architecture is mainly controlled by the tectonic activities in the rifting stages, whereas it is dominantly controlled by sediment supply and sea-level changes in the post-rifting stage.

### *5.3. Hydrocarbon Accumulation Rules and Exploration Direction*

The basin evolution and sequence architecture can control not only the depositional filling process but also the distribution of hydrocarbon resources and the types of traps [33,68–70]. Structural traps are widely developed in the regions of tectonic slope breaks (e.g., steep slope fault belt, steep-fault bending belt, multi-level step-fault belt) where fault traps and faulted anticlinal traps formed (Figures 12 and 13). By contrast, lithological traps have a close relationship with the development of the sedimentary slope breaks (e.g., gentle slope belt, flat continental shelf belt) (Figures 12 and 13). There are two types of lithological traps, including up-dip wedge-out traps and lithologic lenticular traps (Figure 12), but their formation regions are different. Up-dip wedge-out traps were formed in the stratum of Wenchang, Enping, and Zhuhai Formations, whereas lithologic lenticular traps were developed mainly in the Zhujiang Formation (Figure 12). In addition, stratigraphic onlap


traps are developed by subsequent uplift and erosion, especially in the stratum of the Zhuhai Formation in the Qionghai Uplift (Figure 12).

**Figure 12.** The types of traps and their development strata and regions in the Zhu III Depression, PRMB.

**Figure 13.** The type and distribution of oil and gas reservoirs across the representative section in the Zhu III Depression, PRMB. The location of section aa' is shown in Figure 1.

The sequence stacking pattern is closely associated with trap types and theirdistribution [68,69]. In the regions of steep slope fault belts and steep-fault bending belts, three different types of traps are developed, including fault traps at the fault terraces of the boundary faults, updip wedge-out traps in the north, and sublacustrine fan traps in the deep lake (Figure 13). In the multi-level step-fault belts, lobate sand bodies are deposited on each fault terrace where fault traps develop. In the gentle slope belts, up-dip wedge-out traps and lithologic lenticular traps are formed (Figure 13). By contrast, in the flat continental shelf belts, neritic shelf sand bodies developed in the Miocene Zhuhai Formation are favorable for forming lithologic lenticular traps (Figure 13).

In the last few decades, exploration achievements have been mainly achieved in the regions of structural traps [71]. The investigation of depositional evolution and stratal stacking patterns provides valuable insights into guiding the exploration and development [33,68,69]. According to exploration status and characteristics of trap distribution, the hydrocarbon exploration in the Zhu III Depression should be conducted at different levels: (1) The flat continental shelf belt developed in the Miocene Zhujiang Formation should be the most

significant area for future exploration since the neritic shelf sand bodies distributed in the flat continental shelf are characterized by a high-quality reservoir property with average porosity and permeability values higher than 18% and 140 mD, respectively. The oils in the sandstone reservoirs are high maturity (Ro = 0.8~1.3%) and originated from the Enping Formation (Toc = 1.6~2.4%, HI = 233~332 mg/g TOC) [71]. Moreover, the neritic shelf sand bodies are favourable for forming "self-reservoir and self-coverage" lithologic reservoirs (Figure 13). Moreover, compared with the other traps, the hydrocarbon exploration for the neritic shelf sand bodies faces fewer difficulties, such as shallow burial, low exploration cost, and good seismic quality. These advantages profoundly influence the hydrocarbon enrichment and increase the success rate of hydrocarbon exploration, but determining the distribution range of neritic shelf sand bodies is primary. (2) The gentle slope belt in the adjacent region of the Qionghai Uplift should be an important area for future exploration (Figure 13). The up-dip wedge-out traps in this region are key exploration targets for seeking lithologic reservoirs. (3) The steep slope breaks (i.e., steep slope fault belt, steep-fault bending belt) located in the footwalls of the Zhusannan Fault should be scientifically investigated to achieve breakthroughs (Figure 13). The primary difficulty in these regions is poor seismic data quality, and more study is needed before locating an exploration target.

### **6. Conclusions**

During the basin evolution process from the rifting stage to the post-rifting stage in the western Pearl River Mouth Basin (PRMB), in the northern South China Sea, five types of depositional systems were developed, including the fan delta, meandering river delta, tidal flat, lacustrine system, and neritic shelf system. The representative depositional systems changed from the proximal fan delta and lacustrine system in the Eocene–early Oligocene, to the tidal flat and fan delta in the late Oligocene, and then the neritic shelf system in the Miocene. The intense rifting of boundary faults significantly controlled the distribution of depositional systems in the rifting stage, whereas sea-level change and sediment supply became significant controlling factors for stratigraphic architecture in the post-rifting stage.

Six types of slope break belts (i.e., steep slope fault belt, steep-fault bending belt, multilevel step-fault belt, antithetic-fault slope belt, gentle slope belt, flat continental shelf belt) developed in the western PRMB, which exerted significant functions on stratal stacking patterns. Our results highlight the diversity and complexity of sequence architecture in the continental rift basin, which resulted from the comprehensive effects of sea/lake-level changes, faulting, palaeogeomorphology, and sediment supply.

Stratal stacking patterns are closely associated with trap types and their distribution. According to the exploration status and characteristics of trap distribution, the hydrocarbon exploration in the Zhu III Depression should be conducted at different levels: the flat continental shelf belt and the gentle slope belt are recommended as the key targets for exploration, whereas the steep slope break belt requires further investigations to achieve breakthroughs.

**Author Contributions:** Conceptualization, E.L. and Y.D.; methodology, S.C.; software, X.L.; validation, D.Y.; formal analysis, S.C.; investigation, X.S.; resources, Y.D.; data curation, X.S.; writing—original draft preparation, E.L.; writing—review and editing, S.C.; visualization, D.Y.; supervision, Y.D.; project administration, Y.D. and D.Y.; funding acquisition, E.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Natural Science Foundation of China (Nos. 42072142, 41702121, U19B2007).

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We appreciate Zhanjiang Branch Company of China Offshore Oil Corporation for providing the data and permission to publish this paper. We are also very grateful to the reviewers and editors for their contributions to improving this paper.

**Conflicts of Interest:** The authors declared that they have no conflict of interest in this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

### **References**

