On seismic profiles, there are some truncated and incised features in the local area, indicating the presence of SBs (Figure 4). These incisions are overall disconnected and formed an analogy U-shaped undercutting (Figure 4C). Most sequence boundaries have toplap reflection below the interface (Figure 4), and onlaps can be observed above the SBs. It should be noted that truncation only developed in the western part of the Dongsha Rise, the northern region of the Huizhou Depression, and the Huizhou Low Rise in the study area (Figure 2).

From well logs, SBs display three different patterns, largely due to the variability of sedimentary facies in different areas (Figures 5–7):

Scenario I: the SBs separate a coarsening-up sandy interval below from a fining-upward sandy interval above (Figures 5–7). Below the boundary is a reverse cycle in which the number of sandstones increases upward, the proportion of argillaceous rocks decreases, and the amplitude of electrical curves increases upward. Above the boundary is a positive cycle in which the amount of sandstone decreases upward, the proportion of argillaceous rocks increases, and the amplitude of electrical curves decreases upward. The characteristics of this sequence boundary reflect the highstand systems tract (HST) of the underlying sequence and the transgressive tract of the overlying sequence. Moreover, the erosion near the sequence boundary is either minimal or nonexistent.

Scenario II: the SBs separate a muddy interval below from a coarsening-up sandy interval above (Figures 5–7). Below the boundary is a succession of mudstone, with the amplitude of the electrical curve being small. Above the boundary is a fining-upward cycle where the amount of sandstone decreases upward, the proportion of argillaceous rocks increases, and the values of the electrical curve decrease upward, indicating the presence of Transgressive Systems Tract (TST) deposits. This suggests two possibilities: (i) the sand-rich section in the underlying Highstand Systems Tract (HST) had been eroded, leaving only the mudstone section formed in the early stage of the HST, or (ii) the underlying HST deposits formed due to the dominant supply of muddy sediments. There is a certain difference in the characteristics of the underlying mudstone in these two scenarios. The former has a large number of markers indicating shallow water and oxidation exposure, while the latter is mudstone formed in deep water.

Scenario III: the SBs separate a coarsening-up sandy interval below from a serrated muddy interval above (Figures 5–7). Below the boundary, a reverse cycle exists where sandstones increase upward, argillaceous rocks decrease, and the amplitude of electrical curves increases upward. Above the boundary is a mudstone section, characterized by a predominantly serrated electrical curve with a small amplitude. This characteristic of the sequence boundary reflects an abundant supply of terrigenous sandy coarse debris in the HST of the underlying sequence, and a significant supply of terrigenous clasts, primarily argillaceous, in the transgressive system tract (TST) of the overlying sequence. During the HST period, the sufficient supply of terrigenous sandy coarse debris led to a gradual shallowing of the water body as the shoreline migrated seaward, forming a coarsening-upward cycle below the boundary, where the sandstone content gradually increased and the argillaceous content gradually decreased. Conversely, during the TST period, the terrigenous clastic supply was primarily argillaceous, and the water body deepened as the shoreline migrated landward, forming mudstone sections with progressively darker colors above the boundary.

From core data, third-order SBs displayed shoaling water, subaerial exposure, and erosion on core data. The diagnostic feature for water shoaling upward is the presence of calcareous mudstone (Figure 8). Erosion and channel incision occurred were characterized by scour surface (Tavakoli et al., 2018) (Figure 8).

On the seismic section, MFSs are mainly manifested as a low-frequency, highly continuous, and high-amplitude reflections (Figures 5–7). In some areas, the downlaping pattern of the overlying strata and the truncation of underlying strata can be observed (Figures 5–7). On well logs, the MFS was characterized by high gamma value, which separated a fining-upward cycle below and a coarsening-upward cycle above (Tavakoli, 2017). In lithology, MFS lies within a mudstone section, where the abundance of planktonic foraminifera is the maximum.

According to the recognition of SBs and MFSs, the Zhujiang Formation in the study area can be subdivided into five third-order sequences (Figures 5–7). To establish a high-resolution stratigraphic framework, ten systems tracts are further delineated using the sequence boundaries and maximum flooding surfaces within the Zhujiang Formation, primarily dominated by the TSTs and HSTs. Notably, a lowstand systems tract is recognized in the Pearl River Delta system due to the influence of local slope breaks, which provide some local accommodation for the deposition and preservation of LST sediments. Therefore, according to the development characteristics of the five third-order sequences (Figure 3), two organization patterns of systems tracts can be determined:

Type 1 was dominated by a two-fold structure of systems tracts which contained the TST and the HST. The TST is dominated by a transgressive delta or coastal system; the HST is dominated by a highstand delta; and the tidal flat facies and carbonate platform facies can be developed under a special geomorphic background. This type of sequence includes the early SQ1 and SQ2 in the Zhujiang Formation. It reflects that the relative sea level rises and falls slowly and that the sediment supply is sufficient.

Type 2 stratigraphic pattern was featured by the local development of the LST within the low-lying area between the delta front and the carbonate platform. The slope fan and the progradation complex are recognized near the slope or the foot of the slope break zone (Figure 5C). Therefore, the occurrence of LST may be related to the slope break zone.

Deltaic systems are the most prevalent and developing depositional systems in the Zhujiang Formation of the Huizhou Area (Figures 8A, B). In general, four primary subfacies of the deltaic environment can be identified:

The shoreface association of the Pearl River Delta mainly contain two subfacies: foreshore and nearshore. The foreshore deposits lie between the mean high tide line and the mean low tide line. Based on core observations (Figures 8C–H), it is mainly composed of fine-medium sandstone and mainly massive bedding, which is well sorted and subcircular, with the presence of the swash cross beddings. At the top of the thick layer of sand, we often see erosion cutting and a formed erosional contact. Numerous bioclasts were found, and shell fragments were well-preserved (Figure 8).

The nearshore subfacies lies between the mean low tide line and the wave base. Depending on the intensity of hydrodynamic conditions, this subfacies can be further divided into upper nearshore and lower nearshore. The upper shoreface deposits are primarily composed of light brown sandy conglomerates or conglomerate-bearing sandstones, with most gravels being fine-grained. Cross-beddings are prevalent, and the sorting is generally medium (Figure 8C). Well log patterns exhibit coarsening-upward box-shaped motifs with significant grain size variations.

The lower shoreface deposits mainly consist of mudstone, silty mudstone, and argillaceous siltstone, with occasional appearances of light-grey medium to fine sandstone. Massive beedings and parallel stratifications are common, transitioning into lenticular stratifications in the seaward direction. The gamma curve is negative, displays a sharp mouth shape, and is distinctly toothed. It does not show obvious depositional cycles and is primarily composed of silty mudstone (Figure 8C).

During the deposition of the Lower Zhujiang Formation, the tidal flat-lagoon sedimentary system primarily developed in the eastern margin of the Dongsha Rise, specifically within the Huizhou Sag. This development was influenced by the combined effects of the confined topography and tidal currents (Figures 8I–L). Vertically, the tidal deposits within the study area are predominantly characterized by tidal flat facies at the base, primarily constituted by quartz-sandstone interbedded with mudstone layers (Figure 8). The quartz-sandstones are clean and moderately sorted. Moving upwards, these transition into lagoon deposits, the lithology of which is dominated by thick mudstone layers interbedded with thin layers of siltstone and dolomite limestone.

The carbonate systems within the study area primarily comprise subfacies such as open platforms, platform edges, and platform front slopes. They can be distinctly observed in seismic sections near the Dongsha Rise (Figure 10). The open platform is an environment located behind the platform edge, typically characterized by low energy and stable water conditions. Given the relatively good water circulation and normal salinity, this environment is conducive to life. Conversely, the sedimentary structure undergoes significant changes and contains a substantial amount of lime mud, leading to the formation of various types of limestone (Figure 10).

The platform edge reef is located near the wave breaking zone of the platform edge. Reef-building organisms mainly consist of coralline algae (red algae), corals, sponges, mosses, and green algae (Figure 11). These are primarily algae and exhibit a twining structure, crust-shell structure, and tuberculous structure. The gamma ray (GR) is low, the curve is box-shaped and displays micro-dentate characteristics, and the amplitude change is consistent.

Platform edge shoals are generally found on the edge of the platform reef to the side of the platform, representing a shelf transition. These shoals have strong hydrodynamic conditions, with bioclastic shoals being developed and low gypsum content. Under suitable conditions, reef bodies can develop and form, resulting in alternating interbeds of reef and beach deposits (Figure 10).

The shallow marine system represents one of the most distal environments within the study area, with the sandbodies being mostly shore-parallel (Figure 9D). The hydrodynamic conditions of the continental shelf are complex and diverse, including ocean currents, waves, tidal currents, and density currents (Zhuo et al., 2014; He et al., 2017). These processes tend to act within various water depths. In the inner-shelf region, the presence of ripple cross beddings indicates the influence of tide or wave actions. Other than normal wave or tide actions, storms also influence the shallow marine deposits, as indicated by the block cross beddings instead of ripple cross beddings.

Isolated shelf sandbodies are perpendicular to the shoreline and parallel to the direction of the tidal or longshore currents (Figure 9B). Because of the long-term reworking of marine processes, the content of sand is relatively high, and the shelf sandbodies tend to be composed of clean and well-sorted sandstones, which are encased in shelf mudstones. The GR curve corresponding to the shelf mud in the shallow sea has an abnormally low amplitude, reflecting relatively pure thick mudstone deposition under the condition of still water.

Stage 1 primarily involves the deposition of the SQ1 sequence. During this stage, seawater started to infiltrate the Zhu I Depression, resulting in the formation of a relatively thin transition sequence that had a high sand content (Figure 12A). The deposition of the Zhuhai Formation essentially covered the previously existing multi-sag topography, leading to the creation of a gentle shelf environment, particularly evident since the deposition of the Zhujiang Formation. At the same time, the second jump of the mid-ocean ridge caused the Zhu II Depression to significantly subside and the seawater to encroach to the Zhu I Depression. During this period, the ancient Pearl River Delta and the Hanjiang Delta both advanced from the north or west side of the study area, forming a large delta cluster. In front of these two delta systems, shallow marine depositional systems developed due to the insufficient input of continental clastic material. Most of the Dongsha Rise remained exposed, but its function as a sediment source significantly diminished. Instead, the sedimentary systems of the shoreland and tidal flat-lagoon around the uplifts replaced it, rather than the typical delta systems.

Stage 2 encompasses three sequences, namely, SQ4, SQ3, and SQ2. The most prominent characteristic of Stage 2 is the interplay between deltaic and carbonate deposits. As depicted in Figure 12B, the SQ2 sequence underwent the most significant transgression after the Pearl River Mouth Basin entered the comprehensive depression stage, leading to the formation of the largest floating surface MFS18.5 in the Pearl River Mouth Basin. This resulted in the largest transgression systems tract and highstand systems tract (HST) in the Pearl River Mouth Basin. During this period, the ancient Pearl River Delta receded to the northwest, but it remained the primary sedimentary system in the study area. The ancient Hanjiang Delta also retreated from the study area and was only present in the north of the Lufeng Depression. The exposed area of the Dongsha Rise dramatically decreased, and thin layers of limestone began to develop where the shoreland and tidal flat-lagoon sedimentary systems were previously established, forming typical carbonate slope deposits. Towards the end of the transgression, the accumulation of carbonate rocks intensified, resulting in a massive carbonate platform. Importantly, by the end of the transgressive systems tract (TST), although the Dongsha Rise was still dominated by carbonate platform facies, the platform’s area started to decline due to the advancement of the ancient Pearl River Delta.

The following two third-order sequences, SQ3 and SQ4, largely mirror the sedimentary facies distribution of the SQ2 sequence (Figure 12C). However, due to the evolution of the carbonate platform, a restricted trough was formed between the shoreline break and the carbonate platform in the early stage. This led to the emergence of a local lowstand systems tract (LST) above the sequence interface. Influenced by the confined trough, several shelf sand ridges were constructed in front of the ancient Pearl River Delta (Figure 12C).

During Stage 3, primarily characterized by the SQ5 sequence (Figure 12D), all carbonate platforms started to submerge, becoming overlaid by shallow marine deposits. The entire study area was dominated by delta and shallow marine shelf facies, with the shoreland facies persisting in the northern region. With the barrier effect of the carbonate platform no longer present, the delta lobes extended further during the late phase of the HST, culminating in the maximum marine regression of the secondary cycle within the Zhujiang Formation.

In the study area, the sand bodies near the sequence boundaries have favorable conditions to form lithologic traps (Figure 15). During the deposition of the Zhujiang Formation, the Huizhou Area was in a relatively wide and slow continental shelf environment, and the shoreline migration was very sensitive to the changes in the relative sea level, forming stacked deltaic or shoreface sand bodies. The mudstone developed during the transgression can act as a regional seal for the sandstone near the sequence boundary to form lithologic traps. The types of lithologic traps formed near the sequence boundaries mainly include HST sand pinch out traps below the sequence boundary, LST stratigraphic traps, and TST sand pinch out traps above the sequence interface. In the study area, a large number of hydrocarbon discoveries have been made near the sequence boundaries, such as the NSB21, NSB18, NSB17.5, and NSB17.1.

During certain periods of sea-level lowstand (18, 17.5, and 17.1 Ma), carbonate platforms may have been exposed, leading to erosion or freshwater leaching. This exposure resulted in the development of secondary pores in the carbonate rocks near the sequence boundary, forming high-quality reservoirs. As sea levels rose, newly formed carbonates served as a cap layer for the secondary pore beneath the sequence boundaries, thereby creating carbonate lithologic traps. In the final stage of carbonate development, the top layer was directly overlaid by mudstone, providing even better capping conditions.