River patterns transition of subaqueous distributary channels in delta front and its significances for petroleum geology: an insight from Paleogene Yabus Formation in the Sag A of Melut Basin, South Sudan

Abstract

Subaqueous distributary channel sandbodies within delta fronts are crucial reservoirs in continental petroliferous basins. Understanding the spatiotemporal transition of river patterns in these channels is essential for accurate evaluation and prediction of oil and gas reservoirs, as well as for providing direct evidence of basin evolution. In the Yabus Formation of the Sag A of Melut Basin, a comprehensive analysis involving sequence division, sedimentary characteristics, seismic facies, high-resolution reservoir inversion, and sand body distribution revealed significant insights. During the Yabus Formation deposition, three intermediate base-level cycles were identified, each showing transition phenomena in the river patterns of subaqueous distributary channels within the delta front. Clear identification criteria for different river patterns were established. Braided subaqueous distributary channels exhibited dominant vertical accretion, high sand content, significant sandstone thickness, and continuous-strong amplitude seismic reflections. While the braided-meandering transition pattern showed a combination of vertical and lateral accretion, medium sand content, moderate sandstone thickness, and medium continuous-medium strong amplitude seismic reflections. Meandering subaqueous distributary channels were characterized by lateral accretion, low sand content, minimal sandstone thickness, weak continuous-weak amplitude seismic reflections, and mud-rich inversion features. The primary control factor influencing the transition of river patterns in these channels was identified as the long-term base-level cycle, shaped by paleotopography and sediment supply. Braided subaqueous distributary channels emerged as the main exploration interval for structural prospects, serving as lateral high-speed migration pathways. Dendritic braided and meandering transition intervals were deemed favorable for both structure-lithologic prospects and the expansion of new exploration fields and layers.

Graphical abstract

Keywords

Delta front / Subaqueous distributary channel / River patterns transition / Melut basin in South Sudan

1 Introduction

The tempo-spatial transformation of river pattern (Qiu, 1985; Miall, 1996; Yu, 2012; Tang et al., 2016; Chen et al., 2015) has long been recognized by sedimentologist for its crucial role in basin evolution and oil and gas reservoirs evaluation (Blum et al., 2009; Wu et al., 2013; Zheng et al., 2015; Yu et al., 2018). Recent studies have investigated river pattern changes using diverse methods. For example, Yang et al. (2020) have studied river pattern changes and their spatial patterns based on satellite photo analysis; Langat et al. (2020) analyzed the evolution of the Tana River in Kenya and its hydrodynamic characteristics based on remote sensing and GIS. Duvall et al. (2020) took the Marlborough Fracture zone in New Zealand as an example to discuss the relationship between river pattern evolution and geomorphic change. Yu et al. (2020) explored the flow structure of multi branched rivers through simulation experiments. Woolderink et al. (2021) used the Meuse River as an example to explore the relationship between tectonic activity, river slope, and river pattern changes, and believed that river pattern changes are not solely influenced by tectonic activity.

In addition to modern river and simulation experiments, scholars have investigated the evolution and controlling factors of river patterns using outcrop and subsurface data. For example, Tan et al. (2014) analyzed the transformation of braided meander river patterns in the Middle Jurassic Toutunhe Formation, located on the southern margin of Junggar Basin. They examined the characteristics and control factors, thereby reconstructing the sedimentary background of the transformation. Li et al. (2015, 2017, 2022) systematically summarized the laws governing river changes, with a particular focus on the braided meander transition in the Toutunhe Formation of the Junggar Basin and the Lower Shihezi Formation of the Ordos Basin. They established sedimentary sequences and proposed the classification basis for three types of abandoned rivers. He et al. (2018) proposed the control factors and mechanisms for the transformation of braided meandering river pattern based on the underground example of the Guantao Formation in the Qingxi area of Dongying Depression. Additionally, Chen et al. (2021) analyzed the characteristics and control factors of river pattern transformation in fluvial sedimentation, using the Jimidi Formation in the Ruman area of the Melut Basin in South Sudan as a case study.

While these above-mentioned studies primarily focus on river pattern transformation within fluvial facies sedimentation, they offer limited exploration of the evolution of distributary channels and subaqueous distributary channels within deltaic sedimentary systems. Delta sedimentation, crucial for oil and gas reservoirs and coalbed sections (Chang et al., 2022; Lu et al., 2022), is characterized by subaqueous distributary channels and associated bar sandbodies, particularly in shallow water deltaic sedimentary systems during structurally stable periods. Subaqueous distributary channels, being the main sandbody type (Zhu et al., 2021), hold significant implications for oil and gas exploration and development through their river pattern transformations.

The evolving understanding of the spatiotemporal transformation of subaqueous distributary channels and river patterns contributes to the exploration of new layers of structural lithologic oil reservoirs. For instance, within the same research area, interconnected sand bodies of braided river subaqueous distributary channels, lacking lateral sealing zones, primarily form structural traps in the braided river intervals. Conversely, intervals transitioning from braided to meandering patterns, as well as established meandering patterns, develop lateral sealing zones and local cap rocks, offering potential opportunities for structural lithologic traps exploration. Concerning development, different sequences or sand formations within the same block may exhibit varying sand body distribution and interlayer distribution due to the transformation of subaqueous distributary channels, potentially influencing development deployment plans. Spanning an area of approximately 400 km², Sag A currently hosts nearly 10 wells, all of which are fully encompassed by high-resolution 3D seismic data. While successful exploration of the structural trap within the Yabus Formation of the Paleogene in the eastern Fracture zone has been achieved, the exploration breakthrough in the extensive slope area, situated within an oil-rich depression, remains elusive. Exploring the evolution of the delta front in the slope area and assessing whether it possesses reservoir cap rock configuration conditions to form structural lithologic traps or lithologic traps requires further investigation. Such endeavors aim to broaden the scope of structural lithologic trap exploration within the research area and enhance the overall reserve scale.

2 Geological setting

The Melut Basin, suited in the Republic of South Sudan, Africa, is a Mesozoic Cenozoic rift basin controlled by the Central African shear zone (Dou et al., 2007; Dou et al., 2018). Encompassing an area of about 3.3 × 10⁴ km², under a structural framework consisting of four subbasins and two highs (Fig.1), characterized by west faulting and east overlapping (Chen et al., 2018). Its structural evolution of has undergone three episodes of rifting in the Early Cretaceous, Late Cretaceous and Paleogene, and the depression stage since Neogene (Tong et al., 2006; Dou et al., 2006; Fig.1). By the end of 2020, the Melut Basin has been identified approximately 6.2 billion barrels of oil reserves, with significant exploration discoveries concentrated in the northern depression, a typical oil-rich depression (Dou, 2005). Sag A, located in the north-east of the northern depression (on the south-east side of the Palogue oilfield), hosts the basin's sole high-yield condensate oil reservoir. Boasting favorable oil quality and high production rates, it stands as a pivotal region for storage capacity enhancement, increased production, and strategic exploration efforts. During the sedimentary period of the Paleogene Yabus Formation, Sag A was positioned within the south-eastern lobe of the Palogue large delta, primarily representing the delta front subfacies (Tong et al., 2006; Dou et al., 2007; Shi et al., 2021) (Fig.1).

3 Sequence division scheme

Based on Melut Basin’s structural evolution, the logging and seismic reflection characteristics observed in the Yabus Formation of Sag A, a detailed level cycle analysis and sequence division were conducted. The bottom of the Yabus Formation is a local unconformity formed by the incision of subaqueous distributary channels. Seismic profiles reveal a distinct contrast: a continuous strong reflection within the lower Yabus Formation and a weak, continuous medium reflection in the underlying Samma Formation, indicating a truncation contact (Fig.2). Lithologically, the formation displays thick, layered-stacked subaqueous distributary channel sandstones, sharply contrasting with the underlying relatively thick mudstones. The sequence boundary marks the transition between falling and rising semi-cycles of long-term base-level changes. At the top of the Yabus Formation lies a substantial layer of mudstone, serving as a transitional surface between rising and falling semi-cycles of long-term base-level changes (maximum flooding surface). Collectively, logging responses, seismic reflections, and lithological inversion findings all suggest that the Yabus Formation corresponds to a long-term base level rising semi-cycle.

The short-term base-level cycle was subdivided according to the grain size rhythm, sandbody thickness and other characteristics. Due to the migration and oscillation of subaqueous distributary river channel, as well as early sediment incision and erosion, the short-term base-level cycle was dominated by a rising semi-cycle. However, the falling semi-cycle was not well preserved. The Yabus Formation in the slope area of Sag A has a thickness of approximately 500 m. Through analysis of the stacking pattern of short-term base-level cycles and transition surface markers, and considering the stacking pattern of sandbodies, seismic response characteristics (overlapping and truncation; Fig.2), and seismic reflection traceability, the Yabus Formation was subdivided into three medium-term base-level cycles (SQ1, SQ2, and SQ3 upwardly) (see Fig.2 and Fig.3).

4 Sedimentary characteristics

Based on regional sedimentary characteristics (Fig.1), log/logging facies (Fig.3), seismic reflection characteristics (Fig.2), paleogeomorphic background (Fig.4), seismic attribute analysis (Fig.4), typical core photographs (Fig.5), lithofacies types and lithofacies assemblages (Fig.6), combined with previous research results (Shi et al., 2018; Wang et al., 2019; Shi et al., 2021), it is indicated that Sag A in the Melut Basin was located on the front edge of the Palogue large delta formed by the Palogue uplift and its north-east provenance area during the sedimentary period of the Yabus Formation. However, significant changes in the subaqueous distributary channels occurred across different sequences of the delta front. During the SQ1 sedimentation period, exhibiting a braided river subaqueous distributary channel (Fig.5) and the SQ2 sedimentary period exhibiting transition from braided to meandering pattern of subaqueous distributary channel (Fig.5). During the SQ3 sedimentary period, the main subaqueous distributary channel was a meandering pattern (Fig.5).

4.1 Braided subaqueous distributary channel

The subaqueous distributary channel sedimentation is characterized by thick layer of medium-coarse sandstone alternating with thin layers of variegated-light gray mudstone. Each sandbody typically ranges from 10 to 15 m in thickness and displays distinct scouring surfaces at the base, often containing imbricated gravel. Predominant lithofacies includes medium-coarse sandstone with trough cross-beddings (St) and medium sandstone with parallel beddings (Sh), forming both fining-upward and coarsening-upward sequences. The typical lithofacies combination types are Gm → St → Sh → M (braided subaqueous distributary channel) and Sp → Sh (intra-channel bar of subaqueous distributary channel). Longitudinally, it is manifested as the superposition of multiple stages of lithofacies assemblages of braided subaqueous distributary channels and intra-channel bar (Fig.6). In a plane view, braided subaqueous distributary channel was in lateral contact with the intra-channel bar (Fig.7), exhibiting an overall high sand content (60%–90%). The intra-channel bar showed a slightly higher sand content. Logging response typically represented serrated box or funnel shape with high amplitude (Fig.3 and Fig.6). Seismic profile shows approximately symmetric U-shaped incision (dominated by vertical accretion internally). The channel was characterized by large width and moderate incision depth (Fig.7). Seismic reflections generally exhibit continuous strong amplitude (SQ1 in Fig.2), while lithological inversion profile is characterized by vertical stacking and lateral splicing of thick sandstone layers, with well-connected sandbodies and thick sandstone interbedded with mudstone (SQ1 in Fig.2).

4.2 Braided-meandering subaqueous distributary channel

The subaqueous distributary channel sedimentation of braided meander transition river pattern is characterized by the interbedding of medium-coarse sandstone, fine sandstone and mudstone. Compared to braided subaqueous distributary channel section, the single layer sandbodies are thinner, ranging from 6 to 10 m in thickness. The main lithofacies types includes trough cross-bedded medium- to coarse-grained sandstone (St), parallel bedded medium-grained sandstone (Sh), and horizontal laminated fine-grained sandstone (Fl). In addition to the typical rock facies combination types Gm → St → Sh → M and Sp → Sh observed in braided subaqueous distributary channels, a proportion of Gm → St → Fl → M (meandering distributary channels) lithofacies combination types appear in the layers transitioning from braided to meandering subaqueous distributary channels. Longitudinally, it appears as a superposition of multiple braided to meandering transition subaqueous distributary channels combined with sandy intra-channel bars (Fig.6). However, horizontally, the braided meandering subaqueous distributary channels are laterally adjacent to sandy intra-channel bars (Fig.7). The overall sand content was moderate (20%–50%), and the sand content at sandy intra-channel bar is relatively high. Logging response represented serrated box or bell shape with high amplitude (Fig.2 Fig.6). The seismic profile shows an approximately symmetric W-shaped downward incision characterized by vertical and lateral aggradation internally (Fig.7). The seismic reflection was characterized by medium continuous to medium strong amplitude (SQ2 in Fig.2). The lithological inversion profile show interbeddings of mud and sand, with locally observed longitudinal superposition of medium-thick sandstone and lateral splicing, indicating moderate sandbody connectivity (SQ2 in Fig.2).

4.3 Meandering subaqueous distributary channel

The sedimentation of subaqueous distributary channels in the meandering pattern was characterized by interlayer of medium- to fine-grained sandstone and mudstone, with a notable increase in mudstone thickness and a reduction in the thickness of a single sandbodies to about 3–5 m. The primary lithofacies types were trough cross-bedded medium-grained coarse sandstone (St), tabular cross-bedded medium sandstone (Sp), fine-grained sandstone with horizontal laminae (Fl), siltstone with ripple beddings (Sr), mainly in fining-upward sequence. The typical types of lithofacies assemblages were Gm → St → Fl → M (meandering subaqueous distributary channel), St → M → Sp → M → Sp (point bar), and Sr → Fl → M → Sr → Fl (spray sedimentary origin, finger-like logging facies). Longitudinally, it appears as a multi-stage meandering subaqueous distributary channel and an interlayer of bay mudstone between the point bar and subaqueous distributary channel (Fig.6). In the plane view, the meandering subaqueous distributary channel was in lateral contact with the point bar (Fig.7), with an overall low sand content (10%–25%). The point bar exhibits an isolated anomaly with high sand content. The logging response was mainly characterized by serrated bell shape with a high amplitude (Fig.3 and Fig.6), with an asymmetric U-shaped incision on the seismic profile and a lateral aggradation in the internal filling (Fig.7). The seismic reflections are characterized by weak continuity and amplitude (SQ3 in Fig.2). The lithological inversion profile is characterized by mud wrapped by sand, isolated distribution of sandbodies, and large thickness of mudstone interlayer and intercalation. The lateral distribution was stable and the connectivity of the sandbodies is poor (SQ3 in Fig.2).

5 River pattern evolution and control factors of subaqueous distributary channels

5.1 River pattern evolution

The Yabus Formation in the Sag A of the Melut Basin can be divided into three mid-term base-level cycles (named SQ1, SQ2, and SQ3 from bottom to top), and the evolution characteristics of subaqueous distributary channels and river patterns in these three cycles are as follows (Fig.8).

During the SQ1 sedimentary period, the lithofacies combination features a vertical aggradation, which is the development interval of typical braided subaqueous distributary channel. The sand content ranges from 60% to 90%, with an overall high background (60%–80%) and locally abnormal stripped-tongue shaped high sand content (80%–90%) (Fig.8). The thickness of sandstone varies from 20 to 40 m, and the plane distribution trend is similar to the contour map of sand content, indicative of substantial thickness and localized anomalies characteristic of braided rivers (Fig.8). Seismic reflections exhibited continuous strong amplitudes, and lithological inversion profiles typically showed thick sandstone interbedded with mudstone. The braided subaqueous distributary channel and the inner beach bar of the channel are longitudinally stacked and laterally connected, forming a sedimentary unit of sandbodies that are widely connected (Fig.8).

The lithofacies combination during the SQ2 sedimentary period features a vertical and lateral aggradation, belonging to development interval of the braided to meandering transition type subaqueous distributary channel. The sand content is 20%–50%, and the plane distribution trend is characterized by an overall medium sand content background and abnormally high sand content (30%–50%) in a north-west south-east trending strip (Fig.8). The thickness of sandstone is 5–35 m, and the plane distribution trend is similar to the contour map of sand content. Specifically, the background sandstone thickness was 5–10 m, and a high value (20–35 m) zone of dendritic sandstone thickness was developed (Fig.8). The seismic reflection is characterized by medium-medium to strong amplitude, the lithology inversion profile is generally characterized by the interbedding of sandstone and mudstone. Braided to meandering subaqueous distributary channels are locally longitudinally stacked and laterally spliced with the inner beach or point bars of the river, forming a set of sedimentary units with local connectivity and isolation of sandbodies (Fig.8).

During the SQ3 sedimentary period, the lithofacies combination typical of lateral aggradation suggests the development of meandering subaqueous distributary channels. Sand content ranged from 10% to 25%, with an overall low sand content background and isolated areas showing slightly higher sand content (15%–25%) (Fig.8). Sandstone thickness varied from 5 to 25 m, with a distribution trend akin to the contour map of sand content. Overall, sandstone thickness ranged from 5 to 10 m, with isolated crescent-shaped areas exhibiting thicker sandstone layers (15–30 m) (Fig.8). Seismic reflections displayed weak continuity and medium-weak amplitude. Lithology inversion profiles typically showed mud wrapped by sand. The edge point bar of the subaqueous distributary channel formed isolated crescent shapes embedded in the dendritic subaqueous distributary channel, constituting a series of sedimentary units with isolated sandbodies and lateral sealing (Fig.8).

During the sedimentary period of the SQ1 to SQ3 sequences in the Yabus Formation of the Sag A in the Melut Basin, the subaqueous distributary channel river pattern of the delta exhibited vertical evolution characteristics of braided pattern → braided to meandering transition pattern → meandering pattern (Fig.9). Specifically, the sedimentary characteristics (Tab.1) showed a decrease in sand content from 60%–90% to 10%–25%, a thinning of sandstone thickness from 20 to 40 m to 5–25 m, and the transformation of logging facies from box- to bell-shape. The transition of seismic reflection from continuous strong reflection, symmetric U-shaped incision to weakly-continuous medium-weak amplitude, and asymmetric U-shaped incision. The plane morphology of sandbody was evolved from braid shape to embedded crescent thick sandbody. The sandbodies transitioned from being widely connected to isolated sandbodies wrapped in mudstone.

5.2 Controlling factors

During the sedimentary period of the Yabus Formation in the Paleogene of the Melut Basin, the river pattern of the subaqueous distributary channel in the delta front gradually evolved from braided river pattern (SQ1) to meandering river pattern (SQ3). This evolution was primarily influenced by long-term base-level cycles. The sedimentary period of the Yabus Formation was generally a long-term base level rising semi-cycle (Fig.3), and the base level was a dynamic equilibrium surface of sediment erosion and deposition, which was also comprehensively constrained by lacustrine level change, tectonic subsidence, sediment supply, paleo-topography, paleoclimate and other factors (Yu, 2008). The SQ1 sedimentary period was in the early stage of a long-term base level rising semi-cycle (above the sequence boundary), with a relatively steep paleo-terrain slope (approximately 0.54° in the north-west south-east direction of the study area), strong provenance supply (coarse particle size, and poor sorting), and strong incision and erosion of subaqueous distributary channels, making it suitable for the development of braided subaqueous distributary channels. The SQ2 sedimentary period was in the middle of the long-term rising semi-cycle of the base level. The slope of the paleo-terrain was relatively slow (the north-west south-east paleo-terrain slope in the study area was about 0.33°), and the subaqueous distributary channel gradually turned into lateral erosion, entering the braided-meandering transition stage. During the SQ3 sedimentary period, it was in the upper part of the long-term rising semi-cycle of the base level, and the paleo-terrain slope further slowed down (the north-west south-east ancient terrain slope in the study area is about 0.21°). The erosion of subaqueous distributary channels was weak, and there were mainly meandering patterns, with widespread development of flooding plains.

From a natural sedimentary perspective, steep slopes tend to favor coarse-grained braided river patterns, whereas gentle slopes tend to favor medium- and fine-grained meandering river patterns. This phenomenon reflects the selective deposition result of the coupling effect between the quality of the sediment material (sediment supply) and the slope of the terrain (Liu et al., 2019; Yu et al., 2022), providing detailed insights into the dynamic mechanism and mechanical differentiation of detrital material. Modern deposition examples further illustrate that when the topographic slope is relatively steep, regardless of particle size and sediment supply, the dominant river pattern is typically braided, controlled by the inertia force of sediments. Conversely, as the topographic slope decreases, the river pattern tends to transition to braided-meandering and ultimately meandering patterns, controlled by the force of friction between sediments and the slope.

Apart from variations in paleo-topography and provenance supply, factors like lacustrine level, tectonic subsidence and paleoclimate, affecting the long-term base-level cycle exhibited no evident changes in the sedimentary period of the Yabus Formation. This period was in weak rifting period, with weak tectonic movement, wide water area and shallow water body, stable lacustrine level and small tectonic subsidence (Dou et al., 2018). Regarding paleoclimate, the spore-pollen assemblage from the Paleogene Yabus Formation primarily comprised Pteridophyte spores and pollen, including triangular spores and pollen, Graminidites spores and pollen, Laevigatosporites spores and pollen, and Cicatricosisporites spores and pollen. A small quantity of flowering plant spores and pollen, such as Toxicodendro pollen, Crassimarginpollenites pollen, Syncolpate pollen, and Tricolpopollenites pollen, indicated a warm and humid tropical-subtropical climate during this era. Generally, paleoclimate changes from SQ1 to SQ3 in the Paleogene were insignificant. However, during the Eocene Adar Formation’s deposition, there was a marked transition to a dry and hot climate, indicated by the presence of thick brownish-red and brown mudstone (Ye, 2006).

Based on comprehensive analysis, it is believed that the main control factor for the transformation of subaqueous distributary channels was the changes in long-term base-level cycles. In the study area, the changes in long-term base-level cycles were mainly influenced by paleo-terrain and provenance supply. During the sedimentary period of SQ1 to SQ3 of the Yabus Formation, provenance supply was gradually weakened, and the slope of paleo-terrain decreased from steep to gentle (from 0.54° to 0.21°). Furthermore, the river pattern of subaqueous distributary channels in the delta front gradually transformed from braided pattern to meandering pattern.

6 Significance of petroleum geology

The transformation of river patterns in subaqueous distributary channels in the delta front leads to variation in reservoir physical properties, connectivity, and trap types across different river pattern intervals. This variability contributes to the presence of different trap types within different intervals, thereby expanding the scope of exploration. Previously, exploration primarily focused on simple structural traps, but now it has extended to encompass structural-lithologic traps.

The SQ1 sequence in Sag A was characterized by a braided pattern, with longitudinally stacked and laterally spliced sandbodies, large thickness, and good connectivity. This interval primarily served as the main target for structural traps exploration. At the same time, when structural traps were not developed, the types also served as good oil and gas migration and transport layers. The simulation results of oil and gas migration and accumulation during the main hydrocarbon expulsion period in the slope area of Sag A (Fig.10) showed that the oil and gas generated by the source rock of the Cretaceous Renk Formation first migrated longitudinally along the Mishmish Fracture zone. Under the control of the unconformity surface and the thick connected sandbodies of the SQ1 sequence braided pattern (Fig.9), migrated laterally along the transport ridge in the slope area, forming three high-speed migration channels, which are nearly north to south direction. Coincidentally intersecting with the main delta front sandbody of the SQ2 sequence (braided transition layer) in a nearly north-west to south-east direction (Fig.10), oil and gas migrate laterally along the SQ1 connected sandbody. When encountering the SQ2 sequence sand body in local contact with SQ1, it was longitudinally transported to the SQ2 layer section under the control of buoyancy, and filled with a structural lithologic trap controlled by the structural contour line and the lateral sealing zone of the subaqueous distributary bay mudstone (Fig.10). It formed a reservoir formation model of “faults unconformity, and braided pattern connected sandbodies jointly controlling migration and braided -meandering transition layer structural lithologic traps accumulating and forming reservoirs” (Fig.10). SQ3 sequence was dominated by meandering river patter, with low sand content, large thickness and relatively stable mudstone layer, good lateral shielding and vertical sealing conditions. It was the main development interval of lithologic traps. However, as it was far away from hydrocarbon source rocks and lateral oil and gas migration layers, fault lithologic traps can be formed by vertical migration of oil and gas via faults.

In addition to what the case study discussed, research on the Jimidi Formation in the Melut Basin also revealed a transition in river patterns from meandering to braided and back to meandering. This transition caused variations in reservoir properties, connectivity, and types among different layers, notably guiding the identification of relatively low-porosity barrier zones consistently distributed within meandering river formations. Consequently, this insight prompted the expansion of exploration efforts from simple structural reservoir exploration to structural-lithologic reservoir exploration (Chen et al., 2021). Moreover, in the Shawan Formation of the Chunguang Block in the Junggar Basin, there was concurrent development of braided river and meandering river deposits. During this period, sedimentary characteristics of “East braided and West meandering” were observed. The braided river sandbodies were thick and well-connected, serving as crucial channels for oil and gas migration. Meanwhile, the meandering river overflowing mudstones were extensively developed, providing favorable conditions for lithologic reservoir formation and representing the predominant reservoir type (Yue, 2021).

7 Conclusions

The Paleogene Yabus Formation in Sag A of the Melut Basin can be subdivided into three mid-term base-level cycles, each showcasing distinct transformations in the river patterns of subaqueous distributary channels within the delta front. As a result, identification criteria for these different river patterns have been proposed. The lithofacies combination of the braided subaqueous distributary channel (SQ1) was mainly vertical aggradation, with high sand content (60%–90%) and large sandstone thickness (20–40 m). Seismic reflections were characterized by continuous strong amplitude, and the lithological inversion profile was characterized by thick sandstone interbedded with mudstone. The lithofacies combination of the braided-meandering subaqueous distributary channel (SQ2) was characterized by both vertical and lateral aggradation, with moderate sand content (20%–50%) and moderate sandstone thickness (5–35 m). Seismic reflections were characterized by medium-to-medium strong amplitude, and the lithological inversion profile was characterized by the interbedding of sand and mud. The lithofacies combination of the meandering pattern subaqueous distributary channel (SQ3) was mainly characterized by lateral aggradation, low sand content (10%–25%), moderate sandstone thickness (5–25 m), weak continuity to medium weak amplitude in seismic reflection. The lithologic inversion profiles were characterized by mudstone interbedded with sandstone.

The primary factor driving the transformation of subaqueous distributary channels was the variation in long-term base-level cycles, primarily influenced by changes in paleo-terrain and provenance supply. During the sedimentary period of SQ1 to SQ3 in the Yabus Formation, the provenance supply was gradually weakened. The slope of paleo-terrain was decreased (from 0.54° to 0.21°), resulting in the gradual transformation of the river pattern of subaqueous distributary channels in the delta front from braided to meandering pattern.

The sandbodies within the layers of braided subaqueous distributary channel displayed longitudinally stacked and laterally spliced, characterized by large thickness and good connectivity. These layers emerged as primary exploration targets and lateral migration channels for structural traps. Meanwhile, the “branch-shaped” braided transition layers exhibited moderate sand content and sandstone thickness, featuring localized connectivity and sealing properties. Positioned adjacent to the underlying braided river pattern high-speed migration layers, they offered promising exploration prospects for structural lithologic traps.

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