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Article

The Genesis of a Thin-Bedded Beach-Bar System Under the Strike-Slip Extensional Tectonic Framework: A Case Study in the Bohai Bay Basin

by
Jing Wang
1,
Youbin He
1,*,
Hua Li
1,
Bin Feng
2,
Zhongxiang Zhao
1,
Xing Yu
3 and
Xiangyang Hou
4
1
School of Geosciences, Yangtze University, Wuhan 430100, China
2
Changqing Division, China Petroleum Logging Co., Ltd., Xi’an 710021, China
3
China United Coalbed Methane Corporation, Ltd., Beijing 100016, China
4
PetroChina Tuha Oilfield Company, Hami 839009, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(14), 7964; https://doi.org/10.3390/app15147964
Submission received: 21 May 2025 / Revised: 10 July 2025 / Accepted: 15 July 2025 / Published: 17 July 2025
(This article belongs to the Special Issue Advances in Reservoir Geology and Exploration and Exploitation)
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Abstract

The lower sub-member of Member 2, Dongying Formation (Paleogene) in the HHK Depression hosts an extensively developed thin-bedded beach-bar system characterized by favorable source rock conditions and reservoir properties, indicating significant hydrocarbon exploration potential. Integrating drilling cores, wireline log interpretations, three-dimensional seismic data, geochemical analyses, and palynological data, this study investigates the sedimentary characteristics, sandbody distribution patterns, controlling factors, and genetic model of this lacustrine beach-bar system. Results reveal the following: (1) widespread thin-bedded beach-bar sandbodies dominated by fine-grained sandstones and siltstones, exhibiting wave ripples and low-angle cross-bedding; (2) two vertical stacking patterns, Type A, thick mudstone intervals intercalated with laterally continuous thin sandstone layers, and Type B, composite sandstones comprising thick sandstone units overlain by thin sandstone beds, both demonstrating significant lateral continuity; (3) three identified microfacies: bar-core, beach-core, and beach-margin facies; (4) key controls on sandbody development: paleoenvironmental evolution establishing the depositional framework, secondary fluctuations modulating depositional processes, strike-slip extensional tectonics governing structural zonation, paleobathymetry variations and paleotopography controlling distribution loci, and provenance clastic influx regulating scale and enrichment (confirmed by detrital zircon U-Pb dating documenting a dual provenance system). Collectively, these findings establish a sedimentary model for a thin-bedded beach-bar system under the strike-slip extensional tectonic framework.

1. Introduction

The beach-bar system represents a typical sand-rich sedimentary body in coastal zones and shallow lacustrine environments. Geologists have systematically investigated its genetic mechanisms [1,2] and depositional processes [3,4]. With the advancement of hydrocarbon exploration, global clastic petroleum exploration has progressively entered the phase of exploring subtle reservoirs. It is now recognized that lacustrine beach-bar sandbodies, widely developed in faulted lacustrine basins, constitute potential high-quality reservoirs. For instance, breakthroughs have been achieved in the hydrocarbon exploration of beach-bar sands within the Green River Formation of the Uinta Basin (USA), and in the Bohai Bay, Junggar, and Qaidam Basins (China) [5]. This success establishes the beach-bar system as a critical target for lithologic reservoir exploration in mature oilfields, demonstrating broad development prospects.
As a significant reservoir type in continental lacustrine basins, research on the beach-bar depositional system still exhibits notable discrepancies in classification schemes and depths. Classification criteria vary considerably as scholars have proposed similar or divergent schemes [6] based on lithological characteristics (e.g., carbonate versus clastic beach-bars), planar positions (e.g., shore-attached, nearshore, offshore, and isolated beach-bars), etc. From the perspective of sandbody thickness, the system can be categorized into thick-bedded (>10 m average single-layer thickness) and thin-bedded types. The thick-bedded type typically forms composite sedimentary bodies tens of meters thick via vertical stacking, with well-developed internal interbeds, and can be sub-divided into progradational and retrogradational stacking patterns. Conversely, the thin-bedded type exhibits frequent sand–mud interbedding, with bar sands averaging 3 to 8 m and bench sands < 2 m per layer [7,8]. Whilst the thick-bedded beach-bar, characterized by favorable reservoir properties, is a primary exploration target, the thin-bedded variant poses significant challenges due to its limited thickness and rapid lateral variability. These technical constraints result in comparatively limited research on its genetic mechanisms in complex tectonic settings [9,10,11].
The beach-bar system within the Paleogene of the HHK Depression, Bohai Bay Basin, exemplifies an atypical thin-bedded system. It features limited sand layers (average single-layer thickness: 1 to 8 m) but exceptional lateral continuity and stability. Production data indicate that this reservoir exhibits high stability, productivity, and significant oil column heights, making it a key target for reserve growth. However, unlike typical thin-bedded beach-bars with frequent sand–mud intercalations, this deposit comprises thick mudstone sequences interbedded with laterally persistent sandstone layers. Furthermore, it developed under a strike-slip extensional tectonic framework, implying a complex genetic mechanism [12,13,14]. Consequently, research on such thin-bedded yet laterally extensive systems remains scarce. Studying this system not only enriches beach-bar depositional theory but also facilitates a strategic shift in subtle reservoir exploration from “thick-bedded targets” to “thin-bedded, widespread plays,” offering new pathways for lithologic trap exploration in complex tectonic zones [15,16].
Developing under the synergistic control of multiple factors (paleoclimate, paleo-water depth, sediment provenance, and tectonic activity), the beach-bar system serves as a vital hydrocarbon reservoir in lacustrine basins. Conventional genetic studies rely primarily on lithological identification, well-log facies analysis, and three-dimensional seismic interpretation [17,18]. Although effective in delineating macroscopic sandbody distribution, these approaches inadequately constrain paleoenvironmental parameters quantitatively and exhibit limited precision in provenance tracing within complex tectonic settings. This dual limitation hinders detailed analysis of beach-bar formation processes [19,20]. To overcome these methodological constraints, this study employs the following multi-dimensional technical framework: Quantitative paleoenvironmental reconstruction, utilizing the abundance and assemblage characteristics of micro-paleontological fossils (e.g., algae, spores, and pollen) to establish evolutionary curves of the paleo-lacustrine environment to quantitatively restore paleo-water depth. This provides high-resolution constraints on the depositional dynamics governing beach-bar development. Precise provenance tracing, addressing multi-source convergence challenges in complex tectonic regions, employs LA-ICP-MS zircon U-Pb dating to systematically acquire detrital zircon age spectra [21,22]. Correlation of these spectra and isotopic signatures with potential source terranes enables effective sediment source tracing and elucidation of provenance-tectonic coupling mechanisms. Through multi-parameter cross-validation and multi-scale data fusion, this integrated approach achieves synergistic quantification of the controlling factors (paleoenvironment, provenance, tectonics). The resulting models provide a scientific basis for understanding beach-bar sandbody genesis in complex geological settings and hold significant practical implications for hydrocarbon exploration target prediction.
The HHK Depression is situated in the southeastern Bohai Bay Basin (Figure 1a,b), covering approximately 3300 km2 (Figure 1c). It is bounded to the south by the Kendong Uplift and the Laibei Uplift, to the north by the relatively Bonan Uplift, and is axially traversed by the Tan-Lu Fault Zone [23]. The depression exhibits a composite fault system comprising NNE–NE trending strike-slip fault systems and EW–NEE trending extensional fault systems, which partition the depression into the following four secondary structural units: the northern steep slope zone, southern gentle slope zone, central strike-slip zone, and central uplift zone. These faults further sub-divide the depression into southwestern subsag, northwestern subsag, and eastern sag (Figure 1c).
The Paleogene sequence in the HHK Depression (from bottom to top) comprises the Shahejie Formation Members 4 to 1 (E2s4-E2s3-E3s2-E3s1), and Dongying Formation Members 3 to 1 (E3d3-E3d2-E3d1), overlain by Neogene Guantao (Ng) and Minghuazhen (Nm) formations and Quaternary (Q) deposits, absent the Kongdian Formation (E1k). Notably, the fluvial-deltaic, fan-delta, and shore-shallow lacustrine beach-bar facies are extensively developed within the Shahejie and Dongying Formations. Through integrated analysis of three-dimensional seismic data, well logs, and lithological records, the following six key chronostratigraphic surfaces in the Paleogene were identified: the base boundary of Shahejie Member 2 (SB5, T5), the top boundary of Shahejie Member 2 (SB4, T4), the base boundary of Dongying Member 3 (SB3, T3), the intra-Dongying Member 3 boundary (SB3L, T3L), the base boundary of Lower Dongying Member 2 (SB3M, T3M), and the top boundary of Lower Dongying Member 2 (SB3U, T3U) (Figure 1d).
During the Paleogene, lithospheric extension and thinning in eastern China were induced by roll-back of the Pacific Plate subduction beneath the Eurasian Plate [24]. The Bohai Bay Basin, located within the superimposed zone of the right-lateral strike-slip motion along the Tan-Lu Fault Zone and destruction of the North China Craton, developed a unique composite stress field dominated by NW–SE extension and NE-oriented shear, with its Cenozoic tectonic history comprising four superimposed phases. The initial Rifting Phase (E1k-E2s3, 65–38 Ma) was characterized by basement fault activation and regional extension-driven subsidence [25,26]. This was followed by the Fault-Depression Phase (E3s2 to E3d, 38–24.6 Ma), dominated by strike-slip faulting and localized fault-controlled depression (Figure 1d).
This research aims to establish a comprehensive genetic model for the thin-bedded beach-bar system in the E3d2L of the HHK Depression, addressing critical knowledge gaps in its sedimentology and exploration potential. To achieve this goal, the following four integrated objectives are defined: first, to characterize sedimentology through integrated core–log–seismic analysis by deciphering depositional architecture, microfacies types, and stacking patterns of thin-bedded beach-bars; second, to quantify controls on sandbody development by determining coupling mechanisms among paleoenvironmental factors (e.g., paleobathymetry), strike-slip tectonics, provenance supply, and paleoclimate shifts; third, to develop a genetic tectono-sedimentary model elucidating unique spatial-temporal distribution mechanisms of thin, laterally continuous beach-bars under strike-slip extensional settings; and fourth, to predict exploration targets through the identification of high-potential zones for lithologic reservoirs in analogous strike-slip basins with validation of predictive universality.

2. Materials and Methods

The integrated dataset comprises three-dimensional seismic data, well logs, core samples, and laboratory analyses including grain size distributions, paleontological assemblages, and geochemical parameters. Within the study area containing 29 drilled wells, 16 wells with complete stratigraphic sub-division data and geophysical logs—including spontaneous potential (SP), natural gamma ray (GR), microlog, and microresistivity—were selected for detailed analysis. These logs provide the fundamental basis for high-resolution stratigraphic correlation and serve as critical constraints for mapping planar sandbody distributions. Log facies interpretation further enables depositional facies characterization.
Core observation, description, and cast thin-section analysis of target intervals from the 16 wells were systematically conducted, with emphasis on data from key cored wells (B8-2-a, C4-2-a, C4-3-a, C4-4-e, and C3-1-a). Analyzed attributes encompass lithology, compositional heterogeneity, and sedimentary structures. Continuous core sections document vertical grain size variations, whose cyclicity patterns provide diagnostic criteria for analyzing sandbody architecture and defining depositional microfacies. This approach proves particularly essential in frontier exploration areas with sparse well coverage.
Full-azimuth three-dimensional seismic coverage exists across the study area, with data acquired at a 2 ms sampling rate and processed to a 12.5 m (in-line) × 25 m (cross-line) bin size using three-dimensional prestack time migration (PSTM). Seismic reflection configurations were analyzed to establish a regional framework for depositional facies interpretation. Integrated with seismic profile characteristics, paleotopography and paleogeomorphology were reconstructed through horizon flattening techniques within the GEOFRAM three-dimensional interpretation system [27,28], thereby enabling the discrimination of paleo-hydrodynamic conditions and basin physiography (Figure 2).
Integrated paleoclimate and paleoenvironmental proxies—including abundant lacustrine algal fossils, palynomorph assemblages, and trace element data from the Paleogene succession—were analyzed using complete datasets from four key wells (C3-1-a, C4-4-3, C4-9-f, and B9-4-b). Sampling details are as follows: Well C3-1-a: 83 cuttings samples from 2895 to 3980 m depth; Well C4-4-3: 26 cuttings samples from 2880~3360 m depth; Well C4-9-f: 24 cuttings samples from 2360~2960 m depth; and Well B9-4-b: 12 cuttings samples from 2725~3005 m depth. A total of 145 cuttings samples were collected at 20 m intervals, spanning the Shahejie Formation Members 2-1 (E3s2–E3s1) and Dongying Formation Members 3-1 (E3d3–E3d1).
Palynofloral assemblages serve as sensitive paleoclimatic indicators. Palynological analysis provides critical chronostratigraphic constraints on vegetation dynamics, constituting a primary methodology for paleoclimate reconstruction [29,30]. Systematic quantification of palynological datasets from the HHK Depression, Bohai Bay Basin, revealed six diagnostic palynofacies assemblages from Shahejie Member 3 through the Dongying Formation. During Dongying Formation deposition, palynofacies indicated deciduous broad-leaved dominance under a subtropical climate, with gymnosperm components showing progressive proliferation of Tsuga and Picea. This succession documents a gradual cooling trajectory within a persistently warm–hot regime, characterized by a climatic shift from arid–thermal to humid–warm conditions with quasi-cyclic oscillations (Table 1).
Synthesis of geological evidence confirms that the HHK Depression comprises continental lacustrine sediments ranging from shallow to deep lake facies. Comparative analysis of basin morphology, scale, and hydrology demonstrates close analogs between the target paleolake and documented modern lacustrine systems in terms of morphometric configuration, depositional patterns, and hydrodynamic regimes [30,31,32,33], enabling paleobathymetric reconstruction via integrated geochemical–paleontological proxies.
These analyses provide key constraints for paleoenvironmental reconstruction as the Fe/Mn ratio serves as a robust geochemical proxy for lacustrine bathymetric zonation: <30 = deep-lake, 30~50 = semi-deep lake, and >50 = shallow-lake facies [34,35,36]. An integrated paleobathymetric methodology combining trace element geochemistry with ostracod differentiation indices was established based on the following quantitative modern lacustrine analogs [37]:
H(s) = −ΣPiInPi(i = 1, 2, …, s)
where
H(s): Dominance differentiation entropy of protozoa.
Pi: The quantity of organisms of the ith species.
ni: The proportion of the total number of individuals in the entire group, N (Pi = ni/N).
Detrital zircon, as a highly stable heavy mineral, preserves primary U-Pb age signatures of source rocks during sedimentary transport. Systematic analysis of zircon age spectra across structural domains enables reconstruction of provenance evolution. Samples were collected from potential source terrains corresponding to major structural units. U-Pb isotopic analysis was performed via laser ablation inductively coupled with plasma mass spectrometry (LA-ICP-MS) targeting oscillatory-zoned grains free of fractures and inclusions. Analytical conditions included the following: Laser system LSX–213 G2+ (beam diameter: 30 μm) (Teledyne CETAC Technologies: Bozeman, MT, USA), ICP-MS (Agilent 7800) (Agilent Technologies: Santa Clara, CA, USA). Standardization: Plesovice zircon (U-Pb ratios), 91500 zircon, and NIST 610–614 glasses (elemental concentrations). To minimize common Pb contamination, only concordant ages (90–100% concordance) were retained, whereas discordant data (<90% concordance) were excluded. Age calibration employed 207 Pb/206 Pb correction [37,38,39]. Data reduction utilized ICP-MS-Data-Cal for isotopic/elemental calculations and Isoplot 3.0 for concordia diagrams, probability density plots, and weighted mean ages. This protocol complies with international microbeam geochronology standards, ensuring data accuracy and cross-laboratory comparability.
Detrital zircon LA-ICP-MS U-Pb dating was performed on 16 samples from 4 representative wells across structural domains (Northern Steep Slope: B8-2-a, B8-2s-b; Central Uplift: C3-1-a; and Southern Gentle Slope: KL1-1-a). Of the 614 analytical spots, 443 concordant points (concordance > 90%) yielded valid age constraints, representing a reliability rate of 72.1% (Table 2).

3. Results

3.1. Sedimentary Characteristics and Microfacies Types

Previous studies have established multiple classification schemes for beach-bar systems based on compositional characteristics, spatial distribution, provenance conditions, and hydrodynamic regimes [40]. The bar facies can be sub-divided into the following three microfacies: (1) bar core (high energy), (2) bar margin (transitional), and (3) interbar (low energy). Similarly, the beach facies comprise (1) beach core, (2) beach margin, and (3) interbeach, totaling six microfacies types. The vertical succession exhibits the following characteristic stacking pattern: interbar (interbeach) → bar margin (beach margin) → bar core (beach core) → bar margin (beach margin) → interbar (interbeach). Microfacies classification schemes for beach-bar systems vary according to regional depositional environments. The vertical succession-based framework outlined above represents a comprehensive sub-division methodology [41,42].
Integrated analyses of thin-bedded beach-bar sedimentary characteristics in the study area reveals that the E3d2 sandbodies exhibit non-stacked depositional architecture, characterized by thin sand layers (single sandbody thickness < 8 m) interbedded with thick shore-shallow lacustrine mudstones [43]. The absence of bar margin, interbar, and interbeach microfacies supports the following refined four-tier classification scheme: (1) bar core, (2) beach core, (3) beach margin, and (4) shore-shallow lake facies. This scheme aligns with the depositional framework of the study area (Table 3, Figure 3, Figure 4 and Figure 5).
A systematic investigation integrating litho-electric characteristics, seismic responses, and seismic sedimentology was conducted for the Paleogene Dongying Formation’s lower submember (E3d2L). The thin-bedded beach-bar system comprises the following three microfacies: bar core, which are thick sandbodies (> 8 m thickness) with massive bedding and low-angle cross-stratification; beach core, which are laterally continuous thin sandbodies (3 to 8 m thickness) exhibiting wave-ripple cross-bedding; and beach margin, which are marginal wedge-shaped sand accumulations (<3 m thickness) with bioturbation structures. These three dominant sedimentary microfacies exhibit distinct sedimentological and geophysical signatures (Figure 3, Figure 4 and Figure 5), characterized by: lithofacies, which are fining-upward sequences from bar core to beach margin; electrofacies, which are bell-shaped (bar core), funnel-shaped (beach margin) GR log motifs; and seismic attributes, which are high-amplitude continuous reflections for the bar core and discontinuous variable amplitudes for the beach margin.

3.1.1. Bar Core

The bar core microfacies develop in the central lacustrine basin of the study area’s E3d2L submember, comprising predominantly fine-grained, medium–fine-grained, and medium-grained sandstones with minor gravelly sandstone. Its coarser grain size relative to beach sands reflects higher-energy depositional conditions. Vertically stacked thick sandstone beds (>8 m single-layer thickness) intercalated with thin mudstone interlayers form lenticular sandbodies exhibiting flat bases and convex tops in cross-section (Figure 3). Diagnostic sedimentary structures include reverse grading sequences, dominant parallel and massive bedding, minor low-angle cross-bedding, and associated soft-sediment deformation features. These characteristics correspond to box-shaped-to-funnel-shaped and bell-and-finger composite gamma ray log motifs, collectively documenting high-density unidirectional flows punctuated by wave reworking.

3.1.2. Beach Core

The beach core microfacies primarily develop peripherally to bar core deposits, exhibiting vertical symbiosis with bar facies but with reduced thickness. Dominated by thin-bedded fine-grained sandstones and siltstones, it displays a characteristic vertical profile of thick mudstone intervals intercalated with thin sandstone layers. Individual beds range from 3 to 8 m in thickness, typically forming peripheral accumulations around thick bar sandbodies. Where overall sand thickness diminishes, it may constitute independent flat-topped convex lensoidal deposits. Under progressively waning hydrodynamic energy, diagnostic sedimentary structures include low-angle (5–12°) lacustrine cross-bedding, wave ripple lamination, and bioturbation features. These sedimentological signatures correspond to high-amplitude funnel-shaped and finger-shaped gamma ray log motifs (Figure 4), collectively recording wave reworked depositions along bar margins.

3.1.3. Beach Margin

The beach margin microfacies develop peripherally to the bar core, forming sheet-like distributions with reduced sandbody thicknesses compared to the bar core. Dominated by siltstones, argillaceous siltstones, and silty mudstones, its vertical profile comprises thick mudstone intervals interbedded with thin (1 to 3 m) siltstone and calcareous/argillaceous siltstone laminae. These deposits transition outward into typical shallow-lacustrine mudstones under low-energy conditions. Diagnostic sedimentary structures include intense bioturbation, deformation bedding, horizontal laminations, and wave-generated ripple cross-stratification. These features correspond to low-amplitude bell-shaped and micropyramidal gamma ray log motifs (Figure 5), recording deposition by suspension settling punctuated by weak wave reworking.

3.2. The Vertical Combination of Thin-Bedded Beach-Bar System

Integrated sedimentological analysis reveals that the E3d2L submember thin beach-bar system exhibits distinct lateral and planar variations in cross-well profiles and microfacies maps constructed perpendicular to the paleoshoreline. Beach cores display mutual stacking geometries, whereas bar cores exhibit central-thickened/lateral-thinned configurations with pronounced vertical stacking and thickness variability. Laterally continuous thin sandbodies demonstrate remarkable distribution stability, preferentially developing along the central strike-slip zone and central uplift zone. Within the northern central strike-slip zone, lobate-shaped bar cores form thickened sand accumulations, surrounded by banded beach cores and large-area sheet-like beach margin deposits (Figure 6a,b).
Unlike composite beach-bar systems with complex internal architectures, the HHK Depression succession lacks multiphase superposition characteristics. Its simple vertical organization comprises two end-member types: Thin-layer stable types, which are shore-shallow lacustrine thin sandbodies (3 to 8 m single-layer) bounded by thick mudstones (>100 m cumulative thickness), forming “thin-sand/thick-mud” alternations. Thick-thin superposition types, which are interbedded thick sandbodies (>8 m average thickness) and thin sands (3 to 8 m) containing mudstone interlayers, capped by variably thick shore-shallow lacustrine muds (Figure 6c).

3.3. Genetic Analysis of Sandbodies

The thin-layer beach-bar system within the E3d2L submember of the HHK Depression is comprehensively controlled by paleoclimate, paleowater depth, tectonic activity, and provenance supply. Its unique sedimentary processes under specific geological conditions result in diagnostic characteristics of an independent thin-bedded beach-bar system.

3.3.1. Palaeoclimate

Palynomorph assemblage analysis (Table 3) enabled paleoenvironmental reconstruction using the following key indicators: (1) paleotemperature (thermophilic/mesothermic/eurythermic taxa), (2) paleohumidity (hydrophilic gymnosperms/angiosperms vs. xerophytes), and (3) paleosalinity inferred from algal assemblages (brackish-saline/brackish-freshwater/freshwater algae) [43,44]. Quantitative percentage statistics and ratio histograms document temporal shifts in thermal–mesothermic climates, humid–xerophytic conditions, and saline–freshwater environments (Figure 7).
Integrating diagnostic palynological signatures reveals a subtropical climate transitioning from dry–hot to warm–humid. A secondary climatic oscillation occurred during the Zone III~V palynozone transition of E3s1 to E3d1 depositional period, characterized by freshwater algal proliferation indicating lake dilution, increased hydrophilic angiosperm/gymnosperm ratios, and expanded thermophilic flora. This event culminated in peak paleotemperature/humidity followed by cooling, reflecting a shift from subtropical warmth to cooler–humid conditions (Figure 7).

3.3.2. Paleobathymetry

Climatic cycles exerted fundamental control on Paleogene lake-level fluctuations in the HHK Depression. During E3d3 to E3d2L deposition, progressive lake deepening occurred overall. We applied an integrated paleobathymetry restoration method utilizing trace element geochemistry and paleontological proxies [36,37,38,39,43]: (1) lake-depth zonation was determined via Fe/Mn ratios (mean 89.4; range 72.4–103.1), consistently indicating shallow lacustrine conditions throughout the Dongying Formation; (2) ostracod assemblage differentiation was quantified using established equations, with water depths calibrated against reference depth–differentiation relationships (Table 4); (3) paleobathymetric mapping revealed mean depths of 0 to 30 m for the Shahejie Formation and 0 to 40 m for the Dongying Formation, representing a 10 m basin-ward deepening trend, with progressive deepening from E3d3 to E3d2L. Spatial analysis demonstrates the following: Maximum depths (30~50 m) in northwestern/southwestern sub-depressions. Intermediate depths (10~25 m) along northern/southern slope zones (Figure 8a). Shallowest depths (10~20 m) in central strike-slip and uplift zones (Figure 8b).
Beach-bar sands preferentially accumulated in wave-agitated shallow waters (<25 m) across central strike-slip/uplift zones and the western northern steep slope. Transgressive system tract development during lake expansion generated rapid accommodation increase while diminishing sediment supply [45], facilitating shore-shallow lacustrine sedimentation (Figure 8b). Wave-reworked deltaic sands were redistributed as beach-bar deposits, forming laterally extensive but vertically isolated sandbodies. The central strike-slip/uplift zones—characterized as underwater structural highs with shallow bathymetry and enhanced wave energy—developed extensive thin beach-bar systems in these shallow lacustrine settings (Figure 8c).

3.3.3. Tectonic Activity

Tectonic activity fundamentally controls sedimentary system evolution and sandbody distribution in the HHK Depression [46,47]. The Paleogene structural framework features a “steep northern margin, gentle southern slope, western deep depression, eastern shallow platform, with central uplift” configuration (Figure 1). The western segment of the intra-basin Tan-Lu strike-slip fault generated NNE-trending master faults and EW-oriented branching faults, sub-dividing the depression into the following: northern steep slope (eastern/western segments); southern slope (eastern/western segments); central strike-slip zone; central uplift zone; northwestern sub-depression; southwestern sub-depression; and eastern depression (Figure 1c).
Seismic stratigraphic interpretation reveals listric master faults forming synthetic/antithetic “Y”-shaped, graben–horst, and step-like fault assemblages (Figure 2). Right lateral strike-slip motion produced en echelon arranged linear master faults along the central structural belt, with EW-trending subsidiary faults emanating from the central uplift (Figure 8c).
Kinematic analysis [39] demonstrates that E–W normal faults and NNE strike-slip faults jointly govern beach-bar sandbody distribution. During the Dongying Formation deposition (rift-depression transition phase), fault reactivation intensified tectonic activity. Paleogeomorphic reconstructions and seismic profiles (T3 to T3U) delineate secondary structural units (Figure 8c), dominated by the following two fault sets: NNE-trending throughgoing faults controlling deep architecture and strike orientation, and E–W faults modulating local relief. The NE–SW-trending basin exhibits a “southern high-northern low, central uplift-peripheral sag” geometry [48].
Tectonism drove paleotopographic development, directly influencing sandbody distribution. From E3d3 to E3d2ᴸ, the central strike-slip and uplift zones experienced sustained uplift, creating trough-margin highs. The central strike-slip zone formed an underwater structural high, while the central uplift zone partitioned northern/southern provenance areas. During E3d2L deposition, the paleotopographic relief (time-domain) ranged around 100~500 ms TWT. Despite significant uplifts in central zones, the northwestern sub-depression accumulated thicker sediments in topographically low settings. Beach-bar sands preferentially developed on structurally elevated trough-margin highs (central strike-slip/uplift zones), correlating with stratigraphic thinning in central troughs versus peripheral depressions (Figure 8c,d).

3.3.4. Provenance Supply

Detrital zircon LA-ICP-MS U-Pb dating was conducted on thin sandstone layers of the Dongying Formation to compare zircon age discrepancies in target strata across the study area [49], delineate sediment provenance zonation, and clarify sediment supply sources for beach-bar deposits in the central uplift zone. Integrated analysis of zircon age distributions from five wells (E3d2L, E3d3) reveals similar age spectra, with U-Pb ages ranging from 65 to 3850 Ma, predominantly clustered in the following three intervals: 65~135 Ma, 500~1000 Ma, and 1800~3100 Ma. Weighted mean ages for these intervals are 137.3 Ma, 689 Ma, and 2339 Ma, respectively (Table 5; Figure 9).
The southern gentle slope zone (Well KL1-1-a; Figure 9c) and northern steep slope zone (Wells B8-2-a, B8-2s-b; Figure 9b) exhibit pronounced age peaks across all three intervals. In contrast, the central uplift zone (Well C3-1-a; Figure 9a) displays dominant peaks at 500–1000 Ma and 1800–3100 Ma. Comparative analysis of zircon age distributions among the central uplift zone, southern provenance (Well KL1-1-a), and northern provenance (Wells B8-2-a, B8-2s-b) demonstrates consistent age trends for Mesozoic, Paleozoic, Neoproterozoic (Pt3), and Mesoproterozoic/Paleoproterozoic/Neoarchean (Pt2/Pt1/Ar2) zircon populations.
Zircon age signatures in the E3d2L suggest that mixed provenance contributed from northern and southern sources to beach-bar deposits in the intra-depression uplift zones (central uplift and strike-slip zones). Paleogeomorphic analysis further indicates (Figure 9d) the following: The western segment of the northern steep-slope zone features abrupt topography with limited sediment transport and minor wave reworking. The eastern segment of the northern steep-slope zone, connected to the central strike-slip fault zone, exhibits gentler gradients and enhanced sediment delivery to beach-bar systems. The southern gentle-slope zone hosts large-scale braided river deltas, providing extensive and stable sediment supply for beach-bar development.

4. Discussion

Genetic mechanism analysis indicates a subtropical climate during stable beach-bar deposition in the E3d2L of the HHK Depression, with a climatic shift from dry–hot to warm–humid from E3s2 to E3d2L. Under the persistent extension strike-slip tectonic stress field, increased fault activity rates in the Dongying Formation, sustained intra-depression uplift, gradual water deepening, sufficient local sediment supply from delta fronts, and enhanced wave reworking collectively promoted beach-bar development. The late lacustrine flooding event in E3d2L facilitated beach-bar preservation, forming extensive thin-bedded (<5 m), laterally continuous beach-bar sandbodies within uplifted zones.
Paleoenvironmental evolution and secondary fluctuations provided unique depositional conditions. Tectonic controls manifest in the following three key aspects: (1) sandbody distribution constrained by sub-tectonic units; (2) sandbody thickness modulated by differential fault displacement; (3) delta-front sheet sand dimensions dependent on regional provenance supply, while sandbody enrichment is governed by local reworking processes. The coupling of these factors establishes the genetic model for thin stable beach-bar deposits in the HHK Depression. This genetic model exhibits significant differences from conventional lacustrine beach-bar systems, as contrasted below (Table 6).
Integrated studies of paleoclimate, paleobathymetry, tectonics, and provenance support a braided-delta beach-bar sedimentary model under strike-slip faulting (Figure 10). This model synthesizes spatial sediment distribution patterns, paleoclimatic constraints, and lacustrine paleogeomorphology, objectively delineating controls on beach-bar distribution. Spatiotemporal analysis reveals the following distinct evolutionary phases: During the Shahejie Formation deposition, ample sediment supplies developed fan-delta to braided-delta systems extending southward to the central structural ridge, forming interconnected depositional systems with southern braided deltas [50,51].
In the Dongying Formation, weakened northern sediment supply and gentler slopes promoted braided-delta beach-bar systems throughout the depression. Sub-tectonic units exerted primary control, with the northern steep slope (eastern/western segments) and southern slope (eastern/western segments) being braided-delta shallow lacustrine systems, and the central strike-slip and uplift zones being shore-shallow lacustrine beach-bar systems.
This study reveals the compartmentalization effects of sub-tectonic units (central strike-slip/uplift zones) generated by strike-slip faults on beach-bar sandbody distribution, challenging conventional “tectonic slope-break control” paradigms. Through integrated provenance and paleogeomorphic analysis, we establish a “dual-provenance-paleotopography coupling model” that elucidates thin-layer beach-bar enrichment mechanisms. Notably, synergistic interactions between lacustrine flooding events and warm–humid transitional climates provide unique preservation conditions for thin beach-bars, distinguishing them from arid climate analogs. The model offers predictive insights for shallow water beach-bar exploration in strike-slip extensional basins globally.

5. Conclusions

The lower submember of Member 2, Dongying Formation (E3d2L), in the HHK Depression contains a thin-bedded beach-bar system characterized by thin sandbodies (3 to 8 m thickness), limited stratigraphic layers (1~2 layers), and exceptional lateral continuity. The following three sedimentary microfacies are identified within this system: bar-core facies exhibiting > 8 m single-layer thickness, beach-core facies with 3 to 8 m thick sands, and beach-margin facies comprising < 3 m accumulations. These microfacies display two distinct vertical stacking patterns, the “Thin-layer Stable Type” featuring thick mudstone intervals intercalated with isolated sandstone layers, and the “Thick-Thin Superposition Type” characterized by alternating thick and thin sand units.
The development of this unique depositional system is governed by synergistic interactions among four fundamental controls. A subtropical paleoclimate regime shifting from dry–hot to warm–humid conditions—marked by secondary fluctuations during the Zone III~V palynofacies transition—established the essential environmental foundation. Concurrently, paleobathymetric analysis confirms prevailing shallow lacustrine conditions (mean Fe/Mn = 89.4), with optimal beach-bar deposition concentrated in the 10~20 m depth range of the central strike-slip and uplift zones. Tectonically, strike-slip extensional stress partitioned sub-units through sustained uplift, generating fault-controlled intra-depressional highs that dictated sandbody distribution patterns. Provenance constraints from detrital zircon U-Pb age spectra—displaying diagnostic peaks at 137 Ma, 689 Ma, and 2339 Ma—further validate a dual mixing system where braided river deltas along the southern gentle slope supplied substantial sediments.
Key innovations emerge from this integrated analysis, most notably the establishment of a novel sedimentary model for thin beach-bars in strike-slip extensional settings. This framework challenges conventional “tectonic slope-break control” paradigms by proposing a “Strike-slip Partitioning + Paleotopography + Climate Transition Synergy” coupling mechanism. The model demonstrates that intra-depressional highs (central strike-slip/uplift zones) function as primary depositional loci, while synergistic preservation driven by late E3d2L lacustrine flooding coinciding with warm–humid climatic transition enables unprecedented large-scale deposition of thin-bedded systems.
Collectively, these findings deliver significant multidisciplinary implications. Theoretically, this work provides a genetic paradigm for lacustrine, thin beach-bar formation, advancing sedimentological understanding in strike-slip basins and refining beach-bar classification schemes. For petroleum exploration, it identifies central strike-slip/uplift zones as high-potential targets, strategically shifting exploration paradigms from “thick-focused” to “thin-widespread” lithologic reservoir plays. Methodologically, the integration of palynological climate proxies, zircon provenance tracing, and quantitative paleogeomorphic restoration establishes a transferable analytical framework applicable to depositional systems in complex tectonic settings globally.

Author Contributions

Conceptualization, J.W. and Y.H.; methodology, H.L. and B.F.; software, X.Y., X.H., B.F. and Z.Z.; validation, H.L., X.Y. and X.H.; formal analysis, X.Y. and X.H.; investigation, Y.H., H.L., J.W. and B.F.; resources, Z.Z., B.F., X.Y. and X.H.; data curation, X.Y.,X.H. and B.F.; writing—original draft preparation, J.W.; writing—review and editing, J.W. and Y.H.; visualization, Y.H., H.L., J.W. and Z.Z.; supervision, Y.H., H.L. and J.W.; project administration, H.L. and Y.H.; funding acquisition, H.L. and Y.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded in part by a grant from the National Key Technologies R & D Program of China during the 14th Five–Year Plan Period (KJGG2022–0303, No. CCL2022TJT0NST1867) and The National Natural Science Foundation of China (grants No. 42272115 and 42272113).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

We would like to extend our gratitude to CNOOC (China) Tianjin Branch Bohai Petroleum Research Institute for their invaluable administrative and technical support, which facilitated the completion of this study. Additionally, we appreciate the kind donations of experimental materials provided by the School of Geosciences, Yangtze University, without which the research would not have been possible.

Conflicts of Interest

Author Bin Feng is employed by the Changqing Division, China Petroleum Logging Co., Ltd. Author Xing Yu is employed by the China United Coalbed Methane Corporation, Ltd. Author Xiangyang Hou is employed by the PetroChina Tuha Oilfield Company. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Regional structural location and sequence stratigraphic column of the HHK Depression (The term “HHK Depression” is a pseudonym used under project confidentiality requirements). (a) Regional location within the Bohai Bay Basin; (b) regional structural location of the HHK Depression; (c) structural sub-division within the HHK Depression; (d) composite sequence stratigraphic column of the Paleogene in the HHK Depression.
Figure 1. Regional structural location and sequence stratigraphic column of the HHK Depression (The term “HHK Depression” is a pseudonym used under project confidentiality requirements). (a) Regional location within the Bohai Bay Basin; (b) regional structural location of the HHK Depression; (c) structural sub-division within the HHK Depression; (d) composite sequence stratigraphic column of the Paleogene in the HHK Depression.
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Figure 2. Representative three-dimensional seismic reflection profile across the HHK Depression (AA′: Nearly south-north reflection seismic profile; BB′: Nearly west-east reflection seismic profile, location see Figure 1c).
Figure 2. Representative three-dimensional seismic reflection profile across the HHK Depression (AA′: Nearly south-north reflection seismic profile; BB′: Nearly west-east reflection seismic profile, location see Figure 1c).
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Figure 3. Core facies, lithology, and electrical properties of the bar core in the E3d2L submember, HHK Depression.
Figure 3. Core facies, lithology, and electrical properties of the bar core in the E3d2L submember, HHK Depression.
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Figure 4. Core facies, lithology, and electrical properties of the beach core in the E3d2L submember, HHK Depression.
Figure 4. Core facies, lithology, and electrical properties of the beach core in the E3d2L submember, HHK Depression.
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Figure 5. Core facies, lithology, and electrical properties of the beach margin in the E3d2L submember, HHK Depression.
Figure 5. Core facies, lithology, and electrical properties of the beach margin in the E3d2L submember, HHK Depression.
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Figure 6. Spatial distribution and vertical stacking patterns of beach-bar sandbodies in the E3d2L submember, HHK Depression. (a) Sandbody correlation well-tie profile; (b) depositional microfacies map; (c) vertical stacking characteristics diagram.
Figure 6. Spatial distribution and vertical stacking patterns of beach-bar sandbodies in the E3d2L submember, HHK Depression. (a) Sandbody correlation well-tie profile; (b) depositional microfacies map; (c) vertical stacking characteristics diagram.
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Figure 7. Paleoclimatic evolution characteristics of the HHK Depression during the Paleogene. (a) Paleosalinity change curve; (b) paleohumidity change curve; (c) paleotemperature change curve.
Figure 7. Paleoclimatic evolution characteristics of the HHK Depression during the Paleogene. (a) Paleosalinity change curve; (b) paleohumidity change curve; (c) paleotemperature change curve.
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Figure 8. Paleowater depth and paleogeomorphology of the HHK Depression during the Paleogene. (a) Paleobathymetry characteristics of the Shajie Formation; (b) paleobathymetry characteristics of the E3d2L submember; (c) paleomorphological recovery map of the E3d2L submember; (d) contour map of stratigraphic thickness of the E3d2L submember.
Figure 8. Paleowater depth and paleogeomorphology of the HHK Depression during the Paleogene. (a) Paleobathymetry characteristics of the Shajie Formation; (b) paleobathymetry characteristics of the E3d2L submember; (c) paleomorphological recovery map of the E3d2L submember; (d) contour map of stratigraphic thickness of the E3d2L submember.
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Figure 9. Detrital zircon age spectrum of the Dongying Formation in the HHK Depression. (a-1) C3-1-a, 3450; (a-2) C3-1-a, 3515; (b-1) B8-2-a, 3165; (b-2) B8-2s-b, 3190; (c-1) KL1-1-a, 2670; (c-2) KL1-1-a, 2775; (d) histogram and moving average trendline of zircon age distribution.
Figure 9. Detrital zircon age spectrum of the Dongying Formation in the HHK Depression. (a-1) C3-1-a, 3450; (a-2) C3-1-a, 3515; (b-1) B8-2-a, 3165; (b-2) B8-2s-b, 3190; (c-1) KL1-1-a, 2670; (c-2) KL1-1-a, 2775; (d) histogram and moving average trendline of zircon age distribution.
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Figure 10. Development models of sandbodies of the Palaeogene in the HHK Depression, Bohai Bay Basin. (a) Fan delta and braided river delta systems of the Shajie Formation; (b) braided river delta and beach-bar system of the Dongying Formation.
Figure 10. Development models of sandbodies of the Palaeogene in the HHK Depression, Bohai Bay Basin. (a) Fan delta and braided river delta systems of the Shajie Formation; (b) braided river delta and beach-bar system of the Dongying Formation.
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Table 1. Paleocene spore–pollen assemblage zones in the HHK Depression.
Table 1. Paleocene spore–pollen assemblage zones in the HHK Depression.
Climate ClassificationPollen AssemblageClimateStrata
VIIGymnosperms Pinus (two-needled pine)–Pinus (single-needled pine)–TsugaWarm and humidE3d1 + E3d2
AngiospermsJuglans–Carya, with an increase in Juglans
VIGymnosperms Pinus (two-needled pine)–Pinus (single-needled pine)–Tsuga–PiceaWarm and humidE3d2
AngiospermsJuglandaceae–Juglans–Ulmus, with an increase in Juglans
VGymnosperms Pinus (two–needled pine) and Pinus (single–needled pine) are relatively reduced, while the Cedrus genus is developingTemperate and humid E3d2 + E3d3
AngiospermsWalnut–Carya–Ulmus, with an increase in Tilia
IIIGymnosperms Pinus (two-needled pine)–Tsuga–PiceaSubtropical and warmE3d3 + E3s1
AngiospermsAcer–Quercus prinoides–Ulmus, with significant increases in Ulmus wislizeni, Quercus, and small-leaved Ulmus
IIGymnosperms Pinus (two-needled pine) continues to develop, the quantity of Tsuga increases, and the genus Ephedra shows significant growthSubtropical and aridE3s2 + E3s1
AngiospermsSmall Heinz oak powder–Quercus–Ulmus–smaller elm, with an increase in Liquibarbar and Rutaceae
IGymnosperms Pinus (two–needled pine)–Pinus (single–needled pine)–Gap pine powderSubtropical and aridE2s3 + E3s2
AngiospermsQuercus (medium oak)–Quercus (small Heinrich’s oak)–Quercus (small oak)–Ulmus (elm)
Table 2. Statistical Information of Detrital Zircon LA-ICP-MS U-Pb Dating Samples.
Table 2. Statistical Information of Detrital Zircon LA-ICP-MS U-Pb Dating Samples.
WellDepth Range/mSampling Depth/mNumber of Concordant PointsValid Data (Concordance > 90%)Percentage
C3-1-a34503450936165.6%
351535151098073.4%
KL1-1-a26702670927076.1%
277527751056461.0%
B8-2-a3164–31673165926772.8%
B8-2s-b3190–3195319012310182.1%
Table 3. Identification chart for beach-bar system microfacies in the E3d2, HHK Depression.
Table 3. Identification chart for beach-bar system microfacies in the E3d2, HHK Depression.
Sedimentary CharacteristicsBar CoreBeach CoreBeach Margin
LithologyMedium-to-fine sandstoneSiltstone and argillaceous siltstoneSiltstone, silty mudstone, and mudstone
Thickness>8 m3 to 8 m<3 m
Textural MaturityModerate (moderately sorted; moderate to good roundness)Moderate (moderately sorted; finer grain size)Moderate–low (poorly sorted)
Sedimentary StructuresParallel bedding, tabular cross-bedding, and wavy cross-beddingHorizontal bedding, wavy bedding, and small-scale cross-beddingLenticular bedding, bioturbation structures, and wavy bedding
Paleogeomorphic PositionPaleogeomorphic highsRelatively gentle paleogeomorphic highsMargins of relatively gentle paleogeomorphic highs
Distribution PatternLocalized, lobate, or irregular ellipticalSheet-like or elongated banded around the bar coreContiguous distribution around the beach core
Logging ResponseBlocky/box-shaped and compositeFunnel-shaped or finger-shapedFinger-shaped
Hydrodynamic ConditionsStrongModerateWeak
Typical WellsB8-2-aC4-2-aC4-3-a
Table 4. Characteristics of paleobathymetry in the HHK Depression during the Paleogene.
Table 4. Characteristics of paleobathymetry in the HHK Depression during the Paleogene.
FormationWellFe/MnH (s)Paleobathymetry (m)FormationWellFe/MnH (s)Paleobathymetry (m)
E3s1B7-2-c96.91.12717E3d2C4-9-d293.691.38627.7
E3s1B6-2-bD96.751.12117.5E3d2B3-3-a91.921.6433.6
E3s1C4-9-d94.471.3725.1E3d2C4-9-f92.371.6132.1
E3s1C4-9E-a95.971.2220.1E3d2C4-9E-a91.021.7336.6
E3s2C4-9-e99.930.566.9E3d3B6-5-a92.011.6333.3
E3s2B7-2-c96.781.0917.4E3d3C4-9-d93.501.4628.33
E3s2C4-9E-a97.920.9613.6E3d3C4-9-e93.991.3826.7
Table 5. Statistical Analysis of Detrital Zircon U-Pb Age Populations Determined by LA-ICP-MS.
Table 5. Statistical Analysis of Detrital Zircon U-Pb Age Populations Determined by LA-ICP-MS.
WellSampling Depth/mZircon Age Range (Ma)Geological Age
CenozoicMesozoicPaleozoicPt3Pt2/Pt1/Ar2
C3-1-a3450––570–10001800–31000001150
3515––500–10001800–31000051659
KL1-1-a267065–250570–10001800–3850007954
2775135–250570–10001800–3100021952
B8-2-a316565–250570–10001800–31000351445
B8-2s-b319065–250500–8001800–310001121573
Table 6. Comparative characteristics of beach-bar systems.
Table 6. Comparative characteristics of beach-bar systems.
FeatureHHK E3d2ᴸ Beach-Bar SystemConventional Lacustrine Beach-Bar Systems
Climate ContextDry–hot → Warm–humid transitionArid-dominated conditions
TectonicsStrike-slip extensional tectonic frameworkSimple fault-slope control
Sandbody DistributionThin-bedded (3 to 8 m) and laterally extensiveThick–thin interbedded architecture, frequent vertical stacking, and rapid lateral variation
ProvenanceDual sources with paleotopographic couplingSingle dominant source
InnovationStrike-slip partitioning + Paleotopography + climate transition synergyTectonic–sedimentary binary control
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Wang, J.; He, Y.; Li, H.; Feng, B.; Zhao, Z.; Yu, X.; Hou, X. The Genesis of a Thin-Bedded Beach-Bar System Under the Strike-Slip Extensional Tectonic Framework: A Case Study in the Bohai Bay Basin. Appl. Sci. 2025, 15, 7964. https://doi.org/10.3390/app15147964

AMA Style

Wang J, He Y, Li H, Feng B, Zhao Z, Yu X, Hou X. The Genesis of a Thin-Bedded Beach-Bar System Under the Strike-Slip Extensional Tectonic Framework: A Case Study in the Bohai Bay Basin. Applied Sciences. 2025; 15(14):7964. https://doi.org/10.3390/app15147964

Chicago/Turabian Style

Wang, Jing, Youbin He, Hua Li, Bin Feng, Zhongxiang Zhao, Xing Yu, and Xiangyang Hou. 2025. "The Genesis of a Thin-Bedded Beach-Bar System Under the Strike-Slip Extensional Tectonic Framework: A Case Study in the Bohai Bay Basin" Applied Sciences 15, no. 14: 7964. https://doi.org/10.3390/app15147964

APA Style

Wang, J., He, Y., Li, H., Feng, B., Zhao, Z., Yu, X., & Hou, X. (2025). The Genesis of a Thin-Bedded Beach-Bar System Under the Strike-Slip Extensional Tectonic Framework: A Case Study in the Bohai Bay Basin. Applied Sciences, 15(14), 7964. https://doi.org/10.3390/app15147964

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