Integrated Analysis of Tectonic Evolution and Hydrocarbon Potential in the Aonan Sag, Northern Songliao Basin: Insights from Rift-Phase Structural Controls

Abstract

In the northern part of the Songliao Basin, the Aonan Depression is a significant structural feature. It plays a crucial role in the relationship between tectonic evolution and deep natural gas accumulation mechanisms. While previous studies have revealed faulting and sedimentary characteristics in the region, the precise connection between tectonic evolution and hydrocarbon accumulation, particularly for deep natural gas, has not been systematically studied. This research uses structural modeling and stratigraphic restoration to reconstruct the tectonic evolution of the third section of the Shahezi Formation and develops a 3D geological model of the study area. The results show that the tectonic evolution of the Aonan Depression can be divided into three stages: rifting, sagging, and inversion. Each stage significantly controls hydrocarbon distribution. A self-sourced, self-storage gas system formed in the Shahezi Formation, while the Denglouku and Yingcheng formations exhibit a source–reservoir combination. The study reveals how tectonic evolution influences hydrocarbon accumulation and distribution on a regional scale. Based on these findings, a “rift-controlled source, sag-controlled storage, and structure-controlled trapping” accumulation model is proposed, providing new insights into tectonic evolution and hydrocarbon accumulation mechanisms, with important implications for similar basins’ exploration and structural studies.

1. Introduction

The Songliao Basin, located in the Meso-Cenozoic rift zone, exhibits a complex structural framework shaped by multiple tectonic processes [1]. The Aonan Sag is a key target in this region, because the evolution of rift-stage tectonics is crucial for evaluating the hydrocarbon potential of deep strata [2,3]. Most studies on the accumulation mechanisms in deep-basin hydrocarbon systems focus on the post-rift phase, which includes fault-sealing and sedimentary facies analysis [4,5]. However, there are still gaps in the understanding of the syn-rift controls related to these systems [6,7].

We established a structural framework that accurately characterizes the deep rift sedimentary sequence of the Aonan Sag. Using balanced cross-section analysis, fault activity assessment, and 3D structural modeling, we restored the paleostructure of the third member of the Shahezhi Formation (K₁sh₃). The study focuses on the tectonic evolution of the Aonan Sag since the rifting phase, clarifying how tectonic evolution controls the formation and transformation of hydrocarbon migration pathways.

2. Regional Geological Setting

An area of 385 km² is covered by the Aonan Sag, which is situated in the depression belt of the northern Songliao Basin (Figure 1a).

The Aonan Sag’s formation and evolution are controlled by the Songliao Basin’s overall tectono-sedimentary cycles. During the Late Paleozoic to Early Mesozoic, the basin lay at the northern margin of the North China Plate, southeastern edge of the Siberian Plate, and western side of the Jiamusi-Bureya Block, within a compressional orogeny zone, and the closure of plate boundaries in the Late Permian to Early Triassic formed a unified continental basement. From the Late Jurassic, regional extension initiated NE-trending rift basins with widespread normal faults in rift-fill strata; starting with the Denglouku Formation (K1d), the basin entered a subsidence phase evolving into fault-controlled sag basins, with a identified fault-sag transition stage featuring sedimentary filling with both rift and sag properties [8,9]. The basin gradually infilled with expanding sedimentary coverage, and early normal faults’ influence diminished. From the Late Cretaceous (≈84–66 Ma), a shift in the subduction mode of the Pacific Plate beneath the Eurasian Plate caused a regional transition from extension to compression. This tectonic inversion led to local reversal of early normal faults within the Aonan Sag, forming small-scale thrust structures (Figure 1b) [10].

The syn-rift lithostratigraphic units are defined as the sedimentary units deposited during the rifting phase of basin evolution. These units include the Early Cretaceous Huoshiling Formation (K₁h), Shahezi Formation (K₁sh), and Yingcheng Formation (K₁yc), which consist of interbedded pyroclastic rocks, clastic sediments, and coal seams. The post-rift lithostratigraphic units are defined as the sedimentary units deposited during the sag phase following rifting in basin evolution. These units include the Denglouku Formation (K₁d), Quantou Formation (K₁q), Qingshankou Formation (K₂qn), Yaojia Formation (K₂y), and Nenjiang Formation (K₂n), which overlie the syn-rift stratigraphy.

The Aonan Sag preserves a complete “rift-sag” depositional sequence. During the rift phase, the basin was primarily controlled by NNE-trending normal faults. The overall tectonic framework consists of an E-W graben (double-fault structure) and N-S half-graben (sag-type faulting). During this stage, structural highs and depositional centers migrated along the strike of the western controlling faults. Sedimentation mainly occurred on the hanging wall of the western faults. The structural highs were generally located in the northeast, controlled by the underlying ancient uplift and the obstructing northeastern boundary faults.

3. Methods

By constructing a 3D structural model based on the balanced section approach, the tectonic evolution of the target stratigraphic interval in the study area was dynamically and systematically reconstructed. Fundamental stages including data acquisition, model construction, evolutionary tracking, and result validation were embraced in the research methodologies (Figure 2), thereby facilitating precise modeling and multi-scale analysis of tectonic evolution.

Compared with traditional single methods, limitations in inversing geological structure evolution are mitigated by multi-disciplinary integration. Regional geological history is reconstructed by characterizing structural stages and key events, corroborating the coupling of tectonic evolution and hydrocarbon accumulation. A precise framework for predicting energy accumulation zones is established, and new insights into tectonic controls on hydrocarbon entrapment are offered by this approach, providing an innovative solution for exploring oil–gas potential in complex geological settings.

3.1. Balanced Section Method

The balanced section method, which adheres to the principle that material amounts remain constant, employs fundamental geometry and computer tools to return distorted rock layers to their original shapes. First presented by Chamberlain [11] and refined by Dahlstrom [12], this method is currently used by oil geologists to investigate subsurface features in sedimentary basins. It functions by adhering to four key principles: area conservation, compression balancing, motion alignment, and rock layer length constancy. To determine the occurrence of geological changes, two primary techniques—forward modeling and backward inversion from available data—are employed [13].

A layer-by-layer methodology was applied to analyze sedimentary strata deformation, integrating three primary datasets: sedimentary facies, borehole lithology, and 3D seismic imaging. Erosion volumes and fault kinematics are calculated by the inversion method, while original stratigraphic geometries are reconstructed by the forward method. Precise modeling of complex deformation stages is enabled by this integration. Through the synthesis of multi-source geological data, a robust framework for understanding basin evolution and sedimentation patterns is provided by this technique.

3.2. Key Technologies for the Restoration Process

3.2.1. The Amount of Erosion Has Recovered

The stratigraphy of the study area is complete in terms of its vertical succession. But some strata, particularly in the K₁sh₃ and K₁sh₄ intervals, have experienced significant erosion. The seismic reflections of the stratigraphy from the syn-rifting phase of the Aonan Sag are continuous and stable. Based on this, the stratigraphic thickness analysis method was employed to restore the erosion amounts. The core logic of this method is to calculate the erosion amount by subtracting the present thickness from the original thickness. Seismic profile interpretation results are used to identify unconformities and the extension trends of eroded strata. The erosion thickness is determined by comparing the thickness of the residual strata with adjacent layers. In subsequent studies, the accuracy of the erosion restoration will be improved by integrating compaction correction and tectonic evolution analysis [14,15].

(A′) and external (A) are erosion reference points. Thinning rate (K) and erosion magnitude (H_b) were calculated as below:

A stable stratigraphic thickness reference point B (Figure 3a), where H = thickness at A′, H′ = thickness at B, and L = distance between A′ and B. Thinning rate formula:

K = (H − H′)/L × 100%

(1)

Along the eroded segment, measurement points (C_i) were set from A′, with L_i = distance from A′ to C_i. Original thickness at C_i:

H_i = H − K × L_i

(2)

Erosion magnitude:

H_b = Hi − h_i

(3)

Calculated H_b values were spatially interpolated to generate erosion distribution maps, visualizing erosion intensity gradients and linking them to tectonic activities [16]. Seismic thickness results were validated against acoustic travel-time methods [17], ensuring reliability. Multi-method integration strengthens tectonic interpretation accuracy.

Figure 3. (a) Schematic diagram of stratigraphic erosion quantification using thickness trend [14]. (b) Schematic diagram of sandstone compaction processes [18].

Figure 3. (a) Schematic diagram of stratigraphic erosion quantification using thickness trend [14]. (b) Schematic diagram of sandstone compaction processes [18].

3.2.2. De-Compaction Correction

During burial, increasing overburden reduces stratal porosity and thickness (Figure 3b). Basin-wide burial depth variations cause differential compaction, necessitating decompaction correction [18,19]. Decompaction restores paleothickness by assuming constant framework volume, with porosity–depth relationships described by:

φ = φ₀e^−CH

(4)

where H represents the depth, φ represents the porosity at depth H, φ₀ represents the porosity at the surface, and C represents the compaction coefficient.

For irreversible compaction with constant framework volume, let Z and Z′ denote current and decompacted burial depths of the stratal top surface, and T, T′ the corresponding thicknesses. The depth–porosity function φ(x) defines the relationship:

$${\int }_Z^{Z+T}\left[1-\phi \left(x\right)\right]dx={\int }_Z^{z^{\prime }+T^{\prime }}\left[1-\phi \left(x\right)\right]dx$$

(5)

The compaction coefficient is:

C = (T − T′)/T′

(6)

Original thickness reconstruction involves the following: compaction profiles are calculated from multi-well data to define target formation compaction trends, extrapolated to regional scales, and compared with current thicknesses [20,21].

3.2.3. Fault Removal and Fold Removal Recovery

To restore paleogeological features, distortions from faulting and folding are first removed. The research area is dominated by normal faults and connected fold systems. During modeling, flexural slip restoration and oblique shear analysis are used to characterize these structures. Stratal position shifts refer to the vertical and horizontal displacement of strata caused by faulting, primarily due to normal faults in the study area. These movements are reversed in the restoration process to remove the effects of faulting.

Prior work by Gibbs, Withjack, and Peterson is built on by the oblique shear method to model hanging-wall stratal displacement above faults. Based on measured fault displacement, 3D stratal movement along fault surfaces is tracked, and this motion is reversed to remove faulting effects [22,23]. This reversal is applied only to hanging-wall rocks, maintaining constant rock volume (Figure 4a) [24,25].

Fold effects are removed by flexural slip via simultaneous target/template layer restoration. The rock bundle is treated as a sliding system with layers divided into an equal-thickness parallel slices. Lower slices are adjusted using template layer thickness as a reference during flattening, keeping reference plane positions unchanged, while upper layers spread from this plane to maintain constant template area (Figure 4b).

3.3. 3D Structural Modeling Technology

3D structural modeling, a core component of geoscience modeling, is utilized to generate digital representations of subsurface structures. Geological and geophysical data are integrated to visualize geological bodies, fault networks, and layer surfaces, with spatial relationships accurately represented. These models are essential for basin evolution reconstruction, hydrocarbon trap quantification, and geological risk prediction.

The modeling process is implemented through a point–line–plane approach [28]. Data are first collected and preprocessed, with horizon closure ensured through structural analysis and fault interpretation. Time-domain profiles are converted to depth domain using interval velocity fields, and point data are resampled into triangular meshes. Finally, paleostructural arrangements are established through tectonic restoration, with model accuracy verified through comprehensive testing (Figure 5).

4. Results

Tectonic restoration in the research area involves denudation quantification, decompaction, fault removal, and fold removal. The assumptions made for the erosion restoration include treating sediment deposition as isopachous, with the eroded thickness estimated based on the remaining stratigraphic thickness. This assumption is grounded in the principle that sediment deposition is relatively uniform across the basin, as supported by regional geological observations. The decompaction parameters were derived from porosity data obtained from well logs in the study area. Fault and fold restoration relied on the reliability of structural interpretation results, which were further validated using flexural slip and oblique shear methods for 2D cross-section restoration. These results were cross-verified with 3D structural restoration, and consistent trends in overlapping areas confirm the reliability of the restoration outcomes.

4.1. The Restoration Results of 3D Structural Modeling

Based on the restoration results of these denudation thickness data, the paleotectonic characteristics of the main strata were successfully reconstructed and integrated to create a 3D model system in this study. Additionally, the paleotectonic morphology of K₁sh₃ at the end of the sedimentation period was correctly reproduced.

The target interval for this study is the K₁sh₃ Member in the Aonan Sag. The reconstructed model (Figure 6) undergoes four stages:

Due to significant topographic undulations in the 3D stratal model, comprehensive structural features cannot be directly presented in 3D. Instead, geological evolutionary profiles are combined with planar contour maps. Structural traits of each evolutionary stage are visualized by extracting contour lines from K₁sh₃ 3D models at different phases (Figure 7). The study’s main structural evolution phases are better depicted by this approach, integrated with 2D evolutionary profiles and descriptions. The following content results from this analytical method [29].

4.2. Analysis of the Tectonic Evolution of the K₁sh₃ Member in the Aonan Sag

The bottom surface’s structural pattern shows alternating uplifts and depressions near the end of K₁sh₃ sedimentation. The middle section is uplifted toward both flanks, while the southwest remains low. Thick sedimentation in the rift basin is primarily controlled by the F1 and F2 faults (Figure 7a).

During the K₁y sedimentation phase, NNE-NE structural trends are exhibited on the top surface of the K₁sh₃ Member, with uplift extending from southwest to northeast (Figure 7b). Minor variations in thickness are observed during K₂n sedimentation, reflecting localized, uneven subsidence (Figure 7c). Following K₂n deposition, tectonic influence on the K₁sh₃ Member becomes limited. The current tectonic configuration, characterized by localized subsidence, inherits the morphology from earlier stages of faulting (Figure 7d).

These observations are further supported by an analysis of fault displacement in stratigraphic profiles, especially during the middle part of the Shahezi Formation. Larger fault displacements during this period correspond to the most intense fault activity, highlighting the control of faulting on sediment thickness variations. The influence of tectonic forces on sedimentation and erosion is evident from the progressive increase in erosion with stratigraphic depth during Shahezi Formation deposition. Erosion is particularly pronounced in the area influenced by the northeastern paleo-uplift, which underwent a major uplift phase between the Shahezi and Yingcheng Formations.

4.3. Cross-Sectional Analysis of Key Evolutionary Stages in the Aonan Sag

4.3.1. The East–West Cross-Section (A–A′)

The half-graben rift basin evolution is controlled by F1 and F2 faults via differential motions (Figure 8a). High-angle normal faulting on western boundary fault F1 occurs during early rifting, forming a rift basin with faulted western margin and overlapped eastern edge. Downthrown strata reach 3000 m. Central basement uplift during middle rifting causes east-west sedimentary hiatus and core layer thinning to 1200 m. F2 activity increases during late rifting, thickening eastern boundary strata to 2800 m and forming east-faulted/west-uplifted tectonic inversion. Post-K₁y erosion occurs due to continuous central uplift, followed by a depression stage with localized subsidence. Basin inversion and anticlinal uplift occur during late K₂n, with expansion slowing. Fault activity and thickness variations align with K₁sh₃ Member paleostructures in the 3D model.

4.3.2. The North–South Cross-Section (B–B′)

Syn-sedimentary fault F7 exerts primary control over the southern sedimentary system (Figure 8b). During the early rifting stage, the downthrown side of F7 accumulated a sediment thickness of 4000 m—twice that of the upthrown side—resulting in a north-shallow/south-deep half-graben structure. In the middle rifting stage, uplift along the sag’s north-south flanks triggered erosion of the upper layers within the K₁sh₃ Member. By the late rifting stage, central basin thickness reached 2500 m, with progressive southward thinning indicating depocenter migration toward the north.

3D structural modeling of the K₁sh₃ Member’s top surface reveals contour alignments with profile-scale anticlines and compressional faults. These features reflect a stress field transition from NW-SE extension to NE-SW uneven subsidence, consistent with evolving tectonic dynamics.

5. Discussion

5.1. Tectonic Evolutionary Sequence Construction Based on Tectonic Restoration Results

Tectonic restoration reveals the Aonan Sag’s tectonic evolution in three stages:

(1)

Syn-rift stage (K₁h–K₁yc)

During the first rifting phase (K₁h deposition), basin architecture was controlled by a syndepositional normal fault system (Figure 9). Basin-controlling faults had listric geometry: steep top segments (60–80° dip) flattening downward, causing footwall (800 m) to hanging-wall (1500 m) thickness differences.

During K₁sh deposition, widespread extensional faulting occurred at the peak activity. Early-stage westward depocenter migration resulted from stronger western asymmetric extension. Asymmetric half-graben basins and alternating horst-graben structures formed via late-stage eastern fault reactivation. Regionally, major faults were reoriented to NNE by the Yanshanian Orogeny (Jurassic–Cretaceous) via stress field rotation.

From late K₁sh to late K₁yc, fault activity declined as regional stress shifted from extension to compression, forming compressional fault systems and anticlines [30]. Multistage erosion of K₁sh resulted from border fault activity, differential subsidence, and tectonic uplift. Basin-bounding fault borders (NW and SE) have >300 m maximum denudation, with residual layers thinning to 100–200 m (Figure 10).

(2)

Post-rift stage (K₁d–K₂n)

As the basin entered thermal sinking, regional extension diminished. Fault activity decreased (F2 slip rates <20 m/Ma), with isolated reactivation of inherited major faults. The basin transitioned to broad subsidence, marked by widespread continuous strata, while depocenter migration resulted from localized differential subsidence (Figure 10).

(3)

Structural inversion (K₂s–Present)

Compressional deformation occurred in the Songliao Basin due to Mongol-Okhotsk Ocean closure and Pacific Plate subduction. In the Aonan Sag, pre-existing normal faults underwent positive inversion, forming thrust faults and anticlines. This phase records the tectonic shift from extension to intracontinental contraction with strike-slip deformation, featuring reactivated fault zones (Figure 10).

5.2. Tectonic Restoration Results Tectonic Control on Source, Reservoir, Cap Rocks, and Traps

According to previous studies, the Aonan Sag is located in the southern part of the Gulong area. In this region, source rocks are scattered across several small rift basins, with thicker source rocks and better accumulation potential in the southern and parts of the Gulong rift. The rift-phase source rocks mainly include the I Member of the K₁h Formation, K₁sh Formation, and II Member of the K₁yc Formation, with the K₁sh Formation being the primary source rock. It consists predominantly of widespread lacustrine mudstones.

Fault control and lacustrine sedimentation provide favorable conditions for source rock accumulation. At the end of the rift phase, the formation of conglomeratic reservoirs in the K₁yc Formation created space for oil and gas storage. During the sag phase, rapid burial led to a significant increase in temperature and pressure of the source rocks, initiating the peak hydrocarbon generation phase.

Additionally, the transgressive systems tract deposits of the I member of the K₁q Formation formed extensive, thick lacustrine dark mudstones and oil shale (100–140 m thick), which represent the most stable and high-quality cap rocks in the basin, effectively controlling hydrocarbon migration in the reservoirs. This led to the establishment of a tectonically driven accumulation model.

5.2.1. Source–Reservoir–Cap Rock Assemblage

The source–reservoir–cap assemblages in the Aonan Sag feature multi-layered source rocks, diverse reservoirs, and regional cap rocks with strong sealing capacity.

During rifting, deep-lake to sub-deep-lake facies formed in K₁sh due to extensional activity of NNE-trending basin-controlling faults (F1, F2), where reduced energy conditions facilitated organic matter preservation [31,32]. Source rocks include three intervals: the 1st Member of the K₁h, K₁sh, and 2nd Member of the K₁yc. K₁sh dark mudstone is the main contributor, with maximum single-layer thickness 22 m, 49.7% mudstone proportion, average TOC 0.58%, and Ro 0.5–1.7%.

Reservoir development is primarily controlled by tectonics and sedimentary systems. Syn-sedimentary faults govern the distribution of fan delta sand bodies (15–25 m thick), while tectonic stress during rift inversion stages leads to the formation of fracture-matrix dual-porosity systems (permeability >10.5 mD) in volcanic rocks and tight sandstones. [33]. Regression-stage secondary pores and fractures contribute to Class II tight gas reservoirs (porosity 4.2–4.8%). The regional cap rocks include the 2nd Member of the K₁d (mudstone thickness 200–300 m, breakthrough pressure >15 MPa) and K₁sh mudstones, which exhibit stable sealing capacity during the maximum flooding phase (displacement pressure >12 MPa). The reservoir–cap analysis reveals two major combinations formed during the rifting stage: First, thick dark mudstone source rocks in the central Shahezi Formation, along with underlying early coarse-grained sandstone reservoirs, form a “self-sourced, self-storage” assemblage. Second, the mudstone of the second member of the Denglouku Formation (breakthrough pressure 18 MPa) acts as a cap rock, while the overlying Shahezi top sandstone and Yingcheng Formation volcanic rocks (porosity 10%) serve as reservoirs, forming a “source below, reservoir above” assemblage.

5.2.2. Types of Traps

The dense fault zone was identified through coherent attributes (Figure 11a), serving as the basis for trap characterization in the study area. Trap development is structurally controlled by two dominant mechanisms: reverse drag anticline traps and stratigraphic traps.

During the syn-rift, reverse drag anticline traps formed on the downthrown side of F2 due to syn-sedimentary fault activity, with slip rates exceeding 120 m/Ma. These traps exhibit closures greater than 50 m and are laterally sealed by both the fault zone and the K1d second member mudstone cap rock, which has a breakthrough pressure over 15 MPa and a mudstone docking probability exceeding 80%.

Stratigraphic traps are associated with tectonic uplift during the rift–depression transition, characterized by truncated unconformities near the K₁d base. These traps cover effective areas larger than 20 km², with stratigraphic dips surpassing 15°. 3D seismic attribute analysis demonstrates that trap distribution in the post-rift transition zone is structurally controlled, making them high-priority targets for deep gas exploration [34,35,36].

5.3. Genetic Model of Reservoir Formation in Aonan Sag During Rift Period

Deep gas reservoir migration occurs via rift-stage fault systems, connecting reservoirs to source rocks. Primary gas reservoir models are disrupted by faults, promoting secondary gas reservoir development.

Based on tectonic evolution’s control over hydrocarbon accumulation elements, a “rift-controlled source, depression-controlled reservoir, structure-controlled accumulation” model is proposed (Figure 11b).

(1)

During the syn-rift, the source is regulated by extensional structures: deep-lake to sub-deep-lake facies sedimentation occurs in double-faulted basins from crustal extension. The term “syn-rift” refers to the period of crustal extension and faulting, during which sedimentary basins form and sediments are deposited, particularly in the Aonan Sag. Source rock growth and evolution are governed by fine-grained sediment deposition and subsidence from fault activity. Fan-delta front sand bodies and volcanic reservoirs form along fault zones via multidirectional sediment sources, laying the “source–reservoir” material basis.

(2)

During the post-rift, reservoir and cap are regulated by stable structures: in rift-depression transition, the basin enters depression with reduced tectonic activity. The term “post-rift” refers to the period following rifting, characterized by reduced tectonic activity, thermal subsidence, and the formation of reservoirs and cap rocks. Early reservoirs are covered by shore-shallow-lake-to-delta sequences, forming regional mudstone cap rock in the K₁d second member. Coupling of Denglouku reservoir-cap sealing and K₁sh source rock peak hydrocarbon generation (Ro = 1.0–1.3%) creates a gas charging window. Reservoir–cap configuration is solidified by tectonic stability [37].

(3)

Hydrocarbon migration and accumulation are governed by tectonic regimes: Oil and gas migrate to high-lying tectonics. The term “high-lying tectonics” refers to structural highs and uplifts formed by extensional and compressional forces. In the Aonan Sag, these elevated regions, located in the northeastern part of the basin, result from thermal subsidence and tectonic inversion. This process is enhanced by a composite migration system comprising faults (conduits or barriers), sand bodies (lateral carriers), and unconformities (regional migration highways), which collectively redistribute hydrocarbons according to tectonic-induced potential gradients. Oil–gas enrichment in K₁d bottom unconformity stratigraphic traps exemplifies tectonic framework control, resulting from coupling structural highs, efficient channels, and effective reservoir–cap assemblages [38,39,40,41].

5.4. Assessment of Hydrocarbon Accumulation Potential

A “structural-sedimentary-reservoir” comprehensive evaluation system was built using key indicators: fault movement magnitude, mudstone cap rock sealing capacity, trap effectiveness, and source–reservoir–cap spatio-temporal matching. This system is based on integrated geological–geophysical evaluation of K₁sh.

First, prioritize regions with high-intensity fault zones (fracture line density > 1 fracture/m). Second, focus on fault-block traps with optimal geometric parameters (closure height 70–860 m, areal extent > 5 km²), prioritizing zones exhibiting equilibrium between fault conduit efficiency and preservation conditions (Figure 12a). Finally, constrain high-efficiency “source–reservoir–cap” matching systems by reservoir–cap combinations. Reservoir types include volcanic or unconventional tight sandstones; cap rock configurations consist of K₁sh lacustrine mudstones or K1d second member mudstones (thickness 200–300 m) (Figure 12b) [42].

Analysis based on described variables shows significant hydrocarbon exploration potential in K₁sh during Aonan Sag’s faulted period.

5.5. Regional Tectonic Implications and Comparison with Typical Petroliferous Basins

The evolution of the Aonan Sag from local rifting to regional thermal subsidence represents a classic tectonic response to a shifting intracontinental stress field. This rift-to-subsidence transition is well-documented in global analogues, which provide critical insights into its petroleum system. The Cretaceous Xujiaweizi Fault-Depression in the Songliao Basin offers a prime example. There, the cessation of active rifting and volcanism initiated a long period of stable thermal subsidence. This quiescent, low-energy depositional environment was fundamental for the accumulation of high-quality source rocks [43].

The North Sea Basin presents a powerful parallel, characterized by multiple rift-subsidence cycles driven by regional extension. The syn-rift phases were dominated by normal faulting, forming half-graben settings that host coarse-grained clastic reservoirs. Conversely, the post-rift thermal subsidence phase, induced by mantle cooling, facilitated the widespread deposition of fine-grained, organic-rich source rocks. These analogues reveal a governing principle directly applicable to the Aonan Sag: rifting controls reservoir development, while thermal subsidence governs source rock deposition. This provides a quintessential model for understanding the co-evolution of tectonics and petroleum systems in rift basins [44].

6. Conclusions

An improved approach integrating balanced cross-sections, 3D modeling, and hierarchical restoration was applied to analyze the tectonic evolution of the Aonan Sag during the rifting phase in the northern Songliao Basin. This study reveals staged activities of basin-controlling faults and their impacts on paleostructures and source–reservoir–cap configurations, thereby clarifying the rifting-to-depression transition mechanism.

A “rift-controlled source, depression-controlled reservoir, structure-controlled accumulation” hydrocarbon accumulation model was established, highlighting exploration potential within rift-phase structures. The integrated methodology overcomes limitations of single-technique approaches, significantly improving the accuracy of complex rift basin restoration. It provides a novel strategy for gas exploration in the northern Songliao Basin, with implications for survey design, efficiency enhancement, and basin-wide exploration optimization.