The MXN sub-sag, nestled within the prolific hydrocarbon terrain of the Bohai Sea area, forms an integral part of the sedimentary tectonic framework of the Bohai Bay Basin (BBB). This Cenozoic basin, stretched along China’s eastern coast, encompasses a vast expanse, estimated at approximately 200,000 km², and is noted for its complex rifted structure shaped through multi-phase tectonic evolutions (Lu et al., 1997; Chi and Zhao, 2000; Hao et al., 2007; 2009; Schlische and Withjack, 2009; Henza et al., 2010; Yin, 2010; Zuo et al., 2011; Xu et al., 2014; Chang et al., 2018; Song et al., 2019; Jin et al., 2020; Ye et al., 2020; Yu et al., 2020) (Figure 1A). Enveloped by ancient uplifts such as Yan Shan, Taihang Shan, Jiaoliao, and Luxi, the BBB is dissected into multiple subbasins and uplifts, including Liaodong Bay, Liaohe, Bozhong, and others, housing significant hydrocarbon reserves (Hao et al., 2009; 2011; Li et al., 2017; Shi et al., 2019; Yu et al., 2020; Zeng et al., 2022).

Residing at the basin’s margin, the MXN sub-sag is a testament to the regional geological complexity, flanked by prominent mountains and demonstrating substantial hydrocarbon accumulations with crude oil and natural gas proven reserves (Jiang et al., 2016; Hou and Hari, 2014). As a Cenozoic sub-sag, it is demarcated by key geological structures, including the Jiaoliao uplift and Bonan Low uplift, and is directly linked to neighboring sub-sags, illustrating a northeastward strip-shaped distribution across 1200 km² (Qi, 2004; Zhou et al., 2010; Zong et al., 2012; Huang et al., 2018) (Figure 1B).

The structural intricacy of the MXN sub-sag is further exemplified by the east branch of the Laizhouwan section within the Tan-Lu Fault Zone (TLFZ), which bisects the sub-sag, creating distinct geological divisions that have played a crucial role in its formation and hydrocarbon potential, including significant oilfields such as Bozhong 36–1 and Penglai 25–6 (Gong et al., 2007; Zhao et al., 2020).

The HHKD sub-sag is located in the eastern Bohai Bay Basin, NE China, sandwiched between the Bonan and Laibei uplifts to the north and south, and connected with the MXN sub-sag to the east (Figure 1B). Two major tectonic systems of east–west extension and northeastward strike–slip structures control the basic tectonic framework of the region (Hsiao et al., 2004; 2010; Liu et al., 2010; Qi and Yang, 2010; Tong et al., 2012; Li et al., 2013; Liu et al., 2015). The structural appearance resembles a graben reworked by slip–strike faults; it is steeper in the south and gentle to the north, higher in the west, and buried deeper in the east (Sun et al., 2017; Liu et al., 2018; Lan and Liu, 2022).

Stratigraphically, the study area hosts both Paleogene and Neogene deposits, with the Paleogene characterized by formations rich in fan- and braided-deltas and lacustrine deposits, while the Neogene features fluvial and deltaic sequences, highlighting the dynamic sedimentary processes that have shaped its geological history (Hou et al., 2001) (Figure 2). The amalgamation of these structural and stratigraphic elements underscores the study area’s significance within the broader context of the BBB’s geological and hydrocarbon framework.

This investigation integrated multidimensional seismic datasets, borehole recordings, and auxiliary geological insights specifically from the HHKD and MXN sub-sags area. A comprehensive spatial coverage was achieved through a high-resolution 3D seismic survey. Detailed lithological interpretations were derived from 15 strategically positioned wells, leveraging an array of logging techniques. The suite of well log data predominantly included gamma-ray (GR) logs, spontaneous potential (SP) logs, acoustic time (AC) logs, among other crucial datasets, providing insights into the primary lithological frameworks within the MXN subregion. The acquisition of both seismic and borehole datasets was facilitated through collaboration with CNOOC (China) Co. Ltd., Tianjin branch.

This method provides thorough geological and spatial knowledge, but it has drawbacks due to seismic resolution limits and the subjective nature of well log interpretation. Seismic data may inaccurately represent stratigraphic features due to complex geology, and well log analysis can be biased by the geologists’ expertise.

Our methodology encompassed a rigorous examination of the spatial and structural dynamics of extensional strike-slip fault systems within the sub-sag. This involved a meticulous interpretation of three-dimensional seismic datasets, including the analysis of sequence boundary variance slices and representative vertical seismic profiles within the established sequence stratigraphic framework. To elucidate the tectono-stratigraphic evolution of the sub-sag, we applied back-stripping analysis. This technique facilitated the construction of balanced cross-sections, enabling a detailed visualization of the basin’s architectural and stratigraphic heterogeneity. Furthermore, our study delved into assessing the impact of extensional strike-slip faulting on the stratigraphic framework, spatial configuration of sub-sags, and the consequent hydrocarbon potential.

The employed analytical methods are robust but have limitations. Seismic data interpretation relies on data quality, which varies and can create uncertainty in fault and boundary analysis. Back-stripping, useful for studying basin evolution, depends on unverifiable assumptions about geological history, which can affect results. The analysis’s success also hinges on the accuracy of seismic and well log data inputs.

The extensional fault system in the HHKD and MXN sub-sags features predominantly EW-NW trending faults, exhibiting significant control over sedimentary and structural patterns. These faults display bifurcation and arc shapes, with echelon or feather-like intersections with secondary faults, observed in various combinations (Figure 3). Their listric and shovel-like profiles cut through deep strata, with near-surface “y” and inverted “y” shapes formed by interactions with secondary faults. The system’s activity varies across sections and periods.

The eastern branch of the Tan-Lu fault delineates the HHKD and MXN sub-sags, running NNE with notable continuity and variability (Figure 3). Vertical main faults slice through deep strata, with their interactions with secondary faults resulting in complex profiles.

The study area features extensional strike-slip faults primarily within the No. 2 sub-branch of the east branch of the Tan-Lu fault zone, characterized by complex structures akin to inverted or semi-flower patterns superposed on listric and shovel-like normal faults. Seismic section HH’ reveals a concentration of these faults in the same sub-branch, where they exhibit simple patterns in deep layers but complex combinations of derivative faults in shallow layers due to extensional, strike-slip, and compressional interactions. The inline 5200 intersects this sub-branch and shows the main fault with a shallow dip angle of 80°–90° that steepens with depth, maintaining strong continuity. In the shallow portions, secondary faults form flower-like structures with the main fault, while at greater depths, they create “Y” shaped configurations (refer to Figures 7E, 8E).

The extensional structural pattern is a structural combination formed under tensional forces. In the study area, this style is mainly composed of normal faults, forming “Y,” inverted “Y,” and flower-like structures. The extensional faults in the HHKD and MXN sub-sags trend nearly EW (Table 2), with secondary faults arranged in echelon or intersecting the main faults in a feather-like pattern. The HHKD sub-sag has three major extensional faults: the Laibei No. 2 fault, the central section of Bonan No. 2, and the eastern section of Bonan No. 1.

The strike-slip structural pattern results from compressional or tensional shear stress, leading to a combination of strike-slip faults. The study area’s strike-slip style is mainly characterized by negative flower structures, flower-like structures, and half-flower structures (Table 2). Strike-slip faults mainly distribute along the eastern branch of the Tan-Lu Fault Zone, trending NNE and controlling the boundary of the HHKD sub-sag. The northern part of the eastern strike-slip fault extends to the Miaoxi northern sub-depression, and the southern part to the Laizhou Bay Depression. Negative flower structures have branching faults upward, forming Y-shaped structures, with the main fault appearing as upright or high-angle listric normal faults. The eastern branch No. One strike-slip fault is primarily characterized by negative flower structures, while the eastern branch No. 2 develops a few such structures (Table 2).

Flower-like structures are combinations of a main fault with multiple secondary faults on the profile. Both the eastern branch No. 2 and its secondary faults develop flower-like structures, whereas the eastern branch No. 1 has fewer flower-like structures. In addition, internal structures within the two sub-depressions also develop, mainly in areas with abundant shallow secondary faults, forming flower-like structures in combination with the main faults locally, as Laibei No. 2 and Bonan No. 1 faults. Half-flower structures are combinations with multiple derivative faults on one side of the main fault, with the derivative faults being normal faults. In the eastern branch strike-slip fault zone, local areas of the eastern branch No. 2 fault develop half-flower structures, with the main fault being steep and the overall fault profile being clear (Table 2).

The overlapping strike-slip structural pattern is a combination of faults formed under both extensional and strike-slip tectonic deformation. Overlapping strike-slip structures are later strike-slip tectonic movements superimposed on early extensional faults, manifesting as flower structures and multi-level “Y”-shaped combination styles. The overlapping strike-slip area is distributed in the central part of the eastern branch strike-slip fault zone between the HHKD and MXN sub-sags, mainly superimposing strike-slip movements on listric and listric normal faults (Table 2).

All three structural patterns are developed in the study area. Among them, the extensional structural pattern is more widespread, mainly distributed along the edges and interiors of the two sub-depressions, trending nearly EW, with long extensions; strike-slip and overlapping strike-slip structural patterns mainly develop along the eastern branch strike-slip fault zone on the edge of the HHKD sub-sag, trending NNE and extending long. The main extensional structures in the HHKD and MXN sub-sags are listric and listric normal faults, with shallow secondary faults forming “Y,” inverted “Y,” and flower-like structures in combination with the main faults; the main strike-slip faults on the profile appear as nearly upright listric faults, combined with shallow secondary faults to form negative flower, half-flower, and flower-like structures; overlapping strike-slip develops in the overlapping area of extension and strike-slip, mainly distributed in the central part of the eastern branch strike-slip transition zone, with extensional structures on both sides being more developed, leading to overlapping, mainly characterized by strike-slip movements superimposing on listric and listric normal faults in the strike-slip zone (Table 2).

Throughout the faulting phase, the boundary faults of the MXN sub-sag were subject to intense activation, markedly influencing sedimentary processes and leading to distinct segmentation within the sub-sags. Initial formation of the MXN sub-sag was observed during the E₂k depositional phase, signified by the rise of the eastern secondary sub-sag. Sedimentary accumulation predominantly occurred within the southern and eastern secondary sub-sags, with sedimentary focus centralized in the eastern section. The E₂k stratigraphy was shaped by three critical faults, notably with the northwest fault acting as the eastern segmental offshoot of the EB-TLFZ (Figure 9).

In the E₂s₄ depositional phase, expansion of the MXN sub-sag was notable, especially within its eastern secondary section. The sedimentary focus gradually migrated northwestward, remaining within the eastern secondary sub-sag. This phase was predominantly governed by a syndepositional fault, with the eastern secondary branch of the EB-TLFZ continuing its evolution and the commencement of its western secondary branch. Additionally, seven distinct faults were instrumental in shaping the development of the eastern secondary sub-sag, marking the initial formation of the southern secondary sub-sag with the sedimentary focus retained in the eastern section (Figure 9).

During the E₂s₃ phase, development of the eastern secondary sub-sag halted, while the southern secondary sub-sag’s evolution proceeded. The period was characterized by the continued evolution of two major strike-slip faults and the influence of seven syndepositional normal faults along with the west secondary branch of the EB-TLFZ, steering the development direction. The sedimentary focus shifted northwestward (Figure 9).

The E₃s_1-2 phase saw the entire region undergoing stable deposition, with the southern secondary sub-sag undergoing adjustments due to two major strike-slip and several normal faults, which also modulated other secondary sub-sags (Figure 9).

Transitioning from the faulting to the depression phase, boundary faults maintained a significant control over sediment deposition, leading to a stabilization of regional depositional thickness and a trend towards uniformity (Figure 9).

In the E₃d phase, the southern secondary sub-sag experienced rapid development and substantial N₁g strata deposition, with a slight southeastward migration of the sedimentary focus. The deposition was primarily controlled by one syndepositional fault alongside two major strike-slip faults (Figure 9).

Post E₃d deposition saw the southeast sag undergoing uplift and erosion, with E₂s₄, E₃s_1-2, and N₁d strata facing differential erosion - E₃s_1-2 experiencing the most severe erosion, followed by E₂s₄, and N₁d experiencing the least (Figure 9).

During the depression phase, Neogene deposition was recorded across all secondary sub-sags, with the sedimentary focus in the northern secondary sub-sag. The N₁g phase featured the thickest strata centrally and in the northwest, tapering towards the southeast, governed by the continued evolution of two major strike-slip faults and several normal faults, which directed sediment deposition in the southern secondary sub-sag. The northern secondary sub-sag was influenced by a singular normal fault (Figure 9).

The N₂m^L phase witnessed uniform deposition across the region, modulated by seven normal faults, with the eastern secondary branch of the EB-TLFZ directing the southern secondary sub-sag.

Presently, post- N₂m^L deposition is characterized by its thickness centrally and in the northwest, thinning towards the southeast, controlled by two major strike-slip faults and adjusted by several normal faults in the eastern secondary sub-sag and other secondary sub-sags. In essence, the MXN sub-sag has undergone a progressive migration of the depositional and subsiding centers westward and northward (Figure 9).

During the E₂k depositional period, NW-oriented initial oblique extension faults were formed, and the strike-slip faults at this stage exist latently in the basement. During this period, the east, and north secondary sub-sag of the MXN sub-sag were formed. The east secondary sub-sag is mainly controlled by the oblique extension and the pre-existing basement strike-slip fault of the East sub-branch, showing a spindle-shaped distribution; while the north secondary sub-sag is controlled by the EW-extension fault (Figure 10A).

During the E₂s₄ depositional period, with the increase of extension displacement, the NE-oriented oblique extension faults continued to develop. At the same time, the East sub-branch strike-slip fault became significantly active under the action of this stress field, showing a right-lateral stepover fault combination mode. It generally bends in an arc shape from south to north, and the end of the segmented fault in the north is accompanied by near EW-oriented secondary extension faults. The west sub-branch strike-slip fault was still latent and inactive during this period. Therefore, in addition to the NE-oblique growth fault controlling the sporadic depression of the hanging wall, in the right-lateral strike-slip deformation of the southern bend of the East sub-branch strike-slip fault, a tensional release type negative unit was formed, that is, a “spindle-shaped” depression was formed. And in the joint part of the two faults in the northern part of the East sub-branch strike-slip fault, controlled by the secondary tensional stress field accompanied by strike-slip shear displacement, a “L-shaped” depression was formed, that is, the North secondary sub-sag of MXN (Figure 10B).

During the E₂s₃ depositional period, the range of fault activity expanded, including the intense activity of the west sub-branch strike-slip fault. But there are still active segments, specifically manifested in the western part of the end of the segmented activity of the strike-slip fault in the HHKD sub-sag, which is accompanied by several relatively large near EW-extension faults, thereby controlling the formation of the “L-shaped” HHKD sub-sag (Figure 11A).

During the depositional period from E₃s₁-E₃s₂ to E₃d, the activity of the strike-slip fault dominated. The East and West sub-branch strike-slip faults penetrated the entire area of the MXN sub-sag and even a larger area of the Bohai Sea. The West sub-branch strike-slip fault showed a complete through-type combination, and the East sub-branch strike-slip fault had a higher degree of connection, extended in a through-type and retained the bifurcated fault traces left by the previous left-step combination mode. Therefore, during this period, the “diamond-shaped” depression, namely, the south secondary sub-sag, was mainly formed in the release bending parts of the two strike-slip faults in the middle and south, similar to the strike-slip fault bend basin morphology (Figures 11B, 12).

Looking at the transformation relationship of different evolution stages of strike-slip faults and oblique extension faults and their control on the characteristics of the depressions formed, in the early stage of basin evolution, that is, the E₂k -E₂s₄ stage and E₂s₃ stage, mainly formed branch “L-shaped” sub-sag and depressurization bending “spindle-shaped” sub-sag, the depression is mainly controlled by the segmentation expansion mechanism of the strike-slip fault, and the scale of the depression is relatively large. In the middle period of basin evolution, that is, E₃s₁, E₃s₂, and E₃d stages, the main formation is depressurization “diamond-shaped” depression, and the scale of the depression is also relatively large (Figure 13).

Since the Cenozoic era, the southwestern depression of the temple has been profoundly transformed by the forces of early extensional fault stretching, numerous phases of tectonic uplift, and late-stage strike-slip faulting. It has developed a series of strike-slip faults atop a foundation of pre-existing faults from the Paleogene period, resulting in a collection of blocks with compressional and torsional characteristics, alongside groups of fault-related folds and reverse faulted anticlines (Figure 14). Influenced by the differing features of the eastern and western branches of the strike-slip faults, the genesis and types of structural closures within various tectonic zones exhibit considerable variation.

Notably, the eastern branch of the strike-slip fault zone is distinguished by a fan-shaped arrangement of faults, with clear signs of early extensional and late compressional-torsional deformations. The strike-slip faults, with their curved segments and “right-lateral left-stepping” overlap zones, have given rise to a series of significant faulted anticlines and faulted block closure groups. For example, the Longkou 31-C structural zone in the southern part of the depression is developed against the backdrop of the southward extension of the Jiao-Liao uplift to the secondary depression, situated in the dual overlap zone of the strike-slip faults. Late-stage strike-slip adjusting faults and configurations have led to the formation of inherited faulted anticline closures at both deep and shallow levels (Figure 14).

The Longkou 31-B structure, located within the depression area, falls within the strike-slip pressure transition zone, forming a “right-lateral left-stepping” block closure adjacent to the eastern branch’s strike-slip fault. The Penglai 25-A structural zone in the north of the eastern branch of the strike-slip fault lies within the overlap zone of the fault, where the Paleogene has experienced multiple episodes of compression, uplift, and erosion. Since the initiation of the new tectonic movement, intense clockwise compressional strike-slip actions have facilitated the emergence of large faulted anticline closures in the area’s shallow layers (Figure 14).

The eastern and western sides of the western branch of the strike-slip fault display pronounced differences in bedding and closure characteristics. For instance, the Penglai 31 structural zone on the western side of the strike-slip fault comprises a block closure group formed through the combined effects of the strike-slip fault and its adjusting faults, presenting a “toothbrush-shaped” configuration. The Penglai 25 west structural zone in the northern part of the strike-slip fault features Paleogene structures with compressional and torsional properties, forming a collection of fault blocks and faulted anticline closures. Additionally, the deep-layer development above the central structural ridge in the southern depression is controlled by the Longkou 31-A block and faulted anticline closures, influenced by early (strike-slip nature) faults. This structural zone is adjacent to the center of the depression, characterized by a thick overlying layer and excellent preservation conditions (Figure 14).

In the Bohai Bay Basin, the fault system is quite complex, making faults the main pathways for oil and gas migration. For a fault to be a favorable source of oil, its active periods must coincide with the hydrocarbon expulsion periods. Otherwise, it is considered a poor oil source fault, such as the nearly east-west trending extensional faults like Bonan No. 1 and Laibei No. 2 faults and the eastern branch strike-slip faults, which are good oil source faults. Most oil and gas reservoirs in the study area were formed when large faults provided pathways for early hydrocarbon source rocks to accumulate within traps, forming large oil and gas fields.

Since faults serve as channels for hydrocarbon movement, their effectiveness in aiding oil and gas accumulation depends on the timing of their activity and hydrocarbon expulsion. Extensional faults create structural traps for hydrocarbons due to the stretching of the earth’s crust, and their formation periods are critical for hydrocarbon migration and trapping. Strike-slip faults allow for horizontal strata displacement and enhance vertical hydrocarbon migration to traps, with secondary faults formed by this movement also acting as migration pathways. Extensional Strike-slip Faults facilitate hydrocarbon migration but require suitable reservoir-cap rock combinations to effectively trap hydrocarbons. In summary, the control of various fault systems on hydrocarbon migration and trapping is a critical factor in the development of oil and gas reservoirs in the Bohai Bay Basin.