Since the 1960s, a large amount of oil and gas has been discovered in the Paleogene and Neogene in the Bohai Sea. In the early exploration, a number of oil fields represented by Suizhong 36-1 were discovered with the Paleogene as the primary target. Later, a large number of oil fields represented by Penglai 19-3 were discovered with the Neogene as the primary target. These oilfields became the mainstay for achieving the production of 3000×10⁴ t in 2010 in the Bohai Oilfield of CNOOC. With the deepening of exploration, the difficulty of exploration increases constantly, and it is more and more difficult to discover large-scale dominant structural traps in the middle and shallow layers. Finding new potential traps through new structural study idea is of great significance for the future oil and gas exploration in the Bohai Oilfield of CNOOC.

The structural traps of the Bohai Bay Basin are closely related to faults. A fault is a structure that represents evident displacement along the fault surface in a geological body. The greater the displacement, the more evident the seismic response characteristics of the fault, and the easier it is to identify the fault, that is, the fault in traditional understanding (for the convenience of discussion, it is referred as an explicit fault). On the contrary, when the displacement of the fault is beyond the resolution of seismic data, the fault is not easy to identify, and is referred to as the “subtle fault” in this paper. There is no absolute limit value between them, subtle faults and explicit faults are often closely related to geological knowledge and seismic data. Researchers in China and abroad have made many researches on the structural characteristics and formation mechanism of subtle faults. Foreign researchers mainly focus on experimental simulation and genetic mechanism of subtle faults in specific background. Among them, Crook and Hardy proved through numerical simulation that steep basement normal faults cause structural deformation of caprock during the increase of displacement^[1,2,3]. In the process, as the shear strain strength increases, different structures would be formed in multiple evolution stages, including very insignificant anticline structure in the early stage, discrete fault zone (concealed fault zone) formed in the middle stage, and deep and large fault formed in the late stage. When studying the triangular shear structural deformation caused by the basement involvement structure, Mitra and Miller found that the activity of vertical basement fault, low-angle reverse fault, and high-angle normal fault would give rise to fault trend zone in the corresponding overlying caprock^[4]. When studying the structural evolution of fault belts related to salt structures, Jackson and Rotevatn et al. revealed that different styles of salt-related fault systems would be formed during the period when the salt structure controlled by the basement was weak; on the plane, they appeared as some scattered small faults distributed along the echelon fault zones^[5]. When analyzing the structure of the Gulf of Thailand, Morley et al. reached the finding that the oblique extension of the existing faults might be the cause of potential strike-slip faults^[6]. The domestic researchers in China mainly focused on the classification, development characteristics and identification technology of subtle faults. Wang Weifeng and others suggested that the weakly deformed structural belts or fault trend zones formed in the caprock of the sedimentary basin due to regional or local stress field or basement fault activity were the products of the early or middle stage of formation and evolution of fault zones, which were characterized by strong concealment and no evident identification marks, and could be divided into different levels according to the scale of the fault trend zones^[7]. Lu Gangchen et al. divided the identified subtle fault zones into basin-level, depression-level, sag-level, sub-sag-level, and trap-level subtle fault zones according to their scales^[8]. Niu Chengmin et al. elaborated on the identification marks and methods of subtle faults and their controls on hydrocarbon accumulation in the Bohai Sea^{[9,10,11,12,13]}. Based on domestic and foreign researches, different researchers put forward the concept of subtle fault from different perspectives. The definitions mostly focused on the description of some specific phenomena in the early stage of fault development. There are still some controversies on the genetic mechanism of subtle fault. Especially in the exploration practice of oil and gas, the structural characteristics, genetic types and distribution pattern of subtle faults, and even the control effect of subtle faults on hydrocarbon accumulation have been rarely studied.

From the basic point of the development and evolution of faults, the rock geological body that undergoes deformation and fracture has experienced several stages, namely, relatively uniform formation, tectonic stress concentration zone (fault trend surface), subtle fault (fault with small displacement) and explicit fault (fracture with large displacement). Explicit faults and subtle faults do not have a clear boundary. The two have the same genetic mechanism but behave differently as they are in different stages of evolution. With the advancement of geophysical methods and technology, subtle faults can also be identified as explicit faults. Explicit faults with larger scale have evident control on hydrocarbon accumulation. In comparison, subtle faults have more complicated development mechanisms, more diverse development characteristics, and more difficult identification methods. Starting from the practice of oil and gas exploration, a subtle fault is defined as the fault occurring objectively in the crustal rock mass, but in the initial stage of rupture and with displacement not enough to form an evident rupture structural response based on the current seismic data, and thus it has strong concealment and is difficult to identify from geological knowledge and seismic data. It should be pointed out that the subtle fault with relatively small displacement is different from the joint that has no displacement, and can be regarded as the advanced stage of the further development and evolution of the joint. Such subtle faults exist objectively in large numbers and have important control on trap formation and hydrocarbon accumulation. However, limited by geological knowledge and seismic data, subtle faults are frequently overlooked in the practice of oil and gas exploration.

Based on the interpretation of the new merged 3D seismic data in the Bohai Sea, this study summarizes the characteristics and structural styles of subtle faults on seismic sections based on related structural theories, analyzes the development process and genetic mechanism of subtle faults in the Bohai Sea, establishes a set of structural analysis system for subtle faults in the Bohai Sea, puts forward the ideas and methods to identify subtle faults, and looks close into the control of subtle faults on oil and gas accumulation. In recent years, the abovementioned method has been applied to the oil and gas exploration practice in the Bohai Sea. As a result, a large number of structural traps have been discovered in places where traditional seismic interpretation methods did not find structural traps before, and the area of individual structural traps has changed from 2.3 km² to over 50.0 km². The method has directly and effectively guided the exploration in mature areas, and four hundred million-ton high-yield oilfields, including the Bozhong 29-6, Luda 25-1, Bozhong 36-1, Kenli 9-1, Kenli 9-5/6, and Jinzhou 25- 1W, Penglai 20-2 and a series medium and small oilfields previously not observed have been discovered.

Geographically, the Bohai Sea is part of the Bohai Bay Basin, which is a Cenozoic intracontinental rift basin developed on the background of the North China Craton Basin. It is surrounded by structural units such as the Yanshan fold belt, the Taihang Mountains and Taihang piedmont fault zones, the Jiaoliao uplift, and the Qinling-Dabie orogenic belt. Since the Late Paleozoic, the North China Plate in which the Bohai Sea is located has been successively affected by the long-range collisions of the northern Siberian Plate, southern Yangtze Plate, eastern Pacific Plate, and Indian Plate, bringing about active movements of deep mantle and strike-slip faults which are the active boundary of the plate. Since entering the Cenozoic, the extension and strike-slip pulling caused by mantle uplift under the background of oblique subduction of the Pacific plate have become the major tectonic mechanism dominating the development and evolution of the basin. The two tectonic stresses, shear and extension, superimposed and interwove, resulting in the formation of current tectonic framework of strike-slip faults and extensional faults (Fig. 1). The Tancheng-Lujiang strike-slip fault zone and the Zhangjiakou-Penglai strike-slip fault zone, which are two giant strike-slip fault systems in eastern China, converge and coexist in the Bohai Sea, becoming the most special structural elements in the Bohai Sea and resulting in the ubiquitous development of strike-slip faults in the Bohai Sea. The NNE- trending Tancheng-Lujiang strike-slip fault system is divided into west, middle and east branches from south to north across the Bohai Sea. The NSW-trending Zhangjiakou-Penglai strike-slip fault system, with two major branches, is distributed in the middle of the Bohai Sea. The intertwined development of the two strike-slip faults in the Bohai Sea controlled the early basin structure and the distribution of fault systems, and played an important role in the formation of the Neogene shallow large-scale structural traps and hydrocarbon accumulation during the neotectonics in the Bohai Sea.

Compared with the fault-fold double-layer structure of the traditional passive continental margin fault basin, the structural evolution of the Bohai Sea area has two major characteristics: multi-cycle superimposition and strong neotectonics. Since the Cenozoic, the overall structural evolution of the Bohai Sea area can be divided into the following five stages: (1) first episode of rift (38-65 Ma) during the deposition of the Paleocene-Eocene Kongdian Formation-Sha 3 Member; (2) thermal subsidence depression period (32.8-38.0 Ma) after the first episode of rift during the deposition of the Eocene Sha 2 and Sha 3 Members; (3) the second episode of the rift during the deposition of the Oligocene Dongying Formation (24.6-32.8 Ma ago); (4) thermal subsidence depression period (5.1-24.6 Ma) after the second episode of rift during the deposition of the Guantao Formation-lower member of the Minghuazhen Formation; (5) the neotectonics transformation stage from the depositional period of the upper member of the Minghuazhen Formation to present (5.1 Ma to recent). On the one hand, the neotec-tonics resulted in the activation and upward extension of margin-controlling faults or the partial activation of basement faults; on the other hand, the strike-slip faults reactivated to form a large number of flower-like structures, bringing about a large number of new faults in the shallow Neogene, which are characterized by small fault displacement and strong concealment. In summary, the dual-source dynamic basin formation mechanism, the intertwined development of large-scale strike-slip faults, and the strong transformation of neotectonics in the Bohai Sea since the Cenozoic have resulted in the new development of many faults of different scales, laying the foundation for extensive development of subtle faults.

The reason why the subtle faults are highly concealed is that the displacement of faults is small or the strength of the formation deformation is small, which leads to weak seismic response difficult to be identified. However, subtle faults are not intrinsically different from explicit faults in the genetic mechanism. From the perspective of tectonic genesis, the formation of different forms of structures such as faults, folds, and diapirs is all controlled by tectonic stress, stress imposing boundary, and stratum medium properties. As for the subtle faults in the Bohai Sea, their formation is mainly controlled by the complex strike-slip faults controlled by pre-existing fault patterns, properties of diverse stratum rock media and strong neotectonic movement.

The two giant strike-slip faults, the NNE-trending Tancheng-Lujiang fault and the NSW-trending Zhangjiakou-Penglai Fault converge in the Bohai Sea to form a pair of conjugate strike-slip faults, resulting in extensive development of strike-slip faults and extremely rich structural styles in the Bohai Sea. The Tancheng-Lujiang Fault has been closely related to the structural evolution of the Bohai Sea since it was formed in the Mesozoic. The fault experienced strong sinistral strike-slip activity in Mesozoic, the sinistral strike slip weakened during the Paleogene rift period and transformed to dextral strike-slip. The strike-slip activity intensified again during the Neogene neotectonics period. The east, middle, and west branches of the Tancheng-Lujiang strike-slip fault are generally strong in the east and weak in the west, and strong in the south and weak in the north. The Zhangjiakou-Penglai Fault is a NWW-trending sinistral fault active in the Cenozoic and is divided into two branches entering into the Bohai Sea from Tianjin, passing through the southern and central parts of the Bohai Sea and extending to the northern part of the Shandong Peninsula. The Zhangjiakou-Penglai fault system is developed and evolved on the basis of a series of Paleogene sub-sag control faults in northwest-west trend. It was mostly extensional in the Paleogene, and sinistral strike-slip in the Neogene, bringing a large number of flower-shaped or flower-like structures. The Zhangjiakou-Penglai fault system in the Bohai Sea is strong in the west and weak in the east. The Tancheng-Lujiang Fault and Zhangjiakou-Penglai Fault vary in strike-slip strength in different evolution periods and regions, but during the neotectonics period, they both had strong strike-slip, resulting in the formation of a large number of new faults. The strike-slip faults in different strikes and combinations show clear zoning feature in the Bohai Sea. The Liaodong Gulf area has a series of parallel overlapping strike-slip faults in the north-northeast strike. In the eastern Bohai, a number of NNE-trending and NEE-trending strike-slip faults connect into a broom-shaped strike-slip fault pattern. In the western part of the Bohai Sea, affected by the NNE-trending Tancheng-Lujiang strike-slip fault zone and the NW-trending Zhangjiakou-Penglai fault zone, the strike-slip faults conjugate and dock with each other. In the southern Bohai Sea, the Tancheng-Lujiang strike-slip fault is divided into three branches that pass through the area almost parallel, and the two fault systems, the NEE-trending fault and the nearly NW-trending fault perpendicularly intersect to form an “H” pattern. In summary, the strike-slip faults of multiple trends and multi-stress nature intersect and coexist, resulting in complex distribution of pre-existing faults, the diversity of tectonic stress boundary conditions and stress transmission types between different blocks, thereby laying structural framework for the development of a variety of Neogene subtle faults.

Media with different properties will deform differentially under the same stress system. As a special kind of material medium, the properties of crustal rocks are closely related to factors such as lithology combination, burial depth, confining pressure and temperature, and diagenetic stage. Generally speaking, rocks with shallow burial, high argillaceous content, and early diagenesis stage have brittle-plastic or plastic characteristics, strong stress absorption, and plastic deformation usually, but are not prone to brittle fracture. The rocks with large burial depth and high degree of consolidation in late diagenesis stage mostly show brittle deformation and weak absorption to stress, and are prone to brittle fracture to form faults. Affected by the superimposition of multi-stage tectonic movements, the sags in the Bohai Sea differ widely in development and evolution, with burial depths ranging from more than 10000 m (the Bozhong Sag) to less than 4000 m (the Miaoxi Sag). From the perspective of sedimentary system, the Neogene Minghuazhen Formation in the Bohai Sea area mainly has ultra-shallow delta and meandering river sedimentary systems under ultra-shallow lacustrine background. The sedimentary system is generally characterized by mud content greater than 60%, relatively shallow burial depth, and strong absorption capacity to tectonic stress, and thus isn’t likely to form evident brittle fracture and fracture displacement accumulation, so the faults have no obvious seismic responses and are subtle. From the perspective of structural unit, the hard basement in the convex area has stronger direct stress transmission than plastic base in the depression area. Therefore, under the same structural stress intensity background, fractures in the depression area and slope zone are more subtle. In summary, the basement type, lithology combination and diagenetic evolution differences result in diverse rock properties and stress-strain characteristics, and lay the material basis for the development of subtle faults in the Bohai Sea.

The Bohai Bay Basin, which is the thinnest continental crust area in eastern China and the center of sedimentary subsidence migration and evolution, is different from traditional rift basins with few faults developing and exponential decrease of subsidence rate after the depression period^[14,15]. The Bohai Sea entered a period of generalized neotectonics in the Neogene, with especially strong tectonic movements since 5.1 Ma ago. The strong reactivation of faults and generation of a large number of new faults are the most intuitive and important manifestations of neotectonics movement. On the one hand, the neotectonics caused the Neogene subsidence center and the depocenter to further significantly migrate to the Bozhong Sag; on the other hand, it resulted in strong Neogene fault activity in the Bohai Sea and the formation of densely distributed new faults on plane. The formation of tectonic pattern in the Bohai Sea is mainly controlled by three dynamic systems, namely the extrusion from the south to north of the Yangtze Plate in the south, the long-distance compression escape effect of the western Indian Plate, and the continuous subduction of the eastern Pacific Plate, so the faults in the Bohai Sea are developed in multiple phases, and are different in stress properties and activity intensity^{[16,17,18,19,20,21]}. Since the Cenozoic, the direction and rate of subduction of the Pacific Plate have changed several times. From the end of the Paleocene to the beginning of the Eocene, the Tancheng-Lujiang fault zone changed from sinistral strike-slip to dextral strike-slip, accompanied by mantle upwelling; the lithosphere of the Bohai Sea greatly thinned, and the Bohai Sea entered the first peak period of subtle fault development. Entering the Neogene neotectonics, the rifting weakened and the strike-slip activity intensified, and shear stress gradually became dominant. Pre-existing faults in the basement generally suffered tension-torsion transformation, and the development of subtle faults in the Bohai Sea entered another peak period.

A large number of physical simulation experiments have confirmed (Fig. 2) that during the development of basement pre-existing strike-slip faults, “R” shear, “R’” shear and “T” rupture often turn up firstly, and the strike-slip displacement activity is weak at the stage. Only with further increase of strike-slip displacement, “P” rupture and the primary deformation zone (PDZ) will come up^[22,23,24]. Most of the subtle faults in the Bohai Sea are generally closely related to early strike-slip activities. Due to weak strike-slip activity and small displacement, these subtle faults have not yet had high-strength strike-slip PDZ formed, therefore they can not be accurately identified on the seismic profile. However, the echelon faults surrounding them turning up earlier are easy to identify, and can be used as signs to judge potential strike-slip faults. There is a weak to strong stress transition zone between the echelon fault zone and the penetrating strike-slip zone. This type of transition zone is a favorable place to look for potential strike-slip faults. Secondly, tensile stress, another kind of major structural stress, also worked through the entire process of the development and evolution of the Cenozoic in the Bohai Sea. Affected by pre-existing fractures and heterogeneity of the basement, different blocks often have different stretching rates, and thus unbalanced stretching, which in turn gives rise to various types of transition zones and subtle faults adjusting uneven extension between large extensional faults at locations where the stretching rates differ greatly^[25]. The subtle faults of the type are formed in the local stress field and small in scale, therefore difficult to identify on the seismic profile (Fig. 3). Finally, the strike- slip fractional faults formed by the Neogene oblique stretching in the Bohai Sea are also worth of attention. Because the pre-existing faults in the oblique pulling process are at certain angles with the extension direction, part of the strike-slip component will produce strike-slip fractional faults around the pre-existing faults^[26,27,28], which is also a primary type of potential strike-slip fault in the shallow Bohai Sea (Fig. 4). Therefore, under the background of complex mechanical boundary conditions, diverse rock media properties, and different types of tectonic force sources, the re-activation of pre-existing strike-slip faults, the unbalanced extension between large extensional faults, and oblique extension in the tension-torsional stress system are important mechanisms for the development of subtle faults in the Bohai Sea.

The understanding on subtle faults in the Bohai Sea has undergone a process of preliminary exploration, detailed verification and maturity. In the early exploration practice, the identification of subtle faults and technical methods have achieved periodic results. Based on previous studies, we have sorted out four typical geological identification markers of subtle faults, irregular changes in sedimentary stratum thickness, inherited distortions and even sudden changes in occurrence, zonation of planar fault combination, and oil-water system contradiction in the same structure (lithology). We have established a set of subtle fault identification and interpretation methods, including seismic data preprocessing, seismic attribute calculation and optimization, fine fault identification and fault enhancement technology. The results of the research have been published in another article^[13], and the article will not go into details.

It should be pointed out that subtle faults are not in isolated existence, and can be comprehensively identified based on their typical geological characteristics when it is difficult to accurately predict and identify them solely relying on seismic reflection characteristics. On the plane, subtle faults are mostly in the early stages of fault development and evolution, with short extension distances and small scales, therefore these faults have partly hidden and partly visible traces with poor continuity on the seismic variance slices. On the section, the subtle faults have small fault displacements, the two walls of the same fault have small wave impedance difference, therefore it is impossible to form an effective abrupt reflection interface. At the same time, limited by the seismic data processing technology and seismic data resolution, the development characteristics and positions of subtle faults are very subtle. For example, various types of “flower-on- flower” structures appear in the sedimentary caprock on the seismic profile, that is, a certain fault of the early flower-like structure is the primary branch, and it re-activated during the neotectonics period, giving rise to a new stage of flower or flower-like structure (after the deposition of Guantao Formation). The “flower-on- flower” structure often indicates that potential strike-slip faults may exist in deeper formations (Fig. 5a). When there are strike-slip faults on the basement, as rocks in different depths and types differ widely, the same fault show different characteristics at different horizons, with clear traces in deep basement, and gradually weakening and deflection, and becoming subtle in shallow sedimentary formations (Fig. 5b).

For the same fault, it may have different activity intensities and show different characteristics in different sections; on the other hand, under different lithology combinations and burial depths, the differences of media properties make different formations have different strain characteristics to the same tectonic stress. Based on typical geological signs such as the combination of faults on profile and on plane, it is possible to predict and find subtle faults to make up for the drawback of seismic data.

Since the Cenozoic, extension and strike-slip have become the two major structural deformation factors dominating the Bohai Sea, which occurred in different regions and different stages at varying strengths. On the other hand, combinations of faults formed in different periods in space often result in various types of transition zones or accommodation zones. Different types of accommodation zones are mostly areas where the tectonic stress mechanisms transform, compound or weaken, and different types of subtle faults are likely to occur. These subtle faults formed under different strain mechanisms have different distribution regularities.

The strike-slip faults in the Bohai Sea are multi-directional laterally and multi-phase longitudinally. The NNE-trending Tancheng-Lujiang fault system and the NW-trending Zhangjiakou-Penglai fault system traverse the entire Bohai Sea area and intersect with each other, and have experienced multiple periods of strike-slip activity, especially the Tancheng-Lujiang fault system which experienced large-scale sinistral strike-slip activity in the Mesozoic and dextral strike-slip activity in the early Paleogene and neotectonics period respectively. The type of tectonic evolution background provided conditions for the development of two types of subtle faults: the first type of subtle fault includes mainly “P” shear, “R” shear and other small fractures in the strike-slip fault zone that often does not join up at the initial stage of formation. These shears are characterized by weak deformation, small size and strong concealment. The second type turns up in the flanks of large-scale strike-slip faults or the area between multiple strike-slip faults which are often local shear stress concentration zones; the shear stress concentration could cause different degrees of shear fracture of the sedimentary formations, but these areas are often far away from large strike-slip faults, and appear as areas accommodating deformation with no large fault displacement, thus resulting in subtle faults. The subtle fault induced by strike-slip fault system is the most widely developed type of subtle fault in the Bohai Sea. It can be further divided into 6 subtypes: horsetail type, “S” type, “P” shear type, and “R” Shear type, basement strike-slip type, and strike-slip associated type.

Stress release at the end of the strike-slip fault would lead to the development of a large number of arc-shaped normal faults, among which there are often some arc-shaped subtle faults with smaller fault displacements (See ① in Fig. 6). This type of subtle fault often appears at the end of a large strike-slip fault, with a curved shape similar to a horsetail shape and large extension. A typical one is the trap-controlling fault of the Jinzhou 20-2 North Oilfield in the Liaodong Gulf area.

The “S” type subtle faults often occur at the strike bends of strike-slip faults, where local stress conversion gives rise to pressure-releasing structures and derivative subtle faults in turn (See ② in Fig. 6). This type of subtle fault usually has a short extension and only comes up near the pressure-releasing zone. They appear in an echelon formation oblique to the primary strike-slip fault on the plane and are often in high density, for example, the trap-controlling fault in the Luda 21 Oilfield in the Liaodong Gulf area.

The “P” shear subtle fault is the subtle fault composed of P shear rupture formed in the early stage of strike-slip fault. In the later period, the continuous PDZ fault may be formed due to the continuous development of the strike-slip effect, but the “P” fault formed in the early stage still exists and controls the development of trap (See ③ in Fig. 6). This type of subtle fault is small in scale and extension, and has a small angle of less than 15° with the major fault generally, for example, the trap-controlling fault of the Kenli 12-2 structure.

The “R” shear type subtle fault is similar to the “P” shear type fault in formation mechanism, is also fault with smaller displacement formed at the initial stage of strike-slip and retained as subtle fault in the later stage of strike-slip modification (See ④ in Fig. 6). Subtle faults of this type are small in scale and short in extension distance, and have angles of 30°-40° with the major fault on the planar strike, for example, the subtle faults in the Penglai 31-3 south structure area.

The basement strike-slip subtle faults are these developing at the basement of the pre-Cenozoic. The large- scale sinistral strike-slip movement of the Tancheng- Lujiang fault zone in the early Mesozoic gave birth to a large number of basement strike-slip faults. Some of the faults didn’t reactivate during the late dextral strike-slip process, moreover, the later Cenozoic shielding to seismic data makes basement faults vague, leaving potential strike-slip faults in the basement buried hill (See ⑤ in Fig. 6). The major faults of these subtle faults are often distributed in a grid pattern, with large density, large extension, large scale, steep fault surface on the section, and consistent strike, for example the major trap-controlling fault of the Kenli 10-1N Structure.

Subtle faults associated with strike-slip often appear between two or more large-scale strike-slip faults. Because the primary strike-slip strain is absorbed by large boundary faults, micro-distance subtle faults are likely to occur between the large faults due to the weakening of the strike-slip strain, possibly in the form of “R” shear at the initial stage of the formation of strike-slip fault or major strike-slip fault after further development (See ⑥ in Fig. 6). This type of subtle fault is consistent with the boundary strike-slip fault in strike, long in extension distance and low in density. The potential strike-slip fault between Penglai 20-2 oilfield and Longkou 20-5 structure belongs to this type (Fig. 7).

In the Cenozoic, the Bohai Sea was accompanied by strong extensional rift under the action of strike-slip, therefore the extensional fault system is well developed in the Bohai Sea. The Cenozoic suffered multi-stage extensions, especially diagonal extensions in different directions, resulting in the development of different types of subtle faults. The subtle faults developing in the extensional fault system mostly occur in the overlapping areas or striking turning points of large extensional faults. On the one hand, due to the differences in extension direction and quantity of multiple large extensional faults, local shear stress concentration zones are likely to form; on the other hand, stress concentration and transformation of stress properties often occur at the turning points of extensional faults, forming shear stress fields, and leading to rupture of rocks and formation of subtle faults. In the Bohai Sea, subtle faults under the extensional system mainly include two subtypes: normal fault adjustment type and bump transition type.

The development of normal-accommodating subtle faults is because the difference in basement properties or the change of fault direction during the uniform extension process leads to different extension rates of different blocks, and in turn formation of structural adjustment zones with strike-slip fracture property at the junctions of the blocks, in which subtle faults often develop (See ⑦ in Fig. 6). Subtle faults of this type are short in extension, relatively isolated, and located between extensional faults on the plane, and cut through limited horizons on the profile. Most of them appear in the form of interlayer faults, such as the trap-controlling fault in the Penglai 13-2 Oilfield and the potential strike-slip fault in West Bozhong 34-1 structure (Fig. 8).

The development of the bump transition subtle fault is because the local variation of direction of large-scale extensional normal fault causes stress concentration zone and the transformation of stress property. Consequently, the rigid stratum of the upthrown wall basement undergoes rotation of fault block, resulting in the activation of previously existed faults in the block and formation of subtle faults of tension and twist nature (See ⑧ in Fig. 6). In addition, in the downthrown wall of bump transition zone, stress concentration often results in a large number of subtle faults too. These subtle faults may be parallel to the latter and perpendicular to the primary fault. In terms of development location, the type of subtle fault usually appears near large-scale boundary-controlling faults. The subtle faults in the upthrown wall of large-scale fault are large in scale and long in extension, and usually occur in large number due to the rotation of fault block. In contrast, the subtle faults in the downthrown wall are small and scattered, for example, the subtle faults in the hanging wall and footwall walls of the boundary fault of the Shaleitian Uplift.

The subtle faults formed in the strike-slip-extension composite fault system mainly refer to subtle faults developing under the background of tenso-shear structure, or subtle faults developing from pre-existing faults under the effect of late extension. In the exploration practice of the Bohai Sea, these faults mainly come in four types: side-connection transfer type, caprock strike-slip type, tension-torsion transfer type and conjugated type.

Side connection transfer type subtle faults usually occur inside the lateral connection extension-torsion faults. Under later extension-torsion stress, due to the rotation of the fault block in the lateral connection zone, potential normal faults or potential strike-slips would like to occur at the lateral connection location (See ⑨ in Fig. 6). This type of subtle faults usually turn up between large lateral faults and are in arc distribution and in small scale, for example, the potential trap-controlling faults in large number in the Bozhong 36-1 Oilfield.

The caprock strike-slip subtle faults refer to the strike- slip faults at the basement of the pre-Cenozoic. Some of them reactivated in the late period and extended to the caprock. Due to the late tension-torsion tectonic background, the caprock faults experienced vertical displacement. With extensional nature, they have rarely flower- like structures and strike-slip derivative faults. At the same time, affected by the dip angle of the basement fault, they are upright in the caprock (See⑩ in Fig. 6). This type of subtle fault is usually associated with basement strike-slip faults and has long extension in the caprock, upright fault plane, and is isolated. For example, the main blocking faults in Kenli 9-1 Oilfield are this type (Fig. 9).

Extension-torsion transformation subtle faults occur at the intersections of primary strike-slip faults and secondary faults. Under the later extension-torsion structural background, accommodation strike-slip faults parallel to the primary faults develop due to stress conversion near the secondary faults (See ⑪ in Fig. 6). Generally, these subtle faults are characterized by small vertical fault displacement and small extension range and appear near primary strike-slip faults. Strata on two sides of the subtle faults differ widely in occurrence. For example, the strata on the west side of the subtle fault in Bozhong 8-4 Oilfield are steeper, while the strata on the east side are gentler.

Conjugate subtle faults are subtle faults turning up near two intersecting extension-strike-slip faults. Due to the large differences in strike, one of the faults usually has more evident strike-slip nature, resulting in alternate pressure-increase zones and pressure-release zones in the intersection. The rapid transition of stress led to the development of a large number of subtle faults near the primary faults (See ⑫ in Fig. 6). These subtle faults are parallel to or perpendicular to the major faults, large in density and shorter in extension length, for example, Caofeidian 5-5 and Caofeidian 6-4 structures.

The differences in the combination of fault systems in the Bohai Sea result in evident zonation of the three types of subtle faults in the Bohai Sea. The subtle faults controlled by the strike-slip fault system mainly occur in the eastern part of the Bohai Sea area dominated by the NNE-trending Tancheng-Lujiang strike-slip fault system, including the Central Liaoning Sag in the Liaodong Gulf, the Liaodong Uplift, and both sides of East Bohai Sea low uplift and Miaoxi area in the eastern Bohai. The subtle faults controlled by extensional fault system mainly appear near large bulges in the southern and western parts of the Bohai Sea, including the downthrown wall on the southern part of the Shijiutuo Uplift and the periphery of the southern boundary of Shaleitian Uplift and Laibei low uplift. The subtle faults in the strike-slip-extensional composite fault system are widely distributed in the Bohai Sea, especially in the southern Bohai area where the strike-slip and extensional fault systems intersect in “#” pattern (Fig. 6).

Subtle faults and explicit faults belong to the same structural system and are controlled by the same tectonic stress system. Therefore, in terms of the genesis of faults, subtle faults and explicit faults have similar controlling effects on traps, and both can form fault block traps and fault nose traps and structural-lithological composite traps blocked by fault. The trap-controlling characteristics of subtle faults match with their development characteristics. With small scale in general, they cause weak deformation in small amplitude of surrounding strata, so it is difficult to form independent traps by subtle faults themselves. Subtle faults are mostly formed in local stress systems, making them inconsistent in strike with large-scale explicit faults, and most of them intersect at large angles. Therefore, subtle faults and explicit faults can combine or intersect to control traps, thus forming large-scale low-amplitude traps. This greatly increases the possibility to form subtle traps, making the subtle faults important trap-controlling faults in the Bohai Sea.

The traps formed by involvement of different types of subtle faults have their own characteristics. The subtle faults formed by strike-slip, with small vertical fault displacement, are difficult to identify on the seismic profile. With large horizontal displacement and little change to the occurrence of strata, they can bring about structural traps with gentle amplitude and large scale by matching with the occurrence of strata in seemingly impossible sites. For example, in the Penglai 20-2 Oilfield in the eastern part of the Bohai Sea, there was no evident fault identified separating the Penglai 20-2 Structure and the Penglai 20-3 Structure in the eastern part in the early exploration period. The drilling failure of the Penglai 20-3 Structure casted a shadow on the next exploration of the Penglai 20-2 Structure. With the continuous deepening of research on subtle fault identification technology and trap-controlling model, the subtle fault associated with strike-slip on the eastern part of Penglai 20-2 Structure was accurately identified, and so the flat Penglai 20-2 Structure in the western part could form a large-scale structural trap. Subsequent several wells deployed in the lower part of the Penglai 20-2 Structure tapped oil layers successively, finally discovering the Penglai 20-2 Oilfield (Fig. 10).

In the extension-strike-slip composite fault system, fault systems of different strain properties and different strikes often occur. With certain angles in between, the subtle faults, explicit faults/strike-slip faults are likely to intersect, making structural trap “change from small to large”, thus forming large-scale structural trap groups. For example, in the Bozhong 36-1 Structure in the southern part of the Bohai Sea, several lateral adjustment-type and extension-torsion transformation subtle faults have been identified in the composite transition zone between the strike-slip fault system in the east and the extensional fault system in the west. These subtle faults intersect with large-scale explicit extensional faults to form a large-scale contiguous fault-block trap group, which greatly increases the area of the structural traps (Fig. 11).

The subtle faults controlled by extensional fault systems mostly occur between two extensional fault systems, and the structural traps formed by them are relatively small, this is mainly because they are limited in extension length, most of them perpendicular to explicit extensional faults, and relatively isolated, making it difficult for them to form large trap groups.

One of the important factors for the late accumulation of oil and gas under the background of neotectonics in the Bohai Sea is the development of a large number of shallow faults which provide pathways for further adjustment and distribution of oil and gas to the shallow layers. Based on the genetic mechanisms of subtle faults mentioned above, the neotectonics period is also an important period for the development of subtle faults in the Bohai Sea. The large number of subtle faults provide effective channels for the upward distribution of oil and gas. It should be noted that although subtle faults as oil and gas migration channels aren’t as efficient as explicit faults, high density of subtle faults often indicate that explicit faults are more active. Therefore in the areas with dense subtle faults, the tectonic stress is fully released, the oil migration and accumulation are more active, and the transport system formed by subtle faults and explicit faults can further regulate the distribution of oil and gas.

On the other hand, exploration practice has confirmed that the subtle faults developed in the transition zone often have a good coupling configuration with the deep transition zones or convergence ridges, and can form ridge-fault connected high-efficiency oil and gas transport channels. Taking the Kenli 12-2 Structure as an example, the subtle faults communicate with deep mature source rocks, and underwent multiple periods of structural inversion during the deposition period of the Shahejie Formation and the Dongying Formation, forming a deep inversion structural ridge. During the Neogene accumulation period, oil and gas in the northern sag migrated through unconformities toward the deep structural ridge, and the subtle faults communicated with the deep structural ridge to form a ridge-fault connected high-efficiency oil and gas transport channel. The Well KL12-2-1 drilled in the structure revealed that oil and gas have migrated to the Dongying reservoir at a buried depth of 570 m in the area, which confirms the control effect of subtle faults on the oil and gas migration (Fig. 12).

The Yellow River Mouth Sag is located in the southern part of the Bohai Sea, and structurally belongs to the Jiyang Depression. It has the typical features of fault depression with “fault in the north and overlap in the south”. The three branches of Tancheng-Lujiang fault pass through the center of the sag, dividing the sag into three subsags, the eastern, middle, and western ones. The Yellow River Mouth Sag is one of the three largest hydrocarbon-rich sags in the Bohai Sea. By the year of 2010, Bozhong 25-1, Bozhong 25-1 South, Bozhong 28-34, Bozhong 26-3, Bozhong 29-4, and Bozhong 35-2 and Kenli 3-2 oilfields had been discovered, with cumulative proven petroleum geological reserves of nearly 6×10⁸ t, and an annual production capacity of 700×10⁴ t had been built in this area. This area has the highest exploration degree of the middle and shallow layers in the Bohai Oilfield (Fig. 13). From the distribution of oilfields discovered in the Yellow River Mouth area, it can be seen that the oilfields are mainly located in the Tancheng-Lujiang strike-slip fault zone and the northern steep slope zone of the sag, for example, the Bozhong 25-1 Oilfield located in the western branch of the Tancheng-Lujiang fault zone, and the Bozhong 25-1 Oilfield located in the central branch of the Tancheng-Lujiang fault zone, and the Zhong 28-34 oilfield group and Penglai 19-3 Oilfield in the eastern branch of Tancheng-Lujiang fault zone. After more than 30 years of exploration, almost all large-scale structural traps in the area have been drilled, and it is increasingly difficult to discover large- and medium-sized oil and gas fields. By 2010, only rolling exploration targets with an area of about 1 km² were left for oil and gas search, and there were no fault block traps larger than 3 km², with the exploration entering a difficult period.

Since 2010, under the guidance of genetic mechanism of subtle faults and structural analysis, a number of large-scale structural trap groups have been discovered successively in the mature areas of the Bohai Oilfield where no large-scale structural traps had been found after multiple rounds of structural interpretation before. An individual structural trap has changed from the original 2.3 km² to a large fault block trap group of over 50 km². Bozhong 29-6, Bozhong 36-1, Kenli 9-1, and Kenli 9-5, and Bozhong 34-9 oilfields have been found in the Yellow River Mouth area. The subtle fault-related oilfields in the Bohai Oilfield have proven 3P geological reserves of 4.5×10⁸ t cumulatively, and established production capacity of 150×10⁴ t. In the following, the Bozhong 29-6 Oilfield is taken as an exploration example of the subtle fault-related oilfield to discourse.

6.2.1. Exploration history

The Bozhong 29-6 Structure is located in the northern steep slope zone of the middle subsag of the Yellow River sag. Since the drilling of Well BZ29-1-1 in 1983, the area has undergone more than 30 years of exploration. A total of 25 wells have been drilled, and Bozhong 29-1, Bozhong 29-1, Bozhong 29-4, Bozhong 29-5 and Bozhong 35-2 small- and medium-sized oil and gas fields had been discovered. Since 2010, almost all the large-scale traps in the area had been drilled, and it is more and more difficult to discover large and medium-sized oil and gas fields. Following the idea of searching for structural lithological traps, Bozhong 29-1 East and Bozhong 29-4 West two small oil- and gas-bearing structures have been discovered successively. Limited in reserve scale, they are difficult to produce. It is urgent to find large- and medium-sized oilfields in such highly mature exploration areas to drive the development of surrounding small oilfields and improve the economic benefits of the overall development system. On the other hand, affected by the NNE-trending Tancheng-Lujiang and NSW-trending Zhangjiakou-Penglai strike-slip faults jointly, the northern steep slope zone of the middle sub-sag of the Yellow River sag witnessed evolution of tectonic stress in multiple stages and directions, and complex tectonic development and evolution process. Meanwhile, seriously affected by shallow gas clouds, seismic data of this area has poor quality and vague fault imaging, which severely limits the structural interpretation of the area. As a result, large-scale traps haven’t been found after years of exploration.

6.2.2. Neotectonic trap and reservoir discovery

From the perspective of regional tectonic position, the northern steep slope zone of the middle subsag of the Yellow River Mouth Sag is located in the intersection area of NNE-trending Tancheng-Lujiang and NW-trending Zhangjiakou-Penglai two major strike-slip faults, and the South Bohai area had strong strike-slip fault activity during the neotectonics period (Fig. 1). The Bozhong 29-4 and Bozhong 28-34 oilfield groups in the Bozhong 29-6 structural enclosure both have subtle faults discovered in the later oilfield development, while the structural interpretation of the Bozhong 29-6 structural area only showed small scattered fault block groups controlled by two NWW trending faults and no shallow faults, which does not match the geological characteristics of the Bozhong 29-6 structural area and its structural location. On the other hand, the Bozhong 29-6 structural area has local gas clouds. According to the exploration history and geological laws of the Bohai Sea, this is an important sign of concentrated release of tectonic stress and active migration and accumulation of shallow oil and gas. Based on the above analysis, the Bozhong 29-6 structural area, with strong strike-slip activity in the late period and sufficient stress release, has the tectonic stress conditions for the development of subtle faults.

By examining the NNE-trending Tancheng-Lujiang fault zone and the NSW-trending Zhangjiakou-Penglai fault zone in the Bozhong 29-6 structural zone, it is found that the Zhangjiakou-Penglai fault system is “strong in the west and weak in the east” in this area, while the Tancheng-Lujiang strike-slip faults is “strong in the east and weak in the west”. The two groups of strike-slip faults exhibit complementary activities in the area, and the NW-trending Zhangjiakou-Penglai fault system had stronger tectonic activity in the late period, making us realize that identification of the NWW strike-slip fault system and its related subtle faults should be strengthened. In 2016-2017, the seismic interpretation of the newly processed 3D merged seismic data of the exploration area in Southeast Bohai paid more attention to the fine identification of the NW-trending Zhangjiakou- Penglai fault system more active during the neotectonics and its related subtle faults. It was found that several faults in the northwest-trending Zhangjiakou-Penglai strike-slip fault system were highly superimposed on the plane and had evident signs of late activity. According to the genetic mechanisms of subtle faults, the accommodation zone formed by this type of strike-slip superimposed area is the favorable place for the development of subtle faults. Under the guidance of abovementioned geological model, seismic technology was researched to identify and interpret subtle faults controlled by the strike-slip fault system, and finally a batch of subtle faults were discovered in the shallow Neogene. From the distribution of the newly discovered subtle faults in the Bozhong 29-6 structural area, the subtle faults are mainly located between or in the flanks of multiple NW-trending Zhangjiakou-Penglai faults. The faults are mostly in NE-NNE- trend and are mostly “P” shear, convex transition and tension-torsion conversion types. Based on the interpretation of the NW-trending Zhangjiakou-Penglai fault system and its related subtle faults in the Bozhong 29-6 structural area, the structural trap group appears as a large-scale fault block trap group controlled by four large-scale NW-trending Zhangjiakou-Penglai strike-slip faults and their derivatives, or adjusted branches. The overall area of the structural trap group on the T0 seismic reflector is over 50 km², thus realizing the discovery and confirmation of the large-scale structural trap group in this area (Fig. 14). Taking the steep slope zone with most developed subtle faults as an example, the subtle faults and explicit faults in the steep slope zone intersect at high angles or almost perpendicularly, forming interconnected independent fault block trap groups, with a single fault block trap area of 2.0-4.6 km². Taking the Well BZ29-6S-1Sa controlled by subtle faults in the steep slope zone as an example, the well encountered oil layers of 32.4 m thick; Well BZ29-6-7 controlled by subtle faults in the uplift area encountered an oil layer of 41.9 m thick, and has petroleum geological reserves of nearly 1200×10⁴ m³, and 3P geological reserves of 2370×10⁴ m³, making this well block the one with the highest abundance of reserves in the oilfield.

6.2.3. Oil migration through subtle faults and discovery of shallow oil reservoirs

Exploration has confirmed that the zones between and flanks of boundary faults striking NWW in the Bozhong 29-6 Oilfield are the area with the highest density of subtle faults and also the highest abundance of oil and gas reserves. The oil and gas migrate vertically through the NW-trending boundary fault, and then further migrate to the shallow layer through branch faults or subtle faults, and accumulate in the central part of the fault system into a reservoir. In the area adjacent to the footwall of boundary fault in the uplift zone, there is a mesh-carpet transport system, the oil and gas migrate along the boundary faults to the shallow layer, then migrate along the three-dimensional network formed by the unconformity surface and sandy conglomerate of the Guantao Formation, and accumulate in the structural highs at last. The development position and degree of structural ridges control the degree of hydrocarbon enrichment. The Bozhong 29-6 Oilfield shows “vertical connected ridge-fault joint control” oil and gas accumulation model. The lower member of the Minghuazhen Formation is a structural lithological reservoir controlled by dual factors of structure and lithology, and the Guantao Formation is a layered reservoir controlled by structure (Fig. 15).

The fine identification and interpretation of subtle faults makes the trap area of the Bozhong 29-6 structure and the scale of oilfield reserves increase significantly. In the end, the Bozhong 29-6 Oilfield has overall 3P geological reserves of nearly 1.2×10⁸ t discovered and has entered into the development stage. The successful discovery of the Bozhong 29-6 Oilfield confirmed the guiding role of the subtle fault interpretation model on the interpretation of structural traps and the control of subtle faults on shallow oil and gas accumulation.

Based on the comprehensive summary of exploration practice and development characteristics of subtle faults in the Bohai Oilfield in the past 10 years, it is made clear that complex strike-slip fault patterns, diverse stratigraphic rock media, and strong neotectonics are the fundamental conditions for the development of subtle faults in the Bohai Sea. The reactivation of pre-existing strike-slip faults, unbalanced extension between large extensional faults, and oblique extension under the tension-torsional stress system are the important genetic mechanisms of subtle faults in the Bohai Sea. Based on the development characteristics and genetic mechanisms of subtle faults in the Bohai Sea, the authors sort out four major signs of subtle faults in the Bohai Sea, namely, unreasonable change in sedimentary strata thickness, inherited stratigraphic distortion and even sudden change in stratum occurrence, and zoning characteristic of plane fault combinations, and the difference of oil-water system not resulted from lithological variations in the same structure (lithology). Seismic attribute calculation and optimization, fault fine identification and fault enhancement technology have worked well in identifying subtle faults. Subtle faults in the Bohai Sea can be divided into strike-slip fault-controlled type, extensional fault-controlled type, and strike-slip-extension composite type according to their tectonic origin and development location. The different types of subtle faults have evident zoning in the Bohai Sea. Subtle faults have a significant control effect on the formation of structural traps and oil and gas migration. The matching of subtle faults with explicit faults and stratigraphic occurrence has high efficiency in forming traps, and can greatly expand the scale of structural trap groups.

The Bohai Oilfield is still in the exploration stage dominated by structural traps. Fine structural interpretation is still an important guarantee for increasing the success rate of exploration. In recent years, with the continuous advancement of geological theory and technology, the analytical thinking and interpretation model of subtle faults in the Bohai Oilfield have become increasingly mature, and many areas have got breakthroughs in oil and gas exploration, and have become important areas for reserves increase in mature areas. The exploration of subtle fault development areas in the Bohai Sea has fully confirmed the important controlling effect of subtle faults on oil and gas accumulation, effectively expanding the exploration space of the highly mature area in the Bohai Sea, and has guiding significance to the oil and gas exploration in the Bohai Sea or even in eastern China.