Continental fault basins are characterized by complex tectonic features and rapid spatial variation of sand bodies^[1,2,3]. The degree of hydrocarbon enrichment is controlled by multiple factors, such as fault activity^[4,5,6], sand body distribution^[7,8,9,10], and sedimentary system^{[11,12,13,14,15]} etc. In recent years, the coupling relationship between structural and sedimentary factors in fault basins has become the focus of exploration research. Researchers have put forward a series of concepts such as "theory of hydrocarbon enrichment in favorable facies of slope areas of fault lake basins", "sequence structure pattern of continental fault basins", "sand control by structural slope break " etc.^{[16,17,18,19,20,21,22,23]}, which have provided theoretical basis for the in-depth exploration of complex fault basins. With the increasing complexity of exploration targets, more and more attention has been paid to the role of fault-sand assemblage pattern (also known as fault-sand collocation, fault-sand assemblage mode^{[24, 25]} or fault combination type^[26], collectively referred to as fault-sand assemblage pattern) in hydrocarbon accumulation. A lot of research in this regard has been carried out, mainly including classification^[25,26] according to the assemblage relationship between fault and sand occurrence. The previous researches showed that the fault-sand assemblage mode determined the trap type and distributed location of oil and gas^{[25, 27, 28]}, and controlled the migration of oil and gas both laterally^[24] and vertically in terms of time^[29] and space^{[30,31,32,33,34]}. The research results of the fault-sand assemblage pattern have significantly promoted the fine exploration of hydrocarbon in the mature areas of eastern fault basins.

Qikou Sag, belonging to the Bohai Bay Basin, is a typical dustpan-shaped fault depression with developed faults. In the past, a number of fault blocks, fault noses and anticlinal reservoirs were discovered following the idea of searching oil on the basis of structure. But after more than 50 years of exploration, it is more and more difficult to find such reservoirs and the scale of reserve increment is becoming smaller and smaller. Binhai fault nose is located on the eastern flank of central uplift zone of Qikou Sag. The shallow Minghuazhen & Guantao Formations in Neogene Gangdong Oilfield and deep lower sub-member of the First Member of Shahejie Formation in Madong Oilfield discovered are all structural reservoirs. Hydrocarbon shows or sporadic oil and gas layers were found in some wells in the layer systems between them, but they weren’t paid much attention to due to the lack of structural traps. Recent studies show that there are multiple sand body zones in this area. These sand body zones connect with oil source faults, which is conducive to the formation of structural lithologic reservoirs. Oil and gas exploration in this area carried out under the guidance of fault-sand assemblage pattern idea has ended with important discoveries, i.e. of 29 wells drilled, 26 wells have tapped industrial oil flow, with 30 million tons of reserve increment confirmed.

In this work, on the basis of previous knowledge, fracture elements and sand body distribution characteristics of Binhai fault nose in Qikou Sag were counted by using 3D seismic and logging data of more than 80 wells to find out the assemblage pattern between faults and sand bodies. It is confirmed that dominant hydrocarbon migration faults and dominant sand bodies jointly control the oil and gas accumulation and enrichment. Moreover, the accumulation control patterns by fault-sand in the Binhai fault nose are determined, in the hope to provide guidance and reference for fine exploration of Bingai fault nose in Qikou Sag and complex fault zones in continental rift lake basins.

Qikou Sag is a rift lake basin developed in Meso-cenozoic. The sag has undergone multiple stages of tectonic transformation and thus complex fault characteristics^[35,36,37]. Binhai fault nose in the study area is a large nose-like structure^[38] attached to the downthrown side of the Binhai fault (Fig. 1). With a tectonic area of 260 km², it connects with Gangxi uplift in the west and gradually transits eastward to the main sag of Qikou. The Binhai fault nose is dominated by Paleogene- Neogene sediments. From bottom to top, are member three, member two, member one of Shahejie Formation (hereinafter referred to as "Sha 3", "Sha 2" and "Sha 1"), and Dongying Formation of Paleogene as well as Guantao and Minghuazhen Formations of Neogene. The study area, rich in hydrocarbon resources, is a multi-layer oil-bearing composite oil and gas accumulation area. The vertical distribution of oil and gas varies greatly. The overlying Gangdong Oilfield of the fault nose is a complex fault block type Neogene oilfield which has been developed for more than 50 years. In the early stage of exploration, following the thinking of structural hydrocarbon prospecting, the anticlinal Madong and Maxi Oilfields were discovered in the lower sub-member of Sha 1 in the middle part of the fault nose. After that, few discoveries have been made and exploration has fallen into stagnation. On the basis of previous fundamental study on structural characteristics and sedimentary systems etc.^{[39,40,41,42]} as well as the study on accumulation control by fault-sand, the fault-sand assemblage patterns and their control on hydrocarbon accumulation have been examined in detail in this work.

Since Paleocene, a large number of normal faults^[35] have been generated in the Binhai area of Qikou Sag under the tensional-torsional regional stress background, mainly including Gangdong Fault (F1), Gangdong Fault Branch (F2), Tangjiahe Fault (F3) and a series of secondary faults have been derived from both sides of them (Fig. 2). Statistical analysis of fracture elements shows that the Binhai fault nose mainly has two groups of faults in NE and EW strikes (Table 1). The main faults, including Gangdong (F1), Tangjiahe (F3), etc. in NE strike control the formation and evolution of the fault nose (Fig. 2). The Gangdong Fault is the largest one in the study area, extending about 20 km across the whole Binhai area. Tangjiahe fault intersects obliquely with the Gangdong Fault in its north side. In addition, on the downthrown side of the main fault in the eastern part of the fault nose, there is a group of near EW secondary faults. These faults are small in scale, with the extension length of 2.8-8.7 km. They intersect obliquely with the main fault, forming a broom-like fault system convergent in the west and spreading in the east. Controlled by the two sets of fault systems, the fault nose in the Binhai area has different structural characteristics in different segments. The western segment is simple in structure, with a complete shape of the fault nose. The eastern segment is fragmented, forming a fault nose structure complicated by faults.

Fault activity plays an important role in sedimentary filling, trap formation, as well as oil and gas migration and accumulation in fault basins. Therefore, the study of fault activity law has practical significance for oil and gas exploration. In this study, the activity characteristics of the faults in the study area were counted by the fault growth index method and the average activity rate method. It can be seen from the activity analysis diagram of the faults (Fig. 3) that the differences of growth index, drop and activity rate indicate the activities of the main faults have obvious difference in time and space. The Gangdong Fault (F1) came up earlier and began to move during the sedimentary period of Sha 3. The activity rate is the highest in the middle segment of the fault, reaching 200 m/Ma, and gradually weakens to both sides. The activity rate of the fault changes rapidly along the strike direction, showing typical characteristics of section-wise development. During the sedimentary period of Sha 2, the activity rate of the fault decreased, but the activity rate of the middle section (100 m/Ma) was still higher than that of the east and west sides (less than 50 m/Ma). During the sedimentary period of Sha 1, the Gangdong Fault began to move intensely. The middle segment of the fault had the highest activity rate of 250 m/Ma, and the western segment had an average activity rate of 100 m/Ma. The eastern segment had the smallest activity rate of 50 m/Ma on average. During the sedimentary period of the Dongying Formation, the Gangdong Fault was still very active, with activity pattern similar to that of the sedimentary period of Sha 1, featuring high activity rate in the middle and gradual dropping rate on both sides. During the sedimentary period of Guantao Formation, Qikou Sag changed from rifting to depression, and the fault system basically stopped moving. During the sedimentary period of Minghuazhen Formation, the Gangdong Fault began to move again, but with activity intensity decreasing greatly. The fault activity mainly concentrated in the middle section, at the rate of up to 50m/Ma. The east and west sides of the fault were almost inactive during this period (Fig. 3a). Compared with the Gangdong Fault, the Gangdong branch fault (F2) and the Tangjiahe fault (F3) feature late active period and low activity intensity (Fig. 3b and 3c). These two faults began to develop during the sedimentary period of Sha 1, and were 60 m/Ma and 90 m/Ma in activity rate, respectively. During the sedimentary period of Dongying Formation, Gangdong branch fault and Tangjiahe fault were most active, with a maximum activity rate of 140 m/Ma and 120 m/Ma respectively, and they dropped in activity intensity significantly during the Neogene sedimentary period. The series of secondary faults in the eastern segment of the fault nose have activity pattern similar to that of the Gangdong branch fault and the Tangjiahe fault (Table 1). These faults generally appeared fairly late, and began to move from the end of Sha 1 to the sedimentary period of Dongying Formation. They were active mainly in the sedimentary period of Dongying Formation, at the activity rate of 30-110 m/Ma, and weakened in activity during Neogene. Generally, the Gangdong Fault is an inherited synsedimentary fault, which controls the overall structural pattern of the study area. The series of faults in the eastern section came up late, cutting and thus making the structure more complex of the eastern section of the fault nose.

The differences in the distribution and evolution of faults make the fault nose tectonic units different in features. Spatially, the Binhai fault nose can be divided into three segments: the west, the middle and the east according to the difference in tectonic characteristics (Fig. 2).

(1) Western segment: The western segment of the fault nose is characterized by simple structure and obvious inheritance from bottom to top. From the seismic profile (Fig. 4a), the south-east-strike Gangdong Fault has a low activity intensity and a small drop in the western part of the fault nose. The fault drop in each sedimentary period is less than 400 m. The strata on the two sides of the fault are roughly the same in thickness, with no obvious synsedimentary phenomenon. Only in the sedimentary period of Sha 1-Dongying Formation, the strata in the downthrown side increased in thickness. On the whole, the western part of the fault nose has few secondary faults. Only on the downthrown side of the Gangdong Fault, there are a few "prematurely weakening" secondary faults in antithetic dipping, which constitute an "asymmetric" graben structure with the Gangdong Fault.

(2) Middle segment: There are deep and shallow two sets of fault systems in the middle segment of the fault nose. The deep structure is simple, with only one main fault, Gangdong Fault. From the seismic profile (Fig. 4b), the strong activity of Gangdong Fault in the middle part of the fault nose results in large stratum drop in the deep part. The stratum drops are more than 1,000 m during the sedimentary period of Dongying Formation and Sha 1, showing clear synsedimentation of faults, and a large nose-like structure controlled by a single main fault is formed in the deep part of the middle segment of the fault nose. The shallow formations are fragmented, and the continuous activity of Gangdong Fault derives a series of secondary faults inclining in the same direction or opposite direction on the downthrown side of the fault, which together constitute "flower-like" structures.

(3) Eastern segment: This segment has many faults and complex structural features. The Gangdong Fault began to bifurcate in the eastern part of the fault nose, forming two faults, a main one and minor one. Although the activity of the main fault in the eastern part of the fault nose is weak on the whole, with an activity rate of less than 50 m/Ma in each sedimentary period, the Tangjiahe fault in the North began to develop due to the superimposed transformation of the dextral strike-slip stress, and they are arranged in right echelon with the Gangdong Fault. In addition, there are a large number of secondary faults near the east-west direction on both sides of the NE main fault, forming a broom-like fault system spreading eastward on the plane. It can be seen from the seismic section (Fig. 4c), the main body of the north fault nose is cut by a series of parallel south-dipping consequent faults, forming a complicated fault nose structure, and transiting to the south into a series of north-dipping faults with weak activity. Shallow-level faults incline toward each other and are independent of each other, presenting the structural style of "cabbage" on the whole

In summary, there are two groups of fault systems in Binhai fault nose. The activity period of the faults is the sedimentary period of Shahejie Formation and Dongying Formation. The differences in the distribution and evolution of faults correspond well to the segmentation of the structural characteristics of the Binhai fault nose. The western segment of the fault nose features low activity and simple structure. In the middle section, the faults had high activity intensity, causing strong drop in the deep formations and flower-like structure on seismic profile in shallow formations. The eastern section is controlled by the NE faults such as Gangdong and Tangjiahe and the near-EW secondary faults, forming the complex fault features nowadays.

Controlled by two major provenances out of the basin, the Yanshan fold belt in the north and Cangxian uplift in the west, there develop fan delta, braided river delta and far-shore underwater fan in Binhai fault nose^[43,44] (Fig. 5). The Shahejie Formation is dominated by delta and gravity flow deposits. The sedimentary period from Sha 3 to Sha 2 was the initial expansion period of the lake basin, when braided river delta deposits developed. The underwater distributary channel of braided delta front and mouth bar in the front of braided river delta are the main deposits. The sedimentary period of Sha 1 was the largest lake expansion period, when the types of sedimentary facies changed greatly, and mainly far-shore underwater fan, turbidite channel and other gravity flow deposits developed. Among them, gravity flow channels are obvious, and multi-stage channels superimpose, forming multi-stage fan-like gravity flow reservoir.

Based on the cores and logging data of more than 80 wells, the thickness of sandstone is counted by sand groups. Sha 2 is divided into three sand groups from bottom to top: Bin IV, Bin III and Bin II. The lower sub-member of Sha 1 is divided into four sand groups: Bin I, Ban 4, Ban 3 and Ban 2. The upper sub-member of Sha 1 is divided into four sand groups ①-④, and the thickness of sandstone in them was counted separately. Taking the typical sand groups of the Bin IV of Sha 2 and Ban 2 of the lower sub-member of Sha 1 and ② of the upper sub-member of Sha 1 as examples, the sand body distribution laws of the layer series are analyzed (Fig. 6). During the sedimentary period of Sha 2, mainly braided river delta deposits developed in the study area. The sand bodies are in NE-SW strike, and large in area and range. The sand bodies are thick in two parts in the middle and eastern segments of the fault nose, with the maximum thickest of 30-40 m. Wells qb1701 and bins22-12 located on the downthrown side of the Gangdong Fault in the middle segment of the fault nose revealed thick sandstone in Sha2 (Fig. 6a). According to the relationship between faults and sand bodies, during the deposition of Sha 2, the tectonic activity was weak, and the series of faults in the eastern part of Shahejie Formation didn’t begin to move; only the middle part of Gangdong Fault developed and controlled the filling of sediments, forming a local thick sand body area on the downthrown side.

During the sedimentary period of the lower sub-member of Sha 1, the sedimentary environment of the study area changed and far-shore underwater fan deposits widely developed. The sandstone thickness map of Ban 2 sand group shows that the sandstone body distribution in this period has obvious zonation. In the eastern part of the fault nose, the sand bodies are distributed in NS strike and finger or band shape. The main channel sand bodies are thick, up to 20-30 m. For example, thick sand layers were drilled in Wells gs42x1 and gs64 in the Ban 2 sand group. The sand bodies thin rapidly towards the two sides of the channel and turn into fine grained or muddy sediments. For example Well gs42, which is located in the west of the Well gs42x1, no sand layers have been drilled in the Ban 2 sand layers (Fig. 6b). In the middle part of the fault nose, the sand bodies are in large continuous pieces, and in the same strike of NE-SW as that of the Gangdong Fault. The sand bodies mainly occur in the downthrown side of the Gangdong Fault, with the maximum thickness of 30-40 m, and there is no sand body in the upthrown side. From the perspective of the distribution relationship of fault and sand bodies, the Gangdong Fault had strong control on sand distribution, and its strong activity gave rise to a large accommodation space on the downthrown side, and thus a thick sand body distribution area in the middle part of the fault nose. However, the series of faults in the eastern part of the fault nose just started to develop or have not yet developed in this period, so the sand distribution wasn’t controlled by fault obviously, and the sand body convergence was mainly controlled by paleogeomorphology. With the bending and shifting of the channel, the channel was divided into many branches, distributed in NS strike like fingers along the eastern part of the fault nose.

During the sedimentary period of the upper sub-member of Sha 1, due to insufficient supply of sediments, the sand bodies developed in small scale did not reach the middle part of the Binhai fault nose, and were mainly distributed like fingers along the eastern part of the fault nose in an inherited SN strike. During this period, the controlling effect of fault activity on sand body in the eastern segment of fault nose became stronger, and a local thick sand body area was formed on the downthrown side of the synsedimentary fault. Well gx508 and g17104 drilled in the downthrown side of Gangdong Fault and Gangdong branch fault (Fig. 6c) encountered thick sandstone in the upper sub-member of Sha 1, reflecting the controlling effect of the synsedimentary fault on sand body.

Based on the laws of structure, paleogeomorphology and sand body distribution etc., the sand control mechanism of Binhai fault nose can be classified into two types, fault trough sand control and trench sand control.

Fault trough sand control refers to the thick sand body area formed by sedimentary filling along the downthrown side of synsedimentary fault controlled by paleostructure. During the active process of synsedimentary fault, the paleo-water was shallower on the upthrown side, but deeper on the downthrown side, with a fairly large accommodation space. Therefore, sand bodies were likely to fill along the downthrown side of the fault. It can be seen from the overlap map of paleogeomorphology and sandstone thickness (Fig. 7), during the sedimentary period of the lower sub-member of Sha 1, the strong activity of Gangdong Fault in the middle part of the study area gave rise to a large accommodation space on the downthrown side. The sand unloaded along the direction of the fault strike, forming a continuous piece along the downthrown side of Gangdong Fault, which shows obvious control of the synsedimentary fault on the sand body.

Trench sand control refers to the sediments in band or finger distribution under the control of paleogeomorphology. Gravity flow deposits are widely developed in member 1 of Shahejia Formation in Qikou Sag. Affected by event triggering mechanism, delta front sand bodies formed multi-stage offshore underwater fans in Binhai fault nose. Controlled by the difference in paleogeomorphology, sediments unloaded and filled along the low potential area of paleogeomorphology, forming channel deposits shifting frequently to both sides. Longitudinally, multi-stage channel sand bodies overlap and stretch along the trench, and on the plane, the sand bodies are in finger or band shape and directional arrangement. It can be seen from the overlap map of paleogeomorphology and sandstone thickness that the paleogeomorphological trenches match well sandstone in distribution in the eastern part of the fault nose. Sand bodies are enriched in low-lying areas, while in the protrusion areas between sandstones, the sandstone thins rapidly and pinches out. Generally the sandstone appears in three bands spreading southward.

Fault-sand assemblage reflects the spatial distribution relationship between faults and sand bodies. The difference in assemblage type not only results in different types of traps, but also plays an important role in controlling hydrocarbon migration and accumulation. According to the relationship between fault strike and sand body distribution direction, the ontact area between fault and sand body and other factors, the fault-sand assemblages in Binhai fault nose are divided into two types: consequent and vertical (Table 2).

Consequent fault-sand assemblage pattern: In this pattern, the direction of fault direction intersects with the direction of sand body in an acute angle; controlled by synsedimentary faults, the sand body extends forward along the accommodation space on the downthrown side. The direction of fault guides the direction of water flow, giving rise to a fault trough channel for sand transportation and accumulation. The fault contacts with the sand body in a large area, and the sand body is thicker near the root of fault and thins toward the far end.

Vertical fault-sand assemblage pattern: the direction of fault strike is perpendicular to or obliquely intersected with the direction of sand body at high angle. In this type of fault-sand assemblage pattern, the fault has smaller control on the distribution of sand body, and the convergence of sand body is mainly controlled by paleogeomorphology. Influenced by the northern provenance, the study area developed several stages of sand bodies along the eastern part of the fault nose. During the sedimentary period from the sedimentary end of Sha 1 to Dongying Formation, a series of EW faults cutting the early sand bodies began to develop, forming the fault-sand assemblage pattern with the current fault strike perpendicular to the direction of the sand body.

According to the distribution characteristics of the two different types of fault-sand assemblage patterns, the consequent fault-sand assemblage pattern mainly occurs in the middle part of the fault nose, and is controlled by synsedimentary activities of Gangdong Fault. During the sedimentary period from Sha 2 to the lower sub-member of Sha 1, synsedimentary fault troughs developed on the downthrown side of the fault, where sand bodies appear in continuous large pieces, with large thickness and wide distribution. It can be seen from the well tie cross-section of sand bodies (Fig. 6a): In the middle section of the fault nose, the sand bodies are thick and continuous. Both Well qb1701 and bins22-12 revealed thick sandstone layers continuous laterally from Sha 2 to the lower sub-member of Sha 1, which gradually thin and pinch out at the locations of Wells gs46 and gs27 in the west. Vertical fault-sand assemblage pattern mainly developed in the eastern part of the fault nose in the sedimentary period of Sha 1. The eastern part of the fault nose was late in tectonic activity. During the sedimentary period of Sha 1, the gravity flow channel sand bodies were distributed in NS finger shape controlled by paleogeomorphology. Later, they were cut by a series of EW faults, forming the present complex characteristics of fault-sand assemblage. Multi-stages of sand bodies superimpose over each other, changing fast laterally and thinning rapidly to both sides. It can be seen from the EW cross-section of the well tie sand bodies that (Fig. 6b): The sand bodies in the east part of the fault nose feature large thickness variation and fast transverse variation. Wells g801, gs42X1 and bin 108X1 encountered thick sandstone layers in Ban 2 and Ban 3 sand groups, while Wells gs42 and gs64 between these wells didn’t encounter sandstone in Ban 2 and Ban 3 sand groups, showing the multi-stages of sand bodies appear in finger-like or band-like distribution. In addition, the synsedimentary phenomenon of faults developed on the downthrown side of Gangdong Fault and Gangdong Branch Fault in earlier active period, and local thick sand body areas appear at their roots, especially in the upper sub-member of Sha 1. It can be seen from the cross-section of the NS well tie sand body (Fig. 6c): Wells gx508 and g17104 located on the downthrown side of the Gangdong Fault and the Gangdong branch fault respectively encountered thick sandstone layers in the upper sub-member of Sha 1, reflecting the controlling effect of synsedimentary faults on the sand body.

Previous systematic studies on source rocks in Qikou Sag have confirmed that the oil and gas in the Binhai fault nose mainly originated from Paleogene Sha 3 source rock^[45,46]. The reservoir is a typical reservoir-above-source one, so the degree of oil and gas enrichment is closely related to the vertical transport capacity. Vertical hydrocarbon transport refers to the transmission of faults connecting lower source rocks during hydrocarbon generation and vertical migration. The hydrocarbon transport capacity is related to the match of the source rock and the active periods of faults. The middle and eastern segments of the study area are adjacent to the main sag of Qikou where the source rocks have large thickness of up to 1 200 m, high organic matter abundance and thermal evolution degree, and strong hydrocarbon generation ability. The western part of the fault nose is far from the main sag of Qikou, where the source rocks are thinner, (with a maximum thickness of 500 m) and weaker in hydrocarbon supply capacity accordingly (Fig. 8). From the homogeneous temperature analysis of fluid inclusions in the Binhai area, it can be concluded that the oil and gas in this area were charged in two stages. The first stage charging took place in the late depositional stage of Dongying Formation, and the second stage in the late depositional stage of Minghuazhen Formation^[47]. Only the faults that moved during the oil and gas charging period and cut down to the bottom source rock can be effective oil source faults and contribute to the vertical transmission of oil and gas. Based on the characteristics of source rocks and the activity law of faults in the study area, in the western part of Binhai fault nose, the source rocks are thin and poor in hydrocarbon supply ability, and the faults basically stopped activities during the hydrocarbon generation period, so the vertical transmission ability of oil and gas there was poorer (Fig. 9a). In the middle segment, the source rock is thick, and higher in thermal evolution degree at the burial depth of up to 5 000 m, with a R_o of over 1.3%^[48]. The long-term successive activity of Gangdong Fault matched well with the period of oil and gas charging, so they had strong vertical transmission capacity for oil and gas (Fig. 9b). In the eastern part of the fault nose, the source rock has strong hydrocarbon supply capacity; moreover, it can be seen from the seismic section (Fig. 4c) that a series of consequent southward dipping faults cut downward to Sha 3, communicating the bottom source rock; and the active period of the faults coincided with the oil and gas charging period, so these faults could act as effective oil source faults. The eastern section had strong vertical oil and gas transmission capacity too (Fig. 9c). By looking closely at the configuration between the source rock and faults, four main oil source faults and more than 10 secondary oil source faults have been confirmed in the Binhai fault nose.

In the middle part of the fault nose, the oil and gas is transmitted through one main oil source fault. In the eastern part of the fault nose, many oil-source faults transmitted oil and gas, constituting a composite oil-gas transmission system.

Based on the analysis of structural characteristics and sand body distribution law above, it is made clear that the fault- sand assemblages in Binhai fault nose area can be divided into two types, i.e. consequent and vertical ones. Their spatial configuration leads to the differences in trap and reservoir types, and controls the planar distribution characteristics of oil and gas. The middle segment has primarily consequent fault-sand assemblage, in which the sand body is distributed continuously in large pieces along the direction of fault strike, and in the same direction with the fault, so the fault and sand body contact in large area, making oil and gas charging easier. Large-scale structural-lithologic oil and gas reservoirs have been discovered in the second and lower sub-member of Sha 1, proving that the middle segment of fault nose has oil and gas in large area and wide distribution (Fig. 8). In the eastern part of the fault nose, vertical fault-sand assemblage takes dominance. In this kind of assemblage, the sand body stretches NS in band shape along the fault nose zone. The sand body in the main channel is thick, get thinner rapidly toward the edges on both sides of the channel, and is poor in transverse connectivity of the sand body is poor. A series of lithologic-structural complex traps are formed due to the fault cutting confinement in the EW direction in later period. Vertical intersection of sand bodies and faults results in small contact area between single sand bodies and faults, but faults developed in the eastern region. Multi-stage sand bodies have good spatial distribution relationship with faults. A series of updip pinch-out lithologic reservoirs are formed in the eastern part of the fault nose, which are confined by faults on both sides. The reservoirs are obviously controlled by sand bodies on the plane and distributed in band. Band-like oil and gas reservoirs have been found in the lower and upper sub-members of Sha 1 on the eastern flank of the fault nose in the areas where the fault- sand body configuration is superior (Fig. 8). In contrast, in the western segment of the fault nose, though the fault and sand also appear in consequent assemblage, due to insufficient supply capacity of provenances and weak vertical transmission capacity of oil and gas, the area of oil and gas is small on the whole, and only small lithologic reservoirs are scattered in individual strata.

Faults play an important role in hydrocarbon migration and accumulation in reservoir-above-source oil pools, where the oil and gas generated in the underlying source rock migrate vertically along faults to the overlying reservoir to accumulate. In addition, the migration and accumulation of oil and gas is closely related to the development of caprock. When the fault throw is smaller than the thickness of caprock, the oil and gas, blocked by mudstone caprock, would laterally migrate to the sand bodies on both sides of the fault, and accumulate in nearby traps. When the fault throw is greater than the caprock thickness, the mudstone caprock is partially or completely faulted, so oil and gas can migrate not only laterally along the sand body underlying the caprock, but also up through the mudstone caprock to the shallow reservoir to accumulate. Therefore, the configuration between fault and caprock is the key to whether oil and gas can migrate to the shallow reservoir, thus controlling the vertical distribution characteristics of oil and gas. From bottom to top, several sets of regional mudstone caprocks, including the middle sub-member of Sha 1 and the second member of the Dongying Formation, developed in the Binhai fault nose. The caprocks are thick, continuous and high in sealing capacity. The area with the greatest thickness of mudstone monolayer in the middle sub-member of Sha 1 is located in the eastern part of the Binhai fault nose. The mudstone is 350 m thick at maximum, and more than 250 m thick in most area. The mudstone in the second member of Dongying Formation is thickest in the middle section of Binhai fault nose, with a maximum single layer thickness of 400 m, and an average single layer thickness of more than 200 m. In the middle segment of the fault nose, the Gangdong Fault moved successively long, the fault throws during the sedimentary period of the middle sub-member of the first member of Shahejie Fomation and the second member of Dongying Formation were over 800 m, breaking these two sets of regional mudstone caprocks, so oil and gas generated in the underlying source rock can migrate upward along the Gangdong Fault. As a result, there are many oil and gas layers vertically in the middle section of the fault nose, ranging from the deep Sha 2 and Sha 1 to the shallow Guantao and Minghuazhen Formations (Fig. 9b).

In the eastern part of the fault nose, the fault throw in the sedimentary period of the middle sub-member of Sha 1 reaches 400 m, while in the sedimentary period of the second member of the Dongying Formation, the fault throw decreased due to the decrease of fault activity, and only part of the overlying mudstone caprock was faulted or not faulted. The caprock plays a good shielding role for oil and gas, which makes the oil and gas in the middle section of the fault nose mainly get enriched in Sha 2 and Sha1. The shallow Guantao and Minghuazhen Formations are poor with hydrocarbon enrichment (Fig. 9c).

Through the study of the configuration relationship of reservoir-forming elements, the law of hydrocarbon accumulation and enrichment in the study area is further clarified, and the fault-sand assemblage reservoir control pattern of Binhai fault nose is constructed. Controlled by the differences in structure, sand body and configuration relationship, there are two types of reservoir-forming patterns under the control of fault-sand assemblages in this area, i.e. single main fault hydrocarbon supply-fault sand consequent matching-multi-layer stereoscopic oil-bearing pattern and fault system hydrocarbon supply -fault sand vertical matching-banded overlapping oil-bearing pattern respectively. The former mainly exists in the middle part of the fault nose. The structure of the middle part of the fault nose is simple, and only one main fault, i.e. Gangdong Fault is developed. The braided river delta sand body in Sha 2 and the far-shore submarine fan sand body in the lower sub-member of Sha 1 spread extensively along the downthrow side of the Gangdong Fault. The distribution direction of the sand body is parallel to the strike of the fault, forming consequent fault-sand assemblage. The source rock of Sha 3 is 800-1 200 m thick and strong in hydrocarbon generating capacity. Hydrocarbon generated in the bottom source rocks migrated vertically along the Gangdong Fault and large-scale lithologic-structural reservoirs in the deep second member and lower sub-member of Sha 1. Neogene fault moved successively, cutting through the 400 m thick regional caprock of the second member of Dongying Formation, mak-ing it possible for oil and gas to migrate to Guantao and Minghuazhen Formations, giving birth to the reservoir-forming characteristics of multi-layered stereoscopic oil-bearing reservoirs in the middle part of fault nose (Fig. 9b). The latter is mainly developed in the eastern part of the fault nose, with faults developed and complex structural characteristics. Several faults cut down to the source rocks with a thickness of 800-1 200 m at the bottom to form a complex hydrocarbon transmission network, which matches the NS banded sand bodies formed during the sedimentary period of Sha 1 forming a vertical fault-sand assemblage pattern and a series of lithologic reservoirs developed in overlapping manner along the eastern flank of the fault nose. The Neogene fault activity was weakened, and the hydrocarbon accumulation of Guantao and Minghuazhen Formations was poor during the sedimentary period (Fig. 9c). Because of weak tectonic activity and poor reservoir development, oil and gas are only locally enriched in the western part of the fault nose, and the overall hydrocarbon-bearing property is poor (Fig. 9a).

By analyzing the characteristics of fault-sand assemblage and reservoir control mechanism in complex fault areas of fault-depression lake basin, the main controlling factors and enrichment rules of hydrocarbon accumulation in different areas of Binhai fault nose are defined. Combining with the current exploration status, the main exploration directions for the next step are defined as the fault trough sand body belt of Sha 2 in the middle part of the fault nose and Sha 1 in the eastern part of the fault nose. With the idea of "pending the exploratory deep gas reservoir and concentrating on drilling the lithologic reservoirs in the middle and shallow layers", the overall deployment and rolling implementation are carried out, and 13 exploration wells are deployed and constructed in accordance with the principle of covering both deep and shallow reservoirs as well as stereoscopic multi-target zone drilling. Industrial oil and gas flow was achieved in 11 wells (seven of which have achieved 100 tons/day of high productivity), the success rate of exploration wells was 91.6%, and 20 million tons scale reserve was confirmed in the Binhai fault nose.

A major natural gas breakthrough has been made in the fault trough sand body of the deep Sha 2 in the middle part of the fault nose. The middle part of fault nose is characterized by continuous distribution of sand bodies in the front of braided river delta and large oil and gas bearing area. Under the current situation of proven overall exploration in the lower sub-member of Sha 1, Well qb1701 is deployed aiming at dominant traps in the slope zone of Sha 2. A total of 69.5 m/23 layers of oil and gas are found in the whole well. Oil production is 68.8 m³/d, gas production is 12.2×10⁴ m³/d. The newly added natural gas reserve is 50×10⁸m³, which is expected to be connected with Well bins 22 gas reservoir to form an uncompartmentalized gas field with 10 billion cubic meters reserves. The successful drilling of this well proves that Sha 2 in the middle section of fault nose has the characteristics of overall gas-bearing (Fig. 10).

A breakthrough has been made in the lithologic reservoir in the east flank trench sand body zone of the fault nose. The eastern flank of the fault nose has many faults and is complex in structure. In the past, following the idea of searching oil on the basis of structure, wells were mostly located in the high part of the fault nose, but exploration results were not satisfactory. Recently, the wells were deployed according to the idea of lithologic reservoir in three zonal sandstone belts delineated along the eastern flank of the fault nose, and new breakthroughs have been made. In Well bin107X1 deployed in the western sandbody belt, one oil layer of 12.8 m thick was found. The Dong 3 Member was tested an oil production of 73.7 m³/d and gas production of 12.2×10⁴m³/d. Well g17104 was deployed in the downthrown side of Gangdong branch fault aiming at the upper sub-member of Sha 1. It had 20 oil layers of 75.6 m thick in total from log interpretation. The sand group ② in upper sub-member of Sha 1 was tested an oil production of 188 m³/d and a gas production of 2.15×10⁴ m³/d, making it the first high-yield flowing well of over 200 m³ oil equivalent from the upper sub-member of Sha 1 in the area. Well bin108X1 was deployed targeting the eastern part of the lower sub-member of Sha 1. It had 15 oil and gas layers of 35.3 m thick in total from log interpretation. It was tested in the lower sub-member of Sha 1, obtaining 119 m³ oil and 6.8×10⁴ m³ gas a day. The success of these wells proves that the eastern flank of the fault nose has superimposed band oil reservoirs (Fig. 10).

Since Paleocene, a large number of normal faults have developed in the tensional-torsional regional stress background. The Cenozoic has zoning structural characteristics. In the western segment of the fault nose, the structure is simple, the fault activity is generally weak, and the structural characteristics have obvious inheritance from bottom to top. In the middle part of the fault nose, there are deep and shallow two sets of fault systems, the deep formation is simple in structure, the strong activity of Gangdong Fault results in the formation of a large fault nose structure; whereas in the shallow formation, a large number of secondary faults are created, forming flower- like structure. The eastern segment of the fault nose has complex structure and many faults. Several late active south-dipping faults line up subsequently in NE direction, forming a fault nose structure complicated by faults.

Influenced by the provenance of Cangxian uplift in the west and Yanshan in the north, braided river delta and offshore underwater fan etc. deposits developed from bottom to top in Binhai fault nose. Controlled by multiple factors including paleogeomorphology and paleostructure, there are two kinds of sand control mechanisms: fault trough and trench. There are two kinds of fault-sand assemblage patterns, i.e. consequent and vertical. In the western and middle parts of the fault nose, the consequent fault-sand assemblage pattern develop. The sedimentary sand bodies of the Sha 2 and lower sub- member of Sha 1 spread extensively along the downthrown side of the Gangdong Fault. Whereas the vertical fault-sand assemblage pattern occurs in the eastern part of the fault nose, and the sedimentary sand bodies of Sha 1 appear in NS bands, overlapping each other and changing greatly in lateral direction.

The vertical conductivity, plane distribution and vertical distribution characteristics of oil and gas are determined by the distribution relationship between faults and sand bodies, oil sources and caprocks. Two oil and gas accumulation modes i.e. single main fault hydrocarbon supply-fault sand consequent matching-multi-layer stereoscopic oil-bearing and fault system composite transportation-fault sand vertical matching-banded overlapping oil-bearing are formed, which controls the difference in oil and gas distribution and enrichment degree in the Binhai fault nose. Exploration practice has confirmed the existence of multiple sets of oil-bearing strata in the Binhai fault nose. In recent years, exploration in the deep Sha 2 in the middle section of fault nose, shallow Sha 1 in the east section of fault nose and Dongying Formation has been successful, demonstrating complex fault zones in the old area have good prospects of oil and gas exploration. In the meantime, the shift of oil searching idea from structural reservoir to lithologic reservoir and the establishment of fault-sand assemblage pattern have provided a good reference for further exploration in this area as well as the exploration in similar complex fault basins.