Syndepositional faults (also known as contemporaneous faults, growth faults) are a type of faults that develop simultaneously with sedimentation. Syndepositional faults in a rift basin in extensional environment are generally normal faults which are characterized by the increase of vertical fault throw with depth and thickening strata in the footwall^[1,2]. There are a large number of syndepositional faults in continental rift basins. Large-scale basin-level syndepositional faults and their combinations give rise to different types of slope break belts and have important controlling effects on the sequence architecture, sedimentary system distribution, and reservoir quality in rift basins^{[3,4,5,6,7,8,9]}. Thus they are important research objects for exploring the response of sedimentary process to tectonic activity^[10,11].

Previously, many studies on the control of large-scale syndepositional faults to sedimentary system distribution and reservoir architecture in extensional fault basins and compressional basins have been carried out, and a lot of findings have been achieved^[12,13,14]. In recent years, as the oil and gas exploration and development research in fault basins went further, researchers have found small-scale syndepositional faults in complex fault blocks of rift basins^[15,16]. This kind of syndepositional faults features small scale, limited control range, complicated distribution geometry, and large number^[17]. At present, researches on such small-scale syndepositional faults occurring in local parts mostly focused on the identification and interpretation of the faults^[16,17]. But the controlling effect of the syndepositional faults on the spatial distribution and development of deltaic sand bodies is still poorly understood. At present, the development of complex fault block oilfields is facing severe challenges^[18], so it is urgent to reunderstand the distribution pattern of deltaic sand bodies controlled by syndepositional faults.

In this study, taking Wang 43 fault block in Wangjiagang Oilfield, Dongying depression, Bohai Bay Basin as the research object, we explore the control effect of syndepositional faults in complex fault blocks on deltaic sand bodies by using cores, logging curves, and three-dimensional seismic data.

W43 fault block area of the Wangjiagang Oilfield is located in Chenguanzhuang-Wangjiagang fault structural belt in the southeast of the Dongying depression, Jiyang subbasin, Bohai Bay Basin (Fig. 1). It is a NE-SW long and narrow fault block belt with grabens and horsts (Fig. 2), which is further complicated inside. The W43 fault block is about 12 km², where 282 wells have encountered the target layer (Fig. 2). In the study area, there are a number of small-scale faults. Except for a third-order fault in nearly EW strike, all the other faults are the fourth-order faults in near NE-SW strike and derived fifth-order faults (Fig. 2a), with complex distribution characteristics^[16]. These faults basically started to develop from the late depositional period of the third member of the Paleogene Shahejie Formation, under the regional NW-SE extension and dextral strike slip^[19,20]. The latest research since 2015 reveals that the small-scale faults in W43 fault block have the development characteristics of syndepositional faults^[16], which provides a good case for finding out the control of syndepositional faults on the development of deltaic sand bodies in complex fault blocks.

The oil-bearing series in W43 fault block includes the first member, second member, and third member of the Shahejie Formation at the depth of 1550-2550 m. The target horizon in this study is the upper sub-member of the second member of Shahejie Formation, which contains four sand groups, corresponding to four medium-term base level cycles. This study mainly focuses on the third sand group in the upper sub-member of Sha-2 member (Es₂^S), which can be divided into seven sublayers (Fig. 2b). Dongying depression developed large-scale river dominated delta deposits in the sedimentary period of the second member of Shahejie Formation. The source material mainly came from the Guangrao bulge area in the southeast. During this period, and the delta prograded toward the basin with gentle slope, inheriting the depositional features in the third member of Shahejie Formation^[21,22].

W43 fault block area has typical complex fault block reservoirs, which are at the stage of high water cut at present and have some problems such as prominent development contradictions and weak geological foundation^[16]. The distributary channel sand bodies of delta in the upper submember of the second member of Shahejie Formation are developed but change rapidly in a lateral direction, making them difficult to predict^[22,23]. The characterization and prediction of distributary channel sand bodies is an important content of EOR (enhanced oil recovery) research in complex fault block oilfield^[24]. The subtle changes of sedimentary facies and distributary channel sand bodies in complex areas are controlled by syndepositional faults. It is of great practical significance to analyze the control of syndepositional faults on sedimentary facies and sand bodies for predicting the distribution of distributary channel sand bodies.

In the area with few wells, the logging curves were calibrated on the seismic sections after time-depth conversion to track top surface and bottom surface of the third sand group in Es₂^S (Fig. 3a). By comparing the third sand group in Es₂^S thickness in the hanging wall and footwall of those faults, the syndepositional faults were identified and marked. In the areas with dense wells drilled, the stratigraphic correlation profiles across both sides of the syndepositional faults through the wells also provide a reliable basis for syndepositional fault identification (Fig. 3b). On the basis of this understanding, the expansion index and ancient vertical fault throw of different parts of the syndepositional faults were calculated through multiple sections along the strike of the syndepositional faults. By examining well profiles and seismic profiles across several syndepositional faults, it is found that the syndepositional faults are large in number and small in scale. According to the classification rules of basin structural units^[25], the study area mainly has third- order and fourth-order syndepositional faults (Fig. 4a). Among them, the fourth-order syndepositional faults are mostly in NE strike, with dip northwest or southeast (Table 1). These small- scale extensional faults developed synchronously in the extensional environment, forming fault clusters in the same or reverse strike with the major faults of the Wangjiagang fault belt^[26].

According to activity analysis, syndepositional faults in the study area vary in activity intensity along the strike, (Fig. 4b), which is similar to the activity distribution of large-scale syndepositional faults^[27], but these faults are smaller in scale and have control scope limited inside the complex fault blocks. Syndepositional faults have obvious control on the development of the strata. In the downthrown area, there is a positive correlation between the activity of the syndepositional faults and the strata thickness (Fig. 4c).

The development pattern of syndepositional faults in the study area can be compared with that in the Canyonlands Graben of the Utah State, USA. In this area, small-scale syndepositional faults are mainly formed in extensional environment^[28,29]. Due to the activity of small-scale syndepositional normal faults, many small grabens have been formed. Through satellite image analysis (Figs. 5a and 5b) and topographic survey (Fig. 5c), it is found that the syndepositional faults in this area are dense (Fig. 5b), mainly distributed in NE-SW direction. Different faults have different dip directions and mostly in the NW and SE strikes (Fig. 5c). The syndepositional faults have smaller extensions of about 0.1-6.5 km generally^[28], and the grabens formed by two syndepositional faults are generally 100-300 m wide^[28].

Different parts of one syndepositional fault have different expansion indexes. In the position with high expansion index and strong fault activity, the terrain of the corresponding downthrown area is relatively low, forming larger accommodation (Fig. 5c, EE° profile); in the position with low expansion index and weak fault activity, the terrain of the corresponding downthrown area is relatively high, with less accommodation (Fig. 5c, FF° profile). In addition, different faults differ widely in activity intensity, and faults with strong activity intensity have larger accommodation formed in the downthrown side (Fig. 5c, GG° profile).

Through core observation and description of the second member of Shahejie Formation (Fig. 6) and examination of logging data, different types of delta sedimentary facies and development characteristics of typical sedimentary sequences (Fig. 7) have been figured out.

The delta plain mainly has distributary channel, natural levee and flood plain deposits (Fig. 7a). The distributary channels are mainly composed of siltstone and fine sandstone, with conglomerate locally (Fig. 6a). The grain-size probability accumulation curves of the sandstone in the distributary channels show two-stage characteristic (Fig. 7c). The channel sediment has cross bedding, massive bedding (Fig. 6b, 6d), erosional surface at the bottom, and basal conglomerate (Fig. 6a) near the scour surface. The distributary channel sediments interbed vertically with brownish red mudstone of flood plain facies (Fig. 7a). The natural levee is mainly composed of muddy siltstone and siltstone with small cross bedding (Fig. 6c). The flood plain sediments are dominated by brownish red mudstone with massive bedding (Fig. 6e).

The delta front facies includes mainly subaqueous distributary channel, mouth bar and interdistributary bay (Fig. 7b). The subaqueous distributary channel is mainly composed of fine sandstone (Fig. 6f) with sedimentary structures such as mud gravel, wavy bedding and cross bedding. The subaqueous distributary channels are vertically adjacent to dark mudstone of the distributary bay and mouth bar sediment (Fig. 7b). The mouth bar is mainly composed of fine sandstone with sedimentary structures such as massive bedding, parallel bedding and wavy bedding (Fig. 6g). There is abundant plant debris between the laminae (Fig. 6h). The grain-size probability accumulation curve of sandstone of the mouth bar shows typical three-stage characteristic (Fig. 7d). The interdistributary bay sediment mainly consists of grey silty mudstone, muddy siltstone and greyish green and greyish black mudstone, with gastropod fossils (Fig. 6i) and horizontal bedding.

On the basis of core analysis, using 3D seismic data and seismic sedimentology methods^[30,31], the target horizon was precisely traced in the 90° phased seismic data volume. Strata slices were made along the isochronous stratigraphic interfaces to find out the plane distribution characteristics of amplitude attribute (Fig. 8a). Taking 3.7 sublayer as an example, the distribution characteristics of amplitude attribute are shown in planar view. Higher amplitude area with warm color indicates the development scope of sandstone. Through the description of sandstone distribution, planar distribution of sand bodies on both sides of syndepositional faults was analyzed to provide a reliable basis for the plane distribution of sedimentary facies (Fig. 9a).

As the same formation in the hanging wall and footwall of a syndepositional fault have different thicknesses, the same scale stratum slices in hanging wall and footwall area were made under the constraint of the top and bottom of stratum. This prevents the slices from passing through the layer and ensures the isochronity of the slices. The distributary channel sand bodies in the second member of Shahejie Formation vary rapidly and show complex distribution pattern, and the plane distribution of sedimentary facies is obviously controlled by syndepositional faults (Figs. 9 and 10).

The specific controlling effect is mainly dependent on the dip and the activity difference of different parts of syndepositional faults. Therefore, the controlling effect is mainly shown in the following four aspects (taking 3.7 sublayer as an example).

(1) In the relatively weak activity areas of the north dipping syndepositional faults, because of the small paleo-topography variation between the hanging wall and footwall, local dominant flow paths would be formed. Thus, the distributary channels were more likely to pass through these paths. In these areas, the syndepositional faults mainly affect the direction of the sand bodies. The sand bodies on both sides of the syndepositional faults are similar in shape, but the sand bodies in the hanging wall area are thicker. For example, when the distributary channel passed through the hanging wall area of the north dipping syndepositional fault F1 and F4, and the well block W14-61 located in the footwall area of F3 (the indicated range of the rectangle 1 in Fig. 9b), it passed through the places with weak fault activity marked by the fault activity measurement and entered the hanging wall area.

(2) When the distributary channels flowed from south to north through south-dipping syndepositional faults with weak activity, it is similar to the situation of the north-dipping syndepositional fault, the distributary channels were more likely to pass from the paths into the footwall area. In this case, the syndepositional faults mainly affect the direction of sand bodies. The sand bodies on both sides of the syndepositional fault don’t differ much in thickness and shape. For example, in area near the well W14-51 and end area of footwall in F6 south-dipping syndepositional fault, the distributary channel passed through the areas with weak activity at the edge or middle part of the fault and entered the footwall area (see the indicated range of the rectangle 2 in Fig. 9b).

(3) In the hanging wall area of the north dipping syndepositional faults with strong activity, low topography would occur in local parts, capturing the drainage. This resulted in the passive deflection of the distributary channels in the hanging wall of these syndepositional faults. Compared with the footwall area, the distribution of sand bodies in the hanging wall areas of the syndepositional faults is obviously different. To be specific, the distributary channel sand bodies increase in scale and thickness or vary to delta front mouth bar facies. For example, in the Well W104-X1 area in the hanging wall of the F4 syndepositional fault in the south of the study area, affected by the syndepositional fault, the strike of the distributary channel changed to near east-west direction from north-west passively (see the indicated range of the rectangle 3 in Fig. 9b). Also in the hanging wall area of the F1 syndepositional fault with the largest scale in the study area, the distributary channel is larger in scale or changes in facies to mouth bar deposit in delta front (Fig. 9b, 9c).

(4) In the areas with strong activity of the south-dipping syndepositional faults, low-lying areas would be formed, intercepting sedimentary supply from the S-N oriented deltaic system. This hindered the progradation of the distributary channels^[32], so the "break" of the distributary channel occurs in local parts. Compared with the less developed sand bodies in the footwall area of the south-dipping syndepositional fault, the distributary channel sand bodies in the hanging wall l areas are larger in planar scale and thickness with different shape. For example, in the Well W14-38 area on the hanging wall of F9 fault (see the indicated range of the rectangle 4 in Fig. 9b) and the area near Well W14-11 on the hanging wall of F9 fault (see the indicated range of the rectangle 4 in Fig. 9b), the progradation of distributary channels is restricted by the activity of syndepositional faults, and is mainly distributed on the side of the hanging wall, meanwhile, the distributary channel increases in width and thickness (Fig. 9c).

Based on the analysis of delta sand bodies in the syndepositional fault development area of complex fault blocks, the vertical development pattern of deltaic sand bodies controlled by syndepositional faults was analyzed from two aspects, sand body thickness and stacking style on both sides of the syndepositional faults.

Profiles of sand bodies across syndepositional faults were made by using drilling data of wells in the hanging wall and footwall areas. It is found that the sand bodies in the hanging wall of the syndepositional faults of the study area increase in thickness significantly (Fig. 11a). The main reason is that the continuous activity of syndepositional faults led to more accommodation in the hanging wall areas^[33,34]. Thus, with the continuous activity of syndepositional faults, the thickness of sand bodies near the footwall area increased gradually. From the cross plot analysis of the thicknesses of sand bodies in the sublayers of the third sand group in Es₂^S from 8 drilling wells and the expansion indexes of the syndepositional fault (F19), it is found the thickness of sand body in the hanging wall area is positively correlated with the expansion index of the fault (Fig. 11b). The third sand group in Es₂^S sand bodies in the hanging wall areas are also thicker than those in the footwall areas. The sand bodies in the sublayers in areas with weak activity (expansion index less than 1.1), medium activity (expansion index of 1.1-1.2), and strong activity (expansion index greater than 1.2) are 2.29, 4.31 and 5.15 m thick respectively on average (Fig. 11b).

In the footwall area of the syndepositional fault, the stacking pattern of distributary channel sand body is quite different from that in the hanging wall area of the syndepositional fault. Compared with the hanging wall area of the syndepositional fault, the sand bodies in the footwall area have more layers. Sand body layers of all sublayers of the third sand group in Es₂^S in eight wells each in the hanging wall and footwall area of the fault F19 were counted. It can be seen from most of the wells that the sand body layers in the footwall areas are more than those in the hanging wall areas (Fig. 11c). The ratios of the sand body layers in the hanging wall and footwall areas are larger than 1 in general, and up to 1.75. And the ratio is positively related to the expansion index (Fig. 11d).

In the sedimentary period of the second member of the Shahejie Formation in W43 fault block area, several syndepositional faults combined into special micro-horsts and micro-grabens (Fig. 12), which can be compared with the Cenozoic extensional structures exposed in the field^[35]. According to the analysis of the sedimentary profile (the location of profile HH′ in Figs. 9b and 10), the syndepositional fault in complex fault block area controls the development position and stacking style of deltaic sand bodies in high-frequency cycles (Fig. 12). There is more accommodation in the footwall of a single syndepositional fault (such as F6, F9) and the micro- graben area (between F1 and F19). Therefore, the sand bodies in such areas build up continuously vertically, forming a continuous sand body stacking pattern in hanging wall area (Fig. 12). In contrast, in the micro-horst area (between F1 and F9), the accommodation is smaller than that in the footwall area, so influenced by the sediment supply of the sedimentary system, the sand bodies appear intermittently (Figs. 8b and 12), and are fewer in number and poorer in continuity in the vertical direction.

According to the research results of the above subsurface data, syndepositional faults in the complex fault block area have strong control on deposition. Under the control of two types of fault dip and two types of distribution patterns of fault activity (expansion index), four different types of sand body distribution patterns have been formed near the syndepositional faults (Fig. 13). In general, the distributary channel of delta preferentially passes through the parts with weak activity of south dipping and north dipping syndepositional faults. In the hanging wall areas with strong fault activity of north-dipping syndepositional faults, the sand bodies converge and are more likely to connect, and the distributary channel has passive deflection near the syndepositional fault. In the hanging wall areas with strong activity of south dipping syndepositional faults, the progradation of distributary channel is blocked. The sedimentary process of sand body affected by syndepositional fault determines the particularity of delta sand body distribution in complex fault block area.

Based on the distribution pattern of sand bodies on both sides of syndepositional faults, the distribution pattern of delta sand bodies suitable for the areas with syndepositional faults of complex fault blocks in fault basin was figured out (Fig. 14). In this concept model, the control of syndepositional faults on the plane distribution and vertical stacking of sand bodies are considered. Compared with the regional large-scale syndepositional faults in the fault basin, the small-scale syndepositional faults in the complex fault block have the characteristics of small scale, dense distribution, different dip directions and different activity intensities in different parts. Because the tectonic movement of syndepositional faults and sedimentation happen at the same time, there were paleotopography differences in complex fault blocks, which affected the flow direction and sedimentary process of distributary channels in short-term cycles. The distributary channels are more likely to pass through the positions with weak activity. And they can also be deflected or restricted in the hanging wall area due to the influence of the activity of syndepositional fault. As a result of syndepositional fault activity, more accommodation space would be formed in the hanging wall, so the sand bodies in the hanging wall area are thicker. In the hanging wall of a single syndepositional fault and micro-graben area controlled by two syndepositional faults, multi-stage sand bodies often superimpose on each other continuously, while in the micro-horst controlled by two syndepositional faults, sand bodies appear intermittently in the vertical direction.

The distribution pattern of delta sand bodies considering the effect of syndepositional faults proposed in this study is obviously different from the pattern not considering the effect of syndepositional faults^[36,37,38]. First of all, in terms of flow direction and plane distribution pattern of distributary channel, the traditional sedimentary model is applicable to the area with uniform variation of topography. In such area, distributary channels extend in branch shape, and the sedimentary pattern is controlled by the process of auto cyclicity. In contrast, in the area with syndepositional faults, the flow direction of distributary channels changes near the syndepositional faults, resulting in obvious differences in the shape, scale and sedimentary facies of the deltaic sand bodies on both sides of syndepositional faults. In addition, in the area with syndepositional faults, controlled by the new factor of syndepositional fault activity, accommodation space changes in local parts, forming distribution pattern of sand bodies different from the traditional pattern.

From the above analysis, we can see that the syndepositional fault is an important geological factor that cannot be ignored in the finer-scale description of sand bodies in complex fault blocks. Controlled by the activities of syndepositional faults, the sand bodies in high frequency cycles of lacustrine basin in complex fault block areas have special distribution patterns, which are quite different from the traditional sedimentary model. The results of this study can further deepen the deltaic sedimentary theory of fault lacustrine basin, and provide a new idea for the fine development geological research of complex fault block oilfield.

Taking W43 area of the Wangjiagang Oilfield in the Dongying depression as an example, small-scale syndepositional faults have been identified in complex fault blocks of fault basin using drilling, logging, and three-dimensional seismic data. In the study area, low order syndepositional faults are large in number, small in scale, and different in dip direction, mainly north dipping and south dipping. Along the strike, the syndepositional fault differs somewhat in activity intensity in different positions. The activity of the syndepositional fault made the accommodation space in the footwall area increase and the strata thicken. These characteristics result in the influence of syndepositional faults by the paleotopography of complex fault blocks, and then forming a special distribution pattern of deltaic sand bodies.

The syndepositional faults control the depositional process of deltaic sand bodies in the high frequency cycle of the complex fault block, and then affect the planar distribution of sand bodies near the syndepositional faults. Controlled by the dip and difference of activity intensities in different parts of the syndepositional faults, the distributary channels are likely to pass through the parts with relatively weak activity of south and north dipping syndepositional faults. But in the areas with relatively strong activity of syndepositional faults dipping in the north, the distributary channel often has a passive deflection, the distribution range of sand bodies increases near the hanging wall, and the facies type and sand bodies distribution differ greatly in both sides of these syndepositional faults. In the strong activity parts of south-dipping faults, the distributary channels are restricted in the process of progradation, and sand bodies mainly deposited in the hanging wall area.

The syndepositional faults in the complex fault blocks also control the scale and vertical stacking pattern of deltaic sand bodies. The thickness and scale of sand bodies in the hanging wall of the syndepositional fault are positively correlated with the activity intensity of the syndepositional fault. In the hanging wall of single syndepositional fault and graben area formed by combination of syndepositional faults, sand bodies are thicker and multi-stages of sand bodies superimpose on each other continuously. In the horst area formed by the combination of south dipping and north dipping syndepositional faults, sand bodies are thinner and appear intermittently vertically.