The Hailar Basin in northeastern China is a complex Upper Jurassic–Cretaceous rift basin (Chen et al., 2007; Feng et al., 2011; Sun et al., 2011; Cui et al., 2016) (Figures 1–3) and is similar in terms of structure to the larger-scale Erlian and Songliao Basins (Traynor and Sladen, 1995; Wu et al., 2006; Zhou et al., 2014; Li et al., 2015).

The Hailar Basin includes the hydrocarbon-rich Wuerxun and Beier Sags (Sun et al., 2011; Zhang et al., 2012; Li et al., 2015) which cover areas of ∼2,240 km² and ∼3,010 km², respectively. In the Beier Sag, the Sudeert fault zone (Figure 1C) hosts the Sudeert oil field and is an important hydrocarbon-producing area (Feng et al., 2010; Cui et al., 2016). The NE-SW trending Sudeert fault zone is located in the center of the Beier Sag between the Beixi and Beizhong sub-sags (Figure 1C) and covers an area of 150 km² with multiple fault-bound structural blocks with variable orientations (Figure 1D) (Lin et al., 2008; Wang et al., 2012). The fault zone can be divided into a central fault belt, northern fault terrace, eastern fault terrace, and western fault scarp (Figure 1D). This study focuses on the central fault belt and western fault scarp, which have been divided into three blocks named after the wells (Figure 1D), B28, B14, and B16, from west to east (Kang et al., 2008; Wang et al., 2012).

The basement of Beier Sag is composed of Upper Palaeozoic and Lower Mesozoic granites and Jurassic metamorphic rocks (Chen et al., 2007; Zhou et al., 2014; Yi et al., 2015). The unconformably overlying sedimentary cover is dominated by a Lower Cretaceous succession comprising the Xing’anling (K1x), Nantun (K1n), Damoguaihe (K1d), and Yimin (K1y) Formations (Figure 3). The Xing’anling Formation is in general 100–590 m thick in the Sudeert fault zone and is composed of non-marine siliciclastics from conglomerates through to mudstones (Figure 3) (Wang et al., 2012). Xing’anling Formation is in unconformable contact with the metamorphic rock strata of the Budate Formation at the base. The top also shows a surface with regional unconformity between the Nantun and Xing’anling Formations. Previous studies (Xu et al., 2008a; Wang et al., 2012) divided the Xing’anling Formation into four members, numbered (from youngest to oldest) 1, 2, 3, and 4 (Figure 4A). Members 1 and 2 in the upper part of the Xing’anling Formation are the most important oil-producing units in the Sudeert oil field (Figure 4). Both members are dominated by sandstones and sandy conglomerates with some tuffs; Member 1 is ∼10–250 m thick, while Member 2 is ∼20–150 m thick. Member 3 contains sandy conglomerates and fine and coarse sandstones and is ∼40–200 m thick, with volcaniclastic rocks including rhyolitic tuffs. Member 4 is ∼150–800 m thick and composed of sandy conglomerates and breccias with some shales. Based on well correlations (Figure 4B), Members 1 and 2 were present in blocks B28, B14, and B16 with relatively consistent thicknesses. The deposit thicknesses of Members 1 and 2 of well block B16 were the largest, and the thickness decreased gradually to well block B28 (Figure 4B).

Pre-rift (pre-Cretaceous), syn-rift (Early Cretaceous), and post-rift (Late Cretaceous) phases are recognized in the Beier Sag (Sun et al., 2011; Zhou et al., 2014; Liu et al., 2015), with corresponding early extensional, later oblique-extensional, and late-stage inverted faults (Sun et al., 2011; Figure 2). During the early Cretaceous period in the Xing’anling Formation, the Beier Sag experienced a north-eastward right-lateral extensional, rotational, wrenching, and faulting movement. During this time, the mountains on both sides of the Hailar Basin rose quickly, and sediment accumulated rapidly within the basin, consisting mainly of coarse debris from nearby and multiple sources, forming a set of fan delta sediments. Later (Nantun Formation), the water bodies in various faulted basins deepened and expanded, and the sedimentation was mainly dominated by gravity flows, resulting in nearshore fans, turbidite fans, and fan delta sediments. In the middle and late early Cretaceous period (during the development of the Damoguaihe and Yimin Formations), the basin entered a rapid subsidence stage dominated by shallow lacustrine sedimentation. In the late Cretaceous period (Qingyuangang Formation), the basin entered a shrinking and dying stage of subsidence with the sedimentation mainly dominated by deep and semi-deep lacustrine sedimentary facies (Kang et al., 2013).

Visualization of the drill core data (nearly 100 core samples from 10 wells such as B14X-5-4, B14-X5-6, and D106-2; Members 1–4; Xing’anling Formation, blocks B14, B16, and B28) physical property analysis (Tables 1–4) showed that block B16 has the characteristics of medium-high porosity and low to ultra-low permeability reservoirs, block B14 block is characterized by high porosity and ultra-low permeability, and block B28 features low porosity and ultra-low permeability (Table 1). Excluding the influence of geological processes (e.g., cementation) on the later transformation of reservoir physical properties, the superposition relationships of the lobes and sand bodies of the three blocks also have some influence. The vertical stacking relationships of the sand bodies, indicated by the differences in physical property data from different well blocks and their geological implications for petroleum geology are further addressed in the Discussion section.

Based on the cores of Members 1–4 of Xing’anling Formation from B14-X5-1, B14-X6-1, D106-1, and D110-2 (Figures 6, 7), the petrography and facies types of Lower Cretaceous Xing’anling Formation in the study area were identified according to type, texture, grain size, and sedimentary structure. The core descriptions showed that the lithofacies type of the fan-delta system is complex and includes three rock types (Miall, 1977).

In this study, the vertical sandstone-mudstone distribution characteristics of well correlations (Figures 8A, 9A) (AA’ and BB’ in Figure 1D) were clearly visible in the seismic sections (Figures 8B, 9B) after logging-constraint seismic inversion, which created conditions for identifying the sedimentary systems of well blocks B14 and B16 in the Sudeert fault zone (Figures 8C, 9C; the blocks are marked on Figure 1D).

Compared with mudstones, sandstones have a greater interval velocity (Eberhart-Phillips et al., 1989; Zhou et al., 2014) and can, therefore, be identified by their high acoustic impedance (Tarantola, 1984; Avseth et al., 2016). The lithological characteristics corresponding to different lacustrine sediments influence the seismic response and amplitude. In this study, sandstones with high acoustic impedance were characterized by a yellow color on impedance overlays; mudstones with lower acoustic impedance have dark colors (Figures 8B, 9B).

Based on two seismic profiles from the B14 and B16 well blocks (AA’ and BB’ in Figure 1D) and acoustic impedance data, together with borehole data, the distribution of the Xing’anling Formation across the study area has been reconstructed.

The fault-bound horsts in the seismic profiles are well blocks B16 and B14 (Figures 8C, 9C). A set of high-impedance reflectors appear at the bottom of the stratigraphy (M1–M4 in Figures 8B, 9B) and are interpreted as multi-stage sandstones (Figures 8C, 9C). These sandstones were deposited on top of the uplift of the Basement and their thicknesses increase close to the boundary faults (Figures 8C, 9C).

Some turbidite deposits characterized from core observations by matrix-supported, poorly sorted, and rounded sandy conglomerates occur locally in the footwall near the major faults (Figures 8, 9). The geophysical response characteristics of the turbidite sandstones are energy dispersive, with high acoustic impedance (Figures 8B, 9B).

The dark blue areas in the seismic sections (Figures 8B, 9B) correspond to sandy mudstones and mudstones interpreted to have been deposited in a lacustrine depositional system of deep water.

Fan deltas are widely developed within the Xing’anling Formation (Figures 9C, 10C) and pro-delta, delta front, and delta plain facies were described (Nemec and Steel, 1988; Zakaria et al., 2013; Wei et al., 2016). In the study area, delta front deposits were dominant with distributary channels and mouth bars (Zakaria et al., 2013; Wei et al., 2016).

During the Xing’anling Formation deposition, the fan delta systems were mainly located in the central fault-bound high, including well blocks B28, B14, and B16, adjacent to the boundary faults (Figures 8–11). The plane graphs of the sedimentary facies indicate that large-scale fan delta fronts developed in block B16. The south of block B14 also contains delta front deposits, while several small, isolated delta lobes are present in block B28 (Figures 9C, 10C).

The Sudeert fault zone shows two types of lacustrine deposits: shallow-lacustrine and deep-lacustrine (Figures 6–10). Deep-water lakes are characterized by dark-colored shale with high total organic carbon (TOC) content. The dark shale of the deep-lake facies is the main source rock for hydrocarbons in the Bell Depression (with TOC contents ranging from 0.16% to 4.73%, averaging 2.45%) (Xu et al., 2008b; Liu et al., 2010; Shan et al., 2010).

During the Xing’anling Formation deposition, the north part of blocks B14 and B16 formed the principal depocenter where deep-lacustrine sediments accumulated (Figures 10C, 11). Shallow-lacustrine deposits mainly developed in the south part of the Sudeert fault zone (Figures 10C, 11C).

Sub lacustrine fans developed far from the lake shoreline (Normark and Dickson, 1976), e.g., in the downthrown side of the major faults, e.g., in the north of block B14. The north part of block B14 includes a turbidite sand body surrounded by mudstones from the deep lacustrine system. A few examples of turbidite sand bodies surrounded by pro-delta mudstones are present in the western and northern parts of block B16 (Figures 10C, 11C).

Three types of fan lobes were identified in blocks B28, B14, and B16: 1) isolated, 2) overlapping, and 3) superimposed lobes (Figure 12).

Deposition of the Xing’anling Formation was divided into early syn-rift (Member 4), syn-rift (Members 4 and 3), and rift climax stages (Members 2 and 1) (Figures 5, 13A–C (Shan et al., 2010; Jia et al., 2014; Zhou et al., 2014). The planar distributions and vertical superposition relationships of the fan bodies in different blocks of the Sudeert fault zone were controlled by the ratio of accommodating space (A) to sediment input rate (S). The 3D structure of the top of the Sudeert fault zone shows a feature of “wide in the west and narrow in the east” with respect to the planar distribution of the fan bodies (Figure 12). Flattening the stratigraphy is a common technique in sedimentology to visualize the depositional environment. By flattening the top surface of Member 4 into two sections (with the same depth and scale) of well areas B28 (AA’) and B16 (BB’), the accommodating space during the deposition of the Xing’anling Formation can be compared. The flattened surface is regarded as the lake level during the sedimentation period of the Xing’anling Formation, and the form of the bottom surface can be approximated as pre-sedimentary paleogeomorphic features. Although this method cannot precisely reconstruct the paleogeomorphic features of the Sudeert fault zone during the sedimentation period of the Xing’anling Formation, the size of the accommodating space in well areas B14 and B16 can be qualitatively compared under the assumption that the sediment input rate was constant across the study area. Figure 13 shows that compared with well area B14, the accommodating space in well area B16 has “narrow and deep” characteristics. In terms of sediment supply, the sand body thickness is an indicator of sediment input but does not necessarily reflect the sediment input rate. The sand body thickness in well area B16 was the largest, followed by well area B14, with well area B28 the smallest (Figure 6). This also indicated that the sediment input rate gradually decreased from west to east.

During the early syn-rift stage (Figure 13A), the deposition was mainly controlled by the inherited paleogeography, in which the sediments tend to accumulate in low-lying sags of the Basement (Zhou et al., 2014). Because of weak fault activity and shallow water depths, little deep water clastic deposition occurred in the study area (Figure 13A) (Liu et al., 2010) and shallow-water lacustrine deposition was dominant (Shan et al., 2010; Corella et al., 2016; Ballato, et al., 2017).

During the syn-rift stage (Figure 13B), oblique extension led to the formation of the Hailar Basin as a distinct feature (Ren et al., 2007) and was accompanied by a relative rise in lake level and increased accommodation (Figures 13D, E). Faults in the study area became more active (Shan et al., 2010; Fu et al., 2012), and sediments were largely derived from the southern uplift area (Figure 13B). Some isolated near-shore fan deltas developed in blocks B14, B28, and B16. Some source materials on the steeply dipping eastern shore were transported longer and formed subaqueous and sub-lacustrine fans near the eastern shore, especially in block B16, whose accommodation was the narrowest (Figure 13B).

From the late syn-rift to the rift climax stage, fault-driven subsidence caused a rapid relative rise of lake level (Wang et al., 2007; Yu et al., 2013) as a result of which the accommodation increased and three depo-centers of different sizes developed. According to the chart of sand body deposition thickness (Figures 10A, 11A), it can be intuitively found that the sand bodies of block B16 is the thickest, followed by block B14 and B28 in that order. This also indicates that the sand bodies supply was gradually decreasing from west to east during the sedimentary periods of Members 2 and 1. With an abundant supply of source, many of the near-shore fan deltas developed in blocks B14, B28, and B16 (Figure 13C). As shown in Figures 13D, E, the accommodation of the block B16 area was “narrower and deeper” than that of block B14; therefore, superimposed sand bodies formed more readily (Figure 13C) (Einaby and Ei-aai, 2016; Wang, et al., 2016; Deaf et al., 2020).

The balance between the accommodation generated by tectonic subsidence and lake-level fluctuations versus sediment input may have been responsible for the formation of the different lacustrine facies types in the study area (Lin et al., 2008). Isolated lobes would have developed in response to scarce sand input and slowly increasing accommodation. The sand input was insufficient to form multiphase fan delta lobes and only a few isolated lobes were formed in the western part of the study area, e.g., in block B28. The formation of contact lobes would have been favored by a greater sediment supply and by steadily increasing accommodation across a wider area (Su et al., 2011). When a deltaic channel migrated laterally, it formed a new lobe, which stayed connected to the initial lobe. Block B14 typifies this type of behavior.

Superimposed lobes were formed when adequate sediment input was combined with steadily increasing accommodation in a restricted area. Fan lobes formed during the early stages did not completely occupy the accommodation (Figures 13D, E). Accommodation developed in a narrow area; thus, the channels migrated laterally but formed new lobes, which stayed connected to the former lobes. Late-stage lobes were vertically superimposed on earlier lobes. Block B16 typifies this type of deposition.

The reservoir in the Sudeert oil field is highly heterogeneous and generally poor in physical properties (Wang, 2019). The physical properties of the reservoir are obviously controlled by the regional sedimentary patterns and the combination of sand bodies, consistent with the overlapping rule of sand bodies in each block (Figures 12, 13).

Block B16 showed the best permeability because the lobe sand bodies in this area are superimposed and have good porosity (Figure 14). Meanwhile, the intersections between different fan bodies strengthen the connectivity between different fan sand bodies, thus increasing the reservoir permeability (Figure 14C). The sand bodies in the block B14 lobes have developed better vertical superimposed relationships and high porosity, but in terms of horizontal sand bodies connection, a contact relationship (overlapping) between different lobes only exists on the edge of the lobes. Although the sand bodies inside the fan lobes have good connectivity, the lack of horizontal connection between multiple fan bodies leads to poor permeability (Figure 14C). However, block B28 is the poorest, with fewer fan lobes formed, most of which are solitary fans (Figures 12, 13), leading to poor reservoir physical properties (Figure 14C).

Member 2 in the Sudeert oil field has the highest oil production, mainly due to its high-quality reservoir (Figure 15). The fan body lobes are large, and the main channel is vertically intersected and vertically overlapped. Meanwhile, the shape of the lobes inside the same fan determines the movement of oil and water in this area. High-production wells mainly concentrate in solitary lobes where sand bodies are well developed and have strong connectivity. Excluding the artificial fractures in parts of the well area, the high production occurs mainly due to the formation of solitary lobes during the same period and from the same sources, resulting in little physical property differences in the lobes, which are conducive to the overall movement of oil and water.

In the sedimentation process of oil and gas reservoirs, a “narrow and deep” paleogeomorphic environment, such as the B16 block in the Suderte oil field, is most conducive to the spatial superposition of fan-shaped sedimentary systems, thus forming a favorable development pattern for oil and gas reservoirs. These findings are of inspiring significance to the general public, as they reveal the factors that control the production capacity of oil and gas in basins. A similar example can be found in the Alberta Basin in Canada, where the Devonian and Cretaceous formations contain abundant oil and gas resources (Gibbs and Rakhit, 2019), which are clearly controlled by the sedimentary characteristics of the formations (Posamentier et al., 1992). Due to the large-scale thrusting of the Rocky Mountains towards the basin since the Cenozoic, a large-scale thrusting structural style has formed on the western margin of the Alberta Basin (Michael and Bachu, 2001), similar to the B16 well area in the present study. The “narrow and small” triangular belt structure formed by the reverse thrusting on the western edge of the Alberta Basin is the most favorable area for the accumulation of oil and gas reservoirs (Bennett and Jiang, 2021), as this type of sedimentary space is more conducive to the interlayering of clastic sediments in the reservoir, thus forming high-quality reservoirs.