Introduction

In mature exploration areas, fault-bound traps are increasingly recognized as major growth points for conventional hydrocarbon reserves and important prospects for future exploration (Zhao et al. 2018b; Xue et al. 2019). The dual roles facilitated by faults are of importance in understanding the migration, accumulation, and preservation of hydrocarbons in petroliferous basins (Xu et al. 2019; Fu et al. 2021). Active faults can effectively channel fluid migration, while inactive faults may act as barriers or seals (Allan 1989; Bouvier et al. 1989; Karlsen and Skeie 2006; Cartwright et al. 2007). Fault growth and activation, as well as fault sealing capacity, determine the hydrocarbon potential within fault-bound traps, as evidenced by extensive research and petroleum exploration practices (Manzocchi et al. 2010; Omosanya et al. 2015; Bretan 2016; Xu 2016). Previous investigations into fault-bound traps have primarily focused on two domains: (1) the study of fault growth and activity and their implications for hydrocarbon migration and accumulation, emphasizing qualitative analysis based on tectonic evolution studies (Baudon and Cartwright 2008; Reilly et al. 2016; Cong et al. 2020), and (2) the quantitative assessment of fault-sealing capacity and its influence on the hydrocarbon column height in traps, with a focus on the mechanisms and methodologies of sealing evaluation (Yielding et al. 1997; Sperrevik et al. 2002; Davies et al. 2003; Childs et al. 2009; Fisher et al. 2017; Song et al. 2020). Since the same fault plane can exhibit diverse sealing behaviors at different locations (and/or times) (Luo et al. 2020), previous studies have often overlooked the changes in fault-sealing capacity during fault growth and activation. Futhermore, the impact of such changes caused by fault reactivation on oil and gas migration and accumulation has not been considered. To address these gaps, this study integrated the analysis of fault activity with the evaluation of fault-sealing capacity and basin and petroleum system modeling (BPSM) in order to provide a comprehensive understanding of the dynamic processes of hydrocarbon migration along faults and their accumulation in traps.

The Bohai Bay Basin (BBB), a key petroliferous basin in China (Fig. 1a), has produced over 30% of the country’s total oil production (Hao et al. 2011; Xu et al. 2024). The LK7-6 structure is situated along the southern part of the Bodong Low Uplift, between the Bodong and Bozhong Sags, with favorable conditions for hydrocarbon supply and accumulation (Fig. 1b). The presence of an extensional normal fault as a trap boundary in the LK7-6 structure, along with the variability in hydrocarbon charge across different traps, indicates that this fault significantly influences hydrocarbon migration and accumulation. Given the potential influence of neotectonic movement (after ~ 5.1 Ma) (Gong et al., 2004; Huang and Liu 2014) and and its alignment with the period of hydrocarbon charging (Zou et al. 2011; Xu et al. 2016), the LK7-6 structure serves as an ideal study area for investigating fault reactivation and its control over hydrocarbon migration and accumulation potential.

This study is thus focused on the F3 trap-boundary fault in the LK7-6 structure, with the following aims: (1) to study the growth and activation history of the trap-boundary fault through fault throw analysis; (2) to determine the lithological juxtapositions and shale gouge ratio (SGR) before and after fault reactivation, and dynamically evaluate changes in the sealing capacity; (3) to reconstruct the evolution of hydrocarbon migration and accumulation during fault reactivation by integrating oil-source correlation and basin and petroleum system modeling (BPSM); and (4) to investigate the effect of fault reactivation on sealing capacity and reveal the controlling mechanisms of fault reactivation on dynamic hydrocarbon migration and accumulation. The study is expected to provide new insights into petroleum geological research and exploration in areas with similar geological settings.

Fig. 1
figure 1

(a) Illustration of the Bohai Bay Basin in eastern China (modified from Hao et al. (2009a), showing the location of the study area (indicated by the blue square) within China. (b) The structural map shows Longkou 7 − 6 (LK7-6) and adjacent areas, including faults at the bottom of the Cenozoic strata, major traps (the light yellow areas), and oil fields (the green areas). It also depicts the areas covered by the 3D seismic survey (outlined by green dashed lines), the 2D seismic lines (indicated by blue dotted lines) and the pseudo wells in the Bodong Sag and Bozhong Sah discussed in the text. (c) Cross-section A–A′ across the Bozhong Sag, Bodong Low Uplift, and Bodong Sag (modified from Qi and Yang (2010). See Fig. 1a for location. TWT = two-way traveltime

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Geological settings

Bohai Bay Basin

The Bohai Bay Basin (BBB), a rift basin formed on the top of the North China Craton (Allen et al. 1997; Huang and Pearson 1999; Hao et al. 2007), consists of seven sub-basins: Xialiaohe, Liaodong Bay, Bozhong, Huanghua, Jizhong, Jiyang, and Linqing (Fig. 1a). The tectonic evolution of the BBB since the Cenozoic has been divided into two stages: (1) Paleogene synrift (65.0–24.6 Ma) and (2) Neogene to Quaternary postrift thermal subsidence (24.6 Ma to the present) (Figs. 1c and 2) (Hu et al., 2001; Gong et al. 2010; Hsiao et al. 2010). There are two relatively independent structural deformation systems in the basin province, the extensional and the strike-slip systems (Fig. 1c) (Qi and Yang., 2010). During the synrift stage, a series of grabens and half grabens were formed along the NW- and NE-trending faults as a result of extensional faulting and tectonic subsidence (Huang and Pearson 1999; Qi and Yang 2010; Teng et al. 2014). Thermal subsidence caused these grabens and half grabens to merge into a major joint basin with only one depocenter during the postrift stage (Fig. 1c) (Qi and Yang 2010; Teng et al. 2014). The neotectonic movement (after ~ 5.1 Ma) has caused intense tectonic reactivation (Gong et al., 2004), mainly manifested by basement fault reactivation and shallow fault development (Huang and Liu 2014).

The Paleogene synrift megasequence consists of three predominantly fluvial–lacustrine sedimentary sequences that are restricted to grabens and half grabens, namely, the Kongdian Formation (E1 − 2k), the Shahejie Formation (E2 − 3s), and the Dongying Formation (E3d) (Fig. 2). The third member of the Dongying Fm (E3d3), the first Member of the Shahejie Fm (E3s1), and the third member of the Shahejie Fm (E2s3), which consist of dark mudstones deposited in the semi-deep and shallow lake environments (Fig. 2), have been proven to be the main sources of hydrocarbons in the BBB (Hao et al. 2009b; Tian et al. 2014). The Neogene to Quaternary postrift megasequence contains three fluvial sedimentary sequences, namely, the Guantao Formation (N1g), the Minghuazhen Formation (N1 − 2m), and the Pingyuan Formation (Qp) (Gong 1997; Huang and Liu 2014) (Fig. 2). Of these, the Guantao Fm (N1g) and Minghuazhen Fm (N1 − 2m) have been deposited in the fluvial and deltaic environments, forming a favorable reservoir–cap rock combination (Zhao et al. 2018a; Sun et al. 2020) (Fig. 2).

Fig. 2
figure 2

A generalized stratigraphic column for the Bohai Bay Basin, highlights the major tectonic and depositional events. The figure specifically notes the timing of hydrocarbon generation and the hydrocarbon charging events. A Vsh log around the Guantao Fm (N1g) is also depicted in an enlarged photo on the right side of the stratigraphic column. Abbreviations: Qp = Quaternary Pingyuan Fm; N2mU = the Upper Minghuazhen Fm; N1mL-1 = the first bed of the Lower Minghuazhen Fm; N1mL-2 = the second bed of the Lower Minghuazhen Fm; N1g = the Guantao Fm; E3d1 = the first member of the Dongying Fm; E3d2U = the Upper second member of the Dongying Fm; E3d2L = the Lower second member of the Dongying Fm; E3d3 = the third member of the Dongying Fm; E3s1 = the first member of the Shahejie Fm; E3s2 = the second member of the Shahejie Fm; E2s3 = the third member of the Shahejie Fm; E2s4 = the fourth member of the Shahejie Fm; Ek = the Kongdian Fm

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LongKou 7 − 6 structure

The LK7-6 structure covers an area of approximately 140 km2 and is located at the southern end of the Bodong Low Uplift (Ren et al. 2019) (Fig. 1b). It lies between the Bozhong Sag to the west and the Bodong Sag to the east, providing an adequate supply of hydrocarbon sources. The structural elements consist of an anticline and three basement faults, namely the Bodong No. 1 (BD1), Bodong No. 2 (BD2), and Fault 3 (F3) (Fig. 3a).

Fig. 3
figure 3

Structural contour maps (time domain, ms) at the base of (a) the Cenozoic strata, (b) the Guantao Formation (N1g), and (c) the Lower Minghuazhen Formation (N1mL), showing the structural elements within the LK7-6 structure, including the anticline (highlighted in gray shading) and major faults. (d) Interpreted seismic cross-sectional profile B–B′ in the LK7-6 structure and its adjacent areas. Note the depiction of the structural elements including the Bodong No. 1 (BD1), Bodong No. 2 (BD2), and Fault 3 (F3). See Fig. 1b for location. TWT = two-way traveltime. (e) Structural contour map (depth domain, m) at the top of the Guantao Formation (N1g) of the LK7-6 structure, with the wells utilized in the study. The depicted structural contour map’s location is indicated in Fig. 3c

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The BD1 fault, aligned along an NNE strike, is a regionally extensional normal fault that was inactive during the Cenozoic. (Teng et al. 2019) (Fig. 3d). The BD2 fault is a branching fault in the eastern part of the southern segment of the Tanlu fault, which experienced dextral strike-slip movement (Peng et al. 2009; Teng et al. 2019). It strikes a nearly straight north-south line on the basement structural map and slightly curves clockwise to the north (Fig. 3a). The cross-sectional profile has revealed that the BD2 fault is nearly vertical and exhibits the characteristics of a hybrid flower structure (Huang and Liu 2017) (Fig. 3d). The difficulty in tracing the activity history of strike-slip faults is due to their movement along their strike (Yu et al. 2008). Previous studies have utilized normal faults derived from strike-slip faults to record the activity history of strike-slip faults (Peng et al. 2018; Ye et al. 2021). The available evidence suggests that the BD2 fault and its associated normal faults were highly active during the deposition periods of the first and second members of the Dongying Fm (E3d1 + 2) (30.3–24.6 Ma) and reactivated during the deposition periods from the Minghuazhen Fm (N1 − 2m) to the Pingyuan Fm (Qp) (12.0–0 Ma). Folds associated with strike-slip movement successively developed around the PDZ (Principal Displacement Zone), gradually forming a broad, gentle anticline known as the Bodong Low Uplift (Teng et al. 2019) (Fig. 3a). It can be observed that the strata from the Kongdian Fm (E1 − 2k) to the third member of the Dongying Fm (E3d3) at the top of the anticline are thinnest, or even absent, with gradual thickening toward the limbs (Fig. 3d), indicating that the anticline was formed during the depositional period from the Kongdian Fm (E1 − 2k) to the third member of the Dongying Fm (E3d3) (Teng et al. 2019).

The basement F3 fault intersects the BD2 fault to the northeast and curves slightly counterclockwise to the southwest on the basement structural map (Fig. 3a). It runs essentially parallel to the strike of the Bozhong North (BZN) fault (Figs. 1b and 3a), which has been interpreted as the result of regional northwest–southeast extensional movement (Qi and Yang 2010; Teng et al. 2019). Separation occurs in the shallow F3 fault, which curves counterclockwise to the northeast (Fig. 3b and c). A series of normal faults have developed on the hanging wall of the F3 fault, diverging upward and extending to the seafloor (Fig. 3d). This has resulted in the formation of fault-bound traps, including three traps (M1–M3) in the middle block and four traps (S1–S4) in the south block (Fig. 3e). The S1 trap and F3 fault represent the primary focus of this study (Fig. 3e). The accumulation of hydrocarbons was primarily observed in the sandstones of the Guantao Fm (N1g) in the hanging wall of the F3 fault (Fig. 3e). The target interval for this study is the Guantao Fm (N1g), which has a burial depth of approximately 2200–3600 m and is characterized by sandstones interbedded with the mudstones (Fig. 2). The Guantao Fm (N1g) exhibits excellent reservoir–cap combinations, wherein the reservoir rocks are sandstones and the cap rocks are interbedded mudstones.

Data and methods

Dataset

The study area, as illustrated in Fig. 1b, covers approximately 600 km2 of three-dimensional (3D) seismic data. The dominant frequency in the Cenozoic strata varies with depth, with an average value of approximately 40 Hz. The inline and crossline spacings are 25 and 12.5 m, respectively. With an average velocity of 2000 m/s, the vertical resolution has been determined to be 12.5 m. The time–depth relationship was derived from seven wells (see Fig. 3e for location). The data set comprised logging data from three wells (Well 1, 3, and 9), including acoustic, density, gamma ray, and resistivity. The crude oil was sampled from the reservoir within the Guantao Formation (N1g) of Well 1, at depths between 2606 and 2619 m. The density of this crude oil sample and the oil–water contacts were obtained from production test reports. The above data used in this study were provided by China National Offshore Oil Corporation Ltd. (CNOOC Ltd), Tianjing Branch.

Methods

The following methods were employed in the study, namely fault throw analysis, fault-sealing capacity evaluation, molecular geochemical analysis of crude oil, and basin and petroleum system modeling (BPSM).

Fault throw analysis

Fault throw versus depth (T–z) plots and expansion index (EI) diagrams have been used to evaluate the period of fault activity (Cartwright et al. 1998; Baudon and Cartwright 2008; Ryan et al. 2017). The fault throw versus depth (T–z) plot represents the distribution of fault throw at different reflection horizons and its variation over depth. This trend of variation provides information related to the evolution of the fault and determines whether the fault has been reactivated (Song et al., 2022; Chu et al., 2023). The EI is a measure of the thickness differential between stratigraphic units situated on either side of a fault. It is calculated as the ratio of the thickness of the hanging wall stratigraphic unit to that of the footwall stratigraphic unit. A normal fault with an EI value exceeding 1 indicates the occurrence of syndepositional fault activity (Cartwright et al. 1998; Baudon and Cartwright 2008). An elevated EI value indicates a higher level of fault activity. To better reveal the activity history of the trap-boundary fault in the Cenozoic, eight key seismic horizons were used to determine the fault throw distribution with depth and the variations in the thickness of the corresponding strata. The eight key seismic horizons are T8, T3m, T3u, T2, T1, T0, T02, and T01. The corresponding stratigraphy can be seen in Fig. 2. By calculating the vertical displacements between corresponding strata at different depths on both sides of the fault, fault throw versus depth (T–z) plots were generated.

Fault throw versus distance (T–x) plots are a means of describing variations in throw, whereby a relationship is established between fault throw and distance along the fault. This approach has been employed to examine fault growth and connectivity (Jackson and Rotevatn 2013). The minimum fault throw on the fault versus distance (T–x) plot represents a potential lateral linkage point for fault segments, while the maximum fault throw between two adjacent minimum fault throws indicates the presence of a separate fault segment. The fault throw versus distance (T–x) plot for the F3 fault was generated by using two key seismic horizons (T0 and T2), which were identified as the top and bottom of the target interval (Guantao Fm, N1g), respectively. It should be noted that the sampling of the throw along the fault was conducted in strict accordance with the guidelines outlined in Ze and Alves (2019). The sample spacing of the acquired throw data is approximately 180 m, less than 3% of the total length of the fault.

Fault-sealing capacity evaluation

Fault juxtaposition and fault rock seals are recognized as the two primary types of fault seals (Knipe 1997; Jolley et al. 2007; Faulkner et al. 2010). Juxtaposition seals are formed when reservoir stratigraphic units are juxtaposed with impermeable stratigraphic units. This phenomenon can be identified using juxtaposition and Allan diagrams (Allan 1989; Knipe 1997). Fault rock seals are formed within the fault zone when high capillary threshold pressure fault rocks develop due to deformation (Knipe et al. 1998; Fossen et al. 2007). Therefore, the sealing capacity of a fault rock can be evaluated based on its capillary threshold pressure (Watts 1987; Fisher and Knipe 2001; Fisher and Jolley 2007).

Capillary threshold pressure can be estimated by calculating clay content in fault rocks. Shale Gouge Ratio (SGR) algorithm is widely employed for its effectiveness in calculating the clay content of complex stacked sequences within a fault zone (Pei et al. 2015). It is simply defined as the percentage or ratio of shale or clay that has gouged past a certain point along the fault plane (Yielding et al. 1997) and is given by the following equation.

$$SGR = \varSigma({V}_{sh}\times Z)/T$$
(1)

where T is the fault throw (m), Z is the interval thickness (m), and Vsh is the shale or clay content in the throw interval.

To obtain the juxtapositions and SGR, a 3D geological model was constructed using Midland Valley’s MOVE software, which integrated seismic and borehole data. The model consists of stratigraphic and fault components. The stratigraphy was derived from the depth-converted 3D seismic data and calibrated with borehole data, while the fault was constructed using polygon data for each reflection horizon. This approach allows for the determination of stratigraphic thickness and fault throw. The gamma-ray logs from two key wells, Wells 1 and 3, were used to estimate the shale and clay mineral content, which was then converted to intrastratigraphic Vsh values near the trap-boundary fault using equations from Clavier et al. (1971). The MOVE software enables the restoration of fault throw and physical compaction to a geological time step in the past, thereby facilitating the precise calculation of fault throw and Vsh corresponding to that specific time step. Accordingly, the SGR of the trap-boundary fault in the study area can be effectively evaluated at different geological time through the use of the functions provided by the MOVE software. Once the SGR value has been determined, the maximum buoyancy pressure that can be sealed by the fault can be calculated using the following equation.

$${P}_{b} = 0.1463\times ln\left(SGR\right) - 0.393$$
(2)

where Pb is the buoyancy pressure (MPa), and SGR is the calculated shale gouge ratio (%) along the fault plane.

The above empirical equation was established by Song et al. (2020) to evaluate the fault sealing capacity in the Qinan Sag within the BBB. The Qinan Sag exhibits comparable tectonic evolution characteristics to the study area, with the target interval dominated by the sandstone-mudstone interbedded Guantao Fm (N1g). The equation’s applicability to this study area is supported by the commonality of tectonic and sedimentary features. The equation is based on the assumption that the buoyancy pressure generated by the hydrocarbon column height is sufficient to reach the limit of the fault sealing capacity. This implies that the buoyancy pressure is equal to the capillary threshold pressure of the fault rock. Subsequently, the potential maximum hydrocarbon height trapped by the fault is calculated as follows:

$$H = [0.1463\times ln\left(SGR\right)-0.393]/({\rho}_{w}-{\rho}_{h})g$$
(3)

where SGR is the calculated shale gouge ratio (%), ρh and ρw are the densities of hydrocarbon and water (kg/m3), respectively, and g is the gravitational acceleration.

Molecular geochemical analysis of crude oil

In this study, a crude oil sample was selected for analysis to test saturated hydrocarbon monomers. The crude oil sampled from the Guantao Fm (N1g) reservoir at depths of 2606 to 2619 m in Well 1. The separation of the saturated, aromatic, and resin fractions was achieved through the utilisation of a range of solvents. The molecular compositions of the saturated hydrocarbon fractions were analysed using gas chromatograph–mass spectrometry (GC-MS). The GC-MS analysis was conducted on the Thermo Fisher Trace DSQ II instrument with the HP-5MS fused silica column (60 m × 0.25 mm × 0.25 μm). The injector temperatures was set at 300℃. Helium was employed as the carrier gas, with a flow rate of 1 mL/min. The temperature settings for the GC-MS analysis were as follows: The temperature was initially set at 50 °C for 1 min, then increased at a rate of 15 °C/min to 100 °C, followed by a further increase at a rate of 2 °C/min to 200 °C. Thereafter, the temperature was increased at a rate of 1 °C/min to 315 °C, and then held at this temperature for 20 min. Subsequently, each compound was identified through its retention time using a full scan from m/z 50 to 600. The sterane compounds and terpenes were selected at m/z 191 and 217, respectively (Peters et al. 2005).

Basin and petroleum system modeling (BPSM)

Basin and petroleum system modeling (BPSM) offers a new approach for simulating dynamic geological processes, including hydrocarbon generation, migration, and accumulation (Bora and Dubey 2015). PetroMod is a widely utilized petroleum system modeling tool developed by Schlumberger that supports one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) modeling (Baur et al. 2018). The algorithm characterizes a series of subsurface processes, including burial and thermal histories (Hantschel and Kauerauf 2009; Sweeney and Burnham 1990), and provides a quantitative estimation of hydrocarbon generation, migration, and accumulation (Aziz and Settari 2002; Hantschel et al. 2000; Baur et al. 2018).

In this study, the 1D module of PetroMod was utilized to simulate burial and thermal history. Given the distribution of drilled wells along the periphery of the sag or in uplift areas, and the absence of wells in the center of the sag, two pseudo wells (P1 and P2) were selected to accurately reflect the thermal history and hydrocarbon generation history of the source rocks in the Bodong Sag and Bozhong Sag. The locations of the pseudo wells are illustrated in Fig. 1b. The Petro Charge Express and 2D module of PetroMod were employed to simulate hydrocarbon migration and accumulation. Petro Charge Express is a tool designed for the rapid assessment of secondary hydrocarbon migration. It was utilized to determine the payhway of hydrocarbon migration along the basement structural surface at a critical geological time. The basement structural surface was derived from 3D seismic interpretation data following time-depth conversion. Figure 1b depicts the extent of 3D seismic data coverage, outlined by green dashed lines. The morphology of the basement structure at a critical geological moment was obtained by layer flattening. The BD2 fault was identified based on fault polygons. The range of generation kitchens and generation intensity of the source rocks were derived from Jiang et al. (2016). The 2D geological model was developed using seismic interpretation data after time-depth conversion (Section C–C’ across the S1 trap and intersecting the Well 1, as illustrated in Fig. 1b). The absolute ages of each sedimentary stratigraphic unit and the erosion events were obtained from the established local chronostratigraphic framework, as documented in previous studies (Hao et al. 2009a; Huang and Liu 2014) (Fig. 2; Table 1). The BBB has undergone two significant episodes of uplift and erosion since the Cenozoic. These episodes occurred during the Paleocene (65–54.9 Ma) and the Early Miocene (24.6–16.6 Ma), respectively (Table 1). The erosion thicknesses were derived from previous studies that reconstructed the tectonic subsidence process using vitrinite reflectance (Ro) and apatite fission track analysis (Hu et al., 2001; Huang and Liu 2014). The major lithologies for each layer were assigned based on sedimentary facies (Table 1). In particular, the lithology of the target interval, the Guantao Fm (N1g), was determined to be interbedded sandstones and mudstones based on well log and core data from Well 1 (data provided by CNOOC Tianjin). The lithological properties define the modelled compaction during burial according to Athy’s law (Athy 1930). To construct thermal and maturity models, it is necessary to define the boundary conditions, including paleowater depth (PWD), sediment–water interface temperature (SWIT), and heat flow. The PWD for different periods was referenced from Ye et al. (2021). The SWIT was derived from the average surface temperature history at different latitudes as provided in PetroMod (Wygrala 1989), selecting East Asia, at 38°N latitude. The McKenzie crustal stretching model was applied to invert the heat flow (McKenzie 1978).

Table 1 Summary of events, assigned ages, lithologies, and corresponding formation applied in the geological model
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Results

Fault growth and activation

In this section, the study employed the Fault Throw versus Distance (T–x) plot to investigate the growth and connectivity of faults. The Fault Throw versus Depth (T–z) plot and the Expansion Index (EI) diagram were used to evaluate fault activity. Figure 4a depicts the geometric sketch of the shallow F3 fault on the T2 and T0 seismic horizons, respectively. The T–x plots demonstrate a highly variable throw profile, characterised by three major maximum throws separated by two minimum throws from southwest to northeast (Fig. 4b). The distribution of throws along the fault strike indicates that the shallow F3 fault evolved from separate segmental faults through lateral extension and interconnection at relay zones (Robson et al. 2018; Teng et al. 2019). The number of maximum and minimum throws allows for the interpretation that the shallow F3 fault resulted from the linkage of at least three fault segments.

Three seismic profiles (Line 1, Line 2, and Line 3) were selected for the measurement of vertical displacements and corresponding stratigraphic thicknesses on both sides of Fault 3 (Fig. 4c). Subsequently, the data were plotted in T–z plots and EI diagrams (Fig. 4d). The locations of the seismic profiles are illustrated in Fig. 4e. As shown in Fig. 4d, lines 1 and 2 exhibit comparable variation trends in the T–z plots, with two maximum throws occurring at the base of the Upper Minghuazhen Fm (N1gU) and the first bed of the Lower Minghuazhen Fm (N1mL-1), and two minimum throws occurring at the base of the Upper second member of the Dongying Fm (E3d2U) and the second bed of the Lower Minghuazhen Fm (N1mL-2). Line 3 displays a distinctive profile, exhibiting a single maximum throw at the base of the Lower Guantao Fm (N1gL), which differs from the profiles of lines 1 and 2. The vertical throw profiles for lines 1 and 2 exhibit a variation from large to small between the Kongdian Fm (E1 − 2k) and the Lower second member of the Dongying Fm (E3d2L), whereas line 3 exhibits an inverse trend. During the deposition of the Kongdian Fm (E1 − 2k) to the third member of the Dongying Fm (E3d3), the expansion indexes were greater than 1 at both southwestern sites (lines 1 and 2) and less than 1 at the northeastern site (line 3). It is notable that the expansion indexes of all sites (lines 1, 2, and 3) were greater than 1 during the deposition of the Upper Minghuazhen Fm (N1gU). The results indicate that the F3 fault, located in the southwestern direction, was highly active during the depositional period from the Kongdian Fm (E1 − 2k) to the Lower second member of the Dongying Fm (E3d2L). The level of activity exhibited a gradual decline during the depositional period from the Upper second member of the Dongying Fm (E3d2U) to the first member of the Dongying Fm (E3d1). During the Neogene, the shallow F3 fault continued to grow during the deposition of the Upper Guantao Fm (N1gU) and remained active during the deposition of the Upper Minghuazhen Fm (N2mU).

Fig. 4
figure 4

A detailed examination of the trap-boundary fault activity. (a) A diagrammatic representation of the geometry of the F3 fault on the T2 and T0 seismic reflectors (see Fig. 4e for the location of Fault 3). (b) Fault throw versus distance (T–x) plots for the F3 fault on the T2 and T0 seismic reflectors. (c) Three typical seismic cross-sections (see Fig. 4e for the locations of lines 1, 2, and 3) have been employed to calculate the fault throws corresponding to the various stratigraphic units. (d) Fault throw versus depth (T–z) plots and expansion index (EI) diagrams for the F3 fault. (e) Structural contour map (time domain, ms) at the top of the Guantao Formation (N1g) showing the location of the F3 fault and the profile lines (indicated by the dashed blue lines)

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Dynamic evaluation of fault sealing capacity

The S1 trap is bounded by the shallow F3 fault and is developed in the hanging wall of the fault (Fig. 3e). The stratigraphy of the Guantao Fm (N1g) in the study area is characterised by the interbedding of sandstone and mudstone. Therefore, within the Guantao Fm (N1g), each oil-bearing reservoir within every oil-bearing trap is characterised by an independent oil–water contact. Two typical reservoirs were employed for the evaluation and comparison of fault sealing capacity: the N1g-i (partially charged) and N1g-ii (fully charged) reservoirs. The accumulation of hydrocarbons has been observed in the hanging wall, while the footwall juxtaposed with the reservoirs consists of water pools (Fig. 5). As indicated in Table 2, the top surface and oil–water contact depths of the N1g-i reservoir are − 2260 and − 2290 m, respectively, with the corresponding oil column height being 30 m (Fig. 5; Table 2). The top surface and oil–water contact depths of the N1g-ii reservoir are recorded at − 2560 and − 2600 m, respectively, with the corresponding oil column height being 40 m (Fig. 5; Table 2).

Fig. 5
figure 5

(a) Reservoir profile of the S1 fault-bound trap in the Guantao Formation (N1g). Two distinct reservoirs, N1g-i and N1g-ii, have been identified within the hanging wall of the F3 fault. Each reservoir is characterised by an independent oil–water contact. The top surface and oil–water contact depths of the N1g-i reservoir are − 2260 and − 2290 m, respectively, with the corresponding oil column height being 30 m. The top surface and oil–water contact depths of the N1g-ii reservoir are − 2560 and − 2600 m, respectively, with the corresponding oil column height being 40 m. (b) Structural contour map (depth domain, m) at the top of the Guantao Formation (N1g), with the location of the profile line (indicated by the dotted blue line)

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Table 2 The data set includes information on traps and reservoirs, fluid density, and maximum hydrocarbon column height
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Allan diagrams (Fig. 6c and d) and the distributions of the calculated SGR over the fault plane (Fig. 6e and f) were obtained for the N1g-i and N1g-ii reservoirs (Fig. 6a and b). The Allan diagrams demonstrate the lithological juxtaposition between target intervals on both sides of the shallow F3 fault (Fig. 6c and d). A sand–mudstone juxtaposition is observed at the top of the N1g-ii reservoir (Fig. 6d), indicating a favorable fault juxtaposition seal. The juxtaposition of two highly permeable reservoirs (sand/silt–sand/silt) at the top of the N1g-i reservoir (Fig. 6c) does not necessarily imply that cross-fault leakage will occur. The interbedding of sandstones with mudstones in the Guantao Fm (N1g) along the fault plane results in a widespread distribution of the sand/silt–sand/silt juxtaposition (Fig. 6c and d). Therefore, juxtaposition is an inadequate method for determining the fault seal, and a quantitative analysis using SGR is necessary. The distribution of SGR across the fault plane exhibits considerable variability, with values ranging from 22 to 32% for the N1g-i reservoir and from 38 to 50% for the N1g-ii reservoir (Fig. 6e and f). The calculated SGR values (Fig. 6f) were applied to Eqs. 2 and 3 to determine the estimated Pbs and corresponding oil column heights for the N1g-ii reservoir. The estimated Pbs were found to be 0.129–0.168 MPa, with the corresponding oil column heights being 59–77 m. These values are considerably greater than the actual oil column heights of the N1g-ii reservoir. Therefore, the shallow F3 fault effectively seals the N1g-ii reservoir, resulting in a fully charged trap. In contrast, when the calculated SGR values (Fig. 6e) are applied to Eqs. 2 and 3, the estimated Pbs and corresponding oil column heights for the N1g-i reservoir are calculated to be 0.051–0.105 MPa, and 23–47 m, respectively. This indicates that the shallow F3 fault has a limited sealing capacity for the N1g reservoir. It is notable that a sand–sand juxtaposition is observed at the top of the N1g-i reservoir, situated to the southwest. This corresponds to an SGR of no more than 20% (Fig. 6e), which may be the location of hydrocarbon leakage across the fault. This leakage is likely responsible for the partial charged of the N1g-i reservoir, where the present-day oil column height is 30 m, which is significantly less than the actual trap height of 50 m.

As illustrated in the EI diagrams (Fig. 4d), the shallow F3 fault was reactivated during the deposition of the Upper Minghuazhen Fm (N2mU). The sealing capacity of the shallow F3 fault was shown to have a limited sealing capacity for the N1g-i reservoir after reactivation (Fig. 6e). To elucidate the impact of fault reactivation on the sealing capacity of the fault, this study employed the N1g-i reservoir to analyze and evaluate the lithological juxtaposition and sealing capacity of the trap-boundary fault prior to reactivation. As illustrated in Fig. 6g, the Allan diagram indicates that the predominant lithological juxtaposition was sand–mudstone juxtaposition. This indicates that the shallow F3 fault exhibited effective juxtaposition sealing for the N1g-i reservoir before reactivation. Furthermore, the distributions of the calculated SGR over the fault plane before reactivation (Fig. 6h) indicate that the minimum SGR before reactivation was at least 42%, with the corresponding maximum oil column height calculated using Eq. 3 reaching to 65 m. Therefore, the shallow F3 fault exhibited excellent sealing capacity for the N1g-i reservoir before reactivation.

Fig. 6
figure 6

Evaluation of the sealing capacity of the shallow F3 fault, using two typical reservoirs in the Guantao Formation (N1g) as examples. The structural maps illustrate the location of the shallow F3 fault and trap where (a) the N1g-i and (b) the N1g-ii reservoirs are developed. The oil column heights of these reservoirs are 30 m and 40 m, respectively. Allen diagrams and calculated SGR values were extracted for the fault plane of the shallow F3 fault. After reactivation (present-day), the perspective view of the fault plane diagram of the shallow Fault 3 towards the NW shows (c and d) Allan diagrams and (e and f) calculated SGR values associated with the N1g-i and N1g-ii reservoirs, respectively. Before reactivation (~ 5.1 Ma), the perspective view of the fault plane diagram of shallow Fault 3 towards the NW shows (g) an Allan diagram and (h) a calculated SGR value associated with the N1g-i reservoir. Allan diagrams: sand juxtaposed against sand (orange), sand/silt juxtaposed against silt (pale yellow), sand juxtaposed against mudstone (light grey), silt juxtaposed against mudstone (grey), mudstone juxtaposed against mudstone (dark gray). Calculated SGR values: SGR < 20% (green), SGR > 20% (yellow to red). Light blue and blue lines represent the projection lines of the upper and lower surfaces of the reservoir onto the hanging wall and footwall of the fault, respectively. The hanging wall line is solid, while the footwall line is dashed

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Hydrocarbon source and charge

Source of crude oil

The molecular geochemical characteristics of crude oil can be determined using saturated hydrocarbon gas chromatography–mass spectrometry (GC-MS). By comparing typical biomarker assemblages from crude oil and source rocks, the origin of the crude oil can be further identified (Fig. 7). Figure 7a illustrates representative GC-MS mass chromatograms of m/z 191 and m/z 217 for the crude oil sample, corresponding to terpanes and steranes, respectively. The crude oil exhibits a relatively low tricyclic terpane content, with the low carbon number tricyclic terpanes displaying a normal distribution, dominated by C23 tricyclic terpanes. Additionally, the content of C24 tetracyclic terpane slightly exceeds that of the C26 tricyclic terpane, with their ratio slightly greater than 1.0. The regular steranes exhibit a V-shaped pattern, with C27 exhibiting a slightly higher content than C29. It is noteworthy that the content of gammacerane is very low, while the content of 4-methysteranes is relatively high (Fig. 7a). These geochemical characteristics suggest that the organic matter in the corresponding source rock is predominantly of aquatic origin, particularly from algae, with minimal input from terrestrial organic matter. Figure 7b shows the GC-MS mass chromatograms of m/z 191 and m/z 217 for the source rocks of the third member of the Shahejie Fm (E2s3). The data are derived from the research of Hao et al. (2009b), 2011). The biomarker assemblage of the source rocks also exhibits characteristics such as low gammacerane, high 4-methysteranes, and a low input of terrigenous organic matter (Fig. 7b). Therefore, the oil–source correlation analysis indicates that this crude oil has its origin in the source rocks of the third member of the Shahejie Fm (E2s3) (Fig. 7).

Fig. 7
figure 7

Mass chromatograms of the saturated fractions for terpene (m/z = 191) and sterane (m/z = 217). (a) The crude oil sample from the reservoir of the Guantao Fm (N1g) in the Well 1 and (b) the source rock sample from the third member of the Shahejie Fm (E2s3), as quoted from Hao et al. (2009b), 2011). The location of Well 1 is shown in Fig. 3e. Tricyclic terpenes are indicated by solid dots (C19, C20, C23, and C26). C24Tet refers to C24 tetracyclic terpane; C27, C28, and C29 denote C27, C28, and C29 sterane 20R, respectively

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Hydrocarbon generation and charging history

Hydrocarbon migration follows the generation and expulsion processes. Therefore, the hydrocarbon generation history of the source rocks within the Bodong Sag and Bozhong Sag was initially determined using the Easy%Ro model (Sweeney and Burnham 1990). Two pseudo wells, located on the east and west side of the LK7-6 structure within the Bodong Sag and Bozhong Sag (as shown in Fig. 1b), were selected to reconstruct the burial and thermal history (Fig. 8). The results indicate that the continuous rifting led to the rapid burial of the sag, accompanied by an increase in both the sedimentation rate and temperature. This enabled the source rocks of the third member of the Shahejie Fm (E2s3) in the Bodong Sag and Bozhong Sag to reach their oil generation threshold (Ro = 0.5%) in the Late Paleogene (~ 30.3 Ma), and subsequently peak oil generation (Ro = 1.0%) in the Late Oligocene (~ 24.6 Ma).

Fig. 8
figure 8

The burial and thermal histories of the source rocks in the third member of the Shahejie Fm (E2s3) for two pseudo wells: (a) P1 well, located on the east side of the LK7-6 structure within the Bodong Sag, and (b) P2 well, located on the west side of the LK7-6 structure within in the Bozhong Sag. The locations of the pseudo wells are indicated in Fig. 1b

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In addition, the study of homogenization temperatures of fluid inclusions (both oil and aqueous fluid) provides an accurate determination of the duration of hydrocarbon charging. A sample from Well 1, located near the F3 fault and extracted from the reservoir within the Guantao Fm (N1g) at a depth of 2610 m, exhibits homogenization temperatures ranging from 75 °C to 94 °C. Combined with the burial and thermal history of the well, it is evident that hydrocarbon charging occurred during the deposition of the Upper Minghuazhen Fm (N2mU) and Pingyuan Fm (Qp), from approximately 5.1 Ma to the present (Fig. 9).

Fig. 9
figure 9

Determination of the duration of hydrocarbon charging. (a) A diagram of the burial and thermal history of Well 1 is provided. (b) The homogenization temperature frequency distribution of fluid inclusions from the Guantao Fm (N1g) in the Well 1 at a depth of 2610 m is presented. The location of the Well 1 is indicated in Fig. 3e. It was determined that hydrocarbon charging occurred from approximately 5.1 Ma to the present

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The process of hydrocarbon migration and accumulation

Migration at the peak of hydrocarbon generation

To elucidate the secondary migration at the peak of hydrocarbon generation, the study employed the Petro Charge Express tool to reconstruct the hydrocarbon migration along the basement structure in the Late Oligocene (~ 24.6 Ma) (Fig. 10). The analysis identified the third member of the Shahejie Fm (E2s3) as the primary source rock and confirmed the extent of the generation kitchens in the Bodong Sag and Bozhong Sag. The BD2 fault was assigned an open status by default, in accordance with the activity characteristics identified by Ye et al. (2021). A flow path migration algorithm was employed.

Figure 10 illustrates the structural morphology of the study area during the Late Oligocene (~ 24.6 Ma), along with the hydrocarbon migration pathways and predicted favorable locations for hydrocarbon accumulation. The simulation results indicate that the structural morphology of the Bodong Low Uplift was already formed during the peak of hydrocarbon generation, consistent with the findings of Teng et al. (2019). The hydrocarbons generated from the source rocks of the third member of the Shahejie Fm (E2s3) in the Bodong Sag and Bozhong Sag exhibited preferential migration along the structural surface towards the structural high. In particular, the hydrocarbons generated in the Bodong Sag migrated from the southeast to the northwest, while the hydrocarbons generated in the Bozhong Sag migrated from the southwest to the northeast. In terms of migration distance, the hydrocarbon supply advantage of the Bodong Sag is more pronounced. A potentially sizable hydrocarbon accumulation zone could be formed at the apex of the Bodong Low Uplift. Additionally, two relatively modest accumulation zones may form in the LK7-6 structure, one of which is located within the trap bounded by the F3 fault.

Fig. 10
figure 10

The pathways of the hydrocarbon migration at the basement of the structural high occurred during a critical period, specifically the Late Oligocene (~ 24.6 Ma). The grey–green areas represent the extent of the generation kitchens of the third member of the Shahejie Fm (E2s3) within the Bodong Sag and Bozhong Sag at this time. The thin dark green lines represent the flow paths, indicating the lateral migration pathways of hydrocarbons along the structural surface. The dark green dashed lines at the top of the structure represent the predicted zone of hydrocarbon accumulation. Figure 1b shows the coverage of the simulation result, outlined by green dashed lines

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Migration and accumulation during fault reactivation

This study employed the 2D module of the PetroMod software to assess the evolution of hydrocarbon migration and accumulation during fault reactivation along the F3 fault of section C–C’. The third member of the Shahejie Fm (E2s3) was assigned as the main source rock based on the oil–source correlation (Fig. 7). Its geochemical properties, including total organic carbon (TOC) and hydrogen index (HI), were referred to Jiang et al. (2016) (see Table 1 for details). The invasion percolation (IP) method was employed to model migration along faults, offering enhanced sensitivity to variations in fault properties (Kroeger et al. 2021). The utilisation of locally refined volumetric elements provided a more precise representation of faults than model cell-sized fault elements (Baur and Katz 2018). Since the BD1 fault was inactive during the Cenozoic, it was excluded from the simulation. The BD2 fault and its associated normal faults were set to an open state from 5.1 Ma to the present, in accordance with the activity characteristics identified by Ye et al. (2021). The fault activity parameters of the F3 fault were assigned on the basis of the results presented in Sect. 4.1, with the 5.1 Ma and present-day SGR distributions (Fig. 6) serving as the fault properties.

Using the PetroMod 2D module and based on the IP method, the modeling results of hydrocarbon migration and accumulation in the C–C’ section of the study area were obtained (Fig. 11). This depicts the dynamic evolution of oil migration and accumulation along the trap-boundary fault before and after reactivation. Since the orientation of the C–C’ profile aligns with the migration direction of hydrocarbons generated by the main source rocks in the Bodong Sag, the results exclusively reflect the migration and accumulation of hydrocarbons generated from the Bodong Sag. It can be observed that the BD2 fault and its associated normal faults, which are in an active period, allow the hydrocarbons generated within the sag to migrate across the open faults and subsequently continue migrating preferentially toward the structural high (Fig. 11a and b). At the beginning of the Pliocene (~ 5.1 Ma), prior to the reactivation of the fault, the Neogene fault-bound traps were essentially formed (Gong 2004). During this time, hydrocarbons that had previously migrated vertically along the trap-boundary fault to the shallow Guantao Fm (N1g) began to charge. The results of the fault-sealing evaluation indicate that the F3 fault exhibited favorable lateral sealing at the top of the Guantao Fm (N1g) (Fig. 6h). A certain degree of hydrocarbon accumulation was therefore formed at the top of the Guantao Fm (N1g), and there was no lateral adjustment of hydrocarbons due to the cross-fault leakage during this period (Fig. 11b and d). Following the activation of the fault, it was inevitable that there would be vertical leakage of hydrocarbons along the fault (Fig. 11a). Concurrently, the variation in fault throw of the reactivated F3 fault exerted influence on the lithological juxtaposition to a certain extent (Fig. 6c), and the fault-sealing evaluation indicated a partial fault-sealing failure at the top of the Guantao Fm (N1g) (Fig. 6d). This resulted in the accumulation of hydricarbons at the top of the Guantao Fm (N1g), accompanied by the hydrocarbon cross-fault leakage and lateral readjustment of hydrocarbons during this period (Fig. 11a and c).

Fig. 11
figure 11

2D modeling results depicting the hydrocarbon migration process over time, (a) after and (b) before fault reactivation. Zooming in locally shows the different characteristics of hydrocarbon migration and accumulation along the fault (c) after and (d) before fault reactivation. Before activation, hydrocarbons migrated vertically along the fault to shallow strata and were charged. Due to the favorable lateral sealing of the F3 fault at the top of the Guantao Fm (N1g), no hydrocarbon cross-fault leakage occurred during this period. After fault reactivation, some vertical leakage occurred locally, with lateral seal failure due to altered lithological juxtaposition, resulting in hydrocarbon cross-fault leakage and lateral readjustment. Results from PetroMod 2D simulation. (e) The map illustrates the LK7-6 structure, including faults and major traps (depicted in light yellow) and oil fields (shown in green), as well as the 2D seismic lines (indicated by blue dotted lines)

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Discussion

Role of regional structural relief in hydrocarbon migration and accumulation

Regional structural relief plays a critical role in controlling the pathways of hydrocarbon migration and accumulation (Hindle 1997). Hydrocarbons migrate from high to low fluid-potential areas, with structural highs containing permeable reservoirs being the most favorable zones for hydrocarbon accumulation (England 1987).

The LK7-6 structure is a structural high formed on folds associated with the strike-slip movement of the Tanlu Fault. The study area benefits from a dual hydrocarbon supply from the Bodong Sag and Bozhong Sag (Teng et al. 2019). The source rocks of the third member of the Shahejie Fm (E2s3) in the Bodong Sag and Bozhong Sag are the primary sources of hydrocarbons (Fig. 7) and had reached peak oil generation in the Late Paleogene (~ 24.6 Ma) (Fig. 8). By this time, the basement structural high was also formed (Teng et al. 2019) (Fig. 10). As a high fluid-potential area, hydrocarbons generated by the source rocks in the sags would be expected to migrate preferentially towards the structural highs, which are lower-potential areas. A flowpath migration simulation conducted using Petro Charge Express (Fig. 10) indicates that during the peak oil generation period (Late Paleogene, ~ 24.6 Ma), the hydrocarbons generated from the source rocks in the third member of the Shahejie Fm (E2s3) within the Bodong Sag and Bozhong Sag would migrate laterally along favorable pathways towards the LK7-6 structure. Specifically, the hydrocarbons generated from the source rocks in the Bodong Sag would migrate northwestward to charge the LK7-6 structure, while those from the Bozhong Sag would migrate northeastward to do the same (Fig. 10). This result further confirms that the LK7-6 structure is located at a regional structural high beneficial for hydrocarbon migration and that both the Bodong Sag and Bozhong Sag can contribute hydrocarbons to it. This migrating pattern is expected to continue throughout the maturation period of the source rocks, corresponding to the generation of large quantities of hydrocarbons, thus laying the groundwork for hydrocarbon accumulation.

Dual control of hydrocarbon migration and accumulation by fault reactivation

Fault reactivation may cause a significant decrease in the capillary threshold pressure of the fault rock, inducing a previously quiescent fault to act as a conduit for vertical hydrocarbon migration or leakage (Langhi et al. 2010; Elkhoury et al. 2011). It is critical to determine the temporal relationship between periods of fault activity and hydrocarbon charging. Generally, faults active during the hydrocarbon charging period are prone to facilitating hydrocarbon migration, while those active after the hydrocarbon charging period tend to cause leakage (Aydin 2000; Duran et al. 2013).

Within the LK7-6 structure, hydrocarbon charging of the Guantao Fm (N1g) reservoirs, occurred during the period from approximately 5.1 Ma to the present (Fig. 9). The reactivation of the F3 fault, closely associated with the neotectonic movement in the BBB from approximately 5.1 Ma to the present, coincides with the period of hydrocarbon charging. This temporal overlap suggests that the reactivation of the F3 fault has likely played a dual role in influencing shallow hydrocarbon migration and accumulation.

The F3 fault, a trap-boundary fault, playes a key role in providing effective vertical conduits for hydrocarbon migration (Fig. 11). Nevertheless, some vertical leakage due to reactivation is inevitable, although a certain volume of hydrocarbons may still be preserved. The appearance of the minimum fault throw in the fault throw versus distance (T–x) plot records the early interconnection of initially separated fault segments and indicates the locations of potentially breached fault relays. It is possible that hydrocarbon columns trapped at these locations may provide access to potential leakage pathways. In the study area, one such linkage zone is identified at the high point of the S1 trap, where potential leakage could also occur (Fig. 4a and b).

Moreover, fault throws undergo constant changes during reactivation. The target intervals within the Guantao Fm (N1g) in the study area are primarily characterized by sand–mudstone interbedding, and variations in throw significantly affect lithological juxtaposition. Consequently, the fault-sealing capacity is significantly altered following reactivation. The analysis of the fault seal in the N1g-i reservoir, conducted before (Fig. 6c and e) and after reactivation (Fig. 6g and h), revealed that changes in fault throw and trap structure have resulted in a sand–sand juxtaposition at the N1g-i reservoir, exceeding the seal failure envelope. This has led to cross-fault hydrocarbon leakage, which has ultimately resulted in the present-day trap remaining unfilled. The hydrocarbon leakage resulting from a fault-sealing failure is further demonstrated by the 2D hydrocarbon migration simulation result (Fig. 11a and c). It is evident that not all instances of fault reactivation result in seal failure and subsequent leakage. The N1g_ii reservoir remains unaffected by fault reactivation, thus maintaining its seal integrity (Fig. 6e and f). It can be concluded that the fault reactivation has led to changes in lithological juxtaposition along the fault plane. This may have resulted in the localized seal failures and hydrocarbon leakage, or alternatively, the formation of lateral seals that lead to hydrocarbon enrichment. This mechanism provides a reasonable explanation for the significant variability in reservoir richness observed within the Guantao Fm (N1g) of the LK7-6 structure.

Limitations, and implications for petroleum exploration in fault-bound traps

This limited study utilized the Move software to evaluate the fault-sealing capacity in a dynamic manner and employed BPSM to reconstruct the dynamic evolution of hydrocarbon migration and accumulation. However, it should be recognized that the achieved “dynamic” state can only be restored to the geological condition at a specific geological time step.

A comprehensive 3D model that includes detailed sedimentary unit volumes and complex structural features enhances the precision of hydrocarbon migration and accumulation calculations (Schneider and Wolf 2000). It is important to note, however, that migration and accumulation simulations require greater precision modeling data and greater computational effort than the calculations of burial thermal history and hydrocarbon generation and expulsion history. Currently, the grid resolution of 3D modeling at the basin and petroleum system scale is typically in the range of hundreds of meters. Given that this study focuses on stacked reservoirs and aims to analyze the impact of changes in fault-sealing on hydrocarbon migration and accumulation, the grid resolution of the model needs to be controlled within tens of meters. This precision requirement presents a significant challenge in both reservoir characterization and migration simulation. Therefore, we adopted a more practical and feasible approach: (1) using 1D simulations to determine the timing of hydrocarbon generation and charging; (2) employing the flow path algorithm in Petro Charge Express to rapidly assess the lateral migration of hydrocarbons generated in the sag during critical periods of hydrocarbon generation; and (3) applying the IP algorithm in 2D modeling to focus specifically on the migration and readjustment pathways of shallow hydrocarbons during periods of fault reactivation. It is essential to recognize that the flow path algorithm assumes that hydrocarbon migration pathways are closely related to structural morphology, with the carrier bed simplified as a homogeneous unit, and does not account for variations in porosity and permeability. Nevertheless, we consider this simplification to be an acceptable compromise as it addresses the issue of how hydrocarbons undergo secondary migration after being generated in the sag, including determining the migration direction and characterizing the migration pathways.

Moreover, fault zones are highly complex, and fault rocks exhibit highly variable in permeability (Manzocchi et al. 2008). It must be acknowledged that the models employed to calculate SGR for the evaluation of fault-sealing capacity, as well as to simulate the evolution of hydrocarbon migration and accumulation during fault reactivation, simplify the geometry and property distribution of the trap-boundary fault. This simplification affects the accuracy of fault-sealing capacity evaluation and, to a certain extent, limits the predictive capability for hydrocarbon migration. Nonetheless, it is believed that the methodology adopted in this study provides an acceptable representation of hydrocarbon migration and accumulation during the period of fault reactivation.

The geological and geophysical evidence, along with modeling results, indicate that fault growth and activity may induce various sealing behaviors along the same fault plane at different times and locations. This variability can significantly impact the accumulation and leakage of hydrocarbons within fault-bound traps. Based on the observed relationship between fault dynamics and hydrocarbon migration in this study, it is recommended that exploration efforts in structures similar to LK7-6 should incorporate a detailed understanding of historical and potential fault activity and integrate dynamic changes in fault sealing behavior into exploration strategies. In particular, it is crucial to focus on the temporal correlation between fault reactivation caused by regional tectonic movements and its correlation with hydrocarbon charging, and to fully consider the impact of reactivation on fault seal integrity in complex fault systems. By integrating fault activity history analysis and fault-sealing capacity evaluation with BPSM, the dynamic relationship between fault reactivation and hydrocarbon migration and accumulation can be better investigated. This integration provides a more detailed and dynamic understanding of fault behavior over time than traditional static analysis. Meanwhile, this practical and feasible approach allows for a better prediction of areas with higher leakage risk and directs exploration towards more tightly sealed structures. Through a comprehensive analysis of the interplay between geological processes and their effects on hydrocarbon migration and accumulation, this study provides a foundation for future exploration in the LK7-6 stucture and similar geological settings, enhancing the predictive capabilities and reducing uncertainties in hydrocarbon exploration.

Conclusions

This study uses the trap-boundary fault, Fault 3 (F3) of the Longkou 7 − 6 (LK7-6) structure, as a case study to integrate the analysis of fault activity history and fault-sealing capacity with basin and petroleum system modeling (BPSM) in an innovative way. This integration provides a practical and feasible methodology for investigating the coupling of changes in fault-sealing capacity with the dynamic evolution of hydrocarbon migration and accumulation during fault reactivation. The following conclusions can be drawn:

(1) The F3 trap-boundary fault is an extensional normal fault, with the shallow part linked by three fault segments through lateral propagation. It experienced significant activity during the depositional period from the Kongdian Formation (E1 − 2k) to the lower second member of the Dongying Formation (E3d2L) and was reactivated during the neotectonic period (5.1 Ma to the present).

(2) The sealing capacities of the F3 trap-boundary fault were dynamically evaluated before and after fault reactivation. The sealing capacities of the fault before and after reactivation exhibit very different characteristics in different reservoirs due to the variation in fault throw. The top reservoir within the Guantao Formation (N1g), which had good sealing capacity before reactivation, experienced seal failure afterward.

(3) The source rocks of the third member of the Shahejie Formation (E2s3) within the two main generation kitchens, the Bodong Sag and Bozhong Sag, serve as the primary source of hydrocarbons for the LK7-6 structure. The regional structural high is favorable for hydrocarbon migration and accumulation. Oil charging has occurred from 5.1 Ma to the present, corresponding to a period of fault reactivation.

(4) Fault reactivation plays a dual role in hydrocarbon migration and accumulation. On the one hand, it provides effective vertical conduits for the hydrocarbon migration. On the other hand, it introduces potential vertical leakage and causes partial fault seal failure, thereby undermining fault seal integrity, leading to cross-fault leakage and subsequent lateral redistribution of hydrocarbons.