Hydrocarbon Potential Assessment Methods in Complex Fault Zones: A Case Study of the Southern Pinghu Structural Belt, East China Sea

Abstract

Frequent tectonic activity in rift basins has led to complex fault zones, which have led to extensive hydrocarbon distributions and tremendous resource potential. This study investigated the hydrocarbon potential in the southern Pinghu structural belt, focusing on fault traps in complex fault zones. Through fault sealing analysis and gas detection attenuation methods, this study aims to improve exploration success rates. The application outcomes demonstrate that the Shale Gouge Ratio (SGR) threshold for achieving the lateral sealing of faults in the southern Pinghu structural belt is 34%, with a critical fault throw of 100 m. Regions where the fault’s lateral sealing zone corresponds with areas exhibiting anomalous gas responses are deemed promising for hydrocarbon accumulation. Additional analysis indicates that favorable fault trap development occurs along the foot walls of significant faults, particularly in the eastern sector of the study area. The findings are corroborated by actual drilling data, affirming the efficacy of these methods in pinpointing hydrocarbon traps within complex fault zones and offering valuable insights for their broader global application.

1. Introduction

The exploration and development of hydrocarbons in rift basins is a key effort in the global energy industry, as they contain complex fault zones formed by frequent tectonic activity. These complex fault zones not only contain large hydrocarbon distributions but also have enormous potential for resource development. Technological progress has changed the way in which scholars predict and utilize these resources, making them increasingly predictable and economically feasible. Fault-related hydrocarbon reservoirs dominate in terms of reserves and production, which are crucial for the development and strategic planning of established exploration areas [1,2,3,4,5].

In recent years, the academic community has made significant progress in research on seismic detection technology, the relationship between faults and hydrocarbon accumulation, and the impact of fault sealing on fluid migration [6,7,8,9,10,11,12]. In particular, fault sealing assessment methods, such as the clay smear potential (CSP) [13], shale smear factor (SSF) [14], and SGR [15], have been shown to be effective in areas with complex geological structures. These methods, combined with Allan diagrams [16] and Knipe diagrams [17], have improved the feasibility of studying fault sealing [18]. On the other hand, scholars have made efforts to construct models that characterize the presence of hydrocarbons and to search for possible locations for hydrocarbon accumulation [19,20,21,22,23,24,25]. Although applying certain seismic detection methods to identify hydrocarbons can also improve the success rate of drilling, multiple interpretations of the results are still important aspects that restrict the development of this technology [26,27,28,29,30,31,32].

Despite the Pinghu structural belt having abundant hydrocarbon sources, unobstructed migration pathways, and well-developed reservoirs, there are still issues with the inability to accumulate hydrocarbons. During actual exploration, the activity history and sealing of faults remain key factors that lead to drilling failures. A study of the Pinghu structural belt showed that edge faults may open vertically, while the vertical sealing of the central part of the basin is greater [33,34]. While progress has been made in sealing research, further investigations combining fault seal evaluation with gas detection are needed to enhance trap efficacy.

This study proposes a new method that effectively assesses regional fault sealing by constructing a fault-throw-Knipe-diagram (FTKD) and calibrating them through comparison with Allan diagrams. This paper further integrates the attenuation method for gas detection technology to enhance the exploration efficiency for fault block traps. This research focused on the sealing and gas content in the southern Pinghu structural belt, aiming to provide innovative perspectives and tools for hydrocarbon exploration.

2. Geological Background

The East China Sea Shelf Basin, located at the intersection of the Pacific Plate, the India–Australia Plate, and the Philippine Plate, is a Mesozoic back-arc continental margin fault-depression basin. During the Eocene, with the strong collision between the Eurasian Plate and the Indian Plate, the Banda Sea and Philippine Sea Basins began to expand [35,36,37,38,39]. At this time, the tensile stress in the basin and the fault activity gradually weakened, and the whole basin evolved from a state of tensional subsidence to gradual stabilization.

The tectonic evolution of the East China Sea Shelf Basin has had an important impact on deposition and resource formation in the basin. The Xihu Sag is located in the north–central part of the East China Sea Shelf Basin and is a large-scale Mesozoic and Cenozoic hydrocarbon-bearing sag in the basin (Figure 1a). Affected by the Yuquan Movement at the end of the Eocene and the Longjing Movement at the end of the Miocene, the Xihu Sag experienced a fault depression stage (Tg-T30), a depression stage (T30-T11), and a regional subsidence stage. Strata such as the Pinghu Formation and the Baoshi Formation formed during the Eocene, and these strata recorded the evolution of the basin and the changes in the depositional environment [40,41,42,43,44,45].

The Pinghu structural belt is located in the middle of the western slope of the Xihu Sag (Figure 1b). The Pinghu Formation in the study area is Eocene in age and mainly contains sandstone and mudstone mixed with coal seams. The Pinghu Formation is in a transitional environment between sea and land and can be divided into four parts:the Pingshang Member, the Pingzhong Member, the Pingxiashang Member, and the Pingxiaxia Member (Figure 1d). The sediments located in the lower part of the Pinghu Formation was deposited during the water invasion period, and the strata were mainly composed of a large set of mudstones interbedded with sandstone, while the sediments located in the upper part of the Pinghu Formation was deposited during the water regression period, and the strata consisted mainly of interbedded deposits of sandstone and mudstone, which were deposited in the transitional stage of evolution [46,47,48,49,50]. From the top to the bottom of the Pinghu Formation, the proportion of mudstone in the formation increased.

According to the fault strikes, N–NNE-trending faults have mainly developed in the southern Pinghu structural belt, among which there are several major faults. The strike of this group of faults is consistent with the regional structural strike. In plain view, the faults in the southern Pinghu structural belt gradually spread in arcs that protrude westward from east to west. The other group is distributed in a NW-trending and intersects or cuts the main faults, forming the basic pattern of the southern Pinghu structural belt (Figure 1c). Therefore, in the southern Pinghu structural belt in the Xihu sag, the preserved hydrocarbon reservoirs consist mainly of faulted noses, faulted anticlines, and faulted blocks.

3. Data and Methods

Based on the drilling and logging data obtained from 12 wells and 3D seismic data, this study conducted in-depth research on the geological characteristics and hydrocarbon accumulation potential of the southern Pinghu structural belt. These data were obtained from the Shanghai Petroleum Company.

The minimum SGR required for lateral sealing in the southern Pinghu structural belt was determined using FTKD data from eight wells. The statistical range of the SGR values was used to define the risk throw and the location of overflow points related to the fault trap. Combined with the Allan diagram, the reliability of lateral sealing was analyzed by characterizing the numerical differences, and the fault surfaces were analyzed. The reliable trap range was combined with the abnormal gas-bearing area for consistency analysis with the hydrocarbon-bearing properties of the traps. Fault sealing and gas detection methods can be used to effectively determine the hydrocarbon bearing potential of an enclosed area.

3.1. FTKD

To describe the mudstone content percentage on the fault plane more accurately, Equation (1) is chosen to calculate the SGR in this study.

(1)$\mathrm{S}\mathrm{G}\mathrm{R}={\displaystyle \frac{\sum V_{sh}\times \mathrm{\Delta }Z}{D}}\times 100\%$

In the formula, SGR is the mudstone content percentage in the fault zone, in % (1–100%); $V_{sh}$ is the mudstone content percentage in the formation, in % (1–100%); $\mathrm{\Delta }Z$ is the thickness of the formation within the range of the fault throw, in m (reflecting the size of the differential stratigraphy); and $D$ is the fault throw, in m.

Knipe diagrams that represent the SGR values provide an effective method for accurately evaluating the sealing of different types of faults and reflect the lithological juxtapositional relationship between a certain fault throw and the mud content on the fault surface [8,17,51]. The variation in the SGR along the fault direction with respect to the fault distance reflects the hydrocarbon leakage points [7,52,53].

The FTKD was further developed by adding the longitudinal changes in fault throws to the Knipe diagram containing SGRs. The FTKD relies on four elements to indicate the boundary sealing values of fault traps and changes in the mudstone content percentage at the tops and bottoms of reservoirs. ① The reservoir range indicated by the Knipe diagram (white box range) is the first element of the FTKD (Figure 2a). ② The Knipe diagram showing that SGR is the second element constituting the FTKD (Figure 2b). ③ The mudstone content percentage of the fault is constantly changing vertically and horizontally (variation in the fault throw with the depth of the fault control zone can link the SGR value to the geological condition of the reservoir cap layer). Thus, the effective fault throw of the layer needs to be characterized by the fault throw at the intersection of the contour line of the reservoir top and the fault control zone (Figure 2d,e), which is the third element that constitutes the FTKD. ④ The hydrocarbon display is the fourth element of the FTKD (Figure 2c). After the x-axis value is determined, the position of the y-axis value is determined by projecting it in the direction of the white box (Figure 2c). When incorporating the throw-depth relationship (as shown in Figure 2f) into the FTKD, this principle is likewise observed. Hence, the SGR value within the Cartesian coordinate system can be acquired upon determining the depth, the throw value, and the position of the white box.

In Figure 2f, the point that descends approximately 3780 m corresponds to one of the points in Figure 2b. This point represents the seismic reflection interface T32, which is a significant reservoir of the study. Assessing SGR using multiple continuous points at varying depths is a crucial function of Figure 2c. Figure 2d–f serve as an example to illustrate the origin of a specific value in Figure 2c, ultimately showing that Figure 2c is a representation of the continuously varying SGR values along the fault’s vertical extent.

The FTKD reflects the changes in the mudstone content percentage that are associated with fault control within a certain fault throw and can quickly determine the effective range of the traps in the fault zone.

3.2. Allan Diagram

Allan diagrams are two-dimensional diagrams used to display the changes between the rising and bottom walls of a fault plane. A model is based on profile data obtained along the fault strike and displays the spatial displacements of the fault in a graphical manner. An overlay of seismic profile data on both sides of the fault plane is used to determine the fault plane distribution and juxtaposition of two rock types [16,54].

Logging-constrained inversion was used to obtain data on the geological and lithological changes on both sides of the fault plane. A framework representing the juxtaposition of strata was generated by extracting the stratigraphic changes on both sides of a single fault. The lithological Allan diagram on both sides of the fault can be displayed in the stratigraphic frame. The SGR values on the Allan diagram show the evolution of the lateral sealing capacity of the fault plane. Therefore, this approach can be further utilized to achieve a two-dimensional representation of the impact of mudstone content percentage within a specific range on the fault plane.

3.3. Gas Detection via the Attenuation Method

Gas detection using the attenuation method is a subsurface gas detection technology that is based on the principle of seismic P-wave energy attenuation [55,56]. When gas is present in the ground, the reflection exhibits a certain frequency change; that is, the proportion of low-frequency energy is higher, the proportion of high-frequency energy is reduced, and the peak frequency decreases (Figure 3) [56,57]. To accurately detect underground gas layers, three-dimensional seismic data are processed using signal decomposition techniques, such as wavelet transform, and the seismic signal is decomposed into frequencies ranging from 5–30 Hz. Outliers indicating attenuation can be computed using the difference between a frequency and a size-adjacent decomposition frequency. These attenuation values are reflected both horizontally and vertically, and further superposition with structural maps will provide better indications. We employed the generalized low-frequency shadow hydrocarbon detection technique to predict potential gas reservoirs in deep, unwelled areas, focusing on the characteristic changes in the main frequency of seismic bodies. As seismic waves propagate through layers with varying rock properties and fluid contents, they experience different degrees of attenuation. When seismic waves pass through gas-bearing strata, they typically cause a relative reduction in the energy of high-frequency components and a relative increase in the energy of low-frequency components in the seismic spectrum, resulting in a lower main frequency. The three different curves represent the waveforms of different frequencies of the seismic body, where a higher main frequency indicates poorer gas-bearing strata, and a lower main frequency indicates better gas-bearing strata (Figure 3). The reason for the continuous decline in curve values beyond the main frequency is the attenuation of seismic wave energy on both sides of the main frequency. This method can be used to effectively detect abnormal gases underground and provides reliable technical support for hydrocarbon detection [58].

4. Results and Discussion

In the southern Pinghu structural belt, the mudstone content percentage of the Pinghu Formation is significant, with the mudstone content percentage from 50% to 82%, with a maximum of 60% (Figure 4). In the context of these mudstone content percentage, the numerical characteristics of the reservoir exhibit differences. A sandstone reservoir with strong fault sealing constitutes a stable and effective reservoir, while gas detection anomalies reveal the distributions within the reservoir. In the study area, the transport and preservation conditions jointly affect the accumulation capacity of hydrocarbon in traps. The rapid evaluation method of hydrocarbon properties can further characterize the areas with hydrocarbon accumulations. The area of overlap between the abnormal gas distribution and the fault sealing area constitutes a zone with superior hydrocarbon accumulation conditions in the southern Pinghu structural belt.

4.1. Application of FTKD in Determining the SGR Threshold and Risk Throw

In this study, significant differences in the SGR values of faults across various well areas were observed. However, by analyzing the distributions of the oil, gas, and water layers, the sealing of faults could be indirectly evaluated.

After a comparative analysis of the fault throws of the trap-controlling faults and their SGRs, it is shown that the SGR values of the faults in the areas of wells C11, C5, C6 and C7 are relatively high (SGR > 0.7) (Figure 5a–h). In this region, as the fault throw increases, the SGR at the reservoir increases [53], and the fault throw of the trap-controlling faults boundary is greater than 100 m (Figure 5i). This phenomenon can be attributed to the high mudstone content percentage in these areas (>0.4) and the scarcity of thin layers between sandstone and mudstone (Figure 4), which allows the SGR of the reservoir to easily reach 0.7 or higher. When the faults into the cap rock, the FTKD analysis reveals that the mud content of the fault zone in the vertical upper cap rock is highly consistent with that in the cap rock (Figure 5a–h). In the context of high mudstone content percentage, this indicates that the SGR in the reservoir is low in the vertical direction, and the adjustment or transport effects of small faults on hydrocarbons are limited [8]. Therefore, these faults not only have lateral sealing properties but also exhibit longitudinal sealing properties.

In contrast, in the C9 and C10 well areas, the throws of the majority of trap-controlling faults are typically less than 100 meters, and the mudstone content percentage are lower than that in the C4 well area. The SGR values of these reservoirs are projected for areas with SGRs < 0.7. Compared with those in the C4 well area, the SGR values of the cap rocks in the C9 and C10 well areas are not significantly different from those in the vertical reservoir. However, in the C9 and C10 well areas, the trend in SGR values in reservoirs with increasing fault throws is not significant (Figure 5a,b). In the context of low mudstone content percentage, the FTKD analysis revealed that water layers are mainly distributed in areas with low SGR values. Previous studies have generally shown that the probability of hydrocarbon accumulation increases with increasing SGR [15,52,59]. However, our findings suggest that there may also be water layers in areas with high SGR values, including many gas layers (Figure 5c–g). This discovery is consistent with Zheng’s (2022) understanding of the random fracturing of mudstone smears. This theory proposes that when a fault is continuously sealed, whether the seal is a thick shale layer or a long clay coating, because the coating position is related to its previous position through a normal distribution, there is a possibility of leakage points randomly appearing in the coating, and the height of the gas column that can be sealed is also discontinuous [8]. However, our research further reveals the differences in this relationship under different mudstone content percentage backgrounds, providing a new perspective for understanding the complexity of the potential for trapped hydrocarbons.

The stratigraphic SGR seal values corresponding to wells C9, C10, C11, C12, C4, C5, C6, and C7 were obtained from Figure 5 and the minimum value, which was 34%, was defined as the fault seal threshold in the southern Pinghu structural belt (Figure 5). The fault throw corresponding to the fault sealing value at each well point was calculated, and a 100 m risk throw was obtained (Figure 5i). The significance of the risk throw in its ability to indicate the closure of well-free areas. When the fault throw is lower than the risk throw, the SGR value of the fault plane is low, and the sealing effect is poor. When the fault throw at the edge of the trap is less than 100 m, the trap is open in a lateral direction. Because the SGR threshold is indirectly obtained based on data from hydrocarbon reservoirs, it may be slightly greater than the actual SGR threshold [51]. The risk throw obtained in the northern Pinghu structural belt is 60 m. Further research in the Baoyunting area shows that the fault seal threshold is 30%, which is lower than the threshold in this study area. This indicates that as exploration advances southward, the required mudstone content percentage in fault zones increases, requiring a larger fault throw to seal hydrocarbon. The fault sealing threshold in global closure studies is between 15 and 40%, and the study area has a high fault sealing threshold. The higher sealing threshold in the study area could be attributed to greater hydrocarbon generation intensity in the southern Pinghu structural belt compared to the north. Well-developed source rocks contribute to a taller hydrocarbon column, increasing the breakthrough pressure on fault planes, indicating a need for higher mudstone content percentage in fault zones [8,10,11,12]. This supports the notion that faults in the Pinghu structural belt may have better vertical transduction towards the edges [34]. Additionally, the deepening of the Pinghu Formation from north to south, along with variations in mudstone content percentage and fault pressure, could lead to different rupture states in the fault zone, affecting the sealing threshold [8,13,60,61,62,63]. Further research on source rocks and fault pressure in this area could provide more insights.

For the first time, the FTKD model was applied to study fault sealing, which provides a new method for identifying the SGR fault sealing threshold and quickly determining the sealing degree of reservoirs and cap rocks. Traditional research methods typically rely on a single fault throw and corresponding SGR values to evaluate fault sealing capabilities [64] and classify data based on the reservoir hydrocarbon properties. By employing a FTKD, the continuous changes in SGR values within reservoir segments can be more accurately identified, as illustrated in Figure 2. This approach provides a more precise tool for assessing fault SGR fault sealing thresholds and determining the sealing efficacy of reservoir and cap rock. However, this method does not fully account for the continuous variations in SGR values throughout the history of fault activity, indicating a direction for further optimization.

4.2. Effects of Hydrocarbon Potential Detection Methods

When assessing the hydrocarbon resource accumulations within a trap, the sealing capacity of faults is a key factor in determining the effective range of the trap [8,9,65]. Allan and Knipe diagrams were used to quantitatively analyze the SGR values of the traps (Figure 5h and Figure 6), after which the faults were divided into closed and non-closed types, with a threshold serving as the basis for judgment. The traps affected by the F1 and F2 faults, as well as the main fault on the east side, exhibit fault anticline characteristics (Figure 7a,b). The northern part of the F1 fault is mainly composed of juxtaposed mudstone and mudstone, with an SGR exceeding 50% (Figure 6c,e). In the southern region, sandstone and mudstone are connected, with lower SGR values (Figure 6c,e). This difference reflects the difference in structural activity along the fault line, with more intense stratigraphic activity and a greater fault displacement in the north (Figure 6a,b). The SGR value of the fault boundary of the northern trap exceeds the threshold, indicating lateral sealing (Figure 6c). The F1 fault has obvious leakage points in the Abnormal ② (Figure 6 and Figure 7c). The purple hydrocarbon accumulation trap range is reduced to the vicinity of the F1 fault, and the F2 fault no longer forms the trap boundary (Figure 7c). After evaluating the lateral sealing of the fault, the effective area of the trap decreased significantly [7].

On the basis of fault sealing, gas detection using seismic attenuation further refines the planar range of the hydrocarbon distribution within the trap. Local leakage of faults F2 and F1 resulted in a further reduction in trap area. The Abnormal ①, Abnormal ②, and Abnormal ③ all show gas anomalies (Figure 7d). The Abnormal ① and Abnormal ③ exhibit lateral sealing capabilities, forming a comprehensive and reliable range of hydrocarbon displays (Figure 7c,e). Although the traps in the Abnormal ① and Abnormal ③ areas are continuous under fault seal evaluation, the range after gas detection shows discontinuity. This finding is consistent with previous research on the limited lateral distribution of sand bodies. That is, the sand bodies of the Pinghu Formation in the Pinghu structural belt are well developed but have limited lateral distribution, and thick sand bodies are distributed regionally [49]. Changes in the width and thickness of the sand bodies, as well as contour lines controlled by the overflow point, result in discontinuities in the range of hydrocarbon potential. The lateral non-sealing of hydrocarbon in the Abnormal ② may be due to the influence of other geological factors on the detection methods used. In the reliable trap area, the area that matches the anomaly more accurately indicates the hydrocarbon accumulation area, which is consistent with the actual drilling results (Figure 7e, Table 1).

Scholars hold different views on the factors that affect the hydrocarbon potential of fault traps. Wang (2020), based on well network data combined with fault surface stress, the mudstone content percentage in fault zones (represented by the Allan diagram), and the migration path model, found that faults with SGR values greater than 0.6 act as barriers to seal hydrocarbon in the lateral migration path, while other parts act as channels for lateral hydrocarbon migration [11]. Li (2021) determined the hydrocarbon filling periods and migration paths in the Lishui Depression through hydrocarbon source comparisons, thus identifying the distribution of hydrocarbon reservoirs [12]. The results of this study are consistent with the above conclusions, further confirming the crucial role of fault sealing in hydrocarbon reservoir formation. Our research revealed that the fault cap sealing system is highly important for hydrocarbon conservation in the southern Pinghu structural belt. Because exploration wells are mainly located at high points in structures, researchers are able to determine the Allan and FTKD values of faults near wells to identify the SGR threshold. For areas beyond the coverage of existing well sites, additional exploratory data are needed for calibration and validation.

In the offshore exploration field, to face the challenges of many fault blocks and sparse exploration wells, an innovative method for identifying hydrocarbon properties has been proposed that integrates seismic attenuation gas detection and fault sealing analysis. The purpose of this method is to comprehensively evaluate and accurately identify the hydrocarbon potential within a trap to more precisely locate the hydrocarbon accumulation area. The academic community has conducted extensive research in this area. For example, Gu Zhimeng (2020) improved the consistency between gas detection results and actual drilling by using data smoothing and denoising techniques to enhance hydrocarbon detection information [25]. Pu (2021) used the gas bearing detection method to identify gas reservoirs in the Tarim Basin and troubleshoot multiple gas bearing anomalies through their abnormal display shapes, reservoir cap rock configurations, and regional response ranges [29]. Li Cai (2023) assessed the hydrocarbon potential of traps in the Bohai region by eliminating high-frequency bright spots [24]. Given that the exploration area for the Pinghu structural belt is located underwater, the application of airborne gamma-ray spectral data is limited. In this context, the processing of abnormal data requires a detailed description of the oil, gas, and water layers in the study area. However, in areas with scarce exploration wells, accurate calibration of lithological characteristics faces significant challenges. Considering the favorable gas generation conditions and reservoir characteristics around the Pinghu structural belt, identifying reliable accumulation spaces associated with gas anomalies is highly important for distinguishing gas and water layers (Figure 7). However, this method is limited by certain geological conditions. Provided that the combination of source rock, reservoir rock, and cap rock is well-established, the spatiotemporal relationship between the period of fault activity and the hydrocarbon accumulation period is also of decisive significance for assessing the effectiveness of hydrocarbon traps [66]. Deciphering the fault activity history can reveal the migration pathways and accumulation patterns of hydrocarbons throughout geological history [7,67]. The fault system in the Pinghu structural belt provides the main migration pathways for hydrocarbon. The activity period and sealing properties of faults directly affect the vertical and lateral migration of hydrocarbon and determine the efficiency and direction of hydrocarbon migration along the faults [68]. The main hydrocarbon accumulation period in this area corresponds to the peak period of fault activity. For instance, the Longjing Movement at the end of the Miocene was crucial for the formation of hydrocarbon traps, and the tectonic activities during this period led to significant generation and effective accumulation of hydrocarbon [52,69,70,71]. Fault seals help find traps, but hydrocarbon accumulation needs verification. Seismic attenuation is effective for gas layers, yet affected by source rocks and strata, including local coal seams. Combining fault seal analysis with attenuation data improves offshore hydrocarbon detection in fractured zones. This study provides a new and effective approach for evaluating and identifying the hydrocarbon potential of traps in offshore exploration areas with multiple fault blocks and few exploration wells, which is expected to improve the efficiency and accuracy of hydrocarbon exploration.

4.3. Application in the Southern Pinghu Structural Belt and Hydrocarbon Exploration Risks Analysis

Using the SGR fault sealing thresholds and fault risk throws, it was determined that the northeastern region of the study area is the main area where closed or semi-closed faults are present (Figure 8b). The faults in this area have large fault throws (over 100 m) and are widely distributed. The downstream side of these faults, the east sides, are conducive to the formation of effective fault traps (Figure 1c, Figure 7 and Figure 8b). A comprehensive assessment method was utilized, based on the principle that “abnormal gas response determines trap shape, and fault sealing determines hydrocarbon reservoir range”, to classify the traps in the southern Pinghu structural belt, resulting in the identification of four types of traps: fully controlled, fault controlled, anomalously controlled, and open (Figure 8b, Table 1). Following gas detection via the attenuation method and fault sealing evaluation, it was found that the effective area of locally controlled and completely sealed traps, such as traps 6, 7, 8, and 9, had significantly decreased; moreover, completely enclosed traps, such as traps 12, 13, 14, 15, etc., had no effective area (Figure 8a,b, Table 1).

Therefore, relying on the attenuation method for gas detection results is risky because widespread responses due to abnormal gas contents occur in the southern and western parts of the study area (Figure 8a, Table 1). Traces of volcanic activity in the southwestern part of the study area [70,72,73] (Figure 8b) resulted in visible anomalies in some traps (numbers: 2, 4, 11, and 20), but these traps were ineffective (Table 1). Traces of volcanic activity, low-velocity shale, and low gas saturation formations are the reasons for these abnormal displays [74,75,76,77].

Although faults in high SGR areas have strong sealing properties, due to certain geological conditions (such as mudstone and sandstone distribution and tectonic activity), these areas may not have formed effective hydrocarbon accumulations, leading to trap failure. For example, although they were completely enclosed, the fault block traps on the east side (numbers 12, 13, and 14) failed to form gas layers (Table 1, Figure 8a,c). This may be due to the well-sealed faults that developed on both sides of the area (Figure 8b,d), which seal not only the control trap faults but also the hydrocarbon source fault [78]. Continuously closed faults can hinder the lateral migration of hydrocarbons and affect the potential for hydrocarbon accumulation in traps. These unfavorable traps can be identified by using the advantage of the gas detection method (Figure 8).

Compared to those in located enclosed areas, traps bounded by fault-extinguishing sections are completely ineffective and serve only as pathways for hydrocarbon migration [53]. For example, in the C7 well area, the SGR value of the middle section of the F1 fault is close to the sealing threshold, and some layers are locally controlled by the fault (Figure 6, Figure 7c, and Figure 8b). In the C10 and C12 wells in the southern part of the study area, the T32 interface fault is fully opened, and the hydrocarbon reserves are poor (Figure 8a,b, Table 1). The trap boundaries for these open fault traps are usually formed by the ends of the faults (Figure 1c and Figure 8b).

In the study area, the transport and preservation conditions jointly determine whether hydrocarbon can be stored in traps. In the delta sedimentary system of the Pinghu structural belt, coal seams and carbonaceous mudstones are well developed, and the thickness of hydrocarbon source rocks is high, indicating significant hydrocarbon source rock potential [79,80]. The vitrinite reflectance values of the source rock system are high, reaching the threshold of hydrocarbon generation in the early stages [81,82]. Slopes have natural advantages in hydrocarbon migration, and regional compression promotes the filling of secondary uplifts [83]. The reservoir conditions in the Xihu Sag are good, especially since the sand dunes in the upper and middle sections are relatively high. Previous studies have shown that sand bodies have good porosity and are widely connected and distributed [49,53]. However, although the regional cap rocks P5 [34] and S1 in the Pinghu structural belt are well developed, poor fault sealing can lead to cap rock failure [33]. In the Pinghu structural belt, the transport and preservation conditions jointly determine whether hydrocarbon can be stored in the traps [84].

In this study, a detailed statistical analysis of all the fault traps in the central Pinghu Formation within the Pinghu structural belt was conducted. The analysis results indicate that the overlap between the areas with anomalous gas-bearing response and the fault sealing area indicates good hydrocarbon availability (Table 1). In the three well areas of C4, C6, and C7, the favorable areas of locally controlled and completely enclosed traps are supported by actual drilling results (Table 1). These findings reveal the significant influence of faults on the hydrocarbon potential of traps in the Pinghu structural belt. There are various explanations in the academic community regarding the distribution of hydrocarbon reservoirs in the Pinghu structural belt. Hydrocarbon reservoirs characterized by “faulting, small, poor, and scattered” characteristics have been discovered in the northern Pinghu structural belt [52]. Chen (2022) noted that the source reservoir cap configuration conditions and sedimentary equality in the Pinghu structural belt control the distribution of traps and hydrocarbon reservoirs [85]. Guan (2022) noted through the fault cap configuration based on the mudsmearing factor that the Pinghu structural belt has no ability to prevent the vertical leakage of hydrocarbon in the area [33]. The edge faults of the Pinghu structural belt may be vertically open, and the closer to the center, the greater the vertical closure may be [34]. In the Pinghu structural belt, continuous open faults can serve as part of the transport system, and isolated lateral closed faults can serve as part of the fault cap sealing system of hydrocarbon reservoirs [53]. In the southern Pinghu structural belt, under the underwater geological environment with sparse exploration wells and complex fault systems, the sealed fault control areas in the north and east are considered good hydrocarbon accumulation areas. In contrast, the small faults and volcanic areas in the southern and western regions merit further research to understand the destruction and preservation of hydrocarbon. This research can provide new perspectives on hydrocarbon accumulation mechanisms within the Pinghu structural belt and offer significant guidance for future exploration activities.

5. Conclusions

This study explored and established an effective method for identifying the hydrocarbon potential of fault traps in the Southern Pinghu Structural Belt of the Xihu Sag through quantitative analysis of fault sealing, in-depth analysis of gas-bearing anomalies, and comprehensive study of geological characteristics. Based on these studies, the following conclusions have been drawn:

• (1). This study pioneers the use of the FTKD method for fault sealing analysis, offering a novel approach to track the spatial variability of SGR values in faults. These results reveal that faults exhibit effective sealing when the SGR exceeds 70%, both laterally and vertically. In the southern Pinghu structural belt, a SGR fault sealing threshold of 34% is identified, with a proposed risk throw of 100 m to signify the sealing conditions in zone without wells.

• (2). Using seismic attenuation evaluations for gas detection and evaluating fault sealing, this study effectively distinguished the effective trap area. Fault sealing is vital, particularly in the southern Pinghu structural belt, where the fault-cap sealing system is key to preserving hydrocarbons. This method offers a new way to assess and identify the hydrocarbon potential in complex offshore areas with multiple fault blocks and limited wells.

Author Contributions

D.J. contributed to the manuscript by refining its content and offering substantial support with data. Additionally, D.J. provided the necessary funding for the research. S.Y. wrote the text of the main manuscript and S.Y. and Y.G. prepared Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7 and Figure 8. Y.G. and X.D. collated the project’s methods and verified the reliability of the data and results. R.P. conducted project control and resource support. S.C. and S.S. contributed to the manuscript through the critical roles of writing, reviewing, and editorial refinement. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data are not available due to legal and commercial restrictions.

Acknowledgments

Thank you to Sinopec Shanghai Offshore Oil and Gas Branch for providing the drilling and seismic data.

Conflicts of Interest

Donghui Jiang is from Sinopec Shanghai Offshore Oil and Gas Company, China Petroleum & Chemical Corporation; Shuo Chen and Siyu Su are from Shaanxi Coalfield Geophysical Prospecting and Surveying Co., Ltd.; The other authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Footnotes

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Figures and Table

Figure 1. Location of the southern Pinghu structural belt and its stratigraphic development characteristics. (a) Location of the East China Sea Shelf Basin; (b) unit division of the Xihu Sag structure, where the blue and dark blue areas represent the central bulge zone of the Xihu Sag, the orange−yellow and yellow areas depict the slope positions on both wings, the purple area highlights the Diaoyu Island Uplift to the east in disconformable contact with the depression, and the white area indicates the adjacent uplifted structural zone, showcasing the relationship between the sag and surrounding geological structures; (c) distribution of fault systems in the southern Pinghu structural belt, the gradient from orange to blue in the seismic reflection horizon of T30 signifies the increasing depth; (d) comprehensive bar chart of the Pinghu Formation.

Figure 2. The FTKD of C7 well. (a) Knipe diagram of C7 well; (b) triangle diagram with oblique projection of SGR values; (c) representative maps of different properties of fault planes (the final manifestation of FTKD); (d) T32 structural map showing location of faults and well; (e) profile location map of faults, where red represents the general distribution of faults, green denotes the typical F2 faults, and blue signifies the characteristic F1 faults.; (f) the displacement variation map of F1 fault with depth.

Figure 3. Comparison diagram of the frequency spectra between gas−bearing and non−gas stratum.

Figure 4. Stratigraphic correlation and lithological characteristics of the Pinghu Formation in the southern Pinghu structural belt.

Figure 5. Numerical display of fault closure in the southern Pinghu structural belt. (a–h) SGR value constrained by well location in the study area; (i) comparison diagrams of risk throw and sgr threshold at different well locations.

Figure 6. Allan diagram of the F1 fault in the southern Pinghu structural belt along the strike(Points O and P designate the starting path of the profile, as depicted in Figure 7). (a) Distribution of bottom wall; (b) the distribution of rising wall; (c) the lithological juxtaposition relationship on both sides of the fault plane; (d) seismic profile of the vertical F1 fault T30–T32 SGR response; (e) SGR distribution status at the segment plane.

Figure 7. Abnormal ① characteristics in the southern Pinghu structural belt. (a) Trap morphology; (b) the extent of the structural trap is shown on the T32 structural map; (c) the range of traps constrained by fault sealing is depicted on the T32 structural map; (d) abnormal range of gas content; (e) gas attenuation range on the T32 structural map.

Figure 8. Assessment and prospective reservoir distribution map of the middle section fault in the southern Pinghu structural belt. (a) Gas occurrence display and T32 fault superimposed planar map; (b) Classification display of the T32 fault, gas occurrence range, and closure range display; (c) Seismic profile of gas anomalies in the Pinghu Formation. (d) Possible lateral migration trajectory of the Pinghu Formation along the F to G seismic line.

Table 1

Risk Analysis for Pinghu Formation Fault Traps in Southern Pinghu Structural Belt.

Relationship Between Faults and Traps	The Relationship Between Trap Amplitude and the Height of the Fault Sealing Column (m)	The Relationship Between Trap Area and Actual Oil Bearing Area(m)	Trap Number	Original Trap Elements		Trap Elements are Controlled by Seal		Drilling Display
				Area(km)	ClosingHeight (m)	Area(km)	ClosingHeight (m)
Fully controlled	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	8	2.39	125	2.39	125
			9	11.36	225	11.36	225	gas reservoir
			12	52.60	670	52.60	670
			13	13.70	620	13.70	620
			14	32.37	1000	32.37	1000
			15	10.63	300	10.63	300	water/dry layer
			19	1.60	100	1.60	100
Fault controlled	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	5	2.86	40	0.72	25	gas reservoir
			6	12.91	250	3.17	130	gas reservoir
			7	13.76	240	6.72	125	gas reservoir
			10	3.90	100	1.73	50
			16	32.34	775	6.53	250
			17	8.42	190	0.60	40
open	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	1	2.25	75	0	0
			2	9.06	140
			3	8.20	125
			4	3.58	120
			11	3.35	200
			18	2.69	75			water/dry layer
			20	4.37	175
anomalous controlled	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	2	9.06	140
			4	3.58	120
			5	2.86	40	0.72	25	gas reservoir
			6	12.91	250	3.17	130	gas reservoir
			7	13.76	240	6.72	125	gas reservoir
			8	2.39	125	2.39	125
			9	11.36	225	11.36	225	gas reservoir
			11	3.35	200
			20	4.37	175
[Image omitted. Please see PDF.]

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