Controls of the Sandbody Scale and Fault Throw on the Lithology and Composite Reservoir Formation in the Baoyunting Slope, East China Sea

Abstract

Under the conditions of many faults, sandbodies, and hydrocarbon sources on the slopes of faulted basins where structural traps are scarce, only a few sandbodies are capable of forming hydrocarbon pools, while most sandbodies act as aquifers. This situation presents a challenge for predicting favorable hydrocarbon accumulation areas and understanding controlling factors. The Pinghu Formation reservoirs in the Baoyunting nose structure of the Xihu Sag in the East China Sea exemplify this characteristic. Among the 19 small-scale oil and gas reservoirs discovered in this area, 10 are faulted sandbody composite traps and 9 are lithological traps, while the majority of the remaining sand layers, especially the thick layers, act as aquifers, resulting in significantly lower accumulation probabilities compared to the adjacent northern and southern areas. We analyzed the relationship between the sandstone thickness and the amplitude through the 1-D forward modeling of wells and dissected the 3-D seismic event to obtain the planar distribution of a single sandbody. Further comprehensive research on fault sealing and kinetic reservoir formation processes suggests that the gas pool formation in this area is closely related to fault sealing and lateral oil and gas transport. A small fault-to-caprock ratio is beneficial for the sealing of mudstone caprocks, while a large fault-to-sand thickness ratio is beneficial for the lateral sealing of faults and the formation of fault–sand composite pools. The tidal microfacies sandbody has a small scale, poor lateral transport ability, and a low probability of gas reservoir formation. The barrier and delta front sandbodies have a large scale, good lateral transport, and a high probability of reservoir formation. Based on the above methods, favorable pool formation traps were identified in the area, and high-yield gas wells were drilled.

1. Introduction

Composite traps on faulted slopes of rift basins have become one of the key types of traps for conventional offshore hydrocarbon exploration and development. However, these types of traps are characterized by complex controlling factors and pose significant challenges to hydrocarbon exploration and development [1,2]. In the case of well-developed faults and sandbodies on faulted slope areas without structural traps, there has been no universally applicable method to predict the occurrence of composite and lithological hydrocarbon reservoirs. The characterization of sandbodies using 3-D seismic and sedimentary microfacies is one of the key elements in identifying the presence of composite traps. However, the formation of composite or lithological traps also requires the presence of obstructions on the updip and lateral sides of the sandbodies. Even if traps exist, their actual hydrocarbon accumulation is influenced by factors such as hydrocarbon supply, migration, and preservation conditions [3,4,5]. Although the application of certain seismic hydrocarbon detection methods can assist in improving drilling success rates [6,7], there are limited successful examples of accurately predicting the presence of composite and lithological hydrocarbon reservoirs by considering factors such as faulted sandbody combinations, fault-sealing capacity, sandbody distribution, and hydrocarbon migration.

Typically, exploration wells on the Baoyunting slope are drilled in the structural traps of fault noses or fault blocks. Fault-nose traps are formed by the combination of nose structures and faults that are located updip of the nose. However, upon the discovery of hydrocarbons, the characterization of the reservoirs reveals that their trap types are mostly small-scale faulted sandbody composite traps or lithological traps. This is primarily due to the prevalence of multiple small faults and individual sandbodies. Based on the results of our investigation, the Baoyunting oil and gas field in the Xihu Trough has 19 oil and gas reservoirs in the fault nose and fault block, but after detailed descriptions, 10 are fault sandbody composite traps, 9 are lithologic traps, and none are structural traps. Moreover, the region is characterized by a large nose structure, with two hydrocarbon supply centers located in the northern fault depression and the southeastern deep depression. Many sandbodies and faults exist and are well configured, providing favorable conditions for trap formation. However, only a few thin sand layers contain oil and gas, while most of the sand layers are water-bearing, especially the thicker ones. The main source rocks are in the deeper part below the reservoir or the lower zone of the structure. Therefore, oil and gas must migrate from the low belt to the high belt for accumulation through vertical migration along the faults or stepped passages formed by faulted sandbody combinations. Consequently, the fault-sealing capacity and the sandbody dimensions, along with the lateral migration, become the critical factors controlling the accumulation of fault–sandbody combinations and lithological traps in the area.

This paper primarily focuses on a detailed analysis of four key controlling factors in the Baoyunting area, namely, fault sealing capacity and the microfacies, dimensions, and hydrocarbon lateral migration of sandbodies. This paper explores methods for predicting favorable hydrocarbon reservoirs and their distribution patterns in prospective traps.

2. Regional Geological Background

The Xihu Depression is located in the northwestern East China Sea Shelf Basin, which is adjacent to the convergence zone of the Pacific Plate, Philippine Plate, and Eurasian Plate [8,9,10,11,12,13] (Figure 1). The fundamental structure of the basin was established during the Keelung movement that occurred in the early and middle Mesozoic eras in the East China Sea Shelf Basin. In the Paleogene, a significant number of northeast-trending normal faults were reactivated in this area. In the early Eocene, the Eurasian Plate and the Pacific Plate were relatively active, and the Philippine Sea Basin underwent extension [14,15,16,17,18]. During the sedimentary period of the Baoshi Formation, many northeast-trending contemporaneous normal faults and large sets of sandstone and mudstone interbedded with igneous rocks developed. During the period of late Eocene Pinghu Formation development (approximately 42.5 Ma), the Pacific Plate changed its direction of subduction from NNW to WNW, and the tectonic stress field in the Xihu Sag near the plate boundary transitioned from extensional to compressional [19]. The Baoyunting area was significantly influenced by tectonic activity, and multiple controlling faults developed in the study area. Previous studies have divided the multiple tectonic movements that occurred in the East China Sea Shelf Basin during the Cenozoic era into three stages: the rifting stage, the wrenching stage, and the regional subsidence stage [20,21]. The period of Eocene Pinghu Formation development was during the fault-bending transformation period of the Xihu Sag. Based on previous investigations, the Baoyunting area, situated on top of an ancient nose structure, is concave on the northeast, north, and southeast sides, with an abundant hydrocarbon source-rock supply [22,23,24,25,26,27].

The Baoyunting nose is generally located in the accommodation zone margin (also known as the transition zone) between the gentle Wuyunting slope in the north and the steep Tuanjieting slope in the south (Figure 2).

The Xihu Sag is in the northwestern part of the East China Sea Shelf Basin [28,29,30] (Figure 2), and the Baoyunting oil and gas field is in the middle section of the western slope of the Xihu Sag (Figure 3). This area is characterized by an east-dipping slope with reverse faults and multiple parallel normal faults. Currently, several gas-condensate fields have been discovered in the Pinghu Formation on the western slope of the Xihu Depression. The area benefits from favorable conditions for hydrocarbon generation, migration, and accumulation. It features a petroleum-generating interval in the lower part of the Pinghu Formation and Baoshi Formation, reservoir–seal combinations of interbedded sandstones and mudstones in the middle part, and the migration of oil and gas along faults and stepped passages formed by the sandbodies. The accumulation processes include two stages: the pre-5 Ma Longjing movement and the post-5 Ma Longjing movement [31,32,33,34,35,36,37,38] (Figure 4).

According to the combined profile and plane morphology of the faults, as well as the stratigraphic occurrence, the Baoyunting area can be divided into five secondary structural units: fault fan (FF), high-belt east-dipping fault slope (HFS), fault saddle (FS), fault horst (FH), and low-belt east-dipping fault slope (LFS) (Figure 3). Their distribution and morphological characteristics are as follows:

• The fault fan (concave) area is a fan-shaped eastward-dipping fault slope between the Wubao Fault and the Wu2 Fault that contains wells N3, N4, etc.
• The high-belt easterly dipping fault slope is sandwiched between the NNE-trending easterly dipping Bao5 well fault and the Bao1 well fault, and it contains the B5 well.
• The fault saddle area presents a fault trench in the east–west section and a fault barrier in the north–south section, and it contains wells B1 and B3.
• The fault horst zone is the highest point of the ancient nose uplift; it contains wells B2 and B4, is located in the south block of the Wubao Fault, and is east of the Bao3 Fault, west of the Baoshi Fault, and north of the Baotuan Fault.
• The low-belt east-dipping fault slope is located east of the Baoshi Fault and contains well N1.

3. Data and Methods

The research area currently has full coverage by three-dimensional seismic data, with an area of approximately 1500 km². The main frequency of the target layer is 28 Hz, and the sampling interval is 2 ms. Twenty-five exploration wells have been drilled, including six horizontal wells and some wells with core data. The combined logging and oil and gas testing data are complete.

In this study, synthetic seismograms were created for individual wells to perform the well-to-seismic calibration. The main interfaces related to the target layer were traced, interpreted, and tied. Specifically, T30, T31, T32, T33, and T34 correspond to the top interfaces of the Pinghsang, Pingzhong, Pingshangxia, and Pingxiaxia Members and the Baoshi Formation (Fm), respectively. The thickness of the sand sets within each stratigraphic member generally follows the principle of approximate thickness equality or gradual variation. The Pinghu Formation is divided into 12 sand sets (subsections) from top to bottom, with the Pingshang Member containing the P1, P2, P3, and P4 sand sets, the Pingzhong Member containing the P5, P6, P7, and P8 sand sets, the Pingxiashang Member containing the P9, P10, and P11 sand sets, and the Pingxiaxia Member containing the P12 sand set (Figure 4).

Due to the reduction in the trap area caused by the cutting of sandbodies by faults, to identify the larger-scale lithology and composite traps, continuous individual seismic events of peaks or valleys were traced and interpreted for larger fault blocks with few faults to identify single sand layers under the condition of maximum seismic resolution. This study mainly traced and interpreted the individual events of the Pinghu Formation in the three secondary structural units of the fault fan, fault saddle, and low-belt fault slope in the Baoyunting area. The determination of the relationship between the amplitude and polarity and the sandstone thickness was based on one-dimensional forward modeling, and the type of microfacies was determined by combining logging facies and the area and morphology of root mean square amplitude anomalies.

3.1. Fault Scale Classification

The faults in the Baoyunting area were divided into second-grade and third-grade faults using the geomodeling fault throw map combined with manual fault throw statistics on the structural maps (Figure 3a). The second-grade fault throw is generally greater than 100 m at the T31 interface, and the maximum can reach over 400 m at the T34 interface. The third-grade fault throw generally ranges from 50 to 100 m at the T31 interface, and it may increase to over 100 m at the top of the Baoshi Formation at the T31 interface. The fourth-grade fault throw remains under 50 m. There is no first-grade fault along the sag boundary in the Baoyunting area. Due to the transition from the Baoshi Formation to the Pinghu Formation in the Xihu Sag, the magnitude of the fault throw is related to the horizon and depth, and the deeper and older the fault, the greater the fault throw. Five secondary faults from north to south are the Laibao Fault (Flb), Wubao Fault (Fwb), Baoshi Fault (Fbs), Baotuan Fault (Fbt), and Tuandong Fault (Ftd) (Figure 3), and they have larger fault throws and special significance in controlling reservoirs.

3.2. One-Dimensional Forward Modeling and Sandbody Scale Determination

By means of one-dimensional forward modeling for the individual wells, we adjusted the acoustic (sound) curve values to increase or reduce the sandstone layer thickness based on the acoustic values of the surrounding rock layers on a seismogram of a well; this method revealed a relationship between each sandstone thickness of interest and its amplitude and phase response. According to the statistics, approximately 80% of sandstones have strong amplitude responses (Figure 5), while 20% of sandstones have weak amplitude responses. The polarity and phase angle of sandstones vary with different horizons. The range of amplitude anomalies represents the outline of the distribution of the sandstone layer. Although many geological factors influence the amplitude, such as impedance contrasts, thin-layer interference, and faults, under specific geological conditions where other factors remain constant, the relationship between the amplitude and sandstone thickness from the one-dimensional forward modeling is generally reliable.

A well-to-seismic calibration was conducted to determine the corresponding individual seismic events in the main sandstone layers of interest. By tracking the event peak or trough reflection on the 3-D seismic sections and using the “attribute delineation, well-defined thickness” method, the planar distribution (sandstone thickness map) of the corresponding sandstone layer was delineated. The distribution of every individual event corresponding to each individual sandstone layer at the well point was tracked based on three-dimensional seismic data; its root mean square amplitude was extracted; and, finally, the thickness of the single sand layer, pinchout line, etc., was determined. As different microfacies sandbodies possess different planar shapes and dimensions [39], the sedimentary microfacies of each sandstone layer were determined according to the scale and planar shape of the sandstone layer associated with factors such as logging, core analysis, and sedimentary facies analysis of the depression. The distribution map of the sandbody was overlapped with its top structural map, and the relationship between the upper and lateral pinchouts of the sandbody and the faults was analyzed (Figure 6), providing a basis for distinguishing composite traps from lithologic traps.

3.3. Juxtaposition and Sealing of the Lateral Fault Lithology

Extensive research has been conducted on fault sealing [40,41,42,43,44,45]. D.A. Smith discussed the criteria for determining whether a fault is sealed in 1966 [46]. Allan published Allan diagrams on fault sealing in 1989 [47]. Knipe improved the perspective view of fault sealing in 1997, and Knipe diagrams provide a rapid analytical method for fault sealing [48]. By comparing lithologic data from cuttings and well logs and combining them with structural maps, the relationships of lithological juxtaposition between the drilled sandstone and the opposite block of the fault were plotted. For the sandstone layers containing oil, gas, and water in the Pinghu Fm, Knipe diagrams were applied to determine the relationships of lithological juxtaposition on both sides of the fault plane. Sand–sand juxtaposition was defined as fault opening, while sand–mud juxtaposition was defined as fault sealing [46]. Combined with the structural maps of the top surface of the sand layer, the effective closure range of the sand layer and its consistency with the comprehensive interpretation of the well logging data for oil, gas, and water layers were determined. The fault–sand ratio and fault–caprock ratio of the oil and gas layers, coexisting gas–water layers, gas-bearing water layers, and water layers interpreted through integrated well log interpretations in the study area were statistically analyzed. The relationships between these ratios and the fault–sand and fault–caprock ratios were fitted, and the probability of trap accumulation related to vertical and horizontal fault sealing was analyzed.

To determine whether the two sides of the fault were sand–sand or sand–mud juxtapositions, it was necessary to create a line chart diagram of a fault throw of the target sandstone top and a Knipe diagram of a well that was drilled into this sandstone [47,48]. The fault throw of the fault was calculated based on contour differences on a structural map of the top surface of the sandstone at each point every 50 m or 100 m from one end of the fault to the other. Then, a line chart diagram of the fault throw change along the fault strike was drawn (Figure 7a).

Finally, based on the distribution of the individual sandbodies on the structural maps showing their top surfaces, whether the sandbodies were cut by faults in the updip-parallel and updip-perpendicular directions was determined, and the vertical and horizontal sealing characteristics of the faults and potential distribution patterns of favorable traps and reservoirs in the Baoyunting area were summarized.

4. Relationship between the Sandbody Dimension and Lateral Transport Ability and Pool Formation

The one-dimensional forward modeling results of the N1 well in the research area are used as an example (Figure 4). The 1-D forward modeling results indicated a positive correlation between the P5 sandbody and the amplitude. The P9 sand layer also showed a positive correlation (Figure 4). Overall, approximately 80% of the individual sandbodies in the study area exhibited a positive correlation with amplitude.

In the analysis of the individual 3-D seismic events that occurred in the Baoyunting nose structure, numerous isolated small-scale sand layers that were not cut by faults were identified (Figure 8). These sandbodies, lacking lateral migration, had difficulty capturing mature oil and gas from the lower belt during the two stages of hydrocarbon accumulation.

The analysis of the accumulation process revealed two migration modes for oil and gas accumulation in the Baoyunting nose structure. The first mode was the early-stage (10 Ma) vertical migration facilitated by faults, where mature oil and gas from the deep Pingxia Formation and Baoshi Formation entered the shallow reservoir along the fault plane (Figure 9a,c). The second mode was the late-stage (after the 5.2 Ma Longjing movement) migration of mature oil and gas from the deep depression along faults and step-like sand layers toward the higher structures (Figure 9b,c). Small, isolated sandbodies in the Pingzhong Member on the high belt, if not cut by faults, could not acquire migrating oil and gas through step-like migration. The failure to form reservoirs for some small water-bearing sandstones encountered during drilling in the sparsely faulted blocks can be attributed to this reason.

Based on the area and morphology of individual sandbodies in different sand sets, their sedimentary microfacies types and interconnectivity are determined [39], and their ability to transport oil and gas laterally is further analyzed (Figure 10).

The P8 and P6 sand sets are in the largest areas in the fault fan, fault saddle, and fault slope substructural units, and they are representatives of the sandbodies in the Pingzhong Member. When the ratio of the sandstone area to the structural unit area is more than 70%, the sand set is defined as having good hydrocarbon transport capacity; the sand set is located in the low zone of the fault slope and is named P6. When the ratio of the sandstone area to the structural unit area is within 30–70%, the sand set is defined as having a medium hydrocarbon transport capacity; it is located in the central and western parts of the fault fan and south of the fault saddle, and it is named P8. When the ratio of the sandstone area to the substructural unit area is <30%, the sand set is defined as having poor transport capacity; it is located in the eastern fault fan and north of the fault saddle for most sand sets. The overall transport capacity of the Pingzhong Member is good in the low belt and moderate-to-poor in the middle and high belts. The poor reservoir-forming conditions and low oil and gas discovery rate of lithologic reservoirs in the middle–high belt are related to the isolation of sandbodies and poor lateral transport capacity. The lower belt has good transport and good reservoir-forming conditions for the Pingzhong Member (Figure 10).

In the Pingxiashang Member, the sand sets with the largest areas in the fault fan, fault saddle, and fault slope substructural units are P10 and P9. These sand sets can be regarded as representatives of sandbodies in the Pingxiashang Member in each fault block. Similar to the method for evaluating the transport capacity in the Pingzhong Member, areas with good transport ability are distributed in the low fault slopes and western fault fan. The area with moderate transport capacity is located in the middle of the fault saddle and on the local low fault slope. The area with poor transport capacity is located in the eastern part of the fault fan and the southern part of the fault saddle. The overall transport capacity of the Pingxiashang Member is evaluated as moderate (Figure 10b). Due to the lateral difference in the transport capacity in the middle–high belt, some isolated lenticular sandbodies lack hydrocarbon supply and accumulation and contain water, while some sandbodies can form reservoirs as a result of good transport conditions.

5. Relationship between the Fault Throw and Pool Formation

5.1. Control of the Offset-to-Caprock Ratio on Fault Sealing

Overall, the longitudinal and lateral sealing of faults in the area plays a crucial role in hydrocarbon accumulation. The smaller the fault throw is, the more favorable it is for the vertical sealing of faults and the formation of reservoirs in the Pingzhong Member. Most faults step upward until they intersect the Pingshang Member, so the P5 mudstone at the top of the Pingzhong Member becomes a quasiregional caprock that ranges from 20 to 70 m [49], forming a reservoir–caprock combination with the sandstone in the Pingzhong Member. Statistics show that when the ratio of the fault throw to the thickness of the P5 mudstone (we refer to it as the offset–caprock ratio, which is the same below) is less than 1.8, the fault has vertical sealing; otherwise, the fault opens vertically [50]. In the case of the B4 well, where the thickness of the P5 mudstone is small and the Wubao fault on its west side exhibits a large fault throw, the offset–caprock ratio exceeds 2, leading to overall poor hydrocarbon accumulation in the Pingzhong Member of the well. In the Baoyunting area, most drilled wells reveal a cumulative thickness of more than 100 m for the P4–P5 caprock, indicating favorable sealing conditions. Therefore, the key factor for hydrocarbon accumulation in this area is the lateral sealing of faults, as a larger fault throw favors lateral sealing and hydrocarbon accumulation. However, statistics show that the lateral sealing of faults in the Pingbei area is positively correlated with the size of the fault throw. The larger the fault throw is, the more conducive it is to the lateral sealing of faults. Therefore, faults play the role of a double-edged sword in reservoir formation.

5.2. Variation in the Lateral Sealing of the Faults

The key factor for reservoir formation in the Baoyunting area is the lateral sealing of the fault. Under the condition of basic vertical fault sealing, fault-related trap formation and hydrocarbon accumulation must rely on lateral sealing. However, the lateral sealing varies with the change in the fault throw along the strike.

Through an analysis of unsuccessful wells in the Baoyunting area, it was observed that the main reason for the water-bearing rocks in the Pinghzong Member of the wells was the small fault throw. Not only did the member fail to meet the lateral sealing requirements of the sand–mud juxtaposition, but it also failed to meet the lateral sealing requirements of the sand–sand juxtaposition with a significant shale–gouge ratio (SGR). For example, on the top T32 structural map, the maximum fault throw did not exceed 100 m, except in the case of the Wubao Fault (Fwb, Figure 3). When the Knipe diagram was utilized and the fault throw along the strike was analyzed, it became evident that the water-bearing sandstone encountered in the Pingzhong Member in wells drilled in the high and middle belts of the Baoyunting nose did not meet the sand–mud juxtaposition conditions required for lateral sealing along the faults.

Taking the P6 water-bearing sandstone in the B5 well as an example, the reason why the fault in the high and middle belts of the Baoyunting structure does not seal laterally and cannot form reservoirs is explained in Figure 11.

Unlike the middle-to-high belt, the low belt of the Baoyunting nose uplift is characterized by the presence of the Baoshi Fault (Fbs) with a large offset. The P5–P8 sand layer on the east side of the fault is abutted by the mudstone on the west side of the fault and is sealed laterally. As a result, the P5–P8 sand layer of the N1 well was totally charged by gas (Figure 7a,b). The P9 sand layer is also mainly abutted with mud but partially with a few thin sandstones. In the same way that the faulted interval has a high shale content with an average of more than 70% (equivalent to the SGR), the P9 thick sand layer is also laterally sealed into reservoirs (Figure 7d).

In the case of sand–sand juxtaposition on both sides of a fault, a high SGR can also help with lateral sealing and reservoir formation. Yielding et al. (1997) proposed using the SGR of the fault belt to assess its sealing capacity. The key factor determining the lateral sealing capability of the fault belt is the mud content within it [43,45]. The SGR is considered the most reliable parameter for characterizing the overall mud content within the fault belt. The SGR limit was defined for reservoir formation in the Heishimiao reservoir in the southern Changyuan area of the Daqing oilfield as 37.5% [51]. International researchers have mentioned SGR limits of 15–20% [43] and 25–30% [52] for reservoir formation. In the Baoyunting area, the mudstone content of the Pingzhong Member ranges from 40% to 80%, which can be viewed as the maximum SGR value. Previous statistical studies have shown that the lowest value of the SGR for lateral sealing under sand–sand juxtaposition conditions in the Pinghu structural belt is approximately 30% [53] (Figure 12).

5.3. Relationship between the Fault–Sand Ratio and Reservoir Formation Probability

Statistics show that there is a good positive correlation between the fault–sand ratio and the probability of reservoir formation in the middle section of the Pinghu Formation in the Pinghu structural belt [50]. The lower the sand ratio and the thinner the sandbody in this area, the greater the probability of lateral sealing and reservoir formation of faults (Figure 13).

The concepts of the fault–sand ratio (Equation (1)) and the probability of reservoir formation (Equation (2)) can be used to express their relationship. The term F_cc represents the fault–sand ratio (also known as the coefficient of hydrocarbon accumulation controlled by the faults), N_p represents the probability of reservoir formation, L represents the fault throw, and H represents the sand thickness.

$$F_{cc}=\frac{L}{H}$$

(1)

$$N_p=\frac{Number of oil and gas layers}{Number of oil, gas, and water layers}$$

(2)

The fault throw of the nearest up-dipping fault of each sandbody is read on the structural map of the sandbody, and the F_cc values are calculated. For a certain interval of F_cc values, the number of layers corresponding to the oil, gas, and water and the probability of reservoir formation are calculated (the numbers in Figure 13 are the number of oil, gas, and water layers in that interval), and a cross-plot of the average F_cc and average N_p values is created (Figure 13).

The fault–sand ratio in the Pingzhong Member of the region generally increases as the age of the layer increases and the sealing of the faults improves. The Pinghu Formation is part of the transitional period deposits between the rift and the depression basin, and most of the fault throw decreases upward and stops shifting near the Pingshang Member. Therefore, the fault throw in the Pingzong Member is smaller than that in the Pingxia Member, the lateral sealing of faults becomes better downward, and lateral sealing is better in the Pingxia Member.

Why does the lateral sealing of the fault in the Pingzhong Member of the study area increase as the normal fault throw increases? This is related to the sandstone content and assemblage in the Pingzhong and Pingxia Members in the area. Affected by the structural and sedimentary background of this area, these primary controlling faults played a crucial role in the formation of the fault-related traps. In the Pingzhong Member in this region, the sandstone contents range from 20% to 40%, and the individual sand layers generally have thicknesses of 5 to 20 m. The total thicknesses of the Pingzhong Member are 150 to 200 m, while the underlying Pingxiashang Member has thicknesses of 150 to 200 m and consists mostly of massive mudstones (Figure 14a). Under the situation of an east-dipping slope cut by parallel faults, when the fault throw at the bottom of the Pingzhong Member is less than 100 m, the sand layer in the Pingzhong Member is likely to abut the sandstone on the other side of the fault without lateral sealing, forming a water layer (Figure 14a). When the fault throw on the bottom of the Pingzhong Member exceeds 150 to 200 m, the sandstone layer of the Pingzhong Member can effectively abut the mudstones of the Pingxiashang Member, which results in favorable lateral sealing and reservoir formation (Figure 14b).

6. Possible Reasons for the Deterioration of the Reservoir Formation Conditions of the Baoyunting Gas Field

The Baoyunting oil–gas field is located in an accommodation margin between the gentle slope of the Wuyunting gas field to the north and the steep slope of the Tuanjieting gas field to the south [54,55]. Although it exhibits a nose structure, the number of discovered gas reservoirs in this transitional zone is far less than those in the Wuyunting and Tuanjieting gas fields. This might be attributed to the unique geological conditions of the slope type in this transitional zone. Specifically, the small fault throws in the transitional zone result in poor lateral sealing and hinder the formation of composite reservoirs. Additionally, the slope in the transitional zone lacks major river system injection and large-scale delta or barrier sandstone reservoir occurrence. The ancient nose structure leads to relatively shallow water depths in the transitional zone, accompanied by small-scale tidal-channel lens-shaped sandstone deposited mainly under supratidal to intertidal environments. As a result, the transitional zone exhibits limited lateral gas transport and entrapping abilities, resulting in a lower probability of forming lithological gas reservoirs.

During the depositional period, the fault throw in this area is generally smaller compared to the steep and gentle slopes in the north and south areas. Additionally, there is a lack of major water influx and the formation of large deltaic sandbodies, as these bodies primarily develop along the axial, steep slope, and gentle slope of a half-graben basin (Figure 2). On the structural map showing the top of the Pinghu Formation in the Baoyunting area, the fault throws associated with traps (such as fault-block traps, fault-nose traps, and fault–sand composite traps) in the Pingzhong Member are generally less than 100 m (Figure 3), resulting in poor potential for reservoir formation.

7. Prediction of Favorable Oil and Gas Accumulation Areas

According to previous resource assessments, the natural gas resources on the western slope of the Xihu Sag can reach 2485 billion cubic meters (gas equivalent). Currently, the proven natural gas reserves are 69.5507 billion cubic meters (gas equivalent), with a proven rate of approximately 2.8%. There are still many oil and gas reservoirs to be discovered, but there are no structural traps available for drilling, and composite traps have become important exploration targets. The application of this research method can significantly improve the detection rate.

7.1. Basis and Method for Identifying Favorable Oil and Gas Traps

The source rock of the Pinghu Baoshi Formation coal measures in the Xihu sag is widely distributed, and it has been generating and supplying hydrocarbons for a long time. Delta and tidal channels and barrier sandbodies are the main reservoirs interbedded in mudstone. Oil and gas migrate vertically and horizontally along faults and sandbodies in a stepped manner toward the western high belt of the slope and are enriched in lithologic and composite traps. The Baoyunting area has two stages of reservoir formation, and the low-maturity oil and gas filling reservoir before the 5.2 Ma Longjing movement was created during an early stage of reservoir formation. The regional compression and uplift of the Longjing movement resulted in the formation of reverse structures and the fault throw, leading to the destruction or adjustment of earlier oil and gas reservoirs. After the Longjing movement, the slope of the traps was basically stable, and a large amount of highly mature wet gas was recharged or invaded again as part of the late secondary reservoir formation. The gas–oil ratio in this area is high, and the area is mainly composed of condensate gas reservoirs, indicating that the late stage of reservoir formation was dominant. Due to the cutting of sandbodies by faults, most oil and gas reservoirs have the characteristics of “faulted, small, poor, and scattered”, and it has been found that in all gas reservoirs, fault–sand composite traps and lithologic traps account for approximately half of the total number of traps (Figure 15).

Based on factors such as fault lateral sealing and sandbody distribution, which are the most important factors controlling reservoirs, the identification of favorable reservoirs in this area is mainly based on the following aspects:

Lithological traps include lenticular sandstone traps and traps that are mainly located in fault blocks with few faults. Lithological traps are mainly located in three secondary structural units: the fault fan to the north of the Wubao Fault, the fault saddle between the Wubao Fault and the Baoshi Fault, and the eastward-dipping fault slope located in the lower belt on the east side of the Baoshi Fault. The individual seismic event microfacies analysis shows that these secondary structural units have upward and lateral pinched-out sandbodies, which have the potential to form lithological or composite traps (Figure 15).

The composite trap is located on the east side of faults with larger fault throws, such as the Wubao Fault, the Baoshi Fault, and the Baodong Fault. In these areas, there are sandbodies formed in depositional environments, such as delta front mouth bars, distributary channels, tidal channels, and barrier sand bars, in the middle and lower sections of the Pinghu Formation. Because of the large fault throw, the trap has the ability to block the upward or lateral inclination of the sandbody (Figure 6 and Figure 16).

Because faults with small fault throws generally do not have lateral sealing, they may transport oil and gas to adjacent layers, destroy reservoir formation, and have destructive effects on both composite and lithologic traps. However, in the dip direction of the sandbody, the sandbody can have faults with small fault throws, which instead have a vertical oil and gas migration effect (Figure 15).

Due to the stepwise migration of oil and gas from low to high belts in the Pinghu structural belt, faults have an interception effect on oil and gas. Therefore, the secondary faults in the eastern low belt of the study area have a higher filling intensity and a higher probability of reservoir formation compared to the traps controlled by the western secondary faults (Figure 15 and Figure 16).

7.2. The Low Belt in the Middle Part of the Pinghu Formation Dominates the Accumulation

The 3-D seismic analysis of a single sand layer in the middle section of the Pinghu Formation indicates that large sandbodies, such as deltas and barriers, are developed in the Baoyunting low belt. The low belt is sandwiched by the Baoshi Fault and the Wudong Fault, both of which extend in the NE direction, with a large fault throw. Fbs fault T32 in the upward-dipping direction of the sandbody has a fault throw of more than 50 m, a large blank area, and an undeveloped internal small fault throw. The Baoyunting low belt is the most likely location in the study area to develop large-scale lithologic traps. This area is close to the deep concave source rock and has the greatest potential for reservoir formation (Figure 8 and Figure 9).

Overall, the pool formation characteristics of the low-slope zone are as follows: ① It is a good reservoir, P4–P6 are delta mouth bar facies, and P7 and P9 are barrier sandbar microfacies; the trap area is relatively large; if the distributary channel facies is not cut by faults, the maximum area of a single trap can exceed 20 km². ② It is adjacent to the hydrocarbon generation center of the deep concave zone, providing a good hydrocarbon supply. ③ The upward Fbs and Fwd have a large fault throw and possess high potential for lateral sealing without interference from small faults.

Based on a comprehensive analysis, it is suggested that, for the middle member of the Pinghu Formation, the low slope of the Baoyunting structure is in a favorable geographical position to form lithological and sand–fault composite traps (Figure 15 and Figure 16).

7.3. Favorable Traps in the Lower Pinghu Formation Are Dominant in the High Belt of the Baoyunting Nose

Through single-sandbody analysis, we found that the high belt is mainly composed of small sandbodies in tidal channels and tidal flats. The high belt also has the characteristic of a small fault throw. Isolated small sandbodies have difficulty forming reservoirs due to the lack of lateral gas enrichment, and traps related to faults have difficulty forming reservoirs due to the small fault throw and poor lateral sealing (Figure 16). However, for the lower member of the Pinghu Formation of the Baoyunting nose, the lateral sealing of the faults has improved due to the downward increase in the fault throws and is over 150 m. Furthermore, as the lower member of the Pinghu Formation is close to the deep source rock, the hydrocarbon supply conditions are relatively good. Therefore, although the sandstone of the lower member is not as developed as that of the middle member of the Pinghu Formation, it has good reservoir-forming conditions (Figure 6 and Figure 16).

The interpretation of the individual seismic event indicates that the P9, P10, and P12 sand sets in the lower member of the Pinghu Fm of the Baoyunting nose have a larger area, good lateral oil and gas migration and supply conditions, and a high probability of reservoir formation (Figure 8b).

Based on the comprehensive analysis, it is suggested that, for the Pingxia Member of the Baoyunting area, the slope of the high belt is a favorable location for the formation of lithological and fault–sand composite traps.

8. Conclusions

1. The fault throw and sandbody size are the two most important factors controlling reservoirs. The larger the fault–sand ratio is, the better the fault sealing performance and the more conducive it is to forming a fault–sand composite oil and gas reservoir. Larger-scale sandbodies, such as those formed by barrier bars and delta fronts, have good lateral migration and charging of oil and gas, making it easy to form lithological oil and gas reservoirs.

2. The larger fault–sand ratio is prone to composite reservoir formation, which is related to the sand–mud interbedding in the Pingzhong Member and the large mudstone in the Pingxia Member. When the fault throw is small, there is a higher probability of sand–sand juxtaposition on both sides of the fault, which results in a lower likelihood of hydrocarbon accumulation. The presence of densely distributed small faults on the slope is unfavorable for the formation of composite trap hydrocarbon reservoirs.

3. In the Baoyunting area, the Pingzhong Member is predominantly characterized by the lens-shaped isolated sandstones formed by tidal channels. These isolated sandbodies in the mid- to high-elevation belt have limited hydrocarbon accumulation potential due to the poor migration conditions of the source faults and the source rocks with relatively low positions. In contrast, the lower belt features large barrier and deltaic sandbodies, which are closer to the mature source rocks and exhibit favorable migration and accumulation conditions.

4. The individual event interpretation of sand layers by seismic data and fault lateral sealing analysis is an effective method for identifying composite and lithologic traps in the Baoyunting area. Under the conditions of well-developed faults and sandbodies in the slope zone of a fault depression and the lack of structural traps, this method has important reference significance in predicting favorable traps and pools.