The most prominent structure in the study area is the “V”-shaped Central Canyon. It quickly widens upward, and the fill sediments onlap onto its walls (Figures 3A,C, 6). The strata are mainly parallel/subparallel along the axis of the Central Canyon (Figure 5), and tapering/onlapping seismic reflections are only occasionally observed along the axis (Figure 5). Many boreholes have targeted the hydrocarbon-rich Central Canyon and show that the strata mainly comprise fine-grained mudstone (Su et al., 2014; Liang et al., 2020). Coarse-grained siltstone and sandstone are also observed within the Central Canyon (Figure 3C), which is the main hydrocarbon reservoir (Wang Z. et al., 2015).

Multiple MTDs characterized by chaotic/blanking seismic reflections are observed above the Central Canyon, separated by continuous seismic surfaces (Figures 3A, 5, 6). This study is focused on the lowermost MTD, here named as MTD-a, which is directly draped on the Central Canyon (Figure 3A).

Many irregular blocks, characterized by short enhanced-amplitude, negative-polarity seismic reflections are identified inside the MTD-a (Figures 3A, 5–7). Although there is no well directly through these blocks, referencing the Well Cc1’s layered structure leads to a reasonable assumption that the blocks are mainly composed of sandstones (Figure 3). The blocks are surrounded by fine-grained mudstone (chaotic seismic reflections). The head zone of MTD-a has not been imaged, because of the limitation of 3D seismic coverage (Figure 4). It may have originated from the slope of QDNB to the north (Figure 1) and flowed toward the southeast part, judging from the extension of MTD-a (e.g., the direction of the lateral margins) (Figure 4). In the 3D seismic coverage, MTD-a could be divided into three subzones, such as a northern highly deformed zone, a blocked zone, and a southern highly deformed zone (Figure 4). No macroscopic blocks could be observed within the northern highly deformed zone, and it is mainly composed of chaotic seismic reflections (Figure 6). In the blocked zone, blocks float within the fine-grained strata (mudstone) and they are separated by high-angle thrust faults (Figure 5). There are suspected blocks observed on the variance slices in the highly deformed zone to the south (Figure 4). However, the seismic identification of these structures is not definitive (Figures 3A, 6).

A total of 306 seismic-scale irregular blocks were recorded in the study area, characterized by enhanced seismic reflections as mentioned above, and are observed in the toe zone of MTD-a, especially those close to the southeastern boundary (blocked zone; green dashed zones in Figure 4A). However, the blocks are rarely observed at the distal termination part of MTD-a (southern highly deformed zone; Figure 4A). They are mainly bounded by thrust faults to their northwestern and southeastern sides and surrounded by chaotic seismic reflections (Figures 6, 7). In other words, they “float” within the chaotic seismic reflections. Although they are bounded by the thrust faults, the blocks are close to each other and some of them are even directly connected (Figure 7).

The irregular blocks are distributed in a linear fashion in plain view with orientations of E–W at the northwestern corner of the blocked zone and NE–SW at the left blocked zone (Figure 4). The dips of blocks are perpendicular to their strike extensions, and a few blocks are nearly horizontal elongation without any dips (Figure 5). The lengths of blocks range from ∼0.20 to ∼6.34 km with an average of ∼1.48 km, while the blocks’ widths are between ∼0.10 and ∼2.05 km, with an average of ∼0.27 km. There is no apparent relationship between the lengths and widths of blocks (R² = 0.03; Figure 8A). The blocks cover an average area of ∼0.44 km² ranging from ∼0.02 to ∼4.24 km². The areas of blocks are moderately related to their lengths (R² = 0.54; Figure 8B) and widths (R² = 0.50; Figure 8C). The average height of blocks is ∼42.4 m (∼30.3 ms twt), and thus the total volume of blocks (total area × average height) is ∼5.68 km³.

A total of 350 seismic-scale thrust faults are identified in the blocked zone, which separate the blocks apart (Figures 5, 7). Most of the thrust faults penetrate the whole MTD-a and terminate at the upper surface of the underlying Central Canyon strata (Figure 6). However, some small-scale thrust faults are mainly located in the upper part (above the blocks) of MTD-a (Figure 7B). The lengths of thrust faults range between ∼0.27 and ∼4.92 km with an average of ∼1.4 km (Figure 8D). The strikes of thrust faults mainly extend between 40° and 70° (∼84% of the total thrust faults) (Figure 8D). Less thrust faults strike 90°–147°, and they usually have short vertical extensions (<1.0 km; Figure 8D). Accordingly, the thrust faults mainly incline 130°–160° (37% of the total thrust faults) and 320°–340° (45% of the total thrust faults; Figure 8E). The dips of thrust faults range from ∼25° to ∼87°, and most of them (90%) are between ∼30° and ∼70° (Figure 8F).

Enhanced seismic anomalies characterized by negative reflections are observed in the Central Canyon and MTD-a’s blocks (Figures 3A, 5–7), which is similar to the typical seismic characteristics of free gas (e.g., Judd and Hovland, 2007; Løseth et al., 2009; Sun et al., 2017). In fact, the exploration well Cc1 drilled through these enhanced seismic anomalies and confirmed that they are gas-charged sandstones (Figures 3A,C). Furthermore, these gas-charged sandstones characterized by sharp decreases of gamma ray (GR) and density (RHOB) are surrounded by fine-grained mudstones (Figure 3B). Accordingly, the irregular blocks within MTD-a could be interpreted as the counterparts of sandstones (Figure 3C). Moreover, the enhanced negative seismic reflections of blocks (Figures 3A, 5–7) indicate that free gas also charged into them. The average porosity of sandstone from well Cc1 is ∼29.6%, and that of gas saturation is ∼71.2%; hence, the volume of free gas stored within the MTD-a’ blocks can be as high as ∼1.2 × 109 m³.

Fine-grained MTDs usually have decreased porosities and permeability, because of shear compaction and dewatering during their emplacement (Piper et al., 1997; Shipp et al., 2004; Sawyer et al., 2007; Dugan, 2012; Sun and Alves, 2020). Therefore, they usually act as effective seal to hinder the vertical migration of fluids (Alves et al., 2014; Sun and Alves, 2020). Remnant blocks that break through the MTDs could occasionally support the vertical fluid migration (Gamboa and Alves, 2015; Cox et al., 2020). However, there are no reports about vertical fluid flow in the compressional toe zone of MTDs as yet, where the compressional stress in the toe zone is proposed to strengthen the seal capacity of sediments.

Gas-charged blocks within MTD-a indicate that significant amounts of hydrocarbon have leaked from the gas field underlying the MTD-a (Figures 5–7). Because the blocks are surrounded by fine-grained sediments and are not directly connected with the underlying gas reservoir, the thrust faults are believed to act as the primary pathways for vertical fluid migration. Normally, the thrust faults in the compressional environment would be tight/closed to barrier fluids (Elmore et al., 2003; Micklethwaite, 2008). However, the thrust faults, likely with fractured medias, are mechanically weak zones (e.g., Lacroix et al., 2014; Cook et al., 2020), and they would likely reactivate under overpressure due to the underlying accumulation of hydrocarbons. Moreover, the blocked zone has weaker deformation compared to the northern and southern highly deformed zones where the failed sediments are probably fully mixed (Figure 4). The less mixture of sediments in the blocked zone would partly keep the fabrics of the strata, which is also in favor of reactivation of the thrust faults. This study documents the thrust faults at the compressional zone of MTDs to serve as fluid pathways for the first time. It also indicates that the fabric heterogeneity plays an important role on the seal completeness, and certain factors (e.g., overpressure) may trigger the disintegration of seal, even for the fine-grained sediment-dominated compressional zones of MTDs.

The hydrocarbon migration and accumulation system in the study area can be updated through this study together with previous studies (Zhu et al., 2009; Li et al., 2017). The deeply buried coal-bearing strata of the lower Oligocene Yacheng Formation serve as the main source rock in the QDNB (Huang et al., 2003). The source rock entered the peak gas-generation window during the late Miocene and Pliocene (Zhu et al., 2009). Hydrocarbon (gas) from the source rock migrated upward mainly along tectonic faults (Figure 6A) and charged into the reservoir of the Central Canyon from the Pliocene to the present (Wang et al., 2014). Within the Central Canyon, the interconnected coarse-grained siltstone/sandstones provided the lateral and vertical hydrocarbon migration pathways (Figure 9). Finally, the hydrocarbon accumulated at the topmost part of the Central Canyon where the reservoir is directly capped by the compressional zone of the fine-grained MTD-a (Figures 6, 9). The compressional zone of MTD-a is composed of a series of thrust faults and coarse-grained blocks, as mentioned above (Figure 3). Accompanying the gradual accumulation of hydrocarbon, overpressure would increase and finally exceed the yield strength of thrust faults. Therefore, the thrust faults would reactivate and arrow hydrocarbons would migrate upward along these faults and charge the blocks, as observed in this study (Figures 7, 9). This study indicates that a large amount of hydrocarbons (∼1.2 × 109 m³) has probably leaked from the main reservoir of the gas field. Furthermore, the hydrocarbon system including the hydrocarbon migration, accumulation, and leakage in the study area is probably still dynamic, and special attention should be paid to the seal completeness where thrust faults occur within the MTDs.