## 4 Sequence Stratigraphic Analysis of Volcanic Rock-Evaporite-Clastic Rock Symbiotic Zone
### Sequence Stratigraphic Framework  
Based on the analysis of the different types of sequences in different blocks, the sequence interpretation of key survey lines was determined by selecting representative cross-well sections that ran through the whole zone to clarify the sequence correlation of the work area further, determine the well-seismic correlation relationship of the sequence, and establish a unified sequence stratigraphic framework of the volcanic rock-evaporite-clastic rock symbiotic zone. The third-order sequence interface was identified in the fourth member of the Shahejie Formation through well-seismic integration, the stratigraphic superposition pattern, and rock-electric response characteristics, and then two third-order sequences and six system tracts were divided (Figure 14) and combined with the thickness map of the fourth member of the Shahejie Formation in the south of Laizhou Bay, the stratigraphic distribution law in the stratigraphic framework is analyzed (Figure 15).  
During the SQS\({}_{4}\)L sequence period, influenced by volcanic activities, the sequence developed in the southern gentle slope belt in a small range, belonging to a composite sequence composed of volcanic rocks and sedimentary rocks. Volcanic rocks were deposited near the strike-slip zone in the west, which were far from volcanic sources and were less affected by volcanic activities. Due to shallow water bodies, sedimentary strata were exposed from the water surface and subjected to denudation, thus developing incompletely, and the eastern strike-slip zone was dominated by clastic rock sedimentation. The seismic profile revealed thick saline lake sequence stratigraphic sedimentation in the southern subdepression, presenting poorly continuous-weak amplitude seismic reflection characteristics. The lake TST in this well profile was relatively thin, but its thickness was stable. Mudstone sedimentation played a dominant role, intermingled with a small amount of thin sandstone; the lake level rose; and the water body deepened. The thickness of the HST increased gradually from west to east, and it was denudated around Well 4 in D structural belt.  
Compared with SQS\({}_{4}\)L, the SQS\({}_{4}\)U sequence is thinner overall, but the formation thickness is stable, with good east-west connectivity and strong comparability. On the gentle slope belt, the strata in the denudation area at the elevated position are generally thin. The strike-slip zone in the east is close to the sedimentary center, the stratum is thick and proximal with a short sediment transport distance, and thick sandstone and mudstone are interbedded. Overall, the stratum in the east is thick, the onlap phenomenon is pinched out on the slope belt, the stratum in the west is supplied by distal sources, and it is thin in the higher terrain area. The system tract is underdeveloped; the HST is denudated in the elevated position, namely, at Well 4 in the D structural belt; and the TST stratum is thin. Thick carbonate lake sequence stratigraphic sedimentation is found in the center of the southern subdepression, and the TST stratigraphic thickness in other areas is thickened. The thickness of the HST formation is the largest. The phenomenon of onlap pinch-out can be seen on the gentle slope, and the eastern strike-slip zone is dominated by clastic rock sedimentation.  
Figure 14: Cont.  
Figure 14: The isochronous sequence stratigraphic framework of the study area: (**a**) southern slope zone–southern sub-depression-eastern slip zone connected well profile; (**b**) seismic interpretation profile of the southern slope zone–southern sub-depression-eastern slip zone.