## 5 The Impact of Sequence on Petroleum Reservoir Development  
The sequence stratigraphy of the Guantao Formation obviously controlled the sedimentary filling evolution in the study area. The base-level variation during the long-term cycle controlled the type and development degree of the sedimentary facies, while base-level variation during the mid-term cycle controlled the type and development degree of the sedimentary microfacies. As the long-term base-level rose, the sedimentary facies in the study area also shifted from alluvial fans to braided fluvial and ultimately to meandering fluvial facies. With the fluctuation in the base-level in the mid-term cycle, the type, development degree, and distribution of the sedimentary microfacies also changed accordingly. In the alluvial fan facies, the stream flow deposits and overflow sand bodies gradually decreased until disappearing with the rise in the base-level from SNG1 to SNG5. Within the braided fluvial facies, the development of the center bars and braided channels gradually decreased with the rise in the mid-term cycle base-level from SNG4 to SNG7. The meandering fluvial channels increased as the base-level decreased from SNG8 to SNG 10. The evolution of the sequence stratigraphy also controlled the distribution of high-quality reservoirs in the Guantao Formation. When the base-level was at a low level, a large amount of sand bodies developed, which can form favorable reservoirs for petroleum. The SNG1 to SNG3 cycles were mainly filled with alluvial fan sediments, with well-developed sand bodies, which comprise the main oil- and gas-bearing layers in the study area. Meanwhile, during the evolution of sequence stratigraphy, base-level cycles can control the combination of reservoirs and seals. At the transition surface of the long-term  
Figure 13: Distribution of sedimentary microfacies in each short-term cycle. Orange arrows show the microfacies changes in mid- term cycles and blue arrows show the microfacies changes in long-term cycle. cycle, which corresponds to the maximum lake-flooding surface in the Guantao Formation in the study area, there was a continuous development of thick floodplain mudstone, which can serve as stable regional seal for petroleum reservoirs. This seal combined with the underlying main oil-bearing layers in SNG1 to SNG3, forming a good storage space for the accumulation of oil and gas. At the top of each mid-term cycle, the elevation of the base-level can form relatively stable local seals in the study area. Good reservoir-seal combinations were formed in SNG1 to SNG3, ensuring that oil and gas were not easily dispersed but migrated to the reservoir layers. The bottom boundary of the Guantao Formation (interface T1) serves as a third-order sequence boundary and has an unconformity contact relationship with the underlying strata. This unconformity surface served as a dominant migration channel in the study area, connecting the reservoir and source rocks and expanding the spatial range of oil and gas migration. Oil and gas migrated through the unconformity surface (third-order sequence boundary) at the bottom of the Guantao Formation in the Binxian uplift to the sand body in MNG1. The interlayers at the top of the mid-term cycles served as seal layers to prevent oil and gas from escaping. The large section of mudstone at the transition of the base-level surface in the long-term cycle further served as a stable seal layer to preserve oil and gas. Therefore, oil and gas were accumulated in the MNG1 cycle strata, becoming the main oil- and gas-producing layer in the area. Therefore, the sequence stratigraphy plays an important role in the development of petroleum reservoirs. It can be seen that the long-term cycle controls the distribution of sedimentary facies, while the mid-term and short-term cycles control the distribution of sedimentary microfacies. When the base-level is at low levels, a large amount of sand bodies develop, forming favorable reservoirs for petroleum.