After more than 50 years of exploration in Shijiutuo uplift within Bohai bay basin,three large oilfields with 100 million tons reserves have been discovered in the anticlinal traps of higher structural position.Conversely,the slope region in the lower structural position exhibits no significant discoveries in the last decade due to the lack of structural traps and the relatively less exploration efforts.As the improvement of exploration activities in Bohai sea area,the focus has transitioned comprehensively into the lithological exploration phase.A profound and systematic study has been undertaken to investigate the shallow hydrocarbon migration,lithological trap characterization,and the hydrocarbon accumulation law in the axial fault slope region of the eastern segment of Shijiutuo uplift.The results show that:①The northern and southern flanks of the study area are gripped by hydrocarbon-rich depressions.The late-stage vigorous activities of major faults along the uplift boundary and the widespread development of near-source fans in the steep slope zones make the uplifted area as the final direction of substantial hydrocarbon migration and accumulation.②The major fault along the uplift boundary exhibits the lowest opening coefficient in Guantao Formation,where the conglomeratic sandstone reservoir serves as the main lateral migration transportation for hydrocarbon migration atop Shijiutuo uplift.③The combinations of transport ridge gradient and cutting ridge fault in Guantao Formation control the vertical migration of hydrocarbon in the axial slope region.Under the conditions of slope gradient less than 1°,and the fault length surpasses 2.50 km,along with the fault distance exceeding 40 m during the accumulation period,the slope region is conducive to the occurrence of significant vertical hydrocarbon migration,challenging the traditional understanding of"no retention during passage"in the axial fault slope region.④In the uplift slope region,the sand bodies obliquely intersecting the structural ridge direction are conducive to hydrocarbon preservation.Simultaneously,the proposal introduces the spectral bluing technique constrained by well data for high-resolution processing,facilitating effective characterization of lithological traps in the slope area.This approach allows for the preferential selection and drilling deployment of oblique sand bodies demonstrating favorable preservation conditions.Under the guidance of the aforementioned new insights and technologies,QHD 27-3 large oilfield was successfully evaluated in the axial slope region of the eastern segment of Shijiutuo uplift in 2023.This breakthrough represents a significant advancement in the shallow-depth exploration of mature areas,offering valuable insights for sustained oil and gas discoveries in the highly mature exploration zones of Bohai bay basin.
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