Expulsion process of overpressure fluids indicated by vertical venting structures in the Dongfang area of the Yinggehai Basin, offshore South China Sea

• Two types of vertical venting structures rooted in Miocene interval.
• The Miocene sands fluid pressure has reached the minimum stress magnitude.
• Water phase overpressures of flank sandstones vary with the uplift heights.
• Fluid direction shifted from basin margin to central areas during anticline uplift.

Two types of vertical venting structures (pipes and gas chimneys) were identified within the Miocene-Pliocene succession of the Dongfang area, Yinggehai Basin, using an integrated approach that combined the seismic data interpretation with the analysis of borehole measurements. The location and distribution of these vertical venting structures can be associated with either stratigraphic or structural elements in the study area. These vertical venting structures were rooted in Miocene interval, which are characterized by abnormally high pressures and high temperatures. The crestal pressures of the Miocene sands have reached the least principal stress and can induce hydraulic fracturing. Consequently, hydraulic fracturing is suggested to be the primary genetic mechanism of these vertical venting structures in the Dongfang area. External factors, such as the up-lifting and stress compression of the Miocene interval along the anticline flank during the Pliocene, have driven the water phase overpressures of flank reservoirs varying with the uplift heights, and the stronger uplift contributed high efficient focused flowing to initiate hydraulic fracturing on crests. On the upper slope of western basin margin, the upward terminuses of the venting structures are adjacent to the base of the Pliocene, while on the crestal positions of the central anticline, their terminuses are adjacent to the base of the Upper Pliocene interval or above it. This regional difference indicates that the fluid direction shifted from the basin margin to the basin central areas within Miocene interval during the Pliocene uplift.