--> Expulsion Process of Overpressures Indicated by Vertical Venting Structures in the Dongfang Area of Yinggehai Basin, Northwestern South China Sea

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Expulsion Process of Overpressures Indicated by Vertical Venting Structures in the Dongfang Area of Yinggehai Basin, Northwestern South China Sea

Abstract

An integrated approach, combining seismic data interpretation with boreholes measurements analysis in the Dongfang area of Yinggehai Basin, northwestern South China Sea, has revealed the presence of two types of vertical venting structures (pipes and gas chimneys) within Miocene-Pliocene succession. The location and distribution of these vertical venting structures can be associated with either stratigraphic or structural elements of the study area. All these vertical venting structures rooted in Miocene intervals which were characterized by abnormal high pressure and high temperature. Present crestal pressures of Miocene sands have reached the least principal stress, which can induce hydraulic fracturing. Abnormal low water-phase overpressures stored in the sands with deeper stratigraphic sequence and lower crests, can indicate pressure decline caused by hydraulic fractures. Thus, hydraulic fracturing was suggested as the primarily genetic mechanism of these vertical venting structures in the Dongfang area. External factors, such as focused fluid flow triggered by structure deformations, would have also driven Late Miocene-Pliocene fluids expulsion. On basin margins (upper of west slope), upward terminals of these venting structures were adjacent to the base of Lower Pliocene, while in the central area (on the anticline) their terminals are adjacent to the base of Upper Pliocene interval or above it. Such a regional difference indicates the fluid direction was shifted from the basin margin to basin central areas within Miocene intervals during Pliocene uplift.