Linking shelf delta to deep-marine deposition in reservoir dispersal of the upper Oligocene strata in the Baiyun Sag, the northern South China Sea

Abstract

Linking shelf delta to deep-marine sediment in the Baiyun Sag (BYS) along the northern South China Sea has led to a great deal of interest, but it remains poorly known. In this paper, we focus on the detailed stratigraphic evaluation and reservoir dispersal of upper Oligocene strata (25.5 to 23.8 Ma, referred to as ZHSQ6), using three-dimensional (3-D) seismic volumes (approximately 2500 km2) calibrated with well-log and core data, to determine internal geometry and sedimentary evolution. Rock velocity estimates from well data show that sedimentary environments may be the most direct controlling factor of wave impedance, reflected polarity and the value of amplitude, which in turn predict the nature of the sedimentary system. Likewise, the analysis of sedimentary sequences reveals that lowstand systems tract and transgressive systems tract consist of inner-shelf deltas. Highstand systems tract, by contrast, contained mid-shelf deltas. A northeast-oriented advantaged longshore current influenced asymmetric sandy ridges with a mixed carbonate–siliciclastic system in the north controlled by paleo-uplift geomorphology. Muddy mass transport deposits (MTDs) and sheet sandstones are both widespread. During falling-stage systems tract, shelf-margin deltas provided coarse grain sediments directly onto the basin floor. Consequently, the volumes of dispersed thick deep-water stacked channels and sand-rich MTDs are significant. The existence of the eastern source clearly increased. Finally, the sediment model for the upper Oligocene Baiyun Sag has been established as an optimal component of source-to-sink systems. This study indicates that sedimentary evolution is controlled by source supply flux and duration, base-level behaviour, topographic gradient and growth faults. Delta front channels and longshore sandy ridges, together with the gully fill, represent the most favourable reservoirs for further hydrocarbon exploration.