Sedimentary architecture and provenance analysis of a sublacustrine fan system in a half-graben rift depression of the South China Sea

Abstract

Research on the sedimentary architecture and provenance of sublacustrine fans is crucial not only to better understand the gravity flow evolution in lacustrine basins but to successfully locate hydrocarbon reservoirs. This study targets at a sublacustrine fan with a large petroleum reserve that has been revealed by drilling within an Eocene lacustrine succession in the Fushan Depression, northern South China Sea. The sedimentary architecture of the sublacustrine fan was investigated through the integration of core, well logs and 3D seismic data, and its provenance was identified by detrital zircon UPb dating. The examination of 117 m long continuous drilling core reveals four major lithofacies in the sublacustrine fan deposits, which are interpreted as deposits of two types of gravity flows (cohesive debris flows and turbidity currents). The cohesive debris flows developed in the lower section are mainly composed of massive gravelly mudstones with various floating clasts and high clay content, whereas the turbidity currents in the upper section are characterized by parallel to ripple bedding and normal graded bedding. These observations reflect a transformation from cohesive debrites to turbidites from deep to shallow water environments. In addition, provenance analysis of 648 detrital zircon UPb ages yield a well-defined relationship between potential provenance areas and sublacustrine fan deposits in the deep lake. The UPb age pattern of the fan is similar to that of northern fan delta deposits, but different from sediments from the western and southern deltas, suggesting that the sublacustrine fan was sourced from the north. Therefore, a deep-lacustrine depositional model is proposed for the sublacustrine fan, showing a gravity-driven evolution process from fan delta front in the northern slope into cohesive debrites and then turbidites in the lake center. This was mainly controlled by fault-related topography and sediment supply. This study highlights the effective use of the detrital zircon UPb dating method to identify the provenance direction of sublacustrine fans, and the proposed depositional model may be applicable to reservoir prediction in other lacustrine basins.