Fluvial morphology and reservoir sand-body architecture in lacustrine rift basins with axial and lateral sediment supplies: Oligocene fluvial-lacustrine succession in the Xihu sag, East China Sea Shelf Basin

The Xihu sag, which is the largest petroliferous sub-basin of the East China Sea Shelf Basin, formed in a continental back-arc setting. The Oligocene Huagang Formation consisting of a fluvial-lacustrine succession deposited during the compressional stage is the prime hydrocarbon-bearing interval in the Xihu sag. A third-order sequence-stratigraphic framework has been built, and component sand-body characteristics were investigated based on seismic attribute analysis and well-log correlation. Two overall upward-fining sequences, and an internal low-accommodation systems tract (LAST) (fluvial successions characterised by amalgamated fluvial channel sand bodies interlayered with rare overbank deposits) and high-accommodation systems tract (HAST) (intervals dominated by overbank or lacustrine deposits) have been identified. The thick, multi-storied channel-fill sandstone bodies deposited along the central depression belt, capped by extensive overbank or lacustrine mud deposits, characterise each sequence and form favourable reservoir-seal associations. Proximal-to-distal changes in lithofacies associations were also analysed. The sequence-stratigraphic and lithofacies analysis suggest the existence of an axial, large-scale river channel system in the Oligocene Xihu sag. On the basis of the restoration of basin geomorphology and seismic facies analysis, the depositional architectures of the axial paleodrainage system have been reconstructed. Overall, the Huagang sequences represent the sedimentary evolution of a large-scale fluvial system sourced from axial and lateral supply areas, to form river deltas into an interior-draining basin-centre lake. Two major fluvio-lacustrine transgression-regression cycles have been recorded. During the transgression cycle, the fluvial morphology was dominated by braided fluvial style; whereas during the regression cycle, the fluvial morphology was characterised by a combination of multiple fluvial channel styles in the LAST, from upstream to downstream low-sinuosity braided, high-sinuosity braided and anastomosing fluvial channel patterns were distributed and then replaced by large-scale lake flooding in the HAST. The braided channel centre, paleobathymetric lows of channel networks and delta-front bodies are sand-prone units. The fluvial sedimentation was governed by multiple parameters: tectonics, paleogeomorphology and climate fluctuations. This integrated study on fluvial sedimentation and evolution of the Oligocene drainage system enable us to propose a conceptual model depicting fluvial channel styles and component sand-body architecture in lacustrine rift basins with axial plus transverse sediment supplies. This model can serve as a reference to illustrate channel-sand-body and associated reservoir architecture in similar types of drainage systems in terrestrial basins.