Fan-delta facies architecture, morphological evolution and sediment delivery in the Oligocene Dongying Formation of the Nanpu Sag, Bohai Bay Basin, China

Abstract

Core, well log and seismic data are used to investigate relationships between fan-delta facies architecture, basin margin morphology and sediment transportation into a deep-lacustrine environment of the Dongying Formation of Nanpu Sag, Bohai Bay Basin. The study interval is subdivided into two fan-delta dominant, third-order sequences of Stratasequence Paleogene Dongying Formation 1 (SQEd 1) (younger) and Stratasequence Paleogene Dongying Formation 2 (SQEd 2) (older). Fan-delta growth occurred in two stages with changing of facies architecture and morphology. During the early stage (SQEd 2), the facies architecture is characterised by smaller and thin, channelised deposits, an extended mouth bar (delta-front sandy deposits) and slump/slides/flood turbidite dominated sandy deposits in the prodelta. However, during the late stage (SQEd 1), the facies architecture is characterised by extended and thicker channelised deposit-dominated area, a local mouth bar (delta-front sandy deposits) and slump/slide-dominated sandy deposits in the prodelta. From the early to late stage, the delta morphology also changed with an upward increase in the delta-plain areal extent and delta-front gradient. Facies architecture variations indicate differences in sediment dispersal. The upward thickening and extending delta plain implies that more sediment was trapped in the proximal area owing to sufficient accommodation space generated by increasing boundary fault displacement. The upward increasing delta-front gradient resulted in differences in the type of prodelta/deep-lacustrine sandy deposits, while grainsize variations of the delta-front deposits affect the prodelta/deep-lacustrine sandy deposits. Boundary fault growth occurred with upward increasing displacement and length with the result that sediment supply outpaced accommodation-space creation and controlled the facies architecture changes. KEY POINTS A two-stage fan delta developed with changing facies architecture is analysed. Facies architecture controlled fan-delta morphology with sediment transport into a deep-lacustrine area. Faults control the facies architecture and sand-rich deposit distribution.