Quantitative Architectural Anatomy of the Meandering Fluvial Deposits of the Neogene Upper Guantao Formation in Zhanhua Sag, Bohai Bay Basin, China

Abstract

The residual oil in the fluvial reservoir of Gudong Oilfield is still widely developed even after decades of development. In order to explore the remaining oil in the subsurface fluvial reservoir, we need to have a better understanding on fluvial sandbody characteristics. This paper aims to demonstrate how to quantify anatomy on meandering fluvial architecture. Sedimentologic data from core descriptions of Guantao Formation in Gudong Oilfield were used to identify architectural elements. The upper Guantao Formation consists of ten lithofacies composing five different types of architectural elements. By integrating the available core, well logs, and seismic data, the fluvial sandbody distribution was analyzed. To characterize the fluvial channel geomorphology, we used seismic inversion to explore the stratigraphic correlation and stacking patterns of different channels, and the sandbody isopach map was obtained by the sandbody thickness in each layer. The architectural elements distribution in the plane was mapped by the sandbody isopach map and its profile characteristics. According to the investigation of meandering fluvial outcrops, subsurface meandering fluvial seismic geomorphology, and modern river morphology, the upstream-bar portions are always eroded because of the downstream channel migration. Statistics on the morphometric parameters (bar width/looplength, upstream deflection angle, downstream deflection angle, etc.) of modern meandering river (Ob River) were obtained from the Google Earth. It is found that the bar width/looplength and upstream deflection angle, upstream deflection angle and downstream deflection angle in modern meandering river have a good relationship, and we can have a quantitative architectural characterization on the point bar. This study quantified lithofacies thickness and proportion in different architectural elements of Guantao Formation, which ensures a high resolution anatomy on the small scale architecture. By using this method, the lithofacies proportion in each architectural element was computed, and through the statistics of architectural elements proportion in the Gudong Oilfield, the different lithofacies proportions in the block 6 can be computed.