199 / 2022-02-28 21:20:31

Sedimentary facies architecture of fluvial reservoirs in responsible for variable ratio of accommodation/sediment-supply

Fluvial succession, sedimentary architecture model, braided river, meandering river, accommodation/sediment-supply ratio

Fluvial successions are important components of the hydrocarbon reservoirs all over the world. Understanding their sedimentary facies architecture is important for the prediction of sedimentary architectures in fluvial reservoirs. However, the sedimentary architectures of fluvial reservoirs are variable with different conditions of accommodation/sediment-supply (A/S) ratio. Therefore, this work aims to establish a set of sedimentary architecture models of fluvial reservoirs, which are in responsible for A/S ratio. For this purpose, a fluvial reservoir case in the Chengdao Oilfield and several real-world rivers are selected. The work performed on the selected cases mainly consists of two parts: recognition of the A/S ratio changes, and characterization of the sedimentary facies architectures that mainly includes channel patterns, geometry and stacked features of their channel sandbodies. The research results and main conclusions are as follows. i) A/S ratio is an important controlling factor of fluvial patterns in meandering and braided river systems. With the rise of A/S ratio, rivers commonly transit between different fluvial patterns in the following order, i.e., from braided river, to wandering river, to braided-meandering river, to classic meandering river and to meandering river with a small sinuosity. ii) The channel-stacked patterns range from multiple channels stacked in a heavy density, two or three channels stacked laterally, and channel isolated in floodplain. iii) In terms of the internal architecture features, braided bifurcations migrate frequently, so erosion/incise surfaces and chutes are very common. There are no classic vertical accretion sandbodies and falling silt layers in braided bars. In successions produced by wandering rivers, mid-channel bars are commonly composed of a set of vertical accretion sandbodies and falling silt layers. Point bars in meandering rivers usually consist of a set of lateral sandbodies and shale drapes.