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Spectrum of Fluvial Systems and Depositional History From High-Resolution Study of Seismic Sedimentology (Lithology and Geomorphology), Neogene, SLT Area, Bohai Bay Basin, China


A 3D survey in SLT area of Bohai Bay Basin provided a rare opportunity to study Neogene fluvial systems and depositional history in wireline-log resolution (2 m). The 3D seismic data are characterized by a high signal-to-noise ratio and a frequency range of 10-100 Hz, with a predominant frequency of 60 Hz. Wireline logs from more than 20 wells with conventional cores ensured a solid calibration of seismic data to lithology and facies. The well and seismic interpretation followed a five-step process. (1) Seismic cube was converted to relative impedance volume, which in turn was correlated to gamma ray logs at well locations by using a well-based lithology-impedance model calibrated by core. As a result, seismic events in the converted volume could be viewed as representing thin-bed lithologic units. (2) The converted volume was properly decomposed into selected panels of high, moderate, and low frequency components. (3) A combined rendering of these panels was realized by using RGB or other color-blending schemes for expanded seismic visual resolution and truer stratal continuity. (4) Stratal slices (Wheeler cube) were made by applying linear interpolation function between reference seismic events. (5) Seismic geomorphologic patterns with lithological designation were analyzed for depositional elements, systems, and history. Our interpretation resolved highly detailed lithologic –geomorphologic models for braided and meandering fluvial systems, which can be easily correlated to modern analogs. The results revealed a complex fluvial depositional history, which involved the development and evolution of different fluvial systems in six fourth-order sequences during a generally transgressive-regressive cycle. In the lowest two sequences, large (>15 km wide) and thick (>30 m) braided fluvial systems dominated. Smaller (3-10 km wide) braided fluvial complexes were identified in the following sequence. As the base-level continued to rise, small braided rivers and complexes (1-5 km wide) developed. During the following sequence underwent a base-level drop, resulting in a system mixed with meander fluvial and seasonal shallow-water deltaic sediments. In the highest (youngest) two sequences, base-level continued to fall, and large meandering fluvial systems (point-bar complexes > 10 km wide) became dominant. The shift from braided system to meandering system was accompanied by shift of sources of sediment supply, both in direction and distance.