Seismic characterization of the depositional evolution history of the organic shale reservoir is essential for reservoir quality evaluation and geologic model building in unconventional plays. However, a direct link between the depositional environment and seismic elastic responses in organic-rich shales remains unclear. By combining the depositional history and rock-physics analysis, we have determined how the depositional environment affects the elastic characteristics of the Longmaxi-Wufeng Shale in the Fuling gas field, Sichuan Basin, Southwestern China. Sedimentological control on the elastic properties mainly lies in two aspects: First, the distinct elastic features of the overlying turbidity mudstone and the underlying deepwater shelf Longmaxi Formation are primarily caused by the rock structure difference due to water energy; second, within the deepwater shelf siliceous shale formation, the elastic property variations are primarily controlled by the progradation tract system and water depths. We evaluate the effect of two types of quartz (biogenic quartz and detrital quartz) in conjunction with organic matter on the elasticity of organic shale. Furthermore, we determine that the two most commonly used seismic inversion attributes, P-impedances and the VP/VS ratio, can be used to indicate the depositional facies evolution. This also gives insights into using geophysical attributes to directly characterize depositional facies for unconventional shale reservoirs.

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