The 4D seismic signature of gas in thin-bedded geology: results from an outcrop analogue model

Numerous examples of reservoir fields from continental and marine environments involve thin-bedded geology, yet, to our knowledge, the inter- relationship between thin-bedded geology, fluid flow and seismic wave propagation remains unexplored. This paper focuses on the 4D seismic signature of gas saturated thin layers, and addresses the challenge of identifying the relevant scales and properties which correctly define the geology, fluid flow and seismic wave propagation in the field. Based on the study of an outcrop analogue for a thin-bedded turbidite, we model the time lapse seismic response to gas saturation changes for different levels of model scale, from fine scale geological grid to upscaled coarse flow simulation grids. Our results show that multiples and converted waves vastly contribute to the measured amplitudes in the case of thin-bedded geology. Hence, any forward/inverse modelling from the flow simulation to the seismic domains involved in quantitative 4D has to take into account thin layers when these are present in the geological setting.