Moving towards prestack 4D time-shift analysis to maximize the value of 4D seismic data

Most current post-stack time-shift estimation methods assume that the seismic ray paths in the seismic vintages are vertical, which is incorrect. Importantly, the offset/angle and offset/angle and azimuth dependencies of the 4D response can lead to smearing when estimating the 4D signature on the post-stack domain. Although 4D post-stack analysis brings value to the 4D interpretation, it may not always be appropriate for quantitative interpretation, especially for the 4D amplitude response which is sensitive to the error in the time-shift estimation.

In this paper, we propose a pre-stack approach for 4D analysis, aiming to maximize the value of the 4D time-lapse project by estimating and applying the 4D time-shift corrections directly in the pre-stack domain. This will allow us to generate all the 4D attributes in the pre-stack domain and then stack the results for a cleaner 4D reservoir signature. In addition, this approach allows us to compute the gradient of the offset variation in time-shift at each CDP, which serves as a proxy metric for the confidence in the original post-stack time-shifts. Moreover, this approach facilitates the computation of the ellipticity of the chosen 4D attribute by using 2D ellipses fitting of azimuth sectors, providing a measure of the anisotropy of the 4D attributes. We demonstrate the impact of our pre-stack time-shift analysis on the North Sea PRM datasets over two field examples, focusing specifically on waterfront delineation around water injectors in the fields and on the time-shift in the overburden within the reservoir interval, including the 4D amplitude difference.