One of the target formations of the studied field is a clastic reservoir of the Middle Cretaceous. This particular formation is comprised of multiple sand units (bars, channels), with varying thickness and lateral discontinuity, separated by thin shale intervals. Due to data limitation and complex depositional environment, the delineation and reservoir characterization of this clastic sequence is relatively complex. This paper describes the workflow used to extract information about the acoustic properties (acoustic impedance) of the target formation and infer additional knowledge of the reservoir properties (porosity) using a model-based deterministic post-stack seismic inversion.
The area of interest (AOI) is covered by a recent high quality 3D seismic survey which has been processed through true-relative amplitude pre-stack time migration. Few wells containing monopole sonic, density and gamma-ray logs were available in the AOI. A basic feasibility study comprising rock physics, 1D well-log based seismic forward modeling, seismic-to-well tie and wavelet extraction was carried out prior to running model-based acoustic impedance inversion. The input seismic volume was post-stack pre-conditioned prior to inversion in order to enhance the usable frequency bandwidth and to reduce residual noise. Several techniques (with and without wells) were used to extract the seismic wavelet. A range of background (low-frequency) models were constructed using different methods and their impact on the inversion was appraised with the assistance of blind wells and classical inversion diagnostics.
The resulting average map and cross-sections of acoustic impedance (AI) enabled to locate the areas of high porosity (low AI) within the target formation. Consequently, in addition to seismic interpretation, we could optimize the location of the candidate well within the high porosity area. The generated AI helped in refining the seismic interpretation of this clastic reservoir as well.
Despite the complexity and the unpredictability of the Clastic reservoir depositional environment, Post-stack seismic inversion was a powerful tool that helped to locate the areas of good reservoir quality and ultimately to optimize the location of the candidate well.