Walkaway VSP Processing for Anisotropy Characterization - An Integrated Approach with Surface Seismic in the Khazzan Field, Oman
Authors: Al Anboori A. , Cook P. , Johnston R. , Barnes E. , Al Battashi M. , Rufino R. , Guerra R. , Al Badi O. 
Published: October 2023

The giant Khazzan gas field, onshore Oman, has been on production since 2017, and currently produces ∼1.5 bcf/d of gas and associated condensate from the Cambro-Ordovician paralic Barik Formation. It covers an area of 2800 km2. Structural depth is a major uncertainty particularly on the flanks of the field which have poor well control and low structural relief. The increased water risk in these areas and the impact on the recoverable gas and associated liquid content have become key development considerations. The depth uncertainty is primarily driven by limited well control and seismic velocity uncertainty. Therefore to provide localized calibration a walkaway vertical seismic profile (WVSP) was acquired in 2022 in a vertical well to reduce the structural uncertainty via velocity model and seismic anisotropy calibration, and subsequently de-risk the resources and formulate an optimized development plan.

The VSP was acquired using an array of eight 3C geophones and two vibroseis sources; and provided the following measurements: a) zero-offset data along the well to calibrate the vertical velocities and b) walkaway data over three depth intervals (shale prone) and along two orthogonal lines to calibrate polar anisotropy (assuming Tilted Transverse Isotropy "TTI") and azimuthal anisotropy, respectively. The VSP analysis was split into two phases: data processing and anisotropy computation. The data processing workflow included data preprocessing, time picking, 3C orientation along the ray path, 3C wavefield separation, deconvolution and migration. The anisotropy computation employed slowness-polarization methods to locally estimate anisotropy for each depth interval using a deterministic search. A probabilistic Monte Carlo inversion algorithm was also used to quantify uncertainty on the inverted anisotropy parameters. Finally, the VSP results were integrated with well logs and surface seismic.

The VSP data quality is excellent and allowed measuring compressional and shear arrivals on both downgoing and upgoing wavefields. The large-offset acquisition allowed wide incident angles (up to 60° from vertical) to be recorded. The Markov Chain Monte Carlo (MCMC) inversion provided an accurate estimate of anisotropy due to good data quality (shear waves are more sensitive to anisotropy than compressional waves) and wide source-receiver offsets. The VSP showed significant anisotropy in the shallow section, higher than that derived from surface seismic. This is a factor that may contribute to the depth differences observed in the upper stratigraphy between seismic predictions and actuals in new wells.

This is the first walkaway VSP in Block 61. It also represents unique information from the borehole as it improves on surface seismic low-resolution anisotropy, measuring Thomsen parameters (delta, epsilon and tilted axis) for each shale unit. The impact of horizontal stresses (azimuthal anisotropy) was also measured. The VSP added value in calibrating the shallow overburden (confirming high anisotropy) enabling reliable velocity picking in future surface seismic reprocessing activities.