High‐resolution seismic imaging of a deep gas reservoir from azimuthal offset VSP, Saudi Arabia

Abstract

An azimuthal, offset VSP survey was acquired over the deep Unayzah sandstone gas reservoir in central Saudi Arabia to confirm the direction along which a proposed horizontal sidetrack to a vertical delineation well was to be drilled. Futhermore, the well test indicated a probable reservoir boundary less than 100 feet away from the well. It was hoped that some of the offset sections might image a small fault, if such were the cause. In addition to the normal zero-offset VSP conducted at the vertical well, two multi-offset VSPs at orthogonal azimuths, and four singleoffset VSPs at other azimuths were acquired. Due to excellent cased hole conditions for the receivers and uniform sandy terrain for the sources, the VSP records were practically noise-free. The dominant frequency in the VSP data was virtually twice that of the surface seismic data. The offset VSP sections show that the gas-bearing Unayzah-A sandstone reservoir is now better resolved and that it deteriorates to the NE while maintaining both its thickness and porosity towards the NW, the direction in which the horizontal section is proposed to be drilled. The shorter offset sections in each quadrant surrounding the well show the absence of any clear disruptions that may indicate faulting, which leads to the suspicion that the boundary detected from the well test may be due to facies change, cementation, or mineralized fractures associated with a major fault bounding the accumulation to the south.