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Predrill Lithoclassification in the Wolfcamp, Permian Basin, and Other Clay-Rich Plays: The Necessity of Prestack Seismic Inversion

Abstract

Seismic inversion is used for reservoir description and to identify reservoir-quality rock and the best approach for completion.

The Wolfcamp and Spraberry formations of the Midland Basin are quite different from the Wolfcamp and Bone Spring formations of the Delaware Basin. The difference is based on the general lithological composition of the reservoir, stress field, fracture distribution, and pore pressure. The Wolfcamp section is generally described as thick quartz-rich shale with several layers of carbonate bodies intersecting the sequence. The carbonate layers are not continuous bodies and are found near the Wolfcamp A-to-B transition and in the Wolfcamp C unit. Spatially, the volume fraction of carbonate could vary significantly, reducing the availability of the more desirable clastic layers. The volume fraction of clay within the Wolfcamp formation also varies significantly between these two basins. While the volume fraction of clay is high in the Midland Basin, clay volume fraction is relatively low in the Delaware Basin. Variability is also observed in the presence of kerogen, the effective porosity distribution, and the pore-pressure development.

Carbonate has high acoustic impedance and high Vp/Vs, while clay has low acoustic impedance and high Vp/Vs. Quartz-rich rock generally has moderate acoustic impedance and low Vp/Vs. As the rock contains all three different minerals in various qualities, acoustic impedance and Vp/Vs must be studied closely to distinguish between different facies.

Poststack inversion only provides acoustic impedance estimation. It is difficult to distinguish between different facies from poststack inversion, especially in mixed clay or silica systems.

Prestack inversion is crucial for facies identification as acoustic impedance and Vp/Vs can be extracted from inversion, so that detailed study of well measurements, petrophysical interpretation, and prestack seismic inversion can provide an understanding of the distribution of reservoir facies and subsequent estimation of Vcarbonate, Vclay, Vquartz, porosity, and total organic carbon using a petrophysical rock physics model for each facies. As will be shown, these findings are applicable to all clay-rich plays.