Identification of shallow-water-flow sands by Vp/Vs inversion of conventional 3D seismic data

Abstract

Shallow-water flow (SWF) from overpressured, unconsolidated/uncemented sands is a high-profile hazard in deep-water drilling. Thus, accurate predrill seismic prediction of SWF sands is an important practical problem. Laboratory measurements show high seismic compressional-to-shear velocity ([Formula: see text]) ratios in SWF sands. Thus, [Formula: see text] anomalies produced by elastic seismic inversion can provide a quantitative identification of SWF sands before drilling. Three 3D common-angle-aperture prestack time-migrated data sets from the Garden Banks area of the Gulf of Mexico are inverted simultaneously for [Formula: see text]. Two wells provide constraints. High [Formula: see text] values are observed in a known SWF zone in Shell Deepwater Development's well GB920 #1. In Kerr-McGee Oil & Gas Corporation's well GB877 #1, [Formula: see text] anomalies match well with independent predrill SWF sand predictions based on geologic information and seismic amplitudes. High [Formula: see text] anomalies (>9) are used to identify potential SWF sands away from the wells. We conclude that [Formula: see text] values are more diagnostic than seismic amplitudes alone for SWF sand detection, and that low S-impedance is the most salient indicator of SWF potential. Raw elastic seismic-amplitude variation with angle data provide unreliable indicators in cases where SWF sands are thin (where interference occurs between reflections from the layer tops and bottoms) or if viscoelasticity is significant.