Abstract

Approximately 5 miles (8 km) of total wave-field data were acquired by Production Geophysical Services (then Kim Tech., Inc.), using the OMNIPULSE Multi-mode Shear-wave Generator source over the southern end of Lost Hills field, Kern County, California. The quality of the shear-wave sections was excellent. They represent a significant improvement over conventional P-wave sections from the area in that they provide better reflection continuity and imaging of the Lost Hills anticline. A multicomponent VSP, which was acquired close to the line, provided crucial P-wave to S-wave correlation, as well as fracture information. V p /V s ratios computed from interval times ranged from 2.79 to 1.63. An anomalously low V p /V s ratio of 1.65 in the zone of interest (Lower Reef Ridge to McDonald shale), confirmed by multicomponent VSP data, corresponds to the producing interval.Evidence of shear-wave splitting due to azimuthal anisotropy was observed, so the SV-wave and SH-wave data sets were rotated into principal-component axes of N45E for S1 and N45W for S2. The predominant fracture orientation changes from N45E at depth to N45W near the surface. This change in fracture orientation with depth was confirmed by multicomponent VSP data. Delay-time ratios (used as a measure of fracture intensity) ranged from a maximum of 11.71 percent to a minimum of -5.48 percent across the structure. These ratios are interpreted to show changes in fracture intensity and orientation across the structure. Delay-time ratios in the zone of interest were anomalously high (1.55-6.53 percent). Comparison of fracture intensity on the flanks of the structure with that on the crest indicates that the flanks have undergone greater deformation than the crest.The total wave-field data set and associated analyses have provided significant structural and stratigraphic information on the Miocene Monterey formation over the Lost Hills field, highlighting the productive interval.

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