AVO and Anisotropy from Logs and Walkaways

An amplitude versus offset (AVO) case study is presented from the West of Shetlands, UKCS in which pre-drill AVO signatures over a prospect were interpreted to be expressions of hydrocarbons but upon study of the data provided by the well were interpreted as a manifestation of anisotropy. The core from the well proved to be crucial in understanding the non-hydrocarbon AVO signature.The well encountered good quality, high porosity, water wet sand and an underlying tuff sequence. Neither the dipole sonic log nor the check-shot survey revealed any anomalous elastic properties, nor did they detect the presence of anisotropy. Conventional, isotropic modelling using the measured log properties could not match the observed AVO response. Only comparison of the dipole sonic log and the check-shots with the core measurements gave evidence for the presence of substantial elastic anisotropy. Incorporation of anisotropy inferred from core and log measurements in the shales and tuffs gave a reasonable match to the observed AVO. The inferred anisotropics are substantial, averaging 15% for both the compressional and shear velocities and increasing over some depth intervals to 25%. A subsequent walkaway vertical seismic profile (VSP), acquired for the specific purpose of calibrating the AVO response, shows a marked AVO effect at the base of the target sandstone and tuffs, consistent with the surface seismic data. It is clear from this case study and other work that simple transverse anisotropy, for example, produced by the natural layering in shales, can cause first order effects on the AVO response.

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Characterization of dispersion, attenuation, and anisotropy at the Buena Vista Hills field, California

Hackert

Parra

Brown

et al. 2001

GEOPHYSICS

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We create a log of intrinsic dispersion and attenuation for the Antelope Shale formation of the Buena Vista Hills field, San Joaquin Valley, California. High dispersion (or low Q) values correlate with thin sand and carbonate beds within the Antelope Shale. These beds are at least ten times as permeable as the host shale formation, so this effect provides a possible avenue for seismic prediction of permeability. The dispersion log is formed through comparison of crosswell seismic velocities (measured at approximately 1 kHz) and sonic log velocities (measured at approximately 10 kHz). In order to provide a proper basis for comparison, the sonic log must first be adjusted for field anisotropy, scaling effects, and resolution of measurement. We estimate a local shale anisotropy of about 20% based on correlations generated from published measurements of other shale fields. We apply resolution enhancement to capture the thin sand and carbonate beds, and windowed Backus averaging to match the measurement scales. A modeling study verifies the technique, and shows that beds of thickness greater than 30 cm have a measurable signature. The actual resolution is on the order of the crosswell Fresnel length, or about 7 m for the model study.

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AVO and Anisotropy from Logs and Walkaways

Cited by 4 publications

References 16 publications

Effects of sand-shale anisotropy on amplitude variation with angle (AVA) modelling: The Sawan gas field (Pakistan) as a key case-study for South Asia's sedimentary basins

Effects of sand-shale anisotropy on amplitude variation with angle (AVA) modelling: The Sawan gas field (Pakistan) as a key case-study for South Asia's sedimentary basins

Anisotropy and amplitude versus offset: a case history from the West of Shetlands

Characterization of dispersion, attenuation, and anisotropy at the Buena Vista Hills field, California

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