This paper is an extension to previously published work on a Mississippi Lime prospect in north-central Oklahoma. The prospect is data rich: It has high density nodal 3C acquisition with all components processed and jointly inverted. Several pilot holes and laterals were drilled and fully logged with Sonic Scanner and FMI fracture logs.
The present work uses 5 Sonic Scanner and 9 FMI logs (5 vertical and 4 lateral) to calibrate converted shear (PS) anisotropy. Different anisotropic effects were measured over the prospect (36 data points) to find general relationships. These were then calibrated to fracture data from FMI logs (17 data points).
Anisotropic effects investigated were
azimuthal travel time differences in the gathers (i.e. sinusoidal variation),
amount of azimuthal travel time correction after C-wave splitting estimation and compensation,
amount of radial component energy within the reservoir unit,
amount of transverse component energy within the reservoir unit, and
the transverse/radial energy ratio within the reservoir unit.
Dominant fracture directions are noted but not analyzed quantitatively.
In this survey area we found that transverse energy was roughly proportional to radial energy and transverse/radial energy ratio was roughly proportional to azimuthal sinusoidal amplitude and the amount of splitting correction. However, transverse energy was not related to transverse/radial energy ratio, sinusoidal amplitude or amount of splitting correction.
When calibrated to FMI fracture counts, the only reliable measure of fracture density was with the transverse energy. All other data elements had questionable or non-existent relationships. This is in good alignment with theory and provides a reliable benchmark to guide seismic fracture characterization in unconventional plays.
Shear wave splitting, or birefringence, is where shear waves split into two approximately orthogonal polarizations, each having different propagation velocities and thus a time delay between their arrivals at a seismic receiver (Crampin, 2003). Crampin (1994) states that "The shear wave splitting observed along almost all shear wave ray paths in the earth's crust is interpreted as the effects of stress-aligned fluid-filled cracks, microcracks, and preferentially oriented pore space". Further, below a depth of about 1 km the polarization of the fast shear wave is within about 20° of the maximum horizontal stress. If the depth is such that the vertical stress is greater than the minimum horizontal stress then cracks will be vertical in addition to being approximately parallel to the maximum horizontal stress (Crampin, 1990).