--> ABSTRACT: A Coupled Petrophysical-Geophysical Model Demonstrates Velocity Dispersion in a Thin-Bedded, Fluvial Reservoir, by Odom, Richard C.; #90142 (2012)

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A Coupled Petrophysical-Geophysical Model Demonstrates Velocity Dispersion in a Thin-Bedded, Fluvial Reservoir

Odom, Richard C.*1
(1) HWS Petrophysics, Arlington, TX.

The Stratton Field 3-D seismic survey has been publically available and widely distributed and studied for many years. The primary reservoir in this prolific gas play is the Frio formation, a thin-bedded reservoir of fluvial origin. The two-mile by one-mile survey area has good well control with twenty one wells with well-logs, however, the correlation of fluvial reservoirs found in the wells to time-slices of the 3-D seismic has been enigmatic.

Using an interval of the upper-middle Frio formation that has good well coverage, good stratigraphic control and minimal tectonic deformation, a 3-dimensional statistical model was developed to search the 3-D seismic for correlation. The correlation of the well-logs to the VSP time-depth chart was very good; however, the positions of the targeted fluvial objects in the 3-dimensional search showed that the VSP time-depth chart was much faster than the time-depth positions in the 3-D seismic. The noted shifts between VSP and 3-D seismic were on the order of 18mS (TWT) shift in 500 feet; this translates to approximately a 90-foot targeting error. This is a very significant error when targeting the thin fluvial reservoir sands that are typically 30 to 60 feet thick.

In the petrophysical model, a classifier was developed to identify beds based on three predominate facies: reservoir sands, tight sands and floodplain mudstones. This information was used to generate bed thickness distributions for the wells. The scales of the seismic velocities, wavelengths and bed distributions appear scalable to forward modeling and experimental results on velocity dispersion. Based on this finding, an improved time-depth chart is published for this section of the survey based on a linear relation to the target positions.

These results have significant impacts in the proper handling and targeting of seismic data in reservoirs such as the Frio formation: 1. Thin-bed velocity-dispersion is a potent effect in these types of reservoirs; 2. Lateral heterogeneity in dispersive parameters may require special consideration when targeting thin fluvial members. 3. Synthetic seismogram techniques such as Weiner-Levinson wavelets rely on linear statistical models and do not account for dispersion.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California